Spreading of sewage sludges on agricultural land is an attractive sludge management option because it combines beneficial reuse and disposal at the same time. However, it is important to reduce the metal content in the sludge in order to minimize the health hazard associated with metal uptake by plants and its subsequent accumulation in the food chain. Treatment of sludge with acid for metal removal is not practical because a large amount of acid is required. Typically 0.5 to 0.8 g of H2SO4/g dry weight of sludge will be required to achieve over 70% removal of cadmium (Cd), zinc (Zn) and nickel (Ni). Lead (Pb) and copper (Cu) are not significantly removed. A biological process called bacterial leaching, which has been used commercially for extracting copper and uranium from low grade ores, was reviewed and its potential for removing heavy metals from anaerobically digested sewage sludge was investigated. Leaching experiments were conducted and the results showed that about 80 to 90% removal of cadmium, zinc and nickel, and 60 to 70% removal of copper were possible. The acid requirement was significantly reduced because only 0.15 g of H2SO4/g dry weight of sludge was needed.
A biological oxidation process called bacterial leaching was used to remove heavy metals from anaerobically digested sewage sludge. The purpose was to decontaminate the sludge so that it could be utilized on agricultural land. The leaching process was found to be affected by pH, aeration and temperature. At α pH of 4, an aeration rate of 100 cm3 of air per litre of sludge per minute and a temperature of 25°C, the following metal removal efficiencies were observed: cadmium, 80 - 85% ; copper, 66 - 80% ; nickel, 70 - 78% and zinc, 84 - 90%. No significant removal was observed for lead. In addition to removing heavy metals, bacterial leaching preserves the soil conditioning and fertilizing properties of the sludge. According to Ontario guidelines, the sludge used in this study was initially not acceptable for use on agricultural lands. However, after bacterial leaching, all criteria for the application of sludge to agricultural lands were easily met.
The present study was conducted to investigate the effectiveness of the sulphate-reduction pathway in the anaerobic treatment of landfill leachate. The effects of several COD/SO4 ratios (keeping COD constant) and loadings on anaerobic filter performance were studied and compared with the results from anaerobic filters which followed the methanogenic pathway. Results indicated that the treatability of leachate by sulphate reducing bacteria (SRB) was dependent upon the leachate strength. With high strength leachate (COD = 15,000 mg/L) from the Keele Valley Landfill, it was found that at lower COD/SO4 ratios (< or = 1.6) toxic conditions developed in the system that were more inhibitory to the SRB than to the methane producing bacteria (MPB). As the COD/SO4 ratio increased, methanogenesis predominated. No predominance of SRB occurred at any COD/SO4 ratio with high strength leachate. The highest COD removal achieved was about 70% of which 20% was accomplished by the SRB at a COD/SO4 ratio of 1.6 and an organic loading rate (OLR) of 4 kg COD/m3.d. With low strength leachate (COD = 1500-3300 mg/L) from the Brock West Landfill, and a COD/SO4 ratio < or = 1, SRB became predominant. In these anaerobic filters in which SRB were predominant, the SRB reduced the COD as well as the MPB could. Sulphide inhibition did not take place at any loading in units treating low strength leachate. Consequently, both SRB and MPB should function at COD/SO4 ratios between 1 and 3. About 60% COD removal was achieved at a loading of 2.8 kg COD/m3.d and a COD/SO4 ratio of 1.0. However at a loading of 6 kg COD/m3.d only 27% COD removal was achieved, all of it through the sulphate-reduction pathway. These OLR values are comparable to those applied in systems where methanogenesis was dominant. It was also observed that once the methanogens were established in the units, it was not possible to displace them completely. However, where methanogenesis had not been previously established, it was found that sulphate-reduction could be the sole pathway for COD removal. From this study, it can be concluded that there is no advantage to the sulphate-reduction pathway in the anaerobic treatment of landfill leachate. The other options for increasing the loadings, i.e. the use of high surface/volume filter media (to achieve higher biomass concentrations) or high rate systems are likely to be more successful.
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