In the 1850s over 400,000 tonnes of sewage were flushed into the River Thames each day - around 150 million tonnes a year.

The river was biologically 'dead' and the stench was overpowering.

In summer 1858 Parliament had to be suspended because of the vile smell, which has been named the 'Great Stink'.

As a result, Parliament passed an enabling act to raise £3m to build a network of giant intercepting sewers, pumping stations and treatment works, designed by the engineer Sir Joseph Bazalgette.

This network has been much improved and extended over the years, but still forms the backbone of London's sewerage system today.

Sewer overflows

By the time 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 surface water run-off.

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 that climate change would bring less frequent but heavier rainfall or that so many green spaces would be concreted over, preventing natural drainage.

Today, the system is struggling to cope with the demands of 21st century London, and discharges are now happening much more frequently - around once a week on average.

We have developed three major engineering schemes to tackle this.

• Watch our video: London Tideway Improvements - Creating a cleaner, healthier Thames

Background study

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 ensure compliance with the requirements of the EC Urban Wastewater Treatment Directive concerning the collection, treatment and discharge of urban wastewater.

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 engineering consultant Professor Chris Binnie.

The study concluded that preventing discharges from combined sewer overflows alone would be not be enough to achieve new water quality standards. It was agreed that London's major sewage treatment works would also need to be expanded as they needed to treat considerably more wastewater.

Two problems

Overloaded sewage works

Although our sewage works operate well under stable, dry weather conditions, in heavy rainfall they can become overloaded. Excess flows are diverted to storm tanks, for treatment when flows subside.

But once the tanks are full, the only option is to discharge any additional excess flows to the river, to avoid sewage backing up onto the streets or even into people's homes.

Overloaded sewers

Bazalgette's sewerage system was constructed with 57 combined sewer overflow points along the tidal River Thames.

When the sewers fill to capacity after heavy rainfall, any excess sewage is discharged into the river via these overflows, to prevent flooding to buildings and streets. Around 39 million tonnes of untreated sewage is discharged annually, and as little as 2mm of rainfall can trigger a discharge.

Three solutions

In line with the recommendations of the Thames Tideway Strategic Study (see above), we are developing three separate schemes to address these problems:

• A £675million investment to improve London's five principal sewage works - Mogden, Crossness, Beckton, Long Reach and Riverside.
• The Lee Tunnel, to deal with the largest combined sewer overflow point at Abbey Mills Pumping Station, which discharges into the River Lee.
• The Thames Tunnel, to deal with the 34 most polluting combined sewer overflows along the River Thames between west London and Beckton Sewage Treatment Works.

Further information

For more detailed information, including the full Thames Tideway Strategic Study report, please visit the Thames Tunnel consultation website:

• Visit the Thames Tunnel consultation website