Aiming at Complete Nitrogen and Phosphorus Removal from WWTP Effluent – The Limits of Technology

Since November 2006 a large-scale research project is executed at Wastewater Treatment Plant (WWTP) Leiden Zuidwest (within the Rijnland District Water Control Board). This research focuses on advanced removal of nutrients (nitrogen and phosphorus), heavy metals and priority hazardous substances from WWTP effluent with different treatment techniques to reach an effluent quality, which will fulfil possible future demands of the Water Framework Directive (WFD) 2000/60/EC. Within the WFD the focus is more and more on ultra low nitrogen and phosphorus concentrations in receiving surface water. Ultra low nitrogen concentrations can be reached with denitrifying sand filters as a tertiary treatment step. When combining nitrogen and chemical phosphorus removal in a single filter concept, phosphorus limitation may occur. Results show that phosphorus limitation occurs when the PO4-P/NOx-N is below 0.005. Aiming at ultra low phosphorus concentrations can be reached by using phosphorus fractionations and distributions into particulate “organic” phosphorus, dissolved “organic” phosphorus, metal bound phosphorus and orthophosphate for the optimisation of process parameters like initial mixing. Results show that the initial mixing energy has no influence on the phosphorus fractionation and distribution in the upper water layer of a dual media filter.