Reducing production of excess biomass during wastewater treatment

Excess biomass produced during the biological treatment of wastewaters requires costly disposal. As environmental and legislative constraints increase, thus limiting disposal options, there is considerable impetus for reducing the amount of biomass produced. This paper reviews biomass production during wastewater treatment and identifies methods for reducing the quantity of biomass produced. Efforts to reduce biomass production during aerobic metabolism must promote the conversion of organic pollutants to respiration products with a concomitant increase in the aeration requirements. Promoting further metabolism of assimilated organic carbon will release additional respiration products and reduce the overall biomass production e.g. by inducing cell lysis to form autochtonous substrate on which cryptic growth occurs or by encouraging microbial predation by bacteriovores. Uncoupling metabolism such that catabolism of substrate can continue unhindered while anabolism of biomass is restricted would achieve a reduction in the biomass yield. Metabolite overproduction in substrate excess conditions has been demonstrated in several bacterial species and can result in an increase in the substrate uptake while resulting in a decreased yield and increased carbon dioxide evolution rate. Addition of protonphores to uncouple the energy generating mechanisms of oxidative phosphorylation will stimulate the specific substrate uptake rate while reducing the rate of biomass production. Increasing the biomass concentration such that the overall maintenance energy requirements of the biomass within the process are increased can significantly reduce the production of biomass. Suitable engineering of the physical conditions and strategic process operation may result in environments in which biomass production may be reduced. It is noted that as biomass settling characteristics are a composite product of the microbial population, any changes which result in a shift in the microbial population may affect the settling properties. Reduced biomass production may result in an increased nitrogen concentration in the effluent. Anaerobic operation alleviates the need for costly aeration and, in addition, the low efficiency of anaerobic metabolism results in a low yield of biomass, its suitability for wastewater treatment is discussed. A quantitative comparison of these strategies is presented.