The recent Ofwat report proposes a £104 million fine on Thames Water for failing Urban Waste Water Treatment standards. The notice highlights the "extremely concerning" number of failing assets across the Thames Water region. It puts on notice Thames Water's failure to act as a "reasonable company" to ensure assets are designed and constructed to meet the flows received under the range of typical local weather conditions. Of local concern to us is the data also shows that the River Mole has a disproportionate number of these failing assets for a small catchment.
Such a high proportion of failing wastewater treatment assets in the Mole catchment poses a serious threat of increased pollution and harm to the river, wildlife, and damage to the amenity provided by the river to local communities. The data suggests that the River Mole is unusually exposed to the failure of Thames Water assets and vulnerable to the attendant harm caused to public health and amenity, as well as impacts on plant health and aquatic biodiversity.
The River Mole is on the frontline of failing Thames Water assets.
ALL major wastewater treatment works in the Mole catchment are identified as "Sites of Concern".
Concerning Sites
Out of 354 WWTWs in the Thames region there are 157 "sites of concern". This means that they are not meeting, or potentially not meeting, Environment Agency EA permits that regulate, for example, the quality of treated effluent and frequency of storm overflows. So all of the WWTWs in the Mole catchment are, or have been at times, non-compliant.
Most Polluting Sites
Across the Thames region 26 sewage works out of 354 have been identified as being responsible for 50% of the most severe Category 3 and 4 pollution incidents.
The River Mole is only 4% of the Thames catchment and yet accommodates over 15% of these "Most Polluting" wastewater treatment works.
These badly behaved WWTWs are Esher, Holmwood, Crawley and Reigate (Earlswood). This number of sewage works with a history of serious pollution incidents is disproportionate to the size of the catchment. This high number of underperforming sites has a potentially larger impact on smaller rivers like the River Mole.
Frequent pollution incidents and failing treatment works pose a particular risk to the River Mole because it suffers from a flashy regime and discharge falls quickly to very low flow with little natural "baseflow" to keep a healthy discharge flowing.
This means that, throughout much of the Summer, the majority of water in streams relies on treated effluent. The Gatwick Stream, for example, is at least 60% sewage effluent. In smaller tributaries, such as Leigh Brook, the entire flow is treated effluent from sewage works. The quality of the treated effluent, including the effective removal of toxic chemicals, is therefore of paramount importance to these smaller streams hosting treatment works.
The data Ofwat gathered from Thames Water shows the concerning number of failing WWTW assets in the River Mole catchment. The map below shows that all of the WWTWs in the Mole catchment are identified as "sites of concern" meaning they are failing to comply in one way or another with EA permits.
For example, Esher WWTW is identified as one of the 26 "Most Polluting Sites" in the Thames catchment, it is also a Site of Concern for inconsistent performance in hitting permits and it is identified as Non-Compliant due to not meeting Full Flow to Treatment consistently. In addition, the high frequency of storm overflows at Esher triggered a Storm Overflow Assessment Framework (SOAF), notably, yet to be complete.
Reference on the map is made to Full Flow to Treatment (FFT). FFT refers to the maximum amount of wastewater that a treatment plant can handle and treat effectively at any given time. It’s the upper limit of flow that the plant is designed to manage before any excess flow might be diverted or managed differently. FFT permits are set for each WWTW based on peak flow factors and design capacity. If a WWTWs persistently fails to treat all the wastewater entering the site at FFT then this could be a breach of permit.
It is notable that, Thames Water had significant difficulty gathering clear data for Ofwat on the status of their own assets including which are failing and the causes of issues. This is shown by the changing number of Sites of Concern provided to Ofwat over the years.
Four of the "Sites of concern" in the River Mole catchment are also listed by Thames Water as the "Most Polluting" WWTWs, namely: Holmwood, Esher, Reigate and Crawley.
Furthermore, 7 out of 9 of the WWTWs Esher, Leatherhead, Holmwood, Dorking, Horley, Burstow and Reigate (Earlswood) have all triggered the Storm Overflow Assessment Framework which means that the frequency of discharges of untreated sewage (storm overflows) has exceeded a critical threshold (~>20 spills a year) and triggered three formal stages of assessment.
The map also shows trends in storm overflow duration from 2019 to 2023 for each WWTW. and the total hours of untreated sewage discharged for those years. Note that several SOAFs finish at Stage 1 which is correct if the cause of excessive storm overflows was operational (human). It is then expected that Thames Water would resolve the issue internally. Despite this the trend in some storm overflows continues to increase as also shown in the chart below with recent early data from 2024 causing further concern for an upward trend in storm overflow duration in the catchment.
Overall, the data suggests that the WWTW assets in the Mole catchment are, relative to the size of the river, in a significantly worse state than many other parts of the Thames region.
Finally, below is a summary of the causes of this large scale failure of compliance by Thames Water to provide functioning and effective wastewater treatment both in the Mole catchment and across the region. This is not an exhaustive list but a summary of findings mentioned in the Ofwat report.
In terms of Thames Water's own analysis the root causes of failure at their sites of concern are shown in the chart below. It shows the percentage of sites where each cause was an issue. Of interest, is that operational failures and lack of training, including examples such as errors in installations and maintenance, appear to account for the highest percentage of root causes.
In the end, it is unclear how much the Ofwat fine will shake-up Thames Water and lead to more effective wastewater treatment. The Private Eye’s wry analysis suggests the impact will be negligible compared to the massive sums of our money lost forever to banks and shareholder dividends.
Time will tell whether this relatively minor penalty will do much to reset the reckless behaviour of a company in a more responsible direction whose neglected assets have become a significant health hazard to both wildlife and the public. One outcome that looks hopeful is that Ofwat will finally grow some teeth and start regulating Thames Water properly.
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