Removal of heavy metals from anaerobically digested sewage sludge by a new chemical method using ferric sulfate

The removal of heavy metals from anaerobically digested sewage sludge was studied by using ferric sulfate. The addition of ferric sulfate to the sludge caused the acidification of the sludge and the elution of heavy metals from the sludge. The pH of the sludge decreased with an increase in the amount of iron added and with a decrease in the sludge concentration. At a sludge solid concentration of 2% (w/w), the sludge pH dropped below 3 and the elution percentage of cadmium, copper and zinc was more than 80% when the added amount of ferric iron was more than 1.5 g per L of wet sludge. Furthermore, the method using ferric sulfate was compared with that using sulfuric acid at pH 3 in order to clarify the effect of ferric iron as an oxidation reagent on elution of heavy metals. Ferric iron eluted cadmium, copper and zinc more effectively than sulfuric acid. This effective elution of heavy metals was caused by the oxidation of the sludge solid by ferric iron added. From these results, it was concluded that ferric iron played a role to acidify the sludge and to oxidize metallic compounds in the sludge and this new chemical method was useful for the removal of heavy metals from anaerobically digested sewage sludge