Treatment of landfill leachate by methanogenic and sulphate-reducing digestion

Landfill leachate was treated in upflow hybrid (sludge-bed/fixed-bed) anaerobic reactors by both methanogenic digestion and by sulphate-reducing digestion. Steady state removal efficiencies of COD under methanogenic digestion was 81-97%, depending upon organic loading rate; and effective treatment occurred up to a volumetric COD loading rate of 3.75 kg COD . m(-3). d(-1). In sulphate-reducing digesters SO42- was converted stoichiometrically to dissolved H2S, driving organic matter mineralization. Sulphate concentrations of 35, 105 and 175 mM in the feed resulted in SO42- removal efficiencies of 92, 52 and 35% respectively. Steady state removal efficiencies of organic COD in the sulphate-reducing digesters averaged 62% at 35 mM, 80% at 105 mM, and 84% at 175 mM feed SO42- concentrations. The ratio of COD:SO42- (g/g) utilized was dependent upon the concentration of SO42- present, and at 35 M SO42- was 0.67. At an organic loading rate <1 kg COD . m(-3). d(-1) the sulphate-reducing digesters offered organic removal efficiencies comparable to methanogenic digesters, but were less effective at higher loading rates. Propionate accounted for the majority of the residual COD in the effluents from the methanogenic digesters, but acetate in the effluent from the sulphate-reducing digesters. Removal of sulphide from the reactors was achieved by addition of iron and by gassing off sulphide, but there was no increase in COD removal after sulphide removal which indicated that sulphide toxicity was not significant. Both methanogenic and sulphate-reducing reactors completely removed up to 100 ppm Cu, Mn, Ni and Zn added in the feed, indicating that even methanogenic reactors produce sufficient sulphide to precipitate these metals completely.