Anaerobic digestion plant smell is a very highly debated subject, and whenever most new AD (biogas) plants are proposed (and a planning application is submitted) it is a topic of great concern to every local resident. With that in mind, I expected to see any number of articles on this topic when I looked on the web. So, I looked, and I didn't find them.
To my surprise I found that there are numerous website pages written by AD Plant objectors about specific planning applications, and by journalists reporting on what those same AD plant objectors were saying for local papers. But, nowhere did I see an attempt at presenting a rational view on this vexing question. It is an important subject, so I thought that I would write this piece in an attempt to present a "balanced" view on whether anaerobic digestion plants really do smell.
First of all. Let's be perfectly honest, these plants handle organic materials and at times these will already be starting to decompose as soon as they reach the AD Plant site. Once on-site the anaerobic digestion process itself is inherently smelly. Nobody could truly say that there is not a potential odour problem for all biogas plants. Decomposition (rotting) of organic matter produces some of the most offensive odours known to man, and decomposition is what the anaerobic digestion process is all about.
So, anaerobic digestion smells? Well to put a finer point to it, the materials which are fed into a biogas reactor can smell unpleasantly, and the output which is known as "digestate" (simply meaning the liquid and fibre which is left-over after the biogas gas-making reaction has occurred) invariably has a nasty odour, when it is first exposed to the air.
But, that categorically does not mean that an anaerobic digestion PLANT will smell, nor that anaerobic digestion plants are smelly. They can and should be operated responsibly, and with adequate design provisions for covered and air-sealed spaces, where the odour producing activities will take place. The great majority produce less odour than an average Dairy or intensive Chicken rearing farm.
As in all walks of life, some people do fail to run their biogas facilities in a responsible manner, and there are regulations against causing odour nuisance which will always need to be policed alongside other environmental protection legislation. Commercial scale AD Plants in the UK, and in most other jurisdictions too, are subject to permitting requirements, and if these are not met, bodies like the UK Environment Agency have powers to close down the AD facility if an odour-nuisance persists.
Odour that is produced by the AD process, can and must be contained and the ventilated air is filtered to remove any odour, before it is blown out through the ventilation system. The technology is routinely available for this, and when correctly applied, the view of many people is that anaerobic digestion plants don't smell. In truth, they routinely create less odour than the farmyard next door.
For any resident who is concerned about a new AD plant planning application being approved, due to their concerns about smells, we would suggest that they conduct some fairly simple research. Find out where there are already biogas plants in your area, and make a visit. Go, get out of your car for a minute, draw a deep breath, and stand on the public roadside, and draw in the air, and smell for yourself.
In my area there are already 4 anaerobic digestion plants within a mile and a half, of where I live and they are all two or more years old, but I have never smelled them outside of the property where each is located. When I have been detected a suspicious odour I have found that the offending odour was in fact the result of general farming activities and not the AD plant.