Bioproduction of hydrogen from food waste by pilot-scale combined hydrogen/methane fermentation

A pilot-scale two-phase hydrogen/methane fermentation system generated 3.9 L biogas per unit time and reactor volume from food waste, of which the fraction of H₂ was approximately 60% at a hydraulic retention time (HRT) of 21 h. As substrate, 90% of the carbohydrates in the organic compounds were consumed, based on COD removal efficiency, and the hydrogen yield was approximately 1.82 (H₂-mol/glucose-mol). The maximum decomposition rate coefficient of hydrogen fermentation was observed at an HRT of 21 h, indicating that reducing HRTs improves hydrogen production. Over 80% of the methane was produced in the methane fermentation tank and the predominant fraction of organic acids after methane fermentation comprised acetic acid. Based on our economic evaluation, two-phase hydrogen/methane fermentation has greater potential for recovering energy than methane-only fermentation.     

Production of hydrogen and methane by one and two stage fermentation of food waste

Anaerobic digestion is an attractive process for generation of hydrogen and methane, which involves complex microbial processes on decomposition of organic wastes and subsequent conversion of metabolic intermediates to hydrogen and methane. Comparative performance of a sequential hydrogen and methane fermentation in two stage process and methane fermentation in one stage process were tested in batch reactor at varying ratios of feedstock to microbial inoculum (F/M) under mesophilic incubation. F/M ratios influence biogas yield, production rate, and potential. The highest H₂ and CH₄ yields of 55 and 94 mL g⁻¹ VS were achieved at F/M of 7.5 in two stage process, while the highest CH₄ yield of 82 mL g⁻¹ VS in one stage process was observed at the same F/M. Acetic and butyric acids are the main volatile fatty acids (VFAs) produced in the hydrogen fermentation stage with the concentration range 10–25 mmol L⁻¹. Little concentrations of VFAs were accumulated in methane fermentation in both stage processes. Total energy recovery in two stage process is higher than that in one stage by 18%. This work demonstrated two stage fermentation achieved a better performance than one stage process.     

Bioaugmentation on hydrogen production from food waste

The aim of this work was to evaluate the effect of two hydrolytic (Paenibacillus polymyxa and Bacillus subtilis) and two fermentative (Clostridium saccharobutylicum and Clostridium beijerinckii) strains on hydrogen (H₂) production in dark fermentation by batch testing. Food waste was used as a substrate, pretreated anaerobic sludge was used as the inoculum, and different concentrations of the evaluated microorganisms were used. Bioaugmentation with 3.5 × 10⁹ CFU/mL/L_reactor B. subtilis showed the best performance, obtaining a production of 84.5 mL H₂/g SV and a reduction in the lag phase (from 7.9 h in control to 3.5 h). Bioaugmentation with B. subtilis in an anaerobic sequencing batch reactor exhibited a significant effect on volumetric productivity, reaching a maximal increase of 344% of H₂ production in comparison with that obtained without the addition of the strain. The increase in H₂ was observed in a short period of time (4 cycles), after which H₂ production returned to the original H₂ production baseline. During all reactor operations, the main volatile fatty acids produced were acetic acid and butyric acid. Microbial community analysis when bioaugmentation was applied showed an importance of lactic acid bacteria abundance, such as that of Bifidobacterium and Lactobacillus, whose metabolic activity was crucial in reactor performance. The added concentration of microorganisms is a critical parameter for the bioaugmentation process.     

Viability of ultrasonication of food waste for hydrogen production

Batch anaerobic studies were conducted to study the effect of ultrasonication as a pre-treatment method for pulp waste prior to anaerobic hydrogen production. Pre-treatment was conducted by sonicating a 100 mL of pulp waste at different sonication times varying from 0.5 min to 30 min. The ultimate hydrogen production increased with increasing sonication time. The highest ultimate hydrogen production was achieved at a sonication time of 30 min and reflected an 88% increase over the unsonicated food waste, of 80 mL/g VS_added. The highest final VFAs concentration after fermentation (corresponding to 70% increase over the unsonicated food waste) was also achieved at a sonication time of 30 min. There were no significant differences between the acetate-to-butyrate ratios (HAc/HBu) for the all sonication times. The maximum hydrogen production rate at sonication time of 30 min was about 145% higher than that the unsonicated food waste.     

