Any operation dealing with organic wastes should be aware of the explosive hazards which can occur from handling apparently safe organic wastes such as green and food waste. Composting needs air to break down these materials aerobically and all operators know that if the process goes anaerobic that bad smells are caused.

 

But in addition to the smell some methane will be produced which doesn’t smell. Methane is the main constituent of natural gas which we all know is explosive.
In the open air the amounts of methane generated will be very small and will dissipate without problem – however in an enclosed space such as a sump or building it is possible to get an explosive atmosphere which if ignited would cause damage or injury.

In anaerobic digestion plants organic materials are starved of air (oxygen) in order to break down the materials anaerobically. This produces biogas consisting of methane and carbon dioxide which is highly odorous and toxic – and can generate an explosive atmosphere. The biogas is burned to produce energy but the process must be designed and operated safely to avoid explosions which could occur due to the presence of methane.

Dusts can also generate an explosive atmosphere in certain conditions, usually if the dust cloud cannot be seen through over a few meters. Wood, sugar, flour and some metal dusts can produce an explosive atmosphere.

Prevention of explosions is achieved by implementing the requirements of the UK regulations and HSE guidance on this subject. These regulations are known as DSEAR (Dangerous Substances and Explosive Atmospheres Regulations). ATEX is a term often used in explosion safety and comes from a French name ‘Atmospheres Explosive’ given to European Union Directives on the matter. These directives have been made into UK Law by DSEAR.

In very simple terms the requirements are as follows as there are basically two methods of preventing an explosion:
 avoidance of an ignition source and
 avoidance of an explosive atmosphere

Every organic waste operation should consider the potential for explosion occurring in areas of the site and the means required to prevent this. This should be recorded in an Explosion Prevention Document.

Any areas where an explosive atmosphere might exist in normal or abnormal circumstances should be considered. These areas should be zoned and identified by signage. Appropriate intrinsically safe equipment, particularly applicable to electrical items such as switches, motors and lighting must be specified for use these areas.

Further information can be found at http://www.hse.gov.uk/fireandexplosion/index.htm