Bio-electrolytic conversion of acidogenic effluents to biohydrogen: an integration strategy for higher substrate conversion and product recovery

Unraveling the effects and mechanisms of microplastics on anaerobic fermentation: Exploring microbial communities and metabolic pathways

To investigate the effects of microplastics (MPs) on hydrolysis, acidification and microbial characteristics during waste activated sludge (WAS) anaerobic fermentation process, five different kinds of MPs were added into the WAS fermentation system and results indicated that, compared to the control group, the addition of polyvinyl chloride (PVC)-MPs exhibited the least inhibition on volatile fatty acids (VFAs), reducing them by 13.49 %. Conversely, polyethylene (PE)-MPs resulted in the greatest inhibition, with a reduction of 29.57 %. MPs, while accelerated the dissolution of WAS that evidenced by an increase of lactate dehydrogenase (LDH) release, concurrently inhibited the activities of relevant hydrolytic enzymes (α-Glucosidase, protease). For microbial mechanisms, MPs addition affected the proliferation of key microorganisms (norank_f_Bacteroidetes_vadinHA17, Ottowia, and Propioniclava) and reduced the abundance of genes associated with hydrolysis and acidification (pfkb, gpmI, ilvE, and aces). Additionally, MPs decreased the levels of key hydrolytic and acidogenic enzymes to inhibit hydrolysis and acidification processes. This research provides a basis for understanding and unveils impact mechanisms of the impact of MPs on sludge anaerobic fermentation.

Biofuel production for circular bioeconomy: Present scenario and future scope

In recent years, biofuel production has attracted considerable attention, especially given the increasing worldwide demand for energy and emissions of greenhouse gases that threaten this planet. In this case, one possible solution is to convert biomass into green and sustainable biofuel, which can enhance the bioeconomy and contribute to sustainable economic development goals. Due to being in large quantities and containing high organic content, various biomass sources such as food waste, textile waste, microalgal waste, agricultural waste and sewage sludge have gained significant attention for biofuel production. Also, biofuel production technologies, including thermochemical processing, anaerobic digestion, fermentation and bioelectrochemical systems, have been extensively reported, which can achieve waste valorization through producing biofuels and re-utilizing wastes. Nevertheless, the commercial feasibility of biofuel production is still being determined, and it is unclear whether biofuel can compete equally with other existing fuels in the market. The concept of a circular economy in biofuel production can promote the environmentally friendly and sustainable valorization of biomass waste. This review comprehensively discusses the state-of-the-art production of biofuel from various biomass sources and the bioeconomy perspectives associated with it. Biofuel production is evaluated within the framework of the bioeconomy. Further perspectives on possible integration approaches to maximizing waste utilization for biofuel production are discussed, and what this could mean for the circular economy. More research related to pretreatment and machine learning of biofuel production should be conducted to optimize the biofuel production process, increase the biofuel yield and make the biofuel prices competitive.

Electrochemical promotion of organic waste fermentation: Research advances and prospects

The current methods of treating organic waste suffer from limited resource usage and low product value. Research and development of value-added products emerges as an unavoidable trend for future growth. Electro-fermentation (EF) is a technique employed to stimulate cell proliferation, expedite microbial metabolism, and enhance the production of value-added products by administering minute voltages or currents in the fermentation system. This method represents a novel research direction lying at the crossroads of electrochemistry and biology. This article documents the current progress of EF for a range of value-added products, including gaseous fuels, organic acids, and other organics. It also presents novel value-added products, such as 1,3-propanediol, 3-hydroxypropionic acid, succinic acid, acrylic acid, and lysine. The latest research trends suggest a focus on EF for cogeneration of value-added products, studying microbial community structure and electroactive bacteria, exploring electron transfer mechanisms in EF systems, developing effective methods for nutrient recovery of nitrogen and phosphorus, optimizing EF conditions, and utilizing biosensors and artificial neural networks in this area. In this paper, an analysis is conducted on the challenges that currently exist regarding the selection of conductive materials, optimization of electrode materials, and development of bioelectrochemical system (BES) coupling processes in EF systems. The aim is to provide a reference for the development of more efficient, advanced, and value-added EF technologies. Overall, this paper aims to provide references and ideas for the development of more efficient and advanced EF technology.