Effect of inoculum pre-treatment on mesophilic hydrogen and methane production from food waste using two-stage anaerobic digestion

Two-stage anaerobic digestion of food waste was performed using four different inoculum pre-treatment methods to enrich hydrogen (H₂) producing bacteria from sludge. The pretreatments used in this study included heat shock, alkaline treatment, aeration, and a novel pretreatment using waste frying oil (WFO). Alkaline pretreatment and aeration did not completely inhibit methanogens in the first stage while no methane (CH₄) was detected in the reactors cultivated either with heat shock or WFO-pretreated inocula. The highest H₂ and CH₄ yields (76.1 and 598.2 mL/gVS, respectively) were obtained using the inoculum pretreated with WFO. The highest total energy yield (21.96 kJ/gVS) and total organic carbon (TOC) removal efficiencies (95.77%) were obtained using inoculum pretreatment with WFO. The total energy yield trend obtained using the different pretreatments was as follows: WFO > alkaline > heat > aeration > control.     

Supercritical water gasification of food waste: Effect of parameters on hydrogen production

Food waste is a type of municipal solid waste with abundant organic matter. Hydrogen contains high energy and can be produced by supercritical water gasification (SCWG) of organic waste. In this study, food waste was gasified at various reaction times (20–60 min) and temperatures (400 °C-450 °C) and with different food additives (NaOH, NaHCO₃, and NaCl) to investigate the effects of these factors on syngas yield and composition. The results showed that the increase in gasification temperature and time improved gasification efficiency. Also, the addition of food additives with Na⁺ promoted the SCWG of food waste. The highest H₂ yield obtained through non-catalytic experiments was 2.0 mol/kg, and the total gas yield was 7.89 mol/kg. NaOH demonstrated the best catalytic performance in SCWG of food waste, and the highest hydrogen production was 12.73 mol/kg. The results propose that supercritical water gasification could be a proficient technology for food waste to generate hydrogen-rich gas products.     

Thermophilic hydrogen production from starch wastewater using two-phase sequencing batch fermentation coupled with UASB methanogenic effluent recycling

The aim of this study was to evaluate the performance of thermophilic hydrogenesis coupled with mesophilic methanogenesis in which the effluent was recycled to the hydrogen reactor for starch wastewater treatment. With this system, the hydrogen production rate and yield were 3.45 ± 0.25 L H₂/(L·d) and 5.79 ± 0.41 mmol H₂/g COD_added respectively, and thus higher than the values of the control group without methanogenic effluent recycling. In addition, relatively higher contents of acetate and butyrate were obtained in the hydrogen reactor with recirculation. The methane reactors were operated with the effluent from the hydrogen reactor, and methane yield was stabilized at 0.21–0.23 L/g COD_removal in both. Analysis of the microbial communities further showed that methanogenic effluent recirculation enriched microbial communities in the hydrogen reactor. Two species of bacteria effective in hydrogenesis, Thermoanaerobacterium thermosaccharolyticum and Clostridium thermosaccharolyticum, dominated during hydrogen production, whereas archaea belonging to Euryarchaeota were detected and cultured in the methane reactor. The recycled effluent supplied alkaline substrates for the hydrogen producing bacteria. Alkali balance calculations showed that the amount of added alkali was reduced by 88%. This amount, required for hydrogen production from starch wastewater, was contributed by alkali in the methanogenic effluent, (2225 ± 140 mg CaCO₃/L), resulting in lower operational costs.     

Effect of biochar in enhancing hydrogen production by mesophilic anaerobic digestion of food wastes: The role of minerals

The role of minerals in biochar in promoting hydrogen (H₂) production by anaerobic digestion of food waste was investigated. The cultures with the addition of biochar, leached biochar, metal sulphate solution and leached biochar combined with metal sulphate solution, respectively, were placed in bench-scale reactors and incubated at incubator at 32 °C. Daily H₂ production and volatile fatty acids (VFAs) were measured and the cumulative H₂ yield (Y_H) and maximum H₂ production rate (R_H) were calculated. The microbial analysis was performed using Illumina MiSeq sequencing. Biochar addition significantly increased the maximum Y_H by 107% and R_H by 54%. However, the addition of leached biochar only increased the maximum Y_H by 39% and R_H by 45% than control. The primary elements in biochars that contribute to H₂ production (Fe, K and Ca) were shown to increase the acetic acid, butyric acid and prevalence of the H₂ producing bacteria Clostridium butyricum.     

