Hides may be received in any of following conditions: fresh, green-salted, sun-dried or dry salted. Fresh hides are not common. The hides from the slaughterhouses are salted and await shipment to the tanneries. Hides form the foreign countries are likely to be dried, or salted and dried; these are stiff and hard, and require special treatment.

Washing:
All hides are first washed to remove blood, dirt, and free salt. This may be done in the vats or "wash wheel", a large drum mounted on trunnions, which receives the hides and wash water and tumbles them by rotation on it's axis. These drums are characteristic feature of the tannery and are used in many of it's operations.

Soaking:
For the purpose of removing salt and to soften hides, they are next suspended in the vats of cold water, with frequent changing of the water in the vat or countercurrent movement of hides from vat to vat. The soda ash, detergents and bacteria are mixed in water for better results. The duration of soaking depends on the condition of the hides. The usual duration is 12-14 hrs.
The combined waste from the washing and soaking is called "soaks". It contains dirt, dung, blood, some hair, and salt. They are dark, olive-green in colour and highly putrescible. The discharge is intermittent.

Liming:
To swell the skins and so that the tanning agents may penetrate the skin the liming is done. The hides are immersed in the solution of lime and sodium sulfate for about 5-10 days. They are mixed in the proportion of 3.5-4.5% of the total weight of the hides. Out of this 2-2.5% is lime. The liming process may also serve to remove the grease from the hides. The lime waste is one of the heaviest of the waste in the tanning process and accounts for the more conspicuous characteristics of the tannery waste.

Unhairing:
Unhairing is accomplished by the machines in which knife-bearing rollers sweep over the skin and remove the loosened hair and epidermis which are flushed off in the stream of water. The hair is separated and washed in clean water. The waste from this process contains fine hair which escape the screen, together with the particles of epidermis and lime. Discharge is continuous.
The unhaired are then given vigorous washing in clean water to remove lime. This operation may be conducted in drums or in vats supplied with the steady flow of clean water. The waste from the washing operation is relatively large in volume, but of low strength.

Fleshing:
It is the next process. This is the removal of the muscle and fatty tissues adhering to the underside of the hide. In the modern tanning process the fleshing is done on the machine which brings the revolving knives against the hides while the stream of the water carries away the particles removed.

Bating:
Then comes this process whose purpose is to remove the lime absorbed by the hides, and hydrolysis of some protien which are not required in the fine leather. It also restores the thickness of hides.
In modern tanning the bate is the solution of the proteolytic enzyme, such as pancreatic or trypsin, plus the ammonium salt such as ammonium sulfate and ammonium bi-sulfate. The former removes the unwanted proteins such as absorbed lime. As a waste producer, the process is of little importance.

Pickling:
It is an important step in the preparation of the leather. It consists of soaking the skins in the solution of the sulphuric acid and the sodium chloride. The purpose of the pickling is to lower the pH level so that the skins can be chrome tanned. After pickling the skins can be preserved for long times before they can be tanned. The process is performed in the wash wheels or vats, where the skins remain for the 10-16 hrs. the pickling liquor can be brought back to strength and thus can be re-used over a considerable period and therefore discharged only occasionally.

Tanning:
After pickling the skins are tanned. The skins are charged into the drums containing a solution of basic chromium sulfate. Salts like sodium chloride or sulfate is also added in the solution to promote the penetration of the chromium salt into the skin. Tanning is completed in the rotating drum in the few hours. If vats are used then it might take several days. Towards the end of the process the soda ash is added to promote the combination of the tanning agent with the skin substance.