The following article will guide you about the five main waste control measures adopted in tannery industry. The measures are: 1. Good Housekeeping and Water Conservation 2. Recovery and Reuse 3. Process Changes for Reducing Pollution Load 4. Substitution for Penta Chlorophenol (PCP) 5. Typical Wastewater Treatment Schemes.
1. Good Housekeeping and Water Conservation:
As in all waste control programmes, good housekeeping is the first step to prevent wastage of water and materials in tannery. Economical use and reuse of water are necessary to reduce the volume of the wastewater.
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The water usage in tanneries can be considerably reduced by:
(i) Better housekeeping.
(ii) Alteration of processes and low float systems to use less water.
(iii) Segregation of cleaner fractions of the wastewater for direct reuse without treatment.
(iv) Recycle after complete or partial treatment.
The adoption of batch washing as an alternative to continuous rinsing using the lattice door can reduce water consumption. It has been observed that paddles may have some advantages over drums for certain types of production from the point of view of water usage.
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Direct reuse is possible in a tannery since some wash waters are relatively clean, such as from washing after bathing, pickling, neutralising and dyeing which can be used for less important tasks such as washing after soaking and liming and floor washing.
2. Recovery and Reuse:
Segregation of chrome waste stream, chrome recovery and reuse is proved to be one of the viable procedures. The chrome recovery and reuse in medium and large size tanneries in India should be practised because it makes good economical sense and prevents pollution due to chromium.
The general method that may be employed is:
(i) The spent chrome liquor is captured, filtered, precipitated, re-acidified, strengthened and reused as the tanning liquor as depicted in Fig. 21.3.
(ii) Process flow diagram of the chrome recovery plant installed in one of the commercial tannery by Central Leather Research Institute (CLRI) in association with Indo-Dutch Project team under Ganga Action Plan is shown in Fig. 21.4.
As far as possible attempts should be made to separate out and reclaim all solid wastes in a tannery, such as hair, fleshings, trimmings, shavings, lime sludge, tan-liquor sludge, tan bark, tanned trimmings and shavings. Some of these materials have market value. Environmentally sound practices of solid waste disposal are already shown in Table 21.9.
3. Process Changes for Reducing Pollution Load:
It has long been known that the traditional depilation (dehairing) method using lime and sulphide, in addition to being wasteful in regard to chemical usage, is without doubt the largest contributor to high levels of pollution.
Tanneries in advanced countries have for years attempted to utilise new processes, namely enzyme depilation, oxidation processes, dimethylamine or caustic soda as a substitute for lime.
Water can be saved in the deliming, bathing and pickling stages either by recycling of water or regeneration of liquors. Replacement of salt by some other biodegradable chemicals could solve the major pollutional problem of the Indian tanning industry.
Conveyance by refrigerator containers may also be tried to avoid salt problem. Processing fresh hides and skins upto wet blue near the slaughtering place can also be considered.
The British Leather Company, largest privately owned tanning company in the U.K., uses a clean technology for reduction of chromium usage, and in turn, reduction of chromium pollution in its tanneries.
The process is described below:
The level of chromium normally used for high quality leather is between 4% and 5% by weight.
To achieve this, even by the most efficient processing some 30% of the chrome offered to the hide is left in the tanning liquor and wasted. The clean technology employs a two-stage tanning process.
The first stage uses a liquor based on titanium, aluminium and magnesium with no chromium. This is the ICI’TAL’ process. In the second stage, a chromium tan is used with 90% chromium instead of normal 17%.
This results in a leather with a chromium content of about 3% but with characteristics comparable to traditional leather. Residual chrome in the spent liquor is reduced because less chrome is used initially and percentage uptake is greater. The overall effect is to reduce the chromium content of the spent liquor from 1200 to 350 ppm and the level in the final effluent to 10 ppm.
The advantages of this process are:
(i) Chromium level in the discharge is substantially reduced removing a potential constraint on production
(ii) The technology requires no additional capital equipment and can be used with existing plant
(iii) Economic benefits – There are modest savings in tanning reagent costs and no new capital investments. The company can expect to save the £1,60,000 (approx.) which would be required for an abatement plant to achieve the same chromium reduction as that obtained by clean technology.
4. Substitution for Penta Chlorophenol (PCP):
The Government of India has banned the use of pentachlorophenol (PCP) —a suspected carcinogenic chemical used mainly in the treatment of leather. The Central Leather Research Institute (CLRI) has identified another chemical, 2 Thiocyanomethyl, thio- benzothiazole (TCNTB) as a safe substitute.
5. Typical Wastewater Treatment Schemes:
The flow diagrams of typical treatment schemes in some of the tanneries, which are located in Tamil Nadu, are shown in Fig. 21.5 through Fig. 21.8.
