Introduction are not allowed to emit sulphide

Introduction

Tanning is the chemical process that
converts animal hides and skin into leather and related products. The
transformation of hides into leather is usually done by means of tanning agents
and the process generates highly turbid, colored and foul smelling wastewater.
The major components of the effluent include sulphide, chromium, volatile
organic compounds, large quantities of solid waste, suspended solids like
animal hair and trimmings.

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The environmental protection regulations
stipulate that industries are not allowed to emit sulphide and chromium in the
wastewater. For every kilogram of hides processed, 30 liters of effluent is
generated and the total quantity of effluent discharged by Indian industries is
about 50,000 m3 /day. During the tanning process at least 300 kg
chemicals (lime, salt etc.) is added per ton of hides (FAO, 1996). In processes
of production leather tanneries produce all the three categories of wastes:
wastewater, solid waste and air emissions

Today, due to increased pollution as well
as elevated public awareness and consequent demand for protection of the
world’s water resources, different types of treatment techniques that remove
organic matter and nutrients from wastewater have been developed. These removal
processes can be grouped into physio-chemical and biological systems.
Biological processes can be further divided into fixed-film and
suspended-growth systems. In attached growth (or fixed film) processes, the
microbial growth occurs on the surface of stone or plastic media and the
wastewater passes over the media along with air to provide oxygen. In suspended
growth processes, the microbial growth is suspended in an aerated water mixture
where the air is pumped in, or the water is agitated sufficiently to allow oxygen
transfer.

 

Sources of Tannery Waste Water and
Quantity

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. Spent
liquors from 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
concentration.

The 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 nutrients and favorable environments for
bacterial growth. The spent bate liquor contains high amount of organic and
ammonia-nitrogen due to the presence of soluble skin proteins and ammonia
salts.

The spent pickling and chrome tanning
waste comprise a small volume, have a low BOD and contains trace of protein
impurities, sodium chloride, mineral aids and chromium salts, mostly in the
trivalent form. The spent lime liquor contains dissolved 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
high ammonia-nitrogen content.

 

 

 

 

 Environmental Impact of Tannery Wastewater

Leather tanning has been ranked as one of the most polluting
activities compared to other manufacturing sector activities. The tanning
process is almost wholly a wet process that consumes high amounts of water
(about 30-40L of water/kg of hides or skin processed) and also generates about
90% of the water as effluent (World Bank, 1998; IFC, 2007).    A review of the total pollution load
decrease achievable according to the United
Nations Industrial Development Organization  posts
precise data on the abatement achievable through industrially proven low-waste
advanced methods, while noting that even though the chrome pollution load
can be decreased by 94% on introducing advanced technologies, the minimum
residual load 0.15 kg/t raw hide can still cause difficulties when using landfills
and composting sludge from wastewater treatment on account of the regulations
currently in force in some countries. In Kanpur,
the self-proclaimed “Leather City of World” and a city of 3 million
people on the banks of the river Ganges,
pollution levels were so high that, despite an industry crisis, the pollution
control board has decided to seal 49 high-polluting tanneries out of 404 in
July 2009. In 2003 for instance,
the main tannery’s effluent disposal unit was dumping 22 tonnes of
chromium-laden solid waste per day in the open.

Process water consumption, and
consequently wastewater effluent discharges, varies greatly between tanneries,
based on the processes involved, raw materials, and products. Generally, water
consumption is greatest in the pre-tanning areas, but significant amounts of
water are also consumed in the post tanning processes. Thus, effluents
discharged from tanneries are voluminous, highly colored, contain a heavy
sediment load including toxic metallic compounds, chemicals, biologically
oxidizable materials and large quantities of putrefying suspended matter and
when discharged untreated, damages the normal life of the receiving water
bodies and land surface .The lack of effective implementation of legislative
control, poor processing practices and use of unrefined conventional leather
processing methods have further aggravated the pollution problem caused by the
tanning industry.

Tannery wastewater release
high amount of organic matter to the environments. Organic compounds which are
present in these wastes are; polyphenolic compounds, acrylic acids condensates,
aliphatic ethoxylates, fatty acids, dyes, proteins, soluble carbohydrates.
Organic matter in water bodies can be oxidized by oxygen. Therefore, when
released into a water body, organic matters consume oxygen and leave the system
in an oxygen-deprived state.

Tannery wastewaters, which
percolate into ground water for a long period, seriously affect the
groundwater. Chromium and sulphide from the effluent pollute the groundwater
permanently and make it unfit for drinking, irrigation and general consumption.
A single tannery can cause the pollution of groundwater around the radius of
7–8 km. Other negative effects include the loss of land productivity,
retardation of the germination of plants and seeds.

Now a day, there is a growing
environmental pressure against the leather processing activity because of the
rise in salinity and heavy metals in the soil and groundwater. Therefore, to
reduce these and any other impact of tannery effluents, the tannery wastewater as
a whole or individual process streams should be treated.

