Upscaling of biohydrogen production process in semi-pilot scale biofilm reactor: Evaluation with food waste at variable organic loads

The present account focuses on upscaling of biohydrogen (H₂) production at semi-pilot scale bioreactor using composite food waste. Experiments were conducted at different organic load (6, 12, 18, 30, 40, 50 and 66 g COD/l) conditions. H₂ production increased with an increasing organic load up to 50 g COD/l (9.67 l/h) followed by 40 g COD/l (6.48 l/h), 30 g COD/l (1.97 l/h), 18 g COD/l (0.90 l/h), 12 g COD/l (0.78 l/h) and 6 g COD/l (0.32 l/h). H₂ production was affected by acidification (pH drop to 3.96) at 66 g COD/l operation due to the excess accumulation of soluble metabolites (5696 mg VFA/l). Variation in organic load of food waste influenced the overall hydrogen production efficiency.     

Critical assessment of biofilm and suspended growth reactor configurations for acidogenic biohydrogen production using wastewater as a function of redox microenvironment

Comprehensive experiments were designed to study the relative function of two bioreactor configurations viz., biofilm and suspended growth for dark-fermentative hydrogen (H₂) production along with wastewater treatment at two varying feed pH conditions (6 and 7). Both the reactors were operated in sequencing batch mode using anaerobic inoculum after pretreatment (combined treatment: acid-shock, heat-shock and chemical-shock). Biofilm system showed efficient H₂ production over the corresponding suspended growth operation at feeding pH 6. VFA profiles visualized high acetate fraction supporting feasible microenvironment for higher H₂ production. Voltammogram profiles visualized significant variation in the bio-electrochemical behavior with the function of operating pH and reactor configuration. It can be inferred from this study that biofilm systems are efficient for H₂ production particularly at acidiophilic microenvironment.     

Integration of acidogenic and methanogenic processes for simultaneous production of biohydrogen and methane from wastewater treatment

Feasibility of integrating acidogenic and methanogenic processes for simultaneous production of biohydrogen (H₂) and methane (CH₄) was studied in two separate biofilm reactors from wastewater treatment. Acidogenic bioreactor (acidogenic sequencing batch biofilm reactor, AcSBBR) was operated with designed synthetic wastewater [organic loading rate (OLR) 4.75 kg COD/m³-day] under acidophilic conditions (pH 6.0) using selectively enriched acidogenic mixed consortia. The resultant outlet from AcSBBR composed of fermentative soluble intermediates (with residual carbon source), was used as feed for subsequent methanogenic bioreactor (methanogenic/anaerobic sequencing batch biofilm reactor, AnSBBR, pH 7.0) to generate additional biogas (CH₄) utilizing residual organic composition employing anaerobic mixed consortia. During the stabilized phase of operation (after 60 days) AcSBBR showed H₂ production of 16.91 mmol/day in association with COD removal efficiency of 36.56% (SDR_A—1.736 kg COD/m³-day). AnSBBR showed additional COD removal efficiency of 54.44% (SDR_M—1.071 kg COD/m³-day) along with CH₄ generation. Integration of the acidogenic and methanogenic processes enhanced substrate degradation efficiency (SDR_T—4.01 kg COD/m³-day) along with generation of both H₂ and CH₄ indicating sustainability of the process.     

Predominance of Bacilli and Clostridia in microbial community of biohydrogen producing biofilm sustained under diverse acidogenic operating conditions

Microbial community structure of acidogenic biofilm from long-term operated sequencing batch bioreactor producing biohydrogen was analyzed through culture independent technique. Bioreactor was operated under variable operation and substrate conditions for a period of 1435 days. Phylogenetic distribution showed a significant diversity and illustrated the presence of four dominant operational taxonomic units (OTUs) viz., Bacteroidia, Bacilli, Clostridia, Flavobacteria and Aquificae. Dominance of Clostridia and Bacilli classes were observed each with four OTUs. Majority of OTUs were found to produce fermentative H₂. Even at higher load and under diverse operating conditions bioreactor functioned without any process inhibition which indicates the robustness of sustained microbial community. Community structure of bioreactor was comparatively evaluated with other bioreactor producing H₂, operated with same parent culture and conditions but with different substrates, established the dynamics and shift of microbial diversity which corresponded to diverse substrates used for the bioreactor operation.     

