The Treatment of Domestic Sewage

In the treatment of domestic sewage it is usual to try to achieve a considerable reduction in the normal pollutants that occur in crude or raw sewage.

Usually in the U.K., the design criteria for a sewage treatment works is based on the Population Equivalent ( Pe.) loadings to a works, or the actual pollution loads.

A normal person in a dwelling will discharge on average approximately 200 litres of sewage containing 60 grams of BOD5 per day. A dwelling may on average contain upwards of 3 to 4 persons.

Sewage Treatment is normally carried out by the simple processes of, settlement and biological treatment, as the sewage is generally a biological material.

In modern sewage treatment plants, they usually employ various methods of achieving this, all to the same effect.

Some plants are more complex than others, but basically they are all simple, and usually comprise of the following stages to effect a reduction of the pollutants

  • Primary Settlement - to remove the heavier or cruder particles.
  • Biological treatment - to effect a reduction in the biological loading.
  • Final Settlement - to remove any finer suspended solids in the liquid.

In England & Wales it will be up to the Environment Agency, to give a consent to discharge to a watercourse, controlled waters, or underground water.

The final effluent quality will be dependant upon the receiving water or ground strata into which it is discharged.

It is normal in the U.K. in a sewage treatment plant to achieve an effluent quality of at least the following quality:

  • Suspended Solids (S.S.) = 30 mg/l.
  • Biochemical Oxygen Demand (BOD 5) = 20 mg/l.

Some plants offered may well achieve a higher quality of effluent.

If however a more rigorous standard is required, by say the Environment Agency, then these figures may be lower and a standard for Ammonia (as N.) may be imposed on the effluent quality.

For further impartial advice on any problems associated with all types of sewage and effluent treatment contact Johnston Smith Consulting - The Independent Specialists.