Minimising the risk on phosphorus limitation; an optimised coagulant dosage system

In the near future the WWTP Horstermeer will be confronted with more stringent requirements. Important factor in this respect is the European Water Framework Directive 2000/60/EC. Within the WFD approach the focus is more and more on low nitrogen (<2.2 mg N-total/L) and phosphorus concentrations (<0.15 mg P-total/L) in WWTP effluent. Therefore, research has been initiated at the Horstermeer WWTP to investigate the possibilities of simultaneous nutrient removal with filtration techniques. After nitrite accumulation, caused by phosphorus limitation, was measured in the filtrate water of the 1-STEP^® filter, research has started. The objective of this research is to minimise the risk on the inhibition of denitrification by phosphorus limitation and to decrease the dosage of coagulant. Therefore a new coagulant dosage system is developed. The back-ground of the coagulant dosage system, the results and cost savings are discussed in this article. When the PO₄-P/NO_x-N ratio in the WWTP effluent becomes below 0.06 mg/mg phosphorus limitation may occur. The results for phosphorus and nitrogen removal are equal or improved compared to the yearly average of 2008. The results show a more stable removal process and a reduction of the coagulant dosage for the coagulant dosage system of 11% when compared to a continuous dosage ratio of MeP 4 mol/mol.