03 December 2020

Avonmouth explosion: What are biosolids and are they dangerous?

03 December 2020

Police have said the explosion at Wessex Water’s Bristol water recycling centre in Avonmouth happened in a silo used to treat biosolids, though the exact cause of the blast is still to be determined.

– What are biosolids?

Biosolids are “treated sludge” – a by-product of the sewage treatment process.

According to Wessex Water, the sludge is treated in anaerobic digesters – oxygen-free tanks – to produce agricultural fertiliser and renewable energy.

– So how does the sewage treatment process work?

Wessex Water says during the sewage treatment process debris such as rags and large objects are removed first 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 by passing through filters with bacteria growing on them that feed off the waste and clean the water.

The scene of the explosion (PA Wire)

– Why do we use biosolids?

According to Bristol-based waste management service GENeco, biosolids provide a “cost-effective alternative to bagged fertiliser” and help improve the fertility of agricultural land.

Neil Sims, biosolid recycling controller, said: “Increasing the organic matter helps improve soil structure, giving plants better roots and helping them to yield more.”

Sean Hill, director of waste management, adds that recycling sewage sludge helps supply essential nutrients back to the soil and provides “a successful blueprint for a sustainable future” for the planet.

– Are biosolids dangerous?

POLICE Avonmouth (PA Graphics)

Biosolids can produce flammable methane gas when treated with bacteria, though police could not comment as to whether this was the cause of the explosion and said the investigation was ongoing.

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.”

– Will there be any fallout from the explosion in Avonmouth?

Luke Gazzard, from Avon Fire and Rescue Service, said there was not thought to be any further safety concerns to people living in the nearby area following the incident.

There will be an investigation into the blast involving the Health and Safety Executive and a number of agencies.

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