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There has been an increase in the amount of food waste and industrial by-products being used in anaerobic digester (AD) plants in NI.
At the CREST centre at South West Regional College in Enniskillen, researchers test AD plant inputs to find the type and quantity of biogas that can be produced from different materials.
“When we started here in 2015, we were inundated with different types of agricultural crops, but we’ve seen over the last 18 months that operators are turning towards industrial wastes from many different types of processes,” Shane McBrien from CREST said.
There is a wide range of materials that can be used in AD plants to produce biogas.
Methane is the main utilisable biogas which is then used to produce heat and electricity.
The use of food waste and industrial by-products is regulated by the NI Environment Agency.
Licences, as well as a requirement to treat or pasteurise digestate (AD plant end product), can be required for different types of input materials.
McBrien points out that grass silage is not the most economical product to use in AD plants, but it is the most straightforward from a regulatory perspective.
However, he said that more AD plant operators are looking at different types of waste products as supplementary inputs.
Environmental benefit
McBrien maintains that producing renewable energy from waste products, which could otherwise be going for landfill or direct spreading on agricultural land, is the main environmental benefit from the use of AD.
The AD process does not extract phosphorus, so there is little to be gained from a water quality perspective.
Also, nitrogen from AD plant digestate tends to be more volatile than raw cattle slurry or poultry litter, so it can have higher ammonia emissions when it is eventually spread.
“If an operator can get a waste product at a relatively cheap cost, it’s essential to know how beneficial that waste is.
At the CREST centre, we test it and quantify it for gas production, but also to see if it will upset the AD process,” McBrien said.
The product mid-way through the AD process can also be tested to see if the digester is at optimal performance.
If biogas production is too high, or too much hydrogen sulphide is produced, it can kill off microbes needed for the AD process.
Work is also on-going at CREST on the production of formulas which will be able to show biogas production potential from a standard forage analysis result.
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