Maximum Volumetric Productivity Research Articles

Simultaneous hydrogen and methane production by tequila vinasses dark fermentation in series with anaerobic digestion

Biofuels production from agro-industrial residues is a promising alternative for obtaining energy from renewable resources. Here, vinasses (tequila manufacture waste) were used as substrate to an anaerobic system comprised by a Continuous Stirred Tank Reactor (CSTR) with biomass carriers and an Upflow Anaerobic Sludge Blanket reactor (UASB) connected in series. The effect of the organic loading rate on the hydrogen and methane generation was evaluated through increase in fed vinasse concentration. Maximum volumetric production rates of 2.2 L H2/L-d in the CSTR and 0.12 L CH4/L-d in the UASB were obtained when feeding 20 g chemical oxygen demand per liter from vinasse diluted to 60 % of its original concentration. Feeding more concentrated vinasses provoked a decrease in the production of hydrogen and methane, coinciding with a 5 g/L-peak in reducing sugars in the CSTR, suggesting overload and acidification as reasons of the diminished performance. The highest hydrogen and methane equivalent energy obtained in the anaerobic system was estimated to be 2.6 kJ/L-d.

Co-digestion of solid-liquid waste from citrus agroindustrial: Effect of hydraulic retention time and organic loading rate on H2 production in a long-term continuous operation leach bed reactor

The Leach Bed Reactor (LBR) application is a promising alternative for the anaerobic digestion of solid waste and wastewater. In this study, the LBR was continuously operated for 160 days in four different phases, for co-fermentation of solid and liquid waste from the citrus agroindustry, at 37 °C, evaluating the effect of hydraulic retention time (HRT) and organic loading rate (OLR). The maximum volumetric hydrogen production (VHPR) was observed at an HRT of 12h (958.3 mLH2.L-1.d-1); however, the highest hydrogen yield (HY) was identified at an HRT of 24h (104.9 mLH2.g-1CHO). Based on the metabolic characteristics of microorganisms identified in higher relative abundance, by 16S rRNA sequencing, such as Lactobacillus, Caproicoproducens, Bifidobacterium, Olsenella, and Solobacterium, an occurrence of chain elongation pathways in the production of butyric and caproic acids was inferred, with hydrogen production resulting from the lactic acid oxidation. The maximum electricity production potential of 16.3 kWh.m-3 liquor was estimated.

Efficient Biorefinery Based on Designed Lignocellulosic Substrate for Lactic Acid Production

The current study investigated the feasibility of developing and adopting a few state-of-the-art fermentation techniques to maximize the efficiency of the lignocellulosic waste bioconversion. There have been various efforts towards utilizing the fermentable sugars released from the specific parts of lignocellulose, i.e., cellulose and hemicellulose. However, complete utilization of carbon sources derived from lignocellulosic biomass remains challenging owing to the generated glucose in the presence of β-glucosidase, which is known as glucose-induced carbon catabolite repression (CCR). To overcome this obstacle, a novel simultaneous saccharification and fermentation (SSF) of lactic acid was designed by using Celluclast 1.5L as a hydrolytic enzyme to optimize the generation and utilization of pentose and hexose. Under the optimal enzyme loading and pH condition, 53.1 g/L optically pure L-lactic acid with a maximum volumetric productivity of 3.65 g/L/h was achieved during the SSF from the brewer’s spent grain without any nutrient supplementation. This study demonstrated the potential of lactic acid production from the designed lignocellulosic substrate.

Continuous production of succinic acid from glycerol: A complete experimental and computational study

In this work, a bioprocess for the fermentation of A. succinogenes for the production of succinic acid from glycerol was developed, employing a continuous bioreactor with recycle. Moreover, a new bioprocess model was constructed, based on an existing double substrate limitation model, which was validated with experimental results for a range of operating parameters. The model was used to successfully predict the dynamics of the continuous fermentation process and was subsequently employed in optimisation studies to compute the optimal conditions, dilution rate, reflux rate and feed glycerol concentration, that maximise the productivity of bio-succinic acid. In addition, a Pareto front for optimal volumetric productivity and glycerol conversion combinations was computed. Maximum volumetric productivity of 0.518 g/L/h, was achieved at the optimal computed conditions, which were experimentally validated. This is the highest bio-succinic acid productivity reported so far, for such a continuous bioprocess.