Regulation of hydraulic retention time on caproic acid production via two-phase anaerobic fermentation of Chinese cabbage waste with autopoietic electron donors

The effects of hydraulic retention time (HRT) on the performance of two-phase anaerobic fermentation for caproic acid production from Chinese cabbage waste (CCW) were investigated. In the electron donor phase, yeast was inoculated to achieve efficient autopoietic ethanol, providing electron donors for the chain elongation process. Shorter HRT led to drastic fluctuations in microorganisms, thus resulting in lower acid yields at HRT of 6 days. At HRT of 10 days, the balanced collaboration of various key bacteria avoided the accumulation of intermediate by-products, and the caproic acid production reached 4660 mg COD/L, which was 119.5% and 154.8% higher than that at HRTs of 6 and 14 days, respectively. At HRT of 14 days, the low ethanol loading rate resulted in ethanol excessive-oxidation to acetic acid. Acetic acid accounted for 41.5% of the total product, while the selectivity of caproic acid was only 15.3%. The main contributor to the production process of caproic acid was Caproiciproducens, while the Ruminalococcaceae also played a role in the process. This study provided a theoretical basis for the efficient production of caproic acid through continuous fermentation with autopoietic electron donors.

Prospects and trends in bioelectrochemical systems: Transitioning from CO2 towards a low-carbon circular bioeconomy

Resource scarcity and climate change are the most quested topics in view of environmental sustainability. CO2 sequestration through bioelectrochemical systems is an attractive option for fostering bioeconomy development upon several value-added products generation. This review details the state-of-the-art of bioelectrochemical systems for resource recovery from CO2 along with various biocatalysts capable of utilizing CO2. Two bioprocesses (photo-electrosynthesis and chemolithoelectrosynthesis) were discussed projecting their potential for biobased economy development from CO2. Significance of adopting circular strategies for efficient resource recycling, intensifying product value, integrations/interlinking of multiple process chains for the development of circular bioeconomy were discussed. Existing constrains as well as outlook for near establishment of circular bioeconomy from CO2 is presented by weighing fore-sighted plans with current actions. Need for developing CO2-based circular bioeconomy via innovative business models by analyzing social, technical, environmental and product related aspects are also discussed providing a roadmap of gaps to pursue for attaining practicality.

Microbial electrolysis cells for the production of biohydrogen in dark fermentation - A review

To assess biohydrogen for future green energy, this review revisited dark fermentation and microbial electrolysis cells (MECs). Hydrogen evolution rate in mesophilic dark fermentation is as high as 192 m³ H₂/m³-d, however hydrogen yield is limited. MECs are ideal for improving hydrogen yield from carboxylate accumulated from dark fermentation, whereas hydrogen production rate is too slow in MECs. Hence, improving anode kinetic is very important for realizing MEC biohydrogen. Intracellular electron transfer (IET) and extracellular electron transfer (EET) can limit current density in MECs, which is proportional to hydrogen evolution rate. EET does not limit current density once electrically conductive biofilms are formed on anodes, potentially producing 300 A/m². Hence, IET kinetics mainly govern current density in MECs. Among parameters associated with IET kinetic, population of anode-respiring bacteria in anode biofilms, biofilm density of active microorganisms, biofilm thickness, and alkalinity are critical for current density.

Dark fermentative volatile fatty acids production from food waste: A review of the potential central role in waste biorefineries

Volatile fatty acids (VFAs) are high-value chemicals that are increasingly demanded worldwide. Biological production via food waste (FW) dark fermentation (DF) is a promising option to achieve the sustainability and environmental benefits typical of biobased chemicals and concurrently manage large amounts of residues. DF has a great potential to play a central role in waste biorefineries due to its ability to hydrolyze and convert complex organic substrates into VFAs that can be used as building blocks for bioproducts, chemicals and fuels. Several challenges must be faced for full-scale implementation, including process optimization to achieve high and stable yields, the development of efficient techniques for selective recovery and the cost-effectiveness of the whole process. This review aims to critically discuss and statistically analyze the existing relationships between process performance and the main variables of concern. Moreover, opportunities, current challenges and perspectives of a FW-based and fermentation-centred biorefinery layout are discussed.

Nutrient limitation regulates the properties of extracellular electron transfer and hydraulic shear resistance of electroactive biofilm

Understanding the roles of nutrient restriction in extracellular electron transfer (EET) and stability of mixed electroactive biofilm is essential in pollutant degradation and bioenergy production. However, the relevant studies are still limited so far. Herein, the effect of nutrient restriction on the EET pathways and stability of mixed electroactive biofilm was explored. It was found that the electroactive Pseudomonas and Geobacter genera were selectively enriched in the biofilms cultured under total nutrient and P-constrained conditions, and two EET pathways including direct and indirect were found, while Rhodopseudomonas genus was enriched in the N-constrained biofilm, which only had the direct EET pathway. Moreover, multiple analyses including 2D confocal Raman spectra revealed that P-constrained biofilm was rich in extracellular polymeric substances (EPS) especially for polysaccharide, presented a dense and uniform layered distribution, and had better stability than N-constrained biofilm with lower EPS and biofilm with heterostructures cultured under total nutrient conditions.

