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Two-stage biogas production is reported to overcome the drawbacks of productivity in anaerobic digestion (AD). Recent publications indicate an increase in methane yield between 10 and 30% via two-stage AD. However, the industrial acceptance is minimal due to their reliability and operational issues. This paper critically reviews the two-stage AD for biogas production. Some of the research gaps identified in two-stage AD include lack of techno-economic analysis to show the industry about the feasibility of this process. There is a clear trade-off between the increase in the methane yield vs. the cost it takes to build the second digester. Practically, building a second digester is not economically feasible due to economies of scale. Other technical challenges include the recirculation leads to ammonia accumulation in the system, and disturbance in syntrophic relationships of microbes between the two-stages. Techno-economic analysis suggests that two stage AD could be about 3% expensive than a single stage AD. Further detailed analysis is required to show clear evidence about the economics and feasibility of two stage AD. The parasitic energy demand of the two-stage system will be higher than a single stage AD due to the reason that two reactors are involved for mixing or maintaining temperature. Most of the two-stage AD, operates at a different temperature and hence the energy demand will be different for different reactors. Some of the problem in the literature includes assessing the stage wise OLR, HRT data, and TS/VS balance before and after the process. To address these issues, further work is necessary to standardize the way two-stage experiments are carried out including the parameters that are necessary to be measured for reproducibility. • Two-stage AD increases the methane yield between 10% and 30%. • Mass transfer and pilot studies are required for commercialization. • Economics of two-stage processes needs to be verified. • Trade-offs of two-stage AD: methane yield vs. energy balance vs. cost. • Two-stage is capital intensive than a single stage process. [ABSTRACT FROM AUTHOR]

Wastes are unceasingly generated in the world, and most of them can be recycled, reused, or recovered to promote a circular economy. Among waste treatment approaches, the anaerobic digestion (AD) process has been considered as an ideal process due to its ecological benefits (reduction of unpleasant odor, pathogens accumulation, or greenhouse gas emission), social and economic advantages, and energy saving. In addition to biogas production, this process can be used to produce various bioproducts, such as biopolymers, bioplastics, biomass, biofertilizers, and biolipids. Interestingly, the AD process residue or digestate is a nutrient-rich by-product that can be used as a biofertilizer for agronomical purposes to balance N-P cycle in the soils. Despite of numerous benefits of AD, less than 1 % of waste is treated by this process. This process has the potential to be integrated with other waste treatment approaches to increase waste treatment efficiency. Therefore, it is essential to focus on each advantage and clarify ambiguity in order to satisfy more countries for employing AD for waste treatment. In this review, various benefits of AD are evaluated; and its potential impacts on particularly agriculture are examined in detail. Additionally, potential biomass and wastes that can be used for anaerobic digestion worldwide are deliberated. Besides, a critical perspective has been developed on the economic, environmental, and social evaluation of the benefits of AD and, as a final point, focused on an integrated circular cascading approach. [Display omitted] • Anaerobic digestion (AD) is an essential solution for waste treatment is discussed. • Various bioproducts can be produced through anaerobic digestion of organic waste. • Approximately 34 % of worldwide waste is generated by high-income countries. • Integrated AD process can maximize the benefits for waste treatment and recycling. [ABSTRACT FROM AUTHOR]

Total livestock emissions account for up to 14.5% of man-made greenhouse gas emissions. Counteractive measures, such as circular economy concepts and negative emission technologies are necessary to limit global warming below 1.5 °C. Possible treatment options for organic manure include anaerobic digestion, combustion, gasification, hydrothermal liquefaction and composting. The choice of treatment varies depending on the economics, the requirement of a specific product, and sociocultural factors. Commercialization of these treatments needs a blend of appropriate technology, feasible economics, policy support and agreeable socio-cultural conditions. Key findings of this study include the following: 1. Increasing scientific awareness about manure management and treatment; 2. Building a sustainable cooperative model to commercialize technologies; 3. Creating a market for manure recycling products; 4. The role of policy in supporting technologies and consumers; and 5. The codigestion of substrates for better efficacy. Current trends show minimal actions in place as opposed to the high-rate of acceleration that is necessary. • Improper disposal of organic manure could cause unwanted ecological damage. • Recent biorefinery models to recover energy and nutrients are addressed. • Biological treatment of organic manure remains the predominant recycling approach. • Limited knowledge of the potential of biorefinery approaches still exist. • Successful implementation of biorefineries requires policy and social support. [ABSTRACT FROM AUTHOR]

Abstract In order to address existing environmental concerns as a result of non-renewable energy sources and to meet future energy demands, biohydrogen offers a suitable alternative energy reserve. Discrete as well as integrative methods of biohydrogen production have been analyzed over time, optimized for achieving high yields. In addition, key process parameters such as temperature, pH, hydraulic retention time, substrate concentration etc., which influence the rate of production have been clarified. Several studies have exploited industrial waste as feed sources for the production of biohydrogen; however, lower yields from these add an additional requirement for suitable pretreatment methods. The present communication examines various pretreatment methods used to increase the accessibility of industrial wastewater/waste for biohydrogen production. Furthermore, a brief overview addresses challenges and constraints in creating a biohydrogen economy. The impacts of pretreating wastes on biohydrogen generation and the latest trends are also supplied. This study helps in the critical understanding of agro-industrial wastes for biohydrogen production, thereby encouraging future outcomes for a sustainable biohydrogen economy. Highlights • This review focused on biohydrogen production using pretreated industrial waste. • Improved biohydrogen production using various pretreatment technologies. • Discussed the future prospects on sustainable biohydrogen economy. [ABSTRACT FROM AUTHOR]

Biomethane from anaerobic digestion of agricultural feedstock is a versatile energy vector for decarbonising agriculture, heavy transport and heat. To lower costs and increase the emission-savings potential, photosynthetic biogas upgrading, cogenerating microalgae with biomethane is investigated here. In a first-of-its-kind work, this paper reports the enviro-economic performance and the marginal (CO 2) abatement cost (MAC) of a polygeneration plant co-producing energy (biomethane), food (Spirulina powder) and bio-fertiliser (digestate) from agricultural feedstock using photosynthetic biogas upgrading at small, medium, and industrial scales. A negative MAC at industrial scale (3 MW biomethane), highlighted the environmental and economic benefit (net present value > 11.5 million€ and internal rate of return >40%) of the process as a low-carbon technology over conventional biomethane production processes at a biomethane sale price of 3 c€/kWh (comparable to natural gas). The operational expenditure, including the cost of the Spirulina cultivation medium and the plant capacity factor had the highest influence on its profitability. Replacing beef as a complete food with Spirulina powder maximised the emission savings rather than replacing beef protein with Spirulina protein. Economic allocation as opposed to energy allocation ensured that the levelised cost and specific greenhouse gas emissions of biomethane (<5 c€/kWh; < 3.5 gCO 2-eq /MJ), Spirulina powder (<68 €/kg; < 4 kgCO 2-eq /kg) and digestate (<5.60 €/tonne; < 0.41 kgCO 2-eq /kg-nitrogen) are better than market-available alternatives across all scales. Trading emission savings from biomethane in the European Union emission trading system should allow the financial viability of smaller-scale processes by 2030. [Display omitted] • Co-generating biomethane, food and biofertiliser is a viable biorefinery process. • At 3 MW-biomethane scale, a negative marginal abatement cost was modelled. • Levelised costs of biomethane of as low as 2.73 c€/kWh were obtained. • For 300 kW scale, the process was profitable with a biomethane tariff of 7 c€/kWh. • Replacing beef as complete food with Spirulina powder maximised CO 2 savings. [ABSTRACT FROM AUTHOR]

The energy sector contributed to three-fourth of overall global emissions in the past decade. Biological wastes can be converted to useful energy and other byproducts via biological or thermo-chemical routes. However, issues such as techno-economic feasibility and lack of understanding on the overall lifecycle of a product have hindered commercialization. It is needed to recognize these inter-disciplinary factors. This review attempts to critically evaluate the role of technology, economics and lifecycle assessment of bio-waste in two processing types. This includes: 1. biological and, 2. thermo-chemical route. The key findings of this work are: 1. Policy support is essential for commercialization of a waste treatment technology; 2. adequate emphasis is necessary on the social dimensions in creating awareness; and 3. from a product development perspective, research should focus on industrial needs. The choice of the treatment and their commercialization depends on the regional demand of a product, policy support, and technology maturity. Utilization of bio-wastes to produce value-added products will enhance circular economy, which in turn improves sustainability. • Technologies for biological and thermochemical treatment of bio-wastes are reviewed. • Bio-wastes are important sources of biogas and other value-added products. • Complex bio-wastes are appropriate for thermochemical treatments. • Biological and thermochemical treatments of bio-wastes can achieve circular economy. • Selection of bio-wastes treatment technologies requires life cycle assessment study. [ABSTRACT FROM AUTHOR]

Algae have long been investigated as a plausible reserve of several biofuel and bioactive compounds attributed to their fast-growing characteristics, shorter doubling time, and capability of accumulating lipids. Compounds extracted from algae are being studied in various sectors namely, pharmaceutical, cosmetics, cancer biology, nanoscience, food industry, etc. In view of the rich potentials of algae, this present review is aimed to highlight the significance of different cultivation aspects of microalgae like open pond and photobioreactor and advantages and disadvantages thereof. This state-of-the-art review provides the limitations of energy (biodiesel, bioethanol, biohydrogen, biomethane) products obtained from the algae in a perspective of shifting lab-scale into a field scale. In addition to the cultivation systems and biofuels, several non-energy products or value-added products obtained from algae were critically compared and presented. Data from plethora literatures discussing the advanced methods for the extraction of omega-3, omega-6 fatty acids, vitamins and nanoparticles from algae have been discussed extensively. Further, bioactive compounds extracted from several algal strains were listed. Considering the health benefits, anti-angiogenic, and anti–cancer properties of algal bioactive compounds were described along with other industrial applications. Overall, this comprehensive review will help in understanding status of algal biofuel, cultivation systems, metabolites and their application for the betterment of the human society. • Types of microalgal cultivation systems and their challenges were discussed. • Limitations of various biofuels produced from microalgae were presented. • Omega-3 fatty acids, vitamins and nanoparticles from algae were summarized. • Anticancer, antiangiogenic activities of microalgal products were specified. • Economics and environmental impacts of algal bio-refinery were analyzed. [ABSTRACT FROM AUTHOR]

In the transition to a climate neutral future, the transportation sector needs to be sustainably decarbonized. Producing advanced fuels (such as biomethane) and bio-based valorised products (such as pyrochar) may offer a solution to significantly reduce greenhouse gas (GHG) emissions associated with energy and agricultural circular economy systems. Biological and thermochemical bioenergy technologies, together with power to gas (P2G) systems can generate green renewable gas, which is essential to reduce the GHG footprint of industry. However, each technology faces challenges with respect to sustainability and conversion efficiency. Here this study identifies an optimal pathway, leading to a sustainable bioenergy system where the carbon released in the fuel is offset by the GHG savings of the circular bio-based system. It provides a state-of-the-art review of individual technologies and proposes a bespoke circular cascading bio-based system with anaerobic digestion as the key platform, integrating electro-fuels via P2G systems and value-added pyrochar via pyrolysis of solid digestate. The mass and energy analysis suggests that a reduction of 11% in digestate mass flow with the production of pyrochar, bio-oil and syngas and an increase of 70% in biomethane production with the utilization of curtailed or constrained electricity can be achieved in the proposed bio-based system, enabling a 70% increase in net energy output as compared with a conventional biomethane system. However, the carbon footprint of the electricity from which the hydrogen is sourced is shown to be a critical parameter in assessing the GHG balance of the bespoke system. Image 1 • A circular cascading bio-based system for advanced biomethane production is proposed. • The benefits of biomass pretreatment by deep eutectic solvents are shown. • Biochar produced from digestate can be used to enhance biogas upgrading and production. • Final net energy output of the proposed system can be potentially improved by 70%. [ABSTRACT FROM AUTHOR]