A COST EFFECTIVE METHOD FOR DETOXIFICATION OF SLUDGE CONTAINING CHROMIUM

The industrial wastewater treatment plant discussed in this paper utilizes processes that remove dissolved metals and suspended solids from the wastewater and generate sludge containing heavy metals, including chromium. Occasionally, the concentration of chromium in the Toxic Characteristics Leaching Procedure (TCLP) extract from the sludge exceeded the regulatory limit of 5 mg/L and resulted in the solid waste exhibiting the characteristic of toxicity. The technical and economic feasibility of a process for conditioning the chromium-laden sludge was investigated. To define the design parameters, our laboratory-scale study addressed: 1) the chromium reduction efficiency of various chemicals, 2) the amount of chemicals required to render the sludge non-hazardous, 3) sludge conditioning efficiency of the process in the presence of other heavy metals, and 4) the preferred type and dosage of chemical.Our results revealed that the chromium-laden sludge could be made non-hazardous by chemical conditioning. The chromium reduction efficiency of triple super phosphate (TSP) is higher than that of either calcium carbonate or magnesium hydroxide, and magnesium hydroxide is the least effective chemical. Based on the laboratory-scale study results, a pilot-scale sludge conditioning system was designed and installed. Available information from the pilot-plant operation indicates that the conditioning system consistently reduced the concentration of chromium in the TCLP extract below the regulatory limit of 5 mg/L. Cost/benefit evaluation results indicate that annual net savings of about 1,000,000 can be achieved by conditioning the sludge with TSP and disposing it as a non-hazardous material in a landfill. The results obtained from the laboratoryscale work as well as from the pilot-scale operation are described and discussed in this paper.