Trivalent chromium removal by electrocoagulatoon ana characterization of the process sludge

Removal of chromium compounds from wastewater is a known pollution control challenge for environmental engineers. In this present work Cr3+ was removed from wastewater by electrocoagulation (EC) using Al electrodes; in a batch cell. Results indicate that EC with an Al electrode can reduce Cr3+ concentration below 2.0 mg L-1, its discharge limit. At higher stirrer speed, Cr3+ removal increases owing to enhanced contact with the Al3+ species. Cell current density controls the rate of Al dissolution and solution pH that affects the Cr3+ removal significantly. pH elevation during EC is due to accumulation of OH- ions forming a supersaturated solution of Al3+ species. Supersaturation with respect to Al(OHMS) is attributed to incomplete precipitation of aluminum hydroxide in the dynamic (transient) state and subsequent precipitation when cell current is stopped. Sludge produced in the process can be classified as non-hazardous according to the European Waste Catalogue. Disposal cost of this dried sludge is estimated to be $0.144 (INR 7.20) per m(3) of tannery effluent treating an initial solution of 1000 mg L-1 Cr3+ to about 2.0 mg L-1 Cr3+. Batch gravity settling characteristics of the electrocoagulated metal hydroxide sludge (EMHS) at different initial sludge loadings (as generated at different current density) is also investigated. Batch sedimentation flux is reported from experimental settling velocity and concentration of sludge. (c) 2007 Society of Chemical Industry.