Prolonged exposure of mixed aerobic cultures to low temperature and benzalkonium chloride affect the rate and extent of nitrification

• Inhibition of nitrification in enriched nitrifying culture occurred at >5 mg BAC/L.
• The non-competitive inhibition coefficient at 22–24 °C was 1.5 ± 0.9 mg BAC/L.
• Maintenance of a heterotrophic/nitrifying culture at 10 °C led to slow nitrification.
• BAC inhibition of nitrification exacerbated by prolonged low temperature conditions.
• BAC degradation rate by heterotrophs decreased with a temperature decrease to 10 °C.

The combined effect of benzalkonium chloride (BAC) and prolonged exposure to low temperature on nitrification was investigated. Ammonia oxidation at 22–24 °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 °C. Maintaining this culture at 10 °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 °C. BAC was degraded by the heterotrophic population, but its degradation rate decreased significantly as the culture temperature decreased to 10 °C. These results confirm the negative impact of quaternary ammonium compounds on the nitrification process, which is further exacerbated by prolonged, low temperature conditions.