There was an article in the Vancouver Sun yesterday that included the statement: “Composting facilities regularly produce hydrogen sulphide, ammonia, methane and carbon dioxide.” (http://www.vancouversun.com/Early+warning+signs+toxic+mushroom+gases+ignored/7050637/story.html)
It was a story that suggested that three deaths of workers in mushroom composting facilities in 2008 may have been prevented by a greater awareness and communication of the risk. When I read the story, I realized that neither the workers, the owners, the Ministry of Environment, nor Worksafe BC really understood at the time that the real risk at mushroom composting facilities was hydrogen sulphide.
Hydrogen sulphide is a gas associated with the rotten egg smell, which one would think would provide adequate warning, but unfortunately, the human nose can no longer smell hydrogen sulphide at concentrations great than 150 parts per million. Hydrogen sulphide is heavier than air, and can therefore accumulate in depressions, tanks or pits.
After this incident, I prepared a short report on understanding hydrogen sulphide in the mushroom composting industry (http://www.transformcompostsystems.com/articles/Hydrogen%20sulfide%20and%20Mushroom%20Production%20Sept%202008.pdf). Hydrogen sulphide is produced under anaerobic conditions by microbes that use sulphate as electron acceptors. Four conditions must be present for hydrogen sulphide to be produced:
1. Bacteria capable of reducing sulphate to sulphide
2. Anaerobic conditions – no oxygen present where the bacteria are active
3. Adequate carbon for the microbes to utilize as energy sources
4. Adequate sulphate for the microbes to reduce to sulphide
The mushroom composting industry potentially creates the perfect storm for hydrogen sulphide production and accumulation. There is a large amount of “goodie water” used to rewet the straw used in the mushroom compost preparation. This “goodie water” may contain sulfate from the gypsum (calcium sulphate) used in mushroom compost preparation, and adequate microbes and carbon from the poultry manure and straw. This “goodie water” is stored in tanks or pits, where hydrogen sulphide produced in the oxygen starved “water” can accumulate.
Do food waste and yardwaste compost facilities also produce hydrogen sulphide? Yes, the potential is there, but the risks are not nearly as high as with mushroom composting because calcium sulphate (gypsum) is not typically added during the composting process. However there are potential risks as outlined in an email sent to our Canadian composting association in Sept 2008:
Unfortunately, there were two deaths at a foodwaste composting facility in California in 2011. Even though the risks of hydrogen sulphide production and accumulation may be less with food waste composting facilities than with mushroom compost facilities, the risks are still there. As we increase our food waste diversion to composting, we have to be aware of these risks. When our composting process is sufficiently aerobic, the risks are minimal, and our composting facilities do not regularly produce hydrogen sulphide.
The risks are potentially greater with anaerobic digestion facilities, where the objective of the process is to exclude oxygen, thereby increasing the risk of producing hydrogen sulphide. Anaerobic digestion also includes liquid tanks, where hydrogen sulphide may accumulate.
When we know that workers have been hurt or killed, it takes the excitement out of separating our food waste from our trash. This all can be prevented by good management of our composting and anaerobic digestion facilities.