Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results for Cassava Peeling Residue Digestate

*Sammy N. Aso*

## **Abstract**

Circular economic paradigm applies residue from one process as input material for another, fostering sustainable benefits for humanity. Anaerobic digestion (AD) is an attractive technology for biogas production in a circular economy. Digestate is the residual organic matter generated as coproduct of biogas. Because digestate is nutrient rich and largely stabilized, it has varied management options. Digestate is suitable for direct use as bio-fertilizer and is a good amendment material to improve soil physical properties. However, the quality, safety, and utility of digestate are dependent upon the characteristics of feedstock, digester process, pre- and postdigestion treatments. Digestates emanating from AD of animal manure, energy crops, food processing residues, and other feedstocks have been reported in published literature. On the other hand, there is dearth of reports on digestate emanating from AD process that utilized cassava peeling residue (CPR) as sole feedstock. This chapter presents relevant information on digestates including production, feedstock, quality and safety requirements, processing and treatment technologies, regulatory aspects, applications management options, cost implications, as well as challenges and opportunities. In addition, new results of nitrogen (N), phosphorus (P), and potassium (K) compositions of liquid fraction of CPR digestate are reported.

**Keywords:** anaerobic digestion, biofuel, biogas, cassava, cassava peeling residue, CPR, circular economy, digestate, management options, renewability, sustainability

## **1. Introduction**

Linear economic model has been constructed on the premise of production, use, and disposal of used resources as wastes. However, there are serious limitations associated with the linear paradigm. These include nonrenewability, unsustainability, and environmental perturbations characterized by negative impacts on air, eco-diversity, soil, and water quality and safety. On the other hand, circular economic model maximizes the 3 (three)Rs of reduce, reuse, and recycle resources. In particular, circular economy applies residue from one process as input material for another process. This approach delivers sustainable benefits for humanity in terms of air, ecology, energy, environment, food, forest, housing,

sanitation, soil and water quality, safety and security; as well as improvements in animal and human health, economic, social, and industrial developments.

(SEBAC), stirred anaerobic sequencing batch reactor (SASBR), up-flow anaerobic sludge bed (UASB) or up-flow multistage anaerobic reactor

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

Today there are millions of anaerobic digesters (domestic, medium, and largescale versions) operating in the world and generating tremendous amount of biogas. In 2016 for instance, about 60.8 billion m<sup>3</sup> of biogas (1.31 EJ) was generated worldwide; most of it, 84%; in Europe (54%) and Asia (30%) [8]. The technical status of AD plants varies widely. Advanced state-of-the-art systems are prevalent in Europe and more low-tech installations in Africa, Asia and South America. However, irrespective of the level of sophistication, the two fundamental products

Digestate is the residual organic matter generated as coproduct of biogas production. Digestate is suitable for direct use as bio-fertilizer, as raw material for production of bio-fertilizers, and as amendment material to improve soil physical

1 Agro-industrial residues 61 *Miscanthus sacchariflorus* (Maxim.) Hack silage

2 Animal manure 62 *Miscanthus sinesis giganteus* Silage

4 Biodegradable plastics 64 Mozzarella Cheese Whey 5 Biodiesel wastewaters 65 Municipal solid waste 6 Biowastes 66 Municipal waste water

8 Buffalo farming wastewater 68 Olive oil mill wastewater 9 Buffalo manure 69 Olive Pomace, olive waste 10 Cacao 70 Orange peel waste

15 Cereal-WPS 75 Peeled Cassava wash water 16 Cereals 76 Pharmaceutical industry sludge 17 Cheese Whey 77 *Phleum pratense* L. silage

11 Cardboard 71 Organic fraction of municipal solid waste

(UMAR).

**3. Digestate**

of AD are biogas and digestate.

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

**S/N Feedstock S/N Feedstock**

3 Barley straw 63 Molasses

7 Blood industry residues 67 Oat silage

12 Cattle/cow: manure/slurry 72 Paper 13 Cattle (beef) urine 73 Paper sludge 14 Cereal bran 74 Peach-juice pulp

18 Chicken manure 78 Pig urine

21 Coffee grounds 81 Plum stones

23 Corn cob mix 83 Potato waste 24 Cornmeal 84 Potatoes

**201**

19 *Chroococcus* sp. (algal biomass) 79 Piggery wastewater

20 Coconut chips 80 Pig/swine effluent; manure; slurry

22 Corn 82 Potato chips production residues

On the predicate of biorefinery platform, biotechnological upgrading of biomass via biological, chemical, physical or some combinations of these would create bio-based energy, chemicals, and other beneficial metabolites and products within the domain of circular economic model. In this context, anaerobic digestion (AD) is an attractive technology as it would utilize organic resources in waste streams to generate biogas and digestate. However, the quality of digestate is dependent upon variables such as characteristics of feedstock, digester process, and treatment options. Digestates emanating from AD of animal manure, energy crops, agricultural residues, organic fraction of municipal solid wastes (OFMSW), and other feedstocks have been reported in published literature [1–3]. On the other hand, there is dearth of reports on nutrient properties of digestate generated from AD processes that utilized cassava peeling residue (CPR) as sole feedstock. This chapter presents relevant information on digestates in general, and new results of a technical experiment conducted to secure overview assessment of nitrogen (N), phosphorus (P) and potassium (K) compositions of liquid fraction of CPR digestate.

## **2. Anaerobic digestion (AD)**

AD is a biochemical process that decomposes organic matter to generate flammable biogas and residual digestate. The process is achieved with the assistance of a suite of microorganisms in a near oxygen free environment. Biogas is basically composed of methane and carbon dioxide in the respective range of 40–75% and 25–40%. Other constituents are hydrogen, nitrogen, oxygen, hydrogen sulfide and other trace components ranging from 0.1 to 3% [4]. Successful AD operations are carried out within digester or reactor systems designed to supply nutrients required for metabolic activities of the microbes, as well as prevent conditions or elements that may become stressors or present inhibitory effects. AD digester operations and systems may be classified according to the following [5–7]:


*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

(SEBAC), stirred anaerobic sequencing batch reactor (SASBR), up-flow anaerobic sludge bed (UASB) or up-flow multistage anaerobic reactor (UMAR).

Today there are millions of anaerobic digesters (domestic, medium, and largescale versions) operating in the world and generating tremendous amount of biogas. In 2016 for instance, about 60.8 billion m<sup>3</sup> of biogas (1.31 EJ) was generated worldwide; most of it, 84%; in Europe (54%) and Asia (30%) [8]. The technical status of AD plants varies widely. Advanced state-of-the-art systems are prevalent in Europe and more low-tech installations in Africa, Asia and South America. However, irrespective of the level of sophistication, the two fundamental products of AD are biogas and digestate.

## **3. Digestate**

sanitation, soil and water quality, safety and security; as well as improvements in

On the predicate of biorefinery platform, biotechnological upgrading of biomass

AD is a biochemical process that decomposes organic matter to generate flammable biogas and residual digestate. The process is achieved with the assistance of a suite of microorganisms in a near oxygen free environment. Biogas is basically composed of methane and carbon dioxide in the respective range of 40–75% and 25–40%. Other constituents are hydrogen, nitrogen, oxygen, hydrogen sulfide and other trace components ranging from 0.1 to 3% [4]. Successful AD operations are carried out within digester or reactor systems designed to supply nutrients required for metabolic activities of the microbes, as well as prevent conditions or elements that may become stressors or present inhibitory effects. AD digester operations and

• Optimal temperature regimen: psychrophilic (<20°C), mesophilic (30–38°C),

• Total solid (TS) content: wet digestion (TS < 12%), semi-dry digestion (TS

• Feeding mode: batch, fed-batch, semi-continuous, and continuous;

• Process stage or step: single-stage (where all AD processes—hydrolysis, fermentation, acetogenesis, and methanogenesis are executed in one

reactor), and multi-stage (where the processes are separated into two or more

• Fluid-dynamic mode: plug flow, completely stirred or mixed, and hybrid; as

• Digester design: anaerobic baffled reactor (ABR), anaerobic filter (AF), anaerobic dynamic membrane reactor (AnDMBR), anaerobic mixed biofilm reactor (AMBR), completely or continuous stirred-tank reactor (CSTR), covered lagoon, expanded granular sludge bed (EGSB), fixed dome, flexible balloon or tube, floating cover or drum, sequential batch anaerobic composting

systems may be classified according to the following [5–7]:

animal and human health, economic, social, and industrial developments.

**2. Anaerobic digestion (AD)**

*Renewable Energy - Technologies and Applications*

and thermophilic (48–57°C);

reactors);

well as

**200**

12–20%), and dry digestion (TS ˃ 20%);

via biological, chemical, physical or some combinations of these would create bio-based energy, chemicals, and other beneficial metabolites and products within the domain of circular economic model. In this context, anaerobic digestion (AD) is an attractive technology as it would utilize organic resources in waste streams to generate biogas and digestate. However, the quality of digestate is dependent upon variables such as characteristics of feedstock, digester process, and treatment options. Digestates emanating from AD of animal manure, energy crops, agricultural residues, organic fraction of municipal solid wastes (OFMSW), and other feedstocks have been reported in published literature [1–3]. On the other hand, there is dearth of reports on nutrient properties of digestate generated from AD processes that utilized cassava peeling residue (CPR) as sole feedstock. This chapter presents relevant information on digestates in general, and new results of a technical experiment conducted to secure overview assessment of nitrogen (N), phosphorus (P) and potassium (K) compositions of liquid fraction of CPR digestate.

Digestate is the residual organic matter generated as coproduct of biogas production. Digestate is suitable for direct use as bio-fertilizer, as raw material for production of bio-fertilizers, and as amendment material to improve soil physical



properties such as bulk density, hydraulic conductivity, and moisture retention capacity. Digestate is also attributed with improved sustainability and veterinary safety; reductions in odors, weed seeds, plant pathogens, food chain contamination risks and greenhouse gas emissions. The three basic types of digestate are: whole digestate, liquor (liquid fraction) digestate, and fiber (solid fraction) digestate. Whole digestate is the digestate as obtained leaving the digester at the end of AD process. It contains less than 15% dry matter. This whole digestate could be separated into liquid and solid fractions using appropriate technology and method. The liquid fraction constitutes up to 90% of the digestate by volume, contains 2–6% dry matter, particles <1.2 mm in size, and most of the soluble nitrogen and potassium, while the solid fraction retains most of the digestate phosphorus, and contains dry

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

However, the quality, safety, and utility of digestate are dependent upon variables such as feedstock characteristics (pH, chemical composition, carbon-nitrogen ratio (C/N), particle size), digester process (temperature, inoculum, microbial community, hydraulic retention time (HRT)), as well as pre- and post-digestion treatments. Feedstock should possess balanced nutrients, including optimal C/N to satisfy physiological needs of the microorganisms. High or low C/N would disrupt biogasification and lead to reduced biogas output due to low buffer capacity (high C/N) or ammonia inhibition (low C/N). Generally, for biogas production, C/N of 20–30 is considered optimal. For food wastes, C/N of around 15 could be appropriate. Digestates within C/N range of 15–20 are regarded as safe for application to agricultural land without further treatment [11]. When sole feedstock lacks sufficient nutrients for adequate C/N, feedstocks with complimentary nutrients profile are co-digested to offset the limitations. **Table 1** highlights some feedstocks that

Perhaps the most important variable affecting the quality and safety of digestate is feedstock. Starting with a high-quality feedstock would virtually guarantee a safe and quality digestate. Source separation can be used to achieve high purity feedstock. The biological, chemical, and physical properties of digestate may be

governed by regulations and quality assurance systems. The European Union (EU) and many European national governments have hygienic, quality and safety standards for digestate certification that consider feedstock source and other aspects such as digester process, treatment options, handling and storage requirements. The essential quality and safety requirements for digestate destined as biofertilizer must be achieved regardless of the initial raw material. Essential quality and safety parameters include nutrients content, dry matter and organic dry matter contents, homogeneity, pH, purity (free of inorganic impurities such as glass, metal, plastic, and stones), sanitized and safe for soil organisms and the environment with regards to biological status (pathogenic organisms) and chemical status (organic and inorganic contaminants/pollutants). Furthermore, the digestate should be free of odor,

Quality assurance systems for digestate certification may comprise monitoring to ensure control; standardization to ensure repeatable performance; characterization label to identify product fitness; declaration to describe product constituents; application guidelines to ensure safe and proper use; and documentation to prove that the product received required treatments following approved protocols. **Table 2** presents established criteria and characteristics for the production and use of quality and safe digestates. In the EU, conformity with these criteria is enough to

matter content ˃ 15% [9, 10].

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

have been used in AD operations and digestate studies.

**4. Regulations, quality, and safety requirements**

phytotoxicity and weed seeds; and be satisfactorily stabilized.

**203**

#### **Table 1.**

*Feedstocks used in digestate production and studies.*

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

properties such as bulk density, hydraulic conductivity, and moisture retention capacity. Digestate is also attributed with improved sustainability and veterinary safety; reductions in odors, weed seeds, plant pathogens, food chain contamination risks and greenhouse gas emissions. The three basic types of digestate are: whole digestate, liquor (liquid fraction) digestate, and fiber (solid fraction) digestate. Whole digestate is the digestate as obtained leaving the digester at the end of AD process. It contains less than 15% dry matter. This whole digestate could be separated into liquid and solid fractions using appropriate technology and method. The liquid fraction constitutes up to 90% of the digestate by volume, contains 2–6% dry matter, particles <1.2 mm in size, and most of the soluble nitrogen and potassium, while the solid fraction retains most of the digestate phosphorus, and contains dry matter content ˃ 15% [9, 10].

However, the quality, safety, and utility of digestate are dependent upon variables such as feedstock characteristics (pH, chemical composition, carbon-nitrogen ratio (C/N), particle size), digester process (temperature, inoculum, microbial community, hydraulic retention time (HRT)), as well as pre- and post-digestion treatments. Feedstock should possess balanced nutrients, including optimal C/N to satisfy physiological needs of the microorganisms. High or low C/N would disrupt biogasification and lead to reduced biogas output due to low buffer capacity (high C/N) or ammonia inhibition (low C/N). Generally, for biogas production, C/N of 20–30 is considered optimal. For food wastes, C/N of around 15 could be appropriate. Digestates within C/N range of 15–20 are regarded as safe for application to agricultural land without further treatment [11]. When sole feedstock lacks sufficient nutrients for adequate C/N, feedstocks with complimentary nutrients profile are co-digested to offset the limitations. **Table 1** highlights some feedstocks that have been used in AD operations and digestate studies.

### **4. Regulations, quality, and safety requirements**

Perhaps the most important variable affecting the quality and safety of digestate is feedstock. Starting with a high-quality feedstock would virtually guarantee a safe and quality digestate. Source separation can be used to achieve high purity feedstock. The biological, chemical, and physical properties of digestate may be governed by regulations and quality assurance systems. The European Union (EU) and many European national governments have hygienic, quality and safety standards for digestate certification that consider feedstock source and other aspects such as digester process, treatment options, handling and storage requirements. The essential quality and safety requirements for digestate destined as biofertilizer must be achieved regardless of the initial raw material. Essential quality and safety parameters include nutrients content, dry matter and organic dry matter contents, homogeneity, pH, purity (free of inorganic impurities such as glass, metal, plastic, and stones), sanitized and safe for soil organisms and the environment with regards to biological status (pathogenic organisms) and chemical status (organic and inorganic contaminants/pollutants). Furthermore, the digestate should be free of odor, phytotoxicity and weed seeds; and be satisfactorily stabilized.

