Removal of heavy metals from sewage sludge by extraction with organic acids

Waste water treatment in activated sludge plants results in the production of large amounts of surplus sludge. After composting the sludge can be reus ed as fertiliser and soil conditioner in agriculture. Compared to landfilli ng and incineration, utilisation of sludge-compost is a more sustainable tr eatment because it recycles both nutrients and organic matter. However the high levels of heavy metals in sludge frequently prevent the reuse of sludg e compost in agriculture. The extraction of heavy metals from the sludge be fore composting is therefore a necessary step to achieve a more sustainable sludge treatment. Extraction of heavy metals by inorganic acids and comple xing agents has severe drawbacks. Organic acids could be an attractive extr acting agent because the extraction can be performed at mildly acidic condi tions (pH 3-5) and they are biologically degradable. The extraction was stu died for heavy metals Cu and Zn and for competing metals Ca and Fe. The rat e of extraction increases for increasing temperature and citric acid concen tration. Cu can be extracted for 60-70% and Zn for 90-100% by citric acid a t pH 3-4. A first economic valuation of the extraction and subsequent compo sting process showed that the total costs of the treatment process are belo w the costs of incineration. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.