Boy 16, among those killed in wastewater treatment works explosion.

Health and safety executive (HSE) investigations are continuing into the cause of an explosion at a wastewater treatment works that killed four people.

Three Wessex Water employees and a contractor were killed in the blast in a silo that held treated biosolids at a water recycling centre in Kings Weston Lane, Avonmouth in December 2020.

Those who died were teenager Luke Wheaton, 16, Michael James, 64, Brian Vickery, 63, and Raymond White, 57.

Mr James was a contractor working at the site, while Brian Vickery and Raymond White were Wessex Water employees. Luke Wheaton was an apprentice at the firm.

Chief executive of Wessex Water, Colin Skellett, said the firm was "absolutely devastated" by what had happened. He said: “Our hearts go out to the family, friends and colleagues of those who lost their lives during this tragic event.

“I know from the thoughts and comments I have received from so many, that this has affected the whole Wessex Water family. We are determined to find out what happened and why and we will work with the relevant authorities to do just that."

GMB regional secretary, John Phillips,  said:  "This terrible tragedy is another stark reminder of the absolute need to ensure people are able to work in environments where risks to their health and safety are properly controlled.”

Unite regional secretary for the southwest, Steve Preddy, said three of its members working at the site were in a "state of shock".

He said: "One was on site and nearby and witnessed much of what took place, but he is safe and well. We have been in touch with our other members who are thankfully all safe and well, but obviously deeply disturbed and emotionally upset by what has happened."

MP for Bristol North West Darren Jones, said he said that he would call on the HSE, the Environment Agency and others "to ensure that lessons are learnt so that these situations, that shouldn't happen in first place, don't happen again."

One witness to the aftermath described hearing a "boom and echo" which shook the ground, followed by "a lot of commotion.”

Sean Nolan said: "It was quite short-lived, I'd say about two or three seconds. Sort of a boom and echo and then it just went quiet. That was it. There was no smoke, there was no after-effects of it."

Biosolids are "treated sludge" - a by-product of the sewage treatment process. Wessex Water says the sludge is treated in anaerobic digesters - oxygen-free tanks - to produce agricultural fertiliser and renewable energy.

According to Wessex Water, during the sewage treatment process debris is first removed using screens. The sewage flows into tanks where the solids sink to the bottom and are removed as sludge. The sewage is then treated biologically with bacteria that clean the water.

Biosolids can produce flammable methane gas when treated with bacteria, though it is not known if this was the cause of the explosion.