The sample treatment used in process monitoring will effect the result and must be born in mind when interpreting the results. In the case of the final effluent the result should be the best approximation of total phosphorus as that is the consent criteria, so the less filtration the better but the system needs to be protected from potential blockage.
In the experiment above it indicates that filtration of 100, 20 or 10 micron gives the same result. As 0.45 micron is the declared analytical cutoff for 'soluble orthophosphate' level of filtration gives a meaningful result but is it representative of the final effluent?
The removal of phosphate during the sewage treatment process has become a crucial area of interest as more and more works have a Phosphorus discharge consent and with the ever increasing use of detergents containing phosphate the problem is growing. The natural removal of both Nitrate and Phosphate where applicable is the method of choice, however, in most cases it is not possible and therefore the Water Company has to resort to chemicals.
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 is carried out by dosing metal salts most suitably aluminium and iron i.e. aluminium sulphate, sodium aluminate, ferric/ferrous chloride, and ferric/ferrous sulphate. The addition of aluminium or ferric salts has the potential to lower the pH of the effluent although most UK wastewaters 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 proportiona l control algorithm. However recent developments in on-line analysers have made control algorithms based on phosphate loading a practical proposition.
Reliable, accurate and cost effective monitoring of Phosphate and Total Phosphorus is achieved by Partech using the MicroMac analyser, this analyser is successfully making measurement at all stages of the waste water treatment process including the inlet.