How can we turn agricultural waste into an energy source?

26 Feb 2019

Image: © Fabian Faber/

TechWatch’s Emily McDaid talks to AgriAD’s Thomas Cromie about how anaerobic digestion can benefit the renewables sector in Northern Ireland.

Northern Ireland (NI) is drowning in agricultural waste, say the experts. But the situation isn’t all bad. Waste products such as slurry can be converted into valuable chemicals that can be used for fertiliser and other industrial products, such as ammonia.

But the question of how to deal with massive amounts of raw waste is a problem for agricultural producers – and it limits how much they can scale their operations.

Queen’s University Belfast (QUB) scientists are now promoting the idea of a closed loop between NI’s biggest industry, agriculture, and renewable energy. Renewable energy can be created and fed into the grid, benefiting everyone in NI, not just farmers.

It’s about dealing with waste in an environmentally beneficial way.

“Instead of waste being a problem, we should look at it as an energy source,” said Thomas Cromie of AgriAD, and one of NI’s leading experts on anaerobic digestion (AD).

close-up image of middle-aged man with dark hair and eyebrows, wearing lilac shirt and black suit jacket.

Thomas Cromie. Image: TechWatch

So, how does this become a zero-sum game?

Cromie said: “There’s a real question in agriculture about the environmental impact of ammonia emissions released into the atmosphere as waste is breaking down. We want to capture this ammonia as usable energy. Currently, farms are importing ammonia to use as fertiliser – but we can create it right here, instead.”

He continued: “Up ’til now we’ve focused on wind and solar energy in the renewable mix – we need to think about the opportunity for renewables outside electricity creation.”

But creating electricity isn’t all bad. Electricity is the main output of Cromie’s AD plants, dotted around Northern Ireland. He said, at present, “there are 80 to 90 anaerobic digestion plants in NI actively producing biogas and energy”.

Cromie was joined in our interview by professor of chemical engineering at QUB, David Rooney. Prof Rooney put it another way: “We’re looking at taking agriculture resources in NI and tying [them] with wind and solar renewables. Carbon, in essence, is a good energy storage mechanism – nature’s best – and every human is carbon-powered. Carbon has a bad reputation because it’s been overused.”

Grainy aerial image of anaerobic digestion plant, with circular blue containers visible beside a shed construction, on farm site surrounded by green fields.

Aerial image of Bridge Energy’s AD plant. Image: TechWatch

How do AD plants work?

The input to the AD process is agricultural waste, everything from slurry to chicken litter. Following a breakdown process involving natural micro-organisms (including bacteria), the outputs are electricity, green fertiliser and biofuel for transport.

“There are other by-products created, too – like lactic acid, which can be used to manufacture many things including surgical implements,” said Cromie.

Part of the picture is that much of energy policymaking is UK-wide. But NI has a different mix of problems and opportunities. Cromie said: “As much as 27pc of greenhouse gas emissions are agricultural in origin in NI, compared to just 10pc in the UK as a whole. This shows the scale of our farming industry.”

And the scale of AD in NI is not to be ignored. Cromie said that between £400m and £500m was spent in NI over the last three to four years on this form of renewable energy. He said that it’s creating a self-funding economy. “Those AD projects spend £120m per year on operational support – and that money is going directly to farmers, from whom they buy the feedstock, to engineers and agricultural contractors, who operate locally,” he pointed out.

Furthermore, this economy has staying power. “The economy around AD is guaranteed over a 20-year period by governmental policy,” Cromie said.

What’s the wider picture?

“We have to be looking at a contribution of 70 to 80pc from renewables into the overall energy mix,” Cromie said, “in order to be competitive and to clean up our environment. Meanwhile, we have 250,000 surplus tonnes of poultry litter yearly in NI. There’s an opportunity there for the taking. Unless we address this issue, we’ll lose it.”

By Emily McDaid, editor, TechWatch

A version of this article originally appeared on TechWatch

TechWatch by Catalyst covered tech developments in Northern Ireland