Engineering Assessment of Hot-Acid Treatment of Municipal Sludge for Heavy Metals Removal

摘要

The hot-acid method for treating sludge was developed by the Walden Division of Abcor, Inc., to remove heavy metals from municipal wastewater sludge. Investigations by Walden have demonstrated the degrees to which heavy metals are solubilized. Sulfuric acid dosage at about 20 to 30 percent of the sludge dry solids followed by heating to 95C for a 30-minute reaction time are the process design parameters. The process is effective in solubilizing about 50 to 90 percent of the concentration of the selected heavy metals. The nitrogen, phosphorus, and organic contents of sludge are also solubilized to varying degrees. The acid-treated sludge was found to be essentially pathogen-free. Dewatering of the treated sludge was shown to be poor. It was determined that untreated sludge requires about four times as much land as hot-acid treated sludge for land application, based on the EPA stipulated cadmium application rate. Hot-acid treatment would not reduce the sludge's metal levels below the USDA recommended guidelines for land application. However, hot-acid treatment clearly improved marketability of a high metals sludge. The hot-acid treatment of sludge would cost about $440 per ton for a 2-mgd waste-water treatment plant and about $140 per ton for a 20-mgd plant. Although hot-acid treatment is costly when compared to conventional sludge treatment practices, its cost effectiveness may be quite good when compared to other systems aimed at controlling the heavy metal contents of sludge.