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Tracey Logan reports on the regular spillages from London's sewage system into the River Thames and the government's proposed solution. A super sewer has been commissioned, officially known as the Thames Tideway Tunnel. The deepest tunnel ever built under London, the project will cost two billion pounds and not be finished until 2020. At 35km long, it's the deepest tunnel ever built under London and will require engineering at the limits of current technology and with the hefty price-tag will come years of riverside engineering works. Once finished, will it really make the Thames a river fit for salmon, a fish whose numbers reflect the purity of the water, or is such a target unrealistic? Some are asking whether such an expensive sewer upgrade is really necessary.