Treatment procedure of sewage:-

The treatment of sewage is carried in three stages:-

A-Primary treatment

B-Filtration

C-Post filtration treatment

A) Primary Sedimentation:-

• In this stage the large, suspended particles are removed from the sewage.
• This helps to avoid clogging of the media. It also helps to have better oxidation of finely derived matter of sewage in the filter.

B)Filtration:-

The trickling filters remove the total solids of the sewage, but we must know that the purpose of the trickling is to change the characteristics of the suspended solids, rather than to remove them totally from the sewage.

C)Post Filtration Treatment:-

• The secondary clarifiers are located after the units of secondary treatment, known as trickling filters or activated sludge process units.
• It indicates the location of secondary clarifier.
• So the secondary clarifiers are provided for post filtration treatment or for treating the treated sewage.
• For making it good, to discharge it into the natural surface stream.
• These secondary clarifiers are also called as 'final clarifiers' to indicate the post filtration treatment.

Principle:-

• These filters by the function also, known as sprinkling filters.
• The sewage is allowed to sprinkle or trickle over the bed of hard, rough, and coarser material.
• The waste which has been filtered is collected through the under drainage system.
• Under aerobic conditions, the organic matter is oxidised.
• A "bio-film" i.e. bacterial film is developed around the particles of the filtering media.
• The colour of this film is varied i.e. in some cases it is blackish and in other cases it is greenish or yellowish.
• The film consists of the micro organism like bacteria, algae, fungi, protozoa. lichens.etc