Effects of key operational parameters on biohydrogen production via anaerobic fermentation in a sequencing batch reactor

Photo fermentative biohydrogen production potential using microalgae-activated sludge co-digestion in a sequential flow batch reactor (SFBR)

Biohydrogen (bioH₂) is a sustainable energy source that can produce carbon-free energy upon combustion. BioH₂ can be generated from microalgae by photolytic and anaerobic digestion (AD) pathways. The AD pathway faces many challenges when scaling up using different bioreactors, particularly the continuous stirred tank reactor (CSTR) and sequential flow batch reactor (SFBR). Therefore, the performance characteristics of SFBR were analysed in this study using Chlorella vulgaris and domestic wastewater activated sludge (WWAS) co-culture. An organic loading rate (OLR) of 4.7 g COD L^-1 day^-1 was fed to the SFBR with a hydraulic retention time (HRT) of five days in the presence of light under anaerobic conditions. The pH of the medium was maintained at 6 using a pH controller for the incubation period of 15 days. The maximum bioH₂ concentrations of 421.1 μmol L^-1 and 56.6 μmol L^-1 were observed in the exponential and steady-state phases, respectively. The effluent had an unusually high amount of acetate of 16.6 g L^-1, which remained high with an average of 11.9 g L^-1 during the steady state phase. The amount of bioH₂ produced was found to be inadequate but consistent when operating the SFBR with a constant OLR. Because of the limitations in CSTR handling, operating a SFBR by optimizing OLR and HRT might be more feasible in operation for bioH₂ yield in upscaling. A logistic function model was also found to be the best fit for the experimental data for the prediction of bioH₂ generation using co-culture in the SFBR.

Role of residue cornstalk derived biochar for the enhanced bio-hydrogen production via simultaneous saccharification and fermentation of cornstalk

Biochar derived from residue cornstalk left after anaerobic bio-hydrogen production (RCA-biochar) was confirmed to enhance bio-hydrogen production from cornstalk hydrolysate. However, the role of RCA-biochar in simultaneous saccharification and fermentation (SSF) during bio-hydrogen production from cornstalk has not yet been revealed. This study therefore aims to fill this knowledge gap. It was observed that with the increase in RCA-biochar concentration from 0 g/L to 10.0 g/L, the maximal cumulative SSF bio-hydrogen yield varied from 24.3 ± 1.1 mL/g-substrate to 154.3 ± 3.6 mL/g substrate under varying pH values - 5.5, 6.0, 6.5, 7.0. The increasing bio-hydrogen production was observed to correlate with both RCA-biochar level and initial pH. Batch tests confirmed that the initial pH had an obvious effect an saccharification, while RCA-biochar affected anaerobic fermentation a lot. The findings revealed the role of previously unrecognized RCA-biochar in SSF bio-hydrogen production from cornstalk, which can provide an alternative approach for lignocellulosic bio-hydrogen production.

Bio-Hydrogen Production from Wastewater: A Comparative Study of Low Energy Intensive Production Processes

Billions of litres of wastewater are produced daily from domestic and industrial areas, and whilst wastewater is often perceived as a problem, it has the potential to be viewed as a rich source for resources and energy. Wastewater contains between four and five times more energy than is required to treat it, and is a potential source of bio-hydrogen—a clean energy vector, a feedstock chemical and a fuel, widely recognised to have a role in the decarbonisation of the future energy system. This paper investigates sustainable, low-energy intensive routes for hydrogen production from wastewater, critically analysing five technologies, namely photo-fermentation, dark fermentation, photocatalysis, microbial photo electrochemical processes and microbial electrolysis cells (MECs). The paper compares key parameters influencing H2 production yield, such as pH, temperature and reactor design, summarises the state of the art in each area, and highlights the scale-up technical challenges. In addition to H2 production, these processes can be used for partial wastewater remediation, providing at least 45% reduction in chemical oxygen demand (COD), and are suitable for integration into existing wastewater treatment plants. Key advancements in lab-based research are included, highlighting the potential for each technology to contribute to the development of clean energy. Whilst there have been efforts to scale dark fermentation, electro and photo chemical technologies are still at the early stages of development (Technology Readiness Levels below 4); therefore, pilot plants and demonstrators sited at wastewater treatment facilities are needed to assess commercial viability. As such, a multidisciplinary approach is needed to overcome the current barriers to implementation, integrating expertise in engineering, chemistry and microbiology with the commercial experience of both water and energy sectors. The review concludes by highlighting MECs as a promising technology, due to excellent system modularity, good hydrogen yield (3.6–7.9 L/L/d from synthetic wastewater) and the potential to remove up to 80% COD from influent streams.

Investigation of the interaction between lighting and mixing applied during the photo-fermentation biohydrogen production process from agricultural waste

To better clarify the effect of lighting and mixing on the hydrogen production from agricultural waste, experiments under diverse constant and dynamic light intensities and mixing speeds were conducted. Cumulative hydrogen yield, hydrogen production rate, OD₆₆₀, pH, reducing sugar concentration were monitored. Results showed that mixing had positive effects on high substrate concentration. The interaction between lighting and mixing was drawn. Higher light intensity (7000 Lux) was suitable for hydrogen production under mixing. Higher light intensity with higher mixing speed (150 RPM) showed the highest cumulative hydrogen yield of 78.1 mL/g TS. Different periods of hydrogen production process required distinct lighting and mixing. Combined with Gompertz model, dynamic lighting and mixing strategies were discussed. Dynamic light intensity (4000-7000-4000 Lux) accompanied with dynamic mixing speed (50-150-50 RPM) was the optimal condition for PFHP. Highest hydrogen yield of 84.7 mL/g TS and highest light conversion efficiency of 36.32% were obtained.

