`star` Microbes in Sewage treatment
`star` Primary Treatment
`star` Secondary Treatment


● We know that large quantities of `color{Violet}"waste water"` are generated everyday in `color{Violet}"citiies and towns"`.

● A major component of this waste water is `color{Violet}"human excreta"`.

● This `color{Violet}"municipal waste-water"` is also called `color{Violet}"sewage"`.

● It contains large amounts of `color{Violet}"organic matter"` and microbes, many of which are `color{Violet}"pathogenic"`.

● This `color{Violet}"cannot be discharged"` into natural water bodies like `color{Violet}"rivers and streams"` directly .

● Before disposal, hence, sewage is treated in `color{Violet}"sewage treatment plants"` (`color{Violet}"STPs"`) to make it less polluting.

● Treatment of waste water is done by the `color{Violet}"heterotrophic microbes"` naturally present in the sewage. This treatment is carried out in `color{Violet}"two stages"`:

`star` `color{Brown}"Primary Treatment"`
`star` `color{Brown}"Secondary Treatment"`


● These treatment steps basically involve `color{Violet}"physical removal of particles"` – large and small – from the sewage through `color{Violet}"filtration and sedimentation"`.

● These are removed in stages; initially, `color{Violet}"floating debris"` is removed by `color{Violet}"sequential filtration"`.

● Then the `color{Violet}"grit"` (soil and small pebbles) are removed by `color{Violet}"sedimentation"`.

● All solids that settle form the `color{Violet}"primary sludge"`, and the `color{Violet}"supernatant"` forms the effluent.

● The effluent from the `color{Violet}"primary settling tank"` is taken for `color{Violet}"secondary treatment"`.


`star` `color{Brown}"In Aeration Tank"`:

● The `color{Violet}"primary effluent"` is passed into `color{Violet}"large aeration tanks"` where it is constantly `color{Violet}"agitated mechanically"` and `color{Violet}"air is pumped"` into it.

● This allows `color{Violet}"vigorous growth"` of useful `color{Violet}"aerobic microbes"` into `color{Brown}"flocs"` (masses of bacteria associated with fungal filaments to form mesh like structures).

● While growing, these microbes `color{Violet}"consume"` the major part of the `color{Violet}"organic matter"` in the effluent.

● This significantly `color{Violet}"reduces the BOD"` (`color{Brown}"biochemical oxygen demand"`) of the effluent.

● BOD refers to the amount of the `color{Violet}"oxygen"` that would be consumed if all the `color{Violet}"organic matter"` in one liter of water were `color{Violet}"oxidised by bacteria"`.

● The sewage water is treated till the `color{Violet}"BOD is reduced"`.

● The BOD test measures the `color{Violet}"rate of uptake"` of oxygen by micro-organisms in a `color{Violet}"sample of water"` and thus, indirectly, BOD is a measure of the `color{Violet}"organic matter"` present in the water.

● The `color{Violet}"greater the BOD"` of waste water, more is its `color{Violet}"polluting potential"`.

`star` `color{Brown}"In Settling Tank"`:

● Once the `color{Violet}"BOD"` of sewage or waste water is `color{Violet}"reduced significantly"`, the effluent is then passed into a `color{Violet}"settling tank"` where the `color{Violet}"bacterial flocs"` are allowed to sediment.

● This sediment is called `color{Violet}"activated sludge"`.

● A small part of the activated sludge is `color{Violet}"pumped back"` into the aeration tank to serve as the `color{Violet}"inoculum"`.

`star` `color{Brown}"In Anaerobic Sludge Digesters"`:

● The remaining major part of the sludge is pumped into `color{Violet}"large tanks"` called `color{Violet}"anaerobic sludge digesters"`.

● Here, other kinds of bacteria, which `color{Violet}"grow anaerobically"`, digest the bacteria and the fungi in the sludge.

● During this digestion, bacteria produce a `color{Violet}"mixture of gases"` such as `color{Violet}"methane, hydrogen sulphide"` and `color{Violet}"carbon dioxide"`.

● These gases form `color{Violet}"biogas"` and can be used as `color{Violet}"source of energy"` as it is inflammable.

● The `color{Violet}"effluent"` from the secondary treatment plant is generally released into natural water bodies like `color{Violet}"rivers and streams"`.

● This methodology has been `color{Violet}"practiced"` for `color{Violet}"more than a century"` now, in almost all parts of the world.

● Till date, `color{Violet}"no man made technology"` has been able to rival the microbial treatment of sewage.

● Due to `color{Violet}"increasing urbanisation"`, sewage is being produced in much `color{Violet}"larger quantities"` than ever before.

● However the number of `color{Violet}"sewage treatment plants"` has `color{Violet}"not increased"` enough to treat such large quantities.

● So the untreated sewage is often `color{Violet}"discharged directly"` into rivers leading to their pollution and increase in `color{Violet}"water-borne diseases"`.

● The `color{Violet}"Ministry of Environment"` `color{Violet}"and Forests"` has initiated `color{Brwon}"Ganga Action Plan"` and `color{Brown}"Yamuna Action Plan"` to save these major rivers of our country from pollution.

● Under these plans, it is proposed to build a `color{Violet}"large number"` of sewage treatment plants so that only treated sewage may be `color{Violet}"discharged in the rivers"`.