PRETREATMENT OPTIONS WITH RO AS TERTIARY TREATMENT OF DISTILLERY SPENTWASH

Abstract

India is one of the leading country in the field of alcohol production. There are around 360 distilleries in India, producing 2300 liters of alcohol per annum. The wastewater generated (spentwash) per liter of alcohol production is 8-15 liters, characterized by high organic content (BOD and COD), high solids (TSS and TDS), low pH (3- 4) and dark brown color. Bio-methanation is currently employed as a primary treatment step in almost all distilleries to reduce organic pollution load. However, bio-methanation alone does not meet the discharge standards, irrespective of subsequent aeration, thus leaving a need of further treatment. At the tertiary level, reverse osmosis (RO) treatment units have already been installed in many distilleries as well as studied in a number of bench and pilot studies in India. However, fouling has the main problem associated with the membrane technology restricting its use in industrial application. Industrial wastewater is generally highly concentrated leading to rapid membrane fouling which results in reduced permeate water flux and increased trans-membrane pressure. In case of distilleries, as the organic content in the spentwash after biological treatment is quite high, direct application of biologically treated spentwash to RO generally results in choking of the membrane system. The focus of the present study is to evaluate the effectiveness of different pretreatment methods for RO system. To begin with, performance of RO treatment plant of an existing distillery in the vicinity was evaluated. Detailed surface and chemical characterization (FE-SEM, AFM, FTIR and XRD) of the RO membrane, collected from the existing plant, was also performed. The EDX results identified Si, Fe, Ca and Na as the major inorganic foulants. AFM study also supported the deposition of colloids or other foulants on the membrane surface. The XRD results confirmed the presence of calcite and silica compounds in the foulant layer. Low pressure membranes (MF and UF) and electrocoagulation (EC) were studied as pretreatment of RO. Optimization of operating parameter was done for these treatment options in isolation as well as with RO against the displayed response of removal of contaminants (COD, color, TDS and TOC). A comparative evaluation was done in terms of contaminants removal and total water production. Study revealed that EC is more effective in removing organic matter as compared to the UF and MF. Whereas, color and TDS removal was observed more in case of UF. The difference in organic matter removal between UF and EC treatment was around 10-12 % (EC>UF). However, the difference in removal of color and TDS was ii around 5-6% (UF>EC). Permeate flux (recovery) with RO has showed a remarkable increase from 17.5 l/m2/h (direct RO) to 40.5 l/m2/h, 38.0 l/m2/h and 32 l/m2/h when used in combination with EC, UF and MF, respectively. Economic assessment of the proposed treatment schemes was done on the lab scale on the basis of water production cost, which was estimated considering technical assumptions, specification and design parameters of different treatment system. The per cubic meter water production cost for RO system is 470 Rs./m3/m2. The pretreatment reduces the cost up to 50% than RO system alone. Among all pretreatment studies (MF, UF and EC), EC-RO (185 Rs./m3/m2) has been found to be more economical and efficient than UF-RO (220 Rs./m3/m2) and MF-RO (256 Rs./m3/m2) in terms of water production. The capital cost of RO system with pretreatment (MF-RO, UF-RO and EC-RO) observed to be 30% higher than RO system alone. Possible reuse of sludge generated during the EC treatment was explored employing it as a partial replacement of cement while manufacturing the non-constructional building blocks. Mortar specimens with different proportions of sludge ranging from 0 to 15% by weight of cement were tested for density, compressive strength and leachability of heavy metals by standard methods. From mechanical as well as environmental point of view, it was successfully demonstrated that cement–sludge mortar containing 7.5% EC sludge could be used in different applications such as decoration tiles, pot making and fencing of garden without adversely affecting the environment.