Feasibility Study of Biohydrogen Production from Acid Cheese Whey via Lactate-Driven Dark Fermentation

The high loading of lactic acid bacteria (LAB) present in cheese whey still limits its use as hydrogen feedstock. This study aims to investigate the feasibility of producing hydrogen from acid cheese whey via lactate-driven dark fermentation (LD-DF). Mesophilic batch fermentations were performed with delipidated acid cheese whey at a fixed pH of 5.8 and driven by an acidogenic bacterial culture containing LAB and lactate-oxidizing hydrogen producers (LO-HPB). The results obtained indicated that it is technically feasible to produce hydrogen from undiluted cheese whey through lactate oxidation-mediated fermentation. It was elucidated that the acidogenic fermentation of cheese whey followed a two-step lactate-type fermentation, in which fermentable carbohydrates were first converted into lactate, and then lactate was metabolized into hydrogen with the co-production of butyrate. The hydrogen yield and the maximum volumetric hydrogen production rate achieved were 44.5 ± 2.9 NmL/g-CODfed and 1.9 NL/L-d, respectively. Further microbial community analysis revealed that Lactobacillus, Clostridium, and Klebsiella were the dominant bacterial genera when the hydrogen production rate peaked. It was therefore suggested that the metabolic potential behind the association between LAB and LO-HPB was important in driving the two-step lactate-type fermentation. Overall, the LD-DF can be a strategic hydrogen-producing pathway to be implemented with cheese whey.

Biotic and abiotic insights into the storage of food waste and its effect on biohydrogen and methane production potential

The present study investigated the physicochemical and microbiological changes occurring during the storage of simulated restaurant food waste (FW) and how such changes affected its biohydrogen and biogas production potential. FW was stored for 72 h in a closed atmosphere under two different scenarios: i) without and ii) with inoculation of a mixed microbial culture harboring lactic acid bacteria (LAB). Both storage scenarios resulted in similar biotic and abiotic changes in FW. Particularly, FW was pre-acidified and pre-hydrolyzed to some extent during the storage, resulting in a feedstock enriched in LAB (≈ 95 % total relative abundance) and lactate (10.5–12.3 g/L, 87.0–90.5 % selectivity). Biochemical hydrogen potential tests revealed that the use of stored FW resulted in similar or even higher hydrogen production efficiencies compared to that of non-stored FW, achieving up to 60 NmL H2/g VS added and a maximum volumetric hydrogen production rate of 9.7 NL H2/L-d. Metabolically, the conversion of lactate into hydrogen was crucial regardless of the use of non-stored or stored FW, albeit the presence of fermentable carbohydrates in the substrate was also essential either to produce lactate or to co-produce extra hydrogen. On the contrary, biochemical methane potential tests showed that the biogas production potential of FW was not affected by storage, yielding on average 400 NmL CH4/g VS added and revealing that lactate oxidation to methane precursors represented an important step in FW biomethanization.

Optimization of co-culture condition with respect to aeration and glucose to xylose ratio for bioethanol production

ABSTRACT The present study was designed to find a suitable microaerobic condition and ratio of glucose and xylose for maximum ethanol production using co-culture of Saccharomyces cerevisiae and Pichia stipitis. The maximum ethanol concentration and yield were achieved at 0.05 vvm aeration rate and 2:1 glucose/xylose ratio. The co-culture resulted in maximum ethanol concentration, ethanol yield, and volumetric productivity of 12.33 ± 0.10 g/L, 0.43 g/g, and 0.26 g/L/h, respectively. While, the monoculture of P. stipitis resulted in 8.96 ± 0.13 g/L, 0.36 g/g, and 0.19 g/L/h respectively. The fermentation carried out in microaerobic mode delivered 10.68% and 10.56% more ethanol concentration and ethanol yield respectively from glucose compared to the combination of anaerobic and microaerobic mode. Also, the glucose uptake rate increased to 0.83 g/L/h, which corresponds to an improvement of 50.16%, suggesting that the lower microaerophilic condition not only supports P. stipitis metabolism but also does S. cerevisiae to convert glucose faster in a co-culture system. Hence, co-culture cultivation in microaerobic mode would be a better condition to achieve maximum ethanol and productivity.

