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The biotechnological production of fuel and chemicals from renewable, organic carbon-rich substrates offers a sustainable way to meet the increasing demand for energy.

This study presents an integration of acidogenesis (dark-fermentation) and methanogenesis for green hythane/biohythane production from food waste in two stages (S–I and S-II) and phases (P–I and P-II) of operational variations. The regulatory influence of biocatalyst and redox environment on anaerobic fermentation was evaluated through a rapid protocol in the context of biogas up-gradation with reference to bio-hydrogen (bio-H2), biomethane (CH4), bio-hythane (H2+CH4) and their composition (H2/(H2+CH4)) as major markers. Bioreactors with two different parent cultures (heat-shock pretreated and untreated) were operated at pH 6 and 7 in two phases to overcome the impediment of single-phase operation aiming for maximum energy recovery from the untreated substrate of P–I. Integration of S–I with S-II was beneficial to achieve 1.22 times higher cumulative bio-hythane production (4.25 L) compared to S–I (3.47 L) condition alone. The bio-hythane composition mimics the H2 enriched CNG (H-CNG) and showed the potential to be implemented for biogas up-gradation as a tool.

Production of low carbon biofuels and biochemicals from renewable feedstocks using a biological process is considered a sustainable alternative to the fossil-fuel-based linear economy. This study demonstrated the pilot-scale (10 m3) production of biohydrogen (bio-H2) and volatile fatty acids/carboxylic acids (VFA) through acidogenic fermentation (AF) using food waste (FW) as renewable feedstock. Bio-H2 production of 54 288 L (155.10 LH2 per kg COD) along with 25.77 g L−1 of VFA composed of acetic (HAc: 15.2 ± 1.98 g L−1), propionic (HPr: 4.89 ± 1.26 g L−1) and butyric (HBu 5.67 ± 0.96 g L−1) acids was achieved within 48 h of the fermentation period. The further acidogenic process was integrated with a biorefinery platform (methanogenesis + photosynthesis) in a circular loop strategy which helped derive multiple biobased products (CH4, algal biomass, O2 and treated water for reuse) from fatty acid-rich acidogenic effluent and untreated COD of AF. The whole bio-manufacturing unit (acidogenesis + methanogenesis + photosynthesis) converted the renewable feedstock (waste/wastewater) into fuels and platform chemicals, in analogy to a conventional oil refinery with a maximized resource recovery. The life cycle assessment (LCA) tool was employed to study the environmental impact of both the bio-H2 (standalone, ST) and waste biorefinery (WB) processes, and the results depicted that the WB approach offered a relatively low impact (approx. 3.5 fold less than ST). The approach helped determine the flow of carbon and its conversion to products; this aided the reduction of carbon emissions as well as minimized the burden on natural resources with the biosynthesis of green H2 and other value-added products, addressing carbon neutrality with bioeconomy.

Addressing the carbon emissions through microbial mediated fermentation is an emerging interest. Custom designed and fabricated gas fermentation (GF) systems were evaluated to optimize the headspace pressure, pH (6.5, 7.5, and 8.5), fermentation time, and substrate concentration by employing enriched homoacetogenic chemolithoautotrophs in non-genetic approach. Headspace pressure showed marked influence on the metabolic conversion of inorganic carbon to acetic and butyric acids with 26% higher productivity than the control (atmospheric pressure). Maximum volatile fatty acid (VFA) yield of 3.7 g/L was observed at alkaline pH (8.5) under 2 bar pressure at carbon load of 10 g/L, 96 h). Acetic (3.0 g/L) and butyric (0.7 g/L) acids were the major products upon conversion of 85% of the inorganic substrate. A better in-situ buffering (β = 0.048) at pH 8.5 along with higher reductive current (RCC: -4.4 mA) depicted better performance of GF towards CO

Sustainable production of fuels and chemicals is the most important way to reduce the carbon footprint in the environment. Forest based abundant lignocellulosic biomass as a renewable feedstock can be an attractive source of biofuels and biochemicals. This study evaluated the production of hydrogen (H2) along with platform chemicals from an organosol pretreated birch sawdust (SD). Acidogenic fermentation (AF) of pretreated SD resulted in production of green H2 (121.4 mL/gVS) along with short (17.8 g/L) and medium (2.64 g/L) chain carboxylic acids. Further integration of AF with anaerobic digestion (AD) in a biorefinery framework offered production of biomethane (bioCH4: 246 mL/gVS) from the leftover SD from AF. Integration of bioH2 with bioCH4 at different time interval of digestion showed 8–14 L biohythane formation ran with a H2 fraction of 1.6–0.3 H2/(H2 + CH4) documenting energy content of 8–9.08 kJ/gVS.

