Anaerobic Waste – How Anaerobic Digestion Works in Wastewater Treatment

Anaerobic Digestion

Anaerobic waste digestion happens in a closed reactor. Bacteria act upon the organic waste and release plenty of carbon dioxide and methane. The microbial community has only obligate anaerobic and facultative bacteria. As in aerobic chemohetrotrophic metabolism, initially the macromolecules are hydrolyzed. These products are then converted to volatile fatty acids (mainly acetic acid), and alcohols. The organisms responsible for these anaerobic waste reactions are popularly called acid formers.

They obtain energy through oxidation of organic compounds, but do not use oxygen as electron acceptor. Instead, another fragment of the substrate is reduced to anaerobic acids and alcohols through anaerobic processes. These are then metabolized by a second group of obligate anaerobic biomass (the methane formers), and converted to methane gas.

It is estimated that 60 to 70% of methane production in an anaerobic waste reaction is through conversion of acetic acid and the rest through carbon dioxide reduction by hydrogen.

The activities of the methane and acid producing groups of bacteria must be balanced as the former is sensitive to pH changes and works best in pH range 6.8 to 7.5.

Stabilization Ponds

Anaerobic waste stabilization ponds are large and shallow basins with residence times of 12 to 25 days. A variety of microorganisms inhabit such ponds participating in the anaerobic digestion of waste. In addition to the aerobic and anaerobic chemoheterotrophic organisms, a pond has a large variety of photoautotrophic life forms also. Green and blue-green algae are found in abundance in the top layers, maintaining a symbiotic relationship (I am ok, you are ok!) with the bacteria.

At times the pond may also have a significant population of sulfur photosynthetic anaerobic waste water organisms.