Leachate from a municipal landfill was combined with domestic wastewater and was treated in batch activated sludge systems. The effectiveness and applicability of the addition of Powdered Activated Carbon (PAC) to activated sludge reactors was investigated. Isotherm tests were carried out with PAC in order to estimate the extent of adsorption of organic matter onto PAC. Then, in activated sludge reactors COD (Chemical Oxygen Demand) removal and nitrification were studied both in the absence and presence of PAC for comparison purposes. In both cases, Oxygen Uptake Rates (OUR) were measured with respect to time in order to investigate substrate removal and change in microbial activity. Addition of PAC to activated sludge increased COD removal by removing mainly the nonbiodegradable fraction in leachate. The COD decreases in batch reactors were best expressed by a first-order kinetic model that incorporated this non-biodegradable leachate fraction. With added PAC, nitrification was also enhanced. But in all of the batch runs a significant accumulation of NO2-N took place, indicating that the second step of nitrification was still inhibited.