NI farm AD plant ‘producing all electricity required’

Anaerobic digestion (AD) is a technology that allows farmers to harness the energy contained in the slurries produced by their livestock.

A case in point is Co. Down milk producer, Robert Macauley. He farms with his wife Jill and son Samuel close to the town of Rathfriland.

Macauley uses one third of the slurry produced by his 650 dairy cows to, essentially, produce all of the electricity required within the farming operation.

At the heart of this operation is a 44 kilowatt (kW) Biolectric anaerobic digestion system.

Two 22kW combined heat and power (CHP) engines convert the biogas produced in a 600m³, stainless steel digester into three-phase electricity on a continuous 24/7 basis.

Macauley explained: “Approximately 90% of the electricity is used directly on-farm. The other 10% is exported to the grid.

“The AD operation that we are using now replaced an initial plant that was established on the farm back in 2017.”

The Co. Down farmer explained that the AD operation produces electricity on a flat-rate basis.

“However, there are occasions each day when there is a spike in our energy requirements, for example, when we are separating the remaining two thirds of the slurry produced on the farm or grinding grain used in our cow diets,” he said.

“At such times we have no option but to import electricity from the grid.

“However, in overall terms, the digester is producing all the electricity required on the farm on a day-to-day basis.”

Slurry is the only feedstock used by the AD system.

“We divert the slurry required by the plant using channels,” Macauley commented.

The on-fam AD plant in situ at Rosconnor Dairies

“The cubicle house sits above the digester, so the slurry is made available to it on a gravity feed basis,” Macauley said.

“Sometimes we add parlour washings to the slurry in order to speed up flow rates.

“Apart from regularly checking that no blockages build up in the slurry channels, making sure that the feed stock is always getting to the digester is a pretty straightforward process.”

The current AD system was installed on the Macauley farm in 2025.

The contract entered into between Macauley and Biolectric provides for the regular maintenance of the plant.

This includes the installation, including the servicing of the engines every 800 hundred working hours, and the nearby availability of fully qualifies repair teams, if required.

“I have been very impressed with the straightforward management requirements of the plant,” Macauley said.

“This takes only a few minutes each day; most of the time is spent checking the AD app, which was downloaded onto my phone.”

Circular economy

The AD operation on the Macauley farm is at the very heart of what he refers to as a “circular economy”.

Separated slurry solids are used as cubicle bedding while the grinding of purchased grains allows for the home mixing of rations that are of a consistent quality.

Meanwhile, six cuts of grass silage are taken on the farm each year with each receiving an application of separated slurry and or digestate.

Macauley emphasised the importance of making best use of the organic manures produced on the farm.

“Because we are spreading the separated liquid, there is no risk of slurry solids coming back on to the farm when the swards are cut.

“The digestate is stored separately. However, it is an homogenised liquid and is a very valuable fertiliser source. But again there is no risk of slurry solids contaminating the grass that is ensiled.”

But there is no point investing in AD technologies if they do not deliver a realistic payback.

According to Macauley, the farm is saving substantial amounts of money by producing electricity that would otherwise have to be sourced from the grid.

Moreover, the size of this figure is likely to rise, following the recent outbreak of hostilities in the Middle East.

“Using our own bedding for the cubicles also reduces our costs. Separated slurry solids meet this requirement extremely well,” Macauley said.

“Looking to the future, we want to make best use of the hot water that is produced by the CHP engines.

“This will add further to the levels of sustainability that can be achieved within the business.”

In contrast to solar or wind, AD delivers a constant energy supply day and night.

Biolectric’s Joseph Fayolle, was a recent visitor to the Macauley farm.

He confirmed that the two CHP engines driving the AD operation have consistently achieved a 99.6% efficiency rating.

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