Effects of temperature and mean cell residence time on the performance of high-rate biological nutrient removal processes
The effects of temperature and mean cell residence time (MCRT) on processes involved in biological nitrogen and phosphorus removal were investigated by operating pilot-scale continuous-flow reactors over a range of temperatures and MCRITs. Two systems were operated as high-rate University of Cape Town (UCT) biological nutrient removal (BNR) processes. A third system was operated as a conventional, fully aerobic activated sludge system for comparison.
Less aerobic volume was needed to achieve complete nitrification in the BNR system than in the conventional system when temperature and MCRT conditions were suitable for complete nitrification. This occurred at 15 d MCRT and temperatures from 10 to 20 °C., and at 5 d MCRT and 20 °C. However, the BNR system was more susceptible to nitrifier washout at 5 d MCRT and temperatures of 10 and 15 °C. Although less volume was needed for complete nitrification in the BNR system, specific nitrification rates and the degree of nitrification were equal in the two systems when compared on the basis of aerobic MCRT. This phenomenon occurred because the MLVSS concentrations were higher in the aerobic zone of the BNR system than in the conventional system for the same organic loading and total MCRT.
Nitrification and denitrification rates were a function of MCRT and temperature, with temperature having a greater effect at lower MCRTs. Batch experiments showed that anoxic uptake of phosphorus occurred, although at a much lower rate than aerobic uptake.
Biological phosphorus removal was adversely affected by colder temperatures. Operation of the BNR process at the lowest MCRT which provided complete nitrification prevented washout of phosphorus removal organisms, and provided the best combined nitrogen and phosphorus removal when phosphorus removal was COD-limited. Higher MCRIs were optimal under P-limiting conditions.
Anaerobic stabilization ranging from 8% to 27% was measured in the BNR system, and was a function of temperature at a 15 d MCRI. A mechanism for anaerobic stabilization was proposed.
Yield coefficients for the BNR and the conventional system were equal and were 0.41 mgVSS/mgCOD. The decay rate in the BNR system, 0.063 d⁻¹, was lower than the decay rate in the conventional system, 0.110 d⁻¹. This resulted in higher MLVSS concentrations in the BNR system.