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1. Domestic water pollution
Domestic water pollution is mainly caused by sewage.
Sewage is defined as the water-borne waste derived from
home, animal or food processing plants and includes
human excreta, soaps, organic materials, different types
of solids, waste food, oil detergents, paper and cloth.They
are the largest group of water pollutants. Water pollution
is caused by uncontrolled dumping of waste collected
from villages, towns and cities into ponds, streams, lakes
and rivers. A major ingredient of most detergents is
phosphate. Phosphates support luxurious growth of algae.
Algae withdraw large quantities of oxygen from water. It
becomes detrimental to other organisms. Domestic waters
are the primary sources of water pollution.The domestic
sewage contributes to the largest amount of waste.

2. Domestic water pollution
The huge discharge of sewage allows following events
that lead to water pollution.
1) Depletion of Oxygen Contents
2) Promotion of Algal Growth
3) Spread of Infections/Diseases
1) Depletion of Oxygen Contents: The aerobic bacteria
present in water are responsible for the decomposition of
organic matter. The amount of oxygen required for such
biochemical decomposition is called the biochemical
oxygen demand. The quantity of oxygen utilized by the
bacteria for the degradation of organic substance is called
BOD. Thus BOD value can be used as an indicator of
water pollution. Along with BOD, the quantity of oxygen
dissolved in a water body (DO), indicates the quality of
biolife in a water system. DO below 4 to 5 ppm is
detrimental to the system.

3. Domestic Water Pollution
2) Promotion of Algal Growth: Stimulation of heavy
algal growth and shift in the algal flora to the blue green
algae, leading to the formation of obnoxious blooms,
floating scums or blankets of algae results into
eutrophication. Most of the algal bloom do not seem to
be utilized as food by the invertebrates or zooplankton,
thereby, minimizing the predatory control. Biological
decomposition of such algal masses in turn leads to
oxygen depletion. In a poorly oxygenated condition,
fish and other animal die and clean river is turned into a
stinking drain.

4. Domestic water pollution
3. Spread of Infections/Diseases: Micro-organisms,
usually viruses, bacteria, some protozoans and
helminthes occur in water bodies as a result of sewage
disposal. Consumption of contaminated water causes
water borne infectious diseases.
Water borne infectious diseases in man.
• Viruses Viral hepatitis, poliomyletitis
• Bacteria Cholera, typhoid, paratyphoid, dysentery,
diarrhoea
• Protozoa Amoebiasis, giardiasis
• Helminthes Roundworm, hookworm, threadworm

5. Pollution Problems of Leather Industries
The leather industry occupies an important place in the
National economy of every country.
• Tannery effluents contain vegetable tannins and non-
tannins which cause chemical oxygen demand. They also
contain high amounts of proteins. These proteins are
biodegradable and exert high BOD where water from
tanneries contain about 46% proteins, 10% fatty acids,
4% tannins, 20% inorganic volatile solids and about 21 %
total dissolved volatile solids.
• Chlorides, trivalent chromium, nitrogen, phosphorus,
sulphate, ammonium salts, lime etc. are the inorganic
pollutants present in significant quantities. For each kg
salted hide 35 litres of water is required.

6. Pollution Problems of Leather Industries
The discharge of untreated waste waters in a nearby water
body may effect the physical, chemical and biological
characteristic of water. High oxygen demand, high pH,
excessive alkalinity, suspended matter, sulphides are
injurious to fish and other aquatic organisms. The chrome
process involves 250 chemicals including cadmium, arsenic
and chromium. Chromium in particular is used in large
quantities. Tanneries make the ground water unfit for
drinking, and irrigation. Groundwater of several places have
been affected by tanneries. A tannery waste can contaminate
the groundwater in a radius of 8 kms. The effluent from
vegetable tanning is coloured which makes the water turbid.
Turbidity reduces light penetration, thereby reducing the
photosynthetic activity of aquatic plants.