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Special report: Fundamental issues at heart of super sewer

The debate about the proposed £4.1bn Thames Tunnel mega sewer is growing ever more politicised. NCE talks to the experts for and against the controversial super sewer.

In recent weeks Thames Water’s proposed £4.1bn Thames Tunnel has been at the centre of fierce debate.

First, the independent Selborne Commission published its inflammatory report (NCE 3 November) and then University of Sheffield urban water professor Richard Ashley raised questions about the ethics of civil engineers endorsing such an expensive scheme.

Now, as the second round of consultation on the project begins, central and local government bodies, quangos, charities and even Bazalgette’s descendant have all announced their priorities and allegiances.

But as the clamour of voices grows, the argument rolls repeatedly over the same ground, with neither side willing to budge on the fundamental differences of opinion behind their stance on the tunnel. Five polarising issues are at the heart of the argument.


Urban Waste Water Treatment Directive

The European Union (EU) Urban Waste Water Treatment Directive was created to protect the environment from waste water discharges. It is a pertinent piece of legislation for the River Thames, which is annually polluted by 39M.m³ of raw sewage from combined sewer overflows (CSOs).

However, the directive does not specify what volume of overflows is acceptable, leaving the Environment Agency to set its own standards for Thames Water to fulfil. Opponents of the Thames Tunnel argue that the Environment Agency’s criteria are unnecessarily stringent, and have led to an excessively large and expensive solution.

“The higher you set your standards, the more open you are to [EU] infraction proceedings”

Association of Drainage Authorities chief executive Jean Venables

Selborne Commission member, past ICE President and Association of Drainage Authorities chief executive Jean Venables says the problem of what to do about CSOs is largely self-made by the government. “The higher you set your standards, the more open you are to [EU] infraction proceedings,” she says.

The Selborne Commission argues that if the standards were revised down a shorter and cheaper tunnel would become legally acceptable to both the government and Thames Water. “The Environment Agency standards are the starting point [for the project] that needs to be challenged,” says Ashley.

However, Thames Water head of London Tideway Tunnels Phil Stride says the standards are in keeping with international practice. “We feel they’ve been set at a reasonable and realistic threshold,” he says, adding that the standards would still leave 4% of discharges unaddressed.


The perceived urgency of the project is also contentious. Thames Water has repeatedly emphasised the need for a “timely” solution, both to resolve an “unsatisfactory situation” and to comply with the directive.

Venables asks whether the sense of urgency over the project is necessary. “Everybody seems to have assumed a timescale,” she says. “The Department for Environment, Food and Rural Affairs (Defra) mentioned dates but we’ve never seen them go back [and review these].”

Commission chairman Lord Selborne accuses Thames Water and the government of rushing the project and unnecessarily dismissing alternatives because of the timescale.

He claims that the tunnel was chosen due to “ministers’ concern that they were out of time…and something had to be done quickly”. Stride opposes this assertion. “We absolutely disagree with this,” he says, pointing out that the project has been under consideration for more than five years. “It depends on your definition of ‘rushed’.”

A three year postponement of the proposed date for completion of construction on the tunnel, announced this month, only throws more fuel on this particular fire. Does the postponement indicate that the timescale for complying with the directive is flexible, or does the loss of three years mean the speed of implementing a solution is more important than ever?


The Selborne Commission questions the adequacy of a number of models used in assessing options. Venables says models used early on to evaluate potential solutions did not take into account the possibility of reduced flows into the sewers as a result of building sustainable drainage systems (Suds).

She says more modelling should have been done - but Stride argues the modelling done has been sufficient.
“We’ve spent three years building up a very detailed and complex model of London’s sewer system,” he says.

The model takes surface water into account, Stride says, and the company has looked at the effect of using Suds in west London to reduce those flows.

Modelling issues have also been raised over CSO discharges. Independent consultant Chris Binnie says 2006 models of CSOs − on which the decision to commit to the Thames Tunnel was based − were created using monitoring data collected from just 9 of 57 CSOs.

The Selborne Commission calls this a “weak scientific base” for a major investment decision.

Stride says Thames Water has instigated a wider monitoring programme since 2006 and now monitors flows from all 57 CSOs. This has resulted in the estimated amount of sewage discharged into the river being revised up from 32M.m³ to 39M.m³.

However, Thames Water was unable to provide its reports and data on this issue to the Commission within the short period in which it operated, meaning that the new monitoring systems were not analysed in the Selborne report.

Suds viability

The Selborne Commission and other detractors assert that Suds can play a major role in preventing CSO discharges to the river − but Thames Water insists that, while it supports the use of Suds, they do not stack up as a solution in this particular case.

The principal disagreement is spatial. Thames Tunnel opponents cite American cities that have implemented widespread Suds − including Chicago, Milwaukee and Portland − and say those programmes can be imitated.

But Stride argues that London’s density, clay ground, historic buildings and public attitudes prevent this from working.

“The thing that’s absolutely key is how densely populated the urban environment is,” he says. Green roofs are a way around this problem, but Stride says much of central London is made up of historic buildings which tend not to have flat roofs and therefore cannot be retrofitted in this way.

His opponents argue that Thames Water fails to appreciate the versatility and flexibility of Suds options. The Selborne Commission further suggests that existing parks could be used for flood retention.

Stride vehemently asserts that public consultation feedback on the use of park sites showed that the public would not accept this, although Thames Water has not specifically consulted on using parks in this manner.

The complexity of implementing Suds on such a scale − both in terms of construction work and of disconnecting existing drains from the sewer network − is another flashpoint in the argument.

Stride estimates that achieving a “meaningful reduction” to CSO discharges in this way would take 30 to 40 years and that disconnection on such a scale is “simply not feasible”. The work would be “highly disruptive” and would require extensive and widespread road works, Thames Water says.

But Venables says Suds will have to be built on a large scale in the future anyway, and Ashley argues widespread use of Suds would offer more benefits than the Thames Tunnel, such as flood mitigation and amenity.

Future wastewater flows

The future forecast for wastewater flows into the sewers is a deep-seated factor in dictating the scale of solution that is required to tackle CSO discharges.

Thames Water posits that population growth will lead to increased flows, meaning the existing sewer network would be overwhelmed and CSOs would operate more frequently, rather than only in extreme weather events as they do currently.

But Binnie − previously a champion of the Thames Tunnel − controversially retracted his support in September after coming to a rather different conclusion on future flows. Binnie’s own calculations indicate that sewage flows in dry weather conditions are likely to remain below 2006 levels, and he cited this in September as a key factor in his change of opinion.

Predictions on future wastewater flows must take into account surface water runoff, water efficiency and leakage − all factors which are liable to change significantly in coming decades.

Binnie’s own turnaround is evidence of the difficulty of reaching a reliable conclusion, says Venables. “The crystal ball is very cloudy.”


The second phase of public consultation will end on 10 February 2012. After that date, Thames Water will respond to the feedback it has received from both the public and the Selborne Commission − and these five issues will likely figure large in the debate again.

Readers' comments (1)

  • Build it!!!!

    I'm a rower and we have to row through raw sewage every time there's any rain. London's sewers are old and need updating rapidly.

    What's the issue? Isn't Thames Water paying and recouping expenditure from London's below-average water bills by bringing them up to the national average?

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