Electromethanogenesis (EM) process was specifically designed to increase the conversion efficiency of organic substrates towards methane production and was further evaluated towards biogas upgradation applications. Four single-chambered EM systems were operated using designed synthetic wastewater (carbon source: 10 g L−1) to comparatively analyze the effect of applied voltage (AV −0.8 V; CC 100 Ω) and electrode placement on methanogenesis. The experiments were performed in three phases: the biomethanation phase (BM) (without electrodes assembly), electrode phase (MESOC; with electrodes assembly in an open circuit operation) and EM phase (MESCC/AV; electrodes assembly in a closed circuit or applied voltage operation). Electrode placement with a submerged anode and variations in the position of the cathode [partially air-exposed cathode; AV (R1/MESAV) and CC (R2/MESCC)] and completely submerged cathode [AV (R3/IE-MESAV) and CC (R4/IE-MESCC)] were evaluated. The results for the EM phase showed higher total biogas formation in R3/IE-MESAV (1.12 L), followed by R1/MESAV (0.91 L), R4/IE-MESCC (0.88 L) and R2/MESCC (0.59 L). Higher methane content was observed in R3/IE-MESAV (0.88 L) with a conversion efficiency of 76%, followed by R1/MESAV (0.66 L; 60%), R4/IE-MESCC (0.60 L; 56%) and R2/MESCC (0.37 L; 34%). EM operation depicted higher CH4 fraction with the co-generation of lower fractions of H2 and CO2 [R3/IE-MESAV (CH4-78%; H2-5%; CO2-17%), R1/MESAV (CH4-73%; H2-3%; CO2-22%), R4/IE-MESCC (CH4-68%; H2-5%; CO2-28%), R2/MESCC (CH4-63%; H2-4%; CO2-34%)] compared to control operation (CH4-61%; H2-4%; CO2-33%). The polarized microenvironment provided during EM (CC/AV) significantly contributed to biogas upgradation with a simultaneous CO2 reduction as compared to the BM and MESOC phases. Electrochemical characterization showed a high regulatory influence of the electron flux in R3/IE-MESAV, which helped to increase CO2 conversion/utilization compared to other conditions. EM influenced direct interspecies electron transfer (DIET), creating a microbe–electrode synergy leading to neutralize the disruptors influence (from side reactions) on final product formation. This study provides a specific strategy for increasing the overall methane yields with the influence of in situ/ex situ potential, while overcoming the limitations of methanogenesis.

As the circular bioeconomy is gaining momentum around the globe, biorefinery will play a key role in the framework of sustainability as integral component. A self-sustainable biorefinery model using azolla cultivation was designed at the focal point of a circular loop. This model is aimed to enable a cascading valorization of bio-based products through a series of sequentially integrated biological/thermal unit operations. Azolla pinnata with an inherent ability to sequester CO2 along with simultaneous nitrogen fixation capability will serves as a potential and renewable carbon resource (feedstock) for the integrated unit operations viz., acidogenesis, photosynthesis, hydrolysis, and pyrolysis. The cascading loop is initiated by acidogenesis of spent wash (distillery wastewater-DSW) in a semi pilot scale bioreactor for the production of biohydrogen and volatile fatty acids (VFA). Treated spent wash after acidogenesis (TSW) was used as feed for Azolla pinnata cultivation, which eventually depicted good COD and nitrates removal efficiency. Harvested A. pinnata biomass (AB) showed presence of good amount of carbohydrates (237 mg/g) and proteins (160 mg/g) along with 11% of lipids (composed of 22% omega fatty acids) thus making it a good nutritional feed for livestock. AB after lipid extraction was subjected to mild acid-hydrolysis and the hydrolysate was used as substrate for acidogenesis to facilitate circular/closed loop operation. The techno-economics of biorefinery process evaluated using SuperPro Designer software depicted the feasibility of the integrated strategy. This study extends the scope of biobased platform with self-sustainability as core objective.

