Across urban areas of the UK an extensive network of sewers carries wastewater from domestic and commercial properties to sewage treatment works (STWs). Designed originally in the 19th Century, many of these are combined sewers which also collect runoff from road drains and roof gutters. Most of the time, combined sewer systems transport all wastewater directly to a STW, where it is treated and then safely discharged to a water body. However, during periods of heavy rainfall, the volume in the system can exceed the capacity of the sewer, bringing a risk of flooding homes and businesses with diluted but otherwise untreated sewage. To avoid this, combined sewer systems are designed to overflow and release excess wastewater directly to nearby rivers or other water bodies.

The structures through which the excess water is discharged in this way are known as combined sewer overflows (CSOs). There are an estimated 10,000 CSOs in England alone, along with storm overflows and storm tank discharges at STWs. Each should spill only very occasionally, thereby causing as little environmental damage as possible. However, acknowledging that many are spilling more frequently than is desirable, a systematic five stage approach has been identified to prioritise the infrastructure improvements required for those discharging into rivers – the Storm Overflow Assessment Framework (SOAF).


The five stages explained


Stage 1 – Spills are counted using Event Duration Monitoring (EDM). There are trigger numbers (e.g. more than 60 spills in a year based on one year’s EDM data), above which an outfall is highlighted as needing investigation. If the cause was exceptional rainfall, then data from more typical years are sought. If rainfall was not exceptional, it may be a maintenance issue, such as a blockage, a collapsed wall, or leakage into the system, and this can be addressed.  Once these have been eliminated, a hydraulic assessment is required. If the hydraulic capacity (the amount of water that can pass through the system) is identified as too small or insufficient, the Framework will proceed to Stage 2.

Stage 2 – The environmental and aesthetic impact of the outfall is determined. The aesthetic assessment covers visibility and prevalence of sewage litter and sewage fungus. The environmental assessment uses aquatic invertebrates, treating upstream and downstream locations as separate sites and comparing them using outputs from RICT (the River Invertebrate Classification Tool). Only where it is not possible to collect invertebrate samples is a modelling approach, based on water quality, followed. Each of these methods uses a scoring system to calculate the degree to which the downstream site has been affected by the outfall relative to the upstream site, from which an impact score can be calculated, and classification determined.

Stage 3 – Assess improvement options including a cost benefit analysis (BTKNEEC*). If any of the methods applied in Stage 2 show an environmental impact, or if the outfall is situated in an urban area, then an economic assessment is made of overflow improvement.

Stage 4 – A decision is made based on the cost benefit results, with no further action being taken if the cost is disproportionate compared to the environmental benefits.

Stage 5 – The most cost beneficial solution is delivered to reduce environmental impact and/or reduce the frequency of discharges.

How APEM is supporting the water industry

The specialists at APEM can support water companies delivering SOAF. We have been making significant contributions to SOAF Stage 1 and 2 investigations since 2017, carrying out the first large scale implementation of the Stage 2 process and now having already completed over 200 such investigations, including calculating impact scores. We currently have a framework for SOAF Stage 2 delivery on behalf of one major water company and will be delivering many more investigations over the coming year. Our strength in field data collection is enhanced by our industry-leading in-house laboratory scientists, who identify the invertebrates, enabling us to deliver the work entirely in-house. Our breadth of services and office locations make us ideally placed throughout the UK to assist in these critical stages of SOAF. SOAF cannot be rushed – it requires data collected over several years, but we can help in discussing and delivering your needs.

If you would like to discuss SOAF, please contact Dr Michael Dobson today.


*Best Technical Knowledge Not Entailing Excessive Cost

If you have any queries, please contact Dr Michael Dobson, Divisional Director.
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