Simultaneous hydrogen gas formation and COD removal from cheese whey wastewater by electrohydrolysis

Cheese whey (CW) was subjected to DC voltages between 0.5 and 5 V for hydrogen gas production with simultaneous COD removal by electrohydrolysis of CW organics. Hydrogen gas formation and COD removal were investigated at different DC voltages using aluminum electrodes. The highest cumulative hydrogen production (5551 mL), hydrogen yield (1709 mL H₂ g⁻¹ COD), hydrogen gas formation rate (913 ml d⁻¹), and percent hydrogen (99%) in the gas phase were obtained with 5 V DC voltage within 158 h. Energy conversion efficiency reached the highest level (80.7%) at 3 V DC voltage with cumulative hydrogen production of 4808 mL and hydrogen yield of 1366 mL H₂ g⁻¹ COD. Hydrogen gas was mainly produced by electrohydrolysis of CW organics due to low H₂ gas production in water and CW control experiments. The highest COD removal (22%) was also obtained with 3 V DC voltage. Major COD removal mechanism was anaerobic degradation of carbohydrates producing volatile fatty acids (VFA) and CO₂. Hydrogen gas was produced by reaction of protons released from VFAs and electrons provided by DC current. Hydrogen gas production by electrohydrolysis of CW solution was proven to be an effective method with simultaneous COD removal.     

Hydrogen gas production from olive mill wastewater by electrohydrolysis with simultaneous COD removal

Diluted olive mill wastewater (OMW) was subjected to direct current (DC) voltages (0.5-4.0 V) for hydrogen gas production with simultaneous chemical oxygen demand (COD) removal by electrohydrolysis. The highest cumulative hydrogen production (3020 ml) and hydrogen yield (2500 ml H₂ g⁻¹ COD) were obtained with 3 V DC voltage while the highest current intensity (80 mA), percent hydrogen (95%) in the gas phase, hydrogen gas formation rate (614 ml d⁻¹), percent COD removal (44%) and energy conversion efficiency (95%) were realized with 2 V. Hydrogen gas production by electrolysis of water was negligible for all voltages. COD removal from OMW with no DC voltage application was usually lower than that obtained with DC power application. Hydrogen gas production by electrohydrolysis of OMW was proven to be a fast and effective method with simultaneous COD removal.     

Electrohydrolysis of landfill leachate organics for hydrogen gas production and COD removal

Hydrogen gas production with simultaneous COD removal was realized by application of DC voltages (0.5-5.0 V) to landfill leachate. The rate and the yield of hydrogen gas production were investigated at different DC voltages by using aluminum electrodes and DC power supply. The highest cumulative hydrogen production (5000 mL), hydrogen yield (2400 mL H₂ g⁻¹ COD), daily hydrogen gas formation (1277 mL d⁻¹), and percent hydrogen (99%) in the gas phase were obtained with 4 V DC voltage. Energy conversion efficiency (H₂ energy/electrical energy) reached the highest level (80.6%) with 1 V DC voltage. Hydrogen gas production was mainly realized by electrohydrolysis of leachate organics due to negligible H₂ gas production in water and leachate control experiments. The highest COD removal (77%) was also obtained with 4 V DC voltage. Electrohydrolysis of landfill leachate was proven to be an effective method for hydrogen gas production with simultaneous COD removal.     

Hydrogen gas production from waste anaerobic sludge by electrohydrolysis: Effects of applied DC voltage

Waste anaerobic sludge was subjected to different DC voltages (0.5-5 V) for hydrogen gas production by using aluminum electrodes and a DC power supply. Effects of applied DC voltage on the rate and extent of hydrogen gas production were investigated. The highest cumulative hydrogen production (2775 ml), daily hydrogen gas formation (686.7 ml d⁻¹), hydrogen yield (96 ml H₂ g⁻¹ COD) and percent hydrogen (94.3%) in the gas phase were obtained with 2 V DC voltage. Energy conversion efficiency (H₂ energy/electrical energy) also reached the highest level (74%) with 2 V DC voltage application. Control experiments with no voltage application to the sludge yielded almost the same level of COD removal, but no hydrogen gas production. Voltage application to water resulted in much lower hydrogen gas production as compared to sludge indicating negligible electrolysis of water. The results indicated that the sludge was naturally decomposed by the active cells removing COD and releasing hydrogen ions to the medium which reacted with the electrons provided by DC current to produce hydrogen gas. Hydrogen gas production from electrohydrolysis of waste sludge was found to be a fast and effective method with high energy efficiency.     

