Hydrogen fermentation of food waste by alkali-shock pretreatment: microbial community analysis and limitation of continuous operation

Biohythane: a Potential Biofuel of the Future

Today, the world is becoming more dependent on fossil fuels. The major drawbacks of these non-renewable energy resources include an extreme environmental pollution and an extinction threat. Several technologies including microalgal biodiesel production, biomass gasification, and bioethanol production have been explored for the generation of renewable energy especially, biofuels. One such promising research has been carried out in the generation of biohythane which has the potential to become an alternative fuel to the existing non-renewable ones. It has been reported that biohydrogen can be produced from organic wastes or agricultural feedstocks with the help of acidogens. Dark fermentation can be carried out by acidogens to produce biohydrogen under anaerobic conditions by utilizing lignocellulosic biomass or sugarcane feedstocks in the absence of light. The spent medium contains volatile short-chain fatty acids like acetate, butyrate, and propionate that can serve as substrates for acetogenesis followed by methane biosynthesis by methanogens. Therefore, the sequential two-stage anaerobic digestion (AD) involves a production of biohydrogen followed by the biosynthesis of methane. This combined process is termed as a single eponym "Biohythane" (hydrogen + methane). Several studies have demonstrated about the effectiveness of biofuel, and it is believed to have a greater energy recovery, environmental friendliness, and shorter fermentation time. Biohythane can serve as an alternative future green biofuel and solve the present energy crisis in India as well as the entire world.

Sustainable management of food waste; pre-treatment strategies, techno-economic assessment, bibliometric analysis, and potential utilizations: A systematic review

Food waste (FW) contains many nutritional components such as proteins, lipids, fats, polysaccharides, carbohydrates, and metal ions, which can be reused in some processes to produce value-added products. Furthermore, FW can be converted into biogas, biohydrogen, and biodiesel, and this type of green energy can be used as an alternative to nonrenewable fuel and reduce reliance on fossil fuel sources. It has been demonstrated in many reports that at the laboratory scale production of biochemicals using FW is as good as pure carbon sources. The goal of this paper is to review approaches used globally to promote turning FW into useable products and green energy. In this context, the present review article highlights deeply in a transdisciplinary manner the sources, types, impacts, characteristics, pre-treatment strategies, and potential management of FW into value-added products. We find that FW could be upcycled into different valuable products such as eco-friendly green fuels, organic acids, bioplastics, enzymes, fertilizers, char, and single-cell protein, after the suitable pre-treatment method. The results confirmed the technical feasibility of all the reviewed transformation processes of FW. Furthermore, life cycle and techno-economic assessment studies regarding the socio-economic, environmental, and engineering aspects of FW management are discussed. The reviewed articles showed that energy recovery from FW in various forms is economically feasible.

Biotechnological interventions in food waste treatment for obtaining value-added compounds to combat pollution

Over the last few decades, the globe is facing tremendous effects due to the unnecessary piling of municipal solid waste among which food waste holds a greater portion. This practice not only affects the environment in terms of generating greenhouse gas emissions but when left dumped in landfills will also trigger poverty and malnutrition. This review focuses on the global trend in food waste management strategies involved in the effective utilization of food waste to produce various value-added products in a microbiology aspect, thereby diminishing the negative impacts caused by the unnecessary side effects of non-renewable energy sources. The review also detailed the efficiency of microorganisms in the production of various bio-energies as well. Further, recent attempts to the exploitation of genetically modified microorganisms in producing value-added products were enlisted. This also attempted to address food waste valorization techniques, the combined applications of various processes for an enhanced yield of different compounds, and addressed various challenges. Further, the current challenges involved in various processes and the effective measures to tackle them in the future have been addressed. Thus, the present review has successfully addressed the circular bio-economy in food waste valorization.

Upgrading the value of anaerobic fermentation via renewable chemicals production: A sustainable integration for circular bioeconomy

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.

