Sewage Treatment Plant (STP)

Sewage treatment is the process of removing contaminants/ pollutants from wastewater, mainly from household sewage. Sewage is the wastewater discharged from residences, workplaces, and industries that contains human waste, domestic trash, and other impurities. This wastewater treatment process helps to prevent water pollution and protect public health and the environment.

Treatment Process of STP

• Preliminary Treatment
  • Screening, grinding and separating large solids and debris from wastewater is the first and foremost stage in wastewater treatment. Upon arrival to the sewer system, the wastewater is sent through a bar screen, which removes large solid objects such as sticks, large food particles, sand, rags, gravel etc. All the gathered debris from the bar screen and grit tank is usually disposed of at a sanitary landfill.

• Primary Treatment
  • It is the second step in wastewater treatment. It allows for the physical separation of solids and greases from the wastewater. The screened wastewater flows into sedimentation tanks where gravity pushes solids to the bottom and oils and greases to float to the top of the tank. Solids are drawn off the bottom, and grease is skimmed off the top and sent on to be treated as sludge/slurry. The clarified wastewater flows on to the next stage of wastewater treatment. Clarifiers and septic tanks are generally used for the purpose of primary treatment.

• Secondary Treatment (Biological Treatment)
  • It is the biological treatment stage where micro organisms are mixed with wastewater that breaks down organic pollutants present in wastewater. Naturally occurring microorganisms are cultivated and feed on the pollutants in the wastewater. Organic matter is consumed and ammonia is converted to nitrate. Three methods are used for Secondary treatment, Suspended film, Fixed film and Lagoon systems.

• A. Activated Sludge/Suspended Film Systems
  • Here, wastewater is mixed and aerated with microorganisms. The clarified wastewater becomes hazy as microorganisms are poured into the sewage. For several hours, mixers and aerators stir the wastewater-microbe mixture as the organic matter and ammonia is consumed. Then, the microorganisms are removed from the wastewater by allowing the mixture to flow to a settling tank. The microbes sink to the tank bottom, are collected and are returned to the mixing tank. The clear, treated wastewater flows out of the tank to the tertiary stage of treatment.

• B. Trickling filters/ Fixed Film Systems
  • Wastewater is sprayed over rocks, sand or even small bits of plastic that are covered with a thin, microscopic film of microorganisms. As the wastewater flows over the microbial film, organic matter and ammonia are consumed.

• C. Lagoon Systems

Large, shallow ponds collect and hold wastewater for months where naturally occurring microorganisms consume the organic matter and stabilize the wastewater. Lagoon systems may be naturally aerated or be equipped with a mechanical aerator to gently stir the wastewater in order to promote good interaction between the microorganisms and the pollutants.

• Odor Control
  • Odor control in sewage treatment plants is important to reduce nuisance odors that can impact surrounding communities. Some common methods used for odor control are:

·       Chemical Scrubbers: They use a chemical solution to neutralize odorous gases. The gases are passed through the solution, where they react with the chemicals and are converted into non-odorous compounds.

·       Biological Filters: They use micro organisms to break down odorous compounds in the air. The filters contain a media that provides a habitat for the microorganisms to grow and thrive.

·       Activated Carbon Adsorption: Activated carbon is very effective at adsorbing odorous gases. Air is passed through a bed of activated carbon, where the odorous compounds are adsorbed onto the surface of the carbon particles.

·       Ozone Treatment: Ozone is a powerful oxidizing agent that can effectively neutralize odorous compounds. Ozone treatment systems generate ozone gas, which is injected into the air stream to oxidize odorous compounds.