The single bioprocess approach has certain limitations in terms of process efficiency, product synthesis, and effective resource utilization. Integrated or combined bioprocessing maximizes resource recovery and creates a novel platform to establish sustainable biorefineries. Anaerobic fermentation (AF) is a well-established process for the transformation of organic waste into biogas; conversely, biogas CO2 separation is a challenging and expensive process. Biological fixation of CO2 for succinic acid (SA) mitigates CO2 separation issues and produces commercially important renewable chemicals. Additionally, utilizing digestate rich in volatile fatty acid (VFA) to produce medium-chain fatty acids (MCFAs) creates a novel integrated platform by utilizing residual organic metabolites. The present review encapsulates the advantages and limitations of AF along with biogas CO2 fixation for SA and digestate rich in VFA utilization for MCFA in a closed-loop approach. Biomethane and biohydrogen processes CO2 utilization for SA production is cohesively deliberated along with the role of biohydrogen as an alternative reducing agent to augment SA yields. Similarly, MCFA production using VFA as a substrate and functional role of electron donors namely ethanol, lactate, and hydrogen are comprehensively discussed. A road map to establish the fermentative biorefinery approach in the framework of AF integrated sustainable bioprocess development is deliberated along with limitations and factors influencing for techno-economic analysis. The discussed integrated approach significantly contributes to promote the circular bioeconomy by establishing carbon-neutral processes in accord with sustainable development goals.

Over the past few decades, access to abundant and easily available fossil sources aided in remarkable advancements in technologies and rapid urbanization. The unparalleled growth in population and migration of rural residents to the urban areas have transformed cities to hotspots for major anthropogenic activities, seminal contact points between natural and socio-economic activities, and areas responsible for generation of more than 80% of gross domestic product (GDP). This has placed cities at a crossroad of facing environmental grand challenges posed by population expansion, climate change, and depleting resources. Ushered by these problems, cities are directing the course of waste utilization towards sustainable production and consumption strategies by adopting the principles of circular economy (CE). This organic waste generated in huge quantities can be considered as a valuable resource in the urban biocycles. The present article highlights the importance of looping back this biogenic organic waste (domestic) to a certain extent in a decentralized way. It offers an eco-centric view to adopt circular urban food practices by constructing and operating a keyhole garden (KHG) as a case study. This approach works in synergy with nature and adopts the principles of biomimicry and cradle to cradle, wherein waste is reintroduced into the system as input resources or is metamorphosed to valuable materials. It underlines the role of CE approaches to spark this transformation and strategically direct the flow of valuable domestic waste resources to close the urban nutrient loops in a decentralized and regenerative approach. While the solutions to close the loop and recycle wastes address the end of pipe problems, the community-based innovation strategies aim at building resilience, producing food more locally, limiting the use of fertilizers, conserving water, and also combating climate change. Graphical Abstract

Global urbanization has resulted in amplified energy and material consumption with simultaneous waste generation. Current energy demand is mostly fulfilled by finite fossil reserves, which has critical impact on the environment and thus, there is a need for carbon-neutral energy. In this view, biohydrogen (bio-H2) is considered suitable due to its potential as a green and dependable carbon-neutral energy source in the emerging 'Hydrogen Economy'. Bio-H2 production by dark fermentation of biowaste/biomass/wastewater is gaining significant attention. However, bio-H2production still holds critical challenges towards scale-up with reference to process limitations and economic viabilities. This review illustrates the status of dark-fermentation process in the context of process sustainability and achieving commercial success. The review also provides an insight on various process integrations for maximum resource recovery including closed loop biorefinery approach towards the accomplishment of carbon neutral H2 production.

