Proceedings

STW A had been experiencing final effluent ammonia peaks, most pronounced under storm conditions, very close to its consented value for several years. The works is medium-sized with conventional primary tanks and two stage biological filtration – plastic media primary percolating filters and blast furnace slag secondary filters. Comprehensive site sampling revealed the primary percolating filters to be the main underperforming treatment unit compared to the Thames Water filter model predictions. Nitrification inhibition tests were carried out together with investigation of a potential additional organic nitrogen load being brought to the works with the crude sewage. Further assessment, comparing the organic loading and irrigation rates of first stage plastic filters within the Thames Water region was also carried out. This pointed towards the lack of primary filter recirculation as a possible reason for the observed poor performance. A site trial was carried out, where temporary recirculation around one of the primary filters was initiated. A fixed speed pump was used to achieve the target irrigation rate. Results will be presented showing the improved performance, with primary filter effluent ammonia concentrations reaching levels closer to the model predicted. A full scale trial is currently underway.