Thames Water unveils £430 million project to stop London drying out
The project would require a 4.2 kilometre tunnel from Isleworth to Teddington
Thames Water has begun a series of public consultations over its plan to take 75 million litres per day from the Thames at Teddington during a drought and replace it with the same amount of triple-treated sewage.
The Teddington Direct River Expansion Plan requires the construction of a new intake at Teddington, a tunnel to link it to the Thames Lee Tunnel, a tertiary treatment plant at the existing Mogden sewage works in Isleworth and a 4.2 kilometre tunnel to take treated sewage to Teddington, downstream of the intake.
Thames Water takes around 350 million litres of water from the Thames at Hampton in normal conditions. This water is transported to the Lee Valley reservoirs via the Thames Lee Tunnel, a 31-kilomtetre tunnel that was completed in 1959. The water is treated at Coppermills treatment plant in Walthamstow before being distributed around London.
Mogden sewage works, near Twickenham Stadium, is designed to treat three billion litres of sewage per year most of which is piped into the Thames at Isleworth Ait. In 2020, Thames Water said that 3.5 billion litres of untreated sewage entered the Thames from Mogden.
The Teddington plan is designed to provide extra water from the Thames when river levels are too low to supply the Thames Lee Tunnel at Hampton. The Thames at Teddington has a greater flow because it is downstream of the River Mole tributary.
Dozens of Thames Water staff were on hand to answer questions at Isleworth Public Hall on Wednesday, the first of a series of public meetings.
The new treatment plant at Mogden would add a third layer of treatment to what currently takes place there to produce water that could, after further treatment, be used as drinking water. Normal, double- treated sewage would be very damaging for the river ecosystem in a drought as it would not be diluted by fresher water from upstream. The new treatment plant would produce 13 million litres of triple-treated sewage per day on standby which would combine with the double-treated sewage as it enters the Thames at Isleworth. During a drought the plant would be activated to produce 75 million litres of sewage which would be piped to Teddington.
Thames Water expects work to start in 2029 and finish in 2033. The tunnel will be dug with a tunnel boring machine, similar to the ones used for HS2, Crossrail and the Tideway Tunnel. The 4.2 kilometre tunnel will be 3.5 metres in diameter and will travel between 20 and 40 metres underground. The works will be serviced by around 162 heavy good vehicle movements per day mostly around Isleworth but also in Ham and Teddington.
The Teddington plan is designed to ensure that odds of Thames Water introducing sever water use restrictions fall from one in a hundred to one in two hundred by 2035. The £430 million cost was estimated at 2022-2023 prices.
You can find out more about the plan from Thames Water
I have got severe reservations about messing about with Mogden sewage works by installing the tertairy treatment plant on stilts above the existing storm tanks when there is no robust plan in place to deal with the increasingly frequent discharges of "storm effluent" to the Thames when the existing storm tank capacity is overwhelmed. At the public event it appears that no decision has yet been made as to what the tertiary plant will comprise (pilot plant trials are ongoing and no results of trials have been released yet.)
I've also got concerns over the water quality that it is proposed to abstract at Teddington. The current supply to the Hampton reservoirs comes from upstream of Staines before the Colne joins the river. I have big question marks over forever chemicals (from historic fire-fighting foam use at Heathrow) draining into the Crane and the Colne and then into the Thames. The baseline water quality information at Teddington (tucked into one of the voluminous appendices) shows that 61 of the 66 samples taken exceed the Environmental Quality Standard for inland surface waters. Does that mean that further treatment/blending of this water will be necessary when it gets to the Lee Valley ? Nor do we yet know what additional chemicals of concern will be added from the stream out of Mogden which may cause deterioration in the stretch from Teddington, through Richmond to the point in Isleworth where the existing Mogden outfall is.
All in all, I think there are too many unanswered questions at this stage and it is premature to conduct the last statutory consultation before the planning application is submitted for the DCO process.