In March 2007, the Government gave us the go-ahead to design a tunnel solution to substantially reduce the amount of untreated sewage which overflows into the River Thames and River Lee from London's Victorian sewers.

These overflows can occur after as little as 2mm of rainfall.

Currently, the overflows occur more than once a week on average, via 57 combined sewer overflows (CSOs), which were designed by Victorian engineer Sir Joseph Bazalgette over 150 years ago.

They discharge 39 million tonnes of untreated sewage into the River Thames in an average year, and up to three times more in wetter years.

The overflows create health risks to river users, endanger wildlife and release tonnes of 'sewage-derived litter' into the river, such as condoms, sanitary products and toilet wipes.

In order that the UK complies with the Urban Wastewater Treatment Directive (UWWTD), the frequency of these discharges cannot be allowed to continue at their current rate.

By the time Sir Joseph Bazalgette started work, most of London's rivers and streams were carrying both sewage and rainwater.

Even at this time, separating the two would have been almost impossible and so he designed his new sewers to perform the dual function of dealing with 'foul' sewage and rainwater run-off from roads, roofs and pavements.

After heavy rainfall the flows were greater than his sewers could take, so he designed the system to overflow into the River Thames when necessary, to prevent sewage from backing up and flooding streets and buildings.

Of course, at that time, the river was 'dead' and his system was bringing big improvements, even though it overflowed from time to time.

When Bazalgette's network was built, the capital's population was around 2.5 million. He planned for population growth of around four million, not the eight million we now serve.

Nor did he predict climate change or that so many green spaces would be concreted over, preventing natural drainage.

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 of untreated sewage.

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.

Our London Tideway Improvements will ensure that the excellent progress we have made is not reversed.

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

Like many older cities around the world, the vast majority of London is served by a 'combined' sewerage system, which collects foul sewage, as well as rainwater run-off from roads, roofs and pavements.

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

However 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 its tributary the River Lee.

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

We have started a £675million upgrade at five of our largest sewage treatment works, which discharge sewage into the tidal River Thames during heavy rainfall when they are overloaded.

This work is scheduled to be complete by 2014.

We also take action to reduce the impact of discharges into the river.

We own and operate two 'Bubbler' boats, 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 sewerage 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 combined sewer overflow (CSO) discharges. These were launched in September 2007.

The process for setting water bills is determined by Ofwat.

It is, though, undeniable that our 14 million wastewater customers will face higher bills to pay for the proposed Thames Tunnel, 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.

We estimate the cost of our preferred route option for the Thames Tunnel (the Abbey Mills route) to be £3.6billion, and that this will require our average bill to have risen by slightly more than £1 a week by 2018.

This means that the bills of Thames Water customers, which have for many years been among the lowest in the country, would rise to around the national average.

The Urban Wastewater Treatment Directive (UWWTD) concerns the collection, treatment and discharge of urban wastewater and the treatment and discharge of wastewater from certain industrial sectors.

The objective of the Directive is to protect the environment from the adverse effects of those waste water discharges. The Urban Wastewater Treatment Regulations 1994 transpose the UWWTD into English law.

The regulations impose a duty on sewage undertakers to provide and maintain adequate collecting systems and treatment plants, in addition to their duties under other legislation, including the Water Industry Act 1991.

The design, construction and maintenance of collecting systems must be undertaken in accordance with the best technical knowledge not entailing excessive costs, notably regarding volume and characteristics of urban wastewater, prevention of leaks, and limitation of pollution of receiving waters due to storm water overflows.

In 2000, the Thames Tideway Strategic Study was set up to consider the environmental impact of storm discharges to the tidal River Thames and to propose potential solutions that would comply with the EU Urban Wastewater Treatment Directive, which requires that sewage should be properly treated except under 'exceptional' conditions.

Thames Water, the Environment Agency, the Greater London Authority, Defra and Ofwat (as an observer) all contributed to the study, which was independently chaired by Professor Chris Binnie.

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

The study concluded that preventing discharges from combined sewer overflows alone would be insufficient to achieve new water quality standards. It was agreed that London's major sewage treatment works would also need to be expanded.

The study concluded that additional capacity is required, and further treatment improvements are needed to improve the water quality in the river and protect fish life.

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 sewage from rainwater.
Aside from the huge disruption this would cause, the cost (at least £12billion) 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 as there is not enough open space available.

Any limited implementation would be extremely 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 and would require a large fleet of vessels.

We will carry out extensive public consultation prior to our submission for planning approval.

The first round of public consultation took place from 13 September 2010 to 14 January 2011.

During the consultation period we gathered information from not only the local community potentially affected by the construction of the tunnel but also anyone offering relevant information to be taken into account.

We are using this information, where possible, to modify the design of the Thames Tunnel to address any concerns that may arise.

We published a report on the Phase 1 consultation on our Thames Tunnel consultation website in spring 2011.

• Visit our Thames Tunnel consultation website

A revised design will then be the subject of a second round of consultation later in 2011, prior to the planning application being submitted.

Following planning consent, we expect to begin main construction of the Thames Tunnel towards the end of 2013, with completion in 2020.

To construct the Thames Tunnel we are currently looking for main shaft sites which will be access points for the machinery required to build the tunnel and remove the spoil.

Reception sites are also needed, where we will insert the tunnel boring machines.

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 and so the search for these construction sites will be more constrained.

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.
• The project could be referred by the Government to the Infrastructure Planning Commission, which has been established 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).

Irrespective of who considers our application(s), due to be submitted in late 2011 / early 2012, we are committed to a full and proper consultation process.

We want to work in partnership with local authorities, communities and other stakeholders to mitigate any impacts. This is why we have begun our consultation and engagement work at a very early stage.

The statutory planning process will allow the general public to be consulted and to comment on the detail of our proposals.

The purpose of the 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.

We attach a very high priority to the separate, but equally important, problem of sewer flooding to customers' properties.

In the five years to 2010, we have invested over £320million to address the issue for more than 10,000 properties.

By agreement with Ofwat, in the five years to 2015, we have been funded to protect nearly 2,500 homes from the risk of flooding.

We are pleased that our economic regulator Ofwat has recently sanctioned initial funding to plan and develop a major scheme addressing this issue in the area around the Counter's Creek, affecting both the boroughs of Hammersmith & Fulham and Kensington & Chelsea.

This has the potential to start construction in 2015, pending further approvals from Ofwat. Public meetings have been held to brief councillors and local residents from both boroughs on this important initiative.

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.

The proposed Thames Tunnel will provide some help in preventing sewer flooding to properties in certain circumstances.

For example, during high tides the ability of the existing gravity-driven overflows to effectively discharge to the River Thames can be restricted.

Diverting excess flows to the Thames Tunnel during these periods 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, need to access the tunnels for maintenance purposes from time to time.

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, taking into account comments of the local communities affected.

There are many other tunnels under London. To avoid them, the tunnels will have to be constructed up to 75 metres below ground - deeper than all other existing infrastructure under London.

The tunnels are designed to store excess sewage linked to rainfall.

Only after the sewage in the existing network has subsided will it be pumped out of the tunnels for treatment at Beckton.

Additional capacity will be provided at Beckton 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.

Construction of the Thames Tunnel in particular will inevitably result in disruption, but we are determined, where we can, to reduce any impact on local communities.

For example, where practicable, we intend to use the river itself to transport excavated soil.