Storm Overflows & Urban Pollution Management

WRc specialists evaluate risks and consult on environmental compliance surrounding storm overflows, in line with guidance for Urban Pollution Management (UPM).

Combatting storm overflow pollution

Combined sewer overflows (CSOs), storm tanks and pumping station overflows are collectively known as intermittent discharges. During periods of heavy rainfall, they serve as relief points for hydraulically overloaded sewer networks to prevent flooding of properties and buildings and to protect wastewater treatment works.

When storm overflows spill, diluted storm sewage enters the environment, impacting the chemical and microbiological water quality of receiving and downstream water bodies. Effective Urban Pollution Management (UPM) diminishes the environmental harm and ensures that water company discharges allow rivers and coasts to comply with Environment Quality Standards.

How we can help

WRc provides specialist knowledge and the development of modelling tools relied upon by water companies to help plan and target future investment. The task of assessing the quantity of pollutants discharged to receiving waters is challenging; collecting data during storm events is expensive and time-consuming, and historically not all storm overflows have been represented in sewer network models.

We can help you quantify the frequency and duration of spills, spill volume, and pollutant concentration of storm overflows. Our consultancy ensures time-effective, cost-effective compliance with UPM environmental standards.

Guiding environmental recovery

Our specialist consultants are dedicated to guiding the journey towards your environmental goals.

Sharing our knowledge for a cleaner world

Our comprehensive UPM training modules enhance beginners' understanding, confidence and practical experience.

Providing reliable tools for excellence

Our rapid but simple modelling tool, UPM-RAT, allows users to easily evaluate your wastewater performance against UPM standards.

UPM-Rapid Assessment Tool - Supporting better outcomes for river health

The Urban Pollution Management – Rapid Assessment Tool (UPM-RAT) has been developed to support screening-level (Level 1 and 2) water quality assessments in line with UK regulatory requirements. The tool enables practitioners to evaluate whether storm overflows from urban wastewater systems contribute to ecological harm in receiving rivers, using a consistent, evidence-based framework aligned to the UPM approach.

UPM-RAT is designed for use by water companies, environmental consultants and regulators undertaking catchment-based studies where rapid, defensible assessment of intermittent discharges is required.

Methodological Approach
Water Quality Assessment Framework
Targeted Training

UPM-RAT conceptualises the wastewater system as a series of sub-catchments draining to a defined river reach, incorporating:

Wastewater treatment works (WwTWs)
Storm overflows (CSOs)
Surface runoff contributions

Hydraulic inputs can be derived from:

Existing wastewater network models, or
Rainfall-driven flow generation where detailed hydraulic models are unavailable

It requires minimum data inputs, saving time. The tool simulates the interaction between wastewater discharges and in-river processes, enabling the derivation of a baseline water quality position under wet weather conditions.

UPM-RAT is now available for use under licence in the UK. If you are interested in understanding more about the software, licence costs and training opportunities, please contact us at solutions@wrcgroup.com

UPM-RAT evaluates compliance against established UPM standards for intermittent discharges, including:

Fundamental Intermittent Standards (FIS):

Dissolved Oxygen (DO)
Un-ionised Ammonia

99 Percentile Standards:

Biochemical Oxygen Demand (BOD)
Total Ammonia

Outputs are provided as compliance metrics, enabling users to determine whether storm overflow activity results in exceedances associated with ecological harm.

UPM-RAT is configured with defensible default parameters, which can be refined using measured or modelled datasets to better represent system behaviour. Its computational efficiency - 10-year simulations in minutes - enables rapid scenario testing, including:

Population growth
Climate change rainfall impacts
Intervention and optimisation strategies

To complement the software, we provide targeted training to support practitioners in:

Integrating river and wastewater datasets
Developing representative catchment schematics
Designing outline solutions aligned to regulatory expectations
Reporting ecological risk and spill performance with confidence

By simplifying complex assessments while maintaining technical rigour, UPM-RAT enables Utilities, consultants and Environmental Regulators to understand, prioritise and mitigate storm overflow impacts more effectively, helping to deliver measurable improvements in river health across the UK.

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