Treatment of Effluents of Tanning Industry | Environmental Engineering

In this article we will discuss about the treatment and disposal of effluents of tanning industry.

There are large numbers of tanning industries in India. Most of these industries are located in U.P., West Bengal and Tamil Nadu. Due to large export market of leather goods, the number of tanneries increased to cope with the increasing demand.

Due to unplanned growth of these industries, especially the small and cottage scale tanneries in villages and out skirts of towns and owing to lack of facilities for proper disposal of its effluents, serious water pollution and insanitary conditions have taken place. The local authorities and public health department have been greatly concerned about this situation to this problem.

Description of Processes Involved in Tannery:

1. Raw Materials:

In the tanning industry a number of raw materials and chemicals are used. Hides and skins are the principal raw materials. Common salt, wetting agents, lime, sodium sulphide, ammonium chloride, ammonium sulphate, enzymatic products, sulphuric acid, sodium carbonate, dyes and sulphonated vegetable oils are the chemicals used in tannery.

The tanning agents used are vegetable tanning materials, basic chrome or syntans. The amount of these raw materials and chemicals used per unit mass of hide or skin processed varies from tannery to tannery.

2. Beam House Processing:

Hides and skins are usually received in the tannery either in wet-salted or dry-salted condition. They are first soaked in water in pits for 16 to 24 hours with wetting agent. The present trend in some of the tanneries is to use either paddles or drums for the soaking process, which reduces the time of soaking and also economises on water consumption.

After soaking, the hides and skins are given a paint of lime and sodium sulphide on the flesh side. At the end of about 4 to 6 hours the skins are un-haired and put in a suspension of lime in pits along with a small quantity of sulphide. This process is meant for swelling the hide and skin and for loosening the hair. To obtain a uniform thickness of the hide and skin and to clean its surface, the hides and skins then undergo a process known as unhearing and fleshing.

This is done by hand or in machines. After removal of hair and fleshing, the hides and skins are washed, and delimed using either ammonium sulphate or chloride and again washed and scudded. Hides and skins meant of chrome tanning are subjected to a process called ‘bating’ which involves the treatment of the stock with a bating agent, the principal constituent of which is a proteolytic enzyme.

3. Tan Yard Processing:

a. Vegetable Tanning:

Skins and hides after the process of deliming and washing are put in an infusion of vegetable tanning material contained in pits and the goods stay in the vegetable tan liquor for about 10 to 15 days by which time the process of tanning is completed.

With the advent of vegetable tanning extracts and syntans the process has been made simpler by drumming the goods in a concentrated solution of the extract the syntans for a few hours only. Vegetable tanned leathers are then put in myrobalan liquor to impart colour and later oiled with a liberal coat of any suitable oil and finally dried.

b. Chrome Tanning:

After bating, the skins and hides are pickled in sulphuric acid and common salt and later chrome tanned using a solution of basic chromium sulphate or chloride. Chrome liquor used for tanning is prepared by mixing sodium dichromate and sulphuric acid and subsequently reducing the production by the addition of molasses or similar organic material. But now-a-days readymade chrome liquor is available in the market which can be straightaway used. Picking and chrome tanning arc usually carried out in the same drum.

After chrome tanning which takes about 6 hours, sodium carbonate is added to the drum to fix the chrome. The goods are then taken out and piled for 24 hours for complete absorption and fixation of chromium in them.

4. Fat-Liquoring and Dyeing:

The leather are then washed. neutralised, dyed and fat- liquored with an emulsion of sulphonated oils and finally finished into various types of leathers. The leathers are dyed with different dyes in the final stages of finishing.

Sources, Quantity and Characteristics of the Effluents:

1. Soaking Effluent:

Soak liquors contain soluble proteins like albumin and are a major source of proteolytic and other bacteria. Suspended matters like dirt, dung, and blood adhering to the hides and skins are discharged intermittently with the soak liquor. The salinity of soak liquor is very high varying from 15000 to 20000 mg/l(as CI).

Soak liquors undergo putrefaction very rapidly since they contain a large amount of impurities and a favourable pH value for the growth of bacteria. The biochemical oxygen demand (BOD) of the soak liquor is usually between 1100 to 2500 mg/l and the quantity of soak liquor discharged will be 250 to 400 litres per 100 kg of hide or skin tanned.