Rising environmental concerns and the imminent depletion of fossil resources have sparked a strong interest towards the production of renewable energy such as biomethane. Inclusion of alternative feedstock’s such as lignocellulosic biomass could further expand the production of biomethane. The present study evaluated the potential of a novel hybrid organosolv-steam explosion fractionation for delivering highly digestible pretreated solids from birch and spruce woodchips. The highest methane production yield was 176.5 mLCH4 gVS−1 for spruce and 327.2 mL CH4 gVS−1 for birch. High methane production rates of 1.0–6.3 mL min−1 (spruce) and 6.0–35.5 mL min−1 (birch) were obtained, leading to a rapid digestion, with 92% of total methane from spruce being generated in 80 h and 95% of that from birch in 120 h. These results demonstrate the elevated potential of the novel method to fractionate spruce and birch biomass and deliver cellulose-rich pretreated solids with superior digestibility.

Experiments are designed and conducted in two stages where the semi-pilot scale acidogenic bioreactor is primarily operated with food waste (40 g COD/L). Cumulative bio-hydrogen production (CHP) of 59 L and volatile fatty acid (VFA) of 6.1 g/L is observed during the 5th cycle at 48th h of the acidogenic process. Secondly, VFA rich acidogenic outlet (AO) is studied for its real field application as phosphate solubilizing agent (PSA) in the soil for the cultivation of Lagenaria sicerari (bottle guard-BG). Regular addition of varied concentration of AO (Zone A- Control, Zone B- 1%, Zone C- 3%, Zone D- 6%, Zone E- 10% and Zone F- 15%) in the soil resulted in phosphate solubility. Relatively higher phosphate availability is observed in Zone-D (729 mg/g) on 148th day followed by Zone-F (551 mg/g), Zone-E (543 mg/g), Zone-C (450 mg/g), Zone-B (357 mg/g) and Zone-A (155 mg/g). Plant growth is evaluated on a regular basis which depicted that the maximum number of BG were harvested from the plants growing in Zone- D (16) followed by Zone-F (13), Zone-C (12), Zone-E (11), Zone-B (9) and Zone-A (5). Application of VFA rich AO illustrated the beneficial effect on phosphate solubility in soil and uptake by plants, thus the proposed formulation could be considered as an alternative PSA.

Metabolic potential of two different cultures, facultative (FB) and strict anaerobes (AB) under two microenvironments [anoxic (ANOX) and anaerobic (ANA)] was evaluated to understand acidogenic fermentation in a self-induced electrofermentation (EF) system for the production of short-chain fatty acids (SCFA: C2-C4) and biogas. ANA condition positively influenced FB and AB metabolism towards higher acetic (C2:2390 mg/L) and propionic acid (C3: 717 mg/L) production, while butyric acid (C4:1481 mg/L) favored ANOX microenvironment (AB). ANOX microenvironment showed a better self-induced potential compared to ANA metabolism (0.46 V (FBANOX); 0.45 V (ABANOX)). An improved H2 (>30%) fraction was noticed with FB while CH4 production was found favourable with AB. The study illustrated the role of system microenvironment in association with metabolic function towards regulating electrofermentation towards specific products synthesis.

Electrofermentation (EF) was used to overcome the thermodynamic limitations of conventional microbial fermentation and regulate the metabolism towards specific biobased products synthesis. Three single chambered EF systems (Applied potential (AP: −0.6 V), Closed circuit (CC: 100 Ω), Control (C)) were individually operated using composite canteen based food waste (FW) (COD-10 g/l) to comparatively analyze the effect of electrical stimulation on enhancing rate of fermentation. The total volatile fatty acids (VFA) showed higher production in AP (4595 mg/l) when compared to CC (3593 mg/l) and C (2666 mg/l). Fatty acid profiles (C2–C4) with individual compositions showed higher amount of acetic acid (C2: HAc) followed by butyric acid (C4: HBu) and propionic acid (C3: HPr). AP showed higher amount of C2: HAc (3221 mg/l) followed by C4: HBu (900 mg/l) and C3: HPr (474 mg/l), while CC showed higher C2: HAc (2332 mg/l) followed by C4: HBu (1233 mg/l) and C3: HPr (388 mg/l) when compared to C which showed higher C2: HAc (1422 mg/l) followed by C4-HBu (655 mg/l) and C3-HPr (589 mg/l). Co-generation of biogas was higher in AP [bioH2 (26%) and bioCH4 (4%)] followed by CC [bioH2 (22%) and bioCH4 (1.3%)] and C [bioH2 (11%) and bioCH4 (6%)]. Fermentation induced with electron flux mechanism depicted the ability to elongate the chain length. Electrochemical characterization showed distinct disparity in electron transfer rates (Kapp) along with waste remediation. The inferences concluded the key role of electrical stimulation on fermentation to enhance the conversion of conventional substrates to targeted carboxylic acids. Biohythane, an alternate renewable biofuel was also produced when bioH2 and bioCH4 are mixed in appropriate ratios. EF is an emerging technology that could open new frontiers in waste biorefinery by integrating bioprocesses for platform chemicals production through sustainable microbial electrochemical technologies.

