Researchers from the University of Jyväskylä and the Natural Resources Institute Finland have discovered that ash appears to be an excellent additive in the anaerobic digestion process in terms of methane production and the fertilizing properties of the remaining digestate. The research results were recently published in a scientific journal Biomass and Bioenergy.

As the global population increases, efficient food production becomes more critical. Rising fertilizer needs and limited access to mined materials like phosphate -classified by the EU as critical raw materials- highlight the urgency of developing recycled alternatives from local side streams and waste.

- Anaerobic digestion is a well-known biological process where microorganisms break down organic material in the absence of oxygen, producing biogas and nutrient rich digestate. We wanted to find out how does addition of ash affect the biogas production in anaerobic digestion of biowaste and how are the fertilizing properties of the digestate affected, explains Senior Lecturer Siiri Perämäki from the University of Jyväskylä.

The positive effect of ash

Researchers noticed that ash can be used as an additive in anaerobic digestion of biowaste to increase methane production and improve the quality of the digestate as a fertilizer.

- However, too much ash can have the opposite effect on methane production, says principal scientist Saija Rasi from Natural Resources Institute Finland.

The study also found that the raw material used in the combustion process affects the quality of the ash and, consequently, methane production.

- Ash produced from wood combustion was more favorable than ash from peat combustion, which is probably due to the different trace element concentrations in the ash. 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, Rasi explains.

Ash addition increased biomass, flowering and germination

The researchers also observed that ash increased plant growth in a growing trial with mustard plants. Adding ash affects the growing medium by increasing its nutrient and trace element content and organic matter, as well as reducing its acidity. Germination and flowering decreased in all growing media containing digestate but increased with the addition of ash.

- It is likely that the plants with digestate addition allocated more resources to growth than to reproduction. Heavy metal concentrations in growing media remained safe, and no harmful metal accumulation was observed in plants, says Perämäki.

The study shows that ash could be used both in biogas plants to optimize the process and to improve the quality of recycled fertilizers when used in appropriate amounts.

- This supports the principles of the circular economy and offers new opportunities for utilizing side streams, Perämäki notes.

The study was conducted as part of the KIRE project (Circular Economy Ecosystem: Waste and Side Streams as Raw Materials), which was funded by the European Regional Development Fund through the Council of Central Finland, the City of Jyväskylä, Mustankorkea LTD, Helsinki Region Environmental Services (HSY), Saarijärven Kaukolämpö LTD, and Kuopion Energia LTD. Part of the study was carried out as a bachelor's thesis by students at the Department of Environmental Science, which investigated the fertilizing properties of the digestate.