MAIN FEATURE
phosphorus in treated wastewater by 80%by 2038 compared to a 2020 baseline, with an interimtarget of 50% by 2028.
Recent studies carried out by Stormwater Shepherds have indicated that phosphorus pollution is not a major problemfrommost urban surfaces. However, well designed SuDS can help alleviate nutrient pollution where it is a problemin surface water run-off. CIRIA guide C808, Using SuDS to reduce phosphorous in surface water run-off, published in 2022, provides guidance on how to do this. It suggests a treatment train starting by maximising infiltration, followed by sedimentation and the removal of solids and finally the introduction of actively growing plants to take up some of the dissolved phosphorus.
Capturing TSS pollution should generally be the focus of water treatment strategies in SuDS design since this removes both the solid particles and any pollution clinging to them. This can be done using a natural SuDS feature, such as planting for bioretention or a
swale.Manufactured components such as filtration devices, silt traps or vortex separators can also be used upstreamto remove solids before water is discharged into ponds, for instance.
Heavy rain causing stormwater runoff from a road in Hoghton, near Preston,
Lancashire inMay
2022.Photo courtesy of Stormwater Shepherds UK.
can lead to higher concentrations of pollutants fromroads, as they build up for longer before being washed away.
Nutrient pollution, usually nitrogen or phosphorous based, can come from sources such as run-off fromagricultural land where fertiliser has been used or CSOs, where wastewater and surface water are discharged into bodies of water. They can lead to algal blooms, which reduces oxygen and can
6 negatively impact on aquatic habitats.
The Plan forWater reports £2.5bn of planned andmade investment in wastewater treatment works between 2020 and 2025, which it says will halve phosphorous pollution. And it promises legislation to force water companies to make upgrades to nutrient removal near protected habitats. The Government’s Environment Act 2021 set a legally binding target to reduce
| June 2023 |
www.draintraderltd.com
Removal of sediment and solids is also important froma water quantity perspective. Build-ups reduce the capacity of a water storage element, whether natural ormanufactured. How and when to remove sediment should be considered at the design stage and should be part of a planned maintenance regime.
Without ameans of reducing suspended solids upstreamof an attenuation device, sediment build-up within the device can reduce its efficiency over time. Some recently introduced underground SuDS attenuation devices have their own built-in systems for intercepting solids which are fast and simple tomaintain.
Mitigation Indices The SuDSManual sets out a simple
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