A water firm has pleaded guilty to health and safety charges after an explosion at one of its sites killed four workers, including a teenage apprentice.

Luke Wheaton, 16, Ray White, 57, Brian Vickery, 63, and Mike James, 64, died in the blast in a silo at Wessex Water's treatment site in Avonmouth, Bristol, in December 2020.

An Avon and Somerset Police investigation into manslaughter was dropped in 2024 but the Health and Safety Executive (HSE) undertook its own investigation to determine if criminal offences had been committed under health and safety laws.

At a hearing at Bristol Magistrates' Court earlier, Wessex Water pleaded guilty to three charges under the Health and Safety at Work Act.

The charges were failing, between 2013 and 2020, to ensure the health, safety and welfare of its employees, failing to ensure the health, safety and welfare of someone other than an employee and failing to comply with a health and safety notice issued in 2021.

The explosion took place in a silo containing biosolids – recycled organic matter used by farmers as agricultural fertiliser.

The four workers who died were on the roof of the silo when the explosion happened.

It is understood James was a contractor working at the site, while Vickery and White were employees of Wessex Water and Wheaton an apprentice at the firm.

A BBC investigation revealed safety reports had noted serious concerns about conditions in parts of the water treatment plant five years before the explosion.

The documents highlighted a "perfect explosive mixture", "serious risk to personnel" and a smell of gas so strong the authors claimed they could not stay in the area to complete their report.

Wessex Water said it had "robust process controls on site" and the matters identified by the reports had been resolved.

The company said the reports related to a different part of the site - not the part where the blast happened.

After the hearing, HM Principal Inspector from HSE Simon Chilcott said the blast "should never have occurred".

He said: "When people go to work, they expect to come home safely at the end of the day.

"Luke, Ray, Brian and Mike had no reasons to suspect that 3 December 2020 would be any different but - because of failings by Wessex Water - the unthinkable happened; a catastrophic explosion which claimed their lives and permanently altered many others."

He said the investigation by HSE, which was supported by Avon and Somerset Police, found that Wessex Water had made "clear, identifiable failures that could too reasonably have been prevented".

Chilcott said: "Wessex Water Services Limited have today pleaded guilty to three charges under the Health and Safety at Work Act.

"We recognise the importance of this acceptance of guilt by Wessex Water to everyone whose life was shattered by this devastating incident and we hope the result brings them a measure of solace."

In a statement, Wessex Water said: "We accept we fell short of the required standards expected of us at the time and have today pleaded guilty at the first opportunity.

"Since the tragedy, it has been our priority to implement any changes so that nothing like this ever happens again.

"However, in the case of Brian, Ray, Luke and Mike we know that this is too little, too late."

The firm said it had strengthened its safety performance since the explosion by establishing an independent Process Safety Board, creating new internal roles to bring specialist expertise in-house and introducing additional training for employees working at sites where dangerous substances were present.

Avonmouth has also been registered as a lower-tier Control of Major Accident Hazards Regulations (COMAH) establishment, subjecting it to stricter requirements and regulatory inspections.

Judge Lynn Matthews has referred the "complex and sensitive" case to Bristol Crown Court where the next hearing was due to take place on 8 January.

According to the Sentencing Council, Wessex Water could be given an unlimited fine for its failures leading up to the fatal blast.

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