The 875m litres produced so far is said to be so clean it has to be treated with salts and other chemicals to make it taste roughly the same as tap water.
"We've been running the desalination plant intermittently at one-sixth output – not because we've needed to but rather as part of the testing of the works and the training of its personnel. It's there if we need it," said Simon Evans of Thames Water.
The technology, which is mostly used in water-stressed areas of the Middle East, was brought to London after Thames Water argued in a 2006 public inquiry that new sources were needed, with climate change threatening hotter, drier summers and an additional 700,000 people predicted to move to the capital by 2021.
London, which has less annual rainfall than cities such as Athens and Sydney, is classed as "seriously water-stressed" by the Environment Agency, but critics of the Beckton plant – including former mayor of London Ken Livingstone – told the inquiry that desalination was energy-profligate, unnecessary and unsustainable.
Livingstone argued that the company could meet future needs by stopping its leaks and reducing demand. In addition, there were fears that chemical leaks from the desalination plant might pollute the Thames.
"London loses 26% of its total water supply through leakage," London assembly Green party member Darren Johnson told the Guardian. "This compares with Tokyo's 4% leakage rate. Reducing leakage by 1% provides enough water for almost 250,000 people, so fixing the mains must be the priority."
Thames Water said the plant would only be used during "prolonged" dry periods, when it can supply up to 1 million people. "We will only use it when we need it," said Evans.
The plant works by taking water from the river at low tide, filtering it and then pumping it through a membrane perforated by millions of minute holes. "This leaves the water so clean we have to remineralise it," said Evans.
Desalination has become the preferred method for water-stressed cities near to coasts to provide clean water for burgeoning populations. From just a handful of major plants in the 1960s, there are now 15,180 major desalination plants in 150 countries, supplying about 300 million people with daily water.
Most plants are in the Middle East, but the method is now becoming common in Spain and the US, as well as tourist resorts and water-stressed farming areas.
However, even a major desalination plant may not be enough for London. The mayor, Boris Johnson, last week argued that water should be channelled from wetter Wales and Scotland to secure the capital's water supply in the south and east of the country.