EFFECTS OF TEMPERATURE AND MEAN CELL RESIDENCE TIME ON BIOLOGICAL NUTRIENT REMOVAL PROCESSES

The effects of temperature and mean cell residence time (MCRT) on biological nitrogen and phosphorus removal were investigated by operating two pilot-scale continuous-flow reactors in parallel over a range of temperatures and MCRTs. One system was operated as a high-rate Virginia Initiative Plant (VIP) biological nutrient removal (BNR) process, and the other was operated as a conventional, fully aerobic activated sludge process for comparison. Results showed that less aerobic volume was needed for complete nitrification in the BNR process than in the conventional process when conditions of temperature and MCRT were suitable for complete nitrification. However, the BNR system was more prone to nitrifier washout than the conventional system. Nitrification rates and the degree of nitrification achieved by the BNR system and the conventional system were equal when compared on the basis of aerobic MCRT. Enhanced biological phosphorus removal (EBPR) was adversely affected by colder temperatures, with lower MCRTs being most affected. EBPR was not possible at a 5 day system MCRT and 10-degrees-C. Nitrification, however, was more sensitive to MCRT and temperature effects than EBPR under all conditions studied. Operation of the BNR process at the lowest MCRT that provided complete nitrification provided the best combined nitrogen and phosphorus removal when EBPR was chemical oxygen demand (COD)-limited. Higher MCRTs were considered optimal when EBPR was limited by phosphorus because of lower sludge productions.