EFFECT OF COMPOSTING ON SEWAGE SLUDGES CONTAMINATED WITH HEAVY-METALS

The influence of composting on sewage sludges contaminated by Cd or by high concentrations of Cd, Cu, Ni and Zn was studied. Independently o f the contamination which existed the composting process led to a loss of weight, as shown by the decrease in organic matter however metal c ontamination did influence microbial populations and led to lower mine ralization and humification being achieved. Contamination by Cd led to a much lower degree of nitrification during composting. Hydrolytic en zyme activities showed that large quantities of heavy metals had a neg ative effect on urease synthesis. Similarly, contamination negatively affected phosphatase; although not beta-glucosidase, which increased w ith the level of contamination. The composting process rendered insolu ble that portion of the metals which could be extracted with DTPA. CaC l2 or H2O by forming metal-humus complexes. Of the metals which were p resent at high quantity in the sewage sludges (Ni, Cu, Zn and Cd), Ni was the most easily extracted by H2O or CaCl2 and was therefore the mo st weakly bound to the organic matrix Zn, in contrast, remained strong ly bound.