Thames Water has opened mainland UK's first-ever desalination plant was to provide "seriously water-stressed" London with a much-needed back-up supply to use in the event of a drought.

Powered by renewable energy, the Thames Gateway Water Treatment Works, at Beckton in east London, will, when required, turn a mixture of seawater and river water from the tidal River Thames into high-quality drinking water for up to one million Londoners.

The official opening, by The Duke of Edinburgh, marks the latest addition to Thames Water's long-term measures to ensure it can meet the demand for water. The ongoing replacement of London's leaky Victorian water mains has helped cut leakage by more than a quarter in the past five years. But that progress on its own is not enough to ensure London will have enough water in a drought.

The capital is classed by the Environment Agency as "seriously water-stressed", which means that demand could outpace supply in a long dry period - climate change threatens hotter, drier summers and an additional 700,000 people are forecast to move to London by 2021.

The key treatment process in desalination is reverse osmosis, which involves forcing salty water through extremely fine membranes. This tried-and-tested technology is used at 14,000 water treatment plants across the world and has provided crews on Royal Navy ships with fresh water for decades.

However, while most reverse osmosis plants have one or two stages, which yield around half of the source water as drinking water, the £270m Gateway works is the world's first-ever four-stage reverse osmosis system, yielding a far more efficient 85 per cent.

The works will only take in water on the outgoing tide, when it is a third as salty as normal seawater and so requires less energy to treat it. The new works is able to produce 150 Mld when needed, enough to supply 400,000 homes (one million people). Its water will be blended with other supplies, so up to 580,000 properties in northeast London (1.4 million people) will potentially receive it in varying proportions.

Martin Baggs, Thames Water's Chief Executive, said:

"People may wonder why we're equipping 'rainy' London with a desalination plant, something more often associated with the Middle East, southern Europe or ocean-going liners. But the fact is, London isn't as rainy as you might think - it gets about half as much rain as Sydney, and less than Dallas or Istanbul. Water is an increasingly precious resource that we can no longer take for granted.

"Our existing resources - from non-tidal rivers and groundwater - simply aren't enough to match predicted demand in London. That's why we're tapping into the new and limitless resource of the tidal Thames, fed by the rolling oceans beyond, so we can ensure our 8.5 million customers have enough water in future in the event of a drought.

"The 2005/06 drought was too close for comfort, with only a very wet May saving the day, and we never want a repeat of that. It highlighted what we already knew: additional water sources are needed, as well as a lot more work on reducing leakage, to be sure we have sufficient supplies long-term.

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