From start-up to maximum loading: An approach for methane production in upflow anaerobic sludge blanket reactor fed with the liquid fraction of fruit and vegetable waste

This investigation provides a reproducible approach for determining the limits of an upflow anaerobic sludge blanket (UASB) reactor designed for the methanization of the liquid fraction of fruit and vegetable waste (FVWL). Two identical mesophilic UASB reactors were operated for 240 days with a three-day fixed hydraulic retention time and an organic load rate (OLR) increased from 1.8 to 10 gCOD L−1 d−1. Because of the previous estimation of flocculent-inoculum methanogenic activity, it was possible to design a safe OLR for the quick start-up of both UASB reactors. The operational variables obtained from the operation of the UASB reactors did not show statistical differences, ensuring the experiment's reproducibility. As a result, the reactors achieved methane yield close to 0.250 LCH4 gCOD−1 up to the OLR of 7.7 gCOD L−1 d−1. Furthermore, the maximum volumetric methane production rate of 2.0 LCH4 L−1 d−1 was discovered for the OLR ranges between 7.7 and 10 gCOD L−1 d−1. The possible overload at OLR of 10 gCOD L−1 d−1 resulted in a significant reduction of methane production in both UASB reactors. Based on the methanogenic activity of the UASB reactors sludge, a maximum loading capacity of approximately 8 gCOD L−1 d−1 was estimated.

Culture depth effect on Scenedesmus sp. growth, photo-physiology and nutrient removal rate in anaerobically digested abattoir effluent

Here, we evaluated the effect of culture depth on the growth and nutrient removal rate of Scenedesmus sp. grown in anaerobically digested abattoir effluent (ADAE) and cultivated semi-continuously in raceway ponds at depths of 14 cm, 17 cm, 20 cm, and 23 cm during Austral winter and summer. Culture medium pH was kept constant at pH 6.5 for all treatments throughout the experimental period using a pH–stat system. Algal cultures grown in summer had 2.3- 2.7 times higher biomass productivity than the same grown in winter. In both seasons, maximum volumetric productivity of this alga was achieved at 14 cm depth (14 cm depth⩾ 17 cm depth⩾ 20 cm depth⩾ 23 cm depth). However, areal biomass productivity of culture grown at 23 cm depth was 12% and 29% higher than that of culture grown at 14 cm depth in winter and summer, respectively. In addition, nitrogen, phosphorus and COD areal removal rates were significantly higher in cultures operated at 23 cm among all treatments in both seasons. The effective quantum yield (Fq'/Fm') in summer was 23 cm depth = 20 cm depth > 17 cm depth = 14 cm depth while it followed 14 cm depth⩾ 17 cm depth⩾ 20 cm depth⩾ 23 cm depth in winter, indicating significance of operational conditions on algal photosynthesis. The outcome of this study shows that, irrespective of the season, operating the culture in higher depths significantly increased areal biomass productivity as well as areal nutrient removal rates when treating ADAE.

High cell density culture of Neochloris oleoabundans in novel horizontal thin-layer algal reactor: Effects of localized aeration, nitrate concentration and mixing frequency

A horizontal thin-layer algal reactor was designed, constructed and tested for high cell density culture of an oil-rich green alga, Neochloris oleoabundans, at a thickness of 6 mm. A hollow fiber membrane sparger was used for localized aeration with CO2-enriched air, which allowed control of culture pH at a narrow range of pH 7.0–7.5. The optimal nitrate concentration under the investigated conditions was 345 mg-N/L with a cell density of ∼6.6 × 108 cells/mL and biomass concentration of ∼8 g/L. The volumetric and areal biomass productivities were 1.81 g/L/day and 10.85 g m− 2/day, respