Road-verge grass: a potential feedstock for anaerobic digestion

new study from researchers at the University of Leeds has identified the potential for road-verge grass to contribute to the feedstock of anaerobic digesters to generate bioenergy. 

With 270,000 km of rural roads and motorways in the UK, the majority of which have grass verges, verge-grass cuttings could present a valuable resource to the bioenergy sector. 

The study examines the practicalities of using verge-grass in farm-fed anaerobic digesters, including the methane potential of the grass and its digestion performance. The opinions of anaerobic digestor operators are also considered. 

Grass verges are typically cut two or three times a year, with the cuttings left at the roadside for decomposition. However, it has been recognised that removing the cuttings has the potential to increase biodiversity, and there is a use for these cuttings. 

In anaerobic digestion, organic matter like verge-grass cuttings can be converted, in the absence of oxygen, to biogas; a gas made up of methane (CH4) and carbon dioxide (CO2). This provides a source of energy for heat and electricity that has low greenhouse gas emissions. UBoC supported PhD student, Judith Ford, a lead author on the study, said “Verge-grass has the potential to reduce CO2 further if it replaces energy crops as a feedstock for anaerobic digestion, as CO2 is released in the production and transportation of fertilisers used on energy crops, and in their cultivation”. In addition, verge-grass can be collected at the time it is usually cut. 

Brown et al., assessed the potential for verge-grass to be incorporated in the feedstock mix of farm-fed anaerobic digesters, by studying the digestion characteristics of the grass. The theoretical biomethane potential of the grass was estimated and compared to that of current feedstocks. The verge grass was found to have similar biomethane potential to other feedstocks, in both its fresh and ensiled (preserved) form. This digestion performance demonstrates the potential for road-verge grass to be incorporated into feedstock mix for farm-fed anaerobic digestors. 

The research included a case study carried out in Lincolnshire, with Lincolnshire County Council. The authority has 8000 km of rural roads, with grass verges currently maintained twice a year. As part of the study, interviews with local anaerobic digester operators were carried out to investigate their attitudes to the use of verge-grass in bioenergy. Brown et al., found the participants to be largely positive about using verge-grass in their feedstock mix, due to the comparable biomethane potential and digestion characteristics. However, interviews suggested this to be conditional on appropriate management of the chemical contamination associated with the roadside. The participants also stated that in order to use the grass, they would require the harvested grass to be packaged and transported to them due to lack of machinery and labour. The most positive response was received from participants whom had already tested verge-grass in their feed stock mix, suggesting that experience with the grass had built confidence in its potential for generating biogas.

Brown et al., recognise the potential benefits that anaerobic digestion of verge-grass would bring to local authorities. If harvested for bioenergy, verge-grass presents the potential to generate income for local authorities, selling cuttings that would otherwise be left at the roadside. The grass also presents a lower carbon option for anaerobic digestion than energy crops, which helps the local authorities to meet carbon reduction targets. In addition, removing the grass cuttings from the verge has the potential to enhance biodiversity. 

To learn more, the full paper can be accessed here

Brown, A.E., Ford, J.S., Bale, C.S., Camargo-Valero, M.A., Cheffins, N.J., Mason, P.E., Price-Allison, A.M., Ross, A.B. and Taylor, P.G., 2020. An assessment of road-verge grass as a feedstock for farm-fed anaerobic digestion plants. Biomass and Bioenergy138, p.105570.

This research was supported by the United Bank of Carbon.