Circular fertiliser: assessing the potential of digestate for indoor farms

Anaerobic Digestors (AD) produce biogas, as well as a resultant slurry ‘digestate’ which can be rich in nitrogen, phosphorus and potassium content – vital for plant growth. Digestate has been used as a fertiliser for arable crops, but it hasn’t been assessed for use in indoor farming systems. The use of digestate could reduce the demand for synthetic fertilisers (and their associated emissions) within the indoor farming sector, whilst supporting the AD sector producing biogas.

This project will examine the effectiveness of plant based and animal slurry based digestate on an indoor basil farming system. From this, growth trial results comparing the suitability of digestates versus synthetic fertiliser will be published in a peer reviewed paper in an appropriate journal, and a collaborative network of industry, academia, and growers will be enhanced in the arena of AD.

Project lead: Alexandros Stratakos, UWE

Project members: Nicholas Pitts, Scotch Whisky Research Institute, India Langley, LettUs Grow; Lilly Manzoni, LettUs Grow

Findings

• Consistently active loads: All digestate matrices showed high culturable microbial activity (TVC), confirming that digestate should be handled as a biologically active, high-load material across unit operations.
•  LAB as a small but useful signal: Lactic acid bacteria comprise a small fraction of the culturable community but are relatively enriched in whey-associated storage streams; this aligns with mild acidification tendencies and provides an early-warning signal for pH/VFA shifts.
• Pathogen screens: targeted negatives: Listeria monocytogenes and thermotolerant Campylobacter were not detected in any matrix using ISO presence/absence methods,
• A literature review to be shared with the Whisky Industry: This review will add to the body of evidence that is being generated by the Scotch Whisky industry as they investigate using AD plants to process the coproducts of Scotch Whisky production, and as such will facilitate the Industry’s use of this technology.
  

Suggestions for further research

We need long-term studies that track digestate at different sites and times of year, recording simple process facts (like pH, temperature and solids) alongside basic microbiology. This will show which feedstocks (e.g., manure vs whey) and which steps in the process (storage, separation, lagoons) create higher or lower risk. We also need better ways to spot low levels of problem germs early, and practical trials of improvements such as short heat treatments, better mixing, keeping pests and birds away from lagoons, and clearer routines for sampling and record keeping. Finally, studies should link digestate quality to real-world outcomes in farms and indoor systems (crop growth, odour, equipment fouling) and to costs, so producers and buyers can set sensible quality checks, choose the right streams, and time applications safely and efficiently.