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Biomethane produced from food waste is a potential fuel for urban buses in Mexico to reduce greenhouse gas (GHG) emissions in transport. Biomethane from food waste can potentially produce 42.32 PJ per year, equivalent to 6.5% of the energy content of diesel used in transport in 2015. By replacing diesel with biomethane from food waste, a reduction of 17.91 MtCO 2 e can be effected, 6.06% of the 2050 GHG emissions target. The economic feasibility of a biomethane plant for a Mexican city was investigated using two scenarios: co-digestion of food waste and sewage sludge (scenario 1); and co-digestion of food waste and pig slurry (scenario 2), both scenarios utilising anaerobic high density polyurethane digesters. Economic performance based on net present value (NPV) gave a positive outcome for scenario 1 with 33% of the revenue coming from gate fees. The levelised cost of energy (LCOE) for biomethane was $US 11.32/GJ ($US 40c/m 3 CH 4 ). Scenario 2 has a negative NPV; to break even (LCOE) biomethane has to be sold at $US 14.38/GJ ($US 51 c/m 3 CH 4 ). Biomethane from scenario 2 can be economically viable if a subsidy of $US 1.38/GJ is applied, equivalent to 5% of the cost of diesel. [ABSTRACT FROM AUTHOR]

Macro-algae represent an ideal resource of third generation biofuels, but their use necessitates a refinement of commonly used anaerobic digestion processes. In a previous study, contrasting mixes of dairy slurry and the macro-alga Ulva lactuca were anaerobically digested in mesophilic continuously stirred tank reactors for 40 weeks. Higher proportions of U. lactuca in the feedstock led to inhibited digestion and rapid accumulation of volatile fatty acids, requiring a reduced organic loading rate. In this study, 16S pyrosequencing was employed to characterise the microbial communities of both the weakest (R1) and strongest (R6) performing reactors from the previous work as they developed over a 39 and 27-week period respectively. Comparing the reactor communities revealed clear differences in taxonomy, predicted metabolic orientation and mechanisms of inhibition, while constrained canonical analysis (CCA) showed ammonia and biogas yield to be the strongest factors differentiating the two reactor communities. Significant biomarker taxa and predicted metabolic activities were identified for viable and failing anaerobic digestion of U. lactuca. Acetoclastic methanogens were inhibited early in R1 operation, followed by a gradual decline of hydrogenotrophic methanogens. Near-total loss of methanogens led to an accumulation of acetic acid that reduced performance of R1, while a slow decline in biogas yield in R6 could be attributed to inhibition of acetogenic rather than methanogenic activity. The improved performance of R6 is likely to have been as a result of the large Methanosarcina population, which enabled rapid removal of acetic acid, providing favourable conditions for substrate degradation. [ABSTRACT FROM AUTHOR]

Globally, 10% of milk production occurs in countries whereby dairy livestock have pasture access for the majority of the year, leading to the seasonal collection and availability of dairy slurry. This can pose challenges if dairy slurry is used as a feedstock in on-farm biogas plants. On-farm biogas plants may also have limited gas grid access and have to facilitate seasonal gas demands of consumers in their locality. The effects of seasonal gas demand coupled with seasonal slurry availability have yet to be addressed in literature. This work aims to analyse the effects of such seasonal factors, and whether optimised digestate recirculation may alleviate some of these challenges. Countries which employ pasture-based dairy farming require such evidence to successfully develop on-farm biogas plants, reduce fugitive emissions resultant from current manure management practices, and furthermore, produce alternatives to fossil fuels. Ireland is used as a case study in this work and the effects of seasonal or constant slurry availability and seasonal or constant gas demand is investigated. The sustainability of such systems is also evaluated in terms of emissions reductions as per EU regulations. For a farm with slurry production from 100 dairy cows and 10 ha of grass dedicated for anaerobic digestion, the seasonal availability of slurry leads to a 21% reduction in total biomethane production; over 12 times the required digestate recirculation; smaller digester sizes; and increased variation in the organic loading rate throughout the year. Facilitating a seasonal gas demand leads to larger digester sizes due to periods of increased mass flow rates, and large peaks and troughs in biogas flow rates. Seasonal slurry availability increases greenhouse gas emissions by ca. 11 g of CO 2 per megajoule of biomethane produced. Pasture-based AD systems may successfully manage the seasonal availability of slurry by optimising digestate recirculation to lower the solids content of the plant. • Slurry seasonality can result in lower biomethane production. • Slurry and gas demand seasonality necessitates increased digestate recirculation. • Pasture-based farming systems may increase biomethane emissions by 11 g CO 2-eq MJ−1. • System methane losses are crucial for sustainability criteria with seasonal slurry. [ABSTRACT FROM AUTHOR]