Effect of copper on nitrifying and heterotrophic populations in activated sludge

Autotrophic microbial oxidation of ammonia to nitrate is the key process in the removal of ammonia from wastewater. Because of the low growth rate of nitrifying bacteria and their sensitivity toward a wide variety of inorganic and organic compounds, the nitrification process is often most susceptible to toxic compounds in wastewater. Copper is reported under certain conditions to inhibit the activity of nitrifiers. However, the toxicological response of nitrifying populations to copper has not been well understood and the information in the literature is contradictory.{09}In addition to nitrifying organisms, nitrifying activated sludge also contain aerobic heterotrophic organisms that are responsible for BOD removal. There has been widespread belief that the nitrifying bacteria are more susceptible to metal toxicity than the heterotrophic organisms in activated sludge. However, recent studies have contradicted that belief and suggest that the toxic effect levels are about the same for both nitrifying and heterotrophic organisms.; This study was designed to investigate and simulate the steady-state effect of copper discharges on the performance of bench-scale nitrifying activated sludge treatment plants.{09}This effect was studied by determination of the changes in the performance of nitrifying and heterotrophic populations following addition of copper to the influent wastewaters and by determining the copper inhibition of nitrification rates and respiration rates as a function of mean cell residence time (MCRT) of the bench-scale systems.; For five MCRTs studied, IC50 values for nitrifiers ranged from 9.9 mg/L to 13.3 mg/L of total copper in activated sludge while IC 50 values for heterotrophs were between 5.5 and 14.6 mg/L for the same operating MCRRs.{09}Results suggest that nitrifiers and heterotrophs showed different toxic responses to Cu under different MCRRs. For example, under a 15-day MCRT, nitrifiers seemed to be no more sensitive to Cu than heterotrophs. On the other hand, under 30-day MCRT heterotrophs seemed to be more susceptible to Cu than nitrifiers. Over the range of reactor MCRTs studied, IC50 values tended to decrease as MCRT of the system increased, indicating that both nitrifiers and heterotrophs are more susceptible to Cu at higher MCRT. The results also showed that nitrifers had higher IC50 values compare to heterotrophs throughout the range of MCRT tested. This provides further evidence that nitrifiers are not necessarily more susceptible to Cu than heterotrophs. (Abstract shortened by UMI.)...