Explosion at waterworks in Avonmouth, UK

...although the news pictures cannot distinguish between that and one of the digesters nearby.

Either way, it's unusual.

The tank structure must have been breached allowing oxygen to enter the tank and then some spark for ignition....

Those 4 guys must have all been working near or on the roof...

I have heard of cases where the level probe created sparks when the liquid level was below the probe due to static electricity and improperly grounded or corroded connection to ground.

If it is an activated sludge tank,it has compressed air or a mechanical mixer so air will always be present above the sludge,which combined with methane is a combustible atmosphere?

Was there any maintenance or repair work going on in the area? Drilling or Welding perhaps?

...except that’s a final treated sludge holding tank at the back end of the sludge train and a location not normally Ex-zone-classified. The fill is from a conveyor belt in free air.

Looking at the plant layout the sludge goes by gravity into open road vehicle trailers for dispersal into the wider environment.

Unlikely: score 10%.

It's also curious that according to one eye-witness, there was no smoke. That would indicate either a pure gas/oxygen explosion or a simple excessive gas pressure or tank defect.

If, as you said, and the photo verifies, it was open to the air and filled via conveyor, then my guess is that it was overfilled, blocking the exit for gasses sufficiently that pressure was able to build up.

"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."

This was a tank that was used for final anaerobic digestion of biosolids and for producing fuel, that would be methane....so no oxygen in anaerobic digester....I stand by my analysis, one of the workers was a contractor doing work on the tank...In biosolid treatment the sludge must be composted for use as fertilizer, there are at least two methods, anaerobic(without oxygen) and aerobic(with oxygen)....

Anaerobic vs aerobic biosolid treatments

https://www.bbc.com/news/uk-55191585

The location of the source of ignition is a curiosity.

Well if I can ease out on a limb for a moment, It seems to me the witness that said he/she heard a whoosh then an explosion, gives a clue.. I would say the workman had opened the tank for whatever reason, be it on purpose or by accident, and methane was escaping at some point...then some sort of ignition lit the methane now mixing with the air and existing around and above the tank in a considerable quantity of several hundred if not thousand cubic feet....the source of ignition could have been a portable power tool, or perhaps somebody smoking, or even a dropped tool....in any case the whoosh was the initial burning of the escaped gas/air mixture, and the interior of the tank was mixed with just the right combination of methane and air (one to three) to create an explosive mixture, and that was the explosion heard, and the source of the damage now seen to the tank...

Good possibilities.

In the photo,there is a rectangular hole,probably for inspection when the tank is empty.

Perhaps there was residual bio gas in the tank and it mixed with the air coming in from the inspection hole.

The maintenance crew may have lowered a light bulb into the hole for inspection.

If the bulb cracked,it may have sparked the first explosion as well as the second explosion.I am sure the crew dropped the light after the first explosion and it hit the bottom of the tank.

Bio fuel is heavier than air,so the top layer of fuel may have mixed with air first and just sat there,stagnant, until an ignition source set it off.The first small explosion(Whoosh) was followed by a vacuum,which sucked a lot of air into the bottom layer,causing turbulent mixing of the bio fuel with the incoming air,which caused the large explosion.

In addition to the incoming air,residual heat from the first explosion could have ignited the remaining fuel/air mixture,this combined with the pressure and ignition source from the first explosion could have had a Diesel effect on the remaining fuel due to compression heat causing a more powerful explosion.

Static discharge could also have been a source of the ignition,due to most digester tanks having a plastic liner.

If the tank was empty,it was not properly purged before inspection began.

https://www.britannica.com/technology/wastewater-treatment/Dewatering