Removal of Chromium from Metal Finishing Wastewater by Lime Coagulation

Abstract

This study is carried out to assess the levels of total chromium in the chrome plating wastewater and its removal by chemical precipitation using lime water suspensions. The wastewater produced from chrome electroplating is huge and complex which far exceed the assimilative capacity of nature and lead to an irreversible health and ecological consequence. Wastewater samples are collected from Homicho Ammunition Engineering Industry. The physicochemical analysis and heavy metal concentrations determined for different parameters using FAAS and titration. The average results obtained are: total dissolved solids (20,848.20 ± 0.35 mg/L), total suspended solids 889.55 ± 0.43 mg/L, electrical conductivity (182.26 ± 0.11 mS/cm ) , pH (2.64 ± 0.01), Cr+6 (196,960 ± 0.70 mg/L), Cr+3 (117,20 ± 0.55 mg/L), total Cr (2,155,33 ± 428.99 mg/L), Fe (3,514.50 ± 21.55 mg/L), Pb (1,070.75 ± 3.35 mg/L) and Cu (1,239.16 ± 18.74 mg/L). The samples collected starting from 2002 to 2012 were analyzed separately and the results were found to be significantly different (p<0.05) for each parameter. The stored wastewater treated through chemical precipitation by reducing Cr+6 with ferrous sulfate in acidic media, (pH 2 to 3.5) to trivalent chromium. Chemical precipitation process was found to be efficient in the removal of chromium from the effluent at different pH (73.68% at pH 7.5; 82.60% at pH 8 and 99.98% at pH 8.5).The treatment of chromium is pH dependent. The percentage removal achieved for the other metals is Cu 89.71%, Fe 94.82%, and Pb 98.27% at pH 8.5 respectively. The best removal for chromium is 99.98% at pH 8.5 leaving 0.16 mg/L in the filtrate which is below permissible limits as per the Ethiopian Environmental protection guidelines. Therefore, this method of removal was found to be cost effective and environmentally friendly.