Sewage Water Treatments
- Preliminary treatment consists of screening and grit removal. Floating solids such as sticks, rags, cloth, sand, gravel and large food particles are removed at this stage to protect the pipes and pumps in the treatment plant from clogging and ensuring effective treatment of sewage. Treatment equipment used for preliminary treatment include bar screens and grit chambers. The collected debris is disposed into a landfill.
- Primary treatment is used to separate fats, greases and suspended solids from the sewage. A sedimentation tank or primary clarifier is used for this purpose. Sewage or wastewater is held in a tank for several hours during which the suspended solids settle at the bottom and the greases float to the top. The solids collected at the top and bottom are skimmed off and undergo further treatment as sludge. This clarified wastewater flows on to the next stage of sewage treatment.
- Clarified wastewater undergoes secondary treatment, a biological process that utilizes microorganisms to break down organic matter present in the sewage. These microorganisms use the organic matter as their food and convert complex organic compounds into simple, easily settleable matter. There are three ways to accomplish secondary treatment:
1.Fixed film systems: These systems grow microorganisms on substrates like sand, rocks or plastic. The sewage is spread over this substrate, allowing the microorganisms to absorb nutrients and organic matter from the sewage. Examples of fixed film systems include trickling filters, rotating biological contractors and sand filters.
2.Suspended film systems: These systems suspend and stir microorganisms in the sewage or wastewater. The microorganisms absorb and break down organic matter with the help of aeration and agitation. As the microorganisms absorb more organic matter, they grow in size and number. After several hours, these microorganisms settle out as sludge. Some of this sludge is pumped into the incoming sewage to act as "seed" microorganisms while the remaining sludge is sent for further treatment. Examples of suspended film systems include activated sludge process, oxidation ditch, and sequential batch reactors.
3.Lagoons: Shallow basins that allow natural degradation of sewage.
Wastewater obtained at this stage is further treated to produce a liquid effluent that is safe to dispose or be sent for advanced treatment after which the water can be reused. - Tertiary treatment is often employed to enhance the quality of effluent. Typical processes include nutrient removal (phosphorous and nitrogen) and disinfection methods using ultraviolet light, ozone or chlorine.