Quality assurance systems for digestate certification may comprise monitoring to ensure control; standardization to ensure repeatable performance; characterization label to identify product fitness; declaration to describe product constituents; application guidelines to ensure safe and proper use; and documentation to prove that the product received required treatments following approved protocols. **Table 2** presents established criteria and characteristics for the production and use of quality and safe digestates. In the EU, conformity with these criteria is enough to

**S/N Feedstock S/N Feedstock**

*Renewable Energy - Technologies and Applications*

30 Dried blood of slaughterhouse

waste

26 Cover crops 86 Primary sludge 27 Crushed cassava juice 87 Pumpkin waste 28 Dairy manure 88 Rabbit manure 29 Distiller's waste 89 Rape residue

31 Duck slaughterhouse sludge 91 Rice residues

33 Energetic crops 93 Sewage sludge

35 Fennel waste 95 Slaughterhouse waste

37 Food industry residues 97 Solid farmyard manure 38 Food waste 98 Sorghum silage

41 Garden wastes 101 Starch processing wastewater

45 Green waste 105 Sunflower residue, sunflower silage

47 Household kitchen waste 107 *Tetraselmis* sp. (algal biomass) 48 Household waste 108 Thin stillage (bioethanol by-product)

42 Glycerin 102 Straws (cereal, pea) 43 Grape seeds 103 Sugar beet pulp

46 Hemp 106 Tea leaves

49 Human excreta 109 Triticale 50 Human urine 110 Triticale silage 51 Industrial and commercial wastes 111 Turkey manure 52 Jute Caddis 112 Vegetable waste

53 Kitchen waste 113 Vinasse

58 *Medicago sativa* L. silage 118 Wheat

*Feedstocks used in digestate production and studies.*

**Table 1.**

**202**

54 Landscape waste 114 Waste-activated sludge 55 Ley silage 115 Waste potato starch 56 Livestock waste 116 Wastewater 57 Maize stover 117 Wastewater sludge

59 Milk (serum, whey) 119 Yeast production wastewater 60 Millet 120 *Zea mays* L. (corn, maize) silage

*Source: Assembled from scientific literatures in the public domain, most of them cited in this present work.*

34 Energy maize 94 *Sida Hermaphrodita* Rusby silage

36 Fish by-product 96 Sludge from Slaughterhouse wastewater treatment plant

44 Grass (clover, Sudan); grass silage 104 Sugar sorghum (*S. saccharatum* L. Moench.) silage

39 Fruits and distillery by-products 99 Source-separated organic household waste 40 Fruit Marc 100 Source-separated municipal solid waste

32 Edible oil 92 Rye

25 Corn residue 85 Poultry litter/manure/waste

90 Restaurant food waste


ensure that digestate complies with European "End of Waste" criteria; and can be

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

In the context of AD and digestate, we may distinguish between pre- and post-

treatment processes. A pretreatment process refers to a processing operation applied upstream, before the digestate emerges from the digester. This could range from size reduction or thermochemical treatment of feedstock substrate; to process management (such as pH, temperature, and retention time control). On the other hand, a posttreatment process is that processing operation applied downstream of digestate harvest. This may also involve size reduction, other unit operations; composting, and end-product requirements that ensure the digestate sanitation. Post treatment may generate nutrient concentrates, liquid and solid fraction digestates conditioned to standardized biofertilizers, and final liquid effluent that could be discharged into a stream or sewage system. Benefits of posttreatment include enhanced marketability, reductions in handling, storage and transportation

costs/requirements, and compliance with environmental regulations.

Depending on the starting feedstock and desired end product form of the digestate, similar technologies could be used for pre and post treatment processing. Applied technologies and methods may be classified as biological, chemical, or physical. The methods could also be used in combination. Biological treatment could be accomplished with the use of microorganisms and catalysts; chemical treatment with acids, alkalis and oxidants; and physical treatment by mechanical and thermal means. Physicochemical treatment combines physical and chemical techniques. Ammonia fiber explosion (AFEX), and supercritical CO2 explosion are examples. The major classifications of treatment options and associated technolo-

**Technology option Example means/aids**

Biological Bacteria *Clostridium* sp. strains LDC-8-c12, 5-8, CO6-72;

Chemical Acids, organosolvs Inorganic acids (hydrochloric, nitric, phosphoric,

Oxidants Hydrogen peroxide, ozone

Alkalis Ammonia, lime

*Rhodobacter sphaeroides* KD131; Thermosaccharolyticum strain M18

(ATCC 96608), *Pleurotus ostreatus*

and countercurrent modes

ium chloride [Amim][Cl]

Ammonia recovery Ion exchange; scrubbing, stripping, precipitation (struvite) Ionic liquids 1-Butyl-3-methylimidazolium hydrogen sulfate

sulfuric); organic acids (fumaric, maleic). May be used in percolation, plug flow, shrinking-bed, batch,

[bmim]HSO4], 1-ethyl-3-methylimidazolium acetate (EMIM-OAc), 1-ethyl-3-methylimidazolium diethyl phosphate, 3-allyl-1-methyl-1H-imidazol3-

Composting Green waste, vine shoot pruning, wood chips Enzyme *Carbohydrase*, *laccase,* lignin *peroxidase* Fungi *Ceriporia lacerata*, *Ceriporiopsis subvermispora*

used without further waste management controls.

**5. Treatment technology options**

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

gies are presented in **Table 3**.

**Category/ method**

**205**

#### **Table 2.**

*Quality and safety validation criteria for digestates.*

ensure that digestate complies with European "End of Waste" criteria; and can be used without further waste management controls.

## **5. Treatment technology options**

**Criteria Process/parameter Requirements**

Sterilization at 133°C 20 min Weed seeds and sprouting plant parts ≤2/L

Pathogens *E. coli* ≤1000 CFU/g fresh matter

Heavy metals Cadmium (Cd) 0.8–20 mg/kg DM

Non-stone impurities >2 mm (glass, metal,

Stability Volatile fatty acids 0.43 g COD/g VS

liquid, solid), mass of product, total nitrogen, ammonium nitrogen, total phosphorus, total potassium, soluble chloride, soluble sodium, dry matter, volatile solids, pH, bulk density, etc.

plastic, etc.)

etc.

*Quality and safety validation criteria for digestates.*

Declarations Name of producer, type of product (whole,

Odor Free of annoying odors

Chromium (Cr) 75–1000 mg/kg DM Copper (Cu) 75–1000 mg/kg DM Lead (Pb) 80–900 mg/kg DM Mercury (Hg) 0.6–16 mg/kg DM Nickel (Ni) 30–300 mg/kg DM Zinc (Zn) 300–4000 mg/kg DM

Polycyclic aromatic hydrocarbons 3–6 mg/kg DM Dioxins and furans 20 ng TE/kg Chlorinated pesticides 0.5 mg/kg Product Polychlorinated biphenyls 0.2 mg/kg DM Absorbable organic halogens 500 mg/kg DM Linear alkylbenzene sulphonates 1300 mg/kg DM Nonylphenol and nonylphenolethoxylates 10 mg/kg DM DEPH: Di (2-ethylhexyl) phthalate 50 mg/kg DM

Stones > 5 mm 8% m/m dry matter

Respiration rate 16 mg CO2 g VS<sup>1</sup> day<sup>1</sup>

Agriculture (e.g., manure, harvesting by-products, silage, energy crops); animal by-products (e.g., manure, stomach intestine, raw milk); food industry (residues from food industry that contain food grade additives); food related shops (e.g., potatoes, dairy waste, bread, meat remnants, flowers, plants); forrest (e.g., bark, wood chips, sludge from the cellulosic industry); parks, gardens (e.g., leaves, grass); greenhouses (e.g., tops, peat products); households, kitchens, restaurants (e.g., fruit and vegetables residues, food, coffee and tea remainders, egg shells);

Residual biogas potential 0.25 l/g VS

Lime, iron chloride, iron oxide, bentonite, diatomaceous earth

0.5% m/m dry matter

Relevant units where applicable

; mg/(kg DM);

(e.g., kg; kg/m<sup>3</sup>

mg/L; %;)

*Salmonella* spp. Absent in 25 g fresh matter

Hygiene Pasteurization at 70°C 1 h

*Renewable Energy - Technologies and Applications*

Organic pollutants

Inorganic pollutants

Additives and chemicals

Feedstock sources

*Source: [9, 12–16].*

**Table 2.**

**204**

In the context of AD and digestate, we may distinguish between pre- and posttreatment processes. A pretreatment process refers to a processing operation applied upstream, before the digestate emerges from the digester. This could range from size reduction or thermochemical treatment of feedstock substrate; to process management (such as pH, temperature, and retention time control). On the other hand, a posttreatment process is that processing operation applied downstream of digestate harvest. This may also involve size reduction, other unit operations; composting, and end-product requirements that ensure the digestate sanitation. Post treatment may generate nutrient concentrates, liquid and solid fraction digestates conditioned to standardized biofertilizers, and final liquid effluent that could be discharged into a stream or sewage system. Benefits of posttreatment include enhanced marketability, reductions in handling, storage and transportation costs/requirements, and compliance with environmental regulations.

Depending on the starting feedstock and desired end product form of the digestate, similar technologies could be used for pre and post treatment processing. Applied technologies and methods may be classified as biological, chemical, or physical. The methods could also be used in combination. Biological treatment could be accomplished with the use of microorganisms and catalysts; chemical treatment with acids, alkalis and oxidants; and physical treatment by mechanical and thermal means. Physicochemical treatment combines physical and chemical techniques. Ammonia fiber explosion (AFEX), and supercritical CO2 explosion are examples. The major classifications of treatment options and associated technologies are presented in **Table 3**.



slurry [38]. Researchers concluded that digestate enhanced soil biological stability,

application of chemical fertilizers [38]; higher yield of potato (*Solanum tuberosum*) over the application of compost [41]; and 30% increase in yield over farm yard

Digestate is applied in recovery of nutrients, production of fertilizers and volatile fatty acids (VFAs). Livestock manure contains about 49 g N/kg TS and 6 g P/kg TS; energy crops, 17 g N/kg TS and 2.5 g P/kg TS; and agro-wastes, 27 g N/kg TS and 3 g P/kg TS [43]. Much of these nutrients remain in digestate after AD operation. For example, total N, P, and K values for digestates obtained from wet AD of agricultural wastes were reported respectively in the ranges 44–120, 8–42, and 28– 95 g/kg DM [44]. These nutrients could be recovered/harvested with the technolo-

VFAs are important input organic acids used extensively in the bioenergy, food, chemical, cosmetic, pharmaceutical, textile, and other industries. Acetic acid (E 260), propionic acid (E 280) and butyric acid are examples; and are GRAS (generally regarded as safe) rated by the FDA. Acetic acid is used to defend against *Campylobacter*, *Escherichia coli*, *Listeria*, *Salmonella*, and other pathogens in beef, chicken, pork, turkey, carcasses, skin and hides [45]. Butyric acid is used in the textile industry to enhance heat and sunlight resistance of fibers. In the food industry, it is used as additive for flavor formulation and modification [46]. Similarly, propionic acid (E 280) is used as antibacterial and antifungal agent to decontaminate packaging films and coatings, and to protect meat and meat products such as sausages, bologna and ham. VFAs have been harvested from digestates generated from short-term dry AD of swine manure, generated from AD of food waste, and used in recovery of biological nitrogen and phosphorus from

Digestate can be deployed for energy generation. Recirculating digestate into the digester maximizes biogas production, at the same time minimizing methane emissions during digestate storage, transport, and use. Digestate was pyrolyzed (via the use of Pyroformer, quartz rotary kiln reactor, and thermocatalytic reforming reactor) to produce biofuels: pyrolysis oil (biooil) and

pyrolysis gas (syngas). The biooil generated by thermo-catalytic reforming process at 750°C had a higher heating value of 33.9 MJ/kg, and a total acid number of

Algae have widespread applications and potentials in: biofuels, cosmetics,

biofertilizer, infant formulas, nutritional supplements, livestock feeds, chemical and allied industries, and biodegradable packaging. Perhaps more importantly, digestate could be used for the cultivation and production of microalgae. In the context of biorefinery platform and circular economy, various compounds produced by microalgae and their applications have been

for good crop performance. Leaves of alfalfa plant fertilized with digestate had higher contents of N, P, and K in comparison to alfalfa fertilized with mineral fertilizers [40]. Digestate also produced higher yields of dent corn than the

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

On the other hand, fertilizer properties relate to provision of nutrients necessary

microbial biomass and enzymatic activities [39].

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

manure [42].

**6.2 Nutrients recovery**

gies outlined in **Table 3**.

sewage sludge [47–49].

**6.3 Energy production**

4.9 mgKOH/g [50].

reported [51, 52].

**207**

#### **Table 3.**

*Major categories of treatment and technology options for AD and digestate processing.*

## **6. Applications management options for digestate**

In the service of circular economy, there are many applications management options for digestate. These may include algae cultivation, energy production, bioadsorbent production, building materials production, nutrients recovery/production, soil creation and other value-added commodities. Perhaps the two most widely recognized utilities of digestate are as land application for soil amendment and as biofertilizer.

#### **6.1 Biofertilizer and soil amendment**

Technological aids used in modern agriculture such as inorganic fertilizers and antibiotics have negative impacts on soil, water, and air quality and safety, and therefore pose health risks to humans and the ecosystem. Inorganic fertilizers for instance have caused environmental and soil quality degradation, eutrophication and heavy metals pollution. Similarly, field-spreading agricultural land with raw/ untreated manures derived from medicated livestock contributes to dissemination of veterinary antibiotic residues and antibiotic-resistant pathogens. Lincomycin, monensin, and sulfamethazine antibiotics were reported to affect soil microbial community composition and respiration, denitrification and nitrogen transformations [37]. Applications of digestate for biofertilizer and soil amendment purposes could ameliorate some of these adverse effects.

Amendment propensity relates to capability to maintain soil fertility and humus balance. Dairy slurry digestate was found richer in humic substances than raw dairy *Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

slurry [38]. Researchers concluded that digestate enhanced soil biological stability, microbial biomass and enzymatic activities [39].

On the other hand, fertilizer properties relate to provision of nutrients necessary for good crop performance. Leaves of alfalfa plant fertilized with digestate had higher contents of N, P, and K in comparison to alfalfa fertilized with mineral fertilizers [40]. Digestate also produced higher yields of dent corn than the application of chemical fertilizers [38]; higher yield of potato (*Solanum tuberosum*) over the application of compost [41]; and 30% increase in yield over farm yard manure [42].

#### **6.2 Nutrients recovery**

Digestate is applied in recovery of nutrients, production of fertilizers and volatile fatty acids (VFAs). Livestock manure contains about 49 g N/kg TS and 6 g P/kg TS; energy crops, 17 g N/kg TS and 2.5 g P/kg TS; and agro-wastes, 27 g N/kg TS and 3 g P/kg TS [43]. Much of these nutrients remain in digestate after AD operation. For example, total N, P, and K values for digestates obtained from wet AD of agricultural wastes were reported respectively in the ranges 44–120, 8–42, and 28– 95 g/kg DM [44]. These nutrients could be recovered/harvested with the technologies outlined in **Table 3**.

VFAs are important input organic acids used extensively in the bioenergy, food, chemical, cosmetic, pharmaceutical, textile, and other industries. Acetic acid (E 260), propionic acid (E 280) and butyric acid are examples; and are GRAS (generally regarded as safe) rated by the FDA. Acetic acid is used to defend against *Campylobacter*, *Escherichia coli*, *Listeria*, *Salmonella*, and other pathogens in beef, chicken, pork, turkey, carcasses, skin and hides [45]. Butyric acid is used in the textile industry to enhance heat and sunlight resistance of fibers. In the food industry, it is used as additive for flavor formulation and modification [46]. Similarly, propionic acid (E 280) is used as antibacterial and antifungal agent to decontaminate packaging films and coatings, and to protect meat and meat products such as sausages, bologna and ham. VFAs have been harvested from digestates generated from short-term dry AD of swine manure, generated from AD of food waste, and used in recovery of biological nitrogen and phosphorus from sewage sludge [47–49].

#### **6.3 Energy production**

Digestate can be deployed for energy generation. Recirculating digestate into the digester maximizes biogas production, at the same time minimizing methane emissions during digestate storage, transport, and use. Digestate was pyrolyzed (via the use of Pyroformer, quartz rotary kiln reactor, and thermocatalytic reforming reactor) to produce biofuels: pyrolysis oil (biooil) and pyrolysis gas (syngas). The biooil generated by thermo-catalytic reforming process at 750°C had a higher heating value of 33.9 MJ/kg, and a total acid number of 4.9 mgKOH/g [50].

Algae have widespread applications and potentials in: biofuels, cosmetics, biofertilizer, infant formulas, nutritional supplements, livestock feeds, chemical and allied industries, and biodegradable packaging. Perhaps more importantly, digestate could be used for the cultivation and production of microalgae. In the context of biorefinery platform and circular economy, various compounds produced by microalgae and their applications have been reported [51, 52].

**6. Applications management options for digestate**

*Major categories of treatment and technology options for AD and digestate processing.*

biofertilizer.

**206**

*Source: [10, 15, 17–36].*

**Table 3.**

**Category/ method**

Physical Mechanical

**6.1 Biofertilizer and soil amendment**

could ameliorate some of these adverse effects.

