Yorkshire Water has been prosecuted after a storage tank leaked sewage into a watercourse in North Yorkshire.

The company admitted one charge of causing a water discharge activity into Dales Beck, at Dalehouse, without an environmental permit, at Leeds Crown Court on Thursday, 13 July. It was fined £600,000 with more than £28,078 in costs.

The pollution originated from the firm's Hinderwell Waste Water Treatment Works in July 2015. It had leaked from a poorly-maintained, rusting sludge storage tank, which had holes in it.

Initially, Yorkshire Water blamed works at the nearby Potash Mine for the watercourse being discoloured, but four days after the spill they used dye testing to trace the source back to the treatment works.

Richard Bradley, prosecuting for the Environment Agency, told the court that the sewage had a significant impact on the quality of the water in the beck. The watercourse, which passes through a caravan park, was discoloured and had a foul odour. Up to 100 dead fish were seen, and water quality tests revealed high levels of ammonia and low dissolved oxygen levels.

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Downstream, the pollution incident affected water quality in Staithes Harbour, which was a designated bathing water at the time. An 'abnormal situation' was declared for 17 days during the peak holiday season and Scarborough Council erected signs advising the public not to enter the water.

Sewage sludge could clearly be seen in the yard near the sludge storage tank and from there it had entered Dales Beck via a small ditch below the Sewage Works. Yorkshire Water did not immediately clean up the yard because it believed the sludge was contained on the site and would be recirculated back into its treatment process. However, the firm's own site user manual stated that the site drainage is not sealed. This was not picked up by the company and the sewage was in fact discharging into the watercourse.

As well as causing the pollution, Yorkshire Water also failed to act quickly in cleaning it up. In August, the Environment Agency asked the firm to provide a plan for how it was going to clean up the mess, but no reply was received. After further visits and correspondence, the company was still cleaning sewage sludge from the water on 19 October, some three months after the incident.

In mitigation, Yorkshire Water told the court that it had invested in measures to prevent the incident recurring. A gully guard had been installed to contain any future spillages within the site, and an alarm had been installed on the sludge tank. The tank has now been replaced.

Claire Campbell, environmental planning specialist at the Environment Agency, said: "This case demonstrates how important it is that water companies and wider industries maintain their equipment and facilities to a high standard in order to protect the environment. This pollution incident had a significant impact on the ecology of the watercourse, as well as an impact on the amenity of a caravan park and Staithes beach."

(LM/MH)