Synthetic fertilisers are a significant contributor of greenhouse gases in agriculture, and tackling them is vital. In June Nicholas Pitts from the Scotch Whisky Research Institute, India Langley and Lilly Manzoni from LettUs Grow, and Dr Alexandros Stratakos, Associate Professor in Sustainable Agri-Food Production at UWE Bristol, came together at the AFN Network+ Crucible event in Bristol to develop a project on this theme. Their idea, developed over two days, was one of two selected to win funding. Here they explain why assessing the use of digestates in indoor farming is so important.

​LettUs Grow designs and builds aeroponic technology and farm management software for greenhouses and vertical farms. The farms require fertiliser all year round, with many growers reliant on synthetic versions, which are a significant source of greenhouse gas emissions. Developing more climate friendly fertilisers is critical. One possible alternative to synthetic fertilisers is to use digestate from anaerobic digesters. Digestate is in surplus due to the growth of the anaerobic digestion sector, and is rich in nitrogen, phosphorus and potassium – vital for plant growth. But although digestate has been used as a fertiliser for arable crops, it hasn’t been used significantly in indoor farming systems.

“As a technology provider we’re doing research on behalf of growers who have a high reliance on synthetic fertiliser,” says Lilly. “That’s generally quite problematic because it’s a by-product of a chemical process and has quite a significant emissions profile. We can reduce our reliance on synthetic fertilisers by using digestate but one of the challenges with that is establishing whether to use plant or animal based digestate, as each have different nutritional profiles and safety consideration.”

It was India and Lilly’s interest in fertiliser and digestates that led them to Nick. “It was almost like a trading game,” says Lilly. “I remember Nick saying that Scotch Whisky has loads of digestate, what can we do with that? And we use fertiliser in our indoor farms, and it was like who could trade what with who.”

An idea was born to develop a project to examine the effectiveness of plant based and animal slurry based digestate on an indoor basil farming system, and assess them for pathogen risks and nutrient contents, and to test their effectiveness against synthetic fertilisers. The plant based digestate will be provided from a Scotch Whisky distillery anaerobic digestion (AD) plant. The project will also include a literature review to identify locations where indoor farms and anaerobic digestion plants could be beneficially  combined.

“The Scotch Whisky industry produces a lot of digestate through our anaerobic digestion plants,” says Nick. “There’s a lot of research in applying digestates to arable farms but there are limits on where and when you can use digestates in arable farming. With the cost of energy and a  movement to circular economies we might have quite a lot of digestate floating about with only so much being able to be effectively utilised by arable farmers in the immediate surrounding of the distilleries. On the East Coast of Scotland you’ve got distilleries and lots of arable farming but in the future if we have anaerobic digestion plants on the islands and on the West Coast of  Scotland where there is much less arable farming it might make more sense for that digestate to go to more closed agricultural systems that help provide jobs and food to local communities.”

It’s this connection and synergy between the two sectors that excites the team.

“What we have at the moment with agri-food is quite a lot of people working in quite extreme silos,” says India. “There’s a really exciting opportunity here for connections. To have co-location of containerised farms situated close to anaerobic digesters or close to farms that are going to be providing the wheat for distilling. Controlled environment agriculture relies heavily on energy, depending on whether its vertical farming which is electricity heavy, or greenhouses that use gas for heating. Being able to connect that with anaerobic digestion provides a lot of the resources we need along with fertiliser.”

Leading the project is Dr Alexandros Stratakos. “We’re bringing together not just academics, but different stakeholders,” he says. “It won’t just be an academic exercise, the outcomes of the research will be able to more easily translate into changing practices that the industry can adopt, not just farming but the AD industry too. At the end of this project we will have tangible, robust scientific data which will be needed to prove this is safe and feasible.”