A new report for the Environment Agency by consultants, Atkins, which describes the development of a decision support tool to help screen rivers in England for the likelihood and extent of sediment-bound phosphorus being released back to the river water, says that more work is needed to understand the issue.

The report Phosphorus cycling in rivers says discharges from wastewater treatment works (WTWs) are a major contributor to non-compliance with soluble reactive phosphorus SRP standards. UK water companies have put in place a major investment programme since the late 1990s to reduce their emissions. A range of measures have also been introduced to reduce diffuse sources of phosphorus (for example, Catchment Sensitive Farming).

However, the report suggests that the effectiveness of reducing phosphorus loading from these sources might be limited by in situ sediment phosphorus cycling. The focus of the report is to understand the implications of this process in terms of the extent and likelihood of its occurrence and the degree to which it is likely to be important.

The  report also highlights potential areas for further research, suggesting that the magnitude and duration of sediment phosphorus release would be best assessed by identifying a site where phosphorus removal is about to be implemented as part of the AMP6 2015-20 investment programme.

Two questions on benefits of reducing phosphorus sources

The report highlights two questions about the benefits of reducing sources of phosphorus.

• Will the reduction of SRP loads from WTWs or diffuse sources result in the release of sediment-bound phosphorus?

• If sediments release phosphorus, how long will it be before the benefits are realised with respect to lower river SRP concentrations and improvements in ecological status?

Soluble reactive phosphorus (SRP) is one of the parameters used to determine the ecological status of a river. The impact of high concentrations of SRP in rivers includes a shift in the composition and abundance of plant species present, which has an impact on the river’s overall ecology. This is one of the most significant causes of water bodies such as rivers not achieving good ecological status under the Water Framework Directive.

The screening tool will help the Environment Agency and its partners to target resources and identify measures to reduce phosphorus levels in rivers to meet water quality standards.

The phosphorus available in a river is determined by the different sources of phosphorus inputs in the river’s catchment. Sources of phosphorus included in the screening tool are:

• point sources such as discharges from wastewater treatment works

• diffuse sources such as farming

• sediment loads from the catchment

Accumulation of sediment within a river can potentially increase the amount of phosphorus available for water– sediment equilibrium reactions. The screening tool includes an estimate of where phosphorus ‘sinks’ are within a river.

Introducing tertiary phosphorus removal at WwTW makes sediments vulnerable to phosphorus release

The report points out that sediments downstream of a wastewater treatment works act as a sink for phosphorus discharged in waste water. Introducing tertiary phosphorus removal at the wastewater treatment works makes the sediments vulnerable to phosphorus release because reduced phosphorus concentrations in the overlying water cause phosphorus to be released from the heavily phosphorus loaded sediments. An estimate of the phosphate source forms part of the screening tool.

The report concludes that given the initial findings, further investment in model development is difficult to justify until there is better confidence in the formulation of the phosphorus sediment exchange model, and quantification of the interaction between phosphorus in the water column and in the sediment.

Extensive analysis of large WwTW needed to understand the issues

 It suggests that a separate line of investigation to inform the issues would be to carry out an extensive analysis of measured wastewater and river phosphorus concentration downstream of large WTWs where phosphorus stripping has been introduced. By comparing river concentrations immediately after phosphorus removal has been introduced and at later intervals, it should be possible to identify whether phosphorus concentrations were elevated in the early period. This would provide independent evidence on the importance of sediment phosphorus exchange and the duration of these impacts.

The report says that because the signal information on both the wastewater emissions and river concentrations tend to be ‘noisy’, a large dataset would be required. However, this should be available because of the large number of WTWs that have been subject to phosphorus removal in recent years.

Development of such models would, however, be valuable to place sediment phosphorus exchange in the context of the wider picture of phosphorus dynamics in rivers and sediment transport processes.

The report also says that current scientific understanding of the key processes of sediment phosphorus dynamics is poor. While the screening tool provides information on the relative risk of phosphorus sediment impacts, it does not provide a measure of the magnitude of the risk. It suggests that further work is needed and that the magnitude and duration of sediment phosphorus release would be best assessed by identifying a site where phosphorus removal is about to be implemented – as part of Asset Management Programme 6 (AMP6).

Click here to download the full report Phosphorus cycling in rivers