SCOTLAND'S water authority has been hit with a £20,000 fine after admitting serious failings over the pollution of an important habitat for birdlife.

Paisley Sheriff Court heard a sewage treatment tank that had been knocked out by an electrical fault was not repaired for six weeks.

Meanwhile, Scottish Water said its staff had incorrectly set a control valve.

Untreated sewage spilled into the RSPB reserve at Castle Semple Loch in Lochwinnoch, Renfrewshire. The loch, also popular with watersports enthusiasts, closed because of health risks after the effluent polluted the beauty spot in October 2010.

The area, sited within Clyde Muirshiel Regional Park, is a designated site of special scientific interest.

Kate Fleming, prosecuting, told the court an electrical fault knocked out one of two large tanks at sewage treatment works in Lochwinnoch.

She said the fault had been reported but nothing had been done to rectify the problem in the six weeks that followed.

Scaffolding had been required but none provided, and despite investigations no-one was able to explain the delay, she added.

She said the outcome was "foreseeable and predicted by Scottish Water staff", who concluded that, in the event of higher than normal flows into the works, the one remaining tank would not have been able to cope.

The main failure, Ms Fleming said, was an apparent inability to carry out a fairly simple electrical repair to the settlement tank which was "critical to the operation of the system".

Scottish Water, she said, had been "lacking in their response to the risk posed".

Ms Fleming said the site took months to recover after the sludge had been cleared. However, the court heard, there were no long-term effects on the eco system and the main impact had been on public amenities due to the concerns raised over health risks posed.

Steve Matthew, for Scottish Water, told the court the issue was caused by human error and infrastructure problems, rather than negligence.

He said the body had authority to discharge untreated sewage at the site on occasions where the inflow, particularly during exceptionally heavy downpours, was well above normal levels.

An overflow system involved screening of effluent through filters, and at the time, staff had set a control valve incorrectly. This resulted in the spills where untreated sewage engulfed surrounding areas, "smothering" plant life and starving the surrounding water of oxygen.

The quango pled guilty to two charges of causing unauthorised pollution and admitted failing to comply with licence conditions on best practice.

Fining the organisation, Sheriff David Pender said he found it "quite staggering" that it took 44 days for a simple repair.

Scottish Water, which apologised, has since spent £150,000 on improvements, and a similar sum has been set aside for further upgrading work.

A spokesman said: "Scottish Water takes its environmental responsibilities very seriously. We investigated this incident thoroughly and have carried out work to improve our operational equipment at the Lochwinnoch Waste Water Treatment Works to reduce the risk of such an incident happening again."

Craig Harris, head of the Crown Office and Procurator Fiscal Service Wildlife and Environmental Unit, said: "This incident caused a great deal of public concern and disruption. The situation that developed was entirely avoidable."