Anaerobic fermentation is an oxidation process, in which organic compounds, such as e.g. livestock manure and toilet waste, are converted by microbiological processes in absence of oxygen (O₂) to methane (CH₄) and carbon dioxide (CO₂). The produced bio-gas (~ 55 - 75% CH₄) can be used for cooking, heating and light. The methane production contributes to the BOD reduction in digested sludge. It is possible to apply high loading rates to the digester and even recalcitrant materials (e.g. lignin) can be degraded.

Anaerobic digestion produces lower amounts of sludge (3 - 20 times less than aerobic processes), since the energy yields of anaerobic bacteria are relatively low. Most of the energy derived from substrate breakdown is found in the final product, CH₄. As regards to cell yields, 50% of organic carbon is converted to biomass under aerobic conditions. The net amount of cells produced per metric ton of COD destroyed is 20 - 150 kg [44.1-330.75lbs], as compared to 400 - 600 kg [882-1323lbs] for aerobic digestion.

(http://www.united-tech.com/wd-anaerobicdigestion.html). But the process is slower than the aerobic digestion, it is much more sensitive to upsets by toxicants, and the start-up of the process requires a long period of time.

Anaerobic digestion is affected by temperature, retention time, pH, chemical composition of wastewater, competition of methanogens with sulfate-reducing bacteria, and the presence of toxicants. The complete anaerobic digestion is considered to take place in three phases (figure 11):

Hydrolysis: particulate material is converted to soluble compounds that can be hydrolyzed further to simple monomer that are used by fermentation performing bacteria.

Fermentation (also Acidogenesis): amino acids, sugars, and some fatty acids are degraded further to acetate, hydrogen, CO₂, and propionate and butyrate.

Methanogenesis (carried out by methanogens): acetate is split into methane and CO₂; CO₂ reacts further with hydrogen to methane; and acetic acids that are produced in this phase are also transformed to methane.

Figure 11: Steps of anaerobic digestion (Bilitewski et al., 1994)

Process temperature affects the rate of digestion and should be maintained in the mesophillic range 35 - 40 °C (95 - 105 °F) with an optimum of 100 °F. It is possible to operate in the thermophillic range 55 - 65 °C (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored.

Thermophilic digestion operates at temperature ranges of 50-65°C [122°F-149°F]. It allows higher loading rates and is also conductive to greater destruction of pathogens. One drawback is its higher sensitivity to toxicants. Because of their slower growth as compared with acidogenic bacteria, methanogenic bacteria are very sensitive to small changes in temperature.

The hydraulic retention time (HRT), which depends on wastewater characteristics and environmental conditions, must be long enough to allow metabolism by anaerobic bacteria in digesters. The retention times of mesophilic and thermophilic digesters range between 25 and 35 days but can be lower.

Most methanogenic bacteria function in a pH range between 6.7 and 7.4. Acidogenic bacteria produce organic acids, which tend to lower the pH of the bioreactor. Under normal conditions, this pH reduction is buffered by the bicarbonate that is produced by methanogens. Under adverse environmental conditions, the buffering capacity of the system can be upset, eventually stopping the production of methane. Acidity is more inhibitory to methanogens than of acidogenic bacteria. An increase in volatile acid levels thus serves as an early indicator of system upset. Monitoring the ratio of total volatile acids (as acetic acid) to total alkalinity (as calcium carbonate) has been suggested to ensure that it remains below 0.1.

(http://www.united-tech.com/wd-anaerobicdigestion.html)

A wide range of toxicants is responsible for the occasional failure of anaerobic digesters. Inhibition of Methanogenesis is generally indicated by reduced methane production and increased concentration of volatile acids.