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pollution control valve

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cockneyexile | 00:00 Sun 20th Feb 2005 | Travel
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when driving on the m25 i have noticed signs for pollution control valve on the side of the hard shoulder (permanent signs not temporary) what is this and what is it used for ? i know obviously pollution this entrigues me as i have not seen these signs on any other motorway except  the m25

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These are mainly on the south side of the M25, but they are in a few other places as well. By  the motorway, usually when you are on an embankment, there is a larger than normal ditch  called  a balancing tank (although it is just a big ditch). These 'tanks' are used to regulate drainage run off during rain, as the water course the rain drains into may not be able to take all the water at once, so it prevents local flooding. As a consequence of this, if an accident happened on the motorway, say with a petrol tanker or just a ruptured fuel tank, the liquid will find its way into the water course via the balancing tank. By putting a penstock (which is just a flash word for a big valve) on the exit of the tank, you can use the tank to hold the pollutant until it can be safely tankered away. This system is used on newer and recently re built roads. On old roads, the council just chuck a load of sandbags in the ditch to dam it up and hope the ditch will be big enough to hold all the polutant.
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to sddsddean cheers for taking the time to post a reply i find your answer very well researched thanks once again. ricky 
No probs Ricky. I was a Civil Engineer for 10 years, so used to build roads. Now like you, I'm trucking round them.

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