Microbial electrolysis: a promising approach for treatment and resource recovery from industrial wastewater

Wastewater is one of the most common by-products of almost every industrial process. Treatment of wastewater alone, before disposal, necessitates an excess of energy. Environmental concerns over the use of fossil fuels as a source of energy have prompted a surge in demand for alternative energy sources and the development of sophisticated procedures to extract energy from unconventional sources. Treatment of municipal and industrial wastewater alone accounts for about 3% of global electricity use while the amount of energy embedded in the waste is at least 2–4 times greater than the energy required to treat the same effluent. The microbial electrolysis cell (MEC) is one of the most efficient technologies for waste-to-product conversion that uses electrochemically active bacteria to convert organic matter into hydrogen or a variety of by-products without polluting the environment. This paper highlights existing obstacles and future potential in the integration of Microbial Electrolysis Cell with other processes like anaerobic digestion coupled system, anaerobic membrane bioreactor and thermoelectric micro converter.

Application of bioelectrochemical systems to regulate and accelerate the anaerobic digestion processes

Anaerobic digestion (AD) serves as a potential bioconversion process to treat various organic wastes/wastewaters, including sewage sludge, and generate renewable green energy. Despite its efficiency, AD has several limitations that need to be overcome to achieve maximum energy recovery from organic materials while regulating inhibitory substances. Hence, bioelectrochemical systems (BESs) have been widely investigated to treat inhibitory compounds including ammonia in AD processes and improve the AD operational efficiency, stability, and economic viability with various integrations. The BES operations as a pretreatment process, inside AD or after the AD process aids in the upgradation of biogas (CO₂ to methane) and residual volatile fatty acids (VFAs) to valuable chemicals and fuels (alcohols) and even directly to electricity generation. This review presents a comprehensive summary of BES technologies and operations for overcoming the limitations of AD in lab-scale applications and suggests upscaling and future opportunities for BES-AD systems.

Production of n-caproate using food waste through thermophilic fermentation without addition of external electron donors

The effectiveness of producing n-caproate from food waste without external electron donors (EDs) was investigated through batch and semi-continuous fermentation. The maximum concentration of n-caproate reached 10,226.28 mg COD/L during semi-continuous fermentation. The specificity for n-caproate was the highest at 40.19 ± 3.95%, and the soluble COD conversion rate of n-caproate reached up to 22.50 ± 1.09% at the end of batch fermentation. The production of n-caproate was coupled with the generation of lactate as an ED to facilitate chain elongation reactions. When lactate was used as the only substrate, n-butyrate (64.12 ± 20.11%) markedly dominated the products, instead of n-caproate (0.63 ± 0.07%). Microbial community analysis revealed that Caproiciproducens, Rummeliibacillus, and Clostridium_sensu_stricto_12 were the key genera related to n-caproate production. In addition to n-caproate, n-butyrate dominated the products in batch and semi-continuous fermentation with a maximum specificity of 47.59 ± 3.39%. Clostridium_sensu_stricto_7 was committed to producing n-butyrate from lactate.

Biochar boosts dark fermentative H2 production from sugarcane bagasse by selective enrichment/colonization of functional bacteria and enhancing extracellular electron transfer

The influence of biochar (BC) on anerobic digestion (AD) of organic wastes have been widely studied. However, the effect of BC on rate-limiting step during AD of lignocellulosic waste, i.e. the hydrolysis and acidogenesis step, is rarely studied and the underlying mechanisms have not been investigated. In this study, the benefits of BC with respect to dark fermentative hydrogen production were explored in a fermentation system by a heat-shocked consortium from sewage sludge (SS) with pretreated sugarcane bagasse (PSCB) as carbon source. The results showed that biochar boosted biohydrogen production by 317.1% through stimulating bacterial growth, improving critical enzymatic activities, manipulating the ratio of NADH/NAD⁺ and enhancing electron transfer efficiency of fermentation system. Furthermore, cellulolytic Lachnospiraceae was efficiently enriched and electroactive bacteria were selectively colonized and the ecological niche was formed on the surface of biochar. Synergistic effect between functional bacteria and extracellular electron transfer (EET) in electroactive bacteria were assumed to be established and maintained by biochar amendment. This study shed light on the underlying mechanisms of improved performance of biohydrogen production from lignocellulosic waste during mesophilic dark fermentation by BC supplementation.

