Can anaerobic digestion materially reduce a farm’s fertiliser costs? The answer depends on the feedstocks, crops, land availability, digestate analysis, storage and spreading arrangements. However, one established Irish biogas plant reported a particularly striking result: annual fertiliser costs that had previously been about €100,000 were reduced to one-third of that level.
This article examines how digestate can displace some purchased fertiliser, the costs that must be included in any realistic calculation, and the operating experience of the GreenGas farm-scale anaerobic digestion plant in County Limerick. The GreenGas section is based on a Landia case study first published in May 2018.
In brief: Anaerobic digestion does not create nitrogen, phosphorus or potassium. It recovers nutrients already present in the incoming materials and produces digestate in which a proportion of the nutrients is readily available to crops. Used within a sound nutrient management plan, that digestate may reduce the quantity of manufactured fertiliser a farm needs to purchase.
How anaerobic digestion can reduce farm fertiliser costs
Anaerobic digestion breaks down biodegradable material in the absence of oxygen. The process produces two principal outputs: biogas and digestate. Biogas can be used in a combined heat and power unit or upgraded to biomethane, while properly managed digestate can supply crop nutrients and organic matter.
The financial value of digestate comes principally from the nitrogen, phosphate, potash and sulphur that it can supply in place of purchased products. Its value is not simply the price of an equivalent quantity of bagged fertiliser, however. to reduce farm fertiliser costs, the calculation should account for:
- the measured nutrient content of the digestate;
- the proportion of each nutrient that will be available to the crop;
- soil analysis and the nutrient requirement of the crop;
- the cost of digestate storage, transport and application;
- timing restrictions and the available spreading window;
- ammonia losses during storage and spreading;
- the distance between the biogas plant, its stores and the receiving land; and
- the prevailing price of the manufactured fertiliser being displaced.
The result is necessarily site-specific. A plant with suitable land close by, adequate covered storage and efficient low-emission application may capture considerably more nutrient value than a plant that has to transport a dilute digestate over long distances.
Why the fertiliser saving is not a fixed percentage
It would be misleading to suggest that every farm installing an anaerobic digester will achieve the same reduction in fertiliser expenditure. Digestate composition varies with feedstock and process conditions, while crop requirements vary with soil, rotation, yield expectation and previous applications.
Good nutrient planning is therefore central to the saving. Digestate should be sampled and analysed, with application rates matched to crop and soil need. In England, current government guidance requires qualifying digestate supplied to agriculture to be used within a nutrient management plan and at rates that do not exceed soil and crop requirements.
The AHDB Organic Materials Value Calculator can help estimate the first-crop value of nutrients supplied by digestate and other organic materials. It is a valuation aid rather than a substitute for a nutrient management plan.
GreenGas: fertiliser costs reportedly reduced to one third
Sponsored content disclosure: The following GreenGas case study was supplied by Landia, an annual sponsor of this website. It was originally published in May 2018. The historical statements and performance figures in this section are attributed to Landia and the plant representatives quoted. They have not been independently verified by this publication and should not be treated as results that every anaerobic digestion plant will achieve.
Ireland’s first farm-scale anaerobic digestion and biogas plant was approaching its eighth year of operation when Landia published the original case study. The GreenGas plant in County Limerick had been developed by the McDonnell family using manure from its dairy herd and poultry operation, together with other suitable feedstocks.
There was no equivalent of the UK Feed-in Tariff available in the Republic of Ireland when the plant began operating in 2010. The family had nevertheless been planning the project for several years. Planning permission had been obtained in 2003, followed by visits to operating biogas plants in the United States, Australia, Scandinavia and Germany.
That research led to a bespoke plant designed around the farm’s circumstances rather than an expectation that one standard configuration would suit every project.
David McDonnell explained the fertiliser objective:
“The nitrates directive was really beginning to kick in during the late 2000s. These EU directives meant serious implications for farmers, so with our annual fertiliser costs up at around 100,000 Euros, we wanted to look at how we could produce our own, better, eco-friendly fertiliser, which at the same time for us was a natural progression for the farm.
“Thanks to our AD operation, our annual fertiliser costs have now been reduced to one third. In the future we can hopefully look at converting our digestate into a marketable product. We wanted to use the land as best we possibly could and ensure that what came out of our AD plant could be put onto our land.”
This was an operator-reported outcome for the complete AD operation. Cutting Farm Fertiliser Costs: It should not be interpreted as evidence that the fertiliser saving was produced by any single pump, mixer or other component.
Plant design and the importance of lifetime cost
The McDonnells worked with German engineering company Krieg & Fischer on the design but chose to work directly with key equipment suppliers. David McDonnell said lifetime cost and the willingness of suppliers to support the plant were important considerations.