Effects of metals in wastewater on hydrogen gas production using electrohydrolysis

Hydrogen gas was produced from metal plating wastewater by electro hydrolysis. Wastewater contains chrome, copper and nickel metals which can accelerate the production of hydrogen gas. Effects of kind of metals, the voltage and reaction time on percent hydrogen gas (HGP) were investigated. After application of different DC voltages on each metallic wastewater, percent hydrogen gas (HGP), cumulative hydrogen gas volume (CHGV), hydrogen gas formation rate (HFR) and total organic carbon (TOC) removal were also evaluated. Hydrogen gas percent was obtained as %99 at 4 V for chrome plating wastewater while percent hydrogen gas was achieved as 50% H₂ gas at 4 V for copper and nickel metal plating wastewater. Maximum CHGV achieved with 4 V DC voltage for all metal plating wastewater. Maximum CHGV (4000 mL), HFR (985 mL H₂ d⁻¹) and percent hydrogen gas (99%) was observed with chrome plating wastewater at 4 V DC voltage. Hydrogen gas produced from chrome metal plating wastewater using electro hydrolysis method can be efficiently used for fuel cells as a source due to nearly pure hydrogen gas.     

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.     

Hydrogen gas production from vinegar fermentation wastewater by electro-hydrolysis: Effects of initial COD content

Vinegar fermentation wastewater with different initial COD contents (9.66–48.6 g L⁻¹) were used for hydrogen gas production with simultaneous COD removal by electro-hydrolysis. The applied DC voltage was constant at 4 V. The highest cumulative hydrogen production (3197 ml), hydrogen yield (2766 ml H₂ g⁻¹ COD), hydrogen formation rate (799 ml d⁻¹), and percent hydrogen (99.5%) in the gas phase were obtained with the highest initial COD of 48.6 g COD L⁻¹. The highest energy efficiency (48%) was obtained with the lowest COD content of 9.66 g L⁻¹. Hydrogen gas production by water electrolysis was less than 250 ml and wastewater control resulted in less than 25 ml H₂ in 96 h. The highest (12%) percent COD removal was obtained with the lowest COD content. Hydrogen gas was produced by reaction of (H⁺) ions present in raw WW ( pH = 3.0) and protons released from acetic acid with electrons provided by electrical current. Electro-hydrolysis of vinegar wastewater was proven to be an effective method of H₂ gas production with some COD removal.     

Hydrogen gas production by electrohydrolysis of volatile fatty acid (VFA) containing dark fermentation effluent

Dark fermentation effluents of wheat powder (WP) solution containing different concentrations of volatile fatty acids (VFAs) were subjected to low voltage (1–3 V) DC current to produce hydrogen gas. Graphite and copper electrodes were tested and the copper electrode was found to be more effective due to higher electrical conductivity. The effects of solution pH (2–7), applied voltage (1–3 V) and the total VFA (TVFA) concentration (1–5 g L⁻¹) on hydrogen gas production were investigated. Hydrogen production increased with decreasing pH and became maximum at pH = 2. Increases in applied voltage and the TVFA concentration also increased the cumulative hydrogen formation. The most suitable conditions for the highest cumulative hydrogen production was pH = 2, with 3 V applied voltage and 5 g TVFA L⁻¹. Up to 110 ml hydrogen gas was obtained with 5 g L⁻¹ TVFA at pH = 5.8 and 2 V applied voltage within 37.5 h. The highest energy efficiency (56%) was obtained with the 2 V applied voltage and 10.85 g L⁻¹ TVFA. Hydrogen production by electrolysis of water in control experiments was negligible for pH > 4. Hydrogen production by electrohydrolysis of VFA containing anaerobic treatment effluents was found to be an effective method with high energy efficiency.     

Effect of organic fraction of municipal solid waste addition to high rate activated sludge system for hydrogen production from carbon rich waste sludge

Municipal solid waste has been used for bio-methane production for many years. However, both methane and carbon dioxide that is produced during bio-methanization increases the greenhouse gas emissions; therefore, hydrogen production can be one of the alternatives for energy production from waste. Hydrogen production from the organic substance was studied in this study with the waste activated sludge from the municipal wastewater treatment. High rated activated sludge (HRAS) process was applied for the treatment to reduce energy consumption and enhance the organic composition of WAS. The highest COD removal (76%) occurred with the 12 g/L organic fraction of municipal solid waste (OFMSW) addition at a retention time of 120 min. The maximum hydrogen and methane yields for the WAS was 18.9 mL/g VS and 410 mL/g VS respectively. Total carbon emission per g VS of the substrate (OFMSW + waste activated sludge) was found as 0.087 mmol CO₂ and 28.16 mmol CO₂ for dark fermentation and bio-methanization respectively. These kinds of treatment technologies required for the wastewater treatment plantcompensate it some of the energy needs in a renewable source. In this way, the HRAS process decreases the energy requirement of wastewater treatment plant, and carbon-rich waste sludge enables green energy production via lower carbon emissions.     

