The combined effect of benzalkonium chloride (BAC) and prolonged exposure to low temperature on nitrification was investigated. Ammonia oxidation at 22-24 degrees C by an enriched nitrifying culture was inhibited at increasing BAC concentrations and ceased at 15 mg BAC/L. The non-competitive inhibition coefficient was 1.5 +/- 0.9 mg BAC/L. Nitrification tests were conducted without and with BAC at 5 mg/L using an aerobic, mixed heterotrophic/nitrifying culture maintained at a temperature range of 24-10 degrees C. Maintaining this culture at 10 degrees C for over one month in the absence of BAC, resulted in slower nitrification kinetics compared to those measured when the culture was first exposed to 10 degrees C. BAC was degraded by the heterotrophic population, but its degradation rate decreased significantly as the culture temperature decreased to 10 degrees C. These results confirm the negative impact of quaternary ammonium compounds on the nitrification process, which is further exacerbated by prolonged, low temperature conditions. (C) 2014 Elsevier Ltd. All rights reserved.