**3.2.3 Flow-through processes**

The flow-through processes, such as UASB process (Fig. 9) are used only for substrates where most of the organic material is in dissolved form with solids content at maximum 1-5 gL-1. In this substrate category are highly loaded wastewaters of industrial origin (e.g. from beverage industry).

Anaerobic Treatment and Biogas Production from Organic Waste 19

1. Specific Biogas Productivity - SBP (it's also called biogas yield). It is defined as volume of biogas produced per mass of substrate inserted into digester (m3kg-1). There are variations; SBP can be expressed in m3 of gas per kg of substrate: i) (wet) mass, ii) total solids, iii) volatile organic solids or iv) COD. SBP tells us how much biogas was produced from the chosen unit of substrate. Maximum possible SBP for certain substrate is called biogas potential. Biogas potential can be determined by a standard

2. Biogas Production Rate – BPR. It is defined as volume of biogas produced per volume of the digester per day (m3m-3d-1). BPR tells us how much biogas we can gain from the

SBP values of an optimally operating digester reach 80-90 % of the biogas potential. Typical values of SBP for farm waste are shown in Table 1. Typical values of BPR for mesophilic digesters are from 0.9 to 1.3 m3m-3d-1. Lower values indicate the digester is oversized; higher values are rare or impossible, due to anaerobic process failure. For thermophilic or two stage digesters the typical BPR values are from 1.3 to 2.1 m3m-3d-1, respectively. UASB reactors are

Biogas production is rarely constant; it is prone to fluctuations due to variation of loading rates, inner and outer operating conditions, possible inhibitions etc... Therefore, a buffer volume is required for the biogas storage. This enables the biogas user to get a constant biogas flow and composition. Most of the modern biogas plants are equipped with cogeneration units (named also combined heat and power units – CHP) which require constant gas flow for steady and efficient operation. There are several possibilities of biogas storage; roughly they can be divided into low pressure (10-50 mbar) and high pressure storage (over 5 bar). Low-pressure storage is used in on-site installations and for gas grid delivery; high pressure storage is used for long term storage, for transport in high pressure tanks and in installations with scarce space for volume extensive low pressure holders.

Low pressure biogas holders arise in many variations. It is possible to include biogas holder in the design of the digester. The most known is the digester with a movable cover. These digesters are less common, because a movable cover requires increased investment and operating expenditure. More common are external biogas holders that are widely commercially available. An example of a modern biogas holder is presented in Fig. 10.

Low pressure biogas holders require an extensive volume of 30 to 2000 m3 (Deublin and Steinhauser, 2008). Usually the pressure is kept constant and the volume of the bag is varied. High pressure biogas holders are of constant volume and made of steel, they are subject to special safety requirements. They do require more complex equipment for compression and

Biogas contains methane (40-70% by volume) and carbon dioxide. There are also components, present in low concentrations (below 1 %) such as water vapour, substrate micro particles and trace gases. Therefore biogas treatment is necessary to preserve equipment for its storage, transport and utilisation. Solid particles can be filtered out by candle filters, sludge and foam is separated in cyclones. For removal of trace gases, where hydrogen sulphide (H2S) is the most disturbing one due to its corrosion properties, processes like scrubbing, adsorption and absorption are used. In some cases also drying is

expansion of the gas and are more cost-effective for operation and maintenance.

required (usually to the relative humidity of less than 80 %).

much less volume demanding and can achieve a BPR of up to 10 m3m-3d-1.

method (ISO 1998).

active volume of a digester in one day.

Fig. 9. The UASB process

## **3.3 Post-treatment and substrate use**

After the substrate has been digested, it usually needs additional treatment. There are several possibilities of digested substrate utilisation. Most often, especially in the case of farm waste treatment, the digested substrate is used as a fertilizer. It can be used in liquid state or dewatered. Liquid substrate (total solids concentration 1-5 % by mass) is pumped from the post-treatment tank and spread on the fields. However it must be considered that fertilizing is possible only in certain periods of the year (once or twice). The post-treatment container must be designed accordingly. A possible solution is a lagoon, where digested substrate is stored and additionally stabilized and mineralized during the storing time. When using solid substrate (total solids concentration 20-30 % by mass), the digested substrate is mechanically dewatered first (by belt press or centrifuge) and then liquid and solid parts are used separately. Solid digestate after dewatering can be used fresh as a fertilizer, or it should be stabilized by composting (see further section).

Liquid part of the separated digestate can be used in the new substrate preparation as dilution water, however great caution must be given to nutrients or salts build-up and consequently possible inhibition in the anaerobic digestion. Usually only a portion of that liquid is used in the substrate preparation; the rest must be further treated as a wastewater. Typical concentration of the liquid part of digestate is 200-1000 mgL-1 of COD.