Utilizing acid-rich effluents of fermentative hydrogen production process as substrate for harnessing bioelectricity: An integrative approach

Lower substrate degradation is one of the limiting factors associated with fermentative hydrogen production process. To overcome this, an attempt was made to integrate microbial fuel cell (MFC) as a secondary energy generating process with the fermentative hydrogen (H₂) production. The acid-rich effluents generated from the acidogenic sequential batch biofilm reactor (AcSBBR) producing H₂ by fermenting vegetable waste was subsequently used as substrate for bioelectricity generation in single chambered MFC (air cathode; non-catalyzed electrodes). AcSBBR was operated at 70.4 kg COD/m³-day and the outlet was fed to the MFC at three variable organic loading rates. The final outlet from AcSBBR was composed of fermentative soluble acid intermediates along with residual carbon source. Experimental data illustrated the feasibility of utilizing acid-rich effluents by MFC for both additional energy generation and wastewater treatment. Higher power output (111.76 mW/m²) was observed at lower substrate loading condition. MFC also illustrated its function as wastewater treatment unit by removing COD (80%), volatile fatty acids (79%), carbohydrates (78%) and turbidity (65.38%) effectively. Fermented form of vegetable wastewater exhibited higher improvement (94%) in power compared to unfermented wastewater. The performance of MFC was characterized with respect to polarization behavior, cell potentials, cyclic voltammetry and sustainable power. This integration approach enhanced wastewater treatment efficiency (COD removal, 84.6%) along with additional energy generation demonstrating both environmental and economic sustainability of the process.     

Microbial diversity analysis of long term operated biofilm configured anaerobic reactor producing biohydrogen from wastewater under diverse conditions

This communication provides an insight into the composition of the microbial community survived in the biofilm configured anaerobic reactor operated for biohydrogen (H₂) production using wastewater as substrate under diverse conditions for past four years. PCR amplified 16S rDNA product (at variable V3 region using universal primers 341F and 517R) was separated by using denaturing gradient gel electrophoresis (DGGE) to identify the diversity in microbial population survived. The phyologenetic profile of the bioreactor showed significant diversity in the microbial community where major nucleotide sequences were affiliated to Class Clostridia followed by Bacteroidetes, Deltaproteobacteria and Flavobacteria. Clostridium were found to be dominant in the microbial community observed. The controlled growth conditions, application of pre-treatment to biocatalyst, operation with specific pH and variation in substrate composition are reasoned for the robust acidogenic culture identified in the bioreactor. Most of the operational taxonomic units (OTUs) observed in the bioreactor are capable to undergo acetate producing pathway, feasible for effective H₂ production.     

Biological hydrogen production in an anaerobic sequencing batch reactor: pH and cyclic duration effects

An anaerobic sequencing batch reactor (ASBR) was used to evaluate biological hydrogen production from carbohydrate-rich organic wastes. The goal of the proposed project was to investigate the effects of pH (4.9, 5.5, 6.1, and 6.7), and cyclic duration (4, 6, and 8 h) on hydrogen production. With the ASBR operated at 16-h HRT, 25 g COD/L, and 4-h cyclic duration, the results showed that the maximum hydrogen yield of 2.53 mol H₂/mol sucrose_consumed appeared at pH 4.9. The carbohydrate removal efficiency declined to 56% at pH 4.9, which indirectly resulted in the reduction of total volatile fatty acid production. Acetate fermentation was the dominant metabolic pathway at pH 4.9. The concentration of mixed liquor volatile suspended solid (MLVSS) also showed a decrease from nearly 15,000 mg/L between pHs 6.1 and 6.7 to 6000 mg/L at pH 4.9. Investigation of the effect of cyclic duration found that hydrogen yield reached the maximum of 1.86 mol H₂/mol sucrose_consumed at 4-h cyclic duration while ASBR was operating at 16-h HRT, 15 g COD/L, and pH 4.9. The experimental results showed that MLVSS concentration increased from 6200 mg/L at 4-h cyclic duration to 8500 mg/L at 8-h cyclic duration. However, there was no significant change in effluent volatile suspended solid concentration. The results of butyrate to acetate ratio showed that using this ratio to correlate the performance of hydrogen production is not appropriate due to the growth of homoacetogens. In ASBR, the operation is subject to four different phases of each cycle, and only the complete mix condition can be achieved at react phase. The pH and cyclic duration under the unique operations profoundly impact fermentative hydrogen production.     