Effect of bioaugmentation on hydrogen production from organic fraction of municipal solid waste

In the present study, the effect of bioaugmentation with three bacterial species (i.e. E. coli, Bacillus subtilis and Enterobacter aerogenes) on the hydrogen production from organic fraction of municipal solid waste was evaluated at different bacteria/sludge ratios (0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35 and 0.40). Cumulative hydrogen production, lag phases, and maximum hydrogen production rates were analyzed using modified Gompertz model. The highest cumulative and volumetric hydrogen production of 564.4 ± 10.9 mL and 1.61L_H2/L_substrate respectively was achieved for bioaugmentation with Bacillus subtilis at bacteria/sludge ratio of 0.25. The corresponding highest hydrogen yield was 43.68 mLH₂/gCarbo. For bioaugmentation with E. coli and Enterobacter aerogenes, the maximum cumulative hydrogen production of 423.4 ± 10.6 mL and 486.3 ± 10.6 mL respectively was obtained from bacteria/sludge ratio of 0.20. Corresponding highest hydrogen yields were 32.9 mLH₂/gCarbo and 37.1 mLH₂/gCarbo respectively. Bioaugmentation shortened the lag phases and improved COD removal. Volatile fatty acid generation was also improved with the bioaugmentation.     

Synergic effect of various amino acids and ferric oxide on hydrogen production

To overcome nitrogen and iron deficiency in the organic fraction of municipal solid waste, amino acids and ferric oxide were separately added in the feedstock to evaluate their effect on hydrogen production. Furthermore, synergic effect of amino acids and ferric oxide on hydrogen production was evaluated. The co-culture of E. coli and Enterobacter aerogenes was used in the present study. The amino acids were applied in the concentration range of 1.0, 2.5, 5.0, 7.5 and 10.0 g/L while ferric oxide was used in the concentration range of 10, 20, 30, 40, 50, 100, 150, 200 and 500 mg/L. Modified Gompertz model was used to analyze cumulative hydrogen production (P), maximum hydrogen production rate (Rmax) and lag phases (λ). The results exhibited that the hydrogen production was positively affected by each amino acid at every concentration applied. Application of alanine resulted in the highest cumulative and volumetric hydrogen production of 685.4 ± 10.1 mL and 1.9561L_H2/L_substrate respectively which increased to 872.5 ± 10.1 mL and 2.492L_H2/L_substrate for ferric oxide addition along with alanine. COD removal and VFA generation were positively affected by the synergic effect of amino acid and ferric oxide.     

Electro-stimulated biological production of hydrogen from municipal solid waste

Hydrogen production from municipal solid wastes was investigated by applying a weak current (0.06 A) to a slurry of municipal solid waste in an anaerobic reactor at 55 °C using 4 electrodes (carbon graphite for the cathode and platinum electroplated titanium for the anode). Current application to the organic waste stimulated the hydrogen producing bacteria especially bacteria related to the Thermotogales and Bacillus families. Measured hydrogen production rates were comprised between 16 and 41 mL/h. Comparison of bacterial and archaeal communities in methane-producing (control) and electro-stimulated reactors showed similar species but with different dynamics correlated to hydrogen or methane production. Energy efficiency of the overall bioelectrolysis process using municipal solid waste and an applied voltage of 3V was approximately 12.4%, which is relatively low compared to values reported in the literature for organic wastes and can be explained by the low organic carbon content and availability in the municipal solid waste. Results of this study highlight some important operational constraints with respect to electro-stimulated hydrogen production from organic wastes; related in particular to electrode lifetime expectancies. Results nevertheless illustrate the potential for hydrogen production from municipal solid waste as a possible route for energy recovery.     

