Phosphorus Removal from Waste Water

Phosphorus Removal from Waste Water

Phosphorus Removal

Characteristics of Phosphates in WW

Phosphorus is a constituent of municipal wastewater, averaging around 15 - 10 mg/L. It exist in 3 forms Organically -bound Body waste and phosphorus food waste

Polyphosphates-Usedextensivelyin detergents and contributes to about half the phosphorus in WW

Orthophosphates-Resultsdueto biological decomposition of organically bound phosphates and hydrolysis of polyphosphates

Thus,  the  principal  phosphate  found  in  WW

Orthophosphates consist of (phosphate) PO3-4 , HPO42- and H2 PO4-and form chemical bonds with cations and positive radicals.

These compounds are highly soluble, thus negligible removal occurs in primary treatment. However, < 3mg/L is removed in biomass from secondary treatment due to utilization by microorganisms.

At Slightly Acidic pH

Chemical precipitation is the principal method used to remove phosphorus. At slightly acidic pH, orthophosphates combine with trivalent aluminum or iron cations to form a ppt.

Since domestic wastewater only contains trace amounts of iron and aluminum, thus, Alum (aluminum sulphate) or Ferric Chloride will have to be added.

At Higher pH

Calcium forms an insoluble complex with phosphate at pH >9.0. The addition of lime can provide both the calcium and pH adjustments necessary.

 Process Selection

The removal of phosphorus can occur as part of the primary or secondary treatment process or as a tertiary process.The choice of process depends on efficiency requirements,

a.             If up to 1mg/L is acceptable for discharge, iron or aluminum salts added to the primary or secondary process is often done.

b.           If greater efficiency is needed, tertiary system is employed with the addition of lime.