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India has over 300 sugarcane molasses based alcohol distilleries that collectively generate around 40.4 x 1010 L of wastewater (spentwash) annually. The effluent has a high organic load (COD of 80,000–100,000 mg L-1 and BOD of 40,000–50,000 mg L-1), low pH, strong odor and dark brown color. Anaerobic digestion (biomethanation), which is the primary treatment step, removes 80-95% BOD. Subsequent treatment by activated sludge process, biocomposting, incineration etc. is unable to provide a comprehensive solution to distillery wastewater management. A key challenge is the further reduction of organic load and removal of the color imparting melanoidins. Because of the limitations in the existing methods and the increasingly stricter environmental norms, Indian distilleries are urgently exploring alternate approaches to treat this wastewater.
This work focused on integrating physico-chemical and biological methods for the treatment of anaerobically treated distillery wastewater. The removal of melanoidins was studied by adsorption on powdered activated carbon (PAC), both prepared in-house and procured commercially. The effect of electrolytic treatment on the properties of melanoidins and structurally similar humic acids was examined. The biomethanated spentwash was treated in an aerobic membrane bioreactor (MBR) equipped with 30 μm nylon mesh submerged filters using activated sludge from a municipal wastewater treatment plant. A detailed characterization of the mesh filter, PAC and sludge produced in the reactors was also conducted. Because of the long SRT (sludge retention time) in an MBR, degradation of high molecular weight compounds is expected to occur. The effectiveness of PAC supplementation on MBR operation was also investigated.
PAC from phosphoric acid treated bagasse displayed the best decolorization (50%) among the prepared samples; however, the performance of the wood-charcoal based commercial carbons was significantly better (over 85% color removal). Electrolytic reduction of melanoidins resulted in increased hydrophobicity thus leading to enhanced adsorption. Treatment of spentwash in an aerobic MBR resulted in COD removal of 41%, which appeared to be limited by the buildup of recalcitrants in the reactor. Nylon mesh was a good solid-liquid separator, with average suspended solids retention of over 80% at a mixed liquor suspended solids (MLSS) content of 10-12 g L-1. PAC addition did not increase the maximum COD removal; however, it allowed operation at higher loading rates (5.4 kg COD m-3 d-1, instead of 3.4 kg COD m-3 d-1 without PAC). Other benefits of PAC supplementation included a 2.5 fold increase in operation time between filter cleaning, 19-22% increase in mesh critical flux and better sludge filterability. Irrespective of PAC addition, only compounds below 940 Da were degraded in the MBR.
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