**2.2.1 Temperature**

6 Management of Organic Waste

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

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

As with all biological processes the optimum environmental conditions are essential for successful operation of anaerobic digestion (Table 2). The microbial metabolism processes depend on many parameters; therefore these parameters must be considered and carefully controlled in practice. Furthermore, the environmental requirements of acidogenic bacteria differ from requirements of methanogenic archaea. Provided that all steps of the degradation process have to take place in one single reactor (one-stage process) usually methanogenic archaea requirements must be considered with priority. Namely, these organisms have much longer regeneration time, much slower growth and are more sensitive to environmental conditions then other bacteria present in the mixed culture (Table 3).

**Parameter Hydrolysis/ Acidogenesis Methanogenesis**  Temperature 25-35°C Mesophilic: 30-40°C

With cellulose containing substrates (which are slowly degradable) the hydrolysis stage

With protein rich substrates the pH optimum is equal in all anaerobic process stages

With fat rich substrates, the hydrolysis rate is increasing with better emulsification, so

In aspiration to provide optimum conditions for each group of microorganisms, a two-stage process of waste degradation has been developed, containing a separate reactor for each stage. The first stage is for hydrolysis/acidification and the second for acetogenesis/methanogenesis.

that acetogenesis is limiting. Therefore a thermophilic process is advised.

pH Value 5.2-6.3 6.7-7.5 C:N ratio 10-45 20-30 Redox potential +400 to -300 mV Less than -250 mV C:N:P:S ratio 500:15:5:3 600:15:5:3 Trace elements No special requirements Essential: Ni, Co, Mo, Se

Table 2. Environmental requirements (Deublein and Steinhauser 2008)

therefore a single digester is sufficient for good performance.

Thermophilic: 50-60°C

upon, along with any dead bacterial residues constitutes the solid digestate.

its salts) as well as carbon dioxide and hydrogen.

**2.2 Factors that affect anaerobic digestion** 

However, there are some exceptions to the case:

is the limiting one and needs prior attention.

The process will be discussed in detail in section 3.

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 temperatures are rarely applied.

## **2.2.2 Redox potential**

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, nitrite or sulphate.