The first 1,000 m3 digester was supplied by Kirk, with Landia recommended for the mixing system. GreenGas subsequently developed from a 250 kW to a 500 kW plant and then to a reported electrical capacity of 1 MW. A further 1,800 m3 digester was added in 2015.
The plant invested in Landia pumps, side-entry mixers and the Landia GasMix digester mixing system. GasMix recirculates liquid from the digester through an externally mounted chopper pump and mixes it with biogas before returning the flow through nozzles in the tank.
Keeping the principal moving components outside the digester was considered an operational advantage because maintenance could be undertaken without entering or opening the tank and unnecessarily interrupting gas production.
How back-flushing restored the mixing system
According to the case study, GasMix did not initially appear to be achieving its forecast mixing performance. A Landia site visit identified a blockage caused by sulphur accumulation in the pipework.
The pipe was cleared by back-flushing: digestate was forced through it in the reverse direction. The digester then returned to effective mixing. GreenGas subsequently incorporated pipework back-flushing into its preventative maintenance programme.
This is a useful operational lesson extending beyond any one make of equipment. Bear it in mind when considering cutting farm fertiliser costs. A fall in mixing performance does not automatically mean that a pump or mixer is incapable of doing its job. Pipe restrictions, accumulated material, nozzle condition, changing dry solids and feedstock contamination should all be investigated systematically.
David McDonnell commented:
“GasMix having all moving parts on the outside of the digester is a big plus. You don’t have to take a hit on your gas yield. A service agreement was put in place with Landia and an extended two-year warranty, but because of the robustness and reliability of the pumps, the latter was never needed.”
Intermittent mixing and energy consumption
Mixing supports contact between microorganisms and feedstock, helps distribute heat and may reduce floating layers and settled material. It also consumes energy, so continuous high-power mixing would not be the most economical approach to cutting farm fertiliser costs. Here again, Landia has the answer.
At the time of Landia’s report, the GreenGas GasMix system was said to operate for approximately 15 minutes every two and a half hours. The side-entry mixers ran for approximately five minutes every hour. These were the operating intervals reported for this plant in 2018, not universal settings for other digesters.
The appropriate mixing regime depends on tank geometry, feedstock rheology, dry solids, gas production, heat distribution and the mixer design. Operators should base changes on process evidence and seek competent engineering advice rather than copying another plant’s timer settings.
Feedstock control protects both biology and machinery
Plant manager Senan Meade explained that the incoming material could vary considerably. Cattle slurry was typically richer when housed cattle were receiving maize during winter, while straw could be difficult to process. Food waste had previously delivered unexpected contaminants, reportedly including shovels, engine components and even a bowling ball.
GreenGas responded by developing Feedstock Acceptance Agreements with suppliers and monitoring every load. Potential material was rejected where its quality or continuity might threaten plant stability.
“We scrutinise our feedstock, but it changes. We monitor every load. We need to protect our bugs. We have rejected lots of potential feedstock because continuity of supply is so important to us.”
That discipline is directly relevant to profitability and farm fertiliser costs. Gate-fee income from unsuitable material can be outweighed by lost gas production, contamination, unplanned maintenance or digestate-quality problems.
Pasteurisation and useful heat
GreenGas also installed a Landia pasteuriser. The plant moved pasteurisation from the front to the back end of the process so that digestate already at approximately 40°C could be raised to the required treatment temperature, rather than heating incoming material from an ambient storage temperature reported as only 6–10°C.
The change illustrates an important principle in biogas plant design: Reduce farm fertiliser costs – examine the temperature and energy level at every stage before deciding where a heat-consuming process should be placed. Recovering and reusing heat can reduce parasitic demand and improve overall plant economics. At GreenGas, surplus heat was also used by the plant and nearby poultry sheds.
Maintenance, monitoring and gradual optimisation
The 2018 case study described a plant managed through regular inspection and incremental improvement. GreenGas rotated the use of mixers and nozzles, carried out daily oil and grease checks where required, used alarms and remotely monitored the operation by smartphone.
Senan Meade summarised the approach:
“Regular daily checks are very worthwhile, just doing the basics with oil and grease when required. You can catch any problems very early. It’s all about getting the balance right and fine-tuning so that you hit the sweet spot for a smooth-running plant.”
He also stressed avoiding shock loading, maintaining enclosed operations and using carbon filters to control odour. These measures supported stable digestion while helping the plant remain a responsible neighbour.
By the time of the case study, GreenGas reportedly had 23 Landia pumps and mixers on site. David McDonnell said the company’s equipment and technical support had played a part in the plant’s growth and operating efficiency:
“Our gradual investment in what now totals over half a million Euros in Landia’s pumps and mixers speaks for itself, but the back-up, pride and enthusiasm to help us get the very best from our plant is priceless.”
More information about the equipment described in this sponsored case study is available from Landia UK.
