Sewage Treatment Plant - Basic

Sewage Treatment is the process of removing contaminants from wastewater and household sewage, effluents and domestic wastes.

It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. 

Its objective is to produce an environmentally-safe fluid waste stream and a solid waste suitable for disposal or reuse. 

It reduces portable water demand of a building.

The sewage treatment involves three stages, called primary, secondary and tertiary treatment.

 First, the solids are separated from the wastewater stream. 

Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne micro-organisms. 

Finally, the biological solids are neutralized then disposed of or re-used, and the treated water may be disinfected chemically.

The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park.

Dissolved Oxygen: 

Normally, the biodegradable material in the sewage consumes oxygen when it degrades.

If this sewage is released in lakes/rivers, it would draw naturally dissolved oxygen from water, depleting the oxygen in the lake/river. 

This causes death of fish and plants. But the STP provides enough oxygen to digest the biodegradable material in sewage. 

The treated sewage does not need oxygen any longer. Thus it does not affect the aquatic life in lakes and rivers.

Biochemical Oxygen Demand (BOD):

The BOD of the sewage is the amount of oxygen required for the biochemical decomposition of biodegradable organic matter under aerobic conditions. The oxygen consumed in the process is related to the amount of decomposable organic matter. The general range of BOD observed for raw sewage is 100 to 400 mg/L. Values in the lower range are being common under average Indian cities. 

Chemical Oxygen Demand (COD):

The COD gives the measure of the oxygen required for chemical oxidation. In general, the COD of raw sewage at various places is reported to be in the range 200 to 700 mg/L 

Turbidity 

After Treatment < 10 NTU2 The outgoing treated sewage has low turbidity (suspended particles that cloud the water). In other words, we get “clear” water.

This prevents the pipelines from getting clogged by settled sediments. If cloudy water is allowed to reach the lakes and rivers, it blocks the sunlight from reaching the bottom of the water body. 

This stops the photosynthesis process of the aquatic plants, killing them. That in turns stops generation of oxygen as a by product of the photosynthesis process.

Depletion of dissolved oxygen in water kills all fish.