In the service of circular economy, there are many applications management options for digestate. These may include algae cultivation, energy production, bioadsorbent production, building materials production, nutrients recovery/production, soil creation and other value-added commodities. Perhaps the two most widely recognized utilities of digestate are as land application for soil amendment and as

**Technology option Example means/aids**

*Extrusion:* Band, single screw, twin screw *Homogenization:* High pressure homogenizers

*Lysis:* Lysis-centrifuges

*Disintegration/maceration (chipping, grinding, milling,*

*shredding)*:

*Renewable Energy - Technologies and Applications*

*Dewatering*: Centrifuges, gravity tables, presses (belt, filter, rotary, screw)

*Membrane separation:* Electrodialysis, microfiltration, nanofiltration,

*Sonication:* Ultrasound/sonoreactors (bath, flat plate, probe,

Thermal Drying/torrefaction, electric heating, evaporation,

tube)

Physicochemical Expansion/explosion Ammonia fiber expansion/explosion (AFEX), steam

explosion

Irradiation Electron beam, gamma ray

Ball mill, colloid mill, hammer mill, two-roll mill

pervaporation, reverse osmosis, ultrafiltration

hot oil, hot water, hydrothermal, microwave, steam

explosion, supercritical carbon dioxide (SC-CO2)

Technological aids used in modern agriculture such as inorganic fertilizers and antibiotics have negative impacts on soil, water, and air quality and safety, and therefore pose health risks to humans and the ecosystem. Inorganic fertilizers for instance have caused environmental and soil quality degradation, eutrophication and heavy metals pollution. Similarly, field-spreading agricultural land with raw/ untreated manures derived from medicated livestock contributes to dissemination of veterinary antibiotic residues and antibiotic-resistant pathogens. Lincomycin, monensin, and sulfamethazine antibiotics were reported to affect soil microbial community composition and respiration, denitrification and nitrogen transformations [37]. Applications of digestate for biofertilizer and soil amendment purposes

Amendment propensity relates to capability to maintain soil fertility and humus balance. Dairy slurry digestate was found richer in humic substances than raw dairy

## **6.4 Other applications**

Digestates have other utilities and management options. These include applications in aquaculture, gardening and horticulture, and the production of building materials and biochar.

other operating costs (electricity, logistics, regulations), and revenue from products (biogas and digestate). In the case of digestate, feedstock, treatment processes, and the logistics of storage, transport, handling and field application bear crucial concerns. Cost-effective digestate production process is presaged by efficient feedstock collection and sorting operations. A cost benefit analysis of municipal solid waste management system in Yangon, Myanmar, identified weak organizational structure and ineffective collection methods in the existing system that operated with just 32% waste collection efficiency. An alternative system with increased waste collection efficiency was then proposed. The new system required labor and vehicular productivity; using vehicles with container-hoist handling mechanism. The new system reduced operating and other costs associated with the old system by up to 42% [60]. It is noteworthy that consumer and public environmental behavior and

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

cooperation on waste management could be modified by pecuniary and

negative but statistically insignificant effect on WTP [61].

Generally, the farther the distance, the higher the cost.

88)/t for on farm co-digestion [63].

and Siegen [64].

**209**

nonpecuniary information. In Surabaya city, Indonesia, researchers found that in the reference case in which the no information treatment was applied, mean WTP (willingness to pay) for marginal improvements in a waste collection and disposal program was estimated to be US\$ 14.65. The researchers reported that pecuniary information increased WTP by 20.5%, whereas non-pecuniary information had a

A situation where 50% of whole unprocessed digestate was applied on agricultural land near the generating biogas plant and the other 50% transported to a location 20 km away was studied. Cost for digestate utilization near the biogas plant was € 3.34 (US\$ 3.73)/t, and that at a location 20 km away was € 5.47 (US\$ 6.10)/t [62]. This study highlights the impact that location or site of digestate utilization could have on cost. Such distance related cost also applies to feedstock substrate.

Researchers performed specific cost analysis for six scenarios that involved direct land application of digestate as reference, and various treatment technology options that included screw press and decanter centrifuge separation, belt drying, evaporation concentration, purification by ultrafiltration and reverse osmosis, and nutrients recovery by ammonia stripping and precipitation. Result indicated that net specific costs ranged from € 1.94 (US\$ 2.16)/m<sup>3</sup> of digestate for the reference scenario, to € 5.45 (US\$ 6.08)/m<sup>3</sup> for stripping, to € 6.80 (US\$ 7.58)/m<sup>3</sup> for belt dryer [62]. Similarly, the costs of AD were found to vary up to € 109 (US\$ 122)/t of digestate from € 35 (US\$ 39)/t for basic storage of digestate for aerobic conditioning, to € 70 (US\$ 78)/t for digestate ready for direct land application, to € 79 (US\$

Case studies were conducted for separation systems in three regions (Aachen, Borken, and Siegen) of Germany. The researchers determined that investment and variable costs were respectively € 23,000 (US\$ 25,536) and € 0.47 (US\$ 0.52)/m<sup>3</sup> for screw press; € 27,000 (US\$ 29,977) and € 0.48 (US\$ 0.53)/m<sup>3</sup> for screening drum press; and € 163,000 (US\$ 180,970) and € 1.46 (US\$ 1.62)/m<sup>3</sup> for decanter centrifuge. Further analysis revealed the unit cost of digestate disposal for screening drum press varied from € 4.1 (US\$ 4.6)/m<sup>3</sup> in Aachen to € 4.8 (US\$ 5.3)/m<sup>3</sup> in Borken,

The following were reported about AD in the UK. Least cost post treatment technology for digestate derived from a 10% solids content food waste was biological oxidation at £13.18 (US\$ 16.97)/t of feedstock. At 20% solids content, least cost option was direct application of whole digestate to agricultural land at £8.76 (US\$ 11.28)/t. The cost of treating 4000 t of slurry with a mechanical screen separator was £0.44 (US\$ 0.57)/t per year, and treatment with decanting centrifuge cost £2.21 (US\$ 2.85)/t per year. Furthermore, about £3.5M (US\$ 4.5 M) would be required to construct a 1 (one) MWe AD plant utilizing farm wastes as feedstock [65–67].

#### *6.4.1 Biochar*

Biochar (charcoal) is the byproduct of thermal pyrolysis of carbonaceous biomass; and has carbon sink properties. Dairy waste and whole sugar beet digestate biochar were effective in eliminating heavy metals (Pb2+, Cu2+, Ni2+, and Cd2+) from aqueous solutions [53].

#### *6.4.2 Gardening and horticulture*

Due to its organic origin and physicochemical characteristics, digestate is useful in gardening and horticulture. It could be applied in soil creation or remediation, and has found applications in green houses, plant nurseries, and home gardening [54].

## *6.4.3 Building materials*

A 50% substitution of wood with cattle manure digestate produced particleboard panels that met ANSI performance requirements [55]. USDA reported that medium-density fiberboard and wood/plastic composite engineered materials could be created using digestate solids without compromising mechanical or aesthetic values [56].

#### *6.4.4 Aquaculture*

Digestate is better than raw manure in fertilizing fish ponds. Firstly, digestate is hygienic because most of the bacteria, parasites and their eggs are destroyed in the AD process. Thus, pond sanitation is improved; minimizing fish diseases and the cost of veterinary services. Secondly, the digestate is largely stabilized and therefore does not consume and compete with fish for dissolved oxygen. Tilapia, Silver carp, Bighead carp, Silver barb and Mrigal fish species raised in pond fertilized with digestate matured faster and achieved higher net weight gain than counterparts raised in pond fertilized with chemical fertilizer or raw manure. By comparison, while chemical fertilizer increased net yield over raw manure by 27%, digestate increased net yield by 55% [57].

#### *6.4.5 Bio-adsorbents and bedding*

Digestates have been applied as bio-adsorbents to scavenge heavy metals from contaminated soils and water [58], and as chicken litter [54], and other livestock bedding [56, 59].

### **7. Cost implications**

The big picture cost elements relevant to AD systems include land acquisition, site preparation/development, plant and machinery (including digester/reactor, pre and post treatment technologies), personnel, feedstock, environmental impact,

### *Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

other operating costs (electricity, logistics, regulations), and revenue from products (biogas and digestate). In the case of digestate, feedstock, treatment processes, and the logistics of storage, transport, handling and field application bear crucial concerns. Cost-effective digestate production process is presaged by efficient feedstock collection and sorting operations. A cost benefit analysis of municipal solid waste management system in Yangon, Myanmar, identified weak organizational structure and ineffective collection methods in the existing system that operated with just 32% waste collection efficiency. An alternative system with increased waste collection efficiency was then proposed. The new system required labor and vehicular productivity; using vehicles with container-hoist handling mechanism. The new system reduced operating and other costs associated with the old system by up to 42% [60]. It is noteworthy that consumer and public environmental behavior and cooperation on waste management could be modified by pecuniary and nonpecuniary information. In Surabaya city, Indonesia, researchers found that in the reference case in which the no information treatment was applied, mean WTP (willingness to pay) for marginal improvements in a waste collection and disposal program was estimated to be US\$ 14.65. The researchers reported that pecuniary information increased WTP by 20.5%, whereas non-pecuniary information had a negative but statistically insignificant effect on WTP [61].

A situation where 50% of whole unprocessed digestate was applied on agricultural land near the generating biogas plant and the other 50% transported to a location 20 km away was studied. Cost for digestate utilization near the biogas plant was € 3.34 (US\$ 3.73)/t, and that at a location 20 km away was € 5.47 (US\$ 6.10)/t [62]. This study highlights the impact that location or site of digestate utilization could have on cost. Such distance related cost also applies to feedstock substrate. Generally, the farther the distance, the higher the cost.

Researchers performed specific cost analysis for six scenarios that involved direct land application of digestate as reference, and various treatment technology options that included screw press and decanter centrifuge separation, belt drying, evaporation concentration, purification by ultrafiltration and reverse osmosis, and nutrients recovery by ammonia stripping and precipitation. Result indicated that net specific costs ranged from € 1.94 (US\$ 2.16)/m<sup>3</sup> of digestate for the reference scenario, to € 5.45 (US\$ 6.08)/m<sup>3</sup> for stripping, to € 6.80 (US\$ 7.58)/m<sup>3</sup> for belt dryer [62]. Similarly, the costs of AD were found to vary up to € 109 (US\$ 122)/t of digestate from € 35 (US\$ 39)/t for basic storage of digestate for aerobic conditioning, to € 70 (US\$ 78)/t for digestate ready for direct land application, to € 79 (US\$ 88)/t for on farm co-digestion [63].

Case studies were conducted for separation systems in three regions (Aachen, Borken, and Siegen) of Germany. The researchers determined that investment and variable costs were respectively € 23,000 (US\$ 25,536) and € 0.47 (US\$ 0.52)/m<sup>3</sup> for screw press; € 27,000 (US\$ 29,977) and € 0.48 (US\$ 0.53)/m<sup>3</sup> for screening drum press; and € 163,000 (US\$ 180,970) and € 1.46 (US\$ 1.62)/m<sup>3</sup> for decanter centrifuge. Further analysis revealed the unit cost of digestate disposal for screening drum press varied from € 4.1 (US\$ 4.6)/m<sup>3</sup> in Aachen to € 4.8 (US\$ 5.3)/m<sup>3</sup> in Borken, and Siegen [64].

The following were reported about AD in the UK. Least cost post treatment technology for digestate derived from a 10% solids content food waste was biological oxidation at £13.18 (US\$ 16.97)/t of feedstock. At 20% solids content, least cost option was direct application of whole digestate to agricultural land at £8.76 (US\$ 11.28)/t. The cost of treating 4000 t of slurry with a mechanical screen separator was £0.44 (US\$ 0.57)/t per year, and treatment with decanting centrifuge cost £2.21 (US\$ 2.85)/t per year. Furthermore, about £3.5M (US\$ 4.5 M) would be required to construct a 1 (one) MWe AD plant utilizing farm wastes as feedstock [65–67].

**6.4 Other applications**

*Renewable Energy - Technologies and Applications*

materials and biochar.

from aqueous solutions [53].

home gardening [54].

*6.4.3 Building materials*

values [56].

*6.4.4 Aquaculture*

increased net yield by 55% [57].

*6.4.5 Bio-adsorbents and bedding*

bedding [56, 59].

**208**

**7. Cost implications**

*6.4.2 Gardening and horticulture*

*6.4.1 Biochar*

Digestates have other utilities and management options. These include applications in aquaculture, gardening and horticulture, and the production of building

Biochar (charcoal) is the byproduct of thermal pyrolysis of carbonaceous biomass; and has carbon sink properties. Dairy waste and whole sugar beet digestate biochar were effective in eliminating heavy metals (Pb2+, Cu2+, Ni2+, and Cd2+)

Due to its organic origin and physicochemical characteristics, digestate is useful in gardening and horticulture. It could be applied in soil creation or remediation, and has found applications in green houses, plant nurseries, and

A 50% substitution of wood with cattle manure digestate produced particleboard

Digestate is better than raw manure in fertilizing fish ponds. Firstly, digestate is hygienic because most of the bacteria, parasites and their eggs are destroyed in the AD process. Thus, pond sanitation is improved; minimizing fish diseases and the cost of veterinary services. Secondly, the digestate is largely stabilized and therefore does not consume and compete with fish for dissolved oxygen. Tilapia, Silver carp, Bighead carp, Silver barb and Mrigal fish species raised in pond fertilized with digestate matured faster and achieved higher net weight gain than counterparts raised in pond fertilized with chemical fertilizer or raw manure. By comparison, while chemical fertilizer increased net yield over raw manure by 27%, digestate

Digestates have been applied as bio-adsorbents to scavenge heavy metals from contaminated soils and water [58], and as chicken litter [54], and other livestock

The big picture cost elements relevant to AD systems include land acquisition, site preparation/development, plant and machinery (including digester/reactor, pre and post treatment technologies), personnel, feedstock, environmental impact,

medium-density fiberboard and wood/plastic composite engineered materials could be created using digestate solids without compromising mechanical or aesthetic

panels that met ANSI performance requirements [55]. USDA reported that

In the continent of Africa, cost of establishing a 4 m<sup>3</sup> anaerobic digester was found to range from US\$ 555 in Uganda to US\$ 698 in Cameroun to US\$ 979 in Rwanda [68]; while that of founding a family size floating drum plant was estimated at US\$ 1667 [69].

**Issues Challenges and opportunities**

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

etc.).

8.3. E-waste Challenge: problems and dangers of e-waste, heavy industry

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

8.4. Mineral waste Challenge: mining of solid minerals do present health and environmental challenges.

pads, highways) are possibilities.

co-digested manure on farm land.

may come to the rescue (**Table 3**).

8.7. Informal and low status Challenge: AD and digestate are perceived to be in domain of

belong to poor/rural settings. 8.8. Legal barriers Challenge: lack of binding global (and for developing countries,

policy frameworks.

**211**

heterogeneity. 8.6. Unrecovered organic matter Challenge: AD is more adapted to easily putrescible

8.5. Source of feedstock Challenge: the source of digestate feedstock and its treatment

products and components; including electrical and electronic equipment, waste batteries, engine blocks, paint, etc. Opportunity: guidance/support for the informal (non or loosely regulated) establishments, to call attention to dangers and health risks that may be associated with used or discarded electronic devices/items (acids, other chemicals, radioactive materials,

Opportunity: chances to implement safeguards for hazardous minerals and to divert safe wastes to beneficial applications. Examples are uses as substitute for backfill material in open pit mining, landfill, or as grit in construction materials. Production of concrete and brick for structural work (bridges, dams, launch

could present barriers. PAS 110 in the UK does not approve certification for digestate generated from mechanically biologically treated waste. Such digestates require proof of biodegradability test to be considered suitable for recycling; like land spreading. There is also the issue of digestate originating from co-digestion of industrial waste and household waste. In the Netherlands, the desire in AD electricity regime to maximize biogas production by mixing manure with other organic material conflicts with AD biofertilizer rules for spreading digestate from

Opportunity: some of these challenges are consumer-induced barriers and lack quantitative elements. Opportunities might lie in the sociocultural realm, such as modifying social and cultural attitudes and behaviors towards waste and its inherent

carbohydrates (starch, sugar). Recalcitrant lignocellulosic components (lignin, etc.) remain undigested. Efficiency of organic matter conversion was quite low as ˃97% of lignin in maize stover was found undigested [73]. AD could thus lead to unrecovered organic matter still present in digestate Opportunity: prospects for advanced and innovative pretreatment technologies to fractionate, recover, purify and convert lignin or other recalcitrant organics to more digestible biopolymers. Alkaline treatment, gamma irradiation, membrane technologies, organosolv, steam explosion, wet oxidation, etc.

informal waste management system and service; and therefore,

own country) coherent rules, laws, directives, regulations and

Opportunity: the formulation of these guidelines and laws on waste governance system. Implementing appropriate technologies and business models for waste management.

relegated as only appropriate for the rural populace. Opportunity: integration of formal and informal systems. Training to abate misconceptions, lack of awareness, and raise public profile of digestate. These may purge image of biogas and digestate as products that are derived from wastes, and hence

Techno-economic analyses were performed for post treatment technologies used to recover nutrients from the digestates of five full scale farm AD systems. Results showed membrane technology had specific cost of € 6.97 (US\$ 7.72)/m<sup>3</sup> of treated digestate. Drying was estimated at € 5.81 (US\$ 6.44)/m3 , while stripping operated at € 5.44 (US\$ 6.03)/m<sup>3</sup> [70]. In addition, the process economics of membrane-based nutrients extraction and fractionation from dairy manure digestate indicated cost of solid-liquid separation unit to be US\$ 11,000; the microfiltration extraction unit cost US\$ 30,000; the nanofiltration fractionation unit was priced at US\$ 60,000; and the daily cost of operation (chemicals, energy and water) was approximately US\$ 24 [71].

