Emissions of nitrogen (N) and phosphorus (P) from urban wastewater treatment (UWWT) plants

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During the period 1970 to 2000, several countries showed a marked increase in the percentage of population connected to urban waste water treatment plants, especially those employing tertiary treatment involving the removal of nutrients. In the European countries included in Figure 1, the percentage of population connected to tertiary treatment increased from almost 0 to 70 or 80 % depending on the country. Over the same period the discharge of phosphorus and nitrogen from waste water treatment plants decreased by 70 % and 50 % respectively. This difference reflects the fact that nearly all the tertiary treatment plants employ phosphorus removal whilst only some of the plants, in particular the large plants, employ nitrogen removal. These results can be linked with the objectives of some of the Marine Conventions, like HELCOM and OSPAR, that require a reduction by 50 % of emissions of nutrients from land-based sources. The level of emissions, however, is still high and shows that the pressure exerted on the environment from nutrients emitted from UWWT plants may still contribute to eutrophication problems.