FIRE/EXPLOSION
Methane, which is makes up from 0% to 80% of biogas, forms explosive mixtures in air, the lower explosive limit being 5% methane and the upper limit 15% methane. Biogas mixtures containing more than 50 % methane are combustible, while lower percentages may support, or fuel, combustion. With this in mind no naked flames should be used in the vicinity of a digester and electrical equipment must be of suitable quality, normally "explosion proof". Other sources of sparks are any iron or steel tools or other items, power tools (particularly comutators and brushes), normal electrical switches, mobile phones and static electricity.
If conducting a flamability test take a small sample well away from the main digester, or incorporate a flame trap in the supply line, which must be of suitable length (minimum 20 m). View sketches of Flametrap1 and Flametrap2
As biogas displaces air it reduces the oxygen level, restricting respiration, so any digester area needs to be well ventilated to minimise the risks of fire/explosion and asphyxiation.
DISEASE
As Anaerobic Digestion relys on a mixed population of bacteria of largely unknown origin, but often including animal wastes, to carry out the waste treatment process care should be taken to avoid contact with the digester contents and to wash thoroughly after working around the digester (and particularly before eating or drinking). This also helps to minimise the spread odours which may accompany the digestion process. The digestion process does reduce the number of pathogenic (disease causing) bacteria, particularly at higher operating temperatures, but the biological nature of the process needs to be kept in mind.
ASPHYXIATION
Biogas consists mainly of CH4 and CO2, with low levels of H2S and other gases. Each of these components has its own problems, as well as displacing oxygen.
CH4 - lighter than air (will collect in roof spaces etc), explosive (see above).
CO2 - heavier than air (will collect in sumps etc), slightly elevated levels affect respiration rate, higher levels displace oxygen as well.
H2S - (rotten egg gas) destroys olefactory (smelling) tissues and lungs, becomes odourless as the level increases to dangerous and fatal. More details, an actual case and detection equipment are available.
Adequate ventilation, suitable precautions and adequate protective equipment will minimise the dangers associated with biogas, making it a good servant rather than a bad master.
SUMMARY
Like water, electricity, automobiles and most of life biogas is not completely safe, but by being aware of the dangers involved you are well on the way to a safe and happy digestion experience.
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