Waste Management & Research (1995) 13, 103-121 BEHAVIOUR OF HEAVY METALS IN ACTIVATED SLUDGE BIOLOGICAL TREATMENT OF LANDFILL LEACHATE F. Avezz6 ~, G. Bissolotti 2, C. Collivignarelli 3 and A.Volpi GhirardinP ~Department of Environmental Sciences, University of Venice, -'S.I.AD. Bergamo, 3Department of Hydraulic and Environmental Engineering, University of Pavia, Italy. (Received 4 November 1991, accepted in revised form 15 December 1993) The behaviour of heavy metals was studied by carrying out a series of experiments with an activated sludge biological reactor (with pure oxygen), fed with different types of landfill leachate. The leachates used had been previously treated by the wet oxidation process and also by ammonia stripping. The expcrimentation aims were to evaluate both BOD and COD removal rates and the distribution of heavy metals concentrations between tile liquid and solid phases. This latter data was used to confirm a mathematical model which predicts the distribution of heavy metals between the liquid and solid phases of a biological process. 1. Introduction The presence of non-biodegradable toxic substances including heavy metals in industrial or mixed municipal and industrial wastewaters, presents one of the most severe problems encountered in biological treatments, as it concerns toxic effects on the biomass, the influence on effluent quality and possible effluent reuse and also the effect on sludge disposal. It is therefore very important to understand the behaviour of heavy metals in biological processes, in order to be able to predict their effect on the operating conditions of the process and on the quality of the effluents obtained. The extensive literature on the subject (Barth et al. 1964, Oliver & Consgrove 1974, Neufeld & Herrnann 1975, Brown & Lester 1979, Stoveland et al. 1979, Nelson el al. 1981, Lester 1983, Petrasek & Kugelman 1983, Legret et al. 1987) demonstrates only partial knowledge of the mechanisms which occur in the removal of non-biodegradable matter by activated sludge systems. In particular, the influence of process variables on these mechanisms is almost completely unknown. Leachate from the sanitary landfill of municipal solid waste presents interesting treatment problems which have featured in previous studies (Collivignarelli & Bissolotti 1988a,b) and also the successive biological treatment (Avezzl) et al. 1989a, Collivignarelli et al. 1990, Collivignarelli et al. 1991a) of these liquid wastes which are notoriously rich in heavy metals. The pre-treatment processes applied to the leachate prior to its biological treatment have been reported earlier (Collivignarelli & Bissolotti 1988a, b, Avezzfi et al. 1989a). The reasons for the adoption of these processes are explained below, The wet oxidation process is suitable for the treatment of liquid wastes containing high concentrations of non-biodegradable organic material and normally operates at high temperatures (up to 350°C) and high pressures (up to 200 bar) in order to increase the reactivity of the applied oxygen. Our experience has shown that in operating with reduced temperatures and pressures (with obviously less cost and operating problems), it is possible to obtain a marked increase in the biodegradability of the liquid waste 0734-242x/95/020103+ 19 $08.00/0 © 1995 [SWA