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• Energy & chemical inputs, biogenic emissions, and product substitutions dominate environmental performance of biorefineries. • More transparency needed for product substitution modelling to avoid misleading conclusions. • Biogenic carbon capture use/storage could enhance future climate mitigation efficacy of biorefineries. • Accurate representation of biogenic carbon cycling essential to identify role of biorefineries in net zero strategies. Anaerobic digestion (AD) has gained attention in the last decade as a strategic component of the circular bioeconomy (CBE), and Life Cycle Assessment (LCA) has been used to explore the environmental performance of prospective AD-biorefineries. A detailed review of 76 studies underpinned systematic evaluation of environmental hotspots and LCA modelling challenges for AD-biorefinery configurations. Crops constitute the most frequently used feedstocks, while electricity, bioethanol, digestate, animal feed, additives, and platform chemicals represent the most reported products. Feedstock cultivation, energy and chemical inputs, biogenic emissions, and avoided burdens due to product substitution are the main drivers of environmental performance. However, modelling of product substitution, biogas upgrading, digestate management and biogenic carbon cycling is patchy, inconsistent, and often inadequately evidenced. Prospective and dynamic LCA coupled with transparent scenario analysis could address some of these shortcomings, and better represent, inter alia , the climate mitigation efficacy of carbon capture use and storage technologies integrated into AD-biorefineries. These findings provide a guide for LCA practitioners and researchers engaged in the study of biorefinery systems and other CBE measures. [ABSTRACT FROM AUTHOR]

The high moisture content and the potential presence of hazardous organic compounds (HOCs) and metals (HMs) in sewage sludge (SS) pose technical and regulatory challenges for its circular economy valorisation. Thermal treatments are expected to reduce the volume of SS while producing energy and eliminating HOCs. In this study, we integrate quantitative analysis of SS concentration of 12 HMs and 61 HOCs, including organophosphate flame retardants (OPFRs) and per- and poly-fluoroalkyl substances (PFAS), with life-cycle assessment to estimate removal efficiency of pollutants, climate change mitigation benefits and toxicological effects of existing and alternative SS treatments (involving pyrolysis, incineration, and/or anaerobic digestion). Conventional SS treatment leaves between 24 % and 40 % of OPFRs unabated, while almost no degradation occurs for PFAS. Thermal treatments can degrade more than 93% of target OPFRs and 95 % of target PFAS (with the rest released to effluents). The different treatments affect how HMs are emitted across environmental compartments. Conventional treatments also show higher climate change impacts than thermal treatments. Overall, thermal treatments can effectively reduce the HOCs emitted to the environment while delivering negative emissions (from about −56 to −111 kg CO 2 -eq per tonne of sludge, when pyrolysis is involved) and producing renewable energy from heat integration and valorization. [Display omitted] • A wide range of novel characterization factors for PFAS and OPFRs are estimated. • Pyrolysis and incineration degrade from 94 % to 99 % of PFAS, OPFRs and BPA. • HMs are the dominant pollutants affecting human health and freshwater ecotoxicity. • Direct pyrolysis is the best option to deliver negative emissions and abate HOCs. • Conventional treatments leave about 40 % of OPFRs unabated, and no PFAS degradation. [ABSTRACT FROM AUTHOR]

Carbon capture and utilization has been proposed as an essential climate change mitigation strategy, but only a few implemented cases exist. During biomethane production from anaerobic digestion, CO₂ is commonly separated and emitted into the atmosphere, which can be utilized as raw material for various products. This research aims to identify and assess CO₂ utilization alternatives for possible integration with biogas upgrading from anaerobic digestion by developing a soft multi-criteria analysis (MCA). A literature review complemented with stakeholder participation enabled the identification of relevant alternatives and criteria for assessment. Potential alternatives for CO₂ utilization include methane, mineral carbonates, biomass production, fuels, chemicals, pH control, and liquefied CO₂. Results show that although no alternative performs well in all indicators, there is an opportunity for short-term implementation for methane, biomass production, mineral carbonates, liquefied CO₂, and pH control. Moreover, the uncertainty analysis reveals that even though the technologies have a high technological development, more information on critical aspects is still required. The soft MCA provides information to decision-makers, practitioners, and the academic community on learning opportunities of the alternatives and indicators to step from development into implementation. For instance, the method can be used to assess more specific systems with different locations and scales or to direct efforts to ease the implementation of CCU. • Multi-criteria analyses identify development areas for CO₂ utilization options. • Literature review for CO₂ utilization alternatives and their assessment criteria. • Many options for CO₂ utilization from biogas are possible with areas of improvement. • CO₂ methanation is in many aspects a relevant option in biogas upgrading. [ABSTRACT FROM AUTHOR]

High-solid anaerobic digestion (HSAD) is an attractive organic waste disposal method for bioenergy recovery and climate change mitigation. The development of HSAD is facing several challenges such as low biogas and methane yields, low reaction rates, and ease of process inhibition due to low mass diffusion and mixing limitations of the process. Therefore, the recent progress in HSAD is critically reviewed with a focus on transport phenomena and process modelling. Specifically, the work discusses hydrodynamic phenomena, biokinetic mechanisms, HSAD-specific reactor simulations, state-of-the-art multi-stage reactor designs, industrial ramifications, and key parameters that enable sustained operation of HSAD processes. Further research on novel materials such as bio-additives, adsorbents, and surfactants can augment HSAD process efficiency, while ensuring the stability. Additionally, a generic simulation tool is of urgent need to enable a better coupling between biokinetic phenomena, hydrodynamics, and heat and mass transfer that would warrant HSAD process scale-up. • HSAD suffers from limited heat and mass transfer due to moisture deficit. • Reactor designs and operational strategies can mitigate potential challenges. • HSAD reactor control algorithm must be informed with feedstock properties. • Integrating biokinetic models, CFD and ML will improve process simulation. • Novel engineered-materials can improve transport characteristics of HSAD process. [ABSTRACT FROM AUTHOR]