2. Liming Effluent:

Lime liquors contain suspended and dissolved lime and sodium sulphide. The discharge of spent lime liquor is intermittent and its average quantity is about 650 to 1000 litres per 100 kg of fractional in terms of biochemical oxygen demand (BOD) and suspended solids.

This waste may contain high concentration of sulphides when sodium sulphide is used in the liming process. This waste contains high ammoniacal nitrogen content. The BOD varies from 6000 to 9000 mg/l.

3. Un-hairing and Fleshing Effluent:

The effluent from the un-hairing operation is more or less continuous and contains mostly hair and sulphides. Fleshing operation gives rise to an effluent which is also more or less continuous and contains fatty and fleshy matter in suspension.

4. Deliming Effluent:

Spent deliming liquors which are discharged as waste also carry a significant pollution load in terms of BOD. The BOD of this waste varies from 1000 to 2000 mg/l. The quantity of waste discharged is about 700 to 800 litres per 100 kg of hide processed.

5. Spent Bate Liquor:

It has a high amount of organic matter and ammoniacal nitrogen due to the presence of soluble skin proteins and ammonium salts used in bating. BOD is usually low.

6. Spent Pickle Liquor:

These wastes contain high amounts of salt and are acidic. BOD of the waste ranges from 500 to 900 mg/l.

7. Vegetable Tanning Effluent:

The spent vegetable tan liquor is probably the strongest fraction in a composite tannery effluent. Although its discharge is intermittent and constitutes about 10 percent of the total volume of the effluent, its BOD ranges from 6000 to 12000 mg/I.

It has persistent colour which is difficult to remove by chemical or biological method and a characteristic offensive odour. It is acidic in nature. The effluent volume is about 200 to 400 litres per 100 kg of hide or skin processed.

8. Chrome Tanning Effluent:

The spent chrome tan liquor is greenish in colour and highly acidic. The waste contains a high concentration of trivalent chromium ranging from 100 to 200 mg/l. Hexavalent chromium is not generally present in the waste chrome liquor because of the reducing agent used and one-bath process utilised. The average discharge of the waste chrome tan liquor is about 400 to 500 litres per 100 kg of hide of skin tanned. BOD of the waste is usually about 1000 mg/l.

9. Dyeing and Fat-Liquoring Effluent:

The effluents from the dyeing and fat-liquoring processes are generally small in volume and discharged intermittently. The principal components are dye and oily emulsions. Major part of the dye and fat-liquor is fixed on the leather.

10. Composite Effluent:

The composite effluent from a tannery is highly coloured and foul smelling. It is highly alkaline with high amount of suspended and dissolved impurities. BOD of the effluent varies from 2000 to 3000 mg/l.

11. The typical analysis of the individual and composite effluent is given in Table 17. 1.

Pollutional Effects:

1. Effect on Streams:

The constituents generally present in a composite tannery effluent are varied in character and load, and damage a stream when discharged into it untreated. Tannery effluent contains considerable amounts of protein when a hair-pulling unhearing system is used. These proteins are biodegradable and cause a very high oxygen demand.

Spent vegetable tan liquors contain significant portion of non-biodegradable materials which can persist in the streams. The salt and hydrogen sulphide present in tannery effluent may adversely affect the stream quality and cause bad taste and odour.

The effluent carries large amounts of suspended matter in the form of lime suspension, hair fleshing’s, etc. which are detrimental to the utility of the stream in that they can make the water turbid and settle on the bottom, thereby affecting fisheries by covering the bottom of the stream and destroying fish food, bottom fauna or the spawning group of fish.

The effluent from a vegetable tannery is highly coloured, and when discharged into a stream, the colour may persist for a long period. Chrome tan effluent is highly toxic to fish and other aquatic life. Excessive alkalinity, high pH and sulphides may also be transmitted to bathers coming in contact with tannery effluent discharged into a stream.

2. Effect on Land:

The productivity of the soil is found to be decreased when tannery wastes are applied on fields, and some parts of the land become completely infertile. Germination of paddy seeds was found to be not satisfactory.

3. Effect on Ground Water:

Discharge of tannery effluents on land adversely affects ground water due to presence of high amounts of chlorides, chromium etc.

4. Effect on Sewers:

Tannery effluent is known to cause deposition of calcium carbonate inside a receiving sewer and choke it. Lime is converted to calcium carbonate by the carbon dioxide present in the effluent and the hair and fleshing’s help to form a binder with this calcium carbonate which firmly adhere and build up gradually on the inside surface of sewer and ultimately choke it.

