Business Green: Thames Water yesterday cut the ribbon on the UK’s first mainland desalination plant, promising that it would provide much-needed backup supply for the " seriously water-stressed" capital in the event of drought conditions. Powered by biodiesel from used cooking oil, the £270m Thames Gateway Water Treatment Works in east London will be able to turn a mixture of seawater and river water from the Thames into drinking water. Thames expects the Gateway to produce 150 million litres …