The Scottish Environment Protection Agency is investigating the leak at the treatment plant which lasted more than two days.
The failure occurred in one of four sets of pumps feeding sewage into the works in Leith. Seafield is Edinburgh’s main sewage treatment plant and is maintained by the Thames Water-led Stirling Water Consortium on behalf of Scottish Water. Property developer MJ Gleeson and consultants MWH also have a stake in the consortium.
Scottish Water has launched an independent inquiry to ascertain why the lead pump failed, but it is thought to have suffered a catastrophic mechanical failure. Its back-up was also out of action, awaiting the delivery of new parts. Both pumps are of an Archimedes screw-style and highly reliable, said a spokeswoman for Scottish Water.
When the lead pump failed on Friday 19 April, Stirling Water was forced to divert sewage into the Forth to prevent sewers in the city backing up and flooding. The discharge was made from another pumping station on the perimeter of the works, operated by Scottish Water.
Engineers installed six temporary pumps over the weekend and the leak was finally stemmed on Monday morning. The repair was hampered by health and safety concerns which prevented the pumps being run while engineers were in the pump well.
Almost 100 million tonnes of sewage are thought to have been discharged. The council warned visitors to avoid contact with sea water at local beaches and the Food Standards Agency warned against eating fish or shellfish.
However, SEPA thinks the spill is unlikely to have caused lasting environmental damage because it was diluted by the estuary. Solid matter had also been screened out. Warning signs were removed from beaches on Thursday 26 April after sampling showed the water met the guideline standards set by the EU bathing water Directive, but fishing and gathering shellfish were still thought to be inadvisable.
Seafield’s discharge consent permits it to release screened sewage during storms. Deviations from best practice are also allowed following "unavoidable mechanical or electrical breakdown".
SEPA is investigating whether the consent has been breached but cannot comment further at this stage. The Scottish Executive has also asked Scottish Water to report back on its findings.
The broken pump dates from the 1970s but an independent review, part of the consortium’s due diligence before it took over the plant in 1999, judged it fit for purpose, and it was refurbished six months ago. A spokesman for Thames said the consortium has spent £100 million upgrading Seafield and the other plants since it won the 30-year private finance initiative contract. The terms of Seafield’s discharge consent have also been tightened since 2000.
"The residents of Edinburgh still think the [sewage] infrastructure is old and creaking; the reality is very different from that," said the spokesman.
But an Ofwat report published in April shows Scotland’s sewage infrastructure still lagging behind that of England and Wales.1 Despite improvements, samples from Scottish sewage works failed water quality tests ten times more often than English and Welsh plants in 2004. Bathing water breaches were also more common in Scotland with 7% of samples failing compared with 2%.