| Topic: | Re: Defra, EA, Mogden, pipe jacking etc . | |
| Posted by: | Nick Tennant | |
| Date/Time: | 16/03/11 11:48:00 |
| Roland, the Parliamentary statement was by Caroline Spelman on 7 September 2010. See Defra's website for full details: http://ww2.defra.gov.uk/news/2010/09/07/tunnel-sewers/ On Mogden, I was genuinely trying to clarify, not confuse. The requirement that the Environment Agency (EA) has set for the tunnel is to intercept the 34 most polluting CSOs discharging to the river – the most westerly one is at Chiswick. I’d need to confer with a specialist on your question about percentages, but on average, about once every two and a half years we would expect flows reaching Mogden to exceed the capacity of the extended plant and its storm tanks. After that, additional flows would be screened and settled (through the storm tanks) with the excess discharged to the river. But it is important to recognise that by the time we reach that point the entire system will have had a thorough ‘flush through', with the most polluting flows safely captured for treatment. Also, by the time that such discharges are required, the natural flow in the river will have increased considerably, providing greater dilution. On your fundamental point, a connection to Mogden to collect such partially treated discharges is technically possible. The issue is that such a change would require a direction from our regulators that the work was warranted. They would need to look carefully at the cost v benefit. It's unlikely they would conclude that the cost of capturing such intermittent and relatively dilute discharges was justified, but deciding what level of discharge the river can cope with remains the EA's call. I can confirm that if they thought it necessary to achieve further reductions, it would be technically entirely feasible to extend the tunnel to Mogden at a later date. But this would not negate the need for the tunnel to collect the CSO discharges, which are from different catchments and have a heavy ‘polluting load’, especially in the narrower western stretches of the river heavily used for recreation. Creating a separate system for rainwater and foul water would cause huge disruption in many streets, to be effective. Pipe jacking works in locations where you do not need to connect everyone on the street to the pipe. Given the mix of utilities and conditions under London, it’s unlikely to be an effective technique here. We would either need to connect the houses or connect the storm drains to the pipe that was jacked in. |