In this article, That’s Farming, looks at anaerobic digestion and its by-product, digestate.
Ireland has an extensive agricultural and food industry, which is a source of greenhouse gas emissions.
These industries produce significant amounts of dairy, brewing and food processing wastes.
These wastes have the potential to be used appropriately to manage further carbon emissions.
We have the ability to utilise these wastes from the food and agriculture industry to benefit both our agriculture and energy industry.
This occurs through a process called anaerobic digestion. In conjunction with waste management, anaerobic digestion can also assist Ireland to reduce energy demand.
In basic terms, anaerobic digestion involves breaking down these biodegradable organic materials to produce renewable energy in the form of biogas and organic fertiliser.
The organic fertiliser anaerobic digestion produces is otherwise known as digestate.
When digestate is applied to seed, plant surfaces, or soil, it has the ability to promote growth by several mechanisms.
These mechanisms include increasing:
- The supply of nutrients;
- Root biomass or root area;
- The nutrient uptake capacity of the plant.
Digestate is a nutrient-rich substance produced by anaerobic digestion that farmers can use as an organic fertiliser.
What is digestate?
Digestate is a by-product of anaerobic digestion. While both have similarities, it is not compost.
Its use place of synthetic fertilisers creates the opportunity to reduce energy and fossil fuel consumption.
It is made up of by-products such as food waste and dairy produce, and meat and fish governed by Animal By-Products Regulations, ensuring that all products used are pasteurised.
Nitrogen, phosphorus and potassium, which are present in the waste and are utilised during anaerobic digestion, remain in the digestate. These nutrients do not transfer into the biogas.
The process of anaerobic digestion
Anaerobic digestion is a process that involves converting carbon present in the waste to methane and other elements, which are primarily utilised to produce heat and electricity. Bacteria is a core component of this process.
The conversion process focuses on breaking down nutrients that would otherwise be unavailable for plant uptake when applied to the soil.
During this process, microorganisms break down materials in the absence of oxygen.
Two final products are available at the end of this digestion cycle.
Renewable electricity, which is the primary product, is followed by organic fertiliser in the form of digestate.
Spreading digestate on the land
Farmers apply digestate to soils that are deficient in macro-nutrients.
Digestate from anaerobic digestors, which treat by-products of animals, are subject to a grazing ban once digestate is applied to the soil. Livestock should not have access to this land.
To get the maximum benefit from digestate spreading, this organic fertiliser application should take place during periods of maximum crop growth.
Benefits of digestate spreading
Compared to spreading slurry, digestate has become a popular alternative for some farmers.
Other benefits include improvement in soil fertility and crop yields.
According to AWSM Farming, nutrients vary depending on the feedstock. Furthermore, around 80% of total nitrogen is available in food-based digestate.
Phosphate and potash are valuable nutrients that are available in digestate.
In terms of availability, typically 50% of phosphate and 80% of potash is available to the soil.
Farmers have found that digestate spreading is the most economically friendly option.
Options for incorporating digestate into your farm include introducing digestate spreading in collaboration with synthetic fertilisers or in place of synthetic fertilisers.
Anaerobic digestion for agriculture
The government has set out a vision to increase food production levels from Irish agriculture, as outlined in Food Wise 2025.
As a result, the volume of animal manure and organic waste will significantly increase.
These by-products of agriculture and the food industry will require management in a sustainable manner to promote low carbon farming in the future.
According to Cre, anaerobic digestion can be a valuable tool to convert these wastes into utilisable energy, thus achieving smart agriculture.
The Department of Agriculture, Food and the Marine advises that farmers should not spread it in the presence of the following environments:
- Persistent, heavy rainfall;
- In a field where drainage has taken place through pipe or mole drains;
- Where snow or frost is present;
- Within ten meters of any water source;
- Where land is 50 meters from a water supply humans use;
- Finally, where topography is challenging, and high inclines are present.
Similar to synthetic fertilisers, EU statutory regulations control digestate applications to land.
These regulations indicate limits on digestate application rates and periods when farmers may not apply digestate to agricultural land.
Fertiliser application closed periods
|Carlow, Cork, Dublin, Kildare, Kilkenny, Tipperary, Waterford, Wexford, Laois and Wicklow
|October 15th – January 12th
|In counties Clare, Galway, Kerry, Limerick, Longford, Louth , Mayo, Meath, Roscommon, Sligo and Westmeath
|October 15th – January 15th
|In counties Cavan, Donegal, Leitrim and Monaghan
|October 15th – January 31st
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