Preliminary Treatment

Preliminary Treatment of Wastewater- In-depth

Preliminary Treatment of Wastewater:

In the Effluent Treatment Plant, Preliminary treatment of wastewater is done to remove the big-sized solid particle that cannot be removed by wastewater treatment processes. These include rugs, dead animals, woods, plastics, vegetables, and heavy inorganic settle-able solids (such as grit). Solids removed at this stage are generally not recycled and are a by-product of the wastewater treatment process. These solids must be removed to avoid any wear and tear to the downstream mechanical components or to avoid clogging of the pipes. Therefore, preliminary treatment devices are designed to:

  1. Remove or reduce the size of the large sized, entrained, suspended or floating solids in Effluent.
  2. Remove heavy inorganic solids such as metal, glass, sand and gravels. Collectively referred as grits.
  3. Remove excessive amount of oil and grease.

Preliminary treatment does not remove any pathogens and does not reduce BOD, COD, or remove any other toxic compounds. Fluid velocity at this stage is kept sufficiently high enough to avoid settling of organic solids. Flow measuring devices are installed at the inlet of preliminary treatment units. During this stage head loss in fluid flow is observed because of the units used.

In preliminary treatment following units are used.

  1. Screening (Bar Screen/ Coarse Screen)
  2. Comminuting device
  3. Grit chamber and detritor
  4. Dissolved Air Flotation

1. Screening (Bar Screen and Coarse Screen):

Bar Screens at a Drain

Equal spaced Horizontal or vertical bars inclined at an angle are placed at the drain. The main function of the coarse screen is to protect downstream facilities. Screens are generally placed at an accessible location and can be further subdivided into the following three types:
a. Trash Rack: Vertically placed at an angle with spacing around 4 to 10 cm. These are manual screens and are cleaned by hand.
b. Mechanical screens: Motor-operated screens with spacing around 1 to 4 cm.
c. Fine Screens: Usually 3 to 10 mm spacing.

Waste accumulated should be removed periodically which can be disposed of by burial, disintegration, or can be used as fertilizer.

2. Comminuting Devices:

A Layout of a Comminuting device. Courtesy: Beyond Discovery

Comminuting devices or shredders are used to cut or reduce the size of solids. These are used to prevent damage to the plant. To prevent wear and tear to the Comminuting devices they are generally placed after grit chambers. These consist of screening devices and cutting devices. Each Comminuting device must contain a bypass arrangement for maintenance purposes.

3. Grit Chambers:

A layout of a Grit Removal System

Grits are heavy inorganic solids such as sand, metal fragments, stones, eggshells of specific gravity ranging 2 to 2.65. Grit chambers may be horizontal flow type or aerated.
Horizontal flow grit chambers are designed in such a way that inorganic solids will settle and lighter organic solids will re-suspend and carry away with wastewater. In aerated type, grit chamber air is diffused from at a level below the fluid level in the chamber. This reduces the specific gravity of the liquid allowing settling of inorganic solids-free from organic solids. Waste produced from grit chambers is kept covered and disposed of as earth filling or incinerated.

4. Dissolved Air Floatation:

A Layout of the Dissolved Air Flotation System

Dissolved Air Flotation (DAF) is the wastewater treatment process that clarifies by removing suspended matters such as oil, grease, and other solids. The removal is achieved by dissolving air in the wastewater under pressure and then releasing the air at atmospheric pressure in the flotation tank. The released air forms tiny bubbles which adhere to the suspended matter and bring them to the surface, where they can be removed using a skimming mechanism. The feed water sometimes is dosed with a coagulant (Ferric Chloride and Aluminium Sulphate) to flocculate them.
The air mixing mechanism involves collecting a portion of the wastewater from the stream leaving DAF into an air drum, where compressed air is mixed. The wastewater is then released back into DAF at slightly reduced pressure. Forming bubbles. The suspended solids adhere to bubbles causing solids to float to the surface. DAF is widely used in treating the wastewater from Oil refineries, Petrochemical, Paper mills, textile, and tannery sectors. In oil refineries, natural gas is used instead of air as a floatation medium due to explosion risk.


1. Introduction to Preliminary Wastewater Treatment, J. Paul Guyer, P.E., R.A., Fellow ASCE, Fellow AEI.
2. Wastewater Operator Certification Training, Pennsylvania Department of Environmental Protection.

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