Enhanced energy recovery via separate hydrogen and methane production from two-stage anaerobic digestion of food waste with nanobubble water supplementation

This study investigated the enhancement effect of N₂- and Air-nanobubble water (NBW) supplementation on two-stage anaerobic digestion (AD) of food waste (FW) for separate production of hydrogen and methane. In the first stage for hydrogen production, the highest cumulative H₂ yield (27.31 ± 1.21 mL/g-VS_added) was obtained from FW + Air-NBW, increasing by 38% compared to the control (FW + deionized water (DW)). In the second stage for methane production, the cumulative CH₄ yield followed a descending order of FW + Air-NBW (373.63 ± 3.58 mL/g-VS_added) > FW + N₂-NBW (347.63 ± 7.05 mL/g-VS_added) > FW + DW (300.93 ± 3.24 mL/g-VS_added, control), increasing by 24% in FW + Air-NBW and 16% in FW + N₂-NBW compared to the control, respectively. Further investigations indicate that different gas-NBW may positively impact the different stages of AD process. Addition of N₂-NBW only enhanced the hydrolysis/acidification of FW with no significant effect on methanogenesis. By comparison, addition of Air-NBW promoted both hydrolysis/acidification stage and methanogenesis stage, reflecting by the enhanced activities of four extracellular hydrolases at the end of hydrolysis/acidification and coenzyme F₄₂₀ at the end of methanogenesis, respectively. Results from this work suggest the potential application of Air-NBW in the two-stage AD for efficient renewable energy recovery from FW.

Robust operation through effluent recycling for hydrogen production from the organic fraction of municipal solid waste

The stability of fermentative hydrogen production from the organic fraction of municipal solid waste (OFMSW) was evaluated in this work using a strategy of effluent recycling. Three pretreatment conditions were applied on the recycled effluent: a) no heat shock treatment, b) one initial heat shock treatment (90 °C, 30 min) and c) systematic heat shock treatment at the beginning of each fermentation. When a systematic heat shock was applied, a maximal hydrogen yield of 17.2 ± 3.8 mLH₂/gVS was attained. The hydrogen productivity was improved by 331% reaching a stable value of 1.51 ± 0.29 mLH₂/gVS/h, after 8 cycles of effluent recycling. This strategy caused a sharp decrease of diversity with stable co-dominance of hydrogen- and lactate-producing bacteria, ie. Clostridiales and Lactobacillales, respectively. For the other conditions, a sharp decrease of the hydrogen yields was observed showing the importance of applying a heat shock treatment for optimal hydrogen production with effluent recycling.

Biohythane production from food processing wastes - Challenges and perspectives

The food industry generates enormous quantity of food waste (FW) either directly or indirectly including the processing sector, which turned into biofuels for waste remediation. Six types of food processing wastes (FPW) such as oil, fruit and vegetable, dairy, brewery, livestock and finally agriculture based materials that get treated via dark fermentation/anaerobic digestion has been discussed. Production of both hydrogen and methane is daunting for oil, fruit and vegetable processing wastes because of the presence of polyphenols and essential oils. Moreover, acidic pH and high protein are the reasons for increased concentration of ammonia and accumulation of volatile fatty acids in FPW, especially in dairy, brewery, and livestock waste streams. Moreover, the review brought to forefront the enhancing methods, (pretreatment and co-digestion) operational, and environmental parameters that can influence the production of biohythane. Finally, the nature of feedstock's role in achieving successful circular bio economy is also highlighted.

Effects of hydraulic retention time on biohythane production via single-stage anaerobic fermentation in a two-compartment bioreactor

Hythane has been well known as a mixture of hydrogen and methane gases but their production is mostly in a different way. The present study dealt with the potential biohythane production in a two-compartment (lower, hydrogenesis; upper, methanogenesis) reactor via a single-stage anaerobic fermentation at mesophilic temperature. The effect of hydraulic retention time (HRT) was tested at 10-2 d using food waste substrate. HRT 2 d resulted in (1) maximum removal efficiencies for COD, carbohydrate, lipid and protein contents with values of 58.5, 58.4, 62.6 and 79.1%, respectively; (2) peak hydrogen and methane production rates of 714 and 254 mL/L-d, respectively; and (3) biogas contents of hydrogen 8.6% and methane 48.0% in the produced gas. At this HRT, Clostridium sensu stricto 2 and Methanosaeta were dominant species in H₂ and CH₄ compartments, respectively. The novelty of this work is creating a novel two-compartment reactor for single-stage anaerobic biohythane fermentation.

