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, functionning with water levels 
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.
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