Experimental determination of the heterotroph anoxic yield in anoxic-aerobic activated sludge systems treating municipal wastewater

This paper describes experimental research to directly quantify the ordinary heterotrophic organism (OHO) cell yield coefficient under anoxic and aerobic conditions with real wastewater as substrate. Until recently these two values were assumed equal in activated sludge models, despite theoretical predictions that the anoxic yield should be reduced relative to its aerobic value. In this study, parallel aerobic and anoxic batch tests were conducted on the same mixtures of wastewater and mixed liquor drawn from anoxic-aerobic activated sludge systems. By equating the readily biodegradable (RB)COD concentrations in the aerobic and anoxic batch tests, the anoxic yield was determined in terms of the respective measured oxygen and nitrate utilisations and the aerobic yield. Additionally, the aerobic and anoxic yield coefficients were directly determined by using defined quantities of the artificial RBCOD acetate. The wastewater batch tests demonstrated that the OHO anoxic yield is reduced to approximately 81% (0.54) of the aerobic value (0.67 mgCOD/mgCOD). The anoxic:aerobic yield ratio (0.81) was confirmed independently in the acetate batch tests at 0.84, and closely equals that predicted theoretically from thermodynamic and bioenergetic principles (0.83). From these investigations, it is recommended that the OHO anoxic yield be decreased to 0.54 for an aerobic yield value of 0.67 mgCOD/mgCOD.