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Determination of numerical values for the assessment of BMP's

A 41 page report titled Determination of numerical values for the assessment of BMPs has been prepared describing the development of a methodology to theoretically assess the effectiveness of structural BMPs with regard to their treatment of 25 selected stormwater priority pollutants (SSPPs). The result is a prioritisation, in terms of pollutant removal efficiency, of 15 different BMPs, which can inform end-users and other stakeholders of the best available options for the treatment of urban runoff pollutants of particular environmental concern. The limitations of this approach in terms of the variabilities in BMP designs and applications are also fully described.

The biological, chemical and physical processes which can contribute to the removal of pollutants within structural BMPs have been identified and are divided into direct and indirect mechanisms. The former are defined as those which clearly participate in the direct removal of pollutants whereas the second category incorporates those processes which occur as precursors to direct removal and includes adsorption to suspended solids and precipitation.

The direct removal processes include adsorption (to substrate), biodegradation, filtration, volatilisation, photolysis and plant uptake. Volatilisation and photolysis are considered to have lower importance regarding the removal of pollutants compared to the other direct removal processes operating within BMPs and they have therefore been allocated a 0.5 relative weighting in the applied calculation procedure.

All processes have been categorised as high, medium, low and not applicable based on an understanding of their relevance to each BMP. It is acknowledged that there is an element of subjectivity to this and therefore a wide consultation has been carried out and all received comments taken into consideration.

The behaviours of each of the 25 SSPPs with respect to both the direct and indirect removal processes have also been categorised using a combination of reference to the appropriate scientific data and scientific knowledge where specific data are not available.

A single value representing the potential of each BMP to remove each identified SSPP has been generated by combining the two sets of data relating the removal processes to the BMPs and to the SSPPs, respectively. The resulting orders of preference for the use of BMPs for pollutant removal are discussed in terms of the full range of 25 SSPPs as well as for representative particulate associated pollutants (suspended solids) and soluble pollutants (nitrates).

There are clear differences with respect to how the BMPs respond to these different pollutant types but in both cases, infiltration basins and sub-surface flow constructed wetlands are predicted to have the highest removal effectiveness. At the other end of the scale, settlement tanks consistently perform least well.

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Middlesex University
Queensway, Enfield, Middlesex
United Kingdom
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