The biomass fractions of heterotrophs and phosphate-accumulating organisms in a nitrogen and phosphorus removal system

In this study, the biomass fractions of heterotrophs (XH) and phosphate-accumulating organisms (XPAO) were estimated through experimental results and theoretical calculation on the mass balance of organic matter, nitrogen and phosphorus in a pilot scale biological nutrient removal process. The process, comprised of conventional anaerobic–anoxic–aerobic activated sludge and a rotating biological contactor in the aerobic stage, was controlled under steady state under conditions of a solid retention time of 10 days and a dissolved oxygen concentration of 2.0 mg/l in the aerobic stage. For long term operation, the process exhibited high performance in organic matter, nitrogen and phosphorus removal. The removal efficiency was 96.5, 70 and 100% of organic matter, total nitrogen and total phosphorus, respectively. According to the mass balance calculation, approximately 62.0% of organic matter, 48.0% of nitrogen and 47.2% of phosphorus of influent were removed in the form of carbon dioxide, nitrogen gas and polyphosphate, respectively. Regarding XH and XPAO as the major organisms responsible for biomass production, the yield fractions of XH growth in anoxic and aerobic conditions were 48.4 and 39.4%, respectively. Moreover, the yield fraction of XPAO growth in aerobic conditions was 12.2%. These results showed that XH was the major community; moreover, only a minute amount of XPAO was present in this nitrogen and phosphorus removal system for municipal wastewater treatment.