Question

What is primary treatment of sewage?

Answer 1

Wastewater treatment, or sewage treatment, is the removal of impurities from wastewater, or sewage before they reach natural water bodies such as rivers, lakes, estuaries, and oceans.

Complete answer:
Physical, chemical, and biological processes are used to eliminate contaminants and produce treated wastewater that is safe enough to release into the environment. A sewage sludge is a by-product of sewage treatment. The sludge has to undergo further treatment before being suitable for disposal or application to land.
Sewage treatment occurs in three stages, called primary, secondary and tertiary treatment.
Primary treatment- This process eliminates floating material and those that settle due to gravity. It includes physical processes like screening, comminution, grit removal, and sedimentation. The sewage moves through the following instruments: screens, comminutor, grit chamber, and primary clarifier.
The screens are made of long, narrow metal bars, spaced closely. They block floating debris such as wood, rags, and other bulky materials. In new plants, the screens are cleaned mechanically, and the materials are disposed of on plant grounds. The screened materials are then passed into the comminutor, where they are used to grind and shred debris. The shredded material is removed later by sedimentation or flotation processes. Grit chambers are long narrow tanks, which are designed to reduce the flow so that solids such as sand, eggshells, etc. will settle out of the water. Suspended solids are, then, removed from the sewage into sedimentation tanks, also called primary clarifiers. The sewage is placed in the clarifier for two hours for the settling to take place. Mechanically driven scrapers move the collected sludge towards a hopper in the base of the tank from where it is pumped out to sludge treatment facilities. Grease and oil from the floating material are removed or sometimes recovered for saponification (soap-making).

This is followed by secondary and tertiary treatment. Secondary treatment removes dissolved and suspended biological matter. It involves the use of indigenous, water-borne microorganisms in a specific habitat. Before the discharge or tertiary treatment, the microorganisms are removed from the treated water. In tertiary treatment, the treated water is sometimes disinfected chemically or physically, by lagoons and microfiltration, before releasing them into water-bodies.

Note: In primary treatment, heavy solids are allowed to settle to the bottom while oil, grease and lighter solids float to the surface. These materials are, then, removed and the liquid may be discharged or taken for secondary treatment. After the whole treatment process, if it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.