Compost used for growing mushrooms is made from chicken manure, straw, gypsum, and water. At this mushroom composting facility, water (called "process water") was stored in a tank and then pumped to the composting barns through a series of pipes. A shed was built around the pumps and part of the pipes.
On the day of the incident, workers were in the shed trying to clear a blocked intake pipe. The supervisor was at the shed entrance directing two workers who were working on a valve about 8 feet below the doorway. When the pipe was disconnected at the valve, a small amount of liquid started to flow out. As one of the workers pulled straw from the valve, he noticed a strange smell. The worker took a step to leave and then collapsed. The second worker remained with the collapsed worker as the supervisor exited the pump shed to summon help.
Other workers from all three businesses on the site, unaware of the hazardous conditions in the shed, entered the shed to attempt rescue. However, when ambulance and fire crews arrived, the emergency personnel suspected a hazardous atmosphere in a confined space and prevented anyone else from entering the shed. The supervisor was found outside the pump shed, disoriented and in respiratory distress. Five workers were recovered from within the pump shed. Three were found dead and two suffered severe brain injuries as a result of the hazardous atmosphere inside the shed.
Findings as to causes
Findings as to underlying factors
- Hazardous gases were released when the blocked pipe was disconnected at the valve. Anaerobic conditions in the process water recycling system had resulted in the production of hazardous gases, including hydrogen sulphide and carbon dioxide.
- The release of gases from the piping system into the shed, a confined space, created a hazardous atmosphere consisting of hydrogen sulphide gas, oxygen deficiency, or a combination of the two.
- Straw and sludge blocked the intake pipe. The sytem did not prevent solids from entering the system and there was no maintenance program to manage the solids and sludge.
- Anaerobic conditions (without oxygen) developed in the process water recycling system when composting production was reduced. There were no features incorporated to keep the process water uniformly aerated, and as a result the process water became stagnant.
- There were missed opportunities that might have prevented the development of hazardous conditions that led to this incident.
- Minimal attention was paid to occupational health and safety at this workplace.
News release on release of investigation report.
Animation that re-enacts what happened: English, Cantonese, Vietnamese