Sustainable drainage and wastewater management

We collect and treat 4.4 billion litres of sewage from our 15 million customers and use our sewer network to transport it to one of our 349 sewage treatment works. Wherever possible the solid element of sewage (what we call sludge) is used to produce renewable energy, while the accompanying water is treated to a high standard and recycled safely to rivers and streams. We will work to ensure our sewerage network is as resilient as possible and will take advantage of opportunities to deliver more sustainable drainage solutions.

Sewer flooding can occur when the sewer network becomes overwhelmed during intensive rainfall, or because of a blockage or collapse. Although our sewer networks are largely in good condition, the combined effects of urbanisation, climate change, population growth, modern lifestyles and an ageing infrastructure mean many networks handle much larger volumes of wastewater than they were originally built for. In addition, sewer abuse - when the wrong things are put down the drain - also poses a challenge for our network. Sewer flooding can have a catastrophic effect on customers’ homes and livelihoods, as well as posing an environmental risk.

Our performance for Sewer Flooding Other Causes (SFOC), the number of internal flooding incidents excluding those due to overloaded sewers, has improved for the third year running with a 26% reduction since the start of the regulatory period (2015/16). In 2018/19, 1,032 properties suffered sewer flooding – less than the previous year. In 2017/18, 1,062 properties experienced incidents of internal sewer flooding due to other causes.

We’ve improved in our overall performance for 2018 (calendar year) with a 2 per cent reduction in the number of pollution incidents compared with 2017. During 2018, we had zero of the most serious pollution incidents for the first time in ten years. We continue to improve in our self-reporting of pollutions, which increased from 73 per cent in 2017 to 76 per cent in 2018, above the target set by the Environment Agency. Our compliance with numeric discharge permits saw a slight deterioration in 2018, registering 98.9 per cent (four failures) compared with 99.4 per cent (two failures) in 2017.

Our priorities in the future, as stated in our submitted business plan, include reducing pollution incidents by 30% during 2020 to 2025 and aspiring to reduce the number of incidents to zero in the long term. We’re also planning to reduce internal sewer flooding by 20% during 2020 to 2025 to 995 incidents per year.

The figures included in the Annual Report (SC2 - total pollutions including consented) are pollution numbers according to our methodology agreed with OFWAT and do not include pollution from surface water outfalls. This accounts for the differences between the figures in the Annual Report and the table above.

* The Environment Agency categorises pollution incidents on a scale of one to four: Cat 1 - major environmental impact, Cat 2 - significant, Cat 3 - minor, Cat 4 - no impact.

** Figure represents the percentage of the total pollutions (Cat 1-3) that were self-reported by Thames Water to the Environment Agency.

We undertake a range of programmes and activities to maintain and enhance our sewer network to reduce the risk of sewer flooding and pollutions:

Intelligent network

As well as undertaking 602km of planned sewer maintenance, we’re increasingly using digital technology to create a more intelligent network and enable more proactive maintenance and repair to help drive a step change in reductions. We’ve now installed almost 1,000 sewer depth monitors to help us better understand how our waste network is flowing and help reduce pollution incidents. Although in their infancy, we’re already seeing positive results with 179 blockages detected by the monitors in 2018/19.


As well as being in the early stages of creating a more intelligent network, we’ve improved training for customer call agents to help them identify early pollutions. They are being given the tools and ‘know-how’ to use information provided by the caller, combined with the risk profile of the area as categorised on our pollution risk register, to prioritise the most urgent work. During the year, we surveyed 232 and renovated 27 sewers near railways, to ensure resilience of this critical infrastructure.

Sewer abuse

Following the discovery of the ‘Whitechapel fatberg’ in September 2017, we worked with the media, stakeholders and customers to raise awareness of the impact of flushing wet wipes down the toilet and pouring fats, oils and grease (FOG) down sinks. We’re proactively engaging customers to change flushing behaviours with our ‘Bin it, don’t block it’ campaign which highlights the dangers of flushing anything other than the three P’s – pee, poo and (toilet) paper – down the drains. We’ve also worked with over 4,300 commercial kitchens including fast food establishments, hospitals and schools to reduce fat disposal down drains and sewers, however, there’s still a way to go to eliminate fatbergs. The emergence of the ‘concreteberg’ in Islington, London, has also helped raise awareness of the cost to customers of this kind of sewer abuse.


Our customers sometimes unknowingly pollute local watercourses by misconnecting toilets, kitchen and bathroom appliances to surface water drains instead of the foul (wastewater) sewer. We estimate that as many as 60,000 properties across our region are misconnected. Property owners could face the risk of being prosecuted and fined up to £50,000 if they don’t put it right. In 2018/19, we helped our customers fix faulty plumbing at approximately 797 properties across 44 catchment areas – a total area covering more than 32,000 properties. In these areas we identified 2,324 misconnected appliances. We’re continuing to investigate misconnections across other catchments.