 

Effect
of Effluent on Water

                Ground
water is the prime source of drinking water in urban and rural areas of our
country. The quality of drinking water in Indian cities has been deteriorating
in the recent years mainly due to growth of population and improper disposal of
waste water from industries. The groundwater in industrial areas across the
country has undergone severe contamination by industrial waste, effluents and
emissions which are discharged indiscriminately without any regulatory system.
Compared with other kinds of water, groundwater is normally preferred because
it tends to be less contaminated directly by wastes and organisms. However in
the wake of recent industrialization and fast urbanization the quality of
groundwater has become an increasing concern due to contamination by various
toxic chemicals. The surface water quality is affected by
both anthropogenic activities and natural processes. Most of the hazards coming
to human and ecosystem are mostly due to ground water pollution. The untreated
sewage, industrial effluents and agriculture wastes are often discharged into
the water bodies. This contaminated water spread wide range of water borne
diseases. The agricultural fields around these water bodies are affected. The
different types of heavy metals carried from waste water effluent are liberally
let out into the nearby rivers causing contamination in them. Drinking water
may be contaminated by various toxic metals. The impact of the effluents is so
stupendous that the water has become unfit for drinking and irrigation. The
total dissolved solids of the ground water is 17,000 mg/l. Sodium chloride is
the major dominant chemical present in ground water which makes it unsuitable for
drinking and irrigation. A single tannery can cause the pollution of ground
water around the radius of 7-8 kilometers. In Tamil Nadu more than 60 percent
of India’s economically important tannery industries are located, tannery waste
water containing chromium and sodium compounds have contaminated more than
55000 hectares of agricultural and nearby water beds.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Waste-water
treatment unit operations and processes

Physical
unit operations

Screening

Comminution

Flow
equalization

Sedimentation

Flotation

Granular-medium
filtration

 

Chemical
unit operations

Chemical
precipitation

Adsorption

Disinfection

Dechlorination

Other
chemical applications

 

Biological
unit operations

Activated
sludge process

Aerated
lagoon

Trickling
filters

Rotating
biological contactors

Pond
stabilization

Anaerobic
digestion

Biological
nutrient removal

 

Treatment
Methods of Tannery Wastewater

Treatment
of tannery effluent is a challenge because it is a mixture of biogenic matter
of hides, inorganic chemicals and a large variety of organic pollutant with
large molecular weights and complex structures. There are two Different kinds
of treatment techniques. These include mechanical, physio-chemical and
biological treatments

 

Physio-chemical
treatment

Physio-chemical
treatment of tannery effluents consist of coagulation, flocculation,
sedimentation, filtration, air stripping, chemical precipitation, adsorption,
ion exchange, electrochemical (electro-oxidation), and chemical oxidation.
Coagulation-flocculation (CF) has been applied to leather tanning wastewater to
reduce organic load and suspended solids as well as to remove toxic substances.
In addition to coagulation chromium can be reduced by chemical precipitation,
adsorption, and ion exchange. The removal of nitrogen and organic matter from
tannery wastewater can be accomplished using physical, chemical or a
combination of both.

Organic
matter can be reduced either by adding powder activated carbon directly into
biological reactor or by advance oxidation processes (AOPs) such as UV, ozone
(O3), photocatalytic oxidation. The available physio-chemical methods for
ammonia removal include adsorption, chemical precipitation, membrane
filtration, reverse osmosis, ion exchange, air stripping, breakpoint
chlorination and electrochemical oxidation. All the processes which can be used
for removal of both organic matter and nitrogen are simple in principle;
however, they are expensive (high operating and maintenance cost, and
consumption of chemicals) and also produced harmful products.

Now
a days, there is a growing interest in the development of new
technologies and procedures for the purification of this waste. Among these
procedures, biological methods have been recognized as a viable possibility for
the degradation of these wastewaters

 

Biological
treatment

Treatment methods in which the removal
of contaminants is brought about by biological activity are known as biological
unit processes. Biological treatment is used primarily to remove the
biodegradable organic substances (colloidal or dissolved) from wastewater.
Basically, these substances are converted into gases that can escape to the
atmosphere and into biological cell tissue that can be removed by settling.
Biological treatment is also used to remove nutrients (nitrogen &
phosphorus) from wastewater. With proper environmental control, wastewater can
be treated biologically in most cases. Biological treatment methods use
microorganisms, mostly bacteria, in the biochemical decomposition of wastewater
to stable end products. More microorganisms, or sludge’s, are formed and a
portion of the waste is converted to carbon dioxide, water and other end
products. Generally, biological treatment methods can be divided into aerobic
and anaerobic methods, based on availability of dissolved oxygen.

Organic
matter removal from strong wastewater such as tannery effluents is clearly favored
by the combination of processes with and without oxygen. The conventional
biological method like Activated Sludge Process (ASP), Sequential Batch Reactor
(SBR), Up flow Anaerobic Sludge Blanket (UASB)

The
activated sludge process (ASP) is the most common and versatile biological
process used worldwide for the secondary treatment of domestic, municipal and
industrial wastewater. With the course of time, several modifications of the
ASP have been made to improve the degree of treatment in accordance with
stringent effluent standards.

