tannery waste and its disposal

1. INTODUCTION ON TANNERY
WASTE
PREPARED BY – JAY KUMAR
ROLL NO. - 173604

2.  TANNERY WASTE
 Tanning industry involves conversion of animal skin into an end product useful for
making leather goods such as shoes, bags, purses, belts and clothing.
 Tanning of raw skin done either by vegetable tanning process or chrome tanning
process.
 The manufacturing process start with the removal of unwanted layers from the
hide, allowing the tanning agent to penetrate the hide and convert it into a pliable
material, which is finished with various colors and surface finishes.

3.  MANUFACTURING PROCESS

4.  SOAKING
 Soaking the skin in water to remove the preservative salt and to remove the skin’s stiff
nature.
 Soaking may be done for 5-10 days.
 It produces a strong but intermittent waste water stream, because the salt water is
discharged occasionally.
 This stream contains, in addition to the salt, dirt, dung,
and blood.
 It has a dark olive green color.

5.  FLESHING
 Which removes the fatty tissues from the hides.
 This may be done either manually or by mechanical means .
 Continuous flow of water is used in this step in order to carry away the fleshing.
 The results in generating a continuous stream
of waste water, which is quiet putrescent in
nature.

6.  LIMING
 In liming, hides are treated with lime and alkaline reducing agent like sodium
sulphide (Na2S).
 This chemical helps in the swelling process of the hide.
 The hair on the hides or skin are also loosened.

7.  UNHAIRING
 Loosened hair are removed by mechanical or manual mean.
 They are separated from wash water with the help of screens.
 Washing of hides also removes excess lime used earlier.

8.  BATING
 It is done to reduce the pH and It reduces the swelling.
 It peptize the fibres and remove the protein-degradation products.
 Bating makes the grain silky, slippery, smoother and more porous
and diminishes the wrinkles on the
skin or hide.

9.  VEGETABLE TANNING
 Vegetable tanning uses tannins , which occur naturally in the bark and leaves
of many plants.
 Tannins bind to the collagen proteins in the hide and coat them, causing them
to become less water-soluble and more resistant to bacterial attack.
 The process also causes the hide to become more flexible.
 Hides are stretched on frames and immersed for several weeks in vats of
increasing concentrations of tannin.
 Vegetable-tanned hide is not very flexible and is used for luggage, furniture,
footwear, belts, and other clothing accessories.

10.  CHROME TANNING
 The process of chrome tanning is of a shorter duration than vegetable tanning and
produces a more resistant leather.
 After bating, the hides are subjected to pickling, a process in which sodium
chloride(5%-8% strength) and sulphuric acid (0.75% strength) are used.
 The reaction is allowed to continue for 10-16 hours. Tanning is done by charging
the hides in drums containing basic chromium sulphate and sodium chloride.
 The latter promotes penetration of the chromium salt into hides.

11.  There are few special tanning processes aimed at producing white garments,
lather, etc. in which sheepskin is raw hide.
 These skins may be tanned using chrome tanning chemical or alum or even
vegetable tanning agents.
 The operations of soaking, fleshing, liming and unhairing are carried out in the
beam house, bating and washing of hides is done in the scrub house. While
tanning, bleaching, fat liquoring and finishing is done in the tan yard.
 Washing , liming, bating, tanning, bleaching and dyeing produce strong but
intermittent wastes.

12.  SOLID WASTES FROM RAW HIDES

13.  This above waste has commercial value, in that the salt can be recovered and
reused.
 Hair are used for making plaster binders.
 Raw trimmings can be converted into glue and gelatin, or are useful for making
leather board, lime sludge is useful for correcting acidic soils.
 Fleshings are used as the poultry feed.
 Chrome sludge can be converted into bricks and spent vegetable tan bark is used
as fuel.

14.  DISPOSAL OF WASTE WATER
 Tannery waste water may be disposed of on land for irrigation or into municipal
sewers or into the sea.
 These wastes have very have TDS because of the use of salt as preservatives.
 The TDS may be as high as 20,000 mg/L. therefore, the raw hides are dusted
before soaking to remove the dried salt. Further the soaking is done in a series of
tanks and water from the first tank, which has the highest conc. Of salts in it, is
stored in evaporation ponds and the dried salt is disposed of with other
predustings. This step reduces TDS considerably, and now it can be used in
irrigation and cultivation purposes.