There are 57 sewage overflow points into the Thames which dump 16m tonnes of waste into the river each year. Just 2mm of rain can send enough water surging through London's Victorian sewage system to trigger an overflow of pollutants into the Thames. Once there the problem does not disappear, instead it flows up and down the river on the tide for up to three months before finally floating out to sea.
When Joseph Bazelgette designed the 100 miles of Victorian sewers that still serve London, the population of the capital was around 2m people. Now it is 8.2m.
"Bazelgette's system was incredibly well designed," explains Phil Stride, head of Thames Tideway Tunnel. "There is absolutely nothing wrong with what he created. The bricks, the design, the workmanship have all stood the test of time. The problem is we have outgrown it, by some distance."
Drop through a manhole in the pavement near Blackfriars Station in the City and the quality of the workmanship is immediately apparent. Not a brick is out of place, the mortar is solid, nothing has crumbled. The air may not be fresh, but the work is. It is like being transported back to the mid 19th century. Nothing has changed, nothing has been added, nothing taken away. Bazelgette's work has withstood everything London has thrown at it, and proved capable of dealing with everything other than the volume.
But critics of Thames Water's tunnel would argue that Bazelgette's interceptor sewers are still doing the work they were designed for.
When the system was first put into use the storm overflows were designed to deposit waste into the Thames once or twice or a year. The sewage outflows are still functioning perfectly, just with a frequency that has gone up 20-fold. Why build a monstrous tunnel under the Thames when the river does the job anyway?
That then is the first argument against the new system. The second is the cost. At £4.2bn for the tunnel that will run from Acton in west London to Abbey Mills in the east, and a further £600m for the Lee tunnel which will make the final connection to Beckton sewage works, the project is massively expensive.
It will involve the creation of a separate company to sit alongside Thames Water, be funded by debt from the private sector and backed by cashflows from customer bills. Achieving funding of this magnitude is no simple matter in the current economic and financial climate – a problem that has led to interest from the Treasury about finding a Government backed solution.
Either way the cost to Thames Water customers, from those in towns outside London that will see no benefit, to those in the capital that will, is huge.
Thames Water argues that the £80 projected annual increase in bills will only take costs to the same level as those in other areas in the south of England. The company also argues that while customers currently benefit from the work and investment of past, largely Victorian, generations, it is future generations that will largely pay for the super sewer.
But before that can happen 24 construction sites will need to be created along, in and over the Thames, many of them in sensitive, residential or recreational areas. The opposition to these works, many of which will be in place for years to come, is stiff, and could be the most difficult hurdle for Thames Water to overcome.
The first major battle in that war will take place in the next few weeks when the Thames Tideway Tunnel goes in front of the Planning Inspectorate looking for the first stage of its planning permission.
Expect it to raise quite a stink.
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