Frequently asked questions

Like many older cities, much of London is served by a 'combined' sewerage system, which collects domestic and industrial sewage, as well as rainwater run-off from roads, roofs, pavements etc.

This 'wastewater' is conveyed to five main sewage treatment works, where pollutants that would otherwise harm the environment are removed.

But heavier rainfall can fill the system to capacity and, rather than allowing sewage to 'back up' and flood homes, businesses, streets and gardens, it overflows into the River Thames and the River Lee.

This is the way that the system was originally designed to operate, when it was built 150 years ago.

The two proposed tunnels are needed to substantially reduce the amount of untreated sewage overflowing into the River Thames from the capital's Victorian sewers after rainfall.

Currently the overflows occur more than once a week on average, via 57 'Combined Sewer Overflows' (CSOs), which were designed as an integral part of the capital's original network by Sir Joseph Bazalgette over 150 years ago.

Annually, they now discharge an estimated 32 million cubic metres of untreated sewage into the river, enough to fill The O2 arena 15 times. They can occur after as little as 2mm of rainfall.

The overflows create health risks to river users, endanger wildlife and release tones of 'sewage-derived litter' into the river.

The CSOs were an integral feature of the way the sewer system was designed 150 years ago. At that time the River Thames was almost entirely 'dead'. There were also much fewer people in London using less water per head. Additionally there was more green space available to soak up rainfall. As a result of all this the overflows occurred only infrequently.

Nowadays, population growth, the more frequent storms associated with climate change, and the rapid run-off from the increasing proportion of impermeable surfaces are all adding to the problem.

Huge improvements have been made over the last 150 years.

Since privatisation in 1989, major investment at our sewage treatment works has greatly accelerated the clean-up of the River Thames. Biologically dead for many years, there is now a much greater diversity of wildlife in the river that needs to be protected from the increasingly frequent overflows.

But there is a danger that we will lose ground if we don't do more, in the face of population growth and climate change. The proposed tunnels will ensure that the excellent progress we have made is not reversed.

Simply, the system needs more capacity to meet the needs of modern-day London.

Doing nothing is not an option. It would be a very unfair legacy to leave to future generations to sort out.

We are already taking action on a number of fronts to reduce the impact of storm discharges on the River Thames and the River Lee.

Thames Water owns and operates two so-called 'Bubbler' boats - vessels which pump oxygen into the River Thames, when requested by the Environment Agency (EA), to help fish survive the pollution impact of the more significant storm discharges from the sewer network.

For similar reasons, again when requested by the EA, we also inject hydrogen peroxide into the River Thames from strategic points along the foreshore of the river

In addition, we operate two 'Skimmer' boats to reduce litter on the river linked to the Combined Sewer Overflow (CSO) discharges. These were launched in September 2007.

We have also agreed investment with Ofwat, which will significantly improve the treatment provided during wet weather at the five largest sewage treatment works discharging to the tidal River Thames. Already underway, this is scheduled to be complete by 2014.

But these initiatives alone are not enough to tackle the growing frequency of the CSO discharges. The proposed tunnels are needed too.

The process for setting water bills is determined by Ofwat, not Thames Water.

It is, though, undeniable that our 13.6 million wastewater customers will face higher bills to pay for the London Tideway Tunnels, in the same way that people in South West England have paid for improvements to sewage treatment works in the interests of cleaner beaches, at a much higher cost per head.

It is far too early to speculate what the final costs may be.

The UWWTD aims to protect the environment from the adverse impacts of insufficiently-treated urban wastewater. It assumes that all urban wastewater is collected and treated to so-called 'secondary' standards.

It does recognise that untreated storm overflows are sometimes necessary, but obliges member states to take action to limit their impact, where possible.

The UWWTD also specifies a minimum quality of treated effluent.

The proposed tunnels are the result of an extensive series of studies, dating back a number of years, which were undertaken to establish the extent of the impacts from the overflows.

Their frequency and volume had already been a concern for the Government and others, including Thames Water, and this led in 2000 to the Thames Tideway Strategic Study (TTSS), which provided evidence of impacts.

The tunnels emerged from the study's recommendations for how best to address the various impacts of the overflows, and in so doing demonstrate unequivocal compliance with the Urban Wastewater Treatment Directive (UWWTD).

Here is a very brief summary of the other main options considered and why they have been ruled out:

• 'Re-plumbing' London with a new sewer network that separates out sewage from rainwater
  Aside from the huge disruption this would cause, the cost (at least £12 billion) would be prohibitive. Such a huge investment programme could only be implemented over an extended period and would have an unacceptable impact on customers' bills.
• Implementation of Sustainable Drainage System (SuDS)
  Retrofitting these types of systems at the size required in London - an already densely urbanized environment, is impractical - there is not enough open space available. Any limited implementation would be very disruptive and costly, but not very effective at reducing Combined Sewer Overflow (CSO) discharges. The clay sub soils in London also make SuDS a less suitable option.
• Installation of screens at CSOs to reduce the volume of litter reaching the river
  At most of the CSO locations installing screening plant would be prohibitively disruptive. This option would also be incapable of dealing with the growing volumes of excess flows - the screens would rapidly become blocked and risk sewage surcharging back up into buildings and streets across the capital. This approach would not tackle the underlying problem of sewage polluting the river.
• Deploying more vessels on the river to inject oxygen into the river and skim off sewer-related litter
  Given the growing frequency of the overflows a bigger fleet of 'Bubblers' and 'Skimmers' would be a sticking plaster solution, treating only the symptoms of the problem, not the cause. This would not be sufficient to meet the requirements of the Urban Wastewater Treatment Directive (UWWTD) and would require a large fleet of vessels.