Enormous quantity of food waste (FW) is becoming a global concern. To address this persistent problem, sustainable interventions with green technologies are essential. FW can be used as potential feedstock in biological processes for the generation of various biobased products along with its remediation. Enabling bioprocesses like acidogenesis, fermentation, methanogenesis, solventogenesis, photosynthesis, oleaginous process, bio-electrogenesis, etc., that yields various products like biofuels, platform chemicals, bioelectricity, biomaterial, biofertilizers, animal feed, etc can be utilized for FW valorisation. Integrating these bioprocesses further enhances the process efficiency and resource recovery sustainably. Adapting biorefinery strategy with integrated approach can lead to the development of circular bioeconomy. The present review highlights the various enabling bioprocesses that can be employed for the generation of energy and various commodity chemicals in an integrated approach addressing sustainability. The waste biorefinery approach for FW needs optimization of the cascade of the individual bioprocesses for the transformation of linear economy to circular bioeconomy.

H2 partial pressure drives the reduction of carboxylic acid (short chain fatty acids) formed as primary metabolites in acidogenic fermentation to form bioalcohols. Microbial catalysis under the influence of H2 partial pressure was evaluated in comparison with a reactor operated at atmospheric pressure under identical conditions. Carboxylic acid reduction gets regulated selectively by the influence of elevated pressures and redox conditions, resulting in the formation of alcohols. The non-equilibrium of the intra and extracellular H2 ions causes the anaerobic bacteria to alter their pathways as a function of interspecies H2 transfer. Ethanol production was quantified, as acetic acid was the major carboxylic acid synthesised during acidogenesis. H2 pressure influenced the electrochemical activity which was reflected in the distinct variation of the electron transfer rates and the catalytic activity of redox mediators (NAD+/NADH, flavoproteins and iron-sulphur clusters). The bioprocess depicted in this communication depicted a non-genetic regulation of product formation, understanding the acidogenic metabolism and alternate route for alcohol production.

This two-phase, two-stage study analyzed production of biohydrogen and volatile fatty acids by acidogenic fermentation of brewery spent grains. Phase-1 served to optimize the effect of pH (4–10) on acidogenic fermentation; whereas phase-2 validated the optimized conditions by scaling up the process to 2 L, 5 L, and 10 L. Alkaline conditions (pH 9) yielded excellent cumulative H2 production (834 mL) and volatile fatty acid recovery (8936 mg/L) in phase-1. Extended fermentation time (from 5 to 10 days) upgraded the accumulated short-chain fatty acids (C2–C4) to medium-chain fatty acids (C5–C6). Enrichment for acidogens in modified mixed culture improved fatty acid production; while their consumption by methanogens in unmodified culture led to methane formation. Increased CH4 but decreased H2 content enabled biohythane generation. Scaling up confirmed the role of pH and culture type in production of renewable fuels and platform molecules from brewery spent grains.

Anode with good electrocatalytic capabilities is more specifically required to reduce the ohimic losses during microbial fuel cell (MFC) operation. Highly conductive polymers viz., Polyaniline (PANi) and Polyaniline/Carbon nanotube (PANi/CNT) composite were prepared by in situ oxidative chemical polymerization method. Anodes were fabricated independently by coating PANi and CNT/PANi composites on the surface of SSM. The fabricated electrodes were evaluated as anode against stainless steel mess (SSM) as cathode during MFC operation. Maximum bioelectricity generation was observed in SSM-PANi/CNT-anode with power density of 48 mW/m2 and COD removal efficiency of 80% compared with SSM-PANi-anode (38 mW/m2; 65%) and SSM-anode (28 mW/m2; 58%). Bioelectrochemical characterization of the electrode materials using cyclic voltammetry and electrochemical impedance spectroscopy showed high electrocatalytic activity of PANi/CNT composite electrode. The study concluded the efficiency of PANi/CNT composite electrodes as bioanode in operation of MFCs towards achieving increased bioelectricity production along with wastewater treatment.

