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 dispo sal. Changing legislation, rising costs and increasing sludge producti on have caused a shift in emphasis from volume of wastewater turnover and maximal BOD removal to minimisation of the biomass produced. Low t o zero yields are possible by extended aeration type processes and rai sing process operating temperature but these involve large energy inpu ts. Manipulation of the activated sludge process to promote the growth of protozoa, nematodes, rotifers or oligochaeta reduces biomass produ ction by predation of the microorganisms. Promotion of microorganism s pecies that have a high maintenance energy by the provision of support matrices or increasing sludge retention time has the potential to pre vent biomass accumulation. Uncoupling of microbial metabolic pathways by both transition from anaerobic to aerobic conditions and by chemica l addition prevents biomass production successfully. The alterations r equired for increasing retention times or introducing a second bacteri al predation chamber may not be feasible at established plants. The in creased operation costs and capital works may not be economically viab le. The most efficient and cost effective solution has yet to be optim ised.