An experimental approach to produce hydrogen and methane from food waste using catalyst

Anaerobic co-digestion of food waste, cow dung, and sludge solution is experimented in the presence of calcium peroxide (CaO₂) as the catalyst to produce hydrogen and methane as a source of renewable energy. The substrate to inoculum ratios (v/v) of 1:1(S1), 1:2(S2), 1:3(S3), 1:4(S4) and 1:5(S5) are investigated in separate fermentative and methanogenic reactors. The result from the fermentative reactors indicate maximum hydrogen concentration of 26.34% with cumulative yield of 114.1 mL/g total solid (TS) in S3 compared to the other samples. Methanogenic reaction shows the highest methane concentration of 54.13% in S3. The highest daily (average) and cumulative biogas yield of 5.36 mL/g TS and 201.9 mL/g TS respectively are identified in S3. A maximum carbon dioxide concentration of 63.11% is found in S1. Overall, the substrate to inoculum ratio of 1:3 is spotted to be optimal for effective hydrogen and methane production during the anaerobic co-digestion process.     

Fungal solid-state fermentation of food waste for biohydrogen production by dark fermentation

The dark fermentation process was evaluated for biohydrogen production from food waste through fungal solid-state fermentation (SSF). Three fungal cultures (one strain of Aspergillus tubingensis and two strains of Meyerozyma caribbica) were compared, being A. tubingensis the best hydrolyser culture for releasing soluble carbohydrates. The biochemical hydrogen potential of food waste hydrolysate (FWH) at different substrate-inoculum ratios obtained a lower hydrogen yield than untreated food waste (RFW). The highest hydrogen yield value corresponded to treatments RFW-20 and RFW-30 with 77.0 ± 2.6 and 76.9 ± 1.4 mL H₂ normalized by per gram volatile solid added (NmL H₂/gVS_added), respectively. The microbial community of food waste was analysed, being detected lactic-acid bacteria genera as Latilactobacillus and Leuconostoc. The presence of actively growing bacteria during the SSF could explain the lowest hydrogen yield (20.1–36.0 NmL H₂/gVS_added) in the FWH treatment due to the substrate competition between lactic-acid bacteria and hydrogen-producing bacteria, where the lactic-acid bacteria were favoured by their faster growth rate.     

Enhancement split-phase hydrogen production from food waste during dark fermentation: Protein substances degradation and transformation during hydrothermal pre-treatments

Hydrothermal pre-treatment (HTP) is considered to be a promising technique which can be used to improve food waste (FW) dewatering performance and biodegradability. It was critical to selecting HTP conditions to optimize protein degradation, transformation and split-phase biogas production during dark fermentation. The results show that as the temperature increased from 80 to 200 °C, a noticeable decrease in the proportions of crude protein and amino acids within the solid-phase was found, whilst total volatile fatty acids within the liquid-phase from the FW under HTP increased. The tryptophan and tyrosine proportions were higher than that of the control after HTP, whereas the fulvic acid- and humic acid-like substances simultaneously disappeared. The optimum performance of hydrogen production during the liquid-phase dark fermentation after HTP-III (at 150 °C for 60 min with 40% water addition) showed the highest hydrogen production rate and proportion of 15.57 mL/h and 62%, respectively. This contributed to protein hydrolysis and enhancement of the biogas production from FW under HTP.     

Influence of organic load on biogas production and response of microbial community in anaerobic digestion of food waste

Organic load (OL) is one important parameter influencing performance of anaerobic digestion, yet it is unclear how it affects the biogas composition and microbial community. This work investigated the influence of OL on biogas production from food waste (FW) and the response of microbial community. Results showed that the main biogas component was methane at low OLs (<10 g VS/L) while it turned into hydrogen at high OLs (>10 g VS/L). The optimum methane and hydrogen yields were 184.4 and 61.3 mL/g VS, corresponding to the OLs of 4 and 20 g VS/L, respectively. Analysis of microbial community indicated that high OLs leaded to the decrease of hydrolysis bacteria and methanogen while it helped to increase the relative abundance of hydrogen-producing bacteria. This study cast an insight that it is essential to control the OL and reinforce the hydrogenogen to obtain high output of hydrogen energy.     

