The European Union presented in 2000 the European Water Framework Directive (WFD). Within the WFD approach the focus is more and more on ultra low nitrogen (<2.2 mg N-total/L) and phosphorus concentrations (<0.15 mg P-total/L) in waste water treatment plant (WWTP) effluent. Nitrogen concentrations of 2.2 mg N-total/L can be reached with denitrifying filters as a tertiary treatment step. When combining nitrogen and chemical phosphorus removal in a filter, phosphorus limitation may occur. After nitrite accumulation, caused by phosphorus limitation, was measured in the filtrate water of the dual media filter, research was started to locate, to understand and to solve the problem. Profile measurements for nitrate, nitrite and orthophosphorus (PO4-P) combined with COD and O2 were conducted. Results show that the minimal required PO4-P/NOx-N ratio is 0.006 mg/mg after coagulation and flocculation. Profile measurements have proven to be a useful tool. It showed how nitrate and orthophosphorus are removed through the filter bed and it showed the consequences for the PO4-P/NOx-N ratio. When orthophosphorus is removed more rapidly compared to nitrate the PO4-P/NOx-N ratio decreases. When the PO4-P/NOx-N ratio becomes below 0.006 mg/mg for a certain period of time, orthophosphorus limitation occurs. The solution can be either changing the filter bed configuration or decreasing the coagulant dosage.

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