Finally, digestates are used as quilt for cattle bedding and poultry litter due to significant cost offsets to livestock farms. The cost of solid digestate as animal bedding (US\$ 55 per dry ton) is cheaper than the cost of alternative wood-based replacement materials such as wood chips at US\$ 65 per dry ton or sawdust and shavings at US\$ 124 to US\$ 248 per tonne [55, 59].

## **8. Challenges and opportunities**

Digestates have good fertilizer qualities: nutrients, safety and other properties required for soil amendment and plants production. However, relative to mineral fertilizers, digestates are not well known in many countries. Therefore, their potential as mineral fertilizer alternative/substitute is limited. Perhaps, standardized quality assurance and control protocols, regulations, certifications, legal and other institutional management systems organized internationally could help demonstrate digestates' benefits, quality and safety, and thereby engender confidence in their utilization as sustainable fertilizer and soil amendment products. Reconciling and bringing such issues and their benefits to existence present challenges and opportunities. Presented in **Table 4** are some of these challenges and opportunities of the waste, AD and digestate system.


*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*


In the continent of Africa, cost of establishing a 4 m<sup>3</sup> anaerobic digester was found to range from US\$ 555 in Uganda to US\$ 698 in Cameroun to US\$ 979 in Rwanda [68]; while that of founding a family size floating drum plant was esti-

Techno-economic analyses were performed for post treatment technologies used to recover nutrients from the digestates of five full scale farm AD systems. Results showed membrane technology had specific cost of € 6.97 (US\$ 7.72)/m<sup>3</sup> of treated

€ 5.44 (US\$ 6.03)/m<sup>3</sup> [70]. In addition, the process economics of membrane-based nutrients extraction and fractionation from dairy manure digestate indicated cost of solid-liquid separation unit to be US\$ 11,000; the microfiltration extraction unit cost US\$ 30,000; the nanofiltration fractionation unit was priced at US\$ 60,000; and the daily cost of operation (chemicals, energy and water) was approximately

Finally, digestates are used as quilt for cattle bedding and poultry litter due to significant cost offsets to livestock farms. The cost of solid digestate as animal bedding (US\$ 55 per dry ton) is cheaper than the cost of alternative wood-based replacement materials such as wood chips at US\$ 65 per dry ton or sawdust and

Digestates have good fertilizer qualities: nutrients, safety and other properties required for soil amendment and plants production. However, relative to mineral fertilizers, digestates are not well known in many countries. Therefore, their potential as mineral fertilizer alternative/substitute is limited. Perhaps, standardized quality assurance and control protocols, regulations, certifications, legal and other institutional management systems organized internationally could help demonstrate digestates' benefits, quality and safety, and thereby engender confidence in their utilization as sustainable fertilizer and soil amendment products. Reconciling and bringing such issues and their benefits to existence present challenges and opportunities. Presented in **Table 4** are some of these challenges and opportunities

8.1. Concept of waste Challenge: the conventional or customary status of looking at

8.2. Biowaste Challenge: because biodegradable waste could be a source of

waste as a problem presents significant challenge. Opportunity: seeing waste as potential resource would help change perception and attitude, possibly stimulating salient management options. Opportunities may emerge in the areas of prevention, recovery, collection, sorting, reducing, reusing, and recycling. For developing countries these have implications for

heavy metals and polluting organic compounds, it presents challenges to life generally, and to the environment. Opportunity: these challenges create opportunities to develop management options (e.g., biological treatments) to protect life, environment, and to benefit agriculture and ecosystem. Biowaste is reported to have potential to tackle climate change in the areas of nitrous oxide (NO2) emissions mitigation, and sequestration

environmental hygiene and sanitation.

capacity of agricultural soils [72].

, while stripping operated at

digestate. Drying was estimated at € 5.81 (US\$ 6.44)/m3

*Renewable Energy - Technologies and Applications*

shavings at US\$ 124 to US\$ 248 per tonne [55, 59].

**8. Challenges and opportunities**

of the waste, AD and digestate system.

**Issues Challenges and opportunities**

mated at US\$ 1667 [69].

US\$ 24 [71].

**210**


**Issues Challenges and opportunities**

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

4000 μg/kgdw [75].

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

polluting hazards before their use. 8.15. Air pollution Challenge: digestate has potential to emit substances and gasses

routine basis is another prospect area.

8.16. Bad odors Challenge: compared to raw manure slurry, digestate has fewer

and composting plants [78].

would minimize odor issues. 8.17. Bad legacies Challenge: there are challenges associated with bad reputation of

**213**

primary air pollutants [76].

manure of medicated animals that affect soil quality. Twenty five percent of 70 digestate and compost samples assessed in Switzerland contained polycyclic aromatic hydrocarbons (PAHs) concentrations beyond the regulated threshold value of

Opportunity: digestate is a sustainable fertilizer and soil improver; thus, necessary to assure its safety. The potential to contaminate soils with pollutants from digestate application beacons vigilance and chances to develop technical and monitoring strategies that sequester and purge the digestates of

that contaminate the air and influence global warming [11]. Challenges also exist due to lack of practical tools to monitor

Opportunity: advanced methods of digestate management and reutilization to minimize emissions of air pollutants (ammonia: NH3, nitrous oxide: NO2) and greenhouse gases (methane: CH4, nitrogen dioxide: N2O). Strategies may include processing (composting, curing, dewatering); alternative applications (in construction, aquaculture, regeneration activities); and storage. Development of software tools that enable quantitative monitoring of emissions from digestate soil applications on a

bad odors. However, this may not be true when compared to chemical fertilizer. There have been complaints of nuisance odors associated with land-spreading of digestate [77], and at landfills

Opportunity: this problem could be due to spreading practice and/or the spreading of unstable digestates. Application of good timing and spreading techniques (trailing-shoes, injection), and use of stabilized digestates (sufficient HRT, aerobic composting)

AD systems and biogas plants around the world. A study in 2006 found that 60% of 600–700 domestic biogas plants in Ethiopia was not functioning [79]. During the 7 years period from 2009, more than 3600 biogas plants were installed in the Tigray region of Ethiopia; and a 2017 study reported that 58.1% of the installations was not operational [80]. The 21 biogas plants installed by Pakistan council for appropriate technology (PCAT) in the 1970s were reported to have failed to perform [81]. In 1986, a survey of the status of 25 biogas plants in Kenya found 36% to be alive, functional and maintained. Another 36% was described as dead, not functional, and not maintained. Unfinished projects accounted for 8%; while remaining plants were reported in disrepair, with varied patterns of being alive, dead, not functional, and not maintained [82]. The regional bioenergy program of the Latin American energy organization (OLADE), catalogs biogas technology projects in Latin American countries. Experience began in 1953 and by 1986 at least 22 countries including Bolivia, Colombia, Costa Rica, Dominican Republic, Ecuador, Grenada, Guatemala, Guyana, Haiti, Honduras, Nicaragua, Jamaica, and Peru had projects at varying levels of implementation. Out of the 3950 biodigesters inventoried, 60% was found operable and 40% was either shut down or functioning irregularly or completely abandoned [83]. Though China rebounded and emerged as a major reference on


**Issues Challenges and opportunities**

*Renewable Energy - Technologies and Applications*

Challenge: lack of reliable data on waste management systems, design features, standard operating procedures (SOPs), etc. could

Opportunity: waste management value chain information is vital. Quantity, type, economic sector, source, and composition data could guide prioritization of strategies and enable trends forecast that deliver better outcomes. Global exchange of briefs would

characteristics as commercial chemical fertilizers, they are not classified in any way, are poorly developed in most countries, and there is no overall guidance [20, 62, 70]. These barriers restrict

Opportunity: these challenges create opportunities to establish frameworks that enable digestate utilization through standardization, fair comparison, commerce development, and

season, feedstock and degree of upgrading have been reported to challenge and impact digestate prices and marketing [54]. Opportunity: upgraded products offer increased marketability due to their denser nutrients. Marketing to nutrient deficient regions, non-agricultural sectors and purposes represent prospects. Manufacturers of organic soils, particle- and fiberboards, landscapers, and private customers all represent credible

establishing a 4 m<sup>3</sup> AD digester in the continent of Africa ranges from US\$ 555 to US\$ 979 [68]; and the price for a family size floating drum reactor was reported at US\$ 1667 [69]. In Sri Lanka, a family unit digester generating 6–10 m<sup>3</sup> of biogas per day cost Rs. 17,000 (US\$ 5459); and described as difficult proposition for low-income families [74]. In the UK, a 1 MWe AD plant utilizing farm wastes as feedstock cost about £3.5M (US\$ 4.5 M) to construct [67]. Also, costs associated with animal breeding and maintenance (veterinary care, feed, water, etc.) escalate operating costs, and constrain availability of manure for

Opportunity: easing cost barriers would require support with appropriate and necessary interventions (policies, credit facilities, subsidy schemes, preventive maintenance that promote solutions, prolong facilities productive lifespan, and minimize operating costs). Furthermore, transparency on proposals and bidding for new plants and projects could build confidence in the

and rural areas can compromise AD projects. Segregation by infrastructure and income for example could affect waste

Opportunity: prospects for rural development with public utilities, services, and infrastructure (roads, power, water, etc.) These would facilitate logistics for waste collection, AD processes, and digestate handling/evacuation.

Challenge: most of the digestate produced in AD is used for soil amendment and as biofertilizer. There are risks of spreading animal pathogens, heavy metals, and other pollutants on soils due to the presence of these hazards in animal by-products used in AD. Sulfadiazine and oxytetracycline are antibiotics found in

limit exchange of ideas and retard progress.

catalyze spread of best practices.

8.10. Standardization Challenge: although digestate products have similar

utilization and trade.

international trade. 8.11. Marketing Challenge: regional nutrient availability, agricultural structure,

market outlets. 8.12. Cost barrier Challenge: initial investment fund is a major issue. Cost of

feedstock.

process.

8.14. Contamination of agricultural

land

**212**

8.13. Urban and rural dichotomy Challenge: differences between metropolitan, urban, sub-urban,

collection and limit access to feedstock.

8.9. Data and waste reporting

system


**9. Cassava peeling residue (CPR) digestate**

*Challenges and opportunities of the waste, AD and digestate system.*

**Issues Challenges and opportunities**

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

8.20. Nomadic and free-range

8.21. Disparity between developed and developing countries

culture

**Table 4.**

**215**

minimized in the presence of digestate.

good algal productivity.

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

livestock.

in the year 2016 [81].

Opportunity: because algae are exploited for biofuels, and various other useful biotechnological metabolites production by valorization of digestate, the inhibitory effect of digestate on algae cultivation is of practical interest. Therefore, digestate pretreatment or at least its dilution before use [88], would aid

Challenge: many developing nations have nomadic animal husbandry architecture and free-range culture. These make the gathering of animal manure as feedstock for digesters a major challenge. In Pakistan, for example, livestock farmers from time to time relocate to weather conditions more benign to their livestock. However, current digester designs used by rural populations such as the fixed dome and floating drum are sedentary and cannot be readily moved by the farmers with their

Opportunity: perhaps this challenge creates opportunity for a mobile biogas system such as the portable biogas plant reported

Challenge: the economic, political and technological mismatches and divides between industrialized and industrially developing countries are challenging local, regional and international waste management systems. Environmental and health dangers do not know or respect boundaries (local, regional, or international) by land, sea, air or space. Planet earth is perhaps at the cusp of the axiomatic global village and economy. Sooner than later, pollution and instability at one corner of the earth would reverberate and affect other parts (Plastics in the oceans? Heavy metals in food, aquatic and terrestrial biota? Ebola in America? Flood events in Zimbabwe, Mozambique, Puerto Rico and U.S. Virgin Islands? Wildfires in Australia, Brazil, Portugal and USA? Coronavirus (COVID–19) in Japan, Singapore, and USA?). Opportunity: cooperation and support are needed to enable developing nations to leapfrog and shorten the learning curve and development timescales. Developing nations need guidance and assistance to cope with technological demands and challenges, and eschew reinventing the wheel. Waste management offers an opportunity for cooperation among nations for the betterment of

deliver the necessities and requirements of N, P, and K.

N, P, and K are critical macro nutrients for crops production. N is considered the limiting nutrient in growth and yield [89]. P is required for energy transfer, signal transduction, photosynthesis, and macromolecular respiration [90]. K is responsible for metabolism of cell division, enzymatic reactions of amide formation, and amino acid activation during proteins biosynthesis and substrate phosphorylation [91]. To be a credible mineral fertilizer substitute, digestate must have the capacity to

humanity and planet earth.

**Table 1** presented a broad gamut of materials used in biogas and digestate creation. The table covered energy crops, agricultural byproducts, food processing residues, livestock effluents, organic fraction of municipal solid wastes, and pharmaceutical industry sludge. However, cassava peeling residue (CPR) was not


**Table 4.**

**Issues Challenges and opportunities**

*Renewable Energy - Technologies and Applications*

programs could help. 8.18. Low diffusion rate Challenge: in Latin America, the number of rural biogas plants

to inadequate AD diffusion.

norms).

**214**

8.19. Inhibition of microalgae Challenge: it has been shown that the green alga (*Raphidocelis*

household digesters, about 50% of biogas tanks installed from 1958 into the 1970s were abandoned in the 1980s. By 1988 the seven million rural digesters existing in 1980 dropped to 4.7 million [84]. In 1986, a survey of biogas plants in Sri Lanka indicated that 61% was functional. By 1996 only 28.5% of completely surveyed 365 biogas systems was reported functional. At this point 16 units had been abandoned and the success rate for biogas systems implementation was reported as 32.9% [74]. In the Netherlands, for a period of over 30 years beginning in the 1970s, many AD projects using biomass were considerably delayed, suspended, abandoned and out rightly never realized. [85, 86]. These failures and circumstances taken together portrayed negative images and bad legacies for biogas plants. Opportunity: reasons adduced for failures included economic, social, technical, and policy components such as high investment and maintenance costs, urbanization and socio-cultural constraints, poor dissemination strategy, complicated permit regulations, shortage of feedstocks, lack of or inadequate training, poor digester design, etc. These reasons provide opportunities to create circumstances, provisions and tools that would promote and sustain biogas systems. Some examples are mobilization of local and external funds, more business-friendly policies and rules, appropriate and sustainable technologies, technical training, warranties for plant performance. Also, public dissemination of information and follow-up on successful

installed yearly from mid-1985 to 1992 was less than 15% of that installed from 1982 to mid-1985. Challenges included technology adoption, technical manpower and materials of construction. However, non-technical reasons for biogas adoption failures accounted for up to 69%, 50% and 25%, respectively, in Cote d'Ivoire (Ivory Coast), Costa Rica and Tanzania [84]. Unstable institutional environment, lack of network and lobby activities, lack of initiatives between academia, research institutes, private sector entrepreneurs and stakeholders were cited nontechnical reasons. For the Netherlands, apart from technological problems; limited economic feasibility, fragmented support from the government, decreases in energy prices, and lack of financial support which made return on investment uncertain contributed

Opportunity: cooperation between academia, government, industry and other stakeholders (farmers, energy sector, municipalities). Cooperative efforts that landed mutually beneficial outcomes should be highlighted, applauded and replicated. Well planned long-term, clear and supportive arrangements would facilitate continuity. Government policy that guide search for solutions, market formation and resources mobilization. Ease of technology adoption would also require reliable and sustainable infrastructure (technical assistance, manpower, cohesive farming approach with biogas and digestate, integration and dissemination of societal and cultural values and

*subcapitata*) is sensitive to digestate, with ecotoxicity index; EC50 of 0.77% [87]. Similarly, *Scenedesmus bijuga*; and oil-rich *Chlorella* sp., including *C. minutissima* and *C. sorokiniana* were found sensitive to digestate. Also, the dark color of liquid digestate of algal biomass inhibited the growth of *Chroococcus* sp. Therefore, cultivation of algae for value added products recovery could be

*Challenges and opportunities of the waste, AD and digestate system.*

## **9. Cassava peeling residue (CPR) digestate**

N, P, and K are critical macro nutrients for crops production. N is considered the limiting nutrient in growth and yield [89]. P is required for energy transfer, signal transduction, photosynthesis, and macromolecular respiration [90]. K is responsible for metabolism of cell division, enzymatic reactions of amide formation, and amino acid activation during proteins biosynthesis and substrate phosphorylation [91]. To be a credible mineral fertilizer substitute, digestate must have the capacity to deliver the necessities and requirements of N, P, and K.

