Anaerobic digestion is a biological process in which microorganisms break down organic matter without oxygen, producing biogas and nutrient-rich digestate. The team investigated how ash influences this process, focusing on methane yield and fertilizer potential.
They found that moderate ash addition boosts methane output and improves digestate quality. However, excessive amounts suppress methane production. The type of ash also proved decisive: wood-derived ash supported higher methane yields than peat ash, likely due to its trace element composition.
"Adding trace elements in the form of ash has a complex effect on the anaerobic digestion process and is very case-specific, depending on the type of ash, the raw material for anaerobic digestion, and the microbial community," explained Saija Rasi, principal scientist at Natural Resources Institute Finland.
The researchers also ran plant trials with mustard, showing that ash increased biomass, flowering, and germination. Ash enriched the growth medium with nutrients, trace elements, and organic matter while reducing acidity. Although digestate alone reduced germination and flowering, the addition of ash reversed this trend. Importantly, heavy metal concentrations remained within safe limits, with no harmful accumulation observed in plants.
"Biogas plants could use ash to optimize the process and improve the quality of recycled fertilizers when used in appropriate amounts," noted Senior Lecturer Siiri Peramaki from the University of Jyvaskyla. "This supports the principles of the circular economy and offers new opportunities for utilizing side streams."
Research Report:Fly ash addition into anaerobic digestion of biowaste: effects on biomethane production and fertilizing properties of the digestate
Related Links
University of Jyvaskyla - Jyvaskylan yliopisto
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