Sewage can be defined as water or liquid that contains impurities or pollutants in the form of solids, liquids or gases or their combinations in such a concentration that it is harmful if disposed of into the environment directly.
Sewage coming from various resources needs to be treated before being disposed of either in rivers, streams, or on land to make it safer. The treatment of sewage is done to make the sewage harmless to living beings.
The degree of treatment depends on characteristics of source of disposal i.e. either river, stream, or land.
A sewage treatment plant consists of combination of unit processes designed to reduce certain constituents of wastewater to an acceptable limit. And these treatment processes are often classified as:
We will take a detailed look on these treatment processes below
Preliminary treatment consists of the separation of floating materials like dead animals, tree branches, papers, pieces of rags, wood along with heavy settleable inorganic solids. It also helps in the removal of oil and grease from sewage.
Preliminary treatment reduces the BOD of wastewater by about 15 to 30%.
Preliminary treatment include
Screening which removes floating elements like paper, rags, cloths etc. Grit Chamber removes silt, sand and grit. Skimming Tank removes oils and greases.
Primary treatment consists of the removal of large suspended organic solids. The liquid effluent of the primary treatment contains suspended organic matter as it is not removed in this step having high BOD(about 60% of the raw sewage).
Preliminary and primary treatment are sometimes considered as one and can be classified together under primary treatment.
The organic solids which are removed in the primary treatment are stabilized by anaerobic decomposition in a digestion tank or incinerated and the residues are either used for landfills or as a soil conditioner.
Primary treatment doesn’t remove the colloidal and dissolved organic matter. Big materials are removed by screen in preliminary treatment or by grinding to smaller pieces in primary treatment.
Disposal of organic matter is a difficult task because if it disposed of without any treatment then it will decompose and create foul odour. Disposal of inorganic matter is simple task as inorganic matter does not decompose. For the purpose of disposal organic and inorganic matter are removed in two different tanks.
Secondary treatment involves further treatment of the effluent, coming from primary sedimentation tank by biological decomposition of organic matter, which can be carried out either under aerobic of anaerobic conditions.
In these biological units, bacteria will decompose the fine organic matter to produce clearer effluent.
The reactors in which organic matter is decomposed by aerobic bacteria are known as aerobic biological units. These aerobic biological units consist of a) Filters, b) Aeration Tank, c) Oxidation Ponds, and Aerated lagoons. All of these are aerobic units that make use of primary settled sewage(the effluent of primary treatment) as influent and that is why they can be classified as secondary treatment units.
The reactors in which organic matter is destroyed and stabilized by anaerobic bacteria are known as anaerobic biological units. These anaerobic biological units consist of a) Anaerobic Lagoons, b) Septic Tank, c) Imhoff Tank. All of these are anaerobic units but only anaerobic lagoons make use of primary settled sewage hence they can only be considered in secondary treatment. As septic tank and Imhoff tank make use of raw sewage directly that is why they can’t be classified in secondary treatment.
The effluent of secondary treatment consist of very little BOD that is about 5 to 10% of raw sewage which means there is 90 to 95% BOD reduction in secondary treatment.
Tertiary treatment consists of removal of organic matter left after the secondary treatment and also to kill pathogens. The treatment is carried out by chlorination and thus not for disposal of sewage in water but for reuse or for water supply.
The method is also called as The final or advanced treatment. Generally, tertiary treatment is carried out on municipal wastewater to meet effluent standards.
These three different processes contain different combinations of treatment units. These combinations can also be changed and made according to the local needs. The selection of treatment units also depends on topography and geology. For example, trickling filter plant consumes high head and that’s why require steep ground slope to avoid pumping.