 

Organic Matter + Oxidation                                                   CO2 + H2O

 

 

 

 

 

 

 

 

 

 

Technologies for Treatment of Effluent Water

Mechanical Treatment
Technologies

Mechanical
systems utilize a combination of physical, biological, and chemical processes
to achieve the treatment objectives. Using essentially natural processes within
an artificial environment, mechanical treatment technologies use a series of
tanks, along with pumps, blowers, screens, grinders, and other mechanical
components, to treat wastewaters. Flow of wastewater in the system is
controlled by various types of instrumentation. Sequencing batch reactors
(SBR), oxidation ditches, and extended aeration systems are all variations of
the activated-sludge process, which is a suspended-growth system. The trickling
filter solids contact process (TF-SCP), in contrast, is an attached-growth
system. These treatment systems are effective where land is at a premium.

 

Aquatic
Treatment Technologies

Facultative
lagoons are the most common form of aquatic treatment-lagoon technology
currently in use. The water layer near the surface is aerobic while the bottom
layer, which includes sludge deposits, is anaerobic. The intermediate layer is
aerobic near the top and anaerobic near the bottom, and constitutes the
facultative zone. Aerated lagoons are smaller and deeper than facultative
lagoons. These systems evolved from stabilization ponds when aeration devices
were added to counteract odors arising from septic conditions. The aeration
devices can be mechanical or diffused air systems. The chief disadvantage of
lagoons is high effluent solids content, which can exceed 100 mg/l. To
counteract this, hydrograph controlled release (HCR) lagoons are a recent
innovation. In this system, wastewater is discharged only during periods when
the stream flow is adequate to prevent water quality degradation. When stream
conditions prohibit discharge, wastewater is accumulated in a storage lagoon.
Typical design parameters are summarized in Table 13.

Constructed
wetlands, aqua cultural operations, and sand filters are generally the most
successful methods of polishing the treated wastewater effluent from the lagoons.
These systems have also been used with more traditional, engineered primary
treatment technologies such as Imhoff tanks, septic tanks, and primary
clarifiers. Their main advantage is to provide additional treatment beyond
secondary treatment where required. In recent years, constructed wetlands have
been utilized in two designs: systems using surface water flows and systems
using subsurface flows. Both systems utilize the roots of plants to provide
substrate for the growth of attached bacteria which utilize the nutrients
present in the effluents and for the transfer of oxygen. Bacteria do the bulk
of the work in these systems, although there is some nitrogen uptake by the
plants. The surface water system most closely approximates a natural wetland. Typically,
these systems are long, narrow basins, with depths of less than 2 feet, that
are planted with aquatic vegetation such as bulrush (Scirpus spp.)
or cattails (Typha spp.). The shallow groundwater systems use
a gravel or sand medium, approximately eighteen inches deep, which provides a
rooting medium for the aquatic plants and through which the wastewater flows.

 

 

Tolerance
Limits for Effluents from the Tanning Industry in India (Environmental
Protection Rule, 1986)

Characteristics

Into
Inland Surface Waters

Into
Marine Coastal Areas

Color

Absent

Absent

Total
Dissolved Solids

2100

Nil

Suspended
Solids

100

100

BOD

30

100

pH
Value

6.0
to 9.0

6.0
to 9.0

Chlorides

1000

Nil

Total
Chromium

2

2

Chemical
Oxygen Demand

250

250

 

Conclusion

Effective waste-water collection and
treatment are of great importance from the standpoint of both environmental and
public health. Extensive research activity in this field has led to significant
improvement and diversification in the processes and methods used for
waste-water treatment and sludge management. Every
ecosystem relies on water in some regard. And when water is contaminated by
sewage, toxic chemicals, or any number of other man-made forms of waste, those
ecosystems are put at serious risk. In developing countries like India,
the problems associated with wastewater reuse arise from its lack of treatment.
The challenge thus is to find such low-cost, low-tech, user friendly methods,
which on one hand avoid threatening our substantial wastewater dependent
livelihoods and on the other hand protect degradation of our valuable natural
resources. The use of constructed wetlands is now being recognized as an
efficient technology for wastewater treatment. Compared to the conventional
treatment systems, constructed wetlands need lesser material and energy, are
easily operated, have no sludge disposal problems and can be maintained by
untrained personnel. Further these systems have lower construction, maintenance
and operation costs as these are driven by natural energies of sun, wind, soil,
microorganisms, plants and animals. Hence, for planned, strategic, safe and
sustainable use of wastewaters there seems to be a need for policy decisions
and coherent programs encompassing low-cost decentralized waste water treatment
technologies, bio-filters, efficient microbial strains, and organic / inorganic
amendments, appropriate crops/ cropping systems, cultivation of remunerative
non-edible crops and modern sewage water application method

 

 

 

 

References

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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