Southern Water has published the findings of its internal investigation carried out into the cause of the loss of supply that occurred from Otterbourne Surface Water Treatment Works in February 2023.

The loss of supply occurred from the Otterbourne works following a water quality shutdown. It was triggered automatically when water quality (turbidity) levels fell below acceptable standards for water supply.

Following repairs to a filtration tank at the works, a valve was not properly closed, which resulted in water of insufficient quality passing into a second tank.

Anglian Water said the error was detected successfully, the site immediately shut down, without any of the affected water entering public supply. Customers were affected between 15 and 19 February 2023.

The error was caused by water from one of the work’s filters (Rapid Gravity Filters or RGFs) passing into the water supply (contact) tank through an outlet valve that should have been closed. This water is normally held in a separate tank. Turbidity monitors immediately detected a change in water quality and this triggered an automatic shutdown.

The company initially expected around 23,000 homes in parts of Winchester and Southampton would be impacted between the 15th and 19th February 2023. However, through a combination of rezoning and tankering treated water into the network, this was reduced to 15,000. Further supplies were brought in late on the 16th reducing the number of homes impacted still further. The remaining 2,200 were out of supply until the 19th.

Valve controlling flow of water between the RGF filter and contact tank had been operated incorrectly

The investigation has found that the valve controlling the flow of water between the RGF filter and the contact tank had been operated incorrectly, not in compliance with procedures.

The site has a fail-safe mode which automatically shuts the plant down if water quality fails to meet strict regulatory standard designed to ensure water quality - the systems trigger well in advance of a quality failure and have a safety factor of more than 30%.

Between the 10th and 15th February 2023 Anglian Water undertook a range of maintenance tasks at the site. Throughout this period no automatic shutdowns were triggered, there were no sample failures from the proactive sampling regime, and trend data disclosed no operation outside of the very strict standards Anglian Water must meet.

However, the incorrect operation of the valve meant that during this final stage of testing, water containing sand and gravel from the filter tank passed into the contact tank. The presence of sand and gravel triggered the automatic shutdown, stopping any partially treated water entering the supply. Anglian Water’s failsafe modes operated as designed.

The report says the valve could not have been opened if it had been isolated correctly which should have happened when the filter was taken out of service for repairs in 2022.

The investigation has found a sequence of failings, some of which are operational, and some have to do with the “capability and availability” of the water company’s employees.

"February’s incident was a case of human error and there are important lessons to be learned"

Tim McMahon, Southern Water’s Water Director, said:

“We are very sorry to those who were impacted by this disruption. We have carried out a thorough internal review into what happened, in order to prevent it happening in future. It is important we are transparent with customers when we fall short of expectations.

“February’s incident was a case of human error and there are important lessons to be learned. The report has made a number of recommendations, and I’d like to reassure our Hampshire communities that we are already taking action. We are also investing around £350 million over eight years at Otterbourne to improve the resilience of our site.

“In recognition of the distress caused by this incident, particularly as it came only two months after a separate loss of water in the same area, we doubled our guaranteed standards of service payments for customers and provided a £50,000 donation to local food banks.”