Rethinking Wastewater Treatment Plant Effluent Standards: Nutrient Reduction or Nutrient Control?

M surface waters in the world suffer from eutrophication. Major investments in wastewater treatment plants (WWTPs) in developed countries have been made the last decades to meet the regulations enforcing reduction of nitrogen and phosphorus emissions. As a positive result, nutrient levels in receiving surface waters are decreasing. However, blue-green algae blooms appear to occur more often. Nitrogen limitation, as well as increased temperatures, contribute to these bluegreen algae blooms. Blue-green algae blooms can produce metabolites, toxic to many organisms including humans, presenting risks regarding safe drinking water supply and possible health problems for, for example, swimmers. Controlling nutrient levels and ratios are possibly effective means to prevent blue-green algae blooms. Although wastewater treatment plants have always been assessed by their nutrient removal efficiencies, they could also act as points for controlled nutrient release to actively control the nutrient levels and nutrient ratio in receiving surface waters. Wastewater treatment plant operators face strict effluent regulations. In Europe, they have to meet the European Urban Wastewater Treatment Directive and the Water Framework Directive (WFD), while in the United States effluent discharges to surface waters are regulated under the National Pollutant Discharge Elimination System (NPDES) and the Clean Water Act (CWA). These regulations have in common that they focus on effluent limits on a general minimal level and, where necessary, more stringent limits regarding nitrogen and phosphorus removal. This approach, however, can have a downside, which is illustrated in the following situation. European Union countries have to meet the WFD requirements, with the objective to obtain a good status (clear water, without (blue-green) algae (blooms)) of groundwater and surface waters. To get clear water, eutrophication should be reduced. Therefore, in the last few decades efforts have been made to reduce the nitrogen and phosphorus discharges into freshwater systems, with on average quite good results (see Figure 1). However, despite large reductions in nitrogen and phosphorus discharge to freshwater systems, bluegreen algae blooms occur more frequently. Unfortunately, this is the result of the accepted notion among policy makers that eutrophication can be reduced or limited by nitrogen or phosphorus limitation, despite evidence in literature, that eutrophication of freshwater systems cannot be controlled solely by nitrogen limitation. Instead, eutrophication could solely be controlled by phosphorus limitation. In addition, the ratio of nitrogen to phosphorus (N:P ratio) in surface waters should not be neglected. A low N:P ratio favors the growth of blue-green algae with nitrogen fixing capacities compared to other algae. Moreover, low nitrate concentrations can also lead to an increase in release of phosphorus from the sediment, which in turn reinforces a low N:P ratio. If the relative abundance of blue-green algae in the algae community increases, the grazing pressure of zooplankton (such as Daphnia) on algae decreases because the blue-green algae negatively impact the zooplankton. These effects create situations in which blue-green algae dominate, despite overall relatively low nutrient concentrations. To restore the water bodies to the required oligotrophic state, the phosphorus load (phosphorus emissions and release of phosphorus present in the surface water body (mainly present in the sediment bed)) should be reduced and care should be taken that the N:P ratio in the freshwater system stays high enough to prevent a growth advantage for blue-green algae. Surface water nutrient load originates from diffuse sources as well from point sources. E.g. in The Netherlands, WWTPs contribute to 34% of the annual phosphorus load and 14% of the annual nitrogen load, while agriculture contributes 62% for phosphorus and 41% for nitrogen. The diffuse sources are difficult to control actively, but the point source effluent of a WWTP can easily be controlled. This effluent could be used to increase the nitrogen concentration of surface waters by discharging higher nitrate concentrations in spring and summer. The nitrogen should be released during the spring and the