Microbial electrochemical systems represent an innovative platform technology with multifaceted applications in the context of sustainability paving ways towards resource recovery. The chapter gives an overview on the electrofermentation (EF), one of the versatile applications of microbial electrochemical technologies (MET) towards valorization of biofuel and platform chemicals by regulating the electrometabolic rates. The regulation of the rate kinetics by placement of electrodes in a hybrid microbial environment induces electrocatalysis toward specific products formation. The electron acceptor (in the form of electrode) and applied potential eliminates the redox constraints and drives unbalanced reactions for specificity.

Biohydrogen (H2) production from renewable wastewater signifies a sustainable alternative to the fossil fuel-based economy. H2 is deemed to be the futuristic energy carrier with high-energy yield. This communication specifically addresses H2 production through wastewater remediation. A comprehensive emphasis was made on the fermentative routes, viz., light-driven and light-independent processes, encompassing wastewater and mixed culture application as the focal point. Important process parameters that have a critical role on the process efficiency were elaborated. Some of the persistent limitations encountered during the process operation were delineated. Stress was made on the pertinent discussion on biocatalyst pretreatment methods in the context of mixed culture wastewater usage. Reported strategies to overcome the limitations were discussed to make the process economically viable, especially for large-scale applications.

Short chain carboxylic (volatile fatty) acid (VFA) production in mixed microbiomes is majorly limited by the prevalence of methanogenic bacteria and the availability of substrate from waste to the biocatalyst during the fermentation process. To enhance the VFA production from food waste, the present study evaluates a strategy for selective enrichment of the biocatalyst by exposing it to acid-shock, followed by operation under alkaline conditions (pH 10). A comprehensive system based analysis was carried out during the bio-based platform chemical synthesis from waste, in conjugation with microbial profiling and bio-electrochemical analysis. After the selective enrichment of the biocatalyst, enhanced VFA synthesis was conducted with pretreated biocatalyst (PT; 11.1 g L−1) and compared with untreated parent biocatalyst (UT; 6.1 g L−​1). In both systems biohydrogen was the co-product. Variations in the VFA profiles were documented with respect to the biocatalyst used, which influenced the degree of acidification (DOA – PT: 37% and UT: 11%). A high fraction of acetic acid (6.9 g L−1) was observed, followed by butyric acid (2.6 g L−1) and propionic acid (1.3 g L−1) in PT operation, contrary to the control system (acetic acid, 3.9 g L−1, butyric acid, 1.6 g L−1; propionic acid, 0.9 g L−1). Specifically, the PT system showed the biosynthesis of iso-valeric acid: 0.15 g L−1 (C5) and caproic acid: 1.9 g L−1 (C6), which indicates the possibility for chain-elongation through the selective enrichment of the microbial community. The PT system showed Epeak at −0.415 V on the cyclic voltammogram, which corresponds to the involvement of the redox couple, H+/H2, correlating with the enhanced acidogenic process, unlike UT. Tailoring of the parent inoculum (pretreatment) resulted in the enrichment and enhancement of the capabilities of the biocatalyst in secreting the redox mediators, which were not detected in the UT system. Acidogenic firmicutes (spore formers) and fatty acid producing bacteroides were enriched in the PT system along with saccharolytic and proteolytic bacteria (Bacillus cellulosilyticus (alkalophile), Soehngenia saccharolytica, etc.). The presence of Clostridium autoethanogenum and Propionibacterium freudenreichii in the PT system supports effective utilization of complex carbohydrates, facilitating acidification.

