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Designing SuDS to remove pollution from surface water must match likely levels of pollution with the performance of SuDS elements. Advanced Drainage Systems UK manager Stuart Crisp reports.
Sustainable Drainage Systems (SuDS) should be designed with water quantity and water quality in mind. Yet often the water quality element is overlooked, down-specified or removed altogether in a misnomered “value engineering” exercise.
One of the challenges for designers is that the Ciria SuDS Manual C753, only defines the pollutant removal performance of natural SuDS components. For engineered components, manufacturers must demonstrate that those selected will reduce contaminants to acceptable levels.
One way to do that is to check whether the manufactured component is on water industry trade body British Water’s list of assessed surface water treatment devices.
To appear on the list, data relating to a component’s pollution treatment ability must have been independently verified.
Risk-based approach
Ciria’s SuDS manual prescribes a risk-based approach to designing for water quality, defining pollution risks by way of pollution hazard indices. The manual provides indices for a range of land uses and for three types of pollution: total suspended solids (TSS) such as tiny soil particles, metals and hydrocarbons. For instance, a busy public car park at a supermarket or hospital would have indices of 0.7 for TSS, 0.6 for metals and 0.7 for hydrocarbons. The manual also provides generic mitigation indices for natural SuDS components for the three types of pollution.
British Water has published two documents with guidance for calculating mitigation indices for engineered SuDS components. In 2016 it published its
Code of practice for the assessment of manufactured treatment devices designed to treat surface run-off. It followed that up in 2022 with a “how to” guide called Applying the Ciria SuDS Manual (C753) simple index approach to proprietary/manufactured stormwater treatment devices.
The British Water guide provides a method for calculating mitigation indices for TSS, metals and hydrocarbons based on test results derived from its code of practice. This means that manufacturers can calculate and publish mitigation indices for their treatment products so that they can be considered as part of a SuDS management train.
British Water’s website provides a list of engineered SuDS components and their independently verified mitigation indices.
ADS StormTech, with its Isolator Row, was recently added to the list. It is the only system that combines attenuation and pollution removal, often without the need to add a treatment device upstream of the attenuation chambers.
It should be noted that removing TSS is also important from a water quantity perspective. Build-ups can reduce the storage and discharge capacity of a water storage element, whether natural or manufactured.
How and when to remove sediment should be considered at the design stage and should also be part of a planned maintenance regime.
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