Automatic process control for stable bio-hythane production in two-phase thermophilic anaerobic digestion of food waste

The paper reports the results of a long term (310 days) pilot-scale trial where food waste as sole substrate was treated in a two-phase thermophilic anaerobic digestion process. This was optimized for concurrent hydrogen and methane production. First phase's optimization for hydrogen production was obtained recirculating the effluent coming from the methanogenic phase and without the addition of external chemicals. A drawback of such approach is the recirculation of ammonia into the first phase reactor for hydrogen production with possibility of consequent inhibition.     

Progress in the production of hydrogen energy from food waste: A bibliometric analysis

The exponential increase in food waste generation has prompted the scientific community to convert it into value-added resources. Hydrogen energy provides a sustainable option to fossil fuels due to its purity, high energy content, with no emissions other than water vapor. Combining the two aspects, a bibliometric analysis was performed for the conversion of food waste to hydrogen energy to evaluate the research trends based on literature in the Scopus database over the last two decades. The cluster analysis supported with the visualization tool aided in conducting a systematic study revealing growing themes and hot issues. The results showed a growing interest in the conversion of food waste to hydrogen energy research with the number of publications increasing by nearly 50 times in the last two decades. Comprehensive journals like the International Journal of Hydrogen Energy were most popular in publishing articles contributing to almost 30% in the research area. The country-wise analysis revealed that China accounted for more than 25% of the articles published followed by South Korea and India while the USA dominated in terms of the number of citations. Lastly, keyword cluster analysis revealed five major research hotspots for future discussion. The study concludes that further perspectives on fuel delivery, environmental impacts, and social acceptance could aid in positive developments in the biohydrogen energy industry.     

Natural inducement of hydrogen from food waste by temperature control

An easy and simple method of producing H₂ from food waste was devised. Although there was no inoculum addition or pretreatment, food waste was naturally decomposed and converted to H₂ when cultivated at 50-60 °C in anaerobic state. Both the highest H₂ yield of 1.79 mol H₂/mol hexose_added and a production rate of 369.1 ml H₂/L/h were observed at 50 °C. While butyrate was the main by-product of the food waste cultivated at 50 °C, lactate whose producing-reaction is non-hydrogenic was dominant at 35 °C where the worst performance was observed. The degradation efficiency of volatile solids and carbohydrate was similar to 50% and 90%, respectively, at both temperatures. Polymerase chain reaction-denaturing gradient gel electrophoresis analysis clearly revealed that the role of temperature control was the microbial selection. At high temperature, the activity of indigenous lactic acid bacteria was suppressed while H₂-producing bacteria, such as Clostridium sp., Acetanaerobacterium elongatum, and Caloramater indicus, were predominantly cultivated.     

Lime mud from paper-making process addition to food waste synergistically enhances hydrogen fermentation performance

The effect of lime mud from paper-making process (LMP) addition on the H₂ fermentation of food waste (FW) was investigated. It was found that a slight addition of LMP (1.0–4.0 g in 200 g FW) significantly enhanced the H₂ fermentation performance, not only increasing the total amount of H₂ produced but also accelerating the whole reaction, shortening the lag period, and increasing the H₂ production rate. Fermentation stability and microbial germination were also facilitated by LMP addition. This was attributed to the existence of Ca, Fe, Mn and alkaline substances such as CaCO₃ and NaOH. The batch process treating a mixture of FW and LMP was showed that the highest hydrogen production of 137.6 mL H₂/g VS was achieved at final pH 5.0, adding 3 g LMP (in 200 g FW) to the fermentation process, which lag-phase time was about 2.5 h.     

