Of the wet salted hide purchased by the tannery only 30-35% is potentially convertible into leather and of this amount, only some 30% is eventually converted into high quality leather with further 10% yielding lower grade material.
The rest of the hide together with excess processing chemicals and the large volumes of water employed form the solid and liquid residues.
1. Gaseous Wastes:
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Tanneries discharge into the atmosphere odorous gases, smoke and dust. The main source of smells in a tannery is the compounds containing nitrogen and sulphur. The end products of anaerobic decomposition or putrefaction of proteins include indole, skatole, mercaptans and miscellaneous aldehydes, all of which are odorous.
The other odour producing compounds in tanneries include sulphide, fatty acids like butyric acid, valeric acid and caproic acids, solvents, lacquers, formalin and some of the chemicals used in finishing operations.
Smells in tanneries intensify from unhygienic practices in skin and hide processing and delayed disposal of liquid and solid wastes. In many tanneries it is the foul odour which emanate from the putrescible solid and liquid wastes which account for much of the smell traditionally associated with the tanneries.
Immediately after the hide or skin is removed from the animals, decay starts unless it is cured properly. The operations like soaking, liming, deliming, fleshing etc., are the most disagreable steps in leather manufacture.
They involve use of bad smelling materials and production of putrescible organic matter like soak pit sludge, lime sludge, green fleshings, and limed fleshings. During soaking, the removal of curing salt and dehydration of the skin introduces the possibility of bacterial growth and protein putrefaction.
Many unhairing systems in practical use are based on balance between sodium sulphide, sulphehydrate, dimethylamine sulphate and sodium hydrosulphide. Some of these sulphides have potential to liberate hydrogen sulphide when mixed with pickling liquors. Hydrogen sulphide is an extremely bad smelling gas which can be detected in concentrations as low as 0.1 ppm.
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Hydrogen sulphide and organic sulphides cause odour nuisance when present in the air at concentrations of 10 to 150 times smaller than the lowest concentration of sulphur dioxide detectable by smell. There is a possibility of hydrogen sulphide generation during deliming from the sulphides sticking to hide if the limed hides are not properly washed.
Indiscriminate disposal of liquid and solid wastes generated during tanning operations may also give rise to considerable smell. Solid wastes produced during beam house operations are highly putrefactive.
The composite liquid waste let out from a tannery contains large amounts of putrescible organic matter and is highly polluting in nature. Although the relatively high pH of the composite wastes tends temporarily to inhibit putrefaction, the wastewaters are inherently putrescible and eventually become highly offensive. Sludge produced during primary or biological treatment of tannery effluents can also give rise to smells.
2. Liquid Wastes:
In the development of the tanning industry, water plays a vital role as the industry consumes large quantities of water. Approximately 30-40 litres of water are used for processing one kg of raw hide/skin into finishing leather. Most of the Indian tanneries which are located near the river banks or natural water bodies draw surface water.
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Ground water from their own open wells/tube wells existing within their premises is also used by some tanneries. Most of the traditional tanneries do not have overhead water tanks for proper distribution system. Water is being pumped directly to the process and in a few tanneries, it is stored in open cement lined pits and ground level tanks.
In general, the quantity of water usage and nature of wastewater discharge varies from process to process and tannery to tannery and from time to time. Most of the discharges are intermittent.
The average water usage and wastewater discharge per kg of hide/skin for different process are as follows –
(i) Raw to E.I – 25-30 1/kg of raw weight;
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(ii) Raw to wet blue – 25-30 1/kg of raw weight;
(iii) Raw to finish – 30-40 1/kg of raw weight;
(iv) E.I. to finish – 40-50 1/kg of E.I. weight; and
(v) Wet blue to finish – 20-25 1/kg of wet blue weight.
Most of the tanneries neither have proper drainage system for collection of the wastewater nor any effluent treatment system. The wastewater is discharged from various sectional operations intermittently and it takes its own course to the nearby low-lying area neighbouring land, pond, street, roadside etc.
Characteristics of Wastewater:
Characteristics of the effluents vary from tannery to tannery and in any one tannery with respect to time. The wastewater from beam house process viz. soaking, liming, deliming etc. are highly alkaline, containing decomposing organic matter, hair, lime, sulphide and organic nitrogen with high BOD and COD.
The wastewater from tanyard process viz. pickling, chrome tanning are acidic and coloured. Vegetable tan wastewater contains high organic matter. The chrome tanning wastes contain high amounts of chromium mostly in the trivalent form.
The details of the tanning operations, water and other chemicals used, general constituents in the wastewater are furnished in Table 21.1.
Characteristics of the Sectional Wastewater:
The characteristics of the sectional wastewater from the beam house operations viz. soaking, liming, deliming, are given in Table 21.2. Characteristics of the sectional wastewater from tanyard operations viz. pickling, chrome tanning, vegetable tanning, myrob liquor are furnished in Table 21.3. The characteristics of sectional wastewater from finishing operations viz. neutralisations, dyeing and fat liquoring are furnished in Table 21.4.