In autumn 2008 we completed an initial round of briefings for each of the 13 London local authorities potentially directly affected by the construction of the Thames Tunnel.

Three workshops (held in November 2008) gave all these local authorities, plus other pan-London consultees, an opportunity to comment on our proposed 'Site Selection Methodology' and our outline 'Community and Stakeholder Engagement Strategy'.

The Site Selection Methodology sets out the criteria we plan to use when selecting the required construction sites.

Our Community & Stakeholder Engagement Strategy outlines our initial plans for public consultation.

Summaries of both documents are available on our website.

The closing date for comments on the documents was 12 January 2009. Using the feedback we received, we will revise both documents and issue them for consultation before publishing the final versions.

At the moment we genuinely do not know exactly where the sites will need to be.

Applying the site selection methodology, we will first compile a 'Long List' of sites that meet the agreed criteria. Further evaluation will reduce this to a short list, then a 'Preferred List'.

We are committed to a thorough consultation process.

When we publish our preferred list of sites, we will seek the views of the communities potentially affected.  This process is currently expected to start in early 2010, in advance of us submitting a planning application (provisionally scheduled for 2011).

Before we make our decisions, we are determined to listen carefully to the views of people potentially affected. No decisions have yet been taken on which sites will be used.

Through early discussion we hope to establish a consensus on the best way to proceed.

To construct the Thames Tunnel we currently expect to need up to six main shaft sites (including one at Beckton Sewage Treatment Works), which will be access points for the machinery required to build the tunnel.

We also anticipate needing up to five smaller intermediate sites.

In addition we need to connect the most polluting Combined Sewer Overflows (CSOs) to the Thames Tunnel. These are all at existing, fixed locations along the River Thames.

Thames Water operates two pumping stations in Hammersmith.

One is an integral part of the drinking water system and the other pumps storm sewage into the River Thames.

A development located on a section of the Hammersmith Pumping Station (clean water) site was sold in May 2005 for conversion into flats.

The London Borough Hammersmith and Fulham agreed that the sale was the best way of protecting the locally listed building located on the site. A small block of social housing was subsequently built adjacent to the (clean water) pumping station.

We have an obligation to maximise the value of all redundant operational land. The profits from sales help keep customers' bills as low as possible.

We have specifically retained other land potentially surplus to operational requirement, with a view that it may be needed to construct the Thames Tunnel.

No decisions have been taken on the precise route of the proposed Thames Tunnel and therefore, construction sites have not yet been identified.

We are currently undertaking a thorough consultation on a draft site selection methodology with 13 London boroughs, including the London Borough of Hammersmith and Fulham, and other key pan-London planning authorities.

The final document will be published on our website in summer 2009.

At this stage, the final 'route to consent' has not been determined. The potential routes are:

• Individual applications to each local planning authority affected by the tunnel route.
• The Mayor of London and/or Secretary of State for Communities and Local Government could determine that the applications are of strategic importance and call them in for determination.
• An application for development consent could be made to the proposed Infrastructure Planning Commission, which the Government is in the process of establishing, following the introduction of the Planning Act 2008. The Act paves the way for a new system to consider major infrastructure projects (in England and Wales).

It has to be stressed that the purpose of the proposed tunnels is to protect and enhance water quality in the River Thames and River Lee, in line with the requirements of the Urban Wastewater Treatment Directive (UWWTD).

We attached a very high priority to the separate problem of sewer flooding to customers' properties.

We have put forward to Ofwat (in our draft Strategic Business Plan) an extensive programme to alleviate sewer flooding.

By agreement with Ofwat, in the five years to 2010 we are investing over £320 million to address the issue for more than 10,000 properties - a four-fold increase on 2000 to 2005.

In our outline plan to Ofwat for the next Price Review period (2010-15), we have proposed schemes to alleviate the risk of sewer flooding to a further 4,700 homes.

We will carefully co-ordinate work on these schemes with the planning for the Thames Tunnel to ensure we obtain the best overall benefits,

Any additional costs incurred would need to be approved by Ofwat.

Even though not its intended purpose, the Thames Tunnel will provide some help in preventing sewer flooding to properties in certain circumstances.

The ability of the existing gravity-driven overflows to effectively discharge to the River Thames can be restricted by high tides.

Diverting excess flows to the Thames Tunnel during high tides is expected to reduce the sewage backing up in the existing sewer system.

Great care - including detailed screening work - will be taken prior to the start of construction to avoid any such damage.

We are already in touch with Transport for London, British Telecom, Network Rail, the Port of London Authority, Crossrail and others to ensure that the structural integrity of other underground assets is preserved during the construction of the Lee and Thames tunnels.

The tunnels themselves will be deep underground and not visible from the surface.

We will, however, sometimes need to access the tunnels for maintenance purposes. For the longer Thames Tunnel this means there will be a number of points along the route where this facility will need to be provided.

We are keen to work in partnership with all stakeholders to find ways of minimising and/or mitigating the visual impact of these sites, in line with the wishes of the local communities affected.

There are many other tunnels under London. It is the need to avoid them that means the proposed tunnels will have to be constructed up to 75 metres below ground - deeper than all the other existing infrastructure under London.

The proposed tunnels are designed to store excess flows linked to rainfall. Only after the flows in the existing network have subsided will the flows from the London Tideway Tunnels be pumped out for treatment at Beckton Sewage Treatment Works.

The storage capacity of the tunnels will be optimised to avoid the need for excessive treatment capacity. Additional capacity will be provided at Beckton Sewage Treatment Works to fully treat the flow from the tunnels, as well as enabling the plant to treat more sewage from the existing catchment and to improve treatment standards.