Odours: Insufficiently Aerated Composting Can Cause Odours
By Anaerobic-Digestion.com
When it comes to public perception, the fact is that the public frequently object to both Anaerobic Digestion and Composting (Aerobic Digestion) on the basis that they are convinced that both processes will be smelly, and to a large extent this is understandable.
Even quite recently installed composting plants, and regrettably also some “current generation systems”, within the last two to three years (as of writing this in Autumn 2006), have suffered from some quite persistent odour problems, in the UK and Ireland.
It is also a fact that once a composting facility has experienced a short term odour problem, local residents do tend to become sensitised to the potential for these sites to cause bad smells. If that happens then the operator may find that other countryside odours", such as from agricultural sludge ("muck") spreading are forever afterwards wrongly attributed to their activities.
Concerned about odour need not, and really should not, hold back the development of either aerobic or anaerobic composting (anaerobic digestion) plant facilities. Odour prevention is not "rocket science", but design for odour prevention starts with the initial design, and does cost money and need investment, from the outset.
If necessary, and the site is very close to properties, or the incoming feedstock is inherently very odorous during offloading/transfer, composting can be carried inside buildings, and these buildings are then provided with an odour filter which will render an odour free discharge. The building envelope is then maintained at a negative internal air pressure and doors are kept closed, thus no odour need escape.
The origin of foul odours is known and can be readily dealt with, but we must stress the need for good design by experienced process designers and with odour experts also involved. If so, odour generation can be reliably reduced to acceptable levels and bad-neighbour odour problems entirely prevented, but only if the composting process is thoroughly understood by the experts carrying out the design.
Noticeably unpleasant odours are almost wholly the result of the presence of a lack of sufficient oxygen (air) and they are generated in anaerobic conditions, and are not produced where there is adequate airflow right throughout the mass of the compost pile.
How do we avoid odours for low odour (eg green waste composting) rather than spend yet more money on air-tight sheds?
The most important factor is the maintenance of an open structure in the compost by the careful selection, shredding and mixing of compost materials by the plant operator, and avoidance of anaerobic conditions by regular movement of the pile for windrowing. For in-vessel composting this is achieved by the provision of sufficiently powered airflow fans and air distribution systems..
Why are aerobic conditions important?
If aerobic conditions are not maintained, anaerobic (low oxygen) decomposition will take place, and foul odors will be generated. With appropriate management practices, odors can be greatly reduced and even eliminated, to the extent that zero complaints will be received from neighbours.
What exactly is aerobic composting?
Composting is a biological process that reduces organic matter to humus. When a plant or animal dies, the remains are decomposed by soil microorganisms and larger soil fauna and are eventually reduced to an earth like substance called humus. Humus forms a beneficial growing environment for plant roots. Aerobic composting refers to decomposition by microorganisms that require oxygen.
How are anaerobic odours generated?
Anaerobic odours can originate with the incoming feedstocks or bulking agents, which may have been stored without aeration for some time before transport to the composting site.
Once those feedstocks or bulking agents are incorporated into the composting system, subsequent odour problems are usually a result of anaerobic (low oxygen) conditions. These odors include a wide range of compounds, of which the most notorious are the reduced sulphur compounds (i.e. hydrogen sulfide, dimethyl sulfide, dimethyl disulfide, and methanethiol), volatile fatty acids, aromatic compounds, and amines.
Ammonia is the most common odor that can be formed anaerobically as well as aerobically, and is usually more noticeable on the composting site rather than off-site. This is due in part because it is lighter than air and rapidly rises up into the atmosphere. Noticeable ammonia losses are primarily a result of low carbon to nitrogen (C:N) ratio. However, pH is also a contributing factor. If the pH is around 9, there is a reasonable equilibrium. A higher pH forces more ammonium into the gas form which you can smell.
So won't this mean that when we design to carry out Anaerobic Digestion, this will be much more odorous?
No not at all. For anaerobic digestion plants the problem is no better or worse than for the equivalent aerobic composting plant - and will be related to the smell potential of the feedstock. Once the feedstock enters the anaerobic plant digestion vessels these are sealed and fully enclosed, so the anaerobic digestion process itself is actually less, not more likely to ever become an odour nuisance.
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