The main purpose of the tertiary treatment is to remove dissolved nutrients, especially nitrogen and phosphorus which have survived the primary and secondary treatment.
The most important nutrient causing eutrophication is phosphorus. Phosphorus can be removed by precipitation with coagulant, like alum or lime or ferric chloride.
However, very less organic matter is left after the primary and secondary treatment and hence, some amount of organic matter not harmful, needs to be provided.
The purpose of the tertiary treatment is to provide suitable water for drinking. Therefore, besides nutrient removal, some amount of fine suspended solids, some amounts of bacteria, small amount of organic material should also be removed.
Thus, water can be passed through activated charcoal which removes fine suspended solids, some bacteria and small quantities of organic material still present in the water through absorption.
Some absorbents (ALM series) can also be utilized to remove heavy metals, if present. Removal of toxic metals can also be done by ion-exchange process. Finally, the water obtained is disinfected through processes mentioned earlier.
An oxidation pond is a large, shallow pond (0.5 to 1.5 m depth), where the waste water, mainly the sewage water, is treated through its decomposition by microorganisms. The waste water enters the pond at one end and the treated water is removed at the opposite end.
Although waste water treatment in oxidation pond is considered to be aerobic process, both aerobic and anaerobic processes operate together. The aerobic decomposition mainly takes place near the surface and anaerobic process, at the bottom.
The oxygen required for the metabolism of bacteria is obtained from the surface through aeration as well as from the algae present in the pond and in turn the bacteria supplies C02 by decomposing the waste.
For good supply of oxygen by algae, the ponds are made shallow so that sunlight can penetrate well, helping photosynthesis. The waste thus, present at the upper part of the pond, undergoes aerobic decomposition to form C02 and H20.
The solids present in the waste get settled to the bottom and undergo anaerobic decomposition to form CH4, C02 and NH3. The oxidation pond, with both the aerobic and anaerobic conditions prevailing is called facultative pond.
The bad odours due to anaerobic decomposition however, creates problem and thus, waste water before being released into the pond should be treated to remove the solid materials which may settle down and aeration should be provided throughout the pond by mechanical stirring.
Secondly, in winters when photosynthesis gets retarded, oxygen supply is reduced creating an anaerobic condition and causes same problem with odours.
Water treatment in oxidation pond is cheap and the maintenance is simple. However, it requires larger space and causes odour problems.