WASTEWATER TREATMENT PLANT/ SEWAGE TREATMENT PLANT

• Wastewater treatment plants is usually based on the need to reduce organic and suspended solids matters to limit pollution of the environment.To control the water pollution, a proper treatment is required for waste water before discharge it into the natural water bodies.                 
• A waste water plant have a some units namely, 

1. Screening

2. Grit chamber
3. Primary sedimentation tank
4. Secondary sedimentation tank
5. Disinfection tank or Chlorination tank
6. Thickening and sludge dewatering
7. Anaerobic digester
8. Sludge disposal 


The schematic representation of waste water treatment plant is given below.









After collecting the wastewater to treatment plant through drainage lines, Several processes occur on it which are given below.



1.Screening :

Screening is the first unit operation used at wastewater treatment plants. Screening involves the removal of large objects for example nappies, cotton buds, plastics, diapers, rags, sanitary items, nappies, face wipes, broken bottles etc. These solid wastes are removed from the wastewater and then disposed off in landfills. A separate device also use to remove dissolved pollutants from water like oil, grease etc. which is called skinning device.

2. Grit chamber :

After the screening process, the water flows from the grit chamber where it removes sand, gravel, cinder, or other heavy solid materials that are heavier (higher specific gravity) than the organic biodegradable solids in the wastewater. 


3.Primary sedimentation tank :

This process involves the separation of macrobiotic solid matter from the wastewater. Primary treatment is done by pouring the wastewater into big tanks for the solid matter to settle at the surface of the tanks. The sludge, the solid waste that settles at the surface of the tanks, is removed by large scrappers and is pushed to the center of the cylindrical tanks and later pumped out of the tanks for further treatment. The remaining water is then pumped for secondary treatment.


4. Secondary sedimentation tank :

The secondary treatment stage involves adding seed sludge to the wastewater to ensure that is broken down further. Air is first pumped into huge aeration tanks which mix the wastewater with the seed sludge which is basically small amount of sludge, which fuels the growth of bacteria that uses oxygen and the growth of other small microorganisms that consume the remaining organic matter. This process leads to the production of large particles that settle down at the bottom of the huge tanks. The wastewater passes through the large tanks for a period of 3-6 hours. It is also known as biological treatment. 

There are different type of biological units are used.

1. Activated sludge process
2. Oxidation pond
3. Rotating disc biological contractor
4. Tricking filter

5. Disinfection tank or chlorination tank

After the primary and secondary treatment stages are directed to digesters. The digesters are heated at room temperature. The solid wastes are then treated for a month where they undergo anaerobic digestion. During this process, methane gases are produced and there is a formation of nutrient rich bio-solids which are recycled and dewatered into local firms. The methane gas formed is usually used as a source of energy at the treatment plants. It can be used to produce electricity in engines or to simply drive plant equipment. This gas can also be used in boilers to generate heat for digesters.


6. Thickening and sludge dewatering :

The sludge that is produced and collected during the primary and secondary treatment processes requires concentration and thickening to enable further processing. It is put into thickening tanks that allow it to settle down and later separates from the water. This process can take up to 24 hours. The remaining water is collected and sent back to the huge aeration tanks for further treatment.


7. Anaerobic digester :

After the primary and secondary treatment stages are directed to digesters. The digesters are heated at room temperature. The solid wastes are then treated for a month where they undergo anaerobic digestion. During this process, methane gases are produced and there is a formation of nutrient rich bio-solids which are recycled and dewatered into local firms. The methane gas formed is usually used as a source of energy at the treatment plants. It can be used to produce electricity in engines or to simply drive plant equipment. This gas can also be used in boilers to generate heat for digesters.

8. Sludge disposal :

In the last stage, The sludge is treated and sent back into the environment and can be used for agricultural use or land fill.