**1. Introduction**

Organic wastes under consideration are of natural origin that possess biochemical characteristics ensuring rapid microbial decomposition at relatively normal operating conditions. When considering the organic waste treatment we have generally in mind organic mineralization, biological stabilisation and detoxification of pollutants. Most common organic wastes contain compounds that are mainly well biodegradable. They can be readily mineralized either through biological treatment (aerobic or anaerobic), or thermo chemical treatment such as incineration, pyrolysis and gasification. The latter will not be treated in this work. Most organic wastes produced today originate in municipal, industrial and agricultural sector. Municipal waste (as well as municipal wastewater sludge) is generated in human biological and social activities and contains a large portion of organic waste readily available for treatment. Agricultural waste is common in livestock and food production and can be utilised for biogas production and therefore contribute to more sustainable practice in agriculture. Industrial wastes arise in many varieties and are the most difficult for biological treatment, depending of its origin. Namely, many industries use chemicals in their production in order to achieve their product quality and some of these chemicals are present in the waste stream, which is consequently difficult to treat. Recently, organic waste treatment has had a lot of attention, due to possibilities of energy recovery from these wastes as well as to prevent their adverse environmental effects. Energy recovery is possible through controlled release of chemically bound energy of organic compounds in waste and can be retrieved through chemical and biochemical processes. Most of the organic wastes appear in solid form; however contain up to 90% of moisture, therefore thermochemical treatment such as incineration cannot be applied. To address sustainability in the treatment of organic wastes, environmental aspect, energy aspect and economical aspect of the treatment processes should be considered.

Biodegradable organic waste can be treated with or without air access. Aerobic process is composting and anaerobic process is called digestion. Composting is a simple, fast, robust and relatively cheap process producing compost and CO2 (Chiumenti et al. 2005, Diaz et al. 2007). Digestion is more sophisticated, slow and relatively sensitive process, applicable for selected input materials (Polprasert, 2007). In recent years anaerobic digestion has become a prevailing choice for sustainable organic waste treatment all over the world. It is well suited for various wet biodegradable organic wastes of high water content (over 80%), yielding methane rich biogas for renewable energy production and use.

Anaerobic Treatment and Biogas Production from Organic Waste 5

Thus, anaerobic digestion is a renewable energy source in an integrated waste management

system. Also, the nutrient-rich solids left after digestion can be used as a fertilizer.

There are four key biological and chemical stages of anaerobic digestion:

Fig. 1. Anaerobic pathway of complex organic matter degradation

thermal energy and pressure) in nature.

In most cases biomass is made up of large organic compounds. In order for the microorganisms in anaerobic digesters to access the chemical energy potential of the organic material, the organic matter macromolecular chains must first be broken down into their smaller constituent parts. These constituent parts or monomers such as sugars are readily available to microorganisms for further processing. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore hydrolysis of high molecular weight molecules is the necessary first step in anaerobic digestion. It may be enhanced by mechanical, thermal or chemical pretreatment of the waste. Hydrolysis step can be merely biological (using hydrolytic microorganisms) or combined: bio-chemical (using extracellular enzymes), chemical (using catalytic reactions) as well as physical (using

Acetates and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such as volatile fatty acids (VFA's) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilised by

**2.1 Biochemical reactions in anaerobic digestion** 

1. Hydrolysis 2. Acidogenesis 3. Acetogenesis 4. Methanogenesis.


Table 1 shows typical solid and organic substance contents and biogas yields for most frequent organic wastes, treated with anaerobic digestion.

1TS – total solids

2VS – volatile (organic) solids

Table 1. Types of organic wastes and their biogas yield

## **2. Basics of anaerobic digestion**

This section deals with anaerobic waste treatment methods only, as the most advanced and sustainable organic waste treatment method. Anaerobic digestion (WRAP 2010) is *"a process of controlled decomposition of biodegradable materials under managed conditions where free oxygen is absent, at temperatures suitable for naturally occurring mesophilic or thermophilic anaerobic and facultative bacteria and archaea species, that convert the inputs to biogas and whole digestate"*. It is widely used to treat separately collected biodegradable organic wastes and wastewater sludge, because it reduces volume and mass of the input material with biogas (mostly a mixture of methane and CO2 with trace gases such as H2S, NH3 and H2) as by-product. Thus, anaerobic digestion is a renewable energy source in an integrated waste management system. Also, the nutrient-rich solids left after digestion can be used as a fertilizer.

#### **2.1 Biochemical reactions in anaerobic digestion**

There are four key biological and chemical stages of anaerobic digestion:

1. Hydrolysis

4 Management of Organic Waste

Table 1 shows typical solid and organic substance contents and biogas yields for most

**VS2 in TS [%]** 

**Biogas yield (SPB) [m3kg-1 of VS]** 

**[%]** 

Municipal organic waste 15-30 80-95 0.5-0.8 Municipal wastewater sludge 3-5 75-85 0,3-0,5 Brewery spent grain 20-26 80-95 0.5-1.1

Fermentation residues 4-8 90-98 0.4-0.7 Fruit slurry (juice production) 4-10 92-98 0.5-0.8 Pig stomach content 12-15 80-84 0.3-0.4 Rumen content (untreated) 12-16 85-88 0.3-0.6 Vegetable wastes 5-20 76-90 0.3-0.4 Fresh greens 12-42 90-97 0.4-0.8

