Skip to main content

Skimming tank; degree of treatment factors

Short answer

Skimming tank removes floating oil/grease; placed after screens/grit chamber, before primary sedimentation. Degree of treatment depends on effluent standards, sewage characteristics, receiving water flow and uses, disposal on land, climate, cost/resources, and public health/aesthetic requirements.

With the neat sketch, describe the purpose and location of skimming tank. Explain the factors that govern the degree of treatment required of municipal wastewater.

Skimming tank: purpose and location

A skimming tank is a small aerated chamber that removes floating oil, grease, fat and scum. The purpose is to prevent these materials from forming scum in the primary tanks, clogging filters and disturbing biological treatment and sludge digestion.

Location: in the treatment train it is placed after the screens and grit chamber and just before the primary sedimentation tank.

  Screen -> Grit chamber -> SKIMMING TANK -> Primary sedimentation
                                 |
                          grease/scum removal
          air ->  ~~~~~ water level ~~~~~
                 baffle (scum board) | outlet below baffle

Aeration (about 0.3 m³ air/m³ sewage) lifts the grease, and a dip baffle holds it back so it can be skimmed off by hand or a mechanical scraper. It is sometimes combined with the grit chamber.

Factors governing the degree of treatment of municipal wastewater

The degree of treatment (percent removal of pollutants needed) is fixed by the quality required at the point of disposal.

  1. Quality standards: the effluent standards laid down by the authority (for Nepal, the Generic Standards for wastewater discharge of the Government of Nepal).
  2. Characteristics of sewage: BOD, suspended solids, and presence of industrial wastes or toxic compounds.
  3. Receiving water body: its flow (dilution), DO, existing BOD and its self-purification capacity.
  4. Uses of the receiving water: drinking-water source, bathing, fishing, irrigation or industrial use downstream need higher treatment.
  5. Disposal on land: soil type, groundwater level, crop to be irrigated and health risk.
  6. Climate and temperature: higher temperature speeds up oxidation but lowers dissolved oxygen.
  7. Cost and available resources: land, funds, power and skilled operators; the choice between simple (pond) and complex (ASP) plants.
  8. Public health and aesthetic requirements: odour, nuisance, protection of aquatic life.

The more sensitive the use of the water and the smaller the dilution, the higher the degree of treatment (preliminary, primary, secondary or tertiary).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Sanitary Engineering chapter 7 questions →