THE tanneries in India have a reputation that they can do without. In Vellore, Tamil Nadu, the tanneries are working to save their reputation by generating green electricity . They have set up a biomethanation plant which uses the fleshings produced by them to generate electricity for its common effluent treatment plant (CETP). By installing the plant the tannery has been able to do two things -- one, reduce the stench of putrefaction and pollution that the fleshings used to produce, and two, generate electricity from waste.
In India, there are around 3,000 tanneries and more than one third of them are located in Tamil Nadu. These tanneries, mainly concentrated in Melvisharam, process 300,000 tonnes of hide and skin per year and generate around 140 tonnes of fleshing per day. Fleshings are the flesh scrap generated during the process of conversion of skins and hides into leather. With proper means of disposal absent, these fleshing -- a health hazard -- are often thrown indiscriminately, creating an obnoxious smell and an unsightly appearance.
Earlier, the fleshings were used to manufacture glue, but the market for it declined with the emergence of synthetic glue. They were then disposed in landfills, but this contaminated the groundwater, causing the total dissolved solids in groundwater to go as high as 4,900 mg per litre in certain areas, which is about ten times beyond the permissible level. Incineration too had its own problems. "The quantity was too large to manage, it gave an obnoxious odour," says Alwar Ramanujam, assistant director, department of environmental technology at the Central Leather Research Institute (CLRI) in Chennai.
But with the biomethanation plant in place, the people living in the tanneries' vicinity will be able to live in a less polluted environment. The capital cost of the plant is Rs 1.57 crore. The Union ministry of non-conventional energy sources has paid 60 per cent of it, with United Nations Industrial development Organisation providing another 17.5 per cent. The Indian Renewable Energy Development Agency and other beneficiaries have met the remaining cost. "The concept was new to India, so everyone was apprehensive in the beginning," says Ramanujan.
The biomethanation plant, which began operation in January 2000, is designed by a French engineer, Michel Aloy and maintained by around 15 tanneries with the technical assistance from CLRI . It has two digesters of 130 cubic metre capacity and is designed to process five tonnes of waste per day -- three tonnes of fleshings and two tonnes of primary sludge from the treatment plant. The fleshings and other solids are collected daily by trucks from the tanneries and deposited at the plant. The fleshings are then minced to peices of about six micron diameter and then mixed with the primary sludge. After it is homogenised, the mixture is fed into a feed chamber.
The primary sludge from the CETP , which contains 90 per cent liquid, is used to run the plant, thus solving the need to use large quantities of clean water. Operating a biogas plant usually requires equal amount of water and fleshing. "Fortunately, the fleshings also contain around 80 per cent liquid," says P A Shanmugan, senior scientist at CLRI .
From the feed chamber, the substrates are transferred to the first digester. It takes 26 days to fill both the digesters, after which five cubic metres of the substrate is taken out from the second digester and a similar amount is added to the first digester. The fleshings are retained in the digester for 26 days. Biological process then takes place inside the digester at 32-34 C. The bacteria converts organic pollutant to methane. A safety valve releases the gases produced. Apart from lime, which is used to neutralise the acidic content of the flesh, no chemicals are used. The remaining scum is taken out by a centrifugal pump, which separates the solids from the liquid, from the top of the digester. The solid material is then directed back to the bottom of the digester.
The plant generates around 312 cubic metres of gas and 1,200 kwh of power daily, out of which 250 kwh is used to operate the plant. The remaining 950 kwh is used to meet the partial requirement of the CETP . According to Shanmugan, the CETP consumes 7,500 kwh of energy per day.
Though there are around 36 tanneries in the locality that generate a total of around 12-13 tonnes of fleshings per day, the plant can take only upto five tonnes right now. But plans are afoot to set up more on the same lines.
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