The use of co-products for animal feed can potentially have a higher greenhouse gas (GHG) emission and water scarcity offset compared to bio-energy (bio-electricity/fuel) production. We cluster 136 Scotch Whisky distilleries and evaluate the co-product pathways for the production of animal-feed and/or bio-energy at centralised processing facilities. Production of animal feed, and the subsequent displacement of imported animal feed, offered the most significant GHG offset, which was between a factor of c.a. 2.5 to 8 times greater than the bio-electricity/fuel and bio-energy/feed scenarios. This offers significant potential from a global net-zero carbon emissions perspective. However, this comes at a cost to local energy security potential. Bio-electricity produced in the electricity intensive scenarios was 481 GWh per year. This would significantly increase Scotland's bio-energy production and equates to c.a. 5% of Scotland's non-commercial electricity needs. • The study utilises consequential life cycle analysis to explore potential pathways for Scotch whisky distillery co-products. • Results indicate significant energy security and climate mitigation trade-offs between pathways. • Production of animal feed offset up to 8 times more GHG emissions than bio-energy scenarios. • Bio-energy scenarios could potentially meet up to 5% of Scotland's non-commercial electricity needs. [ABSTRACT FROM AUTHOR]

• The potential of implementing management strategies in reducing waste-related greenhouse gas emissions was evaluated. • Three kinds of waste – food, sewage, and plastic were analysed was evaluated using life cycle assessment. • By diverting plastic waste away from incineration, the reduction in GHG emission is 61%. • By diverting food waste away from incineration, GHG emission amount is reduced to 54.69 % of the emission from BAU method. • The GHG emission of sewage sludge and food waste co-digestion reduces the current emission (incineration) by 64.3%. The potential of implementing waste recovery strategies on three kinds of waste – food, sewage, and plastic – in reducing waste-related greenhouse gas emissions in Singapore was evaluated using life cycle assessment. It was found that the proposed strategies are capable of reducing 1,087.80 kgCO 2 -eq per tonne of plastic waste, 53.00 kgCO 2 -eq per tonne of food waste, and 43.86 kgCO 2 -eq for the combination of one tonne of sewage waste and one tonne of food waste. When uncertainties of the three waste streams were considered in sensitivity analysis and projections of waste quantities to 2025 were performed up to a 95% confidence level, it was found that the proposed strategies could reduce about 37.13% of the GHG emissions from the business-as-usual disposal waste technology (that is, incineration). This corresponds to an average reduction of 0.53 million tonnes CO 2 -eq per year, which is equivalent to the annual GHG emissions from electricity consumptions of 256,322 households in Singapore. Graphical Abstract [Display omitted]. [ABSTRACT FROM AUTHOR]

Anaerobic digestion (AD) is at the interface of biowaste management, energy generation, food production and land-based carbon dioxide removal. Strategic deployment of AD requires careful scoping of interactions with prospective alternative biowaste management, energy generation technologies and land uses to ensure effective delivery of climate neutrality and circularity. There remains a need to assess the greenhouse gas (GHG) mitigation efficacy of AD in the context of future alternative (counterfactual) processes associated with differential rates of decarbonisation across energy, waste management and land (including agriculture) sectors. To address this gap, prospective life cycle assessment (LCA) is applied to AD deployment scenarios across three decarbonisation contexts, using the UK as an example. Food waste prevention and diversion to animal feed always achieve more GHG mitigation than AD, even with sustainable intensification of food and feed production. Compared with maize- or grass-biomethane transport fuel, solar electricity generation can avoid 16 times more fossil energy and afforestation can mitigate six times more GHG per hectare of land occupied. Transport biomethane is currently the most effective biogas use for GHG mitigation, but large-scale combustion of biogas for electricity or industrial heat generation is the most effective long-term option as transport is electrified and bioenergy carbon capture & storage (BECCS) is deployed. Prioritising waste prevention and diversion to animal feed (including via insect meal) instead of maximising AD deployment could simultaneously: offset an additional 10–15% of national GHG emissions; meet an additional 2–4% of national energy demand; free enough arable land to provide 20–21% of national recommended protein and kcal intake. However, AD is likely to remain the best option to manage substantial volumes of residual food wastes and manures that will remain available even if ambitious projections on waste prevention and diet change are realised. [Display omitted] • Consequential LCA of anaerobic digestion (AD) in future decarbonisation contexts • GHG mitigation efficacy of AD declines as energy & transport systems decarbonise • AD-crop cultivation is a highly inefficient land use for energy generation and GHG mitigation • Carbon capture & storage could maintain effective mitigation from large-scale biogas use • Sustainable niche for waste-AD alongside waste prevention & diversion to animal feed [ABSTRACT FROM AUTHOR]