£2bn Thames tideway clean-up plan gets the green light

A massive engineering project to increase the capacity of Bazalgette's 140-year-old interceptor sewers which run either side of the river Thames has been given an initial go-ahead by Environment Secretary Margaret Beckett. The plan involves constructing a new tunnel deep beneath the Thames with capacity to handle overflows from sewers during heavy rain.

Thames Water prides itself on the state of the tideway - the stretch of the river which is London's focus and centrepiece. The company's publicity dubs the Thames "the cleanest metropolitan river in the world", and shows its banks teeming with cats, searching for the fish that are returning to the estuary.

There have indeed been massive improvements in the quality of the river since the 1960s when it was almost lifeless, but the company's publicity does not tell the whole story.

In fact, any significant rain over London causes the sewers to overflow into the river, lowering dissolved oxygen levels and polluting the river with bacteria, chemicals and litter.

The EU urban wastewater treatment and water framework Directives are now forcing the Government to find a solution to the problem. A team of experts from the Environment Agency, Ofwat, the water company, the Greater London Authority and the Environment Department (DEFRA) have been examining the options for the past three years. The group, known as Thames tideway strategic study, is due to report later this year.

In March, the plans took a leap forward when Mrs Beckett issued advice on water companies' business plans for the AMP4 asset management planning period from 2005-2010 (see pp 46-47 ). Mrs Beckett instructed Ofwat to include the scheme in Thames Water's business plan for the period - effectively giving the go-ahead for the next phase of planning and design work.

Professor Chris Binnie, the consultant who chairs the study group, said that the optimal scheme was to build a tunnel under the river from Twickenham in the west to the Beckton and Crossness sewage works in the east. Construction was likely to take around six years and was unlikely to begin before 2010.

The scheme will be a massive engineering undertaking and a worthy addition to the work of Victorian engineer Sir Joseph Bazalgette, who built the interceptor sewers which to this day prevent much of London's sewage entering the river.

At some 35km in length and around nine metres in diameter, the tunnel will be capable of holding millions of cubic metres of storm sewage. It will need to be up to 100 metres underground to avoid other tunnels, and will have to follow the course of the river to collect sewer overflows from about sixty different points.

The Agency's tideway technical manager, John Goddard, said: "There are 54 overflows and we are assessing discharges individually. They discharge on a weekly basis - even during moderate rainfall - and anything discharged will move up and down the estuary for 15km on the tide."

"We estimate that each year 12 million cubic metres of storm sewage flow into the river and the biggest discharge in 24 hours was three million cubic metres. We estimate the discharges carry 10,000 tonnes of screenable solids like plastic, needles and faecal matter into the river."

Concerns include litter, health risk from pathogens and the sag in dissolved oxygen levels. Levels can drop as low as 10% in summer which "put at risk the sustainability of fish populations", Mr Goddard said.

The water company currently employs two bubbler barges which pump air into the water. The Agency also regularly uses hydrogen peroxide dosing to keep oxygen levels up.

However, a long-term solution to the sewer overflows is required under the EU urban wastewater treatment Directive, which required Member States to have adequate sewage collection systems in larger sewerage catchments by the end of 2000. By 2015, all waters will also have to achieve good ecological status to meet the requirements of the water framework Directive.

Professor Binnie said: "We have looked at a whole range of options and we will be producing a report in a few months' time. We are hoping for a ministerial decision later this year. The next phase in AMP4 will be the design and planning aspects. Construction would take place mainly in AMP5 and AMP6 [between 2010 and 2020]."

Professor Binnie put the costs at "between £1 and £2 billion", but would not be drawn on the implications for water bills. One source suggested that the project might mean that Thames Water would no longer boast the cheapest combined water and sewerage bills in England and Wales - but it seems the increase might only be around £6-12 per year.