Electro-fermentation for biofuels and biochemicals production: Current status and future directions

Electro-fermentation is an emerging bioporcess that could regulate the metabolism of electrochemically active microorganisms. The provision of electrodes for the fermentation process that functions as an electron acceptor and supports the formation and transportation of electrons and protons, consequently producing bioelectricity and value-added chemicals. The traditional method of fermentation has several limitations in usability and economic feasibility. Subsequently, a series of metabolic processes occurring in conventional fermentation processes are most often redox misaligned. In this regard, electro-fermentation emerged as a hybrid technology which can regulate a series of metabolic processes occurring in a bioreactor by regulating the redox instabilities and boosting the overall metabolic process towards high biomass yield and enhanced product formation. The present article deals with microorganisms-electrode interactions, various types of electro-fermentation systems, comparative evaluation of pure and mixed culture electro-fermentation application, and value-added fuels and chemical synthesis.

A Comprehensive Understanding of Electro-Fermentation

Electro-fermentation (EF) is an upcoming technology that can control the metabolism of exoelectrogenic bacteria (i.e., bacteria that transfer electrons using an extracellular mechanism). The fermenter consists of electrodes that act as sink and source for the production and movement of electrons and protons, thus generating electricity and producing valuable products. The conventional process of fermentation has several drawbacks that restrict their application and economic viability. Additionally, metabolic reactions taking place in traditional fermenters are often redox imbalanced. Almost all metabolic pathways and microbial strains have been studied, and EF can electrochemically control this. The process of EF can be used to optimize metabolic processes taking place in the fermenter by controlling the redox and pH imbalances and by stimulating carbon chain elongation or breakdown to improve the overall biomass yield and support the production of a specific product. This review briefly discusses microbe-electrode interactions, electro-fermenter designs, mixed-culture EF, and pure culture EF in industrial applications, electro methanogenesis, and the various products that could be hence generated using this process.

Enhanced Acetogenesis of Waste Activated Sludge by Conditioning with Processed Organic Wastes in Co-Fermentation: Kinetics, Performance and Microbial Response

Aimed at the low ratio of carbon and nitrogen (C/N, approximately 7/1) of waste activated sludge (WAS), which would inhibit the acetogenesis process during anaerobic fermentation, this study introduced three brewing wastes, including vinegar (VR), stillage (SR) and soy sauce (SSR) residues, to promote acetogenesis by co-fermenting with WAS. Results showed that different brewing wastes contributed differently to the volatile fatty acids (VFAs) yield. The best performance was observed with SSR (4517 ± 367 mg COD/L), particularly rich in C2–C3 VFAs, corresponding to 40% and 52% higher concentrations than with SR and VR, respectively. Meanwhile, the hydrolysis rate constant peaked at 0.0059 h−1 in the SSR test, compared to the sole WAS test (0.0018 h−1). Furthermore, canonical correlation analysis reflected that the functional consortia, known to ferment saccharides/amino acids into C2–C3 VFAs (i.e., Proteiniclasticum, Petrimonas, Cloacibacillus and Gemmobacter), was related to the characteristics of the feedstock.

Waste based hydrogen production for circular bioeconomy: Current status and future directions

The present fossil fuel-based energy sector has led to significant industrial growth. On the other hand, the dependence on fossil fuels leads to adverse impact on the environment through releases of greenhouse gases. In this scenario, one possible substitute is biohydrogen, an eco-friendly energy carrier as high-energy produces. The substrates rich in organic compounds like organic waste/wastewater are very useful for improved hydrogen generation through the dark fermentation. Thus, this review article, initially, the status of biohydrogen production from organic waste and various strategies to enhance the process efficiency are concisely discussed. Then, the practical confines of biohydrogen processes are thoroughly discussed. Also, alternate routes such as multiple process integration approach by adopting biorefinery concept to increase overall process efficacy are considered to address industrial-level applications. To conclude, future perspectives besides with possible ways of transforming dark fermentation effluent to biofuels and biochemicals, which leads to circular bioeconomy, are discussed.

