DESIGN OF EFFLUENT TREATMENT SYSTEM FOR A SMALL SCALE CHROME PLATING UNIT

Abstract

The small scale electroplating industries, although, generate little amount of waste waters, they are highly toxic in nature because of the presence of metals and acids alongwith highly dangerous cyanides. Chromium, being one of the constituents of thp chrome plating effluents•, poses a great problem in disposal due to its toxicity. Chromium is one of the heavy metals which is essential in -micro quantities for biological activities. But when taken in higher concentrations, it becoms toxic, mutagenic, carciogenic and teratogenic. As public health control measures are being enforced progressively in the country, industries will find it necessary to give the required degree of treatment to their effluents. Plating effluents can be treated to remove chromium alongwith other metals by a number of different methods. But the cost of treatment has to be economical in terms of its applications.. Type of. treatment system for a plating unit depends on the average flow rate of the effluents, concentration of toxicants, pH, alongwith other factors. Attempts have been made to design a treatment plant for a typical chrome-plating industry manufacturing rims of cycles and, rikshaws. The effluent flow rate was found to be 0.4 m3/hour with 69.8 mg/1 chromium, 978.7 mg/1 nickel and 1038.7 mg/1 iron concentrations respectively. A comparative study of different treatment methods such as redction and precipitation, ion exchange, evaporative recovery ii and reverse osmosis etc. reveals that reduction and precipitation method would be best suited and most economical as the primary concern is chat of treatment and not of reclamation. Hexavalent chromium was reduced to trivalent chromium with sodium metabisulphite and ferrous sulphate respectively. By the analysis of the results it is observed that reduction with metabisulphite is cheaper than that ferrous sulphate. But ferrous sulphate is recommended for more than one reasons. 1. With ferrous sulfate removal efficiency is better. Plot between residual chromium concentration and dosage of ferrous sulphate is linear. 2. Removal efficiency varies exponentially with dosage of sodium metabisulfite and after a certain dosage efficiency becomes constant. Based on these infdrmation a treatment, plant has been designed. Since flow rate is very small, batch treatment was preferred to continuous treatment. Conventional tanks type treatment system has been designed for mixing reagents and settling. Initial construction cost is about Rs. 10,000/- and per day treatment cost is Rs. 135/-.