Agenda item

Thames Water - Waste Overspill & Woking Waterways Presentation OSC23-005

For the Committee to receive a presentation from, and hold discussion with, Thames Water regarding potential sewage overspill into Woking Borough’s waterways.  A briefing paper is included to provide background.


The Committee welcomed to the meeting representatives of Thames Water, Richard Aylard, External Affairs and Sustainability Director, and Nikki Hines, Social Housing Relationship Manager.

Thames Water had accepted that their performance on sewage disposal and river health had not met either their or their customers’ expectations.  The CEO of Thames Water, Sarah Bentley, had committed to an eight-year turnaround plan beginning in 2021.  As part of the plan, Thames Water committed to reducing the duration of total spillage across their network by 50% on 2020 levels by 2030.  In sensitive catchment areas, referring to chalk and limestone waterways, waterways that run through Sites of Special Scientific Interest and Areas of Outstanding Natural Beauty, and waterways used recreationally, the total duration of spillage would be reduced by 80% on 2020 levels.  The upper river Wey, including the parts that flowed through the Borough, was considered a sensitive catchment area.  Thames Water had begun reporting their spillages voluntarily to the public via a live map. 

The Committee was provided with a general explanation of the process for treating sewage.  A works such as Chobham could process approximately 3,500 homes’ waste.  On entry, sewage would be screened and any items that would block, damage, or otherwise impact the ability to treat the sewage were removed.  The screened sewage would be pumped to filter beds of bacteria that cleaned the sewage.  The treated sewage would then be pumped to a further filter bed before being discharged.  Each stage required a minimum amount of time to be effective.

Thames Water routinely tested the composition of treated sewage but did not routinely test untreated sewage.

There could be several causes of spillage into waterways, the process to initiate and end overspill was automated.  Thames Water’s wastewater responsibility covered both smaller foul water and larger surface water systems.  During periods of rainfall surface water had the potential to enter the foul system due to several factors.  The factors were: ingress from the ground, damaged access points, through holes that were required in manhole covers, and incorrect connections (deliberate or not) made into the sewage network.  Incorrect connections could cause significant amounts of surface water to enter the narrower pipes of the foul water network and quickly cause blockages.

The sewage system had been designed primarily to prevent the backing up of sewage into homes.  After rainfall, due to the increased water levels in pipes and the minimum time required to treat sewage, any additional flow would be diverted to storm tanks at sewage works, so long as a defined amount of sewage, or greater, was being processed.  Once filled the tanks required emptying and cleaning.  Any more excess sewage could not be stored and the system would automatically eject additional sewage into waterways.  The Committee was informed that the composition of discharge was sewage watered down by rainwater.

Sewage treatment works were continually upgraded to ensure sufficient capacity.  Thames Water engaged with local authorities to determine expected future demand and continued to be a statutory consultee on Local Plans.

Following questions, it was confirmed that almost all spillages correlated with excessive rainfall.  Use of storm tanks outside of these instances was required to be reported to the regulator, OFWAT.  Monitoring equipment had only been installed at treatment sites in the last few years.  The installation of monitors had necessitated upgrading some sites as they struggled to process the minimum levels of sewage set even under normal conditions.

The Committee asked for an explanation as to the high number of spillages at the Chobham works.  The reported statistic was hours of discharge, however, flow from each discharge was relatively low.

Although Thames Water were not qualified to provide medical or veterinary advice, it was generally advised if having come into contact with water contaminated by sewage, to wash thoroughly.  Contamination of water could occur from the discharge of untreated sewage from works (and even the possibility of some human sourced bacteria in treated sewage).  However, there were several other common sources of waterway contamination: effluence from farm animals, effluence from wild animals, and common microbial contaminants from mammal urine.

Regarding the lack of statistics at the Wisley sewage treatment works it was explained that the works operated on a different principle.  There was no storm discharge point as all waste received was treated, so called ‘treat-all’ works.

Thames Water representatives emphasised that the Company had a standing policy of co-funding signage with councils to warn residents of potential threat to health on entry where necessary along waterways.

Following Member query, the Thames Water representatives agreed to provide details on the capacity and upgrades at the Chobham site.

Use of rainwater gardens was discussed, including several that had been built in the Borough in recent years.  Richard confirmed that Thames Water supported the principle of their installation and did discuss individual cases with Councils if there was the potential to help fund.

Thames Water had committed to a £9.6 billion spending plan in the period 2020-25.  Part way through the period the plan increased by £2 billion as investors agreed to provide additional funding.  The company had not paid a dividend for the previous five years and almost all profits generated had been reinvested into the company.

Councillor Azad, and the whole Committee, thanked Richard Aylard and Nikki Hines for attending.


That  (i)    the report be noted; and

          (ii)   the presentation be noted.

Supporting documents: