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Anaerobic digestion (AD) can be used as a stand-alone process or integrated as part of a larger biorefining process to produce biofuels, biochemicals and fertiliser, and has the potential to play a central role in the emerging circular bioeconomy (CBE). Agricultural residues, such as animal slurry, straw, and grass silage, represent an important resource and have a huge potential to boost biogas and methane yields. Under the CBE concept, there is a need to assess the long-term impact and investigate the potential accumulation of specific unwanted substances. Thus, a comprehensive literature review to summarise the benefits and environmental impacts of using agricultural residues for AD is needed. This review analyses the benefits and potential adverse effects related to developing biogas-centred CBE. The identified potential risks/challenges for developing biogas CBE include GHG emission, nutrient management, pollutants, etc. In general, the environmental risks are highly dependent on the input feedstocks and resulting digestate. Integrated treatment processes should be developed as these could both minimise risks and improve the economic perspective.

Microbial electrosynthesis (MES) is an innovative technology for electricity driven microbial reduction of carbon dioxide (CO2) to useful multi-carbon compounds. This study assesses the cradle-to-gate environmental burdens associated with acetic acid (AA) production via MES using graphene functionalized carbon felt cathode. The analysis shows that, though the environmental impact for the production of the functionalized cathode is substantially higher when compared to carbon felt with no modification, the improved productivity of the process helps in reducing the overall impact. It is also shown that, while energy used for extraction of AA is the key environmental hotspot, ion-exchange membrane and reactor medium (catholyte & anolyte) are other important contributors. A sensitivity analysis, describing four different scenarios, considering either continuous or fed-batch operation, is also described. Results show that even if MES productivity can be theoretically increased to match the highest space time yield reported for acetogenic bacteria in a continuous gas fermenter (148 g L−1 d−1), the environmental impact of AA produced using MES systems would still be significantly higher than that produced using a fossil-based process. Use of fed-batch operation and renewable (solar) energy sources do help in reducing the impact, however, the low production rates and overall high energy requirement makes large-scale implementation of such systems impractical. The analysis suggests a minimum threshold production rate of 4100 g m−2 d−1, that needs to be achieved, before MES could be seen as a sustainable alternative to fossil-based AA production.

Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon dioxide (CO2) reduction and biomethane (or value-added products) production. There are limited literature critically reviewing the latest scientific development on the Bioelectrochemical (BES) based biogas upgrading technology, including CO2 reduction efficiency, methane (CH4) yields, reactor operating conditions, and electrode material tested in BES reactor. This review analyzes the reported performance and identifies the crucial parameters to be considered for future optimization, which is currently missing. In this review, the performances of BES approach of biogas upgrading under various operating settings in particular fed-batch, continuous mode in connection to the microbial dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other versatile application options associated with BES based biogas upgrading, such as resource recovery, are presented. The three-dimensional electrode materials have shown superior performance in supplying the electrons for the reduction of CO2 to CH4. Most of the studies on the biogas upgrading process conclude hydrogen (H2) mediated electron transfer mechanism in BES biogas upgrading.

Solid waste management is an important environmental and public health challenge, particularly in the developing world. This chapter highlights the importance of organic waste recycling using insects, which can subsequently be utilized as an alternative animal feed or even human food ingredient. In addition, biowaste conversion by exploiting insects is found to be environmentally more beneficial (reduced greenhouse gas emissions) than traditional waste handling, such as composting. Thus, organic waste valorization via insects could be a sustainable measure of biowaste treatment as it generates novel nutrient sources while posing a minimum threat to the environment. Characterizing biowaste as an insect rearing substrate, examining the potential role of the insect gut microbiota on waste degradability and insect biomass and quality, and identifying environmentally friendly residue handling approaches are some of the critical aspects for insect-based biowaste recycling in the future.

Biogas upgrading by employment of bioelectrochemical systems (BESs) is an emerging approach for electricity-based production of biomethane. Recent advances within the field have successfully demonstrated BES for biogas upgrading at laboratory scale under different configurations and operating conditions: in situ, ex situ, batch mode, and continuous mode. This chapter summarizes the development and status of bioelectrochemical biogas upgrading, and includes examples of multifunctional systems combining biogas upgrading with resource recovery. Insights are given into proposed electron transfer mechanisms and reported BES designs for CO2 reduction to methane. BES technology for biogas upgrading has primarily been developed to lab-scale and still has to be further developed to evaluate the current economic perspectives of the technology compared to conventional biogas-upgrading technologies.

Anaerobic digestion (AD) has been widely applied bioprocess to produce the biogas for fuels from organic waste degradation. AD has been integrated with other processes for increasing the digestion efficiency and waste valorization. The integration of AD with other bioprocess optimizes the production of targeted product and reduces the waste. Recently, microbial electrosynthesis (MES) was coupled with AD for the biomethane production, chemical synthesis and resource recovery. MES coupling to AD also gives an opportunity for value-added chemical generation and hence provides additional economic gains of integrated system. In MES, the remaining carbon dioxide (CO2) in biogas is reduced to methane by methanogens utilizing in situ produced hydrogen at cathode, thereby enriching methane content. Furthermore, electroactive microbes could directly accept the electron from cathode to reduce the CO2 to methane and chemicals. Therefore, CO2 fraction in the biogas could be utilized for the further chemical synthesis such as acetate, butyrate. In this chapter, advances on AD technology and MES coupling with AD are thoroughly discussed for the production of fuels and chemicals. The outputs of recent laboratory scale experiments are summarized and discussed. Furthermore, mechanism of CO2 reduction is elaborated with methane and chemical production.

Several pre-treatment approaches have been explored to enhance the anaerobic fermentation kinetics and efficiency, which include thermal-alkaline treatment, free ammonia, sequential ultrasound techniques as well as grinding, and sieving. Additionally, valorization of mineralized compounds and production of reusable water can also be achieved via post-treatments. The post-treatment concept allows preserving or recovery of value-added byproducts in the form of manures, soil conditioners, and renewable energy. In this chapter, we explain the recent advancement in the pre-treatment and post-treatment of anaerobic digestate to enhance the anaerobic process and for the removal of undesirable compounds, recovery of energy, nutrients, and waste stabilization before disposal.

Agriculture-based industries generate significant amounts and varieties of agro-industrial wastes (AIW). AIW, if not treated properly, may cause environmental pollution and pose a harmful effect on human and animal health. Although AIW consist of organic compounds that can pose a risk to the environment, however, if used in the right way can potentially produce bioenergy, biofertilizer, and biofuels. Energy generation using agro-wastes not only fulfill the energy demand locally but also contribute to a significant reduction in greenhouse gas emissions. Bioenergy a possible substitute for fossil-fuels can be generated from AIW residues like rice straw, sweet potato, sugarcane bagasse, sawdust, etc. Nutrient-rich and bioactive compounds obtained from AIW can be recycled as raw materials in the agro-based industries, which can reduce the total production costs. Moreover, agricultural wastes, such as animal manures, agricultural effluents, biosolids, etc., can be used as organic fertilizers. This will help to mitigate the scarcity of organic fertilizers and increase the crop yield and productivity. However, the sorption of heavy metals in fertilizers needs to be considered. AIW can be converted to biodegradable plastic or can be used as a feed-in aquaculture industry. Overall, AIW seems to be a potent source of alternative energy and organic fertilizers.

Syngas from gasification of waste biomass is a mixture of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2), which can be utilized for the synthesis of biofuels such as methane (CH4). The aim of the study research work was to demonstrate how syngas could be methanated and upgraded to natural gas quality (biomethane) in a fed-batch trickle-bed reactor system using either manure – (AD-M) or sludge-based (AD-WW) inoculum as microbial basis. The methanated syngas had a high concentration of CO2 and did not fulfil the criteria for natural gas quality biomethane. Further upgrading of syngas to biomethane could be achieved simultaneously in the same reactors by addition of exogenous H2, resulting in CH4 concentrations up to 91.0 ± 3.5% (AD-WW) and 95.3 ± 1.0% (AD-M). Microbial analysis indicated that the communities differed between AD-M and AD-WW demonstrating functional redundancy among the microbial communities of different inocula.

The biological reduction of CO 2 into multicarbon chemicals can be driven by electrons derived from the cathode of a bioelectrochemical reactor via microbial electrosynthesis (MES). To increase MES productivity, conditions for optimal electron transfer between the cathode and the microbial catalyst must be implemented. Here, we report the development of a 3D-graphene functionalized carbon felt composite cathode enabling faster electron transfer to the microbial catalyst Sporomusa ovata in a MES reactor. Modification with 3D-graphene network increased the electrosynthesis rate of acetate from CO 2 by 6.8 fold. It also significantly improved biofilm density and current consumption. A 2-fold increase in specific surface area of the 3D-graphene/carbon felt composite cathode explained in part the formation of more substantial biofilms compared to untreated control. Furthermore, in cyclic voltammetry analysis, 3D-graphene/carbon felt composite cathode exhibited higher current response. The results indicate that the development of a 3D-network cathode is an effective approach to improve microbe-electrode interactions leading to productive MES systems.

Biological syntrophy can be defined as a nutritional condition where various syntrophic microorganisms mutually synergize their metabolic activities to break down complex organic substrates that cannot be otherwise catabolized by either individually. Direct interspecies electrons transfer (DIET) is a syntrophic metabolism wherein free electrons flow from one cell to another through shared physical, electrical connections without the requirement of reduced electron carriers (redox mediators) like molecular hydrogen or formate or protein released after cell death. In anaerobic digestion (AD), the transfer of electrons between two different syntrophic microbial communities such as bacteria and archaea is a vital process for methanogens to get control of energy barriers and catabolize complex organics which could not be digested by them alone. Studies conducted so far show that DIET can be accelerated by conductive materials like carbon nanotubes, biochar, carbon cloth, granular activated carbon (GAC), and magnetite. The conductive materials mediated DIET has showed to be highly efficient in enhancement of methane yield than indirect interspecies electron transfer (IIET) in case of conventional anaerobic digestion (AD) process. This review summarizes the studies conducted to date on the effect of conductive materials mediated DIET on methanogenesis in the AD. The different types and concentrations of conducting materials affecting (a) DIET, (b) major microbial communities required for carrying out DIET, and (c) the rate of methanation, are critically discussed in the review.

Microbial electrosynthesis is a bioprocess where microbes reduce CO 2 into multicarbon chemicals with electrons derived from the cathode of a bioelectrochemical reactor. Developing a highly productive microbial electrosynthesis reactor requires excellent electrical connection between the electrochemical setup, the cathode, and the microbes. Copper is a highly conductive cathode material widely employed in electrochemical apparatuses. However, the antimicrobial properties of copper limit its usage for bioelectrochemistry. Here, biocompatible reduced graphene oxide coated on copper foam is synthesized as a cathode material for the microbial electrosynthesis of acetate from CO 2 . Dense and electroactive Sporomusa ovata biofilms form on the surface of reduced graphene oxide-coated copper foam electrodes while only scattered and damaged cells cover uncoated copper electrodes. Besides the formation of metabolically-active biofilms, acetate production rate from CO 2 is 21.3 and 43.5-fold higher with this novel composite cathode compared with an uncoated copper foam cathode and a reversed cathode made of reduced graphene oxide foam coated with copper, respectively. The results demonstrate that reduced graphene oxide can be employed as a biocompatible and conductive buffer between microbes and bactericidal electrode materials with excellent electrochemical property to enable highly performant microbial electrosynthesis.

Biogas technology is one of the widely applied anaerobic digestion approaches to harvest methane from different wastes. Recently, methane loss from biogas plants and its environmental and economic consequences have been underlined, but not thoroughly researched. In this investigation, process related CH 4 loss from nine different commercially operating biogas upgrading plants such as water scrubber, amine, and membrane-based plants was examined. The result of the measurements showed an average of 0.81% methane loss with respect to supplied methane to the upgrading plants. A methane loss up to 1.97% was detected in water scrubber methane upgrading technology and up to 0.56% loss from membrane technology, while 0.04% methane loss was detected in amine based upgrading, thus the water scrubber has shown the most detrimental effect as regards methane loss. The regenerative thermal oxidizer was further applied to reduce CH 4 emission by 99.5% of the amount of CH 4 in the waste gas from the upgrading unit, which ensures the sustainable process of biogas production.