Optimizing the production of short and medium chain fatty acids (SCFAs and MCFAs) from waste activated sludge using different alkyl polyglucose surfactants, through bacterial metabolic analysis

Alkyl polyglucose is an environmentally-friendly biosurfactant, which is able to enhance short-chain fatty acids production with different carbon chain lengths and concentrations, during sludge anaerobic fermentation. This presents a promising strategy for sludge re-utilization by effectively converting hazardous sludge into value-added compounds. The maximum yield of short-chain fatty acids produced from sludge was 479.3 and 462.2 mg COD/g VSS, following pretreatment with APG06 and APG1214, respectively. To the best of our knowledge, the short-chain fatty acid production performance by sludge fermentation reported here, achieved a higher level than reported in previous studies. Additionally, these findings indicate that the production of medium-chain fatty acids from sludge can be induced by alkyl polyglucoses. Finally, the microbial community and enzyme activity were also assessed to reveal the mechanism of short-/medium-chain fatty acids biosynthesis under alkyl polyglucose pretreatment. This study demonstrates that alkyl polyglucose provides an environmentally-friendly and effective strategy for enhancing the production of short-/medium-chain fatty acids from waste activated sludge. These findings are useful for the assessment of alkyl polyglucose-assisted production of short-/medium-chain fatty acids, as well as for understanding the interactions between short-/medium-chain fatty acids and microbial communities with key enzymes, to establish short-/medium-chain fatty acids metabolic pathways during sludge fermentation.

Industrial wastewater to biohydrogen: Possibilities towards successful biorefinery route

The aim of this review is to summarize the modern developments and enhancement strategies reported for improving the biorefinery route of industrial wastewater to biohydrogen. Recent developments towards biohydrogen production chiefly involves culture enrichment, pretreatment of biocatalysts, co culture fermentation, metabolic and genetic engineering, ecobiotechnological approaches and the coupling process of biohydrogen. In addition, an overview of dark fermentation, pathways involved, microbes involved in biohydrogen production, industrial wastewater as substrate have been focused. The utilization of organic residuals of dark fermentation for subsequent value added products are highlighted. More apparently, the two stage coupling process and its possibilities towards biorefinery has been reviewed comprehensively. Moreover, comparative energy and economic aspects of biohydrogen production from industrial wastewater and its prospects towards pilot scale applications are also spotlighted. Though all the enhancement strategies have both benefits and disadvantages, coupling process is considered as the most successful biorefinery route for biohydrogen production.

Microbial electrochemical technologies: Electronic circuitry and characterization tools

Microbial electrochemistry merges microbiology, electrochemistry and electronics to provide a set of technologies for environmental engineering applications. Understanding the electronic concepts is crucial for effectively adopting these systems, but the importance of electronic circuitry is often overlooked by microbial electrochemistry researchers. This review provides the background on the electronics and electrochemical concepts involved in the study of microorganisms interacting with electricity, and their applications in microbial electrochemical technology (MET). The potentiostat circuitry is described along with its working principles. Electrochemical analyses are presented together with the rational and parameters employed to study MET devices and electroactive microorganisms. Finally, future directions are delineated towards the adoption of MET, and the related electronics, in environmental engineering applications.

Enhanced quinoline removal by zero-valent iron-coupled novel anaerobic processes: performance and underlying function analysis

Quinoline is toxic and difficult to degrade biologically; thus, it is a serious threat to the safety of ecosystems. To promote quinoline reduction, zero-valent iron (ZVI) was introduced into an anaerobic digestion (AD) system through batch experiments. The performance of three different types of ZVI (i.e., iron powder, iron scrap and rusty iron scrap) on quinoline degradation, methane production, formation of volatile fatty acids (VFAs) and chemical oxygen demand (COD) removal were investigated systematically. Compared to the AD system alone, quinoline and COD removal as well as the production of methane and acetic acid were effectively enhanced by ZVI, especially rusty iron scrap. The removal efficiencies of quinoline and COD were increased by 28.6% and 19.9%, respectively. The enhanced effects were attributed to the high accumulation of ferrous ions and high pH self-buffering capability, which were established by ZVI addition. Furthermore, high-throughput sequencing analysis indicated that the functional microorganisms in the ZVI-AD system were higher than in the AD system, and the added types of ZVI played important roles in structuring the innate microbial community in waste activated sludge (WAS). Especially, high enrichment of microorganisms capable of degrading quinoline, such as Pseudomonas and Bacillus, in the coupled system was detected.

