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Waste water treatment in activated sludge plants results in the production of large amounts of surplus sludge. After composting the sludge can be reused as fertiliser and soil conditioner in agriculture. Compared to landfilling and incineration, utilisation of sludge-compost is a more sustainable treatment because it recycles both nutrients and organic matter. However the high levels of heavy metals in sludge frequently prevent the reuse of sludge compost in agriculture. The extraction of heavy metals from the sludge before composting is therefore a necessary step to achieve a more sustainable sludge treatment. Extraction of heavy metals by inorganic acids and complexing agents has severe drawbacks. Organic acids could be an attractive extracting agent because the extraction can be performed at mildly acidic conditions (pH 3-5) and they are biologically degradable. The extraction was studied for heavy metals Cu and Zn and for competing metals Ca and Fe. The rate of extraction increases for increasing temperature and citric acid concentration. Cu can be extracted for 60-70% and Zn for 90-100% by citric acid at pH 3-4. A first economic valuation of the extraction and subsequent composting process showed that the total costs of the treatment process are below the costs of incineration.

The anaerobic hydrolysis rate of organic solid waste was studied at fixed volatile fatty acid (VFA) concentrations ranging from 3 to 30 g COD/L and fixed pH values between 5 and 7. For separate control of both VFA and pH, a special completely mixed reactor was designed. In this way, it was possible to distinguish between the inhibitory effects of pH, total VFA, and undissociated VFA on anaerobic hydrolysis. It was shown that hydrolysis of the organic solid waste followed first-order kinetics. Using a statistical analysis, it was found that the hydrolysis rate constant was pH dependent but was not related to the total VFA and undissociated VFA concentrations.Note: Calculations were made using average composition of VFA formed during fermentation [acetate:propionate:butyrate:valerate = 0.51: 0.21:0.22:0.06 (g COD/L)] as determined by ten Brummeler et al. (1991) and corresponding dissociation constants of VFA components.