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Tanning Industry

 

Tanning Process 
Sources of Waste Water and their Characteristics

 

 

Skins of cows and buffalos are called "Hides". Skins of goats and sheeps are called "Skins". Tanning is the art of converting animal skins into leather.The tannery wastes are characterized by strong color, high BOD, high pH and high dissolved salts. The tanning process consumes large quantities fo water. Most of this water used is finally discharged as waste water carrying high amounts of suspended solids, dissolved solids, BOD, COD, strong colour, bad smell. Depending upon the type of process 30-40 litres of water is used to process each kilogram of raw hides or skin into finished leather.

 

Tanning Process :

The tanning process consists of three basic stages

                            1.      Preparation of the hides for tanning.

                            2.      Tanning proper.

                            3.      Finishing.

Tannery wastes originate from the beam house and the tanyard.

In the beam house curing, fleshing, washing, soaking, dehairing, lime splitting, bating, pickling and degreasing operations are carried out. In the tanyard, the final leather is prepared by several processes. These include vegetable or chrome tanning, shaving and finishing. The finishing operation includes bleaching, stiffing and fat liquoring and coloring.

Protective treatment administrated soon after the hides and skines are flayed is called curing. Curing involves dehydration of the hide by drying it with salt or air in order to stop proteolytic enzyme degradation. Curing creates an envionment for the hides and skins in which the protein destroying organism cannot  function. Its sole purpose is to ensure that the hides and the skins are protected during transit from slaughter house to the tanneries which are generally located some good distance away. It also facilitates storing.

Fleshing removes the fatty tissues from the skin by mechanical means. Cured hydes and skins arriving at tannary are trimmed to remove long shanks and other unwanted areas.

Washing and soaking remove the dirt, salts, blood, manure and nonfibrous proteins and restore the moisture lost during preservation and storage.

             Unhairing is accomplished by the use of lime, with or without sodium sulfide, this makes the skin more attractive and more amenable to the removal of trace protein impurities.

Lime splitting separates the skin into two layers, one is more valuable grain layer the other the lower or flesh side is called the split.

Bating prepares the hide fir tanning by reducing the pH, reducing the swelling, peptizing the fibres and removing the protein degradation products. 

             Bating is generally accomplished with ammonium salts and a mixture of commercially prepared enzymes, the bathing bath renders the grain sticky, slippery, smoother and more porous increases its width and decreases its wrinkles.

             Pickling generally precedes chrome tanning and involves treatment of the skin with salt and acid to prevent precipitation of the chromium salts on the skin fibers.

              Degreasing removes natural grease, thus preventing formation of metallic soaps and allowing the skin to be more evenly penetrated by tanning liquors. Chrome tanning is used primarily for light leathers while vegetable tanning is preferred for most heavy leather products. The process of chrome tanning is of shorter duration and produces more resistant leather.

The second stage of leather making, the tanning proper, involves the treatment of the hides to make them non putrescible and even soft when dried.

In chrome tanning process the tanning is done in the same vat after one day of pickling  by adding a solution of chromium sulphate. After 4 hours of tanning, the leather is bleached with a dilute solution sodium thiosulphate and sodium carbonate in the same bath. The third stage of finishing consists of stuffing and fat liquoring followed by dyeing. In the former process the tanned leather incorporates oil and grease and thus becomes soft and resistant to tearing. Dying can be done using synthentic dye stuffs.

Depending on the type of product, either vegatable substances containing natural tanning eg. the extract of barks, wood etc. or inorganic chromium salts.

Vegetable tanning produces leathers which are fuller, plumper, more easily tooled and embossed and less effected by body perspiration or changes in humidity. Dyeing to produce the final colored leather product is usually done with basic dyestuffs.

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Sources of waste water and their characteristics
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The waste may be classified as continuous flow waste and intermittent flow waste. Continuous flow waste consists of wash wastes after various processes and comprise of a large portion of the total waste, and are relatively less polluted than the other one. Spent liquors belonging to soaking, liming, bating, pickling, tanning and finishing operations are discharged intermittently. Although these are relatively small in volume, they are highly polluted and contain varieties of soluble organic and inorganic substances.The waste water 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 spent soak liquor contains soluble proteins of the hides, dirt and a large amount of common salt when salted hides are processed.The spent liquor undergoes putrefaction very rapidly as it offers a good amount of nutriants and favourable environments for bacterial growth.

The spent bate liquor contains high amount of organic and amonia-nitrogen due to the presence of soluble skin proteins and amonia salts.

The vegetable tan extract contains tannins and also non tannins and high organic matter. Tannins are of high COD but relatively low BOD, while non tannins including inorganic salts, organic acids and salts and sugar are high both in COD and BOD. The waste water from tanyard process viz. pickiling, chrome tanning are acedic and coloured.

The spent pickling and chrome tanning waste comprise a small volume, have a low BOD and contains trace of proteinic impurities, sodium chloride, mineral aids and chromium salts, mostly in the trivalant form.

The spent lime liquor contains disolved and suspended lime and colloidal proteins and their degradation products, sulphides, emulsified fatty matters and also carry a sludge composed of unreacted lime, calcium sulphide and calcium carbonate. The spent liquor has a high alkalinity, moderate BOD and a high ammonia-nithogen content.

Effects of waste on receiving water and sewers :

Tannery wastes are characterized by high BOD, high-suspended solids and strong color. These wastes when discharge as such deplete the dissolved oxygen of the stream very rapidly, due to both chemical and biological oxidation of sulfur and organic compounds. 

A secondary pollution of the stream may occur due to the deposition of the solids near the discharge point and its subsequent putrefication. Presence of tannins in the raw water renders it unsuitable for use in certain industries.

The tannery waste when discharged into a sewer only chokes the sewer due to the deposition of solids, but also reduces the cross- section of the sewer arising out of the lime encrustation.

Treatment of waste :

The method of treatment of waste may be classified as physical, chemical, and biological. The physical treatment includes mainly screening and primary sedimentation. Screens are required to remove fleshings, hairs and other floating substances. About 98% of the chromium is precipitated in the primary sedimentation tanks and is removed along with the sludge.

Chemical coagulation, with or without prior neutralization followed by biological treatment is necessary for better quality of the effluent. Ferrous sulfate is reported to be the best coagulant for the removal of the sulfides and may be used for the effective removal of color, chromium, sulfides, BOD and suspended solids from chrome tan wastes.

Biological treatment of the tannery waste, in activated sludge process, after mixing with municipal waste water in a suitable proportion, and using acclimatized micro organisms capable to reduce the BOD, COD and tannin by about 90%. Trickling filters may also be used for effective removal of BOD, COD and color.

The low cost biological methods of treatment may effectively be used for the treatment of tannery wastes. Both oxidation pond and anaerobic lagoons are recommended for small and isolated tanners. For further improvement of the effluent quality the anaerobic lagoons may be followed by an aerated lagoon.

 

Composition of composite waste water

pH      9.8 - 11.8
Organic solids 1500 mg/l
Hardness 1600 mg/l
BOD 1000 mg/l
Sulfides   1000 mg/l 
Total solids   6170 mg/l     
Chromium 40 mg/l    

 

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