Heavy metals and disposal alternatives for an anaerobic sewage sludge

Abstract
Treatment of wastewater unavoidably results in the generation of huge quantities of sludge in which pollutants concentrate and which must be disposed of. There may be significant levels not only of salts and organic pollutants, but also of heavy metals, which are of particular concern. The presence of these substances causes environmental damage in soil, plants, groundwater and air. The classical disposal alternatives at hand include thermal treatment (incineration), land application and ocean dumping, all of these being objectionable to some extent because of major problems created by the pollutants in the sludge. Knowledge of the heavy metal content of the sludge is a key step in selecting or in ruling out the various disposal alternatives. An anaerobically stabilized sewage sludge from an urban‐residential wastewater treatment facility has been collected and studied in order to identify the most suitable disposal alternative. The sludge has been analyzed for its calorific value and heavy metal content. Sequential chemical extraction of heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn) has been carried out using selective extractants in order to determine changes which occur in the chemical partitioning of heavy metals in the present use as fertilizer. Significant concentrations of lead, copper and zinc have been found, but metals levels in the sludge are within the limits established for its use in agriculture. Lixiviation of the metals into seawater has also been explored. Lead, cadmium and chromium are noticeably lixiviated.