December 4, 2020

expert reaction to explosion at waste water treatment works in Avonmouth

An explosion occurred yesterday at a waste water treatment works in Avonmouth, killing 4 people and injuring another.

Mr Tony Ennis, Director of HAZTECH Consultants Ltd, said:

“The explosion was in an ‘aerobic digester’ which is designed to break down organic waste materials and generates flammable methane gas during the process.

“Waste is generally fed in at the top of the reactor and methane drawn off at the top.  The spent waste can be used as soil improver in agriculture, and the methane is often used to drive a gas engine to generate electricity.

“Methane is flammable and explosive between 5 – 15% by volume in air.

“It is possible that personnel on the top of the top of the digester were carrying out hot work e.g. drilling or other work which caused a spark and ignition of the methane gas in the top of the digester.

“In normal operation, the gas composition in the digester should not be flammable, but if air had entered the system then this would create an explosible mixture.  The resulting explosion would blow the top off the digester at the weak point where the roof joins the shell.”

Prof Clifford Jones, Visiting Professor, University of Chester, said:

“When I first read of the incident and the tragic multiple fatalities the word ‘overpressure’ entered my mind immediately, and the word ‘blast’ has featured in several of the media statements so far.  Whether an explosion has an overpressure depends on conditions at ignition including fuel-air contacting.  One has to be extremely careful  in arguing from the particular to the general or vice versa.  It has often been noted however that when in a hydrocarbon explosion heat is the lethal factor there are comparable numbers of fatal and non-fatal injuries, whereas when pressure is the lethal factor the numbers of fatal and non-fatal injuries differ more widely.  Up to now we have been informed of four deaths and one non-fatal injury at the Avonmouth explosion.  I want to say no more than that that is consistent with there having been an overpressure.  Whether there was an overpressure can probably be ascertained from the damage profile at the accident scene.

“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.  The HSE have previously put out a document on this: https://www.hse.gov.uk/pubns/risks-from-sewage-sludge-drying-plants.pdf.”

Declared interests

Prof Clifford Jones: “I have no conflict of interests.”

None others received.