Lactate- and acetate-based biohydrogen production through dark co-fermentation of tequila vinasse and nixtamalization wastewater: Metabolic and microbial community dynamics

The aim of this work was to study the metabolic and microbial community dynamics during dark co-fermentation of 80% tequila vinasse and 20% nixtamalization wastewater (w/w). Batch co-fermentations were performed in a 3-L well-mixed reactor at 35 °C and pH 5.5. In correspondence to Illumina MiSeq sequencing and reactor monitoring, changes in metabolites and microbial communities were characterized by three main stages: (i) a first stage during which lactate and acetate producers dominated and consumed the major part of fermentable carbohydrates, (ii) a second stage in which lactate and acetate were consumed by emerging hydrogen-producing bacteria (HPB) in correlation with bioH₂ (100 NmL/L-h or 1200 NmL/L_reactor) and butyrate production, and (iii) a third stage during which non-HPB outcompeted HPB after bioH₂ production ceased. Altogether, the results of this study suggest that cooperative interactions between lactate producers and lactate- and acetate-consuming HPB could be attributed to lactate- and acetate-based cross-feeding interactions.

Enhancement of hydrogen production using untreated inoculum in two-stage food waste digestion

This research investigated the possibility to enhance H₂ production using untreated inoculum in a two-stage hydrogen-methane process from food waste. Batch experiments were conducted to evaluate the H₂ production efficiency at different F/M ratios (ranging from 1:1 to 64:1). The results showed that when a proper F/M ratio was selected, significant H₂ production was feasible to be achieved even inoculated with untreated anaerobic sludge. Among the F/M ratios studied, maximum H₂ yield (217.98 mL H₂ g VS^-1 FW) was found in the digester at the F/M of 64:1, which was 93.75 times higher than that of the digester at the F/M of 1:1. Higher hydrogen yield was achieved at the greater F/M ratio, due to the enrichment of the H₂ producing bacteria and the reduction of the antagonistic bacteria. The two-stage process allowed more stable methane production and higher overall energy yield compared to the single-stage process.

Pretreatment of macroalgal Laminaria japonica by combined microwave-acid method for biohydrogen production

Suitable pretreatment can effectively enhance the fermentative hydrogen production from algae biomass. In this study, combined microwave-acid pretreatment was applied to disintegrate the biomass of macroalgae L. japonica, and dark fermentation in batch mode was conducted for hydrogen production. The results showed that combining microwave pretreatment at 140 °C and 2450 MHz with 1% H2SO4 for 15 min could effectively disrupt macroalgal cells and release the organic matters, and soluble chemical oxygen demand (SCOD) concentration increased by 1.92-fold and achieved 5.12 g/L. During the fermentation process, both polysaccharides and proteins were consumed. Hydrogen production process was dominated by acetate-type fermentation, and the dominance of genus Clostridium contributed to more efficient hydrogen production. After the pretreatment, hydrogen yield increased from 15 mL/g TSadded to 28 mL/g TSadded, and energy conversion efficiency increased from 9.5% to 23.8%. Combined microwave-acid pretreatment is potential in enhancing hydrogen production from the biomass of L. japonica.

Producing desulfurized biogas using two-stage domesticated shear-loop anaerobic contact stabilization system

In this study, a two-stage domesticated shear-loop anaerobic contact stabilization (SLACS) system is introduced as a new reactor design to enhance methane productivity with significant reduction in hydrogen sulphide (H₂S) synthesis. Due to the rich sulfate content in industrial wastewaters, the initial fermentation phase of anaerobic digestion is highly acidifying and often leads to severe performance losses, digester's instability, and even culture crash. The SLACS system functions as a dissimilatory sulfate reduction - methanogenic reactor consisting of two compartments, a shear-loop anaerobic bed (SLAB) unit and an anaerobic plug flow (APF) unit. The functional role of the SLAB unit is not limited to acidogenesis but also sulfidogenic processes, which curtails H₂S generation in the APF unit (methanogenic stage). Experimental observations indicated that pH serves a critical role in the cohabitation of acidogenic and sulfidogenic microbes in the SLAB unit. Although acidogenesis was not influenced by pH within the range of 4.5-6.0, it is vital to stabilize the pH of this unit at 5.4 to establish a steady sulfate reduction of above 75%. The highest desulfurization achieved in this compartment was 88% under a hydraulic retention time (HRT) of 4 h. With an average methane productivity of 256 mL g^-1 VS, the methanogenic performance of the two-stage domesticated SLACS system shows a 32% methanogenic proficiency higher than that of the one-stage digestion system. Microbial community structure within the system carried out via Next Generation Sequencing (NGS) provided qualitative data on the sludge's sulfidogenic and methanogenic performance.

