Avonmouth explosion 2020: what happened at Wessex Water treatment works in Bristol - and what are biosolids?

The silo blast killed four people and injured a fifth

Friday, 4th December 2020, 12:40 pm
Updated Friday, 4th December 2020, 12:44 pm

Police continue to investigate the cause of a large explosion at a waste water treatment works which left four people dead.

Officers declared a major incident after fire crews were called to Wessex Water, in Avonmouth, near Bristol, at 11.20am on Thursday 4 December.

The blast happened in a large chemical tank, known as a silo, that contained treated biosolids at the firm’s water recycling centre.

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The scene in Avonmouth, Bristol, as fire crews, police and paramedics are responding to a large explosion at a warehouse where there have been multiple casualties. (pic: PA)

A fifth person was also hurt in the explosion but their injuries were not believed to be life-threatening. The families have been informed.

What has Wessex Water said?

Wessex Water chief executive Colin Skellett said the company's "thoughts are with the families, friends and colleagues of those involved".

He said: "[We] are absolutely devastated that the tragic incident at our site earlier today (Thursday 3 December] has resulted in four fatalities."

The company is working with the relevant authorities to understand what happened, he added.

What caused the explosion?

Chief Inspector Mark Runacres, from Avon and Somerset Police, said it was a “tragic incident” and the force’s “thoughts and sympathies” go out to the families.

He said: “A thorough investigation will be carried out into what happened, involving a number of agencies and the Health and Safety Executive in due course and we will not be speculating on the cause of the investigation while such work is being undertaken.”

The incident is not being treated as terror-related and there are not thought to be any ongoing public safety concerns, Ch Insp Runacres confirmed.

And added: "I can reassure people living in the nearby area that there is not believed to be any ongoing public safety concerns."

Yet people have been urged to avoid the area until further notice.

What are biosolids?

Biosolids, a by-product of the sewage treatment process, are otherwise referred to as treated sludge which are recycled to land as an organic soil conditioner.

The sludge is treated in oxygen-free tanks to produce agricultural fertiliser and renewable energy, according to Wessex Water.

They are seen as a cheaper alternative to bagged fertiliser.

Are biosolids dangerous?

Biosolids can produce a flammable methane gas when treated with bacteria.

Yet the cause of the explosion is still under investigation and police could not comment on whether this was a factor.

Professor Clifford Jones, visiting professor at the University of Chester, said: “The substance responsible for the explosion might well have been methane which, as is well known, is formed in sewage treatment. Sewage at an advanced state of treatment can form sewage sludge dust, which is capable of a dust explosion.”

Professor Jones has more than 45 years’ experience specialising in fuels and combustion and cites coal science and technology, bush fires and hydrocarbon process safety as topics of major interest.

Why was there an explosion in Avonmouth in 1996?

In 1996, Avonmouth was the setting of another large explosion when a tanker containing what was believed to be epichlorohydrin began to offload at the Albright & Wilson site.

Both the tanker and the storage tank were destroyed after a series of blasts sparked a blaze.

"It was later discovered that the road tanker in fact contained sodium chlorite, which reacts explosively with epichlorohydrin,” a report from the Health and Safety Executive concluded.

“The fire generated a 100m black plume of smoke containing hydrogen chloride, which drifted across the Severn estuary closing local motorways and rail services."