**Table 1** presented a broad gamut of materials used in biogas and digestate creation. The table covered energy crops, agricultural byproducts, food processing residues, livestock effluents, organic fraction of municipal solid wastes, and pharmaceutical industry sludge. However, cassava peeling residue (CPR) was not


**Acknowledgements**

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

**Conflict of interest**

**Author details**

Sammy N. Aso

**217**

All currency conversions to US\$ were based on exchange rate taken at different times and days, during the period of last quarter of the year 2019, from the Foreign

There is no conflict of interest (private or public) associated with this work.

Food Engineering Laboratory, Rivers State University, Port Harcourt, Nigeria

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: sammyasso@yahoo.com

provided the original work is properly cited.

Exchange Converter Site: https://www1.oanda.com/currency/converter/

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

**Table 5.**

*Nutrients values of liquid fraction of cassava peeling residue (CPR) digestate.*

represented in the table. There is a published report on ammonium, potassium, total nitrogen, and total phosphorus contents of digestate generated from co-digestion of human urine, cow dung, and cassava effluent (a mixture of peeled cassava wash water and crushed cassava juice) [92]. CPR is a solid substrate abundantly generated during production of cassava root-based food systems such as gari and starch [93]. The present author is not aware of any report on nutrients value of digestate generated from the AD of CPR as sole feedstock. Therefore, a technical experiment was conducted to secure an overview assessment of N, P, and K compositions of liquid fraction of CPR digestate.

Some results of the research work on CPR as sole substrate for AD were reported earlier. These included proximate properties (e.g., moisture content, total solids, volatile solids), digester performance characteristics (methane content of biogas, pH, discharge effluent COD), feedstock materials, sampling procedures, analyses [94]. Presented in **Table 5** are results of nutrient values of liquid fraction of CPR digestate. **Table 5** results appear to be within the range of some published nutrients values for liquid digestates derived from other feedstocks such as algal biomass (*Chroococcus* sp.) [88], starch processing wastewater [95], source separated household waste [96], as well as liquid and solid manure slurries [97].

## **10. Conclusions**

Cassava (*Manihot esculenta* Crantz) is perhaps third largest source of food energy for humans. Cassava supports the nutrition and subsistence of up to one billion persons in over 100 countries. Also, cassava is gluten free and could thus assuage medical complications for individuals with celiac disease. Cassava root processing byproduct such as CPR has organic matter content with applications in biogas and digestate production. This is a welcome development in views of biorefinery platform and the emergent circular economy. CPR digestate may be applied directly for agronomic uses or treated to generate products with varied applications and utilities. Treatment technologies may be biological, chemical, physical, or some combinations. Global benefits would include carbon sequestration, energy recovery, resource sustainability and recycling, waste reduction, profitability of AD process, biogas facilities, and agricultural systems in general. End effects of climate change mitigation, enhanced energy and food security, environmental and ecological protection, and sustainable development are good news for humanity and planet earth. These outcomes should motivate and provide consumers, farmers, regulators, managers, and other stakeholders in the emergent circular economy with insights to integrate and apply quality, safety, marketing, handling, storage, transportation, compliance with environmental regulations, and cost considerations and requirements strategies for digestate; into a renewable and sustainable energy production and waste management system.

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

## **Acknowledgements**

All currency conversions to US\$ were based on exchange rate taken at different times and days, during the period of last quarter of the year 2019, from the Foreign Exchange Converter Site: https://www1.oanda.com/currency/converter/

## **Conflict of interest**

represented in the table. There is a published report on ammonium, potassium, total nitrogen, and total phosphorus contents of digestate generated from co-digestion of human urine, cow dung, and cassava effluent (a mixture of peeled cassava wash water and crushed cassava juice) [92]. CPR is a solid substrate abundantly generated during production of cassava root-based food systems such as gari and starch [93]. The present author is not aware of any report on nutrients value of digestate generated from the AD of CPR as sole feedstock. Therefore, a technical experiment was conducted to secure an overview assessment of N, P, and K compositions of

**S/N Nutrient Value [mg/L]** Ammonia nitrogen 561 Ortho-phosphorus 20 Potassium 1066 Total Kjeldahl nitrogen 573 Total phosphorus 31

Some results of the research work on CPR as sole substrate for AD were reported earlier. These included proximate properties (e.g., moisture content, total solids, volatile solids), digester performance characteristics (methane content of biogas, pH, discharge effluent COD), feedstock materials, sampling procedures, analyses [94]. Presented in **Table 5** are results of nutrient values of liquid fraction of CPR digestate. **Table 5** results appear to be within the range of some published nutrients values for liquid digestates derived from other feedstocks such as algal biomass (*Chroococcus* sp.) [88], starch processing wastewater [95], source separated house-

Cassava (*Manihot esculenta* Crantz) is perhaps third largest source of food energy for humans. Cassava supports the nutrition and subsistence of up to one billion persons in over 100 countries. Also, cassava is gluten free and could thus assuage medical complications for individuals with celiac disease. Cassava root processing byproduct such as CPR has organic matter content with applications in biogas and digestate production. This is a welcome development in views of biorefinery platform and the emergent circular economy. CPR digestate may be applied directly for agronomic uses or treated to generate products with varied applications and utilities. Treatment technologies may be biological, chemical, physical, or some combinations. Global benefits would include carbon sequestration, energy recovery, resource sustainability and recycling, waste reduction, profitability of AD process, biogas facilities, and agricultural systems in general. End effects of climate change mitigation, enhanced energy and food security, environmental and ecological protection, and sustainable development are good news for humanity and planet earth. These outcomes should motivate and provide consumers, farmers, regulators, managers, and other stakeholders in the emergent circular economy with insights to integrate and apply quality, safety, marketing, handling, storage, transportation, compliance with environmental regulations, and cost considerations and requirements strategies for digestate; into a renewable and

hold waste [96], as well as liquid and solid manure slurries [97].

*Nutrients values of liquid fraction of cassava peeling residue (CPR) digestate.*

*Renewable Energy - Technologies and Applications*

sustainable energy production and waste management system.

liquid fraction of CPR digestate.

**10. Conclusions**

**216**

**Table 5.**

There is no conflict of interest (private or public) associated with this work.

## **Author details**

Sammy N. Aso Food Engineering Laboratory, Rivers State University, Port Harcourt, Nigeria

\*Address all correspondence to: sammyasso@yahoo.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Gómez X, Cuetos MJ, García AI, Morán A. An evaluation of stability by thermogravimetric analysis of digestate obtained from different biowastes. Journal of Hazardous Materials. 2007; **149**(1):97-105. DOI: 10.1016/j. jhazmat.2007.03.049

[2] Tambone F, Orzi V, D'Imporzano G, Adani F. Solid and liquid fractionation of digestate: Mass balance, chemical characterization, and agronomic and environmental value. Bioresource Technology. 2017;**243**:1251-1256. DOI: 10.1016/j.biortech.2017.07.130

[3] Antoniou N, Monlau F, Sambusiti C, Ficara E, Barakat A, Zabaniotou A. Contribution to circular economy options of mixed agricultural wastes management: Coupling anaerobic digestion with gasification for enhanced energy and material recovery. Journal of Cleaner Production. 2019; **209**:505-514. DOI: 10.1016/j.jclepro. 2018.10.055

[4] Salomon KR, Lora ES. Estimate of the electric energy generating potential for different sources of biogas in Brazil. Biomass and Bioenergy. 2009;**3**(9): 1101-1107. DOI: 10.1016/j.biombioe. 2009.03.001

[5] Kusch S, Schäfer W, Kranert M. Dry digestion of organic residues. In: Kumar S, editor. Integrated Waste Management. Vol. 1. Croatia: IntechOpen; 2011. pp. 115-134. ISBN: 978-953-307-469-6. Available from: http://cdn.intechopen.com/pdfs/ 17433/InTech-Dry\_digestion\_of\_orga nic\_residues.pdf

[6] Aso SN, Teixeira AA, Achinewhu SC. Cassava residues could provide sustainable bioenergy for cassava producing nations. Chapter 13. In: Waisundara VY, editor. Cassava. Rijeka, Croatia: IntechOpen; 2018. pp. 219-240. DOI: 10.5772/intechopen.72166

[7] Liu H, Wang L, Zhang X, Fu B, Liu H, Li Y, et al. A viable approach for commercial VFAs production from sludge: Liquid fermentation in anaerobic dynamic membrane reactor. Journal of Hazardous Materials. 2019;**365**:912-920. DOI: 10.1016/j.jhazmat.2018.11.082

Community. 2006;**L36**:25-31. Available from: https://publications.europa.eu/ en/publication-detail/-/publication/ eb5ea96c-1ee9-4654-931a-37a1f581b

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

Biochemical Engineering Journal. 2009;

Ballesteros M, Negro MJ. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresource Technology. 2010;**101**(13):4851-4861. DOI: 10.1016/j.biortech.2009.11.093

**46**(2):169-175. DOI: 10.1016/j.

[18] Alvira P, Tomás-Pejó E,

[19] Hjorth M, Christensen KV, Christensen ML, Sommer SG. Solidliquid separation of animal slurry in theory and practice. A review. Agronomy and Sustainable

10.1051/agro/2009010

15435075.2011.557845

1-18. DOI: 10.1016/j. ultsonch.2010.02.014

Development. 2010;**30**:153-180. DOI:

[20] Mouat A, Barclay A, Mistry P, Webb J. Digestate market development in Scotland. Vol. OPR080-801. Stirling, UK: Zero Waste Scotland; 2010. Available from: http://www.wrap.org. uk/sites/files/wrap/Zero\_Waste\_Scotla nd\_Digestate\_Market\_Development.pdf

[21] Moon HC, Song IS. Enzymatic hydrolysis of foodwaste and methane production using UASB bioreactor. International Journal of Green Energy. 2011;**8**(3):361-371. DOI: 10.1080/

[22] Pilli S, Bhunia P, Yan S, LeBlanc RJ, Tyagi RD, Surampalli RY. Ultrasonic pretreatment of sludge: A review. Ultrasonics Sonochemistry. 2011;**18**:

[23] van der Stelt MJC, Gerhauser H, Kiel JHA, Ptasinski KJ. Biomass upgrading by torrefaction for the production of biofuels: A review. Biomass and Bioenergy. 2011;**35**: 3748-3762. DOI: 10.1016/j. biombioe.2011.06.023

[24] Zhang Q, He J, Tian M, Mao Z, Tang L, Zhang J, et al. Enhancement of methane production from cassava

bej.2009.05.003

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

[13] Petersson A. English summary of SPCR 120—Certification rules for digestate from biowaste by the quality assurance system of Swedish Waste Management. Swedish Gas Centre; 2008. Available from: http://www.fao. org/fileadmin/user\_upload/nr/sustainab ility\_pathways/docs/Certification% 20rules%20for%20digestate%20from%

[14] Siebert S. Quality requirements and quality assurance of digestion residuals in Germany. In: ECN/ORBIT Workshop: The future for Anaerobic Digestion of Organic Waste in Europe. Nüremberg, Germany. 2008. Available from: http:// www.kompost.de/uploads/media/ Quality\_Requirements\_of\_digestion\_ residuals\_in\_Germany\_text\_01.pdf

[15] Al Seadi T, Lukehurst C. Quality management of digestate from biogas plants used as fertiliser. IEA Bioenergy. 2012:4-36. Available from: https://www. researchgate.net/profile/Teodorita\_ Seadi/publication/265227188\_Quality\_ Management\_of\_Digestate\_from\_Bioga s\_Plants\_Used\_as\_Fertiliser/links/54b

62dec0cf26833efd35c9d.pdf

JRC87124.pdf

**219**

[16] Saveyn H, Eder P. End-of-waste criteria for biodegradable waste subjected to biological treatment (compost & digestate): Technical proposals. In: European Commission, Joint Research Centre, Institute for Prospective Technological Studies (IPTS). Seville: Spain; 2014. Available from: http://ftp.jrc.es/EURdoc/

[17] Mottet A, Steyer JP, Déléris S, Vedrenne F, Chauzy J, Carrère H. Kinetics of thermophilic batch anaerobic digestion of thermal hydrolysed waste activated sludge.

32e/language-en

20biowaste.pdf

[8] WBA: World Bioenergy Association. WBA Global Bioenergy Statistics 2018. Stockholm, Sweden: World Bioenergy Association; 2018. Available from: https://worldbioenergy.org/uploads/1812 03%20WBA%20GBS%202018\_hq.pdf

[9] BSI: British Standards Institution. Specification for whole digestate, separated liquor and separated fibre derived from the anaerobic digestion of source-segregated biodegradable materials. PAS 110:2010. British Standards Institution, London, UK. 2010. pp. 60. Available from: http://www.wrap.org.uk/sites/files/ wrap/PAS110\_vis\_10.pdf

[10] Tampio E, Marttinen S, Rintala J. Liquid fertilizer products from anaerobic digestion of food waste: Mass, nutrient and energy balance of four digestate liquid treatment systems. Journal of Cleaner Production. 2016;**125**:22-32. DOI: 10.1016/j.jclepro.2016.03.127

[11] Zeshan S, Visvanathan C. Evaluation of anaerobic digestate for greenhouse gas emissions at various stages of its management. International Biodeterioration & Biodegradation. 2014;**95**(Part A):167-175. DOI: 10.1016/ j.ibiod.2014.06.020

[12] EC: European Commission. Regulation (EC) No. 208/2006 of 7 February 2006 amending Annexes VI and VIII to regulation (EC) No. 1774/ 2002 of the European parliament and of the council as regards processing standards for biogas and composting plants and requirements for manure. Official Journal of European

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

Community. 2006;**L36**:25-31. Available from: https://publications.europa.eu/ en/publication-detail/-/publication/ eb5ea96c-1ee9-4654-931a-37a1f581b 32e/language-en

**References**

jhazmat.2007.03.049

2018.10.055

2009.03.001

nic\_residues.pdf

**218**

[1] Gómez X, Cuetos MJ, García AI, Morán A. An evaluation of stability by thermogravimetric analysis of digestate obtained from different biowastes. Journal of Hazardous Materials. 2007; **149**(1):97-105. DOI: 10.1016/j.

