22 06 2021

Outcomes of the webinar on Anaerobic Co-digestion for roadside grass clippings

The Grassification project aims to explore the potential of roadside grass clippings, transforming them from waste into a valuable resource. During this webinar, project partners discussed the use of roadside grass cuttings as feedstock for energy and how this value chain can be developed further. […]

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The webinar was opened by Marcella Fernandes de Souza, project manager of Grassification at Ghent University, who gave a detailed overview of the project and its multidimensional approach to roadside grass clippings refining processes. With a significant potential, counting around 2 million tons generated yearly only in the 2 Seas region, covered by the Grassification project, roadside grass clippings can be processed to obtain valuable products and stimulate a bio-based and circular economy. With this scope, the project investigated the valorization of roadside grass clippings in anaerobic co-digestion to assess whether this can lead to a stable process.

When it comes to the mowed and collected grass clippings, Flanders has an estimated biogas production potential of 340 GWh per year, which corresponds to ¼ of the current biogas production, said Sam Tessens from Biogas-E. Therefore, the anaerobic digestion of roadside grass clippings can be a relevant opportunity not only to lower the GHG balance but also for closing nutrient and carbon loops. Local municipalities dispose of locally available biomass waste streams and are strongly interested in the anaerobic digestion of roadside grass clippings.

Following up on that, Audrey Miserez introduced the practical aspect of the exploratory digestion test on pilot scale conducted by Inagro in their pocket digester. The aim of this activity was to determine if the co-digestion of roadside grass and VeDoWS manure is stable. The grass was mown in Harelbeke in Octobre with a mowing head adapted by Vandaele. Once delivered at Inagro, the grass was ensiled to ensure continued availability of the feedstock. In April/May, the streams were fed to the digester. Pig slurry was added as an additional input stream to adjust the viscosity and prevent possible operational difficulties with the installation due to the high dry matter content of the faecal fractions. During the 30 days of the experiment, a biogas production between 7.5 to 9 m³/h was obtained. This result was similar to the production obtained when only pig manure was fed to the digester, indicating that the addition of 20% grass did not cause any perturbations to the system.

Technically feasible, the anaerobic co-digestion of roadside grass clippings can have some economic hurdles. Miet Van Dael, from VITO, provided an accurate overview of all the potential bottlenecks that can affect the process. The mowing season, the pre-treatment, the effects in the digester itself and the post-treatment of the digestate were discussed with a focus on the possible additional costs and the benefits for the entire value chain. At a technical level, pollution from litter and sand as well as the harvesting frequency can impact the anaerobic process itself. Nevertheless, the major challenge comes from the legislation, which imposes the post-treatment of digestate when using grass clippings in biogas reactors, so to avoid the transfer of pathogens. Therefore, currently in Flanders the use of grass clippings is economically feasible only in an organic municipal waste digester, which already has connections with composting facilities.

The environmental impact of the anaerobic co-digestion of roadside grass clippings was addressed by Rahul Ravi, a joint PhD student from Ghent University and the University of Copenhagen. Starting with some key assumptions, different scenarios were showcased and it was demonstrated that the co-digestion of roadside grass can bring benefits relative to the chosen baseline case ‘composting’ and that net savings depend mainly on the status of grass and the biogas market.

Lastly, the practical experience of anaerobic digestion of roadside grass was presented by Henry Dymoke from the Scrivelsby AD plant. The British economic case showed that verge biomass can constitute a viable feedstock for anaerobic digestion when combined with utilization by rurally sited AD plants in reasonable proximity. Ensiling and de-littering verge biomass are important prior to feeding into the AD plant to avoid system blockages and the formation of floating mats during the gas production. However, also in the United Kingdom, the regulatory framework poses some challenges to the use of grass in the digestion process.

During the interactive session, participants agreed that legislation is the main bottleneck hindering the use of grass in the anaerobic digestion process. Roadside grass is still classified as a waste stream due to the possible presence of pathogens, so post-treatment remains a major requirement. Therefore, more investigation on the post-treatment of digestate could help to assess the safety of the digestate itself and thus optimize the entire value chain.