Straw is one of the most available agriculture waste materials to be utilized as resource. Straw gasification for syngas production is one of the alternatives for resource recovery. Nevertheless, straw gasification might be problematic due to high alkali content, which may cause fouling of the gasifier reactor. In the present study, straw was gasified at different temperatures; in particular 750 °C, 800 °C, 850 °C, 900 °C, and 950 °C furnace temperature, was evaluated and then compared with wood feedstock using allothermal gasifier. The tested temperatures are not the limiting factor for the wood gasification using olivine (Mg2+,Fe2+)2SiO4 bed material and catalysis. Relatively, lower temperature is appropriate for straw gasification where agglomeration with bed material was observed at a higher temperature.

Biogas technology is one of the widely applied anaerobic digestion approaches to harvest methane from different wastes such as wastewater treatment sludge, agriculture residue, and other biomasses. Biogas can be further purified utilizing biogas upgrading technology so it can be applied as biomethane in vehicles and for gas grid injection. Recently, methane loss from biogas plants and its environmental and economic consequences have been underlined, but not thoroughly researched. In this investigation, process related CH4 loss from nine different commercially operating biogas upgrading plants such aswater scrubber, amine, and membrane-based plants was examined. The result of the measurements showed an average of 0.7% methane loss with respect to supplied methane to the upgrading plants. A methane loss up to 1.7% has been detected in water scrubber methane upgrading technology, while 0.037 % methane loss was detected in amine based upgrading, thus the water scrubber has shown the most detrimental effect as regards methane loss. Finally, the regenerative thermal oxidizer was further applied to reduce CH4 emission by 99.5 % of the amount of CH4 in the waste gas from the upgrading unit.

Biogas upgrading technologies have received widespread attention recently and are researched extensively. Microbial biogas upgrading (biomethanation) relies on the microbial performance in enriched H2 and CO2 environments. In this review, recent developments and applications of CH4 enrichment in microbial methanation processes are systematically reviewed. During biological methanation, either H2 can be injected directly inside the anaerobic digester to enrich CH4 by a consortium of mixed microbial species or H2 can be injected into a separate bioreactor, where CO2 contained in biogas is coupled with H2 and converted to CH4, or a combination hereof. The available microbial technologies based on hydrogen-mediated CH4 enrichment, in particular ex-situ, in-situ and bioelectrochemical, are compared and discussed. Moreover, gas-liquid mass transfer limitations, and dynamics of bacteria-archaea interactions shift after H2 injection are thoroughly discussed. Finally, the summary of existing demonstration, pilot plants and commercial CH4 enrichment plants based on microbial biomethanation are critically reviewed.

The applicability of microbial electrosynthesis (MES) for chemical synthesis from carbon dioxide (CO2) requires improved production and energetic efficiencies. Microbial catalysts, electrode materials, and reactor design are the key components which influence the functioning of such processes. In particular, cathode materials critically impact the electricity-driven CO2 reduction process by microorganisms. Interest in cathode surface modifications for improving MES processes is thus consistently increasing. In this paper, the recent developments and spatial modification of cathode materials for microbial CO2 reduction are systematically reviewed. The characteristics of commercially available materials, their modifications, and developments in new materials that have been used as cathodes for MES are summarized. Key cathode–microorganism interactions that led to improved CO2 conversion are then discussed. The cathode surface modification approaches have focused mainly on improving the surface area and surface chemistry of the materials. Although the modified cathode surfaces improved biofilm growth in direct electron uptake based bioconversions, they have achieved lower acetate production rates than that of hydrogen-based MES processes thus far. Research efforts on different materials suggest that the three-dimensional cathodes that can retain more biomass, in particular in hydrogen-based bioconversions, are promising for further improvements in production efficiencies. Further efforts toward reducing the energy inputs for achieving energetically efficient MES processes by using electrocatalytically efficient cathodes are needed.

