Aquatic Pollution and Coastal Management

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.5.4.Tertiary Treatment
If a high quality of effluent is required, it may be necessary to give further treatment. The aim of the tertiary treatment is ‘polishing’ the secondarily treated sewage to remove suspended solids and nutrients. The liquid derived from secondary treatment may be retained in sedimentation ponds or passed through sand or earth filters to remove suspended solids. To reduce plant nutrients (nitrogen and phosphorus), both chemical and biological methods can be employed.

Phosphorus can be precipitated chemically as calcium phosphate by substances such as lime, ferrous/ferric chloride, sulphate alum, etc at pH above 11.

Al₂(SO₄)₃ + H₂PO₄ + 3H₂ ——> Al(OH)₃ + AlPO₄ + 3SO₄^-2 + 4H₂O

Ammonia can also be stripped off the effluents. In air stripping of ammonia, the ammonium ions are converted into ammonia gas by increasing the pH, the gaseous ammonia is then removed into air by agitating the waste water.

NH₃ + H₂O ——> NH₄ + OH^-