CONDITIONING AND DEHYDRATION OF MUNICIPAL SEWAGE SLUDGES AFTER REMOVAL OF HEAVY-METALS

The wastewater treatment plants produce a considerable quantity of slu dges with high concentrations of heavy metals. The disposal of these c ontaminated sludges limited, in this case, to incineration with ash la ndfilling is a very costly practice for the municipalities. Some decon tamination processes are currently developed to resolve this problem; amongst them, a biological process allowing the solubilization of toxi c metals and the sewage sludge digestion in one step. In the present s tudy, the dehydration technic for the acid sludges resulting from this decontamination process was investigated and a method was proposed to improve its efficiency. The research shows that normal operation cond itions can be used for the dehydration of leached sludges, in industri al scale and with acceptable costs. The results revealed the optimum c onditioning of acid sludges by addition of bentonite and cationic poly mers. The utilization of plate filter-press resulted in sludge cake pr oduction with 19 to 30% siccity and a filtration capacity between 1.5 and 4.0 kg . m(-2). h(-1). On the other hand, laboratory tests showed that the decontaminated sludge pH (between 1.7 and 2.5) and the additi on of ferric chloride do not affect significantly the conditioning of the sludges.