S4: Water Quality

S4

What S4 requires

Source: Defra Non-Statutory Technical Standards for Sustainable Drainage Systems (2015, refreshed 19 June 2025), Standard S4. Implemented through the Simple Index Approach in CIRIA C753 §26 and supported by the SuDS Manual treatment-train guidance.

S4 requires that runoff is treated to remove sediments, heavy metals, and other pollutants before discharge to surface waters or infiltration to groundwater. Treatment is demonstrated using a management train of multiple treatment stages, each removing specific pollutants.

Treatment Train Approach

Rather than relying on a single feature to treat all pollutants, a management train uses sequential stages: initial settlement and filtration in the first feature (e.g., permeable pavement), further filtration and sorption in bioretention, and final polishing in a surface water body or infiltration to ground. Each stage progressively removes contaminants, improving discharge quality.

Simple Index Approach (SIA)

The 2025 National Standards reference the Simple Index Approach (CIRIA C753, Chapter 26) as the primary method for water quality assessment. The SIA assigns each SuDS feature a removal efficiency for sediment (as TSS, Total Suspended Solids), and for metals and hydrocarbons. Features are scored based on removal efficiency (scored 0–4, with 4 being 95%+ removal). A treatment train is assembled so that the product of source, site, and regional control factors yields a total score ≥ 14. This demonstrates that the combination of features achieves adequate treatment.

Hazard-Based Approach

The standard also considers the pollution hazard from the site's land use. Residential areas have lower hazard (mainly sediment and trace metals). Commercial or industrial areas with vehicle movements have higher hazard (hydrocarbons, metals, salts). The treatment requirement is scaled to the hazard: high-hazard sites require more robust treatment trains than low-hazard residential.

Pollutants of Concern

• Total Suspended Solids (TSS): Fine sediment particles. Settled in most SuDS features. Target: > 90% removal.
• Heavy Metals: Copper, zinc, lead, and cadmium. Released from vehicle wear, roofing materials, brake dust. Removed by sorption in engineered growing media and soil. Copper is particularly concerning due to aquatic toxicity.
• Hydrocarbons: Oil, petrol, diesel. Released from vehicle leakage and spillage. Removed by degradation in soil and biofilm growth on SuDS surfaces. Infiltration features with adequate soil/media depth are essential.
• Nutrients (Nitrogen, Phosphorus): From fertilisers, atmospheric deposition, and detergents. Can cause eutrophication in surface waters. Reduced through infiltration (soil uptake) and vegetation uptake in bioretention.

Site Investigation and Design: A qualitative assessment of the site's pollution hazard informs the treatment train requirements. The Water Quality Calculator in the Catchment implements the Simple Index Approach and guides selection of compliant feature sequences.

→ Treatment Train Calculator