SuDS for Water Quality – New Civil Engineer September 2023

Designing SuDS to remove pollution from surface water requires a risk-based approach, matching likely levels of pollution with the performance of SuDS elements. Stuart Crisp, UK manager at Advanced Drainage Systems (ADS), reports.

Sustainable Urban Drainage Systems (SuDS) should be designed with both water quantity and water quality in mind. Yet often the water quality element is overlooked, or 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 performance of natural SuDS components in the removal of pollutants. For engineered components, manufacturers must demonstrate that the component(s) selected will reduce the contaminant types to acceptable levels.

One way to do that is to check whether the manufactured component is on 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 verified by an independent expert.

Risk-based approach

The SuDS Manual prescribes a risk-based approach to designing for water quality, defining pollution risks by way of pollution hazard indices. In Table 26.2, the manual provides the 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 such as a supermarket or hospital, would have indices of 0.7 for TSS, 0.6 for metals and 0.7 for hydrocarbons. The manual then provides generic mitigation indices for natural SuDS components for the three types of pollution in Table 26.3.

British Water, which represents water and wastewater companies, has published two documents which provide guidance on how to calculate mitigation indices for engineered SuDS components. In 2016 it published a Code of Practice for the Assessment of Manufactured Treatment Devices Designed to Treat Surface Run-off. And in 2022 it followed that up with a ‘how to’ guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices.

British Water’s ‘how to’ 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.

On its website, British Water 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, 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 removal of 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.

For more information on Advanced Drainage Systems, visit