Engineers were fighting to stem the flow of millions of gallons of untreated sewage into the Firth of Forth near Edinburgh last night as a health alert was issued for ten miles of coastline.

A pump failure on Friday triggered an emergency overspill from a treatment works and it is estimated that as many as 22 million gallons (100 million litres) of sewage have been discharged from a pipe into the seabed near the Seafield waste-water treatment plant. At one stage 1,000 litres of raw sewage from up to 800,000 homes in the Scottish capital were being pumped out every second.

Although Thames Water, which operates the Seafield plant at Leith on behalf of Scottish Water, said yesterday that the sewage had been “screened” for solids, there were reports of human waste being washed up. Environmental health officials gave warning last night that it was likely to be several days before the waste had dispersed.

Bright yellow posters were erected at beaches and promenades from South Queensferry to the west of Edinburgh to Portobello in the east, Edinburgh’s seaside resort. They warned the public not to take anything from the shore, to avoid picking up or eating any shellfish or fish and to make sure they washed their hands thoroughly after leaving the area.

Jack McConnell, Scotland’s First Minister, called for an “immediate investigation”.

Peter Farrer, general manager for Scottish Water, said that engineers would work through the night to stem the flow. A spokesman for the company said last night that a series of smaller replacement pumps were now “80 per cent operational”.

Gordon Greenhill, Head of Community Safety at Edinburgh City Council, said that the spill raised public health concerns but would not be a long-term environmental problem. He said: “Any raw sewage has human pathogens in it, which have the capacity to make people ill. Raw sewage obviously has E.coli and the implications for the young and the elderly are always there.”

A spokeswoman for Thames Water, which runs the Seafield facility under a Private Finance Initiative scheme, said: “We have a pumping station at Albert Road in Leith, and what normally happens is that sewage that comes underground is pumped from that station up to Seafield treatment works. But because one of the pumps failed we couldn’t get the sewage up to the treatment works and it had to be diverted into the Firth of Forth. If it wasn’t for the emergency outlay it would have started flooding streets and internal property.”

Rob Kirkwood, chairman of Leith Links Residents’ Association, described equipment at the plant as “third world” and said that residents had been complaining about the odour for years.

Thames Water said that the pump involved was installed in the 1970s but emphasised that it had not experienced problems with the equipment at any of its plants in England.

A spokesman for the Scottish Environment Protection Agency confirmed that it had begun an investigation. In theory, it can take action from withdrawing licences to recommending prosecution for negligence or knowingly causing damage to the environment.

The incident was dragged last night into the campaign for the Scottish parliamentary election on May 3. While Mr McConnell seized upon it to show leadership qualities, the SNP said that it raised “serious questions” over the viability of PFI schemes. Murdo Fraser, the Conservatives’ deputy leader in Scotland, used it to high-light his party’s election pledge to remove Scottish Water from state control. “Privately owned water companies in England and Wales offer better quality at a cheaper price,” he said.

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