Effect of Chemical Coagulation Pretreatment on Anaerobic Digestion of Tannery Wastewater

Tanning factory is globally considered to be one of the most resource consuming and pollutants producing industries because it requires, for processing leather, large amounts of fresh water, chemical reagents, and energy, and in addition produces wastes, liquid as well solid, rich in organic matter and heavy metals (e.g., chromium). Such characteristics of tannery wastes make their biological treatment promising as well as challenging. This study deals with the effect of coagulation process as a pretreatment method on real tannery wastewater prior to anaerobic digestion (AD), with the aim of removing chromium and thus limiting its inhibitory effect on biological process. Different concentrations (100, 200, 400, 600, 800, and 1,000 mg=L) of the coagulant FeCl3 were tested, at pH 7.5, with an initial total chromium concentration of 150 mg=L. The efficiency of metal removal was in the range of 0.44–29.38%. Simultaneously, the chemical oxygen demand was reduced by 0.86–28.80%, with the detrimental effect of lowering the total biogas production potential from tannery wastewater. Nevertheless, coagulation pretreatment enhanced the AD process of tannery wastewater, resulting in an increase of the cumulative biogas production by 10% and in a reduction of the digestion time by approximately 25%, in comparison with untreated wastewater. After 37 days of incubation, the maximum specific cumulated volume of biogas produced from the wastewater pretreated with 800 mg=L of FeCl3 was 837 mL=g total volatile solids added with approximately 84% of methane.