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Thames' desalination plant enters choppy waters

News - If drought hits London, the capital could face a water shortage of 236Ml per day.Thames Water says it needs a desalination plant. London Mayor Ken Livingstone says not. Andrew Mylius looks at the evidence.

THE REVELATION last Wednesday that Thames Water had again missed leakage reduction targets set by regulator Ofwat, while raking in a pretax profit of £346.5M, will not help the firm's bid to build a £200M, 140Ml/day desalination plant on the Thames Estuary.

London Mayor Ken Livingstone blocked Thames' planning application for the plant earlier this year, arguing, among other things, that the company squanders six times more water than the facility will produce. Thames appealed against his decision and the application has for the past five weeks been subject to the scrutiny of a public inquiry, which ends today (Thursday).

Thames says that it needs the plant as a stop-gap resource until progress has been made on replacing its Victorian mains network, and until 350Ml/day of new reservoir capacity can be built at Abbingdon, Oxfordshire in about 2020. It adds that it is investing heavily in reducing leakage, with four-fi fths of profi t made in the past few years reinvested in the business.

Without the desalination plant, London's water supply is vulnerable to drought, Thames says. If standpipes have to be used to control water use, there will be a heavy impact on health, the capital's economy and on the environment.

But Livingstone points out that up to a third of water supplied by the plant will be lost through leakage before it reaches customers' taps. The mayor also objects to the plant on the grounds that it will be energy hungry and contribute to climate change.

'Every single day the plant would pump in excess of 150t of carbon dioxide into the atmosphere, ' he says.

'Even if the impact on climate change of the desalination plant is only slight on a global scale. . .

it would send out the worst possible message about the true extent of this country's determination to tackle the root causes and symptoms of climate change.' Livingstone wants to curb water consumption through the blanket metering of businesses and households. The harvesting of rainwater should be made compulsory on all new developments, and grey water recycling ought to become more widespread. Thames should also look at the reuse and purification of outfl ow from wastewater treatment plants, he says.

But Thames Water has countered that when a costbenefi analysis is carried out, increasing the rate at which leaks are found and fixed does not compare well with building the new desalination plant. Nor does the reuse of grey water or recycling water from sewage works, which costs three to four times as much as desalination.

The rainwater harvesting can make useful short-term contributions to saving water, but is not much use during periods of prolonged drought - against which the desalination plant is needed to protect. And the blanket metering of households is diffi ult in London where over half of all properties are flats with shared supply pipes.

If drought strikes London, there is a potential supply shortfall of 236Ml/day. In 2004-5 the dry year demand for London was calculated at 2,255Ml/ day. Yet the water available for supply was only 2,077Ml/ day. Thames wants a margin of 58Ml/day for contingencies.

Statistics show there is a one in 20 year probability of demand exceeding supply.

'If a similar drought to 1944 or 1976 had occurred this summer, we would have been in a Level 4 situation [standpipes] by July, ' warns Thames environment director Richard Aylard.

This would involve extensive periods of rota cuts for two to three months, he says.

A drought like that of 1934 or 1921 could cause a total failure of the supply system, all but emptying London's reservoirs. Rota cuts would have to be imposed for four to five months.

Livingstone says that standpipes would be an 'acceptable inconvenience'. Thames Water's London Resource Zone contains 6.2M customers (see map). But large increases in population have resulted in over 100Ml/day of additional demand in the past decade. London's population in 2004/5 was 8% higher than forecast when Thames drew up its business plan in 1999. London's population is expected to increase by a further 749,000 by 2016.

Demand for treated water will increase by 300Ml/day over 30 years, Thames says, or by 20l per person per day. If demand is unchecked, within 30 years the supply shortfall will be 700Ml/ day.

Livingstone says that the desalination plant will be energy guzzling and spew out greenhouse gases. Thames says that its reverse osmosis plant - a process whereby salt particles are ltered out of the water as it is pumped through the microscopic pores of a specially developed membrane - will use only twice as much as a conventional water treatment plant.

Lower requirements for the plant will be 2.7kWh/m 3, roughly 15% of that used by the thermal distillation process widely used in the Middle East. Energy consumption would be kept to a minimum through a combination of an improved design and the operating regime.

The company plans to abstract water from the Thames during the last three hours of each ebb tide, when the water's salinity is lowest, at 11,000mg/l. Water would then be stored in a 175,000m 3 'buffer' tank, allowing the constant supply of treated water inbetween ebb tides.

Water would then be filtered and treated to remove all but salt, minimising the load placed upon the reverse osmosis stage.

Reverse osmosis is the most energy intensive stage of the water treatment process.

Thames plans to keep this to a minimum by using a linear system, with water passing through four membranes at high pressure. Filtration membranes will have an increasingly small pore size, with 85% of water passing through the membranes.

The remaining 15% of highly saline water will ow out the end of the tration system and be used to convey material filtered out in earlier treatment stages back into the Thames.

This is more energy efcient than single stage ltration as blockages are less likely.

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