Faulty sensor left sewage spill monitoring unavailable for 13 months

A key sensor which records pollution spills from a large treatment works in south Wales was replaced 13 months after it began showing consistently flat readings, indicating a potential fault.

Welsh Water said replacing the sensor at the into Swansea Bay treatment works hadn’t been straightforward and that there wasn’t evidence the issue had adversely affected bathing water quality.

Environment regulator Natural Resources Wales has recorded what it termed a “non-compliance” and is investigating the matter.

The Local Democracy Reporting Service contacted Welsh Water after hearing that spill data at Swansea Bay treatment works, off Fabian Way, had been unavailable from March 3, 2025, because of an issue with its storm tank sensor.

Welsh Water said the sensor measured and recorded the water level inside the storm tank. When the water level reached a predetermined threshold, it said, the system identified this as the point at which a spill began. Each time this occurred it’s logged and reported as a spill event.

Welsh Water said the sensor had shown a consistently flat reading since March 3 last year and that it didn’t believe it had been working after that point.

Furthermore it said the issue had been flagged previously “but a process issue meant it was not fully investigated or resolved at that time”.

Asked if it knew how many many spills there were after March 3, 2025, and how long they lasted for, it said: “Without the accurate storm tank level data, we can’t definitively know when and how long for the site was spilling during this period.”

It said a temporary fix was put in place and that the installation of a replacement sensor, which has back-up technology unlike the original one, was completed on April 28, 2026. That’s more than 13 months after the consistently flat readings began.

The Local Democracy Reporting Service then asked Welsh Water when the temporary fix had been put in place. It said April 15, 2026.

Welsh Water said it notified NRW about the issue on May 1, 2026.

Welsh Water went on to say that throughout this whole period there was a second piece of equipment which monitored levels within the storm tank but didn’t record actual spill events. However, it said this equipment hadn’t been been working due to cable issues but had now been brought back into service.

Welsh Water said replacing the main spill-recording sensor hadn’t been a like-for-like job. This meant extra time was needed to get stakeholder input and approval, ensure there was minimal disruption when work began, and schedule the job “when all necessary personnel and resources were available”.

There’s a defined industry process for recording discharges from assets like the one off Fabian Way. A total of 93 discharges were recorded at Swansea Bay treatment works in 2024 – the year before the flat readings began – and 19 the year before that.

A Welsh Water spokesman said it understood the importance to customers of high-quality bathing water in the area and that it was working hard to invest in infrastructure.

“Maintaining and replacing the monitor on this storm water tank isn’t a straightforward task,” he said. “The monitoring equipment is located at a position where breathing apparatus is needed to access it, and complex engineering is required to maintain or replace the sensor.”

Referring to regulations around spill-detecting sensors which he said allowed some flexibility, he said: “The fault on the monitor on the Swansea Bay storm tank doesn’t mean that we weren’t complying with our regulations during this period.”

He said Welsh Water had “dozens of monitored assets” around Gower and information on their operation was shared with other organisations.

He added: “There is no evidence that the temporary loss of the monitor adversely affected bathing water quality during this period as NRW comprehensively sample in Gower to protect water quality in the bay.”

Environment academics at the University of Wales Trinity Saint David (UWTSD), when asked about the storm tank sensor issue, said operators of wastewater treatment works in general could still notice spills if sensors weren’t working by changes in the “raw water” such as unnatural colours and excessive foam, by unusual flow changes, and via manual laboratory sampling although this wasn’t immediate.

They said, however, that relying on such methods without automated sensors significantly increased the risk of an undetected discharge. “In general no amount of automation would prevent spills if the physical capacity of the plant is limited,” they said in an email to the Local Democracy Reporting Service.

NRW confirmed it was made aware of the storm tank issue on May 1. It said under the terms of the Swansea Bay treatment works permit Welsh Water had to ensure its spill-recording equipment was operational and that any failures should be fixed as soon as reasonably practicable.

It added that Welsh Water didn’t have to notify NRW just because spill recording equipment was faulty unless it was causing or may cause significant pollution.

NRW said it had recorded “a non-compliance” in relation to the Swansea Bay treatment works issue and has asked the utility for details implemented to avoid any recurrence of the sensor problem.

“We are currently investigating therefore are unable to provide further detail at this time,” it said.