(16)The evaluation showed that significant reductions of nitrogen and phosphorus emissions were achieved through the implementation of Directive 91/271/EEC. Nevertheless, urban wastewater treatment plants remain, according to the evaluation, an important pathway of those pollutants into the environment, directly leading to eutrophication of water bodies and seas in the Union. Part of this pollution can be avoided as technical progress and best practices in place show that emission limit values established under Directive 91/271/EEC for nitrogen and phosphorus are outdated and should be strengthened, especially for larger treatment plants. Tertiary treatment should be systematically imposed on all urban wastewater treatment plants of 150 000 p.e. and above, as such plants represent an important remaining source of nitrogen and phosphorus discharge.