Anaerobic digestion and digestate use: accounting of greenhouse gases and global warming contribution

Abstract

Anaerobic digestion (AD) of source separated municipal solid waste (MSW) and use of the digestate is presented from a global warming (GW) point of view by providing ranges of greenhouse gas (GHG) emissions useful for calculation of global warming factors (GWFs), i.e. the contribution to GW measured in CO 2 -equivalents tonne -1 wet waste. The GHG accounting was done distinguishing between direct contributions at the AD plant and indirect upstream or downstream contributions. GHG accounting for a generic AD plant with either biogas utilization at the plant or upgrading of the gas for vehicle fuel -in both cases the digestate was used for fertilizer substitution -resulted in a GWF from -375 (a saving) to 111 (a load) kg CO 2 -eq. tonne -1 wet waste. This large range was a result of the variation found for a number of parameters. In descending order of importance these were: energy substitution by biogas, N 2 O-emission from digestate in soil, fugitive emission of methane, unburned methane, carbon bound in soil and fertilizer substitution. GWF for a specific AD plant was in the range -95 to 28 kg CO 2 -eq. tonne -1 of wet waste. The ranges of uncertainty, especially of fugitive losses of methane and carbon sequestration highly influenced this result. Compared to the few published GWFs for AD, the range of our data was much larger demonstrating the need to use a consistent and robust approach to GHG accounting and simultaneously accept that some key parameters are highly uncertain.

Assessment of micro-scale anaerobic digestion for management of urban organic waste: A case study in London, UK

“…Other authors have studied the GHG reduction potential of AD compared with other treatment methods for MSW (Baldasano and Soriano, 2000, Liu et al, 2012, Møller et al, 2009, Masse et al, 2011 and farm residues such as cattle slurry (Masse et al, 2011), but no previous studies have calculated the GHG reduction from source segregated food waste as per this paper. In comparison to digestion of food waste, the GHG reduction from the AD of MSW compared with landfilling will vary.…”

“…This is because the waste has different characteristics and different treatment is required for the MSW digestate since it cannot be used as a fertilizer due to high levels of contamination. Studies report values of 0.114 (Liu et al, 2012) 0.375 (Møller et al, 2009) and0.55-0.78 (Baldasano andSoriano, 2000) kg CO2eq kg -1 waste treated for MSW. The AD of source segregated food waste produces a high quality digestate with minimal contamination that can be used as a fertiliser and displace the use of mineral fertiliser, resulting in additional GHG reductions.…”

Life Cycle Assessment (LCA) of Light-Weight Eco-composites

“…the unintentional leakage escaped from opened reactor during operation/maintenance where the major component of biogas CH 4 was the main concern. In previous studies, a low fugitive loss was estimated (0-3% of CH 4 produced) (Moller et al, 2009); as stated by IPCC CH 4 emissions could be close to zero ‗where the technical standards for biogas plants ensure…”

“…The exhaust gases from the AD plant include fugitive gaseous emissions from pipes or reactors and gas emitted from fuel or biogas combustion (Moller et al, 2009). As described in section 5.2.1, exhaust gases except those emitted from diesel combustion, were captured in the enclosed AD system and treated prior to being released to atmosphere.…”

“…These together with fugitive gases and emissions from fuel combustion comprised three exhaust gas sources during AD process (Moller et al, 2009). In AD scenario, complete biogas combustion was assumed, thus SO 2 , NO x and CO 2 were considered as main products.…”