*Renewable Energy - Technologies and Applications*

[7] Liu H, Wang L, Zhang X, Fu B, Liu H, Li Y, et al. A viable approach for commercial VFAs production from sludge: Liquid fermentation in anaerobic dynamic membrane reactor. Journal of Hazardous Materials. 2019;**365**:912-920. DOI: 10.1016/j.jhazmat.2018.11.082

[8] WBA: World Bioenergy Association. WBA Global Bioenergy Statistics 2018. Stockholm, Sweden: World Bioenergy Association; 2018. Available from: https://worldbioenergy.org/uploads/1812 03%20WBA%20GBS%202018\_hq.pdf

[9] BSI: British Standards Institution. Specification for whole digestate, separated liquor and separated fibre derived from the anaerobic digestion of source-segregated biodegradable materials. PAS 110:2010. British Standards Institution, London, UK. 2010. pp. 60. Available from: http://www.wrap.org.uk/sites/files/

[10] Tampio E, Marttinen S, Rintala J. Liquid fertilizer products from anaerobic digestion of food waste: Mass, nutrient and energy balance of four digestate liquid treatment systems. Journal of Cleaner Production. 2016;**125**:22-32. DOI:

wrap/PAS110\_vis\_10.pdf

10.1016/j.jclepro.2016.03.127

[11] Zeshan S, Visvanathan C. Evaluation of anaerobic digestate for greenhouse gas emissions at various stages of its management. International Biodeterioration & Biodegradation. 2014;**95**(Part A):167-175. DOI: 10.1016/

[12] EC: European Commission. Regulation (EC) No. 208/2006 of 7 February 2006 amending Annexes VI and VIII to regulation (EC) No. 1774/ 2002 of the European parliament and of the council as regards processing standards for biogas and composting plants and requirements for manure.

Official Journal of European

j.ibiod.2014.06.020

[2] Tambone F, Orzi V, D'Imporzano G, Adani F. Solid and liquid fractionation of digestate: Mass balance, chemical characterization, and agronomic and environmental value. Bioresource Technology. 2017;**243**:1251-1256. DOI:

[3] Antoniou N, Monlau F, Sambusiti C, Ficara E, Barakat A, Zabaniotou A. Contribution to circular economy options of mixed agricultural wastes management: Coupling anaerobic digestion with gasification for

enhanced energy and material recovery. Journal of Cleaner Production. 2019; **209**:505-514. DOI: 10.1016/j.jclepro.

[4] Salomon KR, Lora ES. Estimate of the electric energy generating potential for different sources of biogas in Brazil. Biomass and Bioenergy. 2009;**3**(9): 1101-1107. DOI: 10.1016/j.biombioe.

[5] Kusch S, Schäfer W, Kranert M. Dry digestion of organic residues. In: Kumar S, editor. Integrated Waste Management. Vol. 1. Croatia:

IntechOpen; 2011. pp. 115-134. ISBN: 978-953-307-469-6. Available from: http://cdn.intechopen.com/pdfs/ 17433/InTech-Dry\_digestion\_of\_orga

[6] Aso SN, Teixeira AA, Achinewhu SC.

Cassava residues could provide sustainable bioenergy for cassava producing nations. Chapter 13. In: Waisundara VY, editor. Cassava. Rijeka, Croatia: IntechOpen; 2018. pp. 219-240.

DOI: 10.5772/intechopen.72166

10.1016/j.biortech.2017.07.130

[13] Petersson A. English summary of SPCR 120—Certification rules for digestate from biowaste by the quality assurance system of Swedish Waste Management. Swedish Gas Centre; 2008. Available from: http://www.fao. org/fileadmin/user\_upload/nr/sustainab ility\_pathways/docs/Certification% 20rules%20for%20digestate%20from% 20biowaste.pdf

[14] Siebert S. Quality requirements and quality assurance of digestion residuals in Germany. In: ECN/ORBIT Workshop: The future for Anaerobic Digestion of Organic Waste in Europe. Nüremberg, Germany. 2008. Available from: http:// www.kompost.de/uploads/media/ Quality\_Requirements\_of\_digestion\_ residuals\_in\_Germany\_text\_01.pdf

[15] Al Seadi T, Lukehurst C. Quality management of digestate from biogas plants used as fertiliser. IEA Bioenergy. 2012:4-36. Available from: https://www. researchgate.net/profile/Teodorita\_ Seadi/publication/265227188\_Quality\_ Management\_of\_Digestate\_from\_Bioga s\_Plants\_Used\_as\_Fertiliser/links/54b 62dec0cf26833efd35c9d.pdf

[16] Saveyn H, Eder P. End-of-waste criteria for biodegradable waste subjected to biological treatment (compost & digestate): Technical proposals. In: European Commission, Joint Research Centre, Institute for Prospective Technological Studies (IPTS). Seville: Spain; 2014. Available from: http://ftp.jrc.es/EURdoc/ JRC87124.pdf

[17] Mottet A, Steyer JP, Déléris S, Vedrenne F, Chauzy J, Carrère H. Kinetics of thermophilic batch anaerobic digestion of thermal hydrolysed waste activated sludge. Biochemical Engineering Journal. 2009; **46**(2):169-175. DOI: 10.1016/j. bej.2009.05.003

[18] Alvira P, Tomás-Pejó E, Ballesteros M, Negro MJ. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresource Technology. 2010;**101**(13):4851-4861. DOI: 10.1016/j.biortech.2009.11.093

[19] Hjorth M, Christensen KV, Christensen ML, Sommer SG. Solidliquid separation of animal slurry in theory and practice. A review. Agronomy and Sustainable Development. 2010;**30**:153-180. DOI: 10.1051/agro/2009010

[20] Mouat A, Barclay A, Mistry P, Webb J. Digestate market development in Scotland. Vol. OPR080-801. Stirling, UK: Zero Waste Scotland; 2010. Available from: http://www.wrap.org. uk/sites/files/wrap/Zero\_Waste\_Scotla nd\_Digestate\_Market\_Development.pdf

[21] Moon HC, Song IS. Enzymatic hydrolysis of foodwaste and methane production using UASB bioreactor. International Journal of Green Energy. 2011;**8**(3):361-371. DOI: 10.1080/ 15435075.2011.557845

[22] Pilli S, Bhunia P, Yan S, LeBlanc RJ, Tyagi RD, Surampalli RY. Ultrasonic pretreatment of sludge: A review. Ultrasonics Sonochemistry. 2011;**18**: 1-18. DOI: 10.1016/j. ultsonch.2010.02.014

[23] van der Stelt MJC, Gerhauser H, Kiel JHA, Ptasinski KJ. Biomass upgrading by torrefaction for the production of biofuels: A review. Biomass and Bioenergy. 2011;**35**: 3748-3762. DOI: 10.1016/j. biombioe.2011.06.023

[24] Zhang Q, He J, Tian M, Mao Z, Tang L, Zhang J, et al. Enhancement of methane production from cassava

residues by biological pretreatment using a constructed microbial consortium. Bioresource Technology. 2011;**102**(19):8899-8906. DOI: 10.1016/ j.biortech.2011.06.061

[25] Bustamante M, Alburquerque J, Restrepo A, de la Fuente C, Paredes C, Moral R, et al. Co-composting of the solid fraction of anaerobic digestates, to obtain added-value materials for use in agriculture. Biomass and Bioenergy. 2012;**43**:26-35. DOI: 10.1016/j. biombioe.2012.04.010

[26] Elliott A, Mahmood T. Comparison of mechanical pretreatment methods for the enhancement of anaerobic digestion of pulp and paper waste activated sludge. Water Science & Technology. 2012;**84**(6):497-505. DOI: 10.2175/ 106143012X13347678384602

[27] Liu X, Wang W, Gao X, Zhou Y, Shen R. Effect of thermal pretreatment on the physical and chemical properties of municipal biomass waste. Waste Management. 2012;**32**(2):249-255. DOI: 10.1016/j.wasman.2011.09.027

[28] WRAP: Waste and Resources Action Programme. Enhancement and Treatment of Digestates from Anaerobic Digestion. Banbury, Oxon, United Kingdom: WRAP; 2012. Available from: http://www.wrap.org.uk/sites/files/wra p/Digestates%20from%20Anaerobic% 20Digestion%20A%20review%20of% 20enhancement%20techniques%20and %20novel%20digestate%20products\_0. pdf

[29] Zhao P, Shen Y, Ge S, Chen Z, Yoshikawa K. Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment. Applied Energy. 2014;**131**: 345-367. DOI: 10.1016/j. apenergy.2014.06.038

[30] Peleteiro S, Rivas S, Alonso JL, Santos V, Parajo JC. Utilization of ionic liquids in lignocellulose biorefineries as agents for separation, derivatization, fractionation, or pretreatment. Journal of Agricultural and Food Chemistry. 2015;**63**(37):8093-8102. DOI: 10.1021/ acs.jafc.5b03461

[37] D'Alessio M, Durso LM, Miller DN,

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

thermophilic conditions. Bioresource Technology. 2013;**128**:612-618. DOI: 10.1016/j.biortech.2012.11.002

[44] Monlau F, Sambusiti C, Ficara E, Aboulkas A, Barakat A, Carrère H. New opportunities for agricultural digestate valorization: Current situation and

Environmental Science. 2015;**8**(9):2600.

Broadbent JR. Efficacy of washing meat surfaces with 2% levulinic, acetic, or

decontamination and residual growth inhibition. Meat Science. 2011;**88**(2):

[46] Li C, Wang Y, Xie G, Peng B, Zhang B, Chen W, et al. Complete genome sequence of *Clostridium butyricum* JKY6D1 isolated from the pit mud of a Chinese flavor liquor-making factory. Journal of Biotechnology. 2016;

[47] Huang WW, Huang WL, Yuan T, Zhao ZW, Cai W, Zhang ZY, et al. Volatile fatty acids (VFAs) production from swine manure through short-term

phosphorus resources in the digestate. Water Research. 2016;**90**:344-353. DOI:

dry anaerobic digestion and its separation from nitrogen and

10.1016/j.watres.2015.12.044

biortech.2014.03.088

[48] Wang K, Yin J, Shen D, Na L. Anaerobic digestion of food waste for volatile fatty acids (VFAs) production with different types of inoculum: Effect of pH. Bioresource Technology. 2014; **161**(6):395-401. DOI: 10.1016/j.

[49] Liu H, Han P, Liu H, Zhou G, Fu B, Zheng Z. Full-scale production of VFAs from sewage sludge by anaerobic alkaline fermentation to improve biological nutrients removal in domestic

perspectives. Energy and

DOI: 10.1039/c5ee01633a

lactic acid for pathogen

256-260. DOI: 10.1016/j. meatsci.2010.12.032

**220**:23-24. DOI: 10.1016/j. jbiotec.2016.01.003

[45] Carpenter CE, Smith JV,

[38] Tani M, Sakamoto N, Kishimoto T, Umetsu K. Utilization of anaerobically digested dairy slurry combined with other wastes following application to agricultural land. International Congress

Series. 2006;**1293**:331-334. DOI: 10.1016/j.ics.2006.03.013

[39] Alburquerque JA, Fuente C, Campoy M, Carrasco L, Nájera I, Baixauli C, et al. Agricultural use of digestate for horticultural crop production and improvement of soil properties. European Journal of Agronomy. 2012;**43**:119-128. DOI:

10.1016/j.eja.2012.06.001

119-124. DOI: 10.1016/j. aaspro.2015.12.004

[40] Koszel M, Lorencowicz E.

Agricultural use of biogas digestate as a replacement fertilizers. Agriculture and Agricultural Science Procedia. 2015;**7**:

[41] Garfí M, Ferrer-Martí L, Velo E, Ferrer I. Evaluating benefits of low-cost household digesters for rural Andean

Sustainable Energy Reviews. 2012;**16**(1): 575-581. DOI: 10.1016/j.rser.2011.08.023

Corporate Document Repository. M-09. 1992. ISBN: 92-5-103126-6. Available from: https://www.build-a-biogas-plant. com/PDF/BiogasSustainableDevolpme

[43] Giuliano A, Bolzonella D, Pavan P, Cavinato C, Cecchi F. Co-digestion of livestock effluents, energy crops and agro-waste: Feeding and process optimization in mesophilic and

[42] Marchaim U. Biogas process for sustainable development. FAO

communities. Renewable and

nt.pdf

**221**

Woodbury B, Ray C, Snow DD. Environmental fate and microbial effects of monensin, lincomycin, and sulfamethazine residues in soil. Environmental Pollution. 2019;**246**: 60-68. DOI: 10.1016/j.envpol.2018.

11.093

[31] Carrere H, Antonopoulou G, Affes R, Passos F, Battimelli A, Lyberatos G, et al. Review of feedstock pretreatment strategies for improved anaerobic digestion: From lab-scale research to full-scale application. Bioresource Technology. 2016;**199**: 386-397. DOI: 10.1016/j. biortech.2015.09.007

[32] Liguori R, Faraco V. Biological processes for advancing lignocellulosic waste biorefinery by advocating circular economy. Bioresource Technology. 2016;**215**:13-20. DOI: 10.1016/j. biortech.2016.04.054

[33] Zeng Y, Guardia AD, Dabert P. Improving composting as a posttreatment of anaerobic digestate. Bioresource Technology. 2016;**201**: 293-303. DOI: 10.1016/j. biortech.2015.11.013

[34] Bhutto AW, Qureshi K, Harijan K, Abro R, Abbas T, Bazmi AA, et al. Insight into progress in pre-treatment of lignocellulosic biomass. Energy. 2017; **122**:724-745. DOI: 10.1016/j. energy.2017.01.005

[35] Duque A, Manzanares P, Ballesteros M. Extrusion as a pretreatment for lignocellulosic biomass: Fundamentals and applications. Renewable Energy. 2017; **114**:1427-1441. DOI: 10.1016/j. renene.2017.06.050

[36] Shi L, Xie S, Hu Z, Wu G, Morrison L, Croot P, et al. Nutrient recovery from pig manure digestate using electrodialysis reversal: Membrane fouling and feasibility of long-term operation. Journal of Membrane Science. 2019;**573**:560-569. DOI: 10.1016/j.memsci.2018.12.037

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

[37] D'Alessio M, Durso LM, Miller DN, Woodbury B, Ray C, Snow DD. Environmental fate and microbial effects of monensin, lincomycin, and sulfamethazine residues in soil. Environmental Pollution. 2019;**246**: 60-68. DOI: 10.1016/j.envpol.2018. 11.093

residues by biological pretreatment using a constructed microbial

*Renewable Energy - Technologies and Applications*

agents for separation, derivatization, fractionation, or pretreatment. Journal of Agricultural and Food Chemistry. 2015;**63**(37):8093-8102. DOI: 10.1021/

[31] Carrere H, Antonopoulou G, Affes R, Passos F, Battimelli A,

[32] Liguori R, Faraco V. Biological processes for advancing lignocellulosic waste biorefinery by advocating circular economy. Bioresource Technology. 2016;**215**:13-20. DOI: 10.1016/j.

[33] Zeng Y, Guardia AD, Dabert P. Improving composting as a posttreatment of anaerobic digestate. Bioresource Technology. 2016;**201**:

[34] Bhutto AW, Qureshi K, Harijan K, Abro R, Abbas T, Bazmi AA, et al. Insight into progress in pre-treatment of lignocellulosic biomass. Energy. 2017;

applications. Renewable Energy. 2017;

386-397. DOI: 10.1016/j. biortech.2015.09.007

biortech.2016.04.054

293-303. DOI: 10.1016/j. biortech.2015.11.013

**122**:724-745. DOI: 10.1016/j.

[35] Duque A, Manzanares P, Ballesteros M. Extrusion as a pretreatment for lignocellulosic biomass: Fundamentals and

**114**:1427-1441. DOI: 10.1016/j.

[36] Shi L, Xie S, Hu Z, Wu G, Morrison L, Croot P, et al. Nutrient recovery from pig manure digestate using electrodialysis reversal: Membrane fouling and feasibility of long-term operation. Journal of Membrane Science. 2019;**573**:560-569. DOI: 10.1016/j.memsci.2018.12.037

energy.2017.01.005

renene.2017.06.050

Lyberatos G, et al. Review of feedstock pretreatment strategies for improved anaerobic digestion: From lab-scale research to full-scale application. Bioresource Technology. 2016;**199**:

acs.jafc.5b03461

consortium. Bioresource Technology. 2011;**102**(19):8899-8906. DOI: 10.1016/

[25] Bustamante M, Alburquerque J, Restrepo A, de la Fuente C, Paredes C, Moral R, et al. Co-composting of the solid fraction of anaerobic digestates, to obtain added-value materials for use in agriculture. Biomass and Bioenergy. 2012;**43**:26-35. DOI: 10.1016/j.

[26] Elliott A, Mahmood T. Comparison of mechanical pretreatment methods for the enhancement of anaerobic digestion of pulp and paper waste activated sludge. Water Science & Technology. 2012;**84**(6):497-505. DOI: 10.2175/ 106143012X13347678384602

[27] Liu X, Wang W, Gao X, Zhou Y, Shen R. Effect of thermal pretreatment on the physical and chemical properties of municipal biomass waste. Waste Management. 2012;**32**(2):249-255. DOI:

[28] WRAP: Waste and Resources Action

Treatment of Digestates from Anaerobic Digestion. Banbury, Oxon, United Kingdom: WRAP; 2012. Available from: http://www.wrap.org.uk/sites/files/wra p/Digestates%20from%20Anaerobic% 20Digestion%20A%20review%20of% 20enhancement%20techniques%20and %20novel%20digestate%20products\_0.

