Biogas From Slaughterhouse Waste In Ireland: Assessment Of Potential Energy Recovery Pathways
Authors:   ,   Cork Institute of Technology, Ware Aidan 
Published: August 2024

"This study was undertaken to investigate the potential energy recovery from the anaerobic digestion (AD) of slaughterhouse wastes in Ireland. The research was approached in two ways; laboratory scale evaluation of methane production of the industrial waste streams from cattle, pig and chicken slaughtering facilities and a life cycle assessment (LCA) and energy balance to assess the potential utilisation pathways. This thesis includes; the evaluation of the potential biogas production from individual and combined slaughterhouse waste streams in Ireland on an equal footing through standardised laboratory testing, the effects of mandatory pre-treatments on the AD of slaughterhouse waste streams and the use of LCA to assess the potential biogas utilisation pathways in terms of environmental savings in the Irish energy market. Of the individual waste streams tested, offals resulted in the highest methane yields; 650.9, 518.2, 501.1 mLCH<sub>4</sub> gVS<sup>-1</sup> for cattle, pig and chicken offal respectively. The mandatory pasteurisation applied to the offals under the animal by-products regulations (ABPR) did not result in an increase in methane yield, however it did increase the bioavailability of the organics causing acute LCFA inhibition in some instances. The methane yields achieved from the digestion of the combined waste streams from the sampled slaughtering facilities resulted in increased methane yields when compared to digesting one tonne of the mixture in its individual components according to annual production ratios. A stabilising effect was also seen from the mixing of the individual waste streams from the chicken and pig slaughtering facilities, with long chain fatty acids (LCFA) inhibition periods being reduced when compared to the respective pasteurised offals. The potential energy recovery from the AD of the combined slaughterhouse waste streams through a CHP unit and upgrading to biomethane for use as a transport fuel was assessed in terms of environmental savings as well as potential energy production. Both utilisation pathways investigated proved to be sustainable renewable energy pathways (EROI>l) as well as providing substantial environmental benefits in nearly all of the impact categories considered in the LCA. The optimal utilisation pathway derived from the LCA and energy balance lies firmly in the viewpoint taken of the reader. The CHP option present itself as the optimum pathway for a present day short term localised viewpoint with maximum energy subsidy in the slaughtering facilities 100%, 83.8% and 65% for cattle, chicken and pig facilities respectively. As well as maximum GWP savings, on average 79.49% higher environmental savings across all three sectors when compared to the transport scenario, with mitigation of GWP one of the main driving factors for bioenergy development in the short term due to current European Directives. When looking to a future perspective considering a larger scale as well as future energy markets, the transport scenario offers a sounder solution. When considering future energy systems; the CHP scenario loses its biggest advantage over the transport scenario that being the much higher GWP benefits only 23.6% higher on average across all three feedstocks, as well as the environmental benefits/burdens observed for CHP either reducing (benefits) or increasing (burdens) in comparison to the present day CHP option, due to the increased share of renewable energy in the substituted electricity and heat fuel mixes projected for 2020. While in the future transport scenario the opposite was observed with net environmental burdens decreasing and benefits increasing in comparison to the present day transport scenario."