### **3.4 Biogas production, storage, treatment and use**

When operating a biogas plant, biogas is the main product and considerable attention must be given to its production, storage, treatment and use. Biogas production completely depends on the efficiency of the anaerobic digestion and its microorganisms. Previous sections have shown what conditions must be met to successfully operate anaerobic digestion. There are two distinct parameters that describe the biogas production:

After the substrate has been digested, it usually needs additional treatment. There are several possibilities of digested substrate utilisation. Most often, especially in the case of farm waste treatment, the digested substrate is used as a fertilizer. It can be used in liquid state or dewatered. Liquid substrate (total solids concentration 1-5 % by mass) is pumped from the post-treatment tank and spread on the fields. However it must be considered that fertilizing is possible only in certain periods of the year (once or twice). The post-treatment container must be designed accordingly. A possible solution is a lagoon, where digested substrate is stored and additionally stabilized and mineralized during the storing time. When using solid substrate (total solids concentration 20-30 % by mass), the digested substrate is mechanically dewatered first (by belt press or centrifuge) and then liquid and solid parts are used separately. Solid digestate after dewatering can be used fresh as a

Liquid part of the separated digestate can be used in the new substrate preparation as dilution water, however great caution must be given to nutrients or salts build-up and consequently possible inhibition in the anaerobic digestion. Usually only a portion of that liquid is used in the substrate preparation; the rest must be further treated as a wastewater.

When operating a biogas plant, biogas is the main product and considerable attention must be given to its production, storage, treatment and use. Biogas production completely depends on the efficiency of the anaerobic digestion and its microorganisms. Previous sections have shown what conditions must be met to successfully operate anaerobic

fertilizer, or it should be stabilized by composting (see further section).

**3.4 Biogas production, storage, treatment and use** 

Typical concentration of the liquid part of digestate is 200-1000 mgL-1 of COD.

digestion. There are two distinct parameters that describe the biogas production:

Fig. 9. The UASB process

**3.3 Post-treatment and substrate use** 


SBP values of an optimally operating digester reach 80-90 % of the biogas potential. Typical values of SBP for farm waste are shown in Table 1. Typical values of BPR for mesophilic digesters are from 0.9 to 1.3 m3m-3d-1. Lower values indicate the digester is oversized; higher values are rare or impossible, due to anaerobic process failure. For thermophilic or two stage digesters the typical BPR values are from 1.3 to 2.1 m3m-3d-1, respectively. UASB reactors are much less volume demanding and can achieve a BPR of up to 10 m3m-3d-1.

Biogas production is rarely constant; it is prone to fluctuations due to variation of loading rates, inner and outer operating conditions, possible inhibitions etc... Therefore, a buffer volume is required for the biogas storage. This enables the biogas user to get a constant biogas flow and composition. Most of the modern biogas plants are equipped with cogeneration units (named also combined heat and power units – CHP) which require constant gas flow for steady and efficient operation. There are several possibilities of biogas storage; roughly they can be divided into low pressure (10-50 mbar) and high pressure storage (over 5 bar). Low-pressure storage is used in on-site installations and for gas grid delivery; high pressure storage is used for long term storage, for transport in high pressure tanks and in installations with scarce space for volume extensive low pressure holders.

Low pressure biogas holders arise in many variations. It is possible to include biogas holder in the design of the digester. The most known is the digester with a movable cover. These digesters are less common, because a movable cover requires increased investment and operating expenditure. More common are external biogas holders that are widely commercially available. An example of a modern biogas holder is presented in Fig. 10.

Low pressure biogas holders require an extensive volume of 30 to 2000 m3 (Deublin and Steinhauser, 2008). Usually the pressure is kept constant and the volume of the bag is varied. High pressure biogas holders are of constant volume and made of steel, they are subject to special safety requirements. They do require more complex equipment for compression and expansion of the gas and are more cost-effective for operation and maintenance.

Biogas contains methane (40-70% by volume) and carbon dioxide. There are also components, present in low concentrations (below 1 %) such as water vapour, substrate micro particles and trace gases. Therefore biogas treatment is necessary to preserve equipment for its storage, transport and utilisation. Solid particles can be filtered out by candle filters, sludge and foam is separated in cyclones. For removal of trace gases, where hydrogen sulphide (H2S) is the most disturbing one due to its corrosion properties, processes like scrubbing, adsorption and absorption are used. In some cases also drying is required (usually to the relative humidity of less than 80 %).