10.1016/j.wasman.2011.09.027

Programme. Enhancement and

[29] Zhao P, Shen Y, Ge S, Chen Z, Yoshikawa K. Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment. Applied Energy. 2014;**131**:

[30] Peleteiro S, Rivas S, Alonso JL, Santos V, Parajo JC. Utilization of ionic liquids in lignocellulose biorefineries as

345-367. DOI: 10.1016/j. apenergy.2014.06.038

pdf

**220**

j.biortech.2011.06.061

biombioe.2012.04.010

[38] Tani M, Sakamoto N, Kishimoto T, Umetsu K. Utilization of anaerobically digested dairy slurry combined with other wastes following application to agricultural land. International Congress Series. 2006;**1293**:331-334. DOI: 10.1016/j.ics.2006.03.013

[39] Alburquerque JA, Fuente C, Campoy M, Carrasco L, Nájera I, Baixauli C, et al. Agricultural use of digestate for horticultural crop production and improvement of soil properties. European Journal of Agronomy. 2012;**43**:119-128. DOI: 10.1016/j.eja.2012.06.001

[40] Koszel M, Lorencowicz E. Agricultural use of biogas digestate as a replacement fertilizers. Agriculture and Agricultural Science Procedia. 2015;**7**: 119-124. DOI: 10.1016/j. aaspro.2015.12.004

[41] Garfí M, Ferrer-Martí L, Velo E, Ferrer I. Evaluating benefits of low-cost household digesters for rural Andean communities. Renewable and Sustainable Energy Reviews. 2012;**16**(1): 575-581. DOI: 10.1016/j.rser.2011.08.023

[42] Marchaim U. Biogas process for sustainable development. FAO Corporate Document Repository. M-09. 1992. ISBN: 92-5-103126-6. Available from: https://www.build-a-biogas-plant. com/PDF/BiogasSustainableDevolpme nt.pdf

[43] Giuliano A, Bolzonella D, Pavan P, Cavinato C, Cecchi F. Co-digestion of livestock effluents, energy crops and agro-waste: Feeding and process optimization in mesophilic and

thermophilic conditions. Bioresource Technology. 2013;**128**:612-618. DOI: 10.1016/j.biortech.2012.11.002

[44] Monlau F, Sambusiti C, Ficara E, Aboulkas A, Barakat A, Carrère H. New opportunities for agricultural digestate valorization: Current situation and perspectives. Energy and Environmental Science. 2015;**8**(9):2600. DOI: 10.1039/c5ee01633a

[45] Carpenter CE, Smith JV, Broadbent JR. Efficacy of washing meat surfaces with 2% levulinic, acetic, or lactic acid for pathogen decontamination and residual growth inhibition. Meat Science. 2011;**88**(2): 256-260. DOI: 10.1016/j. meatsci.2010.12.032

[46] Li C, Wang Y, Xie G, Peng B, Zhang B, Chen W, et al. Complete genome sequence of *Clostridium butyricum* JKY6D1 isolated from the pit mud of a Chinese flavor liquor-making factory. Journal of Biotechnology. 2016; **220**:23-24. DOI: 10.1016/j. jbiotec.2016.01.003

[47] Huang WW, Huang WL, Yuan T, Zhao ZW, Cai W, Zhang ZY, et al. Volatile fatty acids (VFAs) production from swine manure through short-term dry anaerobic digestion and its separation from nitrogen and phosphorus resources in the digestate. Water Research. 2016;**90**:344-353. DOI: 10.1016/j.watres.2015.12.044

[48] Wang K, Yin J, Shen D, Na L. Anaerobic digestion of food waste for volatile fatty acids (VFAs) production with different types of inoculum: Effect of pH. Bioresource Technology. 2014; **161**(6):395-401. DOI: 10.1016/j. biortech.2014.03.088

[49] Liu H, Han P, Liu H, Zhou G, Fu B, Zheng Z. Full-scale production of VFAs from sewage sludge by anaerobic alkaline fermentation to improve biological nutrients removal in domestic wastewater. Bioresource Technology. 2018;**260**:105-114. DOI: 10.1016/j. biortech.2018.03.105

[50] Neumann J, Binder S, Apfelbacher A, Gasson JR, Ramírez García P, Hornung A. Production and characterization of a new quality pyrolysis oil, char and syngas from digestate–Introducing the thermocatalytic reforming process. Journal of Analytical and Applied Pyrolysis. 2015; **113**:137-142. DOI: 10.1016/j. jaap.2014.11.022

[51] Singh R, Parihar P, Singh M, Bajguz A, Kumar J, Singh S, et al. Uncovering potential applications of cyanobacteria and algal metabolites in biology, agriculture and medicine: Current status and future prospects. Frontiers in Microbiology. 2017;**8**:515. DOI: 10.3389/fmicb.2017.00515

[52] Koutra E, Economou CN, Tsafrakidou P, Kornaros M. Bio-based products from microalgae cultivated in digestates. Trends in Biotechnology. 2018;**36**(8):819-833. DOI: 10.1016/j. tibtech.2018.02.015

[53] Inyang M, Gao B, Yao Y, Xue Y, Zimmerman AR, Pullammanappallil P, et al. Removal of heavy metals from aqueous solution by biochars derived from anaerobically digested biomass. Bioresource Technology. 2012;**110**: 50-56. DOI: 10.1016/j.biortech. 2012.01.072

[54] Dahlin J, Herbes C, Nelles M. Biogas digestate marketing: Qualitative insights into the supply side. Resources, Conservation and Recycling. 2015;**104**: 152-161. DOI: 10.1016/j. resconrec.2015.08.013

[55] Spelter H, Winandy J, Zauche T. Anaerobically digested bovine biofiber as a source of fiber for particleboard manufacturing: An economic analysis. BioResources. 2008;**3**(4):1256-1266. Available from: https://ojs.cnr.ncsu.edu/ index.php/BioRes/article/viewFile/

BioRes\_03\_4\_1256\_Spelter\_WZ\_ADBF\_ Particleboard/278

www.iea-biogas.net/files/daten-reda ktion/download/Technical%20Brochure s/NUTRIENT\_RECOVERY\_RZ\_web2.

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

Engineering. 2013;**5**(08):506. Available from: http://www.build-a-biogas-plant.c om/PDF/ProblemsAfricanBiogas2013.pdf

Capacity-cost and location-cost analyses for biogas plants in Africa. Resources, Conservation and Recycling. 2010; **55**(1):63-73. DOI: 10.1016/j. resconrec.2010.07.004

[70] Bolzonella D, Fatone F, Gottardo M, Frison N. Nutrients recovery from anaerobic digestate of agro-waste: Techno-economic assessment of full

Environmental Management. 2018;**216**:

[69] Amigun B, Von Blottnitz H.

scale applications. Journal of

[71] Gerardo ML, Aljohani NHM, Oatley-Radcliffe DL, Lovitt RW. Moving towards sustainable resources: Recovery and fractionation of nutrients from dairy manure digestate using membranes. Water Research. 2015;**80**:

[72] Favoino E, Hogg D. Effects of

A specific angle: The potential contribution of biowaste to tackle Climate Change and references to the

soil policy. Proceedings of the

composted organic waste on ecosystems—

International Congress CODIS. 2008; **2008**:145-156. Available from: https:// orgprints.org/13135/1/fuchs-eta

l-proceedings-codis-2008.pdf#page=151

[74] de Alwis A. Biogas—A review of Sri Lanka's performance with a renewable

energy technology. Energy for Sustainable Development. 2002;**6**(1): 30-37. DOI: 10.1016/S0973-0826(08)

60296-3

[73] Hu Y, Shen F, Yuan H, Zou D, Pang Y, Liu Y, et al. Influence of recirculation of liquid fraction of the digestate (LFD) on maize stover anaerobic digestion. Biosystems Engineering. 2014;**127**:189-196. DOI: 10.1016/j.biosystemseng.2014.09.006

111-119. DOI: 10.1016/j. jenvman.2017.08.026

80-89. DOI: 10.1016/j. watres.2015.05.016

[63] Hogg D. Eunomia Research & Consulting. Costs for municipal waste management in the EU. Final Report to Directorate General Environment, European Commission. 2002. Available from: http://ec.europa.eu/environment/ waste/studies/pdf/eucostwaste.pdf

[64] Delzeit R, Kellner U. The impact of plant size and location on profitability of

[65] Baddeley A, Ballinger A, Cessford I, Smith EM, Enviro A. Assessing the costs

biogas plants in Germany under consideration of processing digestates. Biomass and Bioenergy. 2013;**52**:43-53. DOI: 10.1016/j.biombioe.2013.02.029

and benefits for production and beneficial application of anaerobic digestate to agricultural land in Wales. In: Project OMK007-203. WRAP: Waste and Resources Action Programme. Cardiff, UK; 2014. Available from: http://www.wrapcymru.org.uk/sites/ files/wrap/Assessing%20the%20Costs% 20and%20Benefits%20for%20Produc tion%20and%20Beneficial%20Applica tion%20of%20Anaerobic%20Digestate %20to%20Agricultural%20Land%20in

%20Wales%202014.pdf

[66] Møller HB, Lund I, Sommer SG. Solid–liquid separation of livestock slurry: Efficiency and cost. Bioresource Technology. 2000;**74**(3):223-229. DOI: 10.1016/S0960-8524(00)00016-X

[67] Stiles WAV, Styles D, Chapman SP, Esteves S, Bywater A, Melville L, et al. Using microalgae in the circular

economy to valorise anaerobic digestate:

Dissemination and problems of African biogas technology. Energy and Power

Challenges and opportunities. Bioresource Technology. 2018;**267**:

[68] Mulinda C, Hu Q, Pan K.

732-742. DOI: 10.1016/j. biortech.2018.07.100

**223**

pdf

[56] USDA. Uses of solids and byproducts of anaerobic digestion. Farm Energy. 2019;**3**:2019. Available from: https://farm-energy.extension.org/usesof-solids-and-by-products-of-anaerobicdigestion/#Livestock\_bedding

[57] Sophin P, Preston TR. Effect of processing pig manure in a biodigester as fertilizer input for ponds growing fish in polyculture. Livestock Research for Rural Development. 2001;**13**:60. Available from: https://www.lrrd.cipav. org.co/lrrd13/6/pich136.htm

[58] Garcia-Sánchez M, Garcia-Romera I, Cajthaml T, Tlustoš P, Száková J. Changes in soil microbial community functionality and structure in a metal-polluted site: The effect of digestate and fly ash applications. Journal of Environmental Management. 2015;**162**:63-73. DOI: 10.1016/j. jenvman.2015.07.042

[59] Alexander R. Digestate utilization in the U.S. Biocycle. 2012;**53**(1):56. Available from: https://www.biocycle. net/2012/01/12/digestate-utilization-inthe-u-s/

[60] Tin AM, Wise DL, Su WH, Reutergardh L, Lee SK. Cost-benefit analysis of the municipal solid waste collection system in Yangon, Myanmar. Resources, Conservation and Recycling. 1995;**14**(2):103-131. DOI: 10.1016/ S0921-3449(95)80004-2

[61] Setiawan RP, Kaneko S, Kawata K. Impacts of pecuniary and nonpecuniary information on proenvironmental behavior: A household waste collection and disposal program in Surabaya city. Waste Management. 2019;**89**:322-335. DOI: 10.1016/j. wasman.2019.04.015

[62] Drosg B, Fuchs W, Al Seadi T, Madsen M, Linke B. Nutrient recovery by biogas digestate processing. IEA Bioenergy. 2015. Available from: https:// *Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

www.iea-biogas.net/files/daten-reda ktion/download/Technical%20Brochure s/NUTRIENT\_RECOVERY\_RZ\_web2. pdf

wastewater. Bioresource Technology. 2018;**260**:105-114. DOI: 10.1016/j.

*Renewable Energy - Technologies and Applications*

BioRes\_03\_4\_1256\_Spelter\_WZ\_ADBF\_

[56] USDA. Uses of solids and byproducts of anaerobic digestion. Farm Energy. 2019;**3**:2019. Available from: https://farm-energy.extension.org/usesof-solids-and-by-products-of-anaerobic-

digestion/#Livestock\_bedding

org.co/lrrd13/6/pich136.htm

2015;**162**:63-73. DOI: 10.1016/j.

the U.S. Biocycle. 2012;**53**(1):56. Available from: https://www.biocycle. net/2012/01/12/digestate-utilization-in-

[60] Tin AM, Wise DL, Su WH, Reutergardh L, Lee SK. Cost-benefit analysis of the municipal solid waste collection system in Yangon, Myanmar. Resources, Conservation and Recycling. 1995;**14**(2):103-131. DOI: 10.1016/

[61] Setiawan RP, Kaneko S, Kawata K.

Impacts of pecuniary and nonpecuniary information on proenvironmental behavior: A household waste collection and disposal program in Surabaya city. Waste Management. 2019;**89**:322-335. DOI: 10.1016/j.

[62] Drosg B, Fuchs W, Al Seadi T, Madsen M, Linke B. Nutrient recovery by biogas digestate processing. IEA Bioenergy. 2015. Available from: https://

S0921-3449(95)80004-2

wasman.2019.04.015

jenvman.2015.07.042

the-u-s/

[57] Sophin P, Preston TR. Effect of processing pig manure in a biodigester as fertilizer input for ponds growing fish in polyculture. Livestock Research for Rural Development. 2001;**13**:60. Available from: https://www.lrrd.cipav.

[58] Garcia-Sánchez M, Garcia-Romera I, Cajthaml T, Tlustoš P, Száková J. Changes in soil microbial community functionality and structure in a metal-polluted site: The effect of digestate and fly ash applications. Journal of Environmental Management.

[59] Alexander R. Digestate utilization in

Particleboard/278

Apfelbacher A, Gasson JR, Ramírez García P, Hornung A. Production and characterization of a new quality pyrolysis oil, char and syngas from digestate–Introducing the thermocatalytic reforming process. Journal of Analytical and Applied Pyrolysis. 2015;

biortech.2018.03.105

[50] Neumann J, Binder S,

**113**:137-142. DOI: 10.1016/j.

[51] Singh R, Parihar P, Singh M, Bajguz A, Kumar J, Singh S, et al. Uncovering potential applications of cyanobacteria and algal metabolites in biology, agriculture and medicine: Current status and future prospects. Frontiers in Microbiology. 2017;**8**:515. DOI: 10.3389/fmicb.2017.00515

[52] Koutra E, Economou CN,

tibtech.2018.02.015

2012.01.072

**222**

Tsafrakidou P, Kornaros M. Bio-based products from microalgae cultivated in digestates. Trends in Biotechnology. 2018;**36**(8):819-833. DOI: 10.1016/j.

[53] Inyang M, Gao B, Yao Y, Xue Y, Zimmerman AR, Pullammanappallil P, et al. Removal of heavy metals from aqueous solution by biochars derived from anaerobically digested biomass. Bioresource Technology. 2012;**110**: 50-56. DOI: 10.1016/j.biortech.

