Report No FR0465



APRIL 1994



A standardised, robust and cost-effective method for assessing and reporting the environmental impact of combined sewer overflow (CSO) discharges is essential for good practice in urban pollution control. Such a methodology is needed for identifying CSOs that perform unsatisfactorily and for prioritising these for improvement schemes intended to limit pollution, as required by the Urban Waste Water Treatment Directive (CEC 1991).


To develop a standardised, objective assessment procedure to enable the impact of combined sewer overflows (CSOs) to be reported within a regulatory framework.


CSO discharges have long been recognised as having a major contribution to the poor quality of urban rivers. Whilst procedures have been developed independently by NRA Regions for short-term reporting needs, there is a need for a standardised national a pproach for long-term use.


During the course of this project it has been possible to develop an objective methodology to assess the impact of combined sewer overflows (CSOs) on receiving waters.

Existing NRA methodologies differed considerably in approach and complexity but certain factors emerged as being common to most and were identified and agreed as useful measures of impact. These are dry-weather operation, public complaint, sewage fungus and sewage-derived litter.

The use of sediment analysis was explored and rejected as a suitable measure of CSO impact.

Biological information did not add to the final assessment of impact, a finding which is in accordance with the experience of NRA Welsh and (former) South West Regions.


The procedures proposed in this report should be used in future broad-brush assessments of CSO impact undertaken at a catchment or regional scale by the NRA and/or Water Utilities.

A broad-brush impact assessment approach should be used as one of the first stages in any programme of investment to upgrade sewer systems.

NRA Regions and Water Utilities should cooperate fully in the process of CSO assessment to arrive at an agreed prioritisation list.

The NRA should consider the resourcing needed to undertake CSO assessments and plan to have them available for future investment programmes.

Biological information is not included in the recommended methodology but it could be used to gain additional information in cases of uncertainty, and is appropriate for detailed scheme design. Interpretation should be based on either percentage difference of upstream and downstream BMWP score or RIVPACS EQI bandings.

Groups of CSOs discharging in close proximity should be assessed as a single unit.

Once an impact classification list has been developed, prioritisation should be achieved by considering each CSO on a case by case basis taking into account other information on receiving water use and sewer hydraulics (dry-weather flow, population served, sewer capacity and modelling data as available).

During the prioritisation process due consideration should be given to all type(s) of impact identified, e.g. aesthetic, water quality.

Once CSOs have been prioritised in terms of impact, planned improvements should be integrated within overall investment programs.


A review is presented of the types of impacts associated with CSO discharges, followed by a review of existing methods for assessing these impact types. Existing Regional protocols for CSO assessment are described and evaluated. The development of a working methodology is discussed and results presented from field applications. Procedures for classifying and prioritising CSOs in terms of impact are presented.

Copies of the report are available from FWR, price 35.00, less 20% to FWR Members.