Anaerobic Treatment and Biogas Production from Organic Waste 21

balances out, pre-treatment may have benefits such as more stable digested substrate, smaller digesters, pathogen removal etc. There are substrates that require extensive pretreatment; especially this is the case for ligno-cellulosic material (like spent brewery grains, Sežun et al. 2011) that require energy intensive pre-treatment to be successfully digested. In such cases the energy need for pre-treatment must be accounted in the energy production. In many cases it cannot outweigh the economy of the process; it may well happen that the

parasitic energy demand is too high.

Fig. 11. Schematic of a combined heat and power units (CHP) unit

In recent years great interest was taken to biogas injection into natural gas grid. Mainly due to the fact, that global energy efficiency in such cases is usually far greater that at CHP plants. Namely in warmer periods of the year, heat produced in the CHP is largely wasted and therefore unused. Injecting the biogas into natural gas grid assures more than 90% energy efficiency, due to the nature of the use (heat production), even in warmer periods. Consequently the whole biogas production process can be more economic, in some cases even without considerable subsidies as well as more renewable energy is put to the energy supply. Also in most cases, the investment costs of biogas plants may be less, since there is no CHP plant. In order to be able to inject the biogas into natural gas as biomethane (Ryckebosch et al., 2011) grid certain purity standards must be fulfilled, which in EU are determined by national ordinances (a good example is the German ordinance for Biogas injection to natural gas grids from 2008), where responsibilities of grid operators and biogas producers are determined (Fig. 13) as well as quality standards are prescribed (DVGW, 2010). When injecting biomethane into the natural gas grid some biogas must be used for the

After cleaning, biogas is used to produce energy. The most common way is to us all biogas in cogeneration plant in CHP unit to produce power and heat simultaneously (Fig. 11). In this case we can achieve maximum power production and enough excess heat to run the digesters. The energy required for heating the digester is also called parasitic energy. The anaerobic digesters require heat to bring the substrate to operating temperature and to compensate the digester heat losses. The digester also requires energy for mixing, substrate pumping and pre-treatment. The largest portion of heating demands in the digester operation is substrate heating. It requires over 90 % of all heating demands, and only up to 10 % is required for heat loss compensation (Zupancic and Ros 2003). In mesophilic digestion a CHP unit delivers enough heat for operation, while in thermophilic digestion additional heat is required. This additional heat demand can be covered by heat exchange between substrate outflow to substrate inflow -

Usually a conventional counter-currant heat exchanger is sufficient; however a heat pump can be applied as well.

Fig. 10. An example of commercially available biogas holder (Sattler 2011)

Electric energy is also required in digester operation for pumping, mixing and process control and regulation. In practice, no more than 10-15 % of electric energy produced should be used for internal demands.

The pre-treatment process may also require electric or thermal energy. Pre-treatment improves anaerobic digestion and its biogas production. However implications of pretreatment methods must be carefully considered. The golden rule is that pre-treatment should not spend more energy that it helps to produce. If the energy use and production

After cleaning, biogas is used to produce energy. The most common way is to us all biogas in cogeneration plant in CHP unit to produce power and heat simultaneously (Fig. 11). In this case we can achieve maximum power production and enough excess heat to run the digesters. The energy required for heating the digester is also called parasitic energy. The anaerobic digesters require heat to bring the substrate to operating temperature and to compensate the digester heat losses. The digester also requires energy for mixing, substrate pumping and pre-treatment. The largest portion of heating demands in the digester operation is substrate heating. It requires over 90 % of all heating demands, and only up to 10 % is required for heat loss compensation (Zupancic and Ros 2003). In mesophilic digestion a CHP unit delivers enough heat for operation, while in thermophilic digestion additional heat is required. This additional heat demand can be covered by heat exchange

Usually a conventional counter-currant heat exchanger is sufficient; however a heat pump

Fig. 10. An example of commercially available biogas holder (Sattler 2011)

Electric energy is also required in digester operation for pumping, mixing and process control and regulation. In practice, no more than 10-15 % of electric energy produced should

The pre-treatment process may also require electric or thermal energy. Pre-treatment improves anaerobic digestion and its biogas production. However implications of pretreatment methods must be carefully considered. The golden rule is that pre-treatment should not spend more energy that it helps to produce. If the energy use and production

between substrate outflow to substrate inflow -

can be applied as well.

be used for internal demands.

balances out, pre-treatment may have benefits such as more stable digested substrate, smaller digesters, pathogen removal etc. There are substrates that require extensive pretreatment; especially this is the case for ligno-cellulosic material (like spent brewery grains, Sežun et al. 2011) that require energy intensive pre-treatment to be successfully digested. In such cases the energy need for pre-treatment must be accounted in the energy production. In many cases it cannot outweigh the economy of the process; it may well happen that the parasitic energy demand is too high.