[54] Dahlin J, Herbes C, Nelles M. Biogas digestate marketing: Qualitative insights

into the supply side. Resources, Conservation and Recycling. 2015;**104**:

[55] Spelter H, Winandy J, Zauche T. Anaerobically digested bovine biofiber as a source of fiber for particleboard manufacturing: An economic analysis. BioResources. 2008;**3**(4):1256-1266. Available from: https://ojs.cnr.ncsu.edu/ index.php/BioRes/article/viewFile/

152-161. DOI: 10.1016/j. resconrec.2015.08.013

jaap.2014.11.022

[63] Hogg D. Eunomia Research & Consulting. Costs for municipal waste management in the EU. Final Report to Directorate General Environment, European Commission. 2002. Available from: http://ec.europa.eu/environment/ waste/studies/pdf/eucostwaste.pdf

[64] Delzeit R, Kellner U. The impact of plant size and location on profitability of biogas plants in Germany under consideration of processing digestates. Biomass and Bioenergy. 2013;**52**:43-53. DOI: 10.1016/j.biombioe.2013.02.029

[65] Baddeley A, Ballinger A, Cessford I, Smith EM, Enviro A. Assessing the costs and benefits for production and beneficial application of anaerobic digestate to agricultural land in Wales. In: Project OMK007-203. WRAP: Waste and Resources Action Programme. Cardiff, UK; 2014. Available from: http://www.wrapcymru.org.uk/sites/ files/wrap/Assessing%20the%20Costs% 20and%20Benefits%20for%20Produc tion%20and%20Beneficial%20Applica tion%20of%20Anaerobic%20Digestate %20to%20Agricultural%20Land%20in %20Wales%202014.pdf

[66] Møller HB, Lund I, Sommer SG. Solid–liquid separation of livestock slurry: Efficiency and cost. Bioresource Technology. 2000;**74**(3):223-229. DOI: 10.1016/S0960-8524(00)00016-X

[67] Stiles WAV, Styles D, Chapman SP, Esteves S, Bywater A, Melville L, et al. Using microalgae in the circular economy to valorise anaerobic digestate: Challenges and opportunities. Bioresource Technology. 2018;**267**: 732-742. DOI: 10.1016/j. biortech.2018.07.100

[68] Mulinda C, Hu Q, Pan K. Dissemination and problems of African biogas technology. Energy and Power

Engineering. 2013;**5**(08):506. Available from: http://www.build-a-biogas-plant.c om/PDF/ProblemsAfricanBiogas2013.pdf

[69] Amigun B, Von Blottnitz H. Capacity-cost and location-cost analyses for biogas plants in Africa. Resources, Conservation and Recycling. 2010; **55**(1):63-73. DOI: 10.1016/j. resconrec.2010.07.004

[70] Bolzonella D, Fatone F, Gottardo M, Frison N. Nutrients recovery from anaerobic digestate of agro-waste: Techno-economic assessment of full scale applications. Journal of Environmental Management. 2018;**216**: 111-119. DOI: 10.1016/j. jenvman.2017.08.026

[71] Gerardo ML, Aljohani NHM, Oatley-Radcliffe DL, Lovitt RW. Moving towards sustainable resources: Recovery and fractionation of nutrients from dairy manure digestate using membranes. Water Research. 2015;**80**: 80-89. DOI: 10.1016/j. watres.2015.05.016

[72] Favoino E, Hogg D. Effects of composted organic waste on ecosystems— A specific angle: The potential contribution of biowaste to tackle Climate Change and references to the soil policy. Proceedings of the International Congress CODIS. 2008; **2008**:145-156. Available from: https:// orgprints.org/13135/1/fuchs-eta l-proceedings-codis-2008.pdf#page=151

[73] Hu Y, Shen F, Yuan H, Zou D, Pang Y, Liu Y, et al. Influence of recirculation of liquid fraction of the digestate (LFD) on maize stover anaerobic digestion. Biosystems Engineering. 2014;**127**:189-196. DOI: 10.1016/j.biosystemseng.2014.09.006

[74] de Alwis A. Biogas—A review of Sri Lanka's performance with a renewable energy technology. Energy for Sustainable Development. 2002;**6**(1): 30-37. DOI: 10.1016/S0973-0826(08) 60296-3

[75] Brändli RC, Bucheli TD, Kupper T, Furrer R, Stahel WA, Stadelmann FX, et al. Organic pollutants in compost and digestate. Part 1. Polychlorinated biphenyls, polycyclic aromatic hydrocarbons and molecular markers. Journal of Environment Monitoring. 2007;**9**(5):456-464. DOI: 10.1039/ B617101J

[76] Tiwary A, Williams ID, Pant DC, Kishore VVN. Assessment and mitigation of the environmental burdens to air from land applied foodbased digestate. Environmental Pollution. 2015;**203**:262-270. DOI: 10.1016/j.envpol.2015.02.001

[77] Wallace P, Harris G, Frederickson J, Howell G, Tompkins D. Biofertiliser management: Best practice for agronomic benefit & odour control. In: Tompkins D, editor. Project OAV036-210. Cardiff: The Waste and Resources Action Program (WRAP); 2011. Available from: http:// www.wrapcymru.org.uk/sites/files/wra p/Digestate%20odour%20management% 20-%20Cymru.pdf

[78] Cheng Z, Sun Z, Zhu S, Lou Z, Zhu N, Feng L. The identification and health risk assessment of odor emissions from waste landfilling and composting. Science of The Total Environment. 2019;**649**:1038-1044. DOI: 10.1016/j. scitotenv.2018.08.230

[79] Eshete G, Sonder K, ter Heegde F. Report on the Feasibility Study of a National Programme for Domestic Biogas in Ethiopia. SNV Netherlands Development Organization: Addis Ababa, Ethiopia; 2006. Available from: http://www.bibalex.org/Search4Dev/ files/338849/172350.pdf

[80] Berhe TG, Tesfahuney RG, Desta GA, Mekonnen LS. Biogas plant distribution for rural household sustainable energy supply in Africa. Energy and Policy Research. 2017;**4**(1): 10-20. DOI: 10.1080/ 23815639.2017.1280432

[81] Mushtaq K, Zaidi AA, Askari SJ. Design and performance analysis of floating dome type portable biogas plant for domestic use in Pakistan. Sustainable Energy Technologies and Assessments. 2016;**14**:21-25. DOI: 10.1016/j. seta.2016.01.001

Bioresource Technology. 2014;**158**:

*DOI: http://dx.doi.org/10.5772/intechopen.91340*

environmental safety dividends. Environmental Progress & Sustainable Energy. 2019. DOI: 10.1002/ep.13138

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results…*

538-548. DOI: 10.1016/j. biortech.2014.07.086

[96] Haraldsen TK, Andersen U,

[97] Pirelli T, Rossi A, Miller C. Sustainability of biogas and cassavabased ethanol value chains in Viet Nam: Results and recommendations from the implementation of the Global Bioenergy

Partnership indicators. In: FAO Environment and Natural Resources Management Working Paper 69. Rome: FAO; 2018. Available from: http://www.

fao.org/3/i9181en/I9181EN.pdf

Krogstad T, Sørheim R. Liquid digestate from anaerobic treatment of sourceseparated household waste as fertiliser to barley. Waste Management & Research. 2017;**29**:1271-1276. DOI: 10.1177/0734242X11411975

[95] Tan X, Chu H, Zhang Y, Yang L, Zhao F, Zhou X. *Chlorella pyrenoidosa* cultivation using anaerobic digested starch processing wastewater in an airlift circulation photobioreactor. Bioresource Technology. 2014;**170**:

Zinkernagel V, Reents H-J. The impact and the interaction of nitrogen and *Phytophthora infestans* as yield-limiting and yield-reducing factors in organic potato (*Solanum tuberosum* L.) crops. Potato Research. 2006;**49**(4):281-301. DOI: 10.1007/s11540-007-9024-7

174-180. DOI: 10.1016/j. biortech.2014.02.023

[89] Möller K, Habermeyer J,

[90] Shenoy VV, Kalagudi GM. Enhancing plant phosphorus use efficiency for sustainable cropping. Biotechnology Advances. 2005;**23**(7-8):

[91] Sobachkin AA. The physiological role of potassium in increasing the productivity of farm crops. In: Potassium Research and Agricultural Production. Proceedings of the 10th Congress of the International Potash Institute held in June 1974 in Budapest, Hungary. International Potash Institute. 1974. pp. 147-152. Available from: https://www.ipipotash.org/uploads/ udocs/potassium\_research\_and\_ agricultural\_production.pdf

[92] Edith KKN, Francis KY, Martin KK, Felix KK. Characterization of digestates from anaerobic co-digestion of manioc effluent, human urine and cow dung. Journal of Water Resource and Protection. 2019;**11**(06):777-788. DOI:

[93] Aso SN. Food engineering stratagem to protect the environment and improve

10.4236/jwarp.2019.116047

**2**(1):31-36

**225**

the income opportunities of gari processors. Journal of Nigerian Environmental Society (JNES). 2004;

[94] Aso SN, Pullammanappallil PC, Teixeira AA, Welt BA. Biogasification of cassava residue for on-site biofuel generation for food production with potential cost minimization, health and

501-513. DOI: 10.1016/j. biotechadv.2005.01.004

[82] Day DL, Chen TH, Anderson JC, Steinberg MP. Biogas plants for small farms in Kenya. Biomass. 1990;**21**(2): 83-99. DOI: 10.1016/0144-4565(90) 90051-K

[83] Caceres R, Chiliquinga B. Experiences with Rural Biodigesters in Latin America. Dordrecht: Springer; 1986. pp. 150-165. DOI: 10.1007/978-94-009-4313-1\_21

[84] Ni J-Q, Nyns E-J. New concept for the evaluation of rural biogas management in developing countries. Energy Conversion and Management. 1996;**37**(10):1525-1534. DOI: 10.1016/ 0196-8904(95)00354-1

[85] Raven RPJM. Implementation of manure digestion and co-combustion in the Dutch electricity regime: A multilevel analysis of market implementation in the Netherlands. Energy Policy. 2004; **32**(1):29-39. DOI: 10.1016/S0301-4215 (02)00248-3

[86] Negro SO, Hekkert MP, Smits RE. Explaining the failure of the Dutch innovation system for biomass digestion —A functional analysis. Energy Policy. 2007;**35**(2):925-938. DOI: 10.1016/j. enpol.2006.01.027

[87] Tigini V, Franchino M, Bona F, Varese GC. Is digestate safe? A study on its ecotoxicity and environmental risk on a pig manure. Science of the Total Environment. 2016;**551-552**:127-132. DOI: 10.1016/j.scitotenv.2016.02.004

[88] Prajapati SK, Kumar P, Malik A, Vijay VK. Bioconversion of algae to methane and subsequent utilization of digestate for algae cultivation: A closed loop bioenergy generation process.

*Digestate: The Coproduct of Biofuel Production in a Circular Economy, and New Results… DOI: http://dx.doi.org/10.5772/intechopen.91340*

Bioresource Technology. 2014;**158**: 174-180. DOI: 10.1016/j. biortech.2014.02.023

[75] Brändli RC, Bucheli TD, Kupper T, Furrer R, Stahel WA, Stadelmann FX, et al. Organic pollutants in compost and digestate. Part 1. Polychlorinated biphenyls, polycyclic aromatic

*Renewable Energy - Technologies and Applications*

[81] Mushtaq K, Zaidi AA, Askari SJ. Design and performance analysis of floating dome type portable biogas plant for domestic use in Pakistan. Sustainable Energy Technologies and Assessments.

[82] Day DL, Chen TH, Anderson JC, Steinberg MP. Biogas plants for small farms in Kenya. Biomass. 1990;**21**(2): 83-99. DOI: 10.1016/0144-4565(90)

[83] Caceres R, Chiliquinga B. Experiences with Rural Biodigesters in Latin America. Dordrecht: Springer; 1986. pp. 150-165. DOI: 10.1007/978-94-009-4313-1\_21

[84] Ni J-Q, Nyns E-J. New concept for

management in developing countries. Energy Conversion and Management. 1996;**37**(10):1525-1534. DOI: 10.1016/

[85] Raven RPJM. Implementation of manure digestion and co-combustion in the Dutch electricity regime: A multilevel analysis of market implementation in the Netherlands. Energy Policy. 2004; **32**(1):29-39. DOI: 10.1016/S0301-4215

[86] Negro SO, Hekkert MP, Smits RE. Explaining the failure of the Dutch innovation system for biomass digestion —A functional analysis. Energy Policy. 2007;**35**(2):925-938. DOI: 10.1016/j.

[87] Tigini V, Franchino M, Bona F, Varese GC. Is digestate safe? A study on its ecotoxicity and environmental risk on a pig manure. Science of the Total Environment. 2016;**551-552**:127-132. DOI: 10.1016/j.scitotenv.2016.02.004

[88] Prajapati SK, Kumar P, Malik A, Vijay VK. Bioconversion of algae to methane and subsequent utilization of digestate for algae cultivation: A closed loop bioenergy generation process.

the evaluation of rural biogas

0196-8904(95)00354-1

(02)00248-3

enpol.2006.01.027

2016;**14**:21-25. DOI: 10.1016/j.

seta.2016.01.001

90051-K

hydrocarbons and molecular markers. Journal of Environment Monitoring. 2007;**9**(5):456-464. DOI: 10.1039/

[76] Tiwary A, Williams ID, Pant DC, Kishore VVN. Assessment and mitigation of the environmental burdens to air from land applied foodbased digestate. Environmental Pollution. 2015;**203**:262-270. DOI: 10.1016/j.envpol.2015.02.001

[77] Wallace P, Harris G, Frederickson J, Howell G, Tompkins D. Biofertiliser management: Best practice for agronomic benefit & odour control. In: Tompkins D, editor. Project OAV036-210. Cardiff: The Waste and Resources Action Program (WRAP); 2011. Available from: http:// www.wrapcymru.org.uk/sites/files/wra p/Digestate%20odour%20management%

[78] Cheng Z, Sun Z, Zhu S, Lou Z, Zhu N, Feng L. The identification and health risk assessment of odor emissions from waste landfilling and composting. Science of The Total Environment. 2019;**649**:1038-1044. DOI: 10.1016/j.

[79] Eshete G, Sonder K, ter Heegde F. Report on the Feasibility Study of a National Programme for Domestic Biogas in Ethiopia. SNV Netherlands Development Organization: Addis Ababa, Ethiopia; 2006. Available from: http://www.bibalex.org/Search4Dev/

20-%20Cymru.pdf

scitotenv.2018.08.230

files/338849/172350.pdf

10-20. DOI: 10.1080/ 23815639.2017.1280432

**224**

[80] Berhe TG, Tesfahuney RG, Desta GA, Mekonnen LS. Biogas plant distribution for rural household sustainable energy supply in Africa. Energy and Policy Research. 2017;**4**(1):

B617101J

[89] Möller K, Habermeyer J, Zinkernagel V, Reents H-J. The impact and the interaction of nitrogen and *Phytophthora infestans* as yield-limiting and yield-reducing factors in organic potato (*Solanum tuberosum* L.) crops. Potato Research. 2006;**49**(4):281-301. DOI: 10.1007/s11540-007-9024-7

[90] Shenoy VV, Kalagudi GM. Enhancing plant phosphorus use efficiency for sustainable cropping. Biotechnology Advances. 2005;**23**(7-8): 501-513. DOI: 10.1016/j. biotechadv.2005.01.004

[91] Sobachkin AA. The physiological role of potassium in increasing the productivity of farm crops. In: Potassium Research and Agricultural Production. Proceedings of the 10th Congress of the International Potash Institute held in June 1974 in Budapest, Hungary. International Potash Institute. 1974. pp. 147-152. Available from: https://www.ipipotash.org/uploads/ udocs/potassium\_research\_and\_ agricultural\_production.pdf

[92] Edith KKN, Francis KY, Martin KK, Felix KK. Characterization of digestates from anaerobic co-digestion of manioc effluent, human urine and cow dung. Journal of Water Resource and Protection. 2019;**11**(06):777-788. DOI: 10.4236/jwarp.2019.116047

[93] Aso SN. Food engineering stratagem to protect the environment and improve the income opportunities of gari processors. Journal of Nigerian Environmental Society (JNES). 2004; **2**(1):31-36

[94] Aso SN, Pullammanappallil PC, Teixeira AA, Welt BA. Biogasification of cassava residue for on-site biofuel generation for food production with potential cost minimization, health and

environmental safety dividends. Environmental Progress & Sustainable Energy. 2019. DOI: 10.1002/ep.13138

[95] Tan X, Chu H, Zhang Y, Yang L, Zhao F, Zhou X. *Chlorella pyrenoidosa* cultivation using anaerobic digested starch processing wastewater in an airlift circulation photobioreactor. Bioresource Technology. 2014;**170**: 538-548. DOI: 10.1016/j. biortech.2014.07.086

[96] Haraldsen TK, Andersen U, Krogstad T, Sørheim R. Liquid digestate from anaerobic treatment of sourceseparated household waste as fertiliser to barley. Waste Management & Research. 2017;**29**:1271-1276. DOI: 10.1177/0734242X11411975

[97] Pirelli T, Rossi A, Miller C. Sustainability of biogas and cassavabased ethanol value chains in Viet Nam: Results and recommendations from the implementation of the Global Bioenergy Partnership indicators. In: FAO Environment and Natural Resources Management Working Paper 69. Rome: FAO; 2018. Available from: http://www. fao.org/3/i9181en/I9181EN.pdf

**227**

Section 4

Hydroelectric Energy

Section 4
