Effects of trace chromium on organic adsorption capacity and organic removal in activated sludge

Heavy metals are commonly found in municipal sewage that contains industrial effluents. Metal concentrations above toxic levels inhibit biological processes in municipal sewage treatment works and discharge of metals into surface waters can have severe effects on the environment and public health. However, trace metals at sub-toxic concentrations have also been observed to affect COD removal in activated sludge. In this study, the effects of a trace metal (chromium) on activated sludge in a sequencing batch reactor (SBR) were investigated. Chromium-laden wastewater at a sub-toxic level of 0.05 mg/l affected the SBR performance to different extents depending on the hydraulic retention time (HRT). Organic removal in activated sludge was postulated to proceed by a rapid adsorption of organics on the sludge, followed by a slower metabolic assimilation mechanism. Heavy metal acted as a strong competitor for active sites on the sludge, hampering organic adsorption and affected the COD reduction efficiency under short HRTs of 2.5 days and below.