Chemical precipitation method for chromium removal and its recovery from tannery wastewater in Ethiopia

Most of the tanneries in Ethiopia (90%) do not treat and very few (10%) partially treat their effluent before discharging it into the receiving water bodies. The untreated tannery effluent causes tremendous pollution of water resources in the country, especially due to its high organic loading and chromium content. Chromium(III) salts are the most widely used chemicals for tanning processes, but only about 60-70% of total chromium salts reacts with the hides and about 30-40% of the chromium remain in the solids and liquid wastes (especially spent tanning solutions). Therefore, the removal and recovery of the chromium content of these wastewaters is necessary for environmental protection and economic reasons. The purpose of this study was to develop an alternative process for removing and recovering trivalent chromium from tannery wastewater via chemical precipitation with sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2) and magnesium oxide (MgO). The effects of pH, stirring time, settling rate and sludge volume were studied in batch experiments. Flame atomic absorption spectrometry was used to determine Cr concentration. The results showed that removal efficiencies of the precipitating agents NaOH (99.97%), Ca(OH)2 (99.97%) and MgO (99.98%) at optimum pH values were not significantly different. However, there was a significant difference in sludge volume of NaOH (590 mL), Ca(OH)2 (412 mL) and MgO (85 mL). The optimum pH of 9.8-10.3 with removal efficiency of 99.98% and the good sludge with high settling rate and lower volume was obtained using MgO precipitating agent. Hence the MgO was found to be a good precipitating agent for removal and recovery of chromium from tanning wastewater. The basic chromium sulfate (BCS) recovered at pH 2.8 is suitable for tanning mixed with fresh BCS from the market