Microbial electrohydrogenesis linked to dark fermentation as integrated application for enhanced biohydrogen production: A review on process characteristics, experiences and lessons

Microbial electrohydrogenesis cells (MECs) are devices that have attracted significant attention from the scientific community to generate hydrogen gas electrochemically with the aid of exoelectrogen microorganisms. It has been demonstrated that MECs are capable to deal with the residual organic materials present in effluents generated along with dark fermentative hydrogen bioproduction (DF). Consequently, MECs stand as attractive post-treatment units to enhance the global H₂ yield as a part of a two-stage, integrated application (DF-MEC). In this review article, it is aimed (i) to assess results communicated in the relevant literature on cascade DF-MEC systems, (ii) describe the characteristics of each steps involved and (iii) discuss the experiences as well as the lessons in order to facilitate knowledge transfer and help the interested readers with the construction of more efficient coupled set-ups, leading eventually to the improvement of overall biohydrogen evolution performances.

Aquatic weeds as the next generation feedstock for sustainable bioenergy production

Increasing oil prices and depletion of existing fossil fuel reserves, combined with the continuous rise in greenhouse gas emissions, have fostered the need to explore and develop new renewable bioenergy feedstocks that do not require arable land and freshwater resources. In this regard, prolific biomass growth of invasive aquatic weeds in wastewater has gained much attention in recent years in utilizing them as a potential feedstock for bioenergy production. Aquatic weeds have an exceptionally higher reproduction rates and are rich in cellulose and hemicellulose with a very low lignin content that makes them an efficient next generation biofuel crop. Considering their potential as an effective phytoremediators, this review presents a model of integrated aquatic biomass production, phytoremediation and bioenergy generation to reduce the land, fresh water and fertilizer usage for sustainable and economical bioenergy.

Aged refuse enhances anaerobic digestion of waste activated sludge

In this work, a low-cost alternative approach (i.e., adding aged refuse (AR) into waste activated sludge) to significantly enhance anaerobic digestion of sludge was reported. Experimental results showed that with the addition dosage of AR increasing from 0 to 400 mg/g dry sludge soluble chemical oxygen demand (COD) increased from 1150 to 5240 mg/L at the digestion time of 5 d, while the maximal production of volatile fatty acids (VFA) increased from 82.6 to 183.9 mg COD/g volatile suspended solids. Although further increase of AR addition decreased the concentrations of both soluble COD and VFA, their contents in these systems with AR addition at any concentration investigated were still higher than those in the blank, which resulted in higher methane yields in these systems. Mechanism studies revealed that pertinent addition of AR promoted solubilization, hydrolysis, and acidogenesis processes and did not affect methanogenesis significantly. It was found that varieties of enzymes and anaerobes in AR were primary reason for the enhancement of anaerobic digestion. Humic substances in AR benefited hydrolysis and acidogenesis but inhibited methanogenesis. The effect of heavy metals in AR on sludge anaerobic digestion was dosage dependent. Sludge anaerobic digestion was enhanced by appropriate amounts of heavy metals but inhibited by excessive amounts of heavy metals. The relative abundances of microorganisms responsible for sludge hydrolysis and acidogenesis were also observed to be improved in the system with AR addition, which was consistent with the performance of anaerobic digestion.

Enhancement of volatile fatty acid production by co-fermentation of food waste and excess sludge without pH control: The mechanism and microbial community analyses

The study provided a cost-effective and high-efficiency volatile fatty acid (VFA) production strategy by co-fermentation of food waste (FW) and excess sludge (ES) without artificial pH control. VFA production of 867.42mg COD/g-VS was obtained under the optimized condition: FW/ES 5, solid retention time 7d, organic loading rate 9g VS/L-d and temperature 40°C. Mechanism exploration revealed that the holistic biodegradability of substrate was greatly enhanced, and proper pH range (5.2-6.4) was formed by the high buffering capacity of the co-fermentation system itself, which effectively enhanced hydrolysis yield (63.04%) and acidification yield (83.46%) and inhibited methanogenesis. Moreover, microbial community analysis manifested that co-fermentation raised the relative abundances of hydrolytic and acidogenic bacteria including Clostridium, Sporanaerobacter, Tissierella and Bacillus, but suppressed the methanogen Anaerolineae, which also facilitated high VFA production. These results were of great guiding significance aiming for VFA recovery from FW and ES in large-scale.

