Proceedings

Under the Water Framework Directive (WFD), Severn Trent Water (STW) are receiving a number of Total Phosphorus (Total P) permits <1mg/l in AMP6, with more in AMP7. Four existing single oxidation sites and one oxibox site (with Population Equivalents <5000) with Total P permits ranging from 0.5 to 0.7 mg/l have already been commissioned. On all these sites the approach has been taken to phase the solution, with the iron coagulant chemical dosing into the ASP being installed first. Performance was then monitored for 3 to 6 months, to ascertain whether Tertiary Solids Removal (TSR) is required to ensure Total P and iron compliance. After reviewing the data it has been agreed on three sites that TSR is not required. On one site further data is required to make the decision, and the results from the final site are borderline due to sporadic final effluent solids. It has also been discovered that at these lower P levels the normal assumed ratios of Ortho P to Total P no longer apply, meaning obtaining Total P data is essential when making decisions.