Nitrification Inhibition by Heavy Metals in Activated Sludge Process

Nitrification is the most sensitive part in the biologic nutrient removal of wastewaters, since the growth rate of nitrifying microorganisms is very lower and strongly dependent on temperature, substrate concentration, oxygen content, pH, and the presence of inhibiting components. This review addresses the nitrification inhibition by heavy metals. This inhibition can be quantified by different techniques, including screening method, specific oxygen uptake rate (SOUR) assay, pure cultures of Nitrosomonas and Nitrobacter, and stress protein assay. Metal sorption plays an important role in metal uptake by nitrifying biomass. The sorption process is relatively rapid and controlled by various factors such as pH, type and concentration of complexing agents in wastewater, oxidation state of the metal, and redox potential. Metal internalization is the main mechanism which contributes to nitrification inhibition by heavy metals. The slow kinetics of metal internalization implies that metal inhibition can easily be underestimated from short-term batch assays. The nitrification inhibition of copper is unique among the metals studied and the copper toxicity to nitrifying enrichment cultures may involve disruption of the cytoplasmic membrane. Combining the intraparticle diffusion model with the biological toxicity model, inhibition could be predicted.