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The Avonmouth explosion was horrific. But it wasn’t unprecedented.

The business of recycling sewage into fertiliser, and the wider wastewater treatment industry, have been chequered by explosions and industrial accidents

Image: Tom Wren/SWNS

The date of 3 December 2020 will be seared into Bristolians’ minds as a day of tragic loss from an industrial accident – on a scale rarely seen in the UK in modern times. 

Yet the huge blast at a Wessex Water waste treatment plant in Avonmouth followed a number of explosions, spillages and major incidents at similar facilities operated by other companies, in recent years. 

The dirty business of recycling waste, which inevitably entails producing and storing flammable gases, doesn’t come without risk. Plants are designed to prevent explosions, over-pressurisation, asphyxiation and pollution, but accidents do happen. At some sites, storage tanks with steel and concrete roofs have been known to rupture and even blast off, in scenes similar to that in Avonmouth.

With the results of an investigation into the disaster, led by Avon and Somerset Police (ASP) and the Health and Safety Executive (HSE), still pending, the Cable spoke with experts who expressed concern about safety standards at some waste treatment plants. 

Treated sludge

At a press conference on the day of the explosion, ASP’s Chief Inspector Mark Runacres said it had occurred in a silo holding treated biosolids – before being recycled as agricultural fertiliser.

We know Wessex Water treats sewage through a range of methods, including within anaerobic digesters (ADs) – systems that can break down a range of organic matter, such as animal or food waste, in the absence of oxygen to produce biosolids. The resulting ‘treated sludge’ is transferred to storage silos before being sold to farmers. Biogas is produced nearby through anaerobic waste digestion by Wessex Water subsidiary, GENeco. 

Tony Ennis, technical director at Haztech Consultants, is a safety specialist and expert in fires and explosions. While not specifically referring to the Avonmouth plant, he told the Cable some UK AD systems gave cause for concern.  

“I have come across several of these systems and have had concerns over the engineering standards and compliance with DSEAR [Dangerous Substances & Explosive Atmospheres Regulations] in some of the designs, and also a lack of understanding of the flammability and explosion issues around methane gas,” Ennis said.

Luke Wheaton, Ray White, Brian Vickery and Mike James (Image: Avon and Somerset Police)

The Avonmouth explosion occurred in a silo. “[But] some methane will continue to be given off in storage, even after the digestion process,” Ennis added. 

Another possibility is that the material inside the silo was limed as part of the stabilisation process of waste treatment, according to an industry expert who asked to remain anonymous. This process could again emit gases, which without sufficient ventilation, could result in an explosive atmosphere. 

A spokesperson for silo protection manufacturer, Hycontrol, which specialises in preventing over-pressurisation in silos from pneumatic deliveries, told the Cable that pressure build-up in limed silos can be a hazardous issue. The HSE has also previously reported on hazards in storage silos, advising that “good ventilation can prevent accumulations of methane in sludge storage tanks”.

‘Hot work’ theory

One source with insight into the investigation told the Cable there had been a fault with a conveyor belt connected to the silo, which was being fixed by the contractor and workers who were killed in the explosion. 

While this has not been verified, in a previous press interview, safety specialist Tony Ennis suggested ‘hot work’, for example drilling, could have ignited methane gas in the top of the tank. 

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ASP and the HSE have so far refused to comment on what triggered the blast. The authorities have also declined to provide details on the investigation’s timeline or terms of reference. 

It is, though, expected to examine all documents associated with the plant design and operation, through to what happened in the lead-up to the fatal explosion and its causes. This may take many months if not years, based on inquiries following similar incidents. 

“[A] close retrospective examination of the risk assessment at the plant will be a good start in future prevention,” said Clifford Jones, a combustion expert and visiting professor at the University of Chester. 

The hazards of turning excrement into fertiliser and energy

In 2014, a processing tower at an AD plant at Harper Adams University in Shropshire collapsed, spilling toxic slurry across the neighbouring countryside. Luckily, the collapse happened in the early hours and no one was hurt. But in 2017, when a container exploded at ADD firm Bio Dynamic in Nottingham, two workers were less fortunate. Robert Tyrko, who had been carrying out welding work within the confined space of a pasteuriser tank connected to the AD, lost his leg while another man was injured. 

In July 2020, the HSE acted before an accident could occur, serving an enforcement notice to Sinclair Agricultural and Recycling Services (SARS) in Aberdeenshire, after the agency found the AD company had failed to protect staff from a potential explosion. The Environment Agency, meanwhile, has found toxic spills at AD plants and associated sites have caused numerous ‘serious pollution incidents’ over the last decade, with accidents also occurring in connected storage tanks. 

There is also a history of explosions at US waste plants specialising in fertiliser and biogas production. Back in 1987, two workers were killed when a sewage digester exploded, lifting a 30-ton floating cover into the air. And in 2018, a methane buildup and protocol breaches caused separate explosions at wastewater plants in Kansas and Chicago, the latter injuring 10 workers.

Image: Tom Wren/SWNS

“Anaerobic digester fires and explosion[s] are […] only reported if people are killed,” US-based AD safety consultant Glenn Smeaton said of the situation across the Atlantic. “If no one is injured or killed, they clean up the mess as quickly as possible.” 

As to why blasts and spills occur, Smeaton added: “I suspect there are already safety procedures in effect, that if carefully followed would have prevented explosive conditions to exist before starting work.

“Managers and supervisors should be constantly aware of the hazards of workers working to get the job done rather than getting it done safely,” he said.

A shocking reminder

Over the last two decades, ADs have become an established feature of the waste treatment and renewable energy sectors, with 600 plants now operating or in development in the UK. The biogas and biosolids industries have been propelled by the scramble to cap emissions, recycle waste into valuable fertiliser, and tap into profitable renewable energy alternatives. 

The events of 3 December 2020, were a shocking reminder of the dangers that chequer the wastewater treatment industry. Sixteen-year old apprentice Luke Wheaton, and three workers – Ray White, Brian Vickery, and Mike James – lost their lives, and a fifth man escaped with injuries.

In a moving Facebook tribute to Wheaton, the boy’s father wrote: “You gave me and mum 16 great years it should have been many many more, a big hole has been put into our hearts and will never ever be filled.”

A Wessex Water spokesperson described the disaster as a “tragic event”, pointing to the company’s “excellent” safety record and adding that it is working closely with the police and Health and Safety Executive to investigate what happened.

A spokesperson for trade association, the Anaerobic Digestion and Bioresources Association (ADBA) said: “Recent incidents, which might not be directly related to the AD process, have nevertheless strengthened ADBA’s resolve to ensure that the whole AD industry, which has the potential to decarbonise 6% of the UK economy by 2030 and therefore play a critical part in addressing climate change and our food and energy security, operates to high standards of safety and environmental protection.”

The spokesperson added that in recent months there had been a “significant increase” in interest around its Anaerobic Digester Certification Scheme (ADCS), which assesses the safety, environmental and operational performance of AD plants

“We are working hard to ensure that by gaining ADCS accreditation, [operators] commit to raising standards in every aspect of their operation to protect their employees, the environment and the community,” they said.

We want to hear from you. Do you have any information about the investigation, working conditions and incident on 3 December 2020 at Wessex Water? Contact journalist Alon Aviram in confidence:

Clarification: This article was amended to clarify that Hycontrol specialises in preventing silo over-pressurisation from pneumatic deliveries. [18.02.2021]

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