Friday January 28 2022
Anaerobic digestion plants can be a great investment for farmers and other businesses that generate organic waste. Not only do they convert that waste into renewable energy, but they can also generate a return on the initial investment, enabling plant owners to make money over time.
However, many of the feedstocks used by AD plants consist of waste organic matter – often spoiled or out-of-date food, industrial by-products, wastewater, and even slurries and manures produced in livestock farming.
The process also creates a digestate sludge, which can be used as a fertiliser.
So, it is no surprise that some people have concerns about unpleasant odours being given off. Afterall, no matter how attractive an AD plant looks on paper, if bad smells ruin your quality of life, it’s never going to be a great investment.
Your neighbours won’t thank you either!
So, in this article we tackle the topic of offensive odours and how modern AD plants ensure the air surrounding them is kept clear!
There are many potential sources of odour in the production of biogas.
Given the nature of the feedstocks, it is unsurprising that some of them will give off unpleasant aromas, particularly slurries and manures. Food waste too, if particularly rancid, can give smell and wastewater is rarely odour-free. Therefore, an odour management strategy in relation to feedstocks is essential.
However, that is not the only aspect of the process that has the potential to produce nasty niffs.
The degradation of organic waste produces methane, a highly odorous gas, as well as Hydrogen Sulphide which smells like rotten eggs, ammonia, volatile fatty acids, and a range of other rather whiffy compounds.
Finally, the digestate sludge produced by anaerobic digestion also has an aroma, meaning odour control must form an integral part of the entire process, from start to finish.
To ensure bad smells are kept to a minimum, it is important to devise and implement an odour management strategy. Such a strategy should be considered at the design stage of the plant, by identifying all the potential sources of odours, how much of a risk they are, and who will be affected if bad smells are emitted.
Although there are many potential areas where smells could be emitted in the event of a plant failure, in a well-functioning biogas plant, the main areas of focus for an odour management strategy are:
As feedstocks don’t generally need to be transported under pressure, they are not usually contained in sealed vessels and as such, allow odours to escape.
To minimise this, it is therefore important to ensure all processes on a plant take place in enclosed buildings or containers, and these are often kept in negative air pressure states to draw in any odours rather than allowing them to escape. Air can then be ventilated via scrubbers or filters that remove the odorous particles.
Of course, odour management isn’t just a product of the design of the plant. The location of the plant is key too, with planning permission unlikely to be granted for large, commercial biogas plants near to residential areas.
Most plants processing food waste are situated on industrial sites away from large numbers of houses, and AD plants involved in the processing of wastewater tend to be on wastewater treatment sites, again, usually located away from housing.
Farm plants, on the other hand, are often smaller than industrial-scale plants and might be located closer to a village or other type of settlement. But even the smelliest feedstocks – manures and slurries – are common farm smells often released during other operations such as spreading slurry as fertiliser. Therefore, although all efforts should be made to ensure the odour is contained, most people living in the vicinity of working farms are more likely to accept occasional farm-based smells.
It is worth mentioning, however, the many feedstocks used on farm AD plants, such as purposely grown energy crops or straw, have minimal odour.
Unlike the management of feedstocks, the anaerobic digestion process takes place in a vacuum in a sealed tank. This is because the purpose of anaerobic digestion is to capture the biogas produced. It is also the main reason modern AD plants do not typically generate odours during day-to-day operations.
When air needs to be vented, such as when the plant needs cleaning or maintenance work needs to be carried out, it is done via filters that remove the odorous gases. Common ventilation systems include:
Scrubber towers are packed with plastic media through which water containing certain chemicals flows. Noxious air is drawn up through the tower and the chemicals in the water neutralise the ammonia, hydrogen sulphide and other odorous compounds.
The recirculated water then needs to be treated to remove absorbed salts, but this can be done without create further smells.
Activated carbon filters absorb ammonia, hydrogen sulphide and other noxious gases. However, they need changing regularly to prevent them becoming saturated.
Biofilters consist of aerobic bacteria and fungi that oxidise the ammonia and hydrogen sulphide into water and soluble, odourless compounds. They grow on wood chips or similar media which is kept moist to encourage the microorganisms to create a biofilm. The air is drawn up through the medium and the noxious compounds removed.
AD plants relatively close to residential housing might want to take extra precautions around odour management, such as installing atomised nozzles around areas of the plant most likely to emit smells.
These spray a water/chemical solution which is effective at neutralising odours, preventing them from escaping into the wider atmosphere.
Depending on the organic waste feedstock used, the digestate created can have an unpleasant odour. This is particularly the case if processing cattle or pig slurries.
This can be managed in two main ways.
The first is to put the digestate through an additional anaerobic digestion process to remove all the ammonia and hydrogen sulphide. This must take place in a sealed reactor, with the noxious gases filtered out via the processes outlined above.
If, after this, it is stored in a well aeriated container, it should generate little to no odour during storage or when sprayed on the fields.
Alternatively, dewatering the digestate will reduce odour emissions. This creates a more effective fertiliser as it concentrates the nutrients from the feedstock, and it reduces handling, transport, and storage costs of the product.
So, do modern AD plants smell?
The answer is for most of their operational life, they do not.
There is potential for the process to create unpleasant odours, but with an odour management strategy in place, which starts with the design and location of the plant and carries on throughout its entire working life, nasty niffs can be managed.
That said, there are times when plants might emit unpleasant smells due to unscheduled shutdowns, or during repairs, maintenance, or cleaning. But these occasions should be rare, and if handled correctly, short lived.
For advice on managing odours around your AD plant, or if you’re planning to build a plant and need an odour management strategy, get in touch.