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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 SO 2− 4 was converted stoichiometrically to dissolved H 2 S, driving organic matter mineralization. Sulphate concentrations of 35, 105 and 175 mM in the feed resulted in SO 2− 4 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 SO 2− 4 concentrations. The ratio of COD:SO 2− 4 (g/g) utilized was dependent upon the concentration of SO 2− 4 present, and at 35 M SO 2− 4 was 0.67. At an organic loading rate −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.

On-line measurement of hydrogen in the biogas of a continuously stirred laboratory scale sewage sludge digester was conducted to determine whether this variable could provide advanced warning of digester failure. Reactor failure was achieved by decreasing the hydraulic retention time (HRT) from 20 to 8 days. At the onset of digester failure a marked decrease in the volume and methane content of the biogas was accompanied by an increase in volatile acid production and a concomitant decrease in pH. During this period, hydrogen accumulation in the biogas was not evident. An increase in hydrogen was noted, however, well after reactor failure had occurred. Hydrogen concentration in the biogas cannot, therefore, be used as an indicator of incipient digester failure, at least when volumetrically overloaded.

Cores of marine sediment were used in model systems to examine degradation of digested sewage sludge in the marine environment at the sediment-water interface. Models of both accumulative and dispersive sites were used, at sludge loadings greater than those applied at sea disposal sites. Mineralization to carbon dioxide was measured using an infra-red gas analyser, and the proportionate mineralization of the added organics increased as loading rate decreased. About 60% of the added organic carbon was mineralized at the lowest rate applied (0.01 kg solids m −2 · d −1 ) over the 133 days of the experiment. Metals added to the model systems in the sludge (Zn, Pb) were immobilized by the sediment and not exported from the model.

Investigation of the fate of inorganic nitrogen compounds in treated sewage effluent suggested that after discharge into an estuary primary production in the water column was at the expense of ammonium. Nitrate was only utilized during dissimilatory nitrate reduction by the sedimentary bacteria. Experiments with bottom sediments showed that nitrate reduction by the sedimentary bacteria could be described by Michaelis-Menten enzyme kinetics: and the measured kinetic characteristics revealed physiological adaptation to high nitrate concentrations of the nitrate reducing bacterial population near the sewer outfall, compared to the population in the sediment at the less polluted river mouth. It is suggested that the denitrifying capacity of mangrove sediments may make lagooning of secondary sewage effluent in mangrove areas a cheap tertiary treatment process to alleviate coastal eutrophication in the tropics.

Acetylene reduction was shown to occur under a nitrogen atmosphere in slurries of anaerobic saltmarsh sediment. This acetylene reduction was entirely inhibited by 20 m m molybdate, but was stimulated by the presence of a hydrogen atmosphere. Anaerobic heterotrophic nitrogen fixation was, therefore, attributed to the activity of sulphate reducing bacteria which were the predominant nitrogen fixers in this environment.

Sulphate reduction rates and the sulphur geochemistry of the sediments were investigated at two sites in a saltmarsh. Rates of dissimilatory sulphate reduction decreased rapidly with depth at both sites over the top 20 cm of sediment. There was no correlation between the measured rates and the numbers of sulphate reducing bacteria. Sulphate reduction rate also varied seasonally, and total annual sulphate reduction over the top 20 cm of sediment depth was equivalent to 0.44 mg-at. sulphate-S cm−2 of sediment surface (main creek site) and 0.31 mg-at. sulphate-S cm−2 (saltmarsh pan site). These rates are similar to those reported in the literature for similar sediments. Free sulphide was present in the sediment of the saltmarsh pan, but never at the creek site although sulphate reduction rates were similar. Pyrite and acid-volatile sulphide distributions with depth were also different between the two sites. The contrast in geochemistry was attributed to different stages of sulphur diagenesis. A large proportion of the sulphide was lost from the sediment by diffusion, tidal forces not contributing to transport of sulphide. The diagenetic difference between the two sites was attributed to the presence of a benthic algal mat in the saltmarsh pan which retarded diffusive loss of sulphide, maintained higher sedimentary sulphide concentrations, and thus kinetically favoured more rapid diagenesis at this site.