What other farms can learn from the GreenGas result and knowledge you'll need to know to Reduce Farm Fertiliser Costs.
The most useful lesson is not that a particular percentage saving can be promised. It is that fertiliser expenditure, digestate utilisation and digester operation should be planned as connected parts of the same farm business.
1. Establish the baseline
Record the farm’s existing fertiliser purchases by nutrient, crop and field—not merely the total annual invoice. Without a reliable baseline, an apparent saving may reflect changed cropping, lower application, a price movement or reduced acreage rather than improved nutrient recovery.
2. Value nutrients, not tonnes of liquid
Digestate has value because of the crop nutrients it can provide. A large tonnage does not automatically represent a large saving, particularly when transport distances are long or much of the material is water. Use representative analysis and realistic nutrient availability assumptions.
3. Include storage and application
Storage capacity allows digestate to be retained until crop and soil conditions are suitable. Application method also matters. Low-emission spreading can help place nutrients more effectively and reduce ammonia losses compared with uncontrolled surface broadcasting.
4. Protect digestate quality at the inlet
Feedstock acceptance, inspection and supplier agreements reduce the risk of physical contamination and biological instability. They can also protect the future usability and marketability of the digestate.
5. Treat mixing as an operating cost as well as a process requirement
Effective mixing is essential, but its electricity use forms part of plant operating expenditure. Assess performance, maintenance access and lifetime cost—not simply installed motor power or initial purchase price.
Allow for the Landia Gas mix's energy-cost saving features that allow it to be set up to run for short periods only, just long enough in each hour to mix fully, and no more.
6. Verify performance over time
Track purchased fertiliser, digestate analysis, application quantities, spreading costs, crop performance and soil indices from year to year. This provides stronger evidence than relying on a single headline percentage.
A practical fertiliser-cost calculation
A farm can begin with a straightforward annual comparison:
- Calculate the crop requirement for nitrogen, phosphate, potash and sulphur under the farm’s nutrient management plan.
- Estimate the crop-available nutrients supplied by digestate using representative analysis.
- Calculate the amount and cost of manufactured fertiliser displaced.
- Deduct additional storage, analysis, transport, contractor and application costs.
- Allow for nutrient losses and any nutrients applied in excess of crop requirement; excess nutrients should not be assigned a financial saving.
- Compare the net result with the established pre-AD baseline.
In simplified form, cutting farm fertiliser costs:
Net fertiliser saving = value of purchased nutrients displaced − additional digestate handling and application costs.
This calculation should be repeated when fertiliser prices, cropping, feedstock or digestate composition change.
Compliance remains part of the economics
Digestate application is governed by rules intended to protect soil, water and air. Requirements vary by jurisdiction, feedstock and the regulatory status of the material. Operators and land managers should check the current rules applicable to their location rather than relying on a historical case study.
For England, useful starting points include the Environment Agency’s anaerobic digestate resource framework, government guidance on nitrogen fertilisers in Nitrate Vulnerable Zones, and the current guidance on reducing ammonia emissions from organic manures.
Applying more nutrient than the crop and soil require is neither a legitimate saving nor sound environmental management. The value lies in replacing an appropriate amount of purchased fertiliser while maintaining crop performance and protecting natural resources.
Conclusion: reduce fertiliser costs through the whole system
The GreenGas experience shows why digestate can be an important part of farm economics. In the 2018 Landia case study, the plant owner reported that annual fertiliser costs previously running at about €100,000 had been reduced to one third.
That figure belongs to one farm, at one stage in its development. It is not a standard forecast for anaerobic digestion projects, nor can it be attributed entirely to a mixing system. The reported outcome arose from the wider AD operation: recovering nutrients, using digestate on available land, controlling feedstock, maintaining the equipment and continually refining the process.
For another farm, the dependable route to lower fertiliser costs is to analyse its own material, calculate crop-available nutrient value, include every handling cost and manage digestate as carefully as any purchased agricultural input.
For more detail on farm fertiliser costs, read our guide to digestate fertiliser, nutrient value and farm cost savings.
Sources and further information to reduce farm fertiliser costs
- Landia: Ireland’s Landmark AD Plant Continues to Set Standards, 2 May 2018.
- Environment Agency: Anaerobic Digestate Resource Framework.
- AHDB Organic Materials Value Calculator.
- Defra and AHDB: Nutrient Management Planning Tool.
For more information about how to reduce farm fertiliser costs and especially the Landia biogas digester mixer pump design visit www.landia.co.uk – or call 01948 661 200.
For more information about the first on-farm biogas digester design of its type in the Republic of Ireland visit the www.greengas.ie archive here.
The Irish provide nowhere near the incentives we have seen in Northern Ireland although the last few years has seen cutbacks, now things seem to be improving again for NI anaerobic digestion plant subsidies or as in many other EU member states. Come on Irish government. Show us you know what green is…