London Tideway Improvements

We’re working to improve the water and sewer networks across London to make sure they can cope with current and future demands. One of the biggest projects to improve and future-proof our network is the Thames Tideway Tunnel (also known as the “super sewer”). We’re working with Tideway (Bazalgette Tunnel Limited) to deliver the Thames Tideway Tunnel. When it’s complete, together with the Lee Tunnel, it will prevent millions of tonnes of sewage from overflowing into the tidal River Thames every year from the capital’s overloaded Victorian sewer system. The new tunnel will transport sewage to our Beckton treatment works where we’ll return treated and safe water to the environment and use the solid sewage waste (sludge) to generate more renewable energy. Learn more about the tunnel and its progress.

We’re working in one of the most densely populated and urbanised regions in the UK, which places a lot of pressure on drainage. Without action, population growth, urban creep and climate change would increase the likelihood of sewer flooding and pollution. We’re shifting our emphasis on reducing the volume of surface water entering our sewers with sustainable drainage systems (SuDS), creating capacity to manage future challenges in a more cost-effective way.

Managing surface water at source through sustainable drainage systems allows us to not only tackle flood risk, but also increase resilience against climate change. In 2015, we launched a SuDS programme to disconnect 20 hectares of impermeable land from the combined sewer network - one hectare is roughly the size of an international rugby pitch – and we’re on track to deliver this target. Between 2020 and 2025 we’re looking to more than triple our target to 65 hectares, providing a wide range of benefits to our customers and the environment beyond that of improving asset resilience.

We’re working with government, local authorities, organisations, charities and customers to raise awareness and make sure water is managed more sustainably. We share our data and expertise with local authorities and organisations and contribute to the development of local plans on surface water management plans. We continue to work with Defra, WaterUK and developers on the challenges facing SuDS, helping to raise awareness of sustainable water use, and managing the impact of new developments on our sewer network. “Blueprint for Water”, a partnership with eight other water companies and 19 environmental organisations, enables us to share principles to tackle issues such as sustainable drainage as well as pollution through joint working. We also use innovative software to explore SuDS opportunities across our region and contribute to the installation of SuDS schemes.

We’ve developed an innovative multi-purpose planter which collects rainfall from the drain pipe before slowly releasing it back into the sewers to help implement sustainable drainage measures for households. The planter, which helps free up capacity in the network during high rainfall, has been designed to be used in residential gardens, with excess water also captured for future uses such as watering the garden. We exhibited the planter at the BITC Waste to Wealth summit held at Veolia Southwark’s waste management facility in London in November 2018, attracting the interest of both HRH The Prince of Wales, and the then environment secretary Michael Gove. We have already installed a planter at Reading Sewage Treatment Works (see picture below) and will be offering them to customers from April 2020.

We’re continuing to work hard to make sure our sewerage network and treatment sites are as sustainable as possible. Once wastewater has been safely treated at our 349 sewage treatment works, the water is discharged from the sites into local watercourse so it’s really important for us to get our treatment processes right, to make sure the watercourses stay healthy. Water quality standards for discharges are set by the Environment Agency and we must pass every discharge quality test to meet our sewage treatment works compliance commitment of 100%.

We did not meet our target of 100% compliance, with four failures at our 349 sewage treatment works with a discharge permit, and as a result we will return £0.12 million to customers at the end of the regulatory period. The failures were at Deephams, Cholsey, Chertsey and Wolverton Townsend during the first six months of the year – there were no failures during the second six months.

We are continuing to focus on day-to-day performance management of our sites. We completed investigations into the cause of each failure so we could apply the learnings to reduce the likelihood of reoccurrence.

We’ve reached the closing stages of some critical resilience projects, including a major £267 million upgrade at Deephams sewage treatment works in North London to prepare for predicted population growth and reduce odour for customers living nearby. The project will allow the site to serve an increase in population equivalent of 104,000 to accommodate the predicted population growth until 2031.

Since its original construction at the end of the 19th century, housing developments have moved closer to the works, through population growth and urbanisation, making odour management important for local customers. The new site will drive a 99% reduction in odour emissions. During the project, which saw three sludge ‘streams’ combined into two more efficient facilities with higher capacity, the safe treatment of the waste of Deephams’ current 885,000 population equivalent was critical. In a seamless operation, we refurbished the old plant to maximise capacity before directing flows to one half of the site to allow construction of half of the new works. Once this was completed, flow was transferred into the newly built section to enable construction of the other. Recognised for delivering an innovative, sustainable and creative contribution to London, the project won the coveted ‘Greatest Contribution to London’ award at the ICE London Civil Engineering awards in May 2019.

We need to tackle a range of challenges including population growth, climate change, land locking of our sites, ageing assets and striving to meet tighter environmental standards. The scale of these challenges means we need to think differently about the future and look far beyond the conventional five-year price review cycles of the water sector. London’s wastewater system is immense and complex, and significant changes will take time to achieve.

A longer-term approach is needed to manage risks and embrace opportunities for innovation, which is why we’re considering time horizons of 25 and 80 years in a way that reflects water resource planning. The water industry, through WaterUK, has developed an approach for strategic long-term drainage and wastewater management planning (DWMP) and we are applying this methodology to London.