A major spillage of sewage sludge from one of Scottish Water's treatment works caused the public to be banned from a lowland loch, popular as a nature reserve, salmon fishery and water sports centre.

The Scottish Environment Protection Agency (Sepa) barred access to Castle Semple Loch in Lochwinnoch, Renfrewshire, in 2010 because of health risks from the sludge. Scottish Water pled guilty to two breaches of pollution law and was fined £20,000 at Paisley Sheriff Court in April 2014.

The loch was a very sensitive location, according to Sepa's investigating officer, John McKechnie. "By failing to ensure the sewage works were maintained and fully operational, Scottish Water created a very significant pollution event, caused a local amenity to be closed for over a week, and created a great deal of concern amongst members of the local community," he said.

Over a thousand fish were killed by spillages of chlorinated water from a reservoir into the Alva Burn near Alloa in 2013. Scottish Water pled guilty to the toxic pollution and was fined £6,500 in July 2014.

Sepa pointed out that chlorine levels in the burn were more than ten times the lethal limit for young trout and salmon. Earthworms, slugs and insects were also killed, it said, and fish stocks would take "several years" to naturally regenerate.

Sepa's investigating officer, Callum Waddell, accused Scottish Water of failing to treat water with neutralising chemicals before discharging it. "Staff failed to recognise environmental harm, or to mitigate pollution risks," he said.

Scottish Water was fined £12,000 in May 2014 after pleading guilty to polluting Purgatory Burn near Kirkintilloch with a series of sewage discharges in 2013. Sepa inspectors found a bad smell, discoloured water over 200 yards and "a dark plume of contaminated water" going into the River Kelvin.

"The detrimental impact on the Purgatory Burn and River Kelvin as a result of the sewage has been significant," said Sepa's Barry O'Connor. "Evidence gathered by Sepa's ecology team indicated that the watercourse had become grossly polluted."

Scottish Water was fined £6,000 at Stirling Sheriff Court in April 2014 for causing untreated sewage to contaminate the Dragon Burn, a tributary of the River Teith. Downstream from a sewer overflow, the burn ran grey amidst fungus, according to Sepa.

A leak of over 10,000 litres of highly concentrated sulphuric acid from a water treatment plant killed trout in the River Devon in Clackmannanshire in 2011. Scottish Water was fined £10,000 for this and another pollution incident at Alloa Sheriff Court in January 2014.