The Issues

Sewage pollution

Currently the Thames is frequently polluted by sewage overflowing into it. This stems primarily from two sources:

  1. Combined sewer overflows (CSOs)
  2. Sewage Treatment Works (STWs)

Combined Sewer Overflows (CSOs)

These are the legacy of the best engineering practices from the 19th and early 20th centuries. Basically, when it rains heavily, rainwater flows directly into the sewers, causing them to overflow into the Thames[1]. There is little doubt that the most appropriate solution to remedy this is to separate rainwater from sewage and indeed, this is how new sewers are built. However, in many cases, separating existing sewers is deemed too expensive and/or disruptive, so workarounds are required.

Sewage Treatment Works

Some of London’s sewage treatment works (STW) have a poor environmental record, following years of under-investment by Thames Water and are now so overloaded that they regularly overflow into the river, causing massive pollution. These include Mogden, Beckton and three others, which are fortunately now being upgraded. In June 2011 heavy rains caused the Mogden STW alone to spill nearly 50% of a discharge which resulted in serious fish kills.

Climate Change

Although climate change should urgently be mitigated in its own right, it’s clear that the unpredictability of weather patterns it brings have to be reckoned with.[2]


It’s just plain common sense that clean storm water should be embraced as the natural resource it is rather than showed down sewers with possible dire consequences.

In spring 2012, a major drought was looming on the South East of the country. It is interesting to contrast this situation with the aforementioned excess rains which resulted in fish kills only 8 months earlier. Had we harvested this rain or allowed it to filter down to underground aquifers rather than let it run down sewer pipes, might the outcome have been better all round?


The flip side of droughts is the very significant risk of flooding. Although London has been spared so far, it’s by no means safe. Such risks are exacerbated by the concreting over of many naturally porous surfaces, particularly in urban areas. This, in itself, can have many unpleasant side-effects beginning with the drying out and cracking of ground.

Two sides of the same coin…

The short term pollution issues arising from deficient storm water management are linked in more than one way with the longer term consequences of climate change: the looming threat of floods and droughts. Please read on to find out about our proposed solutions

  1. [1] Check Wikipedia for more information
  2. [2] This 2010 Llodys study finds a tenfold increase in extreme weather in East London since the 1960′s and that was even before the most extreme events experienced in 2012 and early 2014. The change is most significant for days of extreme rainfall over 40mm, which recorded a 900% increase. To be future-proof our drainage systems will have to contend with this reality. Perpetuating old, flawed, designs that combine rainwater and sewage in a single pipe is clearly a risky strategy, to say the least…