Farm-scale anaerobic digestion can reduce the carbon footprint of dairy farms

In the framework of the LA project Pocket Power, the greenhouse gas emission reduction potential through farm-scale anaerobic digestion was studied. An anaerobic digestion model was set up and applied to estimate desired methane production in the form of biogas as well as unwanted methane emissions associated with farm-scale digestion of manure.

The effect of temperature and retention time on methane emissions resulting from long-term manure and digestate storage was evaluated. Moreover, the overall carbon footprint of several manure management scenarios for Flemish dairy farms was assessed. The scenarios assessed, differed in the possible presence of a digester as well as in the manure collection and storage method. It specifically concerned these four scenarios:

  1. Dairy cow stable with storage in a manure pit beneath the slatted floor, no digestion;
  2. Dairy cow stable with a manure scraper and external manure storage, no digestion;
  3. Dairy cow stable with a manure scraper for fresh manure collection, farm-scale digester and external digestate storage;
  4. Dairy cow stable with a manure pit beneath the slatted floor (no fresh manure collection), farm-scale digester and external digestate storage.

A reduction in methane emissions was achieved for lower manure storage temperatures (through external storage) and by decreasing the stored manure volume and thus the storage time before (controlled) anaerobic digestion. At the same time, feeding fresh manure induced an increased methane production in the digester. Methane emissions from storage took the largest share in the overall carbon footprint of dairy farms, up to over 80% for open manure storage under relatively warm conditions and without controlled digestion. The lowest carbon footprint was achieved on dairy farms with fresh manure collection by a manure scraper, followed by controlled digestion and storage of the digestate in a gas-tight tank, located outside. More specifically, the overall carbon footprint of dairy farms reduced up to over 50% under specific conditions. To allow for such reduction, controlled digestion must take place in a properly managed and correctly dimensioned reactor as high digester methane losses and low digester retention times increase the carbon footprint significantly.

More details? Read the paper ‘Model-based analysis of greenhouse gas emission reduction potential through farm-scale digestion’ or contact Tine.Vergote@UGent.be

This research was supported by the LA project Pocket Power, with Ghent University and Inagro as project partners, funded by Flanders Innovation and Entrepreneurship (VLAIO, http://www.vlaio.be) and co-financed by Boerenbond, ABS, Biolectric, Continental Energy Systems, Innolab, Vermeulen Construct, United Experts, Biogas-E, Inverde and VLACO.