Analysing and Removing Phosphate in Sewage Treatment

The removal of Phosphate during the sewage treatment process has become a crucial area of interest as more and more works have Phosphorus discharge consent. With the ever increasing use of detergents containing phosphate the problem is growing.

The natural removal of both Nitrate and Phosphate is the method of choice where applicable. However, in most cases this is not possible and therefore water companies have to resort to chemicals, the most commonly used being Ferric Chloride or Ferrous Sulphate. In some cases where the efficiency of iron is much reduced the use of Aluminium Salts has been successfully applied and works normally in conjunction with an iron salt. In all cases the amount of chemical used is critical for the performance of the works, cost control and meeting the metal discharge consent.

The removal of phosphate from the effluent of a sewage works comes under the Urban Waste Water Treatment Directive (UWWTD) (91/271/EEC) and it requires the control of nitrogen and phosphorus input into water bodies by specifying minimum treatment requirements to be achieved, related to the population equivalent (PE), this being the size of the area served, and the nature of the receiving water.

Requirements:

Total Phosphorus 2 mg/l as P 10,000 – 100,000 pe
1 mg/l as P >100,000 pe
Or
80% removal of the influent load

The UWWTD requires measurement of Total Phosphorus concentration, which is determined after oxidation by the measurement of Soluble Reactive Phosphorus (SRP). This is probably the most frequently measured phosphorus determinand and is measured spectrophotometrically by the formation of a phosphomolybdenum blue complex. It is referred to as Reactive Phosphate because some polyphosphates and some organic phosphates can be hydrolysed by the test.

The removal of Phosphate is carried out by dosing metal salts most suitably aluminium, iron and calcium i.e. aluminium sulphate, sodium aluminate, ferric/ferrous chloride, ferric/ferrous sulphate and calcium hydroxide. The addition of aluminium or ferric salts will have the potential to lower the pH of the effluent, although most UK waste waters have sufficient buffering to cope with this effect. Traditionally the metal salts are dosed at a rate determined by a pre-set diurnal profile derived by analysing a series of samples over a period of time or by using a flow proportional control algorithm.