Fermentation experiments were designed to elucidate the functional role of the redox microenvironment on volatile fatty acid (VFA, short chain carboxylic acid) production and co-generation of biohydrogen (H2). Higher VFA productivity was observed at pH 10 operation (6.3g/l) followed by pH 9, pH 6, pH 5, pH 7, pH 8 and pH 11 (3.5 g/l). High degree of acidification, good system buffering capacity along with co-generation of higher H2 production from food waste was also noticed at alkaline condition. Experiments illustrated the role of initial pH on carboxylic acids synthesis. Alkaline redox conditions assist solubilization of carbohydrates, protein and fats and also suppress the growth of methanogens. Among the carboxylic acids, acetate fraction was higher at alkaline condition than corresponding neutral or acidic operations. Integrated process of VFA production from waste with co-generation of H2 can be considered as a green and sustainable platform for value-addition.

In recent times, we are witnessing an exigent need to switch towards employing renewable feedstocks owing to the continuous depletion of fossils as a result of various anthropogenic activities. Abundant and low-cost feedstocks like food wastes (FW) are highly sought after these conditions as an alternative feedstock. According to Food and Agriculture Organization (FAO), nearly 1.3 billion tons of FW is being discarded from various sources including food processing plants, kitchens, restaurants, etc., which contain more than 50% of biodegradable organic content that can be transmuted into high-value biobased products. In the near future, unavoidable FW quantities are bound to increase, making it highly essential to establish sustainable treatment strategies by employing bioprocesses like acidogenesis, photosynthesis, bioelectrogenesis, and bioanoxygenesis. This chapter reviews the biotechnological aspects of converting FW into an array of biobased products elucidating the metabolic pathways and subsequently proposing a biorefinery framework to hollistically utilize waste and develop a sustainable green future.

Clostridium sporogenes BE01, a non-acetone forming butanol producer, can produce hydrogen and volatile fatty acids (VFAs) during butanol fermentation from rice straw hydrolysate. Bio-electrochemical analysis revealed the changes that occurred in the redox microenvironment and electron transport mediators during fermentation at different pH and CaCO3 concentrations. CaCO3 played a very important role in enhancing the production of hydrogen, volatile fatty acids and solvents by stimulating the changes in the electron transport system. The electron transport system mediated by NAD/NADH, flavins, Fe–S clusters, protein bound FAD, and cytochrome complex in C. sporogenes BE01 was analysed by cyclic voltammetry (CV). Electrokinetic analysis revealed that the favorability for redox reactions increased with an increase in pH, and the polarization resistance reduced significantly with CaCO3 supplementation.

The effect of stacking multiple microbial fuel cells for stable power output was evaluated in continuous mode operation. Three single chambered air cathode CMFCs with Nafion (CMFCN), Terry cotton (CMFCT) as membranes and one without membrane (CMFCML) were operated in continuous mode. Maximum power density (PD) and COD removal efficiency was obtained for CMFCN (0.1 W/m2, 50%) followed by CMFCML (0.062 W/m2, 47%) and CMFCT (0.025 W/m2, 39%) and were stable throughout the operation. To increase the power output further, stacking of CMFCs was carried in series/parallel circuitry, which yielded high power density in parallel (2.0 W/m2; 7.2 W/m3) and high voltage in series (1.1 V). Study also evidenced that stacking resulted in high and stable bioelectricity by minimizing the electron losses in comparison to individual CMFCs operation. Stable and high power output signifies the impact of continuous mode operation that constantlty replenishes the substrate.

The efficiency of bioaugmentation strategy for enhancing biohydrogenesis at elevated organic load was successfully evaluated by augmenting native acidogenic microflora with three acidogenic bacterial isolates viz., Bacillus subtilis, Pseudomonas stutzeri and Lysinibacillus fusiformis related to phyla Firmicutes and Proteobacteria separately. Hydrogen production ceased at 50g COD/l operation due to feed-back inhibition. B. subtilis augmented system showed higher H2 production followed by L. fusiformis, P. stutzeri and control operations, indicating the efficacy of Firmicutes as bioaugmentation biocatalyst. Higher VFA production with acetic acid as a major fraction was specifically observed with B. subtilis augmented system. Shift in metabolic pathway towards acidogenesis favoured higher H2 production. FISH analysis confirmed survivability and persistence of augmented strains apart from improvement in process performance. Bio-electrochemical analysis depicted specific changes in the metabolic activity after augmentation which also facilitated enhanced electron transfer. P. stutzeri augmented system documented relatively higher COD removal.

