Since November 2006 a large-scale research project has been carried out at Wastewater Treatment Plant (WWTP) Leiden Zuidwest (within the Rijnland District Water Control Board). This research focuses on advanced removal of nutrients (phosphorus and nitrogen), heavy metals and priority hazardous substances from WWTP-effluent with different treatment techniques to reach an effluent quality, which could be required in the future by the Water Framework Directive (WFD) 2000/60/EC. Within the WFD-approach to guarantee an ecological and a chemical “good status” of the receiving water bodies, the focus is more and more on ultra low phosphorus concentrations in effluent. To be able to reach these stringent goals more insight into phosphorus components in effluent is required. A new method of distribution of phosphorus is used to determine orthophosphate, metal bound phosphorus, dissolved “organic” phosphorus and particulate “organic” phosphorus. This knowledge about the distribution of phosphorus makes it possible to compare different filter concepts and different process parameters, for example flocculation time, initial mixing energy and filtration rates. When comparing (filter concept 1) continuous sand filtration with (filter concept 2) dual media filtration for phosphorus removal, it appears that, a higher percentage of the formed metal bound phosphorus will pass the continuous sand filter. The ortho-phosphorus which is not bound to trivalent metal after coagulation will remain dissolved ortho-phosphorus and will pass the filter bed. This is shown in both filter concepts. The dissolved ‘organic’ phosphorus decreases after flocculation and the particulate ‘organic’ phosphorus increases which suggests that it may be colloidal or associated with colloidal material. With continuous sand filtration 50% of the particulate phosphorus is removed. In the dual media filter even 86% of the particulate phosphorus is removed.

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