Assessment of supercritical water gasification of food waste under the background of waste sorting: Influences of plastic waste contents

Waste sorting is being gradually implemented as a key measure for circular and sustainable development in China, food waste will be separately collected and separated from municipal solid waste (MSW), thus the plastic content in food waste also will be reduced. In this study, supercritical water gasification (SCWG) of food waste with different contents of plastic (0–3.5 wt%) was experimentally investigated to simulate the influence of waste sorting on the food waste treatment. The results showed that lower plastic content in food waste favored higher gas yield and gasification efficiencies. The highest H₂ yield and total gas yield were 3.11 mol/kg and 8.41 mol/kg in the plastic-free case, respectively. When the plastic content decreased from 3.5 wt% to 0 wt%, the cold gas efficiency (CGE), carbon conversion efficiency (CE) and hydrogen gasification efficiency (HE) increased by 125.97%, 173.48% and 94.09%, respectively. However, lower plastic content negatively affected the quality of produced syngas through decreasing H₂ mole fraction and LHV. The solid residues from SCWG of food waste with lower plastic content had higher ratio of fixed carbon to volatile matter (FC/VM). Based on the analysis of pyrolysis properties and combustion behavior, decreasing the plastic content in food waste helped to improve the thermal stability of solid residues. Moreover, lower plastic content resulted in a decrease of total organic carbon (TOC) concentration in liquid effluent, which is favorable for further treatment of liquid effluent.     

Effect of sulfate on hydrogen production from the organic fraction of municipal solid waste using co-culture of E. coli and Enterobacter aerogenes

In the present study, the effect of sulfate on the hydrogen production from the organic fraction of municipal solid (OFMSW) waste using co-culture of Enterobacter aerogenes and E. coli has been studied under varying pH conditions. The presence of sulfate in the feedstock declines hydrogen production efficiency. To evaluate the effect of sulfate on hydrogen production from OFMSW, COD/sulfate ratio of 17.5, 15.0, 12.5, 10.0, 7.5, 5.0 and 2.5 were applied at different pH conditions (i.e. pH 5.5, 6.0 and 6.5). The hydrogen production continuously declined with the decreasing COD/sulfate ratio and increase in pH. The cumulative hydrogen production decreased from 220.8 ± 10.5 mL in control to a minimum of 98.3 ± 10.5 mL, 74.4 ± 10.4 mL, and 44.6 ± 2.6 mL at pH 5.5, 6.0 and 6.5 respectively. The major content of gaseous composition included hydrogen and CO₂ at higher COD/sulfate ratio and low pH, while H₂S formation started with the decrease in COD/sulfate ratio and increase in the pH. Similarly, sulfate removal efficiency was found to be influenced by COD/sulfate ratio and pH condition. Soluble metabolite analysis revealed that total volatile fatty acid concentration was not affected by sulfate addition. Thus, Sulfate removal is essential prior to fermentation in order to improve hydrogen yield.     

Comparison of different electrodes in hydrogen gas production from electrohydrolysis of wastewater organics using photovoltaic cells (PVC)

Electrical power generated by a photovoltaic cell (PVC) was supplied to diluted industrial wastewater in a mechanically mixed and sealed stainless-steel reactor for hydrogen gas production. Three different electrodes, graphite, stainless steel and aluminum rods were used for comparison. Protons released from decomposition of organic compounds and electrons provided by the DC current reacted to form hydrogen gas. The highest cumulative hydrogen gas formation (CHF) was obtained with the aluminum electrode (120 L in 8 days) and the lowest was with the graphite electrode (4 L). Hydrogen gas production from wastewater was 2.4 times higher than that produced from water when aluminum electrodes were used. TOC content of wastewater was reduced from 2400 to 1700 mg L⁻¹ with nearly 29% TOC removal within 6 days. CHF from wastewater was 76 L within 18 days with the stainless-steel electrodes while CHF from water was only 9.5 L. Fermentative hydrogen gas production from wastewater was negligible in the absence PVC. Energy conversion efficiency for hydrogen gas production (hydrogen energy/electric energy) was found to be 74% with the aluminum electrodes.     

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.     

Investigation of the interaction between intermediates from gasification of biomass in supercritical water: Formaldehyde/formic acid mixtures

Supercritical water gasification (SCWG) is a promising technology for converting wet biomass and waste into renewable energy. While the fundamental mechanism involved in SCWG of biomass is not completely understood, especially hydrogen (H₂) production produced from the interaction among key intermediates. In the present study, formaldehyde mixed with formic acid as model intermediates were tested in a batch reactor at 400 °C and 25 MPa for 30 min. The gas and liquid phases were collected and analyzed to determine a possible mechanism for H₂ production. Results clearly showed that both gasification efficiency (GE) and hydrogen efficiency (HE) increased with addition of formic acid, and the maximum H₂ yield reached 17.92 mol/kg with a relative formic acid content of 66.67% in the mixtures. The total organic carbon removal rate and formaldehyde conversion rate also increased to 67.33% and 89.81% respectively. The reaction pathways for H₂ formation form mixtures was proposed and evaluated, formic acid promoted self-decomposition of formaldehyde to generate H₂, and induced a radical reaction of generated methanol to produce more H₂.     