Pretreatment of food waste for methane and hydrogen recovery: A review

Food waste (FW) management by biological process is more attractive and eco-friendly approach than thermo-chemical conversion or landfilling. However, FW composition and physico-chemical and biological characteristics affect the overall biological process in terms of product yield and degradation rate. To overcome this major bottle-neck, the pretreatment of FW is proposed. Therefore this review aims to provide a comprehensive summary of the importance of pretreatment of FW with respect to FW management by anaerobic digestion (AD) and dark fermentation (DF). It also reviews the existing knowledge gaps and future research perspectives for better integration of FW pretreatments for AD and DF, which should include (i) the preservation of carbon mass through freeze and thaw, or drying; and (ii) improve the carbon accessibility through particle size reduction and thermal pretreatments for high-rate bioenergy recovery.

Kinetics and microbial community analysis for hydrogen production using raw grass inoculated with different pretreated mixed culture

In this study, five pretreatment methods (heat shock, acid, base, aeration and gamma radiation) were applied for enriching hydrogen producers from anaerobically digested sludge, aiming to compare their hydrogen fermentation performance using raw ryegrass as substrate. Results showed that various pretreatment methods caused great variations on grass hydrogen fermentation performance. Acid pretreatment was most efficient compared with other tested pretreatment methods, with relevant hydrogen yield of 64.4mL/g dry grass and organics removal of 31.4%. Kinetics results showed that the first-order kinetic model fitted hydrogen evolution better than the modified Gompertz model. Microbiological analysis showed that various pretreatment methods caused great variations on microbial activity and microbial community composition. Clostridium and Enterococcus were two dominant genera, while relative abundances of these two genera varied greatly for different pretreated samples. Difference in microbial activity and microbial community distribution induced by the pretreatment methods might directly cause different ryegrass fermentation performance.

Biohydrogen production from food waste: Current status, limitations, and future perspectives

Among the various biological routes for H₂ production, dark fermentation is considered the most practically applicable owing to its capability to degrade organic wastes and high H₂ production rate. Food waste (FW) has high carbohydrate content and easily hydrolysable in nature, exhibiting higher H₂ production potential than that of other organic wastes. In this review article, first, the current status of H₂ production from FW by dark fermentation and the strategies applied for enhanced performance are briefly summarized. Then, the technical and economic limitations of dark fermentation of FW are thoroughly discussed. Economic assessment revealed that the economic feasibility of H₂ production from FW by dark fermentation is questionable. Current efforts to further increase H₂ yield and waste removal efficiency are also introduced. Finally, future perspectives along with possible routes converting dark fermentation effluent to valuable fuels and chemicals are discussed.

Investigating hydrothermal pretreatment of food waste for two-stage fermentative hydrogen and methane co-production

The growing amount of food waste (FW) in China poses great pressure on the environment. Complex solid organics limit the hydrolysis of FW, hence impairing anaerobic digestion. This study employed hydrothermal pretreatment (HTP) to facilitate the solubilization of FW. When HTP temperature increased from 100 to 200°C, soluble carbohydrate content first increased to a peak at 140°C and then decreased, whereas total carbohydrate content was negatively correlated with increasing temperature due to the enhanced degradation and Maillard reactions. Protein solubilization was dramatically promoted after HTP, whereas protein degradation was negligibly enhanced. The hydrogen and methane yields from hydrothermally pretreated FW under the optimum condition (140°C, 20min) through two-stage fermentation were 43.0 and 511.6mL/g volatile solids, respectively, resulting in an energy conversion efficiency (ECE) of 78.6%. The ECE of pretreated FW was higher than that of untreated FW by 31.7%.