The paper deals with a research programme that the Norwegian environmental control authorities have established in order to be able to recommend how the municipal discharges of nutrients to the North Sea may be reduced by 50 % by 1995. The strategy is to ensure phosphorus removal by chemical treatment for all plants > 5000 pe, and nitrogen removal as well for all plants > 10 000 pe that discharge to the area of influence. One is aiming at compact plants. Pretreatment based on chemical coagulation and biological treatment based on high biomass reactors are investigated experimentally. An economic analysis of using physical/chemical treatment also for the N-removing step is presented. It shows that the physical/chemical methods for N-removal cannot compete economically with the biological methods.

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