Nitrification and heavy metal removal in the activated sludge treatment process

Abstract

A bench-scale, continuous flow, completely mixed activated sludge system was used to investigate the relationship of nitrification and other system parameters to heavy metal removal in the activated sludge waste treatment process. The heavy metals studied were chromium, at concentrations ranging from 0.1 to 10.0 milligrams per liter, and silver, at concentrations ranging from 0.1 to 0.5 milligram per liter in settled primary domestic sewage. Analyses performed on the influent, mixed liquor, return sludge, and effluent included heavy metal concentration, pH, dissolved oxygen, temperature, suspended solids, chemical oxygen demand, nitrogen species, alkalinity, and other system control parameters. Results indicate that the sludge wasting rate is directly related to heavy metal removal. Nitrification is only indirectly related since sludge wasting controls sludge age which in turn determines the degree of nitrification. A theory for the mechanisms contributing to heavy metal removal is developed. The capability of activated sludge to concentrate heavy metals is the main phenomenon involved. Sludge wasting is the key mechanism affecting heavy metal removal from the system.