How much phosphorus needs to be removed to meet the Water Framework Directive in UK Rivers

The challenge

Across the UK many rivers are eutrophic and have phosphorus levels well above the WFD limits for good ecological status. The challenge is to devise a cost effective strategy for P reductions in rivers in order to meet the WFD standards. The problem is exacerbated because there are many sources of phosphorus with the main sources being agricultural runoff and point source effluents from Sewage Treatment Works.

The solution

In partnership with UKWIR, the EA, the EU, NERC and Wessex water, the Universities of Reading and Oxford have developed the INCA (Integrated Catchments) Model to simulate the dynamics and processes of phosphorus transport. The model considers all sources of P in catchments and can be used to assess the relative contributions of P from agriculture and from STWs.

The model can be set up for any river system and scenarios evaluated to investigate the most cost effective mitigation strategies. The model has been applied to the Hampshire Avon and the River Thames and used to assess different levels of P removal at STWs, the effects of reduced fertilizer use or reduced P runoff from agriculture.

In both rivers a combine strategy of a 25% reduction in P from agricultural sources and a significant reduction in P from STWs would be sufficient to meet the WFD instream P standards. A cost effectiveness strategy suggests a combined reduction strategy will be necessary to meet the targets.

Resulting benefits

The benefits of such a P reduction strategy have yet to be fully tested, although the technology for P removal at STWs is well known, and extensive research on farm mitigation strategies has been undertaken. Catchment sensitive farming linked to STWs control will substantially move UK rivers towards the WFD objective of improving aquatic riverine ecology

Future directions

The power of a catchment integrated process based model is that it allows all sources of P to be considered and thus the relative impact of those sources quantified. The translation of the results of the INCA analysis into real catchment P reductions can now be evaluated, but this will require collaboration between the farming community and the Water Companies. This is a challenge but positive results are possible with good will and strategic catchment plans.

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