This slide will tell about how we can purify water with the help of microbes

1. Sewage treatment by
microbes

2. Introduction
 The large quantities of waste water are generated everyday in cities and
towns.
 Human excreta forms a major component of this waste water.
 This municipal waste-water is also termed sewage.
 It consists of large amounts of organic matter and microbes. Many of which
are pathogenic.
 This untreated sewage cannot be discharged directly into natural water
bodies like rivers and streams.
 Hence, before disposal, treatment of sewage is being done in sewage
treatment plants (STPs) to make it less polluting.
 Treatment of waste water is done with the help of heterotrophic microbes
which are naturally present in the sewage.

3. What does sewage treatment include?
 The sewage treatment includes combined actions of:
 Aerobic microbes which firstly reduce the BOD (biochemical oxygen demand)
of the effluent, BOD refers to the amount of the oxygen that will be
consumed if all the organic matter in one liter of water are oxidised by
bacteria. Till the BOD is reduced the sewage water is being treated. The BOD
test is measurement of the rate of uptake of oxygen by micro-organisms in a
sample of water and indirectly, BOD is the measure of the organic matter
present in the water. The greater the BOD of the waste water, more the waste
water has polluting potential.
 And other different kind of bacteria, which grow anaerobically, which do the
digestion of the bacteria and the fungi in the sludge. During this digestion,
bacteria does production of mixture of gases such as methane, hydrogen
sulphide and carbon dioxide. These gases constitute biogas and it can be used
as source of energy.

4. Two stages of sewage treatment
 Primary treatment: These Steps of treatment basically involve removal of
particles whether it is large and small – from the sewage through filtration
and sedimentation physically. Removal is achieved in following stages;
initially, removal of the floating debris is done by sequential filtration.
 Then the grit (soil and small pebbles) are removed with the aid of
sedimentation.
 All the solids settle down as the primary sludge, while the supernatant
constitute the effluent.
 From the primary settling tank, the effluent is conveyed for secondary
treatment.

5. Secondary treatment
 The primary effluent is made to travel into large aeration tanks where it is
agitated constantly in a mechanical manner and air is forced into it.
 This facilitates the growth of useful aerobic microbes vigorously into flocs
(masses of bacteria associated with fungal filaments to form mesh like
structures).
 While these microbes grow, they consume the major part of the organic
matter in the effluent.
 This reduces the BOD (biochemical oxygen demand) of the effluent in a
significant manner.

6. Secondary treatment(continued)
 When the BOD of sewage or waste water is reduced, the effluent is then
made to travel into a settling tank where the bacterial ‘flocs’ are permited to
sediment.
 This sediment is known as activated sludge.
 A small part of this activated sludge is forced back into the aeration tank so
that it can serve as the inoculum.
 The remaining major part of the sludge is forced into large tanks called
anaerobic sludge digesters
 From the secondary treatment plant, the effluent is generally delivered into
natural water bodies like rivers and streams.