2. Tanning is the process that converts the
protein of the raw hide or skin into a stable
material which will not putrefy and is suitable
for a wide variety of end applications. Tanned
material dries out to a flexible form.
That does not become putrid when wetted
5. The preparatory stages are when hide/skin is
prepared for tanning.Many options for
pretreatment of skin exist.Not all of the options
may be performed.Preparatory stages may
Preservation-the hide/skin is treated with a
method which renders it temporarily.
Soaking-water for purpose of washing or
rehydration is reintroduced.
Liming-unwanted proteins and “opening up” is
6. Unhairing-the majority of hair is removed.
Fleshing-subcutaneous material is removed.
Splitting-the hide/skin is cut into two or
more horizontal layer.
Reliming-the hide/skin is further treated to
achieve more “opening up” or more protein
Deliming-liming & unhairing chemicals are
removed this step.
Bating-proteolytic proteins are introduced to
the skin to remove further proteins and to
assist with softening of the pelt.
7. Degreasing-natural fats/oils are stripped
or as much as is possible from the hide
Frizing-physical removal of fat layer
inside the skin.
Bleaching-chemical modification of dark
pigments to yield a lighter coloured pelt.
Pickling-lowering of pH value to the
acidic region. Must be done in presence
Depickling-raising of the pH out of the
14. SOLIDS:-Solids to be found in tannery
effluent fall into several distinct categories:-
SUSPENDED SOLIDS:-Quality of insoluble
matter contained in waste water.
SETTELEABLE SOLIDS:-Quantity of
solids, having tandency of settle down.
GROSS SOLIDS:-They are larger than a
sampling machine can handle.
ex-leather pieces,fleshing residue.
15. BOD:-Tanning waste have a long break
down period,this longer digestion periods
can apply to a variety of chemicals used in
NITROGEN:-Nitrogen is contained in several
Total kjeldahl nitrogen:-Nitrogen contained
in proteinaceous material(liming and
SULPHIDES:-Sulphide content is tannery
effluent result from the use of sodium
sulphide & sodium hydrosulphide.and the
breakdown of hair in unhairing process.
16. NEUTRAL SALTS:-Two common types of salts are
to be found in tannery effluent:-
OIL & GREASE:-During leather manufacture,
natural oils and grease are released from within
pH value:-acceptable limit ranges from 5.5 to
CHROMIUM COMPOUNDS:-Two forms of chrome
are associated with the tanning industry :-
• Chrome 3+(trivalent chrome)
• Chrome 6+(hexavalent chrome)
18. Most solids can be removed using simple
sedimentation techniques with the solids
recovered as slurry or sludge. Very fine solids
and solids with densities close to the density
of water pose special problems. In such case
filtration may be required. Although
flocculation may be used, using alum salts or
the addition of polyelectrolysis.
20. Biodegradable organics material of plant or
animal origin is usually possible to treat
using extended conventional wastewater
treatment processes such as activated sludge
or trickling filters.
21. TREATMENT OF OTHER ORGANICS:-
Synthetic organics material including
solvents, paint, pharmaceuticals,
pesticides, cooking products and so
forth can be very difficult to treat.
Treatment methods are often specific to
the material being treated. Methods
include advanced oxidation processing,
distillation, adsorbtion, nitrification,
incineration, chemical immobilization or
22. TREATMENT OF ACIDS & ALKALIS:
Acids & alkalis can usually be neutralised
under controlled conditions. neutralization
frequently produces a precipitate that will
require treatment as a solid residue that
may also be toxic. In some cases,gases
may be evolved requiring treatment for
the gas stream. Some other forms of
treatment are usually required following
OIL AND GREASE REMOVAL:
Oil and grease can be recovered from open
water surfaces by skimming devices.
23. Chromium, present in tannery effluent is
removed by using chromium salts(chromium
chloride as adsorbate) and cement kiln dust
(a waste from white cement industry) as
Chrome recovery plant,
with a filter press
The treatment of tannery effluents is by now a well established
technology, and modular common effluent treatment plants
servicing traditional tannery clusters or newly created leather
industry zones is a widely accepted approach. However, two issues
still pose serious challenges:
• High TDS (salinity) content, unaffected by treatment. This
problem is especially pronounced in developing countries where
mixing tannery effluent with domestic sewage or its discharge into
the sea is not feasible, and the raw hides and skins are still
preserved by salting. Relocation of tanneries to the seaside is
often not feasible, and desalination of treated effluent by reverse
osmosis is very expensive.
• Utilization or safe disposal of sludge. Cost-effective solutions to
both of these problems are still eagerly awaited.