Technologies for Reduction of Chrome Discharge:
Chromium Tanning salt is used today in more than 85% tanning operations. The Trivalent form is generally used. Chrome wastes are of great concern.
The conventional chemicals and processes used in tanning have a low utilization potential for chromium uptake with hardly 40 -70% of the applied salt being taken up by the leather. This results in the balance finding its way to effluents and creating environmental problems. As per an estimate for 1998, 45000 tonnes of basic chrome sulphate were used out of which 18000 tonnes were released in the waste water streams. It is, however, now possible to increase the uptake of chromium to nearly 98-99%. More than 500 plants have been installed in the India for recovery and recycle of Chromium salts. Cleaner chrome tanning methods based on high exhaustion principles have been evolved. Closed pickle tan loop method gives a saving of about Rs. 2000 per tonne of leather produced. Chromium recovery can be achieved either by a direct recycling of the spent liquor by adding make up quantities. Salts and other impurities tend to accumulate and may have an adverse effect on quality of leather. Recovery of Chromium by alkaline precipitation and consequent dissolution in Sulphuric acid and a reuse of the resultant solution is an efficient methodology. The highly reactive alkalis may give a voluminous sludge . Lime may also give Calcium Sulphate which makes the reuse of chromium difficult. Using MgO as alkali is considered more appropriate for small as well as large tanneries because of ease of operation and low investment costs.
Use of Chromium free tanning has also been advocated as a clean technology option. Synthetic organic tanning agents alone or in a combination with metallic cations can be considered as a substitute for chromium. Vegetable tanning has also got a high pollution potential because of the low biodegradability. Recovery of vegetable tanning floats has been used by ultrafiltration in many European countries and the recovered tannins can be used in the tanning process. Vegetable and aluminum tanning can also produce chrome free leather.
Within the options of clean technology during post tanning operations, absence of chromium during retanning and not using polluting dyestuffs, benzidene dyes and halogenated oils in fat liquors are strongly advocated.
The finishing processes should ideally work on water based finishes. Pigments should not contain heavy metals.
With up to 40 tonnes of water being used to process 1 tonne of hide, there is a need to conserve water. This can be achieved by reduction through low float processing, batch type washing instead of rinsing and combining processes and may give a reduction of about 30% in the water consumption. The recycle of soaking, liming,unhairing, pickling and chrome tanning liquors can also reduce the overall water consumption by 20 -40%. Treated effluents can be also used through membrane filters to further reduce the water consumption.
Utilising Solid Wastes from Tanneries:
Significant amount of solid wastes are produced including trimmings, degraded hide and hair from the beam house process. The solid wastes can represent upto 70% of the wet weight of the original hides. Large amounts of sludges are also generated.
Solid wastes can be utilized to make dog toys, gelatin etc.
The salt recovered from mechanical desalting and solar evaporation pans can be used for curing and pickling or can be disposed off into the sea where feasible.
Fleshing and solid sludge from primary and secondary treatment processes can be utilized to produce biogas. Tannery fleshings which are the major solid wastes emanating from the beam house can be subject to biomethanation. The fleshings are liquefied completely biologically and the resultant liquefied fleshing is treated in anaerobic reactors to produce biogas. Any anaerobic reactor like a U.A.S.B. can be used. The technology is being implemented at Melvisharam, Tamil Nadu. This technology developed by the Central Leather Research Institute is also used for biomethanation of slaughter house and vegetable market wastes.
Sludges from the treatment plant can also be used in brick manufacture, land filling of low lying areas and also as a fertilizer(given that the Chrome content meets the prescribed standards). A special “oxidative” firing and a “reductive” cooling technology has been developed to manufacture bricks ensuring a safe deposition of chromium without any leaching in Industrial Bricks.
Hair could be used in the manufacture of low priced rugs and carpets. Hair protein can be used as a supplement in animal feed. In England, waste hair has been used in the production of flower pots that are easily biodegradable, hair hydrolysate has found limited applications in the pharmaceutical and cosmetics industry, as a thermal and acoustic insulation material and as a manure(30%) supplement with household and garden refuse(70%).
Sludge from lime pits can be used for land filling as well as construction of low priced houses.
Barks and nuts from vegetable tanning could be used as fuel in boilers and brick kilns. The shavings, trimmings and buffings of vegetable and chrome tanned leather could be used in the manufacture of leather boards.