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Presents the findings of a study conducted to examine the efficiency of the combined anaerobic‐aerobic reactors for the treatment of three different tannery industrial wastewaters, focusing on the fate of chromium and other associated metals. Examines elimination rates of such metals with respect to each successive treatment process. Discusses the correlation between the chemical characteristics of the wastewater influent and effluent along with the level of heavy metals in such wastewater and the produced sludge. Confirms that the use of such sludge in agriculture should be restricted and recommends the recovery of Cr from the tanning sludge.

In this study tannery industrial wastewater was supplied from a leather industry plant in the south of Germany. An anaerobic pilot plant was erected for the treatment of this wastewater. Discusses the effect of such an anaerobic process with special reference to the fate of chronium and the other associated heavy metals throughout the process. The studied metals were Cr, Cd, Pb, Ni, Cu, Fe and Zn. The elimination rate of these metals throughout the anaerobic processes from the wastewater and the increasing rate of metal correlations in the sludge were studied extensively. Levels of Cr, Cu, Fe, Cd, Ni, Pb and Zn in the raw wastewater after acidification were 33.72, 0.13, 1.82, 0.06, 0.19, 0.21 and 1.07 mg/l respectively. In the sludge the levels were 6,296, 16, 141, 31, 12.4, 23 and 94 mg/kg dry weight respectively. The levels of these metals were decreased by stabilization. The overall results revealed that a remarkable decrease has been achieved in the reduction level of metals in the final wastewater. The elimination of such metals reached 98 per cent for Cr, 53.8 per cent for Cu, 81.3 per cent for Fe, 16.7 per cent for Cd, 21.1 per cent for Ni, 72.9 per cent for Zn. On the other hand, such metals were concentrated in the sludge, on the dry weight basis, up to the levels of 13,193 mg Cr/kg, 27.7 mg Cu/kg, 348 mg Fe/kg, 3.2 mg Cd/kg, 13.7 mg Ni/kg, 28 mg Pb/kg and 110 mg Zn/kg. Meanwhile, the studied anaerobic process exhibited significant improvement in the quality of the tannery wastewater to meet the guideline characteristics. However, the partial high level of Cr (III) and the other associated metals in the produced sludge strongly militate against the use of such tannery sludges as manure in agriculture.

Investigates anaerobic two‐stage treatment of tannery wastewater. This results in a mean chemical oxygen demand (COD) removal of 60 per cent for tannery B and more than 70 per cent for tanneries A and C with COD_o = 5,710mg l^‐1 and mean detention time held at 3.5 days. Gas production was small, rating an estimated 75 l kg‐1 COD_o with COD_o = 5,710mg l^‐1. Tests simultaneously the influence of quality and quantity of wastewater on COD removal and gas production (multiple regression). Finds significant inhibitory effects were caused by chloride on gas volume and by sulphate on COD removal. Chromium showed no significant effect. Dosage of ferric chloride for removal of the toxic sulphide effected a decrease in gas volume and had no effect on COD removal (analysis of variance).