Grass cuttings (from lawns) 20-37 86-93 0.7-0.8

Grass silage 21-40 87-93 0.6-0.8 Corn silage 20-40 94-97 0.6-0.7 Straw from cereals ~86 89-94 0.2-0.5 Cattle manure (liquid) 6-11 68-85 0.1-0.8 Cattle excreta 25-30 75-85 0.6-0.8 Pig manure (liquid) 2-13 77-85 0.3-0.8 Pig excreta 20-25 75-80 0.2-0.5 Chicken excreta 10-29 67-77 0.3-0.8 Sheep excreta 18-25 80-85 0.3-0.4 Horse excreta 25-30 70-80 0.4-0.6 Waste milk ~8 90-92 0.6-0.7 Whey 4-6 80-92 0.5-0.9

This section deals with anaerobic waste treatment methods only, as the most advanced and sustainable organic waste treatment method. Anaerobic digestion (WRAP 2010) is *"a process of controlled decomposition of biodegradable materials under managed conditions where free oxygen is absent, at temperatures suitable for naturally occurring mesophilic or thermophilic anaerobic and facultative bacteria and archaea species, that convert the inputs to biogas and whole digestate"*. It is widely used to treat separately collected biodegradable organic wastes and wastewater sludge, because it reduces volume and mass of the input material with biogas (mostly a mixture of methane and CO2 with trace gases such as H2S, NH3 and H2) as by-product.

Yeast 10-18 90-95 0.5-0.7

frequent organic wastes, treated with anaerobic digestion.

**Organic waste TS1**

1TS – total solids

2VS – volatile (organic) solids

**2. Basics of anaerobic digestion** 

Table 1. Types of organic wastes and their biogas yield


Fig. 1. Anaerobic pathway of complex organic matter degradation

In most cases biomass is made up of large organic compounds. In order for the microorganisms in anaerobic digesters to access the chemical energy potential of the organic material, the organic matter macromolecular chains must first be broken down into their smaller constituent parts. These constituent parts or monomers such as sugars are readily available to microorganisms for further processing. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore hydrolysis of high molecular weight molecules is the necessary first step in anaerobic digestion. It may be enhanced by mechanical, thermal or chemical pretreatment of the waste. Hydrolysis step can be merely biological (using hydrolytic microorganisms) or combined: bio-chemical (using extracellular enzymes), chemical (using catalytic reactions) as well as physical (using thermal energy and pressure) in nature.

Acetates and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such as volatile fatty acids (VFA's) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilised by

Anaerobic Treatment and Biogas Production from Organic Waste 7

Anaerobic digestion can operate in a wide range of temperature, between 5°C and 65°C. Generally there are three widely known and established temperature ranges of operation: psychrophilic (15-20°C), mesophilic (30-40°C) and thermophilic (50-60°C). With increasing temperature the reaction rate of anaerobic digestion strongly increases. For instance, with ideal substrate thermophilic digestion can be approx. 4 times faster than mesophilic. However using real waste substrates, there are other inhibitory factors that influence digestion, that make thermophilic digestion only approx. 2 times faster than mesophilic.

The important thing is, when selecting the temperature range, it should be kept constant as much as possible. In thermophilic range (50-60°C) fluctuations as low as ±2°C can result in 30% less biogas production (Zupančič and Jemec 2010). Therefore it is advised that temperature fluctuations in thermophilic range should be no more than ±1°C. In mesophilic range the microorganisms are less sensitive; therefore fluctuations of ±3°C can be tolerated. For each range of digestion temperature there are certain groups of microorganisms present that can flourish in these temperature ranges. In the temperature ranges between the three established temperature ranges the conditions for each of the microorganisms group are less favourable. In these ranges anaerobic digestion can operate, however much less efficient. For example, mesophilic microorganisms can operate up to 47°C, thermophilic microorganisms can already operate as low as 45°C. However the rate of reaction is low and it may happen that the two groups of microorganisms may exclude each other and compete in the overlapping range. This results in poor efficiency of the process, therefore these

In the anaerobic digester, low redox potential is necessary. Methanogenic archaea need redox potential between -300 and -330 mV for the optimum performance. Redox potential can increase up to 0 mV in the digester; however it should be kept in the optimum range. To achieve that, no oxidizing agents should be added to the digester, such as oxygen, nitrate,

In microorganism biomass the mass ratio of C:N:P:S is approx. 100:10:1:1. The ideal substrate C:N ratio is then 20-30:1 and C:P ratio 150-200:1. The C:N ratio higher than 30 causes slower microorganisms multiplication due to low protein formation and thus low energy and structural material metabolism of microorganisms. Consequently lower substrate degradation efficiency is observed. On the other hand, the C:N ratio as low as 3:1 can result in successful digestion. However, when such low C:N ratios and nitrogen rich

Table 3. Regeneration time of microorganisms

**2.2.1 Temperature** 

temperatures are rarely applied.

**2.2.3 C:N ratio and ammonium inhibition** 

**2.2.2 Redox potential** 

nitrite or sulphate.

**Microorganisms Time of regeneration**  Acidogenic bacteria Less than 36 hours Acetogenic bacteria 80-90 hours Methanogenic archaea 5-16 days Aerobic microorganisms 1-5 hours

methanogens. The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFA's are generated along with ammonia, carbon dioxide and hydrogen sulphide as well as other by-products.

The third stage anaerobic digestion is acetogenesis. Here simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid (or its salts) as well as carbon dioxide and hydrogen.

The final stage of anaerobic digestion is the biological process of methanogenesis. Here methanogenic archaea utilise the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water. It is these components that makes up the majority of the biogas released from the system. Methanogenesis is – beside other factors sensitive to both high and low pH values and performs well between pH 6.5 and pH 8. The remaining, non-digestible organic and mineral material, which the microbes cannot feed upon, along with any dead bacterial residues constitutes the solid digestate.