Kinetic study of biological hydrogen production by anaerobic fermentation

The growth kinetics of hydrogen producing bacteria using three different substrates, namely sucrose, non-fat dry milk (NFDM), and food waste were investigated in dark fermentation through a series of batch experiments. The results showed that hydrogen production potential and hydrogen production rate increased with an increasing substrate concentration. The maximum hydrogen yields from sucrose, NFDM, and food waste were 234, 119, and 101 mL/g COD, respectively. The low pH (pH<4)$(\mathrm{pH}<4)$ inhibited hydrogen production and resulted in lower carbohydrate fermentation at high substrate concentration. Michaelis–Menten equation was employed to model the hydrogen production rate at different substrate concentrations. The equation gave a good approximation of the maximum hydrogen production rate and the half saturation constant (_Ks)$(K_{\mathrm{s}})$ with correlation coefficient (R²)$(R^2)$ over 0.85. The _Ks$K_{\mathrm{s}}$ values of sucrose, NFDM, and food waste were 1.4, 6.6, and 8.7 g COD/L, respectively. Based on _Ks$K_{\mathrm{s}}$ values, the substrate affinity of the enriched hydrogen producing culture was found to depend on carbohydrate content of the substrate. The substrate containing high carbohydrate showed a lower _Ks$K_{\mathrm{s}}$ value. The maximum hydrogen production rate was governed by the complexity of carbohydrates in the substrate.     

Effect of acid-pretreatment on hydrogen fermentation of food waste: Microbial community analysis by next generation sequencing

This work presents the effect of acid-pretreatment on H₂ fermentation of food waste with detailed microbial information by next generation sequencing. The pretreated food waste at pH 1.0–4.0 was cultivated under mesophilic conditions without external inoculum addition. From the food waste acid-pretreated at pH 1–3, H₂ yields in the range of 1.37–1.74 mol H₂/mol hexose_added were achieved, attaining the highest value at pH 2. Clostridium sp. such as Clostridium acetobutylicum ATCC 824 and Clostridium perfringens occupied more than 70% of total number of sequences at pH 1–3. On the other hand, in the control (no pretreatment) and at pH 4, lactic acid bacteria such as Lactobacillus and Streptococcus were found to be the dominant genus (>90% of total number of sequences), resulting in a low H₂ yield. In addition, the effect of substrate concentration on H₂ fermentation was investigated, and the maximum H₂ productivity was estimated to be 27.2 L H₂/L/d by Andrew's model.     

Effect of organic loading rate on continuous hydrogen production from food waste in submerged anaerobic membrane bioreactor

The characteristics of hydrogen fermentation in a membrane bioreactor (HF-MBR) fed with food waste were investigated at thermophilic condition. The HF-MBR was operated at three different organic loading rates (OLRs) of 70.2, 89.4 and 125.4 kg-COD/m³/day. Biogas production rate increased from 22.4 to 32.8 and 62.5 l/day with OLR. The maximum Hydrogen yield and production rate were 111.1 mL-H₂/g-VS _added and 10.7 l-H₂/L/day at an OLR of 125.4 kg-COD/m³/day. The total carbohydrate degradation was better than 96% throughout the experimental runs. Continuous H₂ production from food waste with CH₄-free biogas was successfully sustained in the HF-MBR for 90 days. The microbial community was dominated by Clostridium sp. strain Z6. The H₂ production was significantly improved by shortening the retention time and increasing the OLRs. The HF-MBR showed an H₂ production capacity at the high OLRs due to its higher cell retention.     

Experimental study on the energy conversion of food waste via supercritical water gasification: Improvement of hydrogen production

In this study, the model food waste was gasified to hydrogen-rich syngas in a batch reactor under supercritical water condition. The model food consisted of rice, chicken, cabbage, and cooking oil. The effects of the main operating parameters including temperature (420–500 °C), residence time (20–60 min) and feedstock concentration (2–10 wt%) were investigated. Under the optimal condition at 500 °C, 2 wt% feedstock and 60 min residence time, the highest H₂ yield of 13.34 mol/kg and total gas yield of 28.27 mol/kg were obtained from non-catalytic experiments. In addition, four commercial catalysts namely FeCl₃, K₂CO₃, activated carbon, and KOH were employed to investigate the catalytic effect of additives at the optimal condition. The results showed that the highest hydrogen yield of 20.37 mol/kg with H₂ selectivity of 113.19%, and the total gas yield of 38.36 mol/kg were achieved with 5 wt% KOH addition Moreover, the low heating value of gas products from catalytic experiments with KOH increased by 32.21% compared to the non-catalytic experiment. The catalytic performance of the catalysts can be ranked in descending order as KOH > activated carbon > FeCl₃ > K₂CO₃. The supercritical water gasification (SCWG) with KOH addition can be a potential applied technology for food waste treatment with production of hydrogen-rich gases.