Continuous hydrogen production from glucose/xylose by an anaerobic sequential batch reactor to maximize the energy recovery efficiency

Fermentation of both glucose and xylose is essential to realize efficient bioconversion of renewable and abundant lignocellulosic biomass to hydrogen.

Effect of hydraulic retention time on suppression of methanogens during a continuous biohydrogen production process using molasses wastewater

This study was undertaken to investigate the reduction of the hydraulic retention time (HRT) to decrease methane generation and recover hydrogen production during the long-term operation of biohydrogen production in a continuous stirred tank reactor (CSTR) using molasses wastewater. Reduction of HRT can be a simple and economic method to immediately control unfavorable methane generated during continuous operation of a hydrogen production system. The steady-state performance of the CSTR showed a hydrogen content of 41.3 ± 3.30% and a hydrogen production rate (HPR) of 63.7 ± 10.01 mmol-H₂L^-1d^-1 under an organic loading rate (OLR) of 29.7 g CODL^-1 at an HRT of 24 h. Increase in the methane level above 40% during long-term operation caused decrease in the hydrogen content and HPR to 5.9 ± 1.6% and 2.1 ± 1.1 mmoL-H₂L^-1d^-1, respectively. When methane increased to a high level over 40%, the CSTR at the HRT of 24 h was operated at the HRT of 12 h. Reduction of the HRT from 24 to 12 h led to decrease in the methane content of 12.1 ± 4.44% and recovery of the HPR value to 48.9 ± 15.37 mmol-H₂L^-1d^-1 over a duration of 13-22 d. When methane is generated in a continuously operated reactor, reduction of the HRT can be an easy way to suppress methanogens and recover hydrogen production without any additives or extra treatments.

Effects of manipulating cyclic duration and pH on fermentative hydrogen production in an anaerobic sequencing batch reactor

The effects of cyclic duration and pH on biological hydrogen production were investigated in an anaerobic sequencing batch reactor. Experiments were conducted using cyclic duration of (4, 8, and 12 h) in combination with pH (4, 5, and 6) in a 3 × 3 factorial design, while hydraulic retention time and organic loading rate were maintained at 24 h and 10.3 g COD L(-1).d(-1), respectively. At pH 4, the effect of cyclic duration on hydrogen production was found to be insignificant. However, in runs with pH 5 and 6, a shorter cyclic duration of 4 h led to lower hydrogen productivity. The operational condition (pH 6, cyclic duration 12 h) induced higher hydrogen production rate of 2.3 ± 0.6 L H2/L reactor.d, whereas higher hydrogen yield of 2.2 ± 0.4 mol H2/mol sucrose was achieved at pH 5 and the same 12 h cyclic duration. The differences in hydrogen production were not statistically significant between 8 h and 12 h cyclic duration. Higher hydrogen production rates were associated with biomass (mixed liquor volatile suspended solids) concentration ranging from 8-13 g L(-1), but further increase in biomass growth was not accompanied by increased hydrogen production. Furthermore, a food-to-microorganism ratio of 0.84 was found to result in higher hydrogen production rate.

The use of the carbon/nitrogen ratio and specific organic loading rate as tools for improving biohydrogen production in fixed-bed reactors

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The effects of the carbon/nitrogen ratios of 40, 90, 140, and 190 on hydrogen production are evaluated by varying the nitrogen source in an upflow fixed-bed anaerobic reactor.

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An optimal C/N ratio of 137 to produce 3.5 mol H₂ mol⁻¹ sucrose is estimated by a mathematical approximation.

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Continuous decreases in the specific organic loading rate as a function of time seemed to be responsible for the instability of the system.

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A microbial biology analysis identified hydrogen-producing and -consuming microorganisms from natural inoculum.

This study assessed the effect of the carbon/nitrogen (C/N) ratio on the hydrogen production from sucrose-based synthetic wastewater in upflow fixed-bed anaerobic reactors. C/N ratios of 40, 90, 140, and 190 (g C/g N) were studied using sucrose and urea as the carbon and nitrogen sources, respectively. An optimum hydrogen yield of 3.5 mol H₂ mol⁻¹ sucrose was obtained for a C/N ratio of 137 by means of mathematical adjustment. For all C/N ratios, the sucrose removal efficiency reached values greater than 80% and was stable after the transient stage. However, biogas production was not stable at all C/N ratios as a consequence of the continuous decreasing of the specific organic loading rate (SOLR) when the biomass accumulated in the fixed-bed, causing the proliferation of H₂-consuming microorganisms. It was found that the application of a constant SOLR of 6.0 g sucrose g⁻¹ VSS d⁻¹ stabilized the system.

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

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

Integrating photobiological hydrogen production with dye-metal bioremoval from simulated textile wastewater

The study reports production of hydrogen in photobioreactors with free (PBR(Fr)) and immobilized (PBR(Imm)) Nostoc biomass at enhanced and sustained rates. Before running the photobioreactors, effects of different immobilization matrices and cyanobacterial dose on hydrogen production were studied in batch mode. As hydrogen production in the PBRs declined spent biomass from the photobioreactors were collected and utilized further for column biosorption of highly toxic dyes (Reactive Red 198+Crystal Violet) and metals (hexavalent chromium and bivalent cobalt) from simulated textile wastewater. Breakthrough time, adsorption capacity and exhaustion time of the biosorption column were studied. The photobioreactors with free and immobilized cyanobacterium produced hydrogen at average rates of 101 and 151 μmol/h/mg Chl a, respectively over 15 days, while the adsorption capacity of the spent biomass was up to 1.4 and 0.23 mg/g for metals and 15 and 1.75 mg/g for the dyes, respectively in continuous column mode.