Oxidation Ponds (or Stabilization Ponds) are a popular sewage treatment method for small communities because of their low construction and operating costs. Oxidation ponds represent 12 per cent (500 numbers) of all sewage treatment plants. New oxidation ponds can treat sewage to Standard B effluent level but require maintenance and periodic desludging in order to maintain this standard.

OPs may comprise one or more shallow ponds in a series. The natural processes of algal and bacteria growth exist in a mutually dependent relationship.

Oxygen is supplied from natural surface aeration and by algal photosynthesis. Bacteria present in the wastewater use the oxygen to feed on organic material, breaking it down into nutrients and carbon dioxide. These are in turn used by the algae. Other microbes in the pond such as protozoa remove additional organic and nutrients to polish the effluent.

There are normally at least two ponds constructed. The first pond reduces the organic material using aerobic digestion while the second pond polishes the effluent and reduces the pathogens present in sewage. Sewage enters a large pond after passing through a settling and screening chamber. After retention for several days, the flow is often passed into a second pond for further treatment before it is discharged into a drain. Bacteria already present in sewage acts to break down organic matter using oxygen from the surface of the pond. Oxidation ponds need to be desludged periodically in order to work effectively.

OPs require large amounts of land and the degree of treatment is weather dependent. They are incapable of achieving a good standard of effluent consistently. It is this variation in performance, which require the gradual phasing out of this type treatment plant.

Depending upon the design. OPs must be desludged approximately every 10 years.

Typical figures for OPs are as follows: