PROCESS OF TANNERY INDUSTRY.
WASTE GENERATION POINTS.
CHARACTERSTICS OF WASTE
WATER, DISCHARGED FROM TANNERY
TREATMENT SCHEMES FOR WASTE WATER.
Tanning and Crusting
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
include: 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
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.
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
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
Crusting is when the hide/skin is
thinned, retanned and lubricating often a
coloring operation is included in the crusting
sub process. The chemicals added during
crusting have to be fixed in place. The
culmination of the crusting sub process is the
drying and softening operation.
For some leathers a surface coating is applied. Tanners
refer to this as finishing. Finishing may include:Oiling
Maximum waste generation points in
tannery industry are processes such as:
Soaking:-generation of waste water.
fleshing:-subcutaneous material waste
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.
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
different compounds: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.
NEUTRAL SALTS:-Two common types of salts are
to be found in tannery effluent:Sulphates
OIL & GREASE:-During leather manufacture,natural
oils and grease are released from within the skin.
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)
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.
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.
Activated sludge is a biochemical process for
treating sewage and industrial waste that
uses air(or oxygen) and micro-organisms to
biological oxidize organic
pollutants,producing a waste sludge(or floc)
containing the oxidizing material.
A trickling filter process consists of bed of
rocks,gravel,slag,peat moss or plastic media over
which wastewater flows downwards and a contacts a
layer of microbial slims covering the bed
media.Aerobic conditions are maintained by forced
air flowing through the bed or by natural convection
air.The process involves adsorption of organic
compounds in wastewater by the microbial slime
layer, diffusion of air into the slime to provide the
oxygen required for the biochemical oxidation of
organic compounds .The end projects include carbon
dioxide gas,water and other products of oxidation.As
the slime layer thickness,it becomes the air to
penetrate the layer and an inner anaerobic layer is
TREATMENT OF OTHER ORGANICS:Synthetic organics material including
cides,cooking products and so forth
can be very difficult to
treat.Treatment methods are often
specific to the material being
treated.Methods include advanced
immobilisation or landfill disposal.
TREATMENT OF ACIDS & ALKALIS:
Acids & alkalis can usually be neutralised
under controlled conditions.neutralisation
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 neutralisation.
OIL AND GREASE REMOVAL:
Oil and grease can be recovered from open
water surfaces by skimming devices.
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
DEMONSTRATION OF EFFECTIVE AND
EFFICIENT TANNERY EFFLUENT TREATMENT
USING AN INNOVATIVE INTEGRATED AND
COMPACT BIOLOGICAL AND PHYSICAL
A significant number of operations within a
tannery are wet operations consuming large
amounts of water, chemicals and energy and
leading to large amounts of polluted water.
Through “process integrated” measures a
significant reduction of water consumption
and pollution load can be achieved, however
tanneries keep producing wastewater
requiring special treatment. In cases where the
potential for “process integrated”
measures has worn out, further pollution
reduction has to be found in the improvement
of end-of-pipe measures.
Compared to Best Available Technology1 the following
effluent quality performance is demonstrated:
- better COD-removal efficiency (+ 5 to 10 %);
- better Chromium removal efficiency (+ 5 to 10 %);
- better Sulphur removal efficiency (+ 30 %);
- equal Nitrogen removal.
This all is being realised against lower total annual
cost than in the BAT situation as a result of:
- lower costs for sludge discharge (reduction sludge
volume 50 % compared to BAT);
- lower costs for discharging waste water to
municipal treatment plant;
- significantly lower chemical consumption.