The digestion of organic material or its degradation where there is no oxygen is an anaerobic process. This process happens in nature in almost all water bodies that have deposits of organic matter. This happens in marshes, ponds, lakes, and even in sewers and sewage treatment systems.
Anaerobic digestion happens as a complex process involving different microbial groups such as
· fermentative bacteria
· hydrogen-producing acetogenic bacteria
· hydrogen-consuming acetogenic bacteria
· carbon dioxide-reducing methanogens
· acetoclastic methanogens
These bacteria consume the waste in water and produce methane and carbon dioxide by converting the biodegradable polymers such as proteins, lipids, and carbohydrates that are represented as suspended particles in the wastewater.
The digestion of these substances happens in multiple steps involving processes such as
· Hydrolysis
· Acidogenesis
· Acetogenisis
· Methanogenesis
The compounds formed from the different stages are described below
In each of the different stages of digestion, the organic substrate is converted to intermediatory compounds till all carbon material is converted to carbon dioxide and methane (CO2 and CH4). Temperature is one of the key factors that influence the efficiency of anaerobic digestion with higher temperature favoring faster digestion. The substrate concentration also has an effect on treatment efficiency in an anaerobic process. The common anaerobic processes are septic tanks, UASB, Imhoff tanks, DEWATS, ecostp, bio-septic tanks, and biogass plants.
Anaerobic processes can only eliminate COD to an extent of 60-75% in the most efficient systems such as UASB and with other types of systems the removal is about 35-50%. There would be a need for additional treatment for higher COD removal and nutrient removal.
There are also processes such as aerobic granular sludge that take advantage of anaerobic digestion in combination with aerobic processes.