Polarized potential and electrode materials implication on electro-fermentative di-hydrogen production: Microbial assemblages and hydrogenase gene copy variation

This study examined the changes in microbial diversity in response to different electrode materials viz., stainless steel mesh (SS) and graphite plate as anodes in two microbial electrolysis cell (MEC) each poised at 0.2V, 0.4V, 0.6V and 0.8V. Changes in microbiota prior to and after pretreatment along with microbiota enriched in response to various poised potentials with SS and graphite are monitored by 16S rRNA gene based DGGE profiling. Significant shifts in microbial community were noticed at all these experimental conditions. Correspondingly, the level of hydrogenase belonging to genera Bacillus, Pseudomonas, Rhodopseudomonas and Clostridium was studied by quantitative real time PCR (RT-PCR) at various applied potentials. DGGE based 16S rRNA gene profiling revealed enriched members belonging to phylum Firmicutes predominantly present at 0.8V in both MECs contributing to high hydrogen production. This study first time explored the growth behavior of mixed consortia in response to poised potentials and electrode materials.

Closed circuitry operation influence on microbial electrofermentation: Proton/electron effluxes on electro-fuels productivity

A novel biocatalyzed electrofermentor (BEF) was designed which uncovers the intricate role of biocatalyst involved in cogeneration of electro-fuels (hydrogen and electricity). The specific role of external resistance (Rext, electrical load) on the performance of BEF was evaluated. Four BEFs were operated separately with different resistances (25, 50, 100 and 200 Ω) at an organic load of 5 g/L. Among the tested conditions, external resistance (R3) with 100 Ω revealed maximum power and cumulative H2 production (148 mW and 450 mL, respectively). The competence of closed circuitry comparatively excelled because it facilitates congenial ambiance for the enriched EAB (electroactive bacteria) resulting high rate of metabolic activity that paves way for higher substrate degradation and electro-fuel productivity. Probing of electron kinetics was studied using voltammetric analyses wherein electron transfer by redox proteins was noticed. The designed BEF is found to be sustainable system for harnessing renewable energy through wastewater treatment.

Evaluation of surfactants on waste activated sludge fermentation by pyrosequencing analysis

The effects of three widely-used surfactants on waste activated sludge (WAS) fermentation and microbial community structures were investigated. Rhamnolipid bio-surfactants (RL) showed more positive effects on WAS hydrolysis and acidification compared to chemosynthetic surfactants, such as sodium dodecylsulphate (SDS) and sodium dodecyl benzene sulfonate (SDBS). The highest SCOD and VFAs concentrations obtained with RL were 1.15-fold and 1.16-fold that of SDS, and up to 1.73 and 3.63 times higher than those obtained with SDBS. Pyrosequencing analysis showed that an evident reduction in bacterial diversity in surfactant-treated WAS. Moreover, acid-producing bacteria (such as Megasphaera and Oscillibacter), detected with RL, were (6.8% and 6.4% in proportion) more abundant than with SDS, and were rarely found in SDBS and the control. The results also revealed that RL allowed efficient hydrolysis enhancement and was favorable to functional microorganisms for further acidification during WAS fermentation.

Single-Stage Operation of Hybrid Dark-Photo Fermentation to Enhance Biohydrogen Production through Regulation of System Redox Condition: Evaluation with Real-Field Wastewater

Harnessing hydrogen competently through wastewater treatment using a particular class of biocatalyst is indeed a challenging issue. Therefore, biohydrogen potential of real-field wastewater was evaluated by hybrid fermentative process in a single-stage process. The cumulative hydrogen production (CHP) was observed to be higher with distillery wastewater (271 mL) than with dairy wastewater (248 mL). Besides H₂ production, the hybrid process was found to be effective in wastewater treatment. The chemical oxygen demand (COD) removal efficiency was found higher in distillery wastewater (56%) than in dairy wastewater (45%). Co-culturing photo-bacterial flora assisted in removal of volatile fatty acids (VFA) wherein 63% in distillery wastewater and 68% in case of dairy wastewater. Voltammograms illustrated dominant reduction current and low cathodic Tafel slopes supported H₂ production. Overall, the augmented dark-photo fermentation system (ADPFS) showed better performance than the control dark fermentation system (DFS). This kind of holistic approach is explicitly viable for practical scale-up operation.

Biohydrogen production: strategies to improve process efficiency through microbial routes

The current fossil fuel-based generation of energy has led to large-scale industrial development. However, the reliance on fossil fuels leads to the significant depletion of natural resources of buried combustible geologic deposits and to negative effects on the global climate with emissions of greenhouse gases. Accordingly, enormous efforts are directed to transition from fossil fuels to nonpolluting and renewable energy sources. One potential alternative is biohydrogen (H₂), a clean energy carrier with high-energy yields; upon the combustion of H₂, H₂O is the only major by-product. In recent decades, the attractive and renewable characteristics of H₂ led us to develop a variety of biological routes for the production of H₂. Based on the mode of H₂ generation, the biological routes for H₂ production are categorized into four groups: photobiological fermentation, anaerobic fermentation, enzymatic and microbial electrolysis, and a combination of these processes. Thus, this review primarily focuses on the evaluation of the biological routes for the production of H₂. In particular, we assess the efficiency and feasibility of these bioprocesses with respect to the factors that affect operations, and we delineate the limitations. Additionally, alternative options such as bioaugmentation, multiple process integration, and microbial electrolysis to improve process efficiency are discussed to address industrial-level applications.

