BACKGROUNDSensitivity of activated sludge to heavy metals may depend on the strength and type of organics in the feed. However, literature data are usually derived from systems with an unknown or variable composition. To fill this gap, this study examined whether in activated sludge systems the inhibitory effect of Ag+ depended on the carbon to nitrogen (COD/TKN) ratio and the type of organic matter in the feed. RESULTSDifferent activated sludges reactors were operated at steady-state conditions. Regularly, sludge samples taken from reactors were contacted with Ag+ in a respirometer and the inhibitory effect was determined by the relative decreases in O-2 consumption and CO2 generation. The inhibitory property of Ag+ drastically increased as the COD/TKN ratio became lower. While sludges fed with acetate, glucose and peptone or glucose alone were equally inhibited when exposed to Ag+, the peptone-fed sludge proved to be much more resistant. It was also clarified whether indicated Ag+ doses had an effect on nitrification, organic carbon removal or both processes. CONCLUSIONTo date, comparative evaluations have rarely been made between different activated sludge systems in understanding the response to silver input. Overall, this study pointed to the need to consider the characteristics of the feed to explain the discrepancies between different activated sludge systems. (c) 2015 Society of Chemical Industry