Clare, The 39 million tonnes of sewage discharged to the tidal River Thames in an average year via the 'Combined Sewer Overflows' (CSOs) is from the areas served by the sewage treatment works over in east London, at  Beckton and Crossness.  In other words, the sewage that overflows into the river via the CSOs should go to these two works, not Mogden. The problem being addressed at Mogden is one of lack capacity at the works, not in the network. The Mogden 'catchment' is only partially a combined system. This means that it is affected by rainfall to a much lesser extent than the Beckton and Crossness works. To try and summarise, the three catchment networks are physically separate. It is not possible for an improvement in the Mogden catchment (which was built later) to have any affect, either positive or negative, on what comes out of the CSOs designed into the Beckton and Crossness catchments.  The upgrade underway at Mogden will provide an improvement to river water quality, but will have no affect on the CSOs the Thames Tunnel needs to intercept.  Basically, the sewers downstream of (roughly) Chiswick north of the river serve Beckton; south of the river the sewage goes (or at least should go, in theory) to Crossness.  Everything upstream is designed to end up at Mogden.  The Environment Agency (EA) has advised there are no 'unsatisfactory' CSOs upstream of Chiswick Ayot. Detailed modelling by the EA and Thames Water shows that if the weather conditions for the last 12 years were repeated in the 12 years after the work at Mogden is complete,  there would be only five discharges. This is a huge improvement towards making the river cleaner, as we all want. But we do need the Thames Tunnel as well. Hope this helps? Nick