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.     

Enhancement of hydrogen production by recycling of methanogenic effluent in two-phase fermentation of food waste

The feasibility of operational strategies was investigated for hydrogen and methane production from food waste. Food waste was heat-treated at 70 °C and fed to a two-phase anaerobic sequencing batch fermenting system. Maximum hydrogen productivity of 1.19 m³ H₂/m³ d was observed at a food waste concentration of 30 g carbohydrate/L, a hydraulic retention time of 2 d, and a solids retention time of 3.4 d. The effluent from hydrogenesis was efficiently converted to methane at an organic loading rate of up to 3.6 kg COD/m³.d. The methanogenic effluent was then recycled to the hydrogenesis reactor without any pretreatment. The recycled effluent not only successfully replaced external dilution water and decreased alkaline dosage by 75%, but also increased hydrogen production by 48%, resulting in hydrogen productivity of 1.76 m³/m³ d. The two-phase digestion with recycling would convert 91% of organic pollutants in food waste to hydrogen (8%) and methane (83%) without any external dilution water.     

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.     

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.     

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.     

Sequential generation of hydrogen and methane from xylose by two-stage anaerobic fermentation

The sequential generation of hydrogen and methane from xylose by two-stage anaerobic fermentation was investigated for the first time in this study. The effects of substrate concentration, bacteria domestication and nitrogen source on hydrogen yield were studied in the first stage. The genetic characterization of the 16S rDNA was used to analyze the flora of strains domesticated with xylose and glucose. The maximum hydrogen yield is 190.6 ml H₂/g xylose when the xylose feedstock concentration is 1% (w/v), hydrogenogens are domesticated with xylose and yeast extract is used as nitrogen source. The soluble metabolite byproducts (SMB) from the hydrogen-producing stage were reutilized by methanogens to produce methane in the second stage. Over 98 wt % of acetate and butyrate in the SMB are reutilized to give a methane yield of 216.5 ml CH₄/g xylose. The sequential generation of hydrogen and methane from xylose markedly increases the energy conversion efficiency to 67.5%.     

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.     

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 hydraulic retention time on hydrogen production and chemical oxygen demand removal from tapioca wastewater using anaerobic mixed cultures in anaerobic baffled reactor (ABR)

This study evaluated hydrogen production and chemical oxygen demand removal (COD removal) from tapioca wastewater using anaerobic mixed cultures in anaerobic baffled reactor (ABR). The ABR was conducted based on the optimum condition obtained from the batch experiment, i.e. 2.25 g/L of FeSO₄ and initial pH of 9.0. The effects of the varying hydraulic retention times (HRT: 24, 18, 12, 6 and 3 h) on hydrogen production and COD removal in a continuous ABR were operated at room temperature (32.3 ± 1.5 °C). Hydrogen production rate (HPR) increased with a reduction in HRT i.e. from 164.45 ± 4.14 mL H₂/L.d (24 h HRT) to 883.19 ± 7.89 mL H₂/L.d (6 h HRT) then decreased to 748.54 ± 13.84 mL H₂/L.d (3 h HRT). COD removal increased with reduction in HRT i.e. from 14.02 ± 0.58% (24 h HRT) to 29.30 ± 0.84% (6 h HRT) then decreased to 21.97 ± 0.94% (3 h HRT). HRT of 6 h was the optimum condition for ABR operation as indicated.     

Different start-up strategies to enhance biohydrogen production from cheese whey in UASB reactors

The effect of different operational strategies and inoculum structure (granules and disaggregated granules) during the start-up of four up-flow anaerobic sludge blanket hydrogenogenic reactors was investigated. The more stable volumetric hydrogen production rates obtained were 0.38 and 0.36 L H₂/L-d, in reactors operated with a constant organic loading rate (OLR) with both inoculum structures, whereas in reactors operated with an increasing OLR methane started to be produced earliest in time. Specific hydrogenogenic activity results proved that the disaggregated inoculum produced a more active biomass than the granular one, but not granule formation was evident. The methane hydrogenotrophic activity was the main limitation of the systems evaluated. In the reactors inoculated with disaggregated sludge the start-up strategy did not influence the bacterial DGGE fingerprint, in contrast to the reactors started-up with granular sludge; members of the Clostridium genus were always present. The results demonstrated that operational conditions during the start-up period are crucial for the production of hydrogenogenic biomass.     

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.     

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.     

Continuous photo-fermentative hydrogen production from lactate and lactate-rich acidified food waste

In the present work, a continuous photo-fermentative hydrogen (H₂) production from lactate was attempted at various hydraulic retention times (HRTs) (48–120 h). Electron balance was made at each operating conditions to elucidate different performances. At 120 h HRT, H₂ production was negligible, while 42% and 52% of substrate electrons diverted towards cell growth and soluble microbial products (SMPs), respectively. After changing HRT to 96 h, H₂ yield jumped to 2.3 mol-H₂/mol-lactate_added with less SMPs production and minimal cell growth. The highest H₂ production rate of 260 mL-H₂/L-fermenter/d was also achieved at 96 h HRT. When HRT was further shortened to 72 and 48 h, H₂ yield dropped to 1.4 and 0.2 mol-H₂/mol-lactate_added, respectively. While almost all of the lactate was degraded at <72 h HRT, only 65% of the lactate was consumed at 48 h HRT. From 200th day, the feedstock was changed to lactate-rich acidified food waste, which was obtained via one-day fermentation of food waste and subsequent centrifugation. At 2 g chemical oxygen demand/L, substrate conversion efficiency reached 35%, which was slightly lower than that of feeding pure lactate. SMPs were found to be mainly consisted of low molecular weight compounds (<500 Da), and the majority of organic matters were aromatic proteins at 120 h HRT and it was shifted to humic-like region in 96 h HRT.     