Characteristics of the Composite Wastewater:
The characteristics of the composite wastewater are governed by the following factors:
(i) Intermittent discharge of wastewater from different sectional operations.
(ii) Wide variation in the volume and quality of wastewater from section to section.
(iii) Partial operations in one tannery and balance operation in another tannery.
It would be difficult to arrive at a realistic characteristic range of the composite effluents to be discharged by various tanning units. However, from the analysis of the wastewater samples collected from various tanneries located in Calcutta and Tamil Nadu region, the general characteristics range of the composite wastewater from raw to finishing process is given in Table 21.5.
The wide variation of BOD, COD, chromium, sulphide and other parametres exhibited in raw to finish composite wastewater is due to the variation in the process, changes in the type, quantity and quality of chemicals used for the process, fluctuations in the volume of water used for process and washings.
3. Solid Wastes:
Solid wastes generated in a tannery can be classified under four heads viz.:
(i) Non-proteinous wastes.
(ii) Non-collagenous protein wastes.
(iii) Untanned collagen.
(iv) Tanned collagen.
Some of these solid wastes or by-products find consumers in the gelatine, glue, textile and artificial leather industries. It is estimated that 33% of the original protein material leaves the tannery in the form of effluent sludge, fleshing, trimmings, shavings etc., of which the greater part would appear as solid wastes. Solid wastes may be produced in a tannery in varying quantities from the following processes.
The salted, “green'” or dried hides are trimmed to remove irregular matter which will produce raw trimmings. Salted hides are then dusted. This operation will produce salt dust containing impurities like blood, dung etc.
Following soaking, the skins or hides are again trimmed. The method of (using lime sulphide, enzymes, or dimethylamine) will determine whether the hair will be recovered as solid waste or pulped into the effluent. Lime sludge is produced in the lime pits which is either discharged into the effluent drains or removed as solid wastes.
After liming, hides and skins are fleshed, producing a waste of flesh with a certain amount of hide pieces. The dehaired and fleshed pelts are then passed through deliming, pickling and tanning operations.
The tanning material may be vegetable, chrome or other agents. In case of vegetable tanning, solid wastes generated are spent tan liquor sludge. After tanning, the leather undergoes a number of machine-shop operations, such as shaving, splitting, buffing, trimming etc., resulting in tanned solid wastes such as split pieces, trimmings and buffing dust.
Sludge is obtained as solid waste from equalisation and settling of wastewaters from different sections of tannery and the biological treatment of tannery wastewaters. All these solid wastes have to be either utilised or disposed off safely without affecting the environment.
Certain solid wastes are utilised in Indian tanneries, due to lack of sufficient market, some of these wastes are discharged into the environment.
Characteristics and Effects of Wastes:
The main pollutants namely BOD, COD, TS, chlorides, sulphates, sulphides and chromium are estimated in terms of kg per tonne of raw hides or skins processed into finished leather. Total average daily discharge of pollutants from the Indian tanning industry is estimated in tonnes (Table 21.6).
The composite wastewater from raw to finishing process is alkaline (pH 7-9) with average contribution of about 575 kg of total solids, 465 kg of dissolved solids, 240 kg of chloride, 135 kg of COD, 100 kg of sulphate, 65 kg of BOD, 7.5 kg of chromium and 4 kg of sulphide per tonne of raw hides/ skins processed into finished leather.
The total pollutional discharge from the Indian tanning industry is high in terms of dissolved solids and chlorides estimated at about 860 tonnes and 360 tonnes per day respectively. The BOD and COD discharges are estimated as high – 100 tonnes and 200 tonnes, respectively, per day.
In addition to other pollutants like sulphate, sulphide etc. about 11 tonnes of chromium is also discharged which causes problems in the disposal of sludge collected in the treatment plants besides involving wastage of a costly chemical.
The number of tanneries in Tamil Nadu is about 600 which is only 30% considering the total number of 2000 tanneries all over India. But the pollutional contribution is more than 45%. Similarly, in U.P. the number of tanneries is about 200 which is only 10% considering the tanneries all over India. But their pollutional contribution is 20%.
This is due to the concentration of more large-scale tanneries in Tamil Nadu and Uttar Pradesh compared to the other States. It is also interesting to note that though the number of tanneries in Karnataka is about 180 which are 9% of the 2,000 total numbers of industries, but their pollutional load is only 3%. This is due to the fact that the tanneries in the State are mostly cottage and small-scale units.
Impact of Gaseous Pollutants on Environment:
The problems associated with gaseous pollutants from a tannery include noxious smell, hydrogen sulphide and dust. The effect of noxious smells on people is primarily a nuisance effect.
The loss of property values near poorly operated tanneries is partly a consequence of offensive odours. Dust problems normally arise in tanneries from leather buffing operations.
Leather dust of finer sizes is reported to be harmful to human health and comfort. Hydrogen sulphide which is liberated during some of the tanning operations is an irritant gas and has a very bad smell. The maximum allowable concentration for 8 hours exposure in working areas is 20 ppm.
