Concerns regarding the environmental sustainability of Irish agriculture have been voiced over the past three-to-four years, prompted by Ireland’s failure to meet EU emission targets and the Harvest 2020 strategy, which prompts expansion across all agricultural sectors.
While critics cite the very high contribution agriculture makes to national greenhouse gas (GHG) emissions (33.3% in 2014), the industry can point to the pasture nature of the majority of agricultural practise in Ireland being far less carbon-intensive than the feedlot systems of other countries. In any case, there is a recognised need to tackle the GHG emissions being generated from the agricultural sector.
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CLICK TO ENLARGE Fig 1: Municipal waste accepted for composting and anaerobic digestion in Ireland, 2007 to 2015-Source EPA 2016[/caption]
At the same time, the EU Landfill Directive has seen a major increase in the amount of source segregated food waste being collected in Ireland (25% since 2013, as illustrated by Fig 1). This waste is currently being treated primarily via composting (>80% of all separated organic waste), with a significant proportion even being exported to the UK for treatment (22% of all organic waste collected in 2015).
Anaerobic digestion (AD) is an alternative treatment method for such organic wastes. Anaerobic digestion can not only stabilise such wastes, but can generate renewable energy in the form of biogas and significantly reduce GHG emissions compared with composting. From a low base, the number of commercial AD facilities treating food waste in Ireland is increasing. As of March 2016, the Department of Agriculture, Food and the Marine has issued licences to eight biogas plants for the treatment of animal by-products (of which food waste would be a subset).
Anaerobic digestion of food waste poses several technical challenges. Such waste is most commonly combined with manure prior to digestion, in order to stabilise the process and improve methane yields. Indeed, there are many commercially operating AD facilities around the world which use manure as a co-substrate (either through co-location with animal production systems or through collection of manures).
Unlike the dairy and beef industries, the Irish pig sector can provide a stable source of manure all year round (with the manure itself being able to generate higher methane amounts than cattle manure).
Mitigating GHG emissions
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Fig 2: 2 Laboratory-scale anaerobic digesters at NUI Galway[/caption]
The need to mitigate GHG emissions from agriculture is driving interest in the potential for on-farm AD plants in Ireland. The increase in the amount of food waste collected in Ireland and the fact that much of this is exported for treatment indicates that there may be commercial opportunities for food waste treatment facilities. The ‘Green Farm’ project aims to assess the technical and financial feasibility of anaerobic co-digestion plants on Irish pig farms.
The project is funded by Science Foundation Ireland and is a collaboration between the Civil Engineering Department of NUI Galway, the Pig Development Department at Teagasc Moorepark and the Department of Science at Waterford Institute of Technology. Thus far, the project has investigated the effects of co-digesting pig manure and food waste on methane yields, process stability and digestate biosafety at laboratory- and pilot-scale.
The use of standard wet AD systems as well as more innovative dry digestion systems has been investigated. High throughput DNA sequencing is being used to analyse how microbial communities are affected by various operational conditions.
A selection of this work will be presented at the upcoming Engineers Ireland-sponsored conference on livestock waste management (Livestock 2016), where leading academic researchers from around the world will convene in Galway to discuss topics such as GHG mitigation, resource recovery, animal waste treatment technologies and government policy on animal waste management.
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Fig 3: Meso-scale anaerobic digester at Teagasc Moorepark, Fermoy, Co Cork[/caption]
The final stages of the ‘Green Farm’ project will consist of the calibration of a mechanistic mathematical model describing the co-digestion of manure and food waste and analysis of the effects of the addition of some trace metals to the co-digestion system. Crucially it will also include a full cost benefit analysis of the concept, which will attempt to identify the market potential for this concept, as well as farm size and locations required to ensure profitability.
The ‘Green Farm’ project aims to provide key information to engineers, farmers, policy makers and the public, which in turn should contribute to the development of a more sustainable, profitable agricultural sector.
For further information and to follow the progress of the project, email Conor Dennehy at c.dennehy2@nuigalway.ie. For further information on Livestock 2016, click here or email Prof Xinmin Zhan at xinmin.zhan@nuigalway.ie.