Sewage Treatment Plant

Microbes in Sewage Treatment

• The municipal waste water, containing large quantities of human excreta is called Sewage.

• Sewage also contains large amounts of organic matter and microbes. Many of them are pathogenic.

• Sewage is treated in sewage treatment plants (STPs) to make it less polluting before releasing into river bodies.

I. Primary treatment or physical treatment

1. This treatment basically involves physical removal of large and small particles from sewage through filtration and sedimentation.

2. First, the floating debris is removed by sequential filtration by passing through wire mesh screens.

3. Then, the grit (soil and small pebbles) are removed by sedimentation in settling tanks. The sediment is called primary sludge and the supernatant is the effluent.

4. The effluent is taken for secondary treatment.

II. Secondary treatment or biological treatment

1. Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply.

2. Useful aerobic microbes grow rapidly and form flocs.

3. Flocs are masses of bacteria associated with fungal filaments to form mesh-like structures.

4. The growing microbes consume organic matter and thus reduce the biochemical oxygen demand (BOD).

5. When BOD of sewage has reduced, the effluent is passed into settling tank.

6. Here, the bacterial flocs settle, and the sediment is called activated sludge.

7. A small part of the sludge is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.

8. In the digesters, other kinds of bacteria, which grows anaerobically, digest bacteria and fungi in sludge.

9. During this digestion, bacteria produce mixture of gases such as methane, hydrogen sulphide and C02 which form the biogas.

Biological Oxygen Demand (BOD)

1. BOD refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water is oxidized by bacteria.

2. BOD measures the amount of organic matter in water by measuring the rate of oxygen uptake by microbes.

3. Higher BOD indicates higher polluting potential.