A life-cycle assessment of greenhouse gas emissions associated with on-farm biogas production

Anaerobic digestion (AD) of liquid dairy manure can generate renewable energy and mitigate greenhouse gas (GHG) emissions. A life cycle assessment (LCA) approach needs to be used to fully assess the AD benefits. However, most LCAs conducted to date have used data from pilot studies. Here, an LCA was conducted using year-round on-farm measurements of 1) emissions from untreated manure, 2) AD first year, and 3) AD fully operational. The data were collected at the same on-farm biogas facility throughout 5 years, starting before the AD became fully operational with addition of industrial food waste (IFW). Plus, soil emissions after land application of untreated manure and digestate were included. The total annual methane emissions from the digestate storage were 1.0 kg CH4 m−3 y−1, in the first year, and 5.6 kg m−3 y−1 in the operational year, while untreated manure emitted 6.6 kg m−3 y−1. The main factor driving digestate CH4 emissions was temperature at 2 m depth (r = 0.98, p < 0.001). Volatile solids (VS) concentration in storage was similar in the first and operational years, but the emissions per VS were much higher in the latter, suggesting that VS is not a good predictor for emissions. In fact, digestate emissions were not correlated to VS concentration (r = 0.37; p = 0.29). The LCA assessed the potential GHG environmental impact of the AD system as a substitute for conventional practices using the robust dataset derived from long-term field studies in Canada. The on-site emissions were higher (> 2x) for AD, but consideration of the avoided off-site emissions resulted in overall GHG emissions 20% lower than the conventional manure treatment. The main emission sources were: storage, fugitive emissions from the anaerobic digesters and the co-generator unit. The environmental benefit came from the avoidance of IFW disposal, grid electricity provision, solid manure stockpiling and land application. The AD could potentially reduce 2.0 t CO2eq cow−1 y−1 from the impact of the conventional manure management practice (9.1 t CO2eq cow−1 y−1). Therefore, the co-digestion of manure and IFW should be encouraged and financially supported by the governmental agencies to mitigate GHG emissions and climate change effect.

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