• Energy Research
• 7. Clean energy close

Abstract The majority of studies analysing the economic potential of biogas systems utilise deterministic models to assess the viability of a system using fixed inputs. However, changes in market conditions can significantly affect the viability of biogas plants, and need to be accounted for. This study assessed the economic potential of undertaking on-farm anaerobic co-digestion of food waste (FW) and pig manure (PM) using both deterministic and stochastic modelling approaches. The financial viability of three co-digestion plants sized to treat PM generated from 521, 2607 and 5214 sow integrated units was assessed. Under current market conditions the largest co-digestion scenario modelled was found to be unviable. Stochastic modelling of four key input variables (FW availability, renewable electricity tariff, gate fees and digestate disposal costs) was undertaken to assess the sensitivity of project viability to changes in market conditions. Due to the high likelihood of accessing sufficient FW, the smallest co-digestion scenario was found to be the least sensitive to any future changes in market conditions. Due to its potential to treat greater amounts of FW than the smallest scenario, a co-digestion plant designed for a 2607 sow farm had the highest revenue generating potential under optimal market conditions; however, it was more sensitive to changes in FW availability than the smaller scenario. This study illustrates the need for farm-based biogas plant projects to secure long-term, stable supplies of co-substrates and to size plants’ capacity based on the availability of the co-substrates which drive methane production (and revenue generation).

Abstract Biosafety of digestate remains one of the major concerns for anaerobic digestion, which affects the downstream management and may determine the success of anaerobic digestion. Inactivation of pathogens during anaerobic digestion has been reported by various studies, but most of them focus on single specific pathogen and/or operation condition, with the overall pictures absent. In this review, a systematic analysis was conducted to assess the effects of different factors on the inactivation of various pathogens during anaerobic digestion, including pathogen types, resistance mechanisms, operation conditions, intermediate products and kinetic models. Statistical analysis was performed to determine the significant influencing factors based on 141 published studies. The results indicated that pathogen types, temperatures and operation modes affected inactivation efficiencies significantly (P non-spore forming Gram-positive bacteria > nematode ≥ Gram negative bacteria ≥ viruses. Thermophilic temperature can inactivate most pathogens effectively, with ambient and mesophilic temperatures not effective. Intermediate products, e.g. volatile fatty acids (VFAs)/ammonia, can contribute to the inactivation of pathogens, and dry anaerobic digestion might be a promising strategy due to the accumulation of VFAs/ammonia. Analysis on eleven commonly used kinetic models showed that Weibull, log-logistic and biphasic models were preferable for simulating the inactivation of most pathogens. Operation conditions of anaerobic digestion need to be optimised for mutually benefiting energy recovery, pathogen inactivation, economic feasibility and biowaste stabilization.

Abstract In this study, the environmental impacts of pig manure direct land application and anaerobic digestion (mono- and co-digestion) for biomethane production followed by digestate land application were evaluated by life cycle assessment, with a focus on the dynamic land application of digestate in terms of the nutrient profiles, soil nutrient status and environmental regulations. Ireland was used as a case study where co-digestion of manure with grass silage is viable, and an Irish emission inventory was established. The results indicate that pig manure and grass silage co-digestion performs best in 9 of the 11 impact categories assessed. Compared with direct land application, mono-digestion of pig manure decreases direct greenhouse gas emissions by 48% (190 tonne CO2e). Co-digestion with grass silage increases the total energy recovery by 226% (1592 MWh) than mono-digestion. The nitrogen available for plants in digestate (41.8%) is a little lower than that in raw pig manure (43.2%) due to the higher ammonia emissions after anaerobic degradation. The environmental impacts of the three pig manure management methods are greatly affected by soil nutrient conditions and regional environmental regulations, which determine the organic fertilizer application rates and consequently the required land areas and transport distances. In this case study, the land application area for digestate is determined by phosphorus acceptance of soils rather than nitrogen or potassium. This study provides practical insights to farmers, the gas industry, and policy makers to conduct effective organic fertilizer application, select optimal AD plant location, and maximize environmental benefits.