Impact of Liquid Wastes on Environment:
Tannery effluents contain vegetable tannins and non-tannins which exert oxygen demand. They also contain high amounts of protein, especially when a hair pulping unhairing system is used.
These proteins are biologically degradable and exert high BOD. About 79-80% of the total dissolved volatile solids in the settled and filtered composite tannery wastes are composed of organic matter present in the form of proteins, fatty acids, ether solubles and tannins and 20-21% composed of other organic compounds.
Chlorides, trivalent chromium, nitrogen, phosphorous, sulphate, ammonium salts, lime etc., are the inorganic pollutants present in significant quantities. These pollutants are of more permanent nature, unless they are in suspended state or precipitated from solution and settled.
Using normal tanning techniques chromium will be present in the trivalent form which will be precipitated in the mixed effluent. The discharge of untreated tannery effluents effects streams, groundwater, land and sewer in which they are discharged.
The discharge of untreated wastewaters in water courses may affect the physical, chemical and biological characteristics of the water and deplete dissolved oxygen from the water bodies.
The high oxygen demand of tannery wastes is due to proteins, fatty matter and tannins. High pH, excessive alkalinity, suspended matter, sulphides are injurious to fish and other aquatic life in streams.
Sulphides present in tannery wastewaters can cause unpleasant odour problems, react with iron and other metals causing black precipitate, render the water unfit for industrial uses and affect fish and other aquatic life.
The sulphide toxicity to fish increase as the pH value is lowered. Nitrogen and phosphorus from tannery effluents encourage uncontrolled growth of algae and other aquatic plants in water bodies.
High amounts of chloride present in tannery wastewaters can make the receiving water less suitable for drinking, industrial and agricultural purposes.
Suspended solids both inorganic and organic present in tannery wastewaters may settle in a stream and affect fisheries by covering the bottom of the stream thereby destroying bottom fauna necessary for fish as food or reduce the spawning ground of fisheries.
In general the toxic effects of chromium salts particularly hexavalent chromium salts towards aquatic life varies with species, temperature, valence of chromium and the complex synergistic and antagonistic effects due to other factors such as hardness.
The effluent from vegetable tanning is coloured and contain some amounts of non-biodegradable matter. When these wastewaters are discharged into streams it is reported that the colour attributed persisted for a long time.
The discharge of untreated tannery wastes into water course may also increase the turbidity of water thereby reducing light penetration and impairing photosynthetic activity of aquatic plants.
Groundwater has been found to be affected where wastewater from tanneries is ponded or lagooned spread out on land or discharged into dry river beds. The groundwater is reported to be rendered unfit for drinking and irrigation where the tanneries are concentrated together.
When the tannery waste gains access to cultivable lands or when the lands are irrigated with such waste, fertility of the soil is reported to have been affected. It may change the characteristics of the soil and interfere with the intake of water by plants. When tannery wastewaters are applied to land, the soil productivity decreases.
The problems associated with discharge of tannery wastes into municipal sewers, include incrustation of sewers, sewer clogging and other forms of interference with sewage treatment. Tannery effluents are known to cause deposition of calcium carbonate with the consequent choking of the receiving sewer.
Lime is converted to calcium carbonate by the carbon dioxide produced by decomposition of the organic matter present in the effluent and the hair and fleshings help to form a binder with this calcium carbonate which firmly adhere and build up gradually on the surface of sewer.
Concrete sewers are likely to suffer damage when they are made to carry sewage containing a high concentration of hydrogen sulphide due to admixture with tannery wastes.
If the tannery waste concentration in domestic sewage is higher, the interference with waste treatment operation could be due to:
(i) Excessive alkalinity or pH.
(ii) Hair and fleshings which form scum.
(iii) Lime sludge and adhering deposits.
(iv) Higher concentration of sulphides and chromium.
(v) Excessive loads of organic matter.
Impact of Solid Waste on Environment:
Generation of solid wastes generated during various tanning operations are not properly utilised or disposed they are likely to cause a number of problems. Salt dust if stored in heaps outside the tanneries is likely to be washed away during rains and cause groundwater pollution.
Hair and lime sludge if discharged along with the effluents are likely to choke the drains. Raw and green fleshings, limed fleshings, splits and trimmings putrify easily and give rise to noxious smells.
In many tanneries, it is the foul odour which emanate from some of these putrescible solid wastes which account for much of the smell traditionally associated with tannery wastes. Vegetable tan bark and vegetable tan sludge if discharged into effluent stream can cause problems in effluent treatment.
Vegetable and chrome tanned shavings and splits do not easily decompose. If they are not utilised, problems of disposal are encountered. Primary and secondary sludges obtained during the treatment of tannery wastes are also putrescible.
The methods adopted for disposal of solid wastes from tanneries vary considerably from area to area and country to country. In general where tanning industries have long been established there usually has been a simultaneous growth in industries which can use these wastes.
This pattern is specially helpful to the tannery industry in the case of obnoxious rag and limed fleshings, trimmings and splits which are lifted by glue, gelatine and fat rendering units.