Difficulties with sewer chokage have been experienced in many places. 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.

Methods of Treatment and Disposal of Tannery Effluents:

1. Prevention and Replacement of Wastes:

i. Reduction of Water Usage in a Tannery:

As in all waste control programmes, good housekeeping is the first step to prevent wastage of water and materials in a tannery. Economical use and reuse of water are necessary to reduce the volume of the effluent.

The water usage in tanneries can be considerably reduced by:

(a) Better housekeeping,

(b) Alteration of processes and low float systems to use less water,

(c) Separation of cleaner fractions of the waste for direct reuse without treatment, and

(d) Recycle after complete or partial treatment.

The adoption of batch washing as an alternative to continuous rinsing using the lattice door can drastically reduce water consumption and has been implemented in European tanneries where it has claimed to have halved 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.

Direct reuse is possible in a tannery since some wash waters are relatively clean, such as from washing after beating, pickling, neutralizing and dyeing which can be used for dirty tasks such as washing after soaking and liming and floor washing.

ii. Recovery and Use of By-Products:

As far as possible attempts should be made to separate out and reclaim all solid wastes in a tannery, such as, hair, fleshing, trimmings, shavings lime sludge, tan-liquor sludge, tanbark, tanned trimmings and shavings. These materials have market value.

(a) Higher final basicity,

(b) Increased temperature,

(c) Shorter floats, and

(d) Longer time of process.

By adopting rapid pit tan fixation of vegetable tanning in leather during vegetable tanning can be improved. Modern technology has long accepted the possibility of combining the fat-liquor and dyeing processes using low float levels and no great problems are encountered in producing a low level of effluent with low pollutant levels. Judicious pH adjustment allows almost 100 percent fixation of reagents during dyeing and fat-liquoring.

iii. Segregation of Salt Laden Effluents:

The salt laden effluents from the soaking and picking operations should preferably be segregated and allowed to evaporate in a shallow impervious pond. Dusting the salted skins carefully will reduce the amount of salt going into solution. Soaking the dusted skins initially in a small volume of water so as to dissolve the salt out in this volume; will reduce considerably the volume of salt laden effluent.

iv. Salt collected after dusting of salted hides and skins should be collected and allowed to go into the liquid wastes. Such salt should be disposed off as solid waste and necessary precautions should be taken to see that it is not reused either for salting of hides or for human consumption.

2. Methods Recommended for Adoption in India:

Reviewing the method mentioned above it is recommended that a complete treatment of effluents of tannery effluents would consist of the following:

(a) Segregation of salt bearing wastes and chrome tan wastes,

(b) Primary treatment of effluents, and

(c) Secondary treatment of effluents.

1. Segregation:

Segregation of various effluents according to their characteristics simplifies the treatment and wherever possible such segregation should be effected. This aspect should be kept in view by new tanneries at the stage planning and layout. Wastes from soaking and pickling should preferably be segregated and allowed to evaporate in a shallow impervious pond. Chrome tanning effluent may be segregated and mixed with lime liquor.

2. Primary Treatment:

This involves removal of suspended solids and partial reduction of BOD and other harmful constituents. Screening for removal of coarser impurities and hair and settling for at least 4 hours in continuous tank form the essential primary treatment of tannery wastes. The sludge from settled vegetable tannery effluents can be dried on sand beds and used as fertilizer for growing forage crops or a soil conditioner.

The sludge from settlement of chrome tannery effluents, after drying, may be used as land fill or disposed off by dumping on a fallow land. Hydrogen sulphide from lime liquor may be removed by aeration using manganese salts as catalyst. In land disposal of dried sludge approval of local authorities should be taken to avoid pollution of ground water.

3. Secondary Treatment:

Secondary methods that can be used for treating settled tannery waste include aerated lagoon or anaerobic lagoon followed by aerated lagoon. Under Indian conditions anaerobic lagoon followed by aerated lagoon with minimum detention of 10 days and 6 days respectively would give good results and the treated effluent could be disposed off. Fig. 17.1 shows flow sheet of the treatment.

In Indian urban areas where the facility of sewerage system is available the above primary treatment of tannery effluent would seem to be adequate for final disposal along with city sewage.

In rural areas the problem is difficult due to non-availability of sewerage. In such a situation both primary and secondary treatment as detailed above would be necessary depending upon the mode of ultimate disposal.