Sewage Treatment | Contents & Methods of Sewage Treatment |

Sewage Treatment.

What is Sewage?

The wastewater from bathrooms, kitchens, lavatories, etc. is called domestic sewage. The wastes which are disposed of from factories, laundries, laboratories, business houses, schools, hospitals, etc. also result in sewage. The spent water from the community as a whole is called sanitary sewage.
Sewage Treatment.


Contents of Sewage.


  1. Urea from urine, proteinaceous matters, detergents, biodegradable feces, animal wastes, fats, carbohydrates, etc. are present in sewage.
  2. Inorganic impurities as nitrates, phosphates, detergents, surfactants, trace metals, other anions, and cations are also part of sewage.
  3. Industrial wastes are very important to generate sewage.

  4. Pathogenic bacteria cause various diseases as follows:
(a) Vibrio cholera. It causes cholera.
(b) Shigella dysentery. It causes bacillary de sentry.
(c) Salmonella typhi. It causes typhoid.



Keeping in view the public health, sewage has to be properly treated.

Methods of Sewage Treatment.


The following two characteristics are kept in mind to understand sewage treatment.
  1. The content of suspended solids.
  2. The biological oxygen demand (BOD) of the sewage.
Some major treatment methods which are generally employed are mentioned below.

Preliminary Treatment:

This treatment involves the removal of gross solids as large floating and suspended solid matter, grit oil, and grease. This is done by passing through screens, skimming tanks, and grit chambers.

Primary Treatment:

In this stage, we remove the remaining suspended settleable solids and reduce the strength of the waste Sedimentation, mechanical flocculation, and chemical coagulation are common processes in this regard. After this treatment, about 60% of the suspended solids, 30% COD, 35% BOD, 10% phosphorous, and 20% total nitrogen is generally reduced.

Secondary Treatment:

Biological processes that involve bacteria and other micro-organisms help to reduce the dissolved and colloidal organic matter. Following sequential changes are brought about.
  1. Reduction of BOD.
  2. Coagulation and flocculation of cólloidal matter.
  3. Degradation of nitrogenous organic matter to ammonia, which is then converted into nitrite and eventually to nitrate.
  4. Oxidation of dissolved organic matter to CO2.
By this secondary treatment, we achieve.
  1. About 80% reduction in COD,
  2. 90% reduction in BOD,
  3. 30% reduction in phosphorous,
  4. 50% reduction in total nitrogen.

Tertiary Treatment:

The main objectives of tertiary treatment processes are the removal of

  1. Bacteria
  2. Fine suspended solids
  3. Dissolved inorganic solids
  4. Final traces of organics materials
Sewage Treatment | Contents & Methods of Sewage Treatment | Sewage Treatment | Contents & Methods of Sewage Treatment | Reviewed by Genuine Chemistry on April 25, 2020 Rating: 5

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