Secondary Wastewater Treatment

Secondary wastewater treatment processes use microorganisms to biologically remove contaminants from wastewater. Secondary biological processes can be aerobic or anaerobic, each process utilizing a different type of bacterial community. Coupled anaerobic–aerobic processes may also be employed under certain circumstances.

Aerobic – Aerobic biological processes are common in municipal wastewater treatment. In an aerobic system, the organic contaminants are converted to carbon dioxide, water, additional microorganisms, and other end products.

Aerobic lagoons – Lagoons are typically large, shallow earthen basins that provide adequate residence time for the wastewater to be treated naturally by both bacteria and algae.

Activated sludge – A suspended-growth aerobic biological process in which microorganisms remove carbonaceous matter from wastewater in an aerobic environment.

Rotating biological contactor – An attached-growth process consisting of a series of closely spaced, parallel discs mounted on a rotating shaft which is partially submerged in the wastewater being treated. Microorganisms grow on the surface of the discs where aerobic biological degradation of the wastewater pollutants takes place.

Trickling filter – An attached-growth process where wastewater is distributed over a fixed bed of media such as rocks, gravel, plastic substrate, etc. The wastewater flows downward over the media surface where microorganisms form a layer of biomass and consume contaminants in the water.

Anaerobic – Anaerobic biological treatment processes employ organisms that function in the absence of molecular oxygen. Anaerobic processes convert organic contaminants to a biofuel gas comprising carbon dioxide, methane, and other end products. Anaerobic processes are generally used to treat high-strength wastewaters where it is impractical to utilize aerobic processes or where producing a biofuel gas is desired. Anaerobic processes also use considerably less energy than mechanical aeration processes. Anaerobic processes are loosely organized as either suspended or attached-growth systems.