Improved hydrogen gas production in electrohydrolysis of vinegar fermentation wastewater by scrap aluminum and salt addition

Scrap aluminum particles and salt (NaCl) were added to the vinegar fermentation wastewater to improve hydrogen gas formation by electrohydrolysis of the wastewater organics. The applied DC voltage and initial COD of the wastewater were constant at 4 V and 33.16 g L⁻¹, respectively. The highest cumulative hydrogen gas formation (2877 mL) was obtained with scrap Al (1 g L⁻¹) and NaCl (1 g L⁻¹) additions within 72 h as compared to 1925 mL H₂ gas formation from raw wastewater. Hydrogen gas formation from Al and NaCl added water was 302 ml as compared to 260 ml from raw water. The highest H₂ gas formation rate (952 mL d⁻¹), the yield (1660 mL H₂ g⁻¹ COD) and the highest current intensity (163 mA) were also obtained with combined effects of scrap Al and NaCl additions. Almost pure hydrogen gas (99%) was produced using the raw wastewater. Initial conductivity of the raw wastewater increased from 1.80 mS cm⁻¹ to 5.01 mS cm⁻¹ with the addition of scrap Al and salt for which the final conductivities were 4.0 mS cm⁻¹ and 6.91 mS cm⁻¹, respectively. The highest energy conversion efficiency was obtained with only scrap Al addition (37.8%) as compared to 30.5% efficiency obtained with Al and salt additions. Additions of NaCl and scrap Al particles was found to be very beneficial for H₂ gas formation by electrohydrolysis of vinegar fermentation wastewater.     

Continuous production of biohydrogen from oil palm empty fruit bunch hydrolysate in tubular photobioreactors

Production of hydrogen by the photosynthetic bacterium Rhodobacter sphaeroides was compared in continuously operated tubular photobioreactors illuminated by natural outdoor sunlight (0.15–66 klux; diurnal cycle) and constant indoor artificial light (10 klux; tungsten lamps). In both cases the operating temperature was 35 °C and the organic carbon source was an acid hydrolysate of oil palm empty fruit bunch (EFB), an agroindustrial waste. In the outdoor photobioreactor, under the best production conditions, the daytime feeding rate of the mixed carbon substrate was 48 mL h^?1 and the average pseudo-steady state hydrogen production rate was 36 mL H₂ L^?1 medium h^?1. The cumulative hydrogen production was 430 mL H₂ L^?1 medium. For the indoor photobioreactor fed at the same rate as the outdoor system, the steady state average hydrogen production rate was 43 mL H₂ L^?1 h^?1 and the cumulative hydrogen production was 517 mL H₂ L^?1 medium. Reducing the feed rate to less than 48 mL h^?1, enhanced the biomass concentration, but reduced hydrogen production in both bioreactors. The sunlight-based cumulative hydrogen production was only about 17% less compared to the artificially lit system, but required only 22% of the electrical energy.     

Microbial electrolysis cell powered by an aluminum-air battery for hydrogen generation,in-situ coagulant production and wastewater treatment

Microbial electrolysis cells (MECs) are an efficient technology for generating hydrogen gas from organic matters, but an additional voltage is needed to overcome the thermodynamic barrier of the reaction. A combined system of MEC and the aluminum-air battery (Al-air battery) was designed for hydrogen generation, coagulant production and operated in an energy self-sufficient mode. The Al-air battery (28 mL) produced a voltage ranged from 0.58 V to 0.80 V, which powered an MEC (28 mL) to produce hydrogen. The hydrogen production rate reached 0.19 ± 0.01 m³ H₂/m³/d and 14.5 ± 0.9 mmol H₂/g COD. The total COD removal rate was 85 ± 1%, of which MEC obtained 75 ± 1% COD removal and 10 ± 1% COD removal was achieved by in-situ coagulating process. The microorganisms removal of MEC effluent was 97 ± 2% through ex-situ coagulating process. These results showed that the Al-air battery-MEC system can be operated in energy self-sufficient mode and recovered energy from wastewater with high quality effluent.