Applied potentials regulate recovery of residual hydrogen from acid-rich effluents: Influence of biocathodic buffer capacity over process performance

An absolute biological microbial electrolysis cell (MEC) was operated for a prolonged period under different applied potentials (Eapp, -0.2V to -1.0V) and hydrogen (H2) production was observed using acid-rich effluent. Among these potentials, an optimal voltage of -0.6 V influenced the biocathode by which maximum H2 production of 120 ± 9 ml was noticed. This finding was corroborated with dehydrogenase activity (1.8 ± 0.1 μg/ml) which is the key enzyme for H2 production. The in situ biocathode regulated buffer overpotentials which was remarkably observed by the change in peak heights of dissociation value (pKa) from the titration curve. Substrate degradation analysis gave an estimate of coulombic efficiency of about 72 ± 5% when operated at optimal voltage. Evidently, the electron transfer from solid carbon electrode to biocathode was analyzed by cyclic voltammetry and its derivatives showed the involvement of redox mediators. Despite, the MEC endures certain activation overpotentials which were estimated from the Tafel slope analysis.

Upflow fixed bed bioelectrochemical reactor for wastewater treatment applications

A cylindrical Upflow Fixed Bed Reactor (UFB-BER) with granular activated carbon, steel mesh electrodes and anaerobic microorganisms, was constructed for analyzing how hydrodynamic parameters affect the reactions involved during wastewater treatment processes for azo dye degradation. Dye removal percentage was not compromised by decreasing HRTm (99-90% upon changing HRTm from 4 to 1h in single pass mode). Using the residence time distribution method for hydrodynamic characterization, it was found that a higher dispersion in the reactor occurs for HRTm=1h, than for HRTm=4h. A kinetic analysis suggests that this dispersion effect could be associated to a higher specific reaction rate dependent on the azo dye concentration.

Continuous volatile fatty acid production from waste activated sludge hydrolyzed at pH 12

This study adopted rapid alkaline treatment at pH 12 to hydrolyze 66% of total chemical oxygen demands. Then the hydrolyzed liquor was fermented in a continuous-flow stirred reactor to produce volatile fatty acids (VFAs) at 8-h hydraulic retention time and at 35 °C. The maximum VFA productivity reached 365 mg VFAs g(-1) volatile suspended solids in a 45-d operation, with most produced VFAs being acetate and propionate, principally produced by protein degradation. The Bacteroidia, ε-proteobacteria and the Clostridia were identified to be the classes correlating with the fermentation processes. The fermented liquor was applied to denitrifying phosphorus removal process as alternative carbon source after excess phosphorus and nitrogen being recycled via struvite precipitation. Fermented liquors from alkaline hydrolysis-acid fermentation on waste activated sludge are a potential renewable resource for applications that need organic carbons.

Systematic approach to assess biohydrogen potential of anaerobic sludge and soil rhizobia as biocatalysts: Influence of crucial factors affecting acidogenic fermentation

A systematic protocol was designed to enumerate the variation in biohydrogen production with two different biocatalysts (sludge and soil) under different pH and organic loads. Both the biocatalysts showed cumulatively higher H2 production under acidogenic condition (pH 6) than at neutral pH condition. The cumulative hydrogen production was non-linearly fitted with modified Gompertz model and statistically validated. Pretreated soil biocatalyst showed relatively higher H2 production (OLR II, 142±5ml) than pretreated sludge (OLR I, 123±5ml); which was evidenced by substrate linked dehydrogenase activity and bio-electrochemical analysis. Experimental results revealed agricultural soil as a better biocatalyst than anaerobic sludge for all the operated process conditions. The voltammogram profiles and Tafel slopes revealed dominance of reductive catalytic activity of the pretreated inoculums substantiating dark-fermentation. Soil consortia showed low polarization resistance (2.24kΩ) and high reductive electron transfer efficiency (1.17 Vdec(-1)) at a high organic load; thus, rebating high H2 production.