Fermentative biohydrogen production by mixed anaerobic consortia: Impact of glucose to xylose ratio

Glucose and xylose are the dominant monomeric carbohydrates present in agricultural materials which can be used as potential building blocks for various biotechnological products including biofuels production. Hence, the imperative role of glucose to xylose ratio on fermentative biohydrogen production by mixed anaerobic consortia was investigated. Microbial catabolic H₂ and VFA production studies revealed that xylose is a preferred carbon source compared to glucose when used individually. A maximum of 1550 and 1650 ml of cumulative H₂ production was observed with supplementation of glucose and xylose at a concentration of 5.5 and 5.0 g L⁻¹, respectively. A triphasic pattern of H₂ production was observed only with studied xylose concentration range. pH impact data revealed effective H₂ production at pH 6.0 and 6.5 with xylose and glucose as carbon sources, respectively. Co-substrate related biohydrogen fermentation studies indicated that glucose to xylose ratio influence H₂ and as well as VFA production. An optimum cumulative H₂ production of 1900 ml for 5 g L⁻¹ substrate was noticed with fermentation medium supplemented with glucose to xylose ratio of 2:3 at pH 6. Overall, biohydrogen producing microbial consortia developed from buffalo dung could be more effective for H₂ production from lignocellulosic hydrolysates however; maintenance of glucose to xylose ratio, inoculum concentration and medium pH would be essential requirements.     

Biomethanation potential of marine macroalgal Ulva biomass in sequencing batch mode: Changes in process performance and microbial community structure over five cycles

Anaerobic digestion (AD) of Ulva biomass, a promising next-generation feedstock for energy production, was investigated in sequencing batch mode. Over five cycles of operation, the methane yield decreased more than twofold (from 0.15 to 0.07 L/g COD_fed), while the organic treatment efficiency (i.e., chemical oxygen demand (COD) removal) remained fairly constant (53.7–61.1%). Such changes in reactor performance were related with structural variations in the microbial community, particularly the bacterial community, with repeated cycles. Methanosaeta- and Methanolinea-related populations were most likely the main aceticlastic and hydrogenotrophic methanogens, respectively, in the reactor. The emergence and prevalence of sulfate-reducing bacteria (SRBs), primarily a Solitalea-related population, most likely resulted in increased consumption of organic substrates for sulfate reduction, rather than methane production, in later cycles. Our observations suggest that the metabolic properties of the reactor changed with the transition of the bacterial community structure over cycles, and the metabolic shift had a negative effect on methanogenesis. The sequencing batch operation strategy applied in this study was not suitable for maximizing methane production from Ulva biomass, although the treatment efficiency was fairly stable. Robust control of SRB activity is necessary for more stable and efficient biomethanation of Ulva biomass in sequencing batch mode.     

Leachate valorization in anaerobic biosystems: Towards the realization of waste-to-energy concept via biohydrogen,biogas and bioelectrochemical processes

Leachate generated in landfills is considered as a hazardous waste stream due to its composition and needs adequate treatment for environmental protection purposes. Nonetheless, a contemporary technology should not only be able to deal with its degradation, but at the same time, recover energy in various forms. Such valorization approaches with priority on these dual-aims are potentially those that rely on anaerobic biosystems. In the literature, processes considered on that matter include fermentative, digestive and bioelectrochemical set-ups to deliver energy-carriers such as biohydrogen (DF), biogas (AD) and electricity (BES), respectively. Moreover, to enhance the global efficiency of leachate utilization, it has been recently trending to develop integrated options by combining these systems (DF, AD, BES) into a cascade scheme. In this review, it is intended to give an insight to the research activities realized in these fields and show possible directions towards the better exploitation of leachate feedstock under anaerobic conditions.