Fig. 11. Schematic of a combined heat and power units (CHP) unit

In recent years great interest was taken to biogas injection into natural gas grid. Mainly due to the fact, that global energy efficiency in such cases is usually far greater that at CHP plants. Namely in warmer periods of the year, heat produced in the CHP is largely wasted and therefore unused. Injecting the biogas into natural gas grid assures more than 90% energy efficiency, due to the nature of the use (heat production), even in warmer periods. Consequently the whole biogas production process can be more economic, in some cases even without considerable subsidies as well as more renewable energy is put to the energy supply. Also in most cases, the investment costs of biogas plants may be less, since there is no CHP plant. In order to be able to inject the biogas into natural gas as biomethane (Ryckebosch et al., 2011) grid certain purity standards must be fulfilled, which in EU are determined by national ordinances (a good example is the German ordinance for Biogas injection to natural gas grids from 2008), where responsibilities of grid operators and biogas producers are determined (Fig. 13) as well as quality standards are prescribed (DVGW, 2010). When injecting biomethane into the natural gas grid some biogas must be used for the

Anaerobic Treatment and Biogas Production from Organic Waste 23

(sulphide and ammonia removal) has been applied. Unpleasant odours mainly originate from storage, disintegration and internal transport of organic waste. These should be carried over in a closed system, equipped with an air collection system fitted with a biofilter or

A quality management system (QMS) specific to a defined digestion process and its resulting whole digestate or any separated liquor and separated fibre, should be established and maintained. Anaerobically digested slurry or sludge contains 2-12 % of solids; wet waste from solid state digestion contains 20-25 % solids. The digestate contains not degraded organic waste, microorganism cells and structures formed during digestion, as well as some inorganic matter. This is potentially an alternative source of humic material, nutrients and minerals to the agricultural soil (PAS, 2010). It may be used directly or separated into liquid and solid part. The liquid digestate is often recycled to the digestion process; some pretreatment may be required to reduce nitrogen or salt content.. Freshly digested organic waste is not stable under environmental conditions: it has an unpleasant odour, contains various noxious or corrosive gases such as NH3 and H2S, and still retains some biodegradability. In certain periods of a year it may be used in agriculture directly, in

Aerobic treatment (composting) is an obvious and straightforward solution to this problem. The composting procedure has several positive effects: stabilization of organic matter, elimination of unpleasant odours and reduction of pathogenic microorganisms to an acceptable level. Composting, applied prior to land application of the digested waste, contributes also to a beneficial effect of compost nitrogen availability in soil. (Zbytniewski

The simplest way is composting of the dehydrated fresh digestate in a static or temporarily turned-over pile. A structural material is necessary to provide sufficient porosity and adequate air permeability of the material in the pile. Various wood or plant processing residues may be used as a structural material like woodchips, sawdust, tree bark, straw and corn stalks provided that the sludge : bulk agent volume ratio is between 1:1 and 1:4 (Banegas et al., 2007). The majority of organic material is contributed by the bulking agent, but significant biodegradation of the digestate organic material also occurs, by means of

The final compost quality depends on the content of pollutants such as heavy metals, pathogenic bacteria, nutrients, inert matter, stability etc. in the mature compost. Typical quality parameters are presented in Table 6. The properties of the compost standard leachate may also be considered. Heavy metals and persistent organic pollutants accumulate in the compost and may cause problems during utilization. Compost quality depends on quality of the input material, which should be carefully controlled by input analysis. Pathogenic bacteria may originate from the mesophilic digestates or from infected cocomposting materials, if applied (e.g. food waste). If thermophilic phase period of the composting process has lasted at least few days, the compost produced may be considered

sanitized and free of pathogens such as *Salmonella, Streptococci* and *coliforms.*

most cases however it must be stabilized before being applied to the fields.

connected with the gas motor air supply.

and Buszewski, 2005; Tarrasón et al., 2008)

natural aerobic microorganisms.

**3.5.1 The anaerobic digestion residue management** 

reactors self-heating. It is advisable to use regeneration (Fig. 12), even in mesophilic temperature ranges, to minimize this expenditure, which on annual basis can contribute to 10-20 % of all biogas production (Pöschl et al., 2010).

Fig. 12. Scheme of the heat regeneration from output to input flows.

Fig. 13. Injection of biogas into natural gas grid (Behrendt and Sieverding 2010)
