An oxidation pond (also called waste stabilization pond) is an artificial pond in which sewage can be retained for a sufficient time to satisfy the biochemical oxygen demand (BOD), and thereby make the sewage non-putrescible.
The purifying action in an oxidation pond can be explained because of a unique relationship between bacteria and algae in shallow ponds.
The bacteria metabolize organic matter releasing nutrients like nitrogen, phosphorous and carbon dioxide.
Algae use these compounds along with energy from sunlight for synthesis of food releasing oxygen in wastewater.
Bacteria take up oxygen released by algae, thus closing the cycle.
This type of relationship between bacteria and algae is called symbiosis.
This is common among organisms living in small ponds and streams, where two or more species live together for mutual growth and development.
Besides these, other microorganisms in wastewater stabilization ponds like protozoa, rotifers etc.
, also live in these waters and feed on algae and bacteria.
Because of the shallow depth, generally less than 2 m, the oxidation ponds act as facultative ponds in which both aerobic as well anaerobic biochemical reactions take place.
As the raw sewage without primary treatment enters the waste stabilization pond, the organic solids settle to the bottom and decompose anaerobically forming a sludge zone at the bottom and producing intermediate products.
The latter are acted upon by facultative and aerobic bacteria, and in the process converted into stabilized nutrient form.
Oxygen is added to the wastewater in the pond by wind action at the surface and from daylight metabolism of algae.