Plantlets of Pfaffia glomerata (Spreng.) were exposed for 28 days to three different metal/metalloid (Hg, Pb and As) with different levels (Hg 1; As 25, 50, 100 and Pb 100 and 400 μM) to analyze the possible phytochelatin initiation and affects on growth and photosynthetic pigments vis-à-vis metal accumulation potential of plants. The plantlets showed significant Hg, As and Pb accumulation in roots (150, 1267.67 and 2129 μg g(-1) DW respectively); however, a low root to shoot metal translocation was observed. It was interesting to note that all tested macronutrient (Mg, K, Ca) was higher in shoots and just opposite in case of micronutrients (Cu, Fe, Zn), was recorded highest in roots. The growth of plantlets (analyzed in terms of length and dry weight) was negatively affected by various metal treatments. In addition, the level of photosynthetic pigments alters significantly in response to all metal/metalloid treatment. In response to all tested metal/metalloids in plants only As induced phytochelatins (PC2, PC3 and PC4) in roots, and in shoots, GSH was observed in all tested metal/metalloids. In conclusion, P. glomerata plantlets could not cooperatively induce phytochelatins under any of Hg and Pb levels.

Sporomusa ovata DSM-2662 produces high rate of acetate during microbial electrosynthesis (MES) by reducing CO 2 with electrons coming from a cathode. Here, we investigated other Sporomusa for MES with cathode potential set at −690 mV vs SHE to establish if this capacity is conserved among this genus and to identify more performant strains. S. ovata DSM-2663 produced acetate 1.8-fold faster than S. ovata DSM-2662. On the contrary, S. ovata DSM-3300 was 2.7-fold slower whereas Sporomusa aerivorans had no MES activity. These results indicate that MES performance varies among Sporomusa . During MES, electron transfer from cathode to microbes often occurs via H 2 . To establish if efficient coupling between H 2 oxidation and CO 2 reduction may explain why specific acetogens are more productive MES catalysts, the metabolisms of the investigated Sporomusa were characterized under H 2 :CO 2 . Results suggest that other phenotypic traits besides the capacity to oxidize H 2 efficiently are involved.

Sulfate-reducing bacteria (SRB) developed biocathodes efficient for reduction of acetic and butyric acids to alcohols and acetone via direct electron transfer reaching current densities of 160-210 A m(-2).

Biomethane is one of the renewable fuels widely utilized for various purposes, including transportation, heating, and power production. Growing interest is currently seen in this biofuel production utilizing waste material such as agricultural residual, sludge and municipal solid waste in anaerobic digestion. However, there is a limitation in using entire organic biomass in anaerobic digestion due to lignocellulose mass and fibrous material, which is hard to digest by microbes. Thus, gasification could be a supplementary technology to utilize residual, digested, and contaminated waste. The coupling of gasification and anaerobic digestion has been tested in lab scale reactor to valorize the waste as biofuel. However, techno-economic evaluation is essential to upscale the technology. Thus, this master's thesis aimed to perform the techno-economic of coupling gasifier and anaerobic digestion to utilize syngas for biomethane production. Three potential scenarios were evaluated for their techno-economic viability. Scenario 1 is based on biomethane generation through syngas fermentation coupled with gasification. Combination of hydrothermal gasification, syngas fermentation, and a steam addition as a hydrogen source make up Scenario 2. In scenario 3, an electrolytic unit was added instead of steam (in scenario 2) as a hydrogen source to produce biomethane. The scenarios' efficiencies range from 13% to 35%. The maximum energy efficiency, 35%, was found in Scenario 3. Furthermore, scenario 1 (237 NOK per litre) was followed by scenario 2 (164 NOK per litre) and scenario 3 (120 NOK per litre) as the scenarios in which the minimum selling price of biomethane decreased. Due to its undiscounted net present value of 76 million NOK compared to the other scenarios, scenario 3 is the most profitable based on the discounted cash flow analysis results. According to sensitivity analysis, the cost of labour and utility had the most impact on the minimum selling price of biomethane.