Wastewater treatment is important to remove as much of the suspended solids known as effluents, that are present in water before this water is discharged back into the environment. Sewage treatment plants help in this kind of treatment of water.

Sewage treatment plants (STPs) are of two types:

1.Chemically treated – In these plants chemicals are added to break down the effluents in water to disinfect it

2.Biologically treated – In biological plants the effluent is broken down using aerobic bacteria

Let us look at three of the most commonly used STPs:

1.Activated Sludge Plants (ASP)

In activated sludge plants, organic matter is present in an aerated basin where micro-organisms metabolize the suspended and soluble organic matter. A part of the organic matter is synthesized into new cells, and the rest is oxidized to CO2 and water to derive energy. New cells are formed in the reaction and removed from the liquid stream in the form of a flocculent sludge in settling tanks. A part of this settled biomass, described as activated sludge is returned to the aeration tank and the remaining forms waste or excess sludge. They need regular servicing every six months depending on the manufacturer and population equivalent size.

Following are the steps that occur in an ASP to treat sewage:

1.Aeration of wastewater in the presence of a microbial suspension

2.Separation of solid and liquid after aeration

3.Discharging the clarified effluent

4.The excess biomass is removed

5.The remaining useful biomass is returned to the aeration tank

II.Rotating Biological Contractors (RCBs)

These plants were the first to enter the domestic market of all sewage treatment plants. They revolutionised domestic sewage treatment. RCBs are three stage treatment plants. The discs treat effluent after it passes the primary settlement tank. Solids form a sludge at the bottom of the primary settlement tank. Wastewater comes in contact with bacteria which are allowed to cultivate on bio discs. The bacteria digest pollutants from wastewater before this water is released into the environment These plants consist of a series of closely spaced, parallel discs which are mounted on a motor-driven rotating shaft that is supported above the surface of wastewater. 40% of the disc area is immersed in wastewater, and these plants usually protrude out of the ground.

III.Submerged Aerated Filter (SAF)

The submerged aerated filter has a settling tank that clarifies solids that slough from the filter. The plant has a 1-4m deep bed made of filter which grows biomass. Air is provided through a blower to the bottom of the bed to support the oxidation process. The air stream helps the efficient mixing of the effluent and disturbance that any excess solids could create from the filter.