The effect of heat-shock treatment to selectively enrich acidogenic, H2 producing consortia was investigated for inoculum preparation and to control the process operation. Long term operation (520 days) in suspended-batch mode bioreactors illustrated relative efficiency and feasibility of heat-shock treated consortia (15.78 mol/kg CODR) in enhancing H2 production (3.31 mol/kg CODR) when compared to parent (control) consortia. On the contrary, substrate degradation was higher in the control operation (ξCOD, 62.86%; substrate degradation rate (SDR), 1.34 kg CODR/m3-day) compared to heat-shock operation (ξCOD, 52.6%; SDR, 1.10 kg CODR/m3-day). Heat-shock pretreatment has resulted in a marked fermentation pathway shift towards acetic-butyric acid type production. The microbial diversity illustrated dominance in the Clostridia class after applying heat-shock pretreatment. The redox catalytic currents and Tafel analysis strongly support the conclusion of an improved biocatalyst performance after pretreatment with regards to H2 production.

The present account focuses on upscaling of biohydrogen (H 2 ) production at semi-pilot scale bioreactor using composite food waste. Experiments were conducted at different organic load (6, 12, 18, 30, 40, 50 and 66 g COD/l) conditions. H 2 production increased with an increasing organic load up to 50 g COD/l (9.67 l/h) followed by 40 g COD/l (6.48 l/h), 30 g COD/l (1.97 l/h), 18 g COD/l (0.90 l/h), 12 g COD/l (0.78 l/h) and 6 g COD/l (0.32 l/h). H 2 production was affected by acidification (pH drop to 3.96) at 66 g COD/l operation due to the excess accumulation of soluble metabolites (5696 mg VFA/l). Variation in organic load of food waste influenced the overall hydrogen production efficiency.

Application of pre-aeration (AS) to waste prior to feeding was evaluated on acidogenic process in a semi-pilot scale biosystem for the production of biobased products (biohydrogen, volatile fatty acids (VFA) and biohythane) from food waste. Oxygen assisted in pre-hydrolysis of waste along with the suppression of methanogenic activity resulting in enhanced acidogenic product formation. AS operation resulted in 97% improvement in hydrogen conversion efficiency (HCE) and 10% more VFA production than the control. Increasing the organic load (OL) of food waste in association with AS application improved the productivity. The application of AS also influenced concentration and composition of fatty acid. Highest fraction of acetic (5.3g/l), butyric (0.7g/l) and propionic acid (0.84g/l) was achieved at higher OL (100g COD/l) with good degree of acidification (DOA). AS strategy showed positive influence on biofuel (biohydrogen and biohythane) production along with the biosynthesis of short chain fatty acids functioning as a low-cost pretreatment strategy in a single stage bioprocess.

The emanations upon combustion of petroleum fuels cause grave pessimistic impact on surrounding ambiance and global climate change. Therefore, the contemporary stance of scientific fraternity is to generate energy and mercantile products through biological methods with waste as a resource. Biohydrogen production from wastewater seems to be a promising green option for sustainable renewable energy. The process is feasible from practical point of view and can be operated under ambient conditions. It has been attracting attention due to its applicability to different types of wastewaters, and the production costs of biohydrogen can compete economically with other traditional methods. However, the crucial challenges like enhancing rate and yield for sustainable biohydrogen production still persist. During fermentation process, the undissociated volatile fatty acids (VFAs) and alcohols accumulate in the system leading to inhibition and redundancy in substrate degradation. Employing integration strategies with other bioprocesses like photo-fermentation or bio-electrochemical systems is the sanguine option to make the process frugally possible.

Enormous quantity of waste/wastewater is being generated each day from domestic and industrial activities, which comprises a good biodegradable fraction with the inherent potential to generate Biobased products including bioenergy. Mitigating waste generation to permissible limits is an essential prerequisite to considerably negate its impact on the environment. This communication makes an attempt to delineate the development of a sustainable remediation strategy with dark-fermentative (acidogenesis) process as focal process coupled with value addition via biohydrogen and carboxylic acid production. To facilitate the commercial viability, the process is networked with biorefinery approach to establish waste remediation with resource recovery in the framework of bio-based economy.