Production of hydrogen and methane from wastewater sludge using anaerobic fermentation

Hydrogen and methane were produced from wastewater sludge using a clostridium strain. The original sludge and pre-treated (acidified, basified and freeze/thawed) sludges were the testing samples. Some pre-treatments enhanced hydrogen yield, whereas other treatments enhanced methane yield. Hydrogen fermentation can be used as a pre-stage to improve subsequent methane production from wastewater sludge.     

Cogeneration of H2 and CH4 from water hyacinth by two-step anaerobic fermentation

A novel reaction mechanism of H₂ and CH₄ cogeneration from water hyacinth (Eichhornia crassipes) was originally proposed to increase the energy conversion efficiency. The glucose and xylose hydrolysates derived from cellulose and hemicellulose are fermented to cogenerate H₂ and CH₄ by two-step anaerobic fermentation. The total volatile solid of hyacinth leaves can theoretically cogenerate H₂ and CH₄ yields of 303 ml-H₂/g-TVS and 211 ml-CH₄/g-TVS, which dramatically increases the theoretical energy conversion efficiency from 19.1% in only H₂ production to 63.1%. When hyacinth leaves are pretreated with 3 wt% NaOH and cellulase in experiments, the cogeneration of H₂ (51.7 ml-H₂/g-TVS) and CH₄ (143.4 ml-CH₄/g-TVS) markedly increases the energy conversion efficiency from 3.3% in only H₂ production to 33.2%. Hyacinth leaves, which have the most cellulose and hemicellulose and the least lignin and ash, give the highest H₂ and CH₄ yields, while hyacinth roots, which have the most ash and the least cellulose and hemicellulose, give the lowest H₂ and CH₄ yields.     

Cogeneration of hydrogen and methane from the pretreated biomass of algae bloom in Taihu Lake

In order to efficiently utilize the biomass waste of algae bloom in Taihu Lake, China and improve energy conversion efficiency, a three-stage process comprising dark hydrogen fermentation with acid-domesticated hydrogenogens, photohydrogen fermentation, and methanogenesis was undertaken. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that algal cells pretreated by microwave heating with diluted acid were degraded into smaller fragments (<5 μm) than those pretreated by steam heating with diluted acid. The microwave pretreatment of algae resulted in higher saccharification efficiency. The domesticated hydrogenogens in presence of acids improved the dark hydrogen production from microwave-pretreated algae biomass and led to a total combined dark and photofermentation hydrogen yield of 283.4 mL/g-total volatile solid (TVS). The energy conversion efficiency of steam-pretreated algae biomass remarkably increased to 47.0% by cogenerating 256.7 mL/g-TVS hydrogen and 253.5 mL/g-TVS methane in the three-stage process: dark-fermentation, photofermentation, and methanogenesis.     

Evaluation of hydrogen producing cultures using pretreated food waste

In order to enhance bio-hydrogen production from food waste, pretreatment methods are widely used. The influence of the initial pH and autoclaving were investigated in batch experiments. Fermentative studies showed that pure cultures like Clostridium beijerinckii could directly utilize raw food waste to produce hydrogen, while other cultures (Clostridium butyricum and Clostridium pasteurianum) could produce hydrogen only after pH adjustment. In this case, the optimal starting pH of the culture was found to be 7. Autoclaving could further enhance hydrogen yields due to increased hydrolysis of food waste. The maximum hydrogen yield was achieved by C. butyricum (38.9 mL-H₂/g-VS_added) after autoclaving food waste with pH adjustment at 7. In addition, the ratio acetic to butyric acid was decreased by autoclaving pretreatment, because butyrate metabolic pathway was favored in the fermentation process. However, suitable pH for bacteria growth and the low ammonia production could be achieved from autoclaving food waste.     

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.     

Effect of storage time and temperature on hydrogen fermentation of food waste

Practically, before being fed to the treatment plant, food waste (FW) is stored for up to a week in a storage tank under ambient temperature condition, which would have an impact on the bioenergy yield. In the present work, FW was stored at different temperatures (5 °C, 20 °C, and 35 °C) for 0 d, 1 d, and 2 d, and it was used as a feedstock for mesophilic H₂ fermentation. H₂ production curves were divided by three groups, finally attaining 1.7–1.8 mol H₂/mol hexose_added, 1.4–1.5 mol H₂/mol hexose_added, and 1.2 mol H₂/mol hexose_added, achieved from the (fresh, FW stored at 5 °C), (FW stored at 20 °C, and 35 °C for 1 d), and (FW stored at 35 °C for 2 d), respectively. The different performance was attributed to the growth of indigenous lactic acid bacteria such as Lactobacillus and Weissella during storage under high temperature condition. In addition, it was found that the activity of homoacetogenic reaction (R17, 4H₂ + CO₂ → Acetate) calculated by establishing metabolic flux balance was different depending on the H₂ production performance. The flux of R17 ranged 0.03–0.06 under low H₂ yield achieved conditions, while it increased to 0.10–0.17 those showing low H₂ yields.