Operational and economic perspectives of pig manure and food waste anaerobic co-digestion
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On-farm anaerobic co-digestion of pig manure (PM) and food waste (FW) is practiced at commercial scale across the world. However, there is a paucity of information regarding how to optimise such co-digestion systems in terms of methane yields, process control, enteric indicator organism removal and digestate disposal. In addition, no analysis of this concept in an Irish economic and regulatory context has been undertaken. In order to identify the most suitable operating conditions for the anaerobic co-digestion of PM and FW, evaluate the viability of using simplified mathematical tools for process simulation, and assess the economic feasibility of on-farm PM/FW co-digestion on Irish pig farms, experiments at laboratory scale and meso-scale were carried out. In the batch scale experiment, the synergistic effects of co-digesting FW and PM were quantified. Co-digestion of PM and FW had synergistic effects on specific methane yields (SMYs) and digestion kinetics. In lab-scale semi-continuous experiments, varying digester feedstock composition from 85 %/15 % to 40 %/60 % PM/FW (volatile solids basis) did not significantly affect digestate biosafety or dewaterability. Decreasing hydraulic retention time (HRT) from 41 to 21 days did not significantly increase the concentrations of the pathogenic indicator microorganisms in digestate. However reducing HRT below 21 days has a significant negative effect on pathogenic indicator microorganisms reduction rates. Decreasing HRT resulted in an increase in the relative abundance of syntrophic acetate oxidising bacteria such as Synergistetes, indicating that hydrogenotrophic methanogenesis may be a key methanogenic pathway at low HRTs. A meso-scale reactor was operated in order to validate a rudimentarily calibrated mathematical model which simulated the co-digestion of PM and FW. The Anaerobic Digestion Model No. 1 provided a somewhat accurate simulation of the system, however more complex parameter optimisation was required to improve model accuracy. An economic model was developed which assessed the financial viability of on-farm biogas plants in Ireland. FW availability was the key factor in determining plant viability. Due to the currently limited amount of FW available for anaerobic digestion, smaller on-farm co-digestion plants were found to be most financially viable as such sites had an increased likelihood of securing sufficient FW.