Low biomass yield activated sludge: A review

The activated sludge process is common for the treatment of wastewater. The process uses a mixed culture of microorganisms that degrade the wastewater aerobically producing carbon dioxide, water and new biomass. Some of the biomass produced is recycled and the rest requires disposal. Changing legislation, rising costs and increasing sludge production have caused a shift in emphasis from volume of wastewater turnover and maximal BOD removal to minimisation of the biomass produced. Low to zero yields are possible by extended aeration type processes and raising process operating temperature but these involve large energy inputs. Manipulation of the activated sludge process to promote the growth of protozoa, nematodes, rotifers or oligochaeta reduces biomass production by predation of the microorganisms. Promotion of microorganism species that have a high maintenance energy by the provision of support matrices or increasing sludge retention time has the potential to prevent biomass accumulation. Uncoupling of microbial metabolic pathways by both transition from anaerobic to aerobic conditions and by chemical addition prevents biomass production successfully. The alterations required for increasing retention times or introducing a second bacterial predation chamber may not be feasible at established plants. The increased operation costs and capital works may not be economically viable. The most efficient and cost effective solution has yet to be optimised.