Biological Treatment of Waste Water
Sewage is the liquid waste released from the community. It contains worthless materials discharged from
industries, kitchens, food industries, health sanitaries, municipalities, etc. The sewage has been treated for
reducing the pollution hazards. The sewage treatment process includes three main steps. They are primary
treatment, secondary treatment and tertiary treatment.
Purpose:
The idea behind all biological methods of wastewater treatment is to introduce contact with bacteria (cells),
which feed on the organic materials in the wastewater, thereby reducing its BOD content. In other words,
the purpose of biological treatment is BOD reduction.
Principle:
Simple bacteria (cells) eat the organic material present in the wastewater. Through their metabolism, the
organic material is transformed into cellular mass, which is no longer in solution but can be precipitated at
the bottom of a settling tank or retained as slime on solid surfaces or vegetation in the system. The water
exiting the system is then much clearer than it entered it.
I.Primary Treatment
It includes the removal of solid wastes floating on the sewage and the other sedimentable wastes from the
sewage. They are removed by screening and sedimentation methods.
1. Screening
It is the first step of the sewage treatment. The sewage is passed through a screen fitted in the opening.
Usually screens of uniform size are used for the screening. The screening of sewage protects the operation
unit from the excessive dumping of solid wastes. The screens are classified into
a. Coarse Screens: Coarse screens are used to remove the floating materials like rags, paper, wood, etc.
from the sewage.
b. Medium Screens: Medium screens are used for removing some amount of organic materials from the
sewage.
c. Fine Screens: Fine screens have perforations of 1.5mm to 3mm size. These screens help in the removal
of some suspended particles from the sewage.
2. Sedimentation
The separation of solid suspended particles from the sewage is called sedimentation. The tank is often
referred to as sedimentation tank or settling tank. Further, it reduces the BOD of sewage water.
II.Secondary Treatment
During the secondary treatment, the non-sedimentable wastes are removed from the sewage by biological
method, that include
mechanical means to create contact between wastewater, cells and oxygen, and those than
don’t.
a) With mechanical means:
1. Activated sludge: This is the most common type. It consists in a set of two basins. In the first, air is
pumped through perforated pipes at the bottom of the basin, air rises
through the water in the form of many small bubbles.
1
These bubbles accomplish two things: they provide oxygen form the air to the water and create highly
turbulent
conditions that favor intimate contact between cells, the organic material in the water and oxygen. The
second basin is a settling tank, where water flow is made to be very quiet so that the cellular material may
be removed by gravitational settling.
2. Trickling filter:
A trickling filter consists in a bed of fist-size rocks over which the wastewater is gently sprayed by a
rotating arm. Slime (fungi, algae) develops on the rock surface, growing by intercepting organic material
from the water as it trickles down.
Diagrammatic representation of
trickling filter
3. Biological contactor:
This is essentially a variation on the trickling filter, with the difference being that solid material on which
slime grows is brought to the water rather than water being brought to it. Rotating disks alternate exposure
between air and water.
III.Tertiary Treatment
The fine colloidal particles in the sewage water are precipitated by adding some chemical coagulants. The
chemical coagulants include alum, ferric chloride, ferric sulphate, etc.
They react with the colloidal particles and form a gelatinous precipitate called flac. Heavy metals are also
removed during this treatment. The treated sewage water is more or less pure. It is fit for agricultural
purpose and drinking.
2
A schematic representation of the various stages in waste water treatment.
Oxidation Ponds







The oxidation pond (sterilization ponds, stabilization ponds, redox ponds, maturation ponds and
sewage lagoons) is a large shallow water body whose depth may range from 2 to 4 feet.
Wastes in the sewage water are oxidized in the ponds.
The sewage is allowed to enter the pond at one site or at several sites or added to the middle of the
pond through pipe lines.
The sewage gets mixed in the pond due to the action of wind on its surface. Some amount of O2
dissolve in the water and diffuses into the different parts of the sewage in the pond. The diffused O2
is enough for the growth of micro-organisms and some primitive plants. These organisms consume
nutrients in the sewage and cause the oxidation of the organic wastes.
At the bottom of the pond anaerobic microbes thrive well and oxidize the organic matter in the pond.
Finally the purified sewage water is discharged into a river or used for irrigation.
The solid fraction of sewage gets deposited at the bottom of the oxidation pond.
The advantages of oxidation ponds are given below:
1. The sewage treatment is very easy.
2. It requires low cost for its-maintenance.
3. No need for skilled supervisions.
4. It has maximum potential to reduce the BOD of
sewage water.
5. It reduces nearly 99% of sewage bacteria from the
sewage water.
6. The algal biomass can be frequently harvested from the pond and used as a source of protein in diet.
3
Sewage Treatment by Anaerobic Digestion
The sewage is treated by using anaerobic micro-organisms. These micro-organisms digest the solid wastes
into simple inorganic chemicals. The essential steps of anaerobic sewage treatment are summarized below:
1. The sewage is collected in a large primary settlement tank. The solid and semi-solid waste materials in
the sewage are allowed to settle at the bottom of the tank.
2. After sedimentation, the sewage is allowed to pass into an aeration tank. An air-compressor is attached
to the aeration tank; it gives proper aeration to the aeration tanks. The aerobic bacteria thrive well and
consume more and more organic matter in the sewage water.
3 . After processing, the water is properly tested for organic matter, ammonia and BOD. If the sewage water
has a low BOD, it indicates the purity of the water.
4. The purified sewage water is discharged into a river or directly used for irrigation.
5. The sedimented sludge coming from the primary settlement tank is allowed to pass into the primary
digester. The digester is a concrete, insulated tank whose capacity ranging from 103 to 104 M3. The
sludge is allowed to remain as such for 20-30 days for anaerobic digestion. The digester is heated to 35°C
by using a heat exchanger.
6. During anaerobic digestion, biogas is also released from the fermentor (digester). This gas is stored in
gas cylinders and used to run engines, heat exchanger and the air-compressor.
7. The digested sludge is transferred to a secondary digester. Here the sludge is allowed to digest itself
under cold condition for about 25days.
The supernatant solution released from the digestion tanks is then allowed to pass into the primary settling
tank. The solid waste is thickened and dried properly. The dried solid waste is used as manure for
agricultural and horticultural crops.
4
Recycling of Waste Water
The possible reuses of sewage are summarized below:
1. The solid waste separated from the sewage is used for improving the soil conditions. It forms a good
manure for agricultural and horticultural crops. It improves the wealth of soil micro flora and mineral
contents of the soil.
2. The solid waste is used as a raw materials for the production of biogas, ethanol, hydrogen, etc.
3. The purified sewage water is used as drinking water for domestic animals and human beings.
4. The sewage water is also used in aquaculture, especially for rearing aquatic fishes and some algae.
5. The purified sewage water is used in industries to cool the industrial machines.
6. The sewage water forms a good source of water for irrigation. Generally, forest crops, fodder crops,
orchards, vineyards, fibre crops, seed crops and
agricultural Crops are irrigated with purified sewage
water.
The sewage water has many advantages over the fresh
water for irrigation:
-It forms an ideal source of water in areas where the
availability of land water is scarce.
-It containes inorganic nutrients like N, P.Ca, Na, K, etc.
increasing the crop yield.
-It is the cheapest source of water in industrial areas.
5