The recirculation of nitrified leachate through landfill sites, followed by in situ denitrification, represents a novel and more sustainable approach for the removal of ammonia from leachate, prior to discharge. The effects of nitrate and leachate supplementation on methanogenesis in Municipal Solid Waste (MSW) were studied in batch cultures. The addition of a range of nitrate concentrations to MSW samples had an inhibitory effect on methanogenesis. The effects were dose-dependent, such that recovery of methane production was recorded within 5 and 23 days with added 100 and 750 mg NO3 dm(-3), respectively. Even after 24 days, no recovery was observed in cultures challenged with 1000 mg NO3 dm(-3). The enumeration of denitrifying bacteria in a range of fresh, actively methanogenic and aged, well-decomposed MSW confirmed the potential of MSW for rapid denitrification. Methanogenesis was not inhibited by the addition of leachate (20-100% strength) that contained high concentrations of VFAs. However, when the same leachate was supplemented with nitrate (250 mg NO3 dm(-3)), methanogenesis was inhibited by the addition of leachate concentrations greater than or equal to20%, which was attributed to inhibition of denitrification by VFAs. Propionate accumulated, confirming the importance of methanogenesis as an electron sink. With the removal of nitrate and the recovery of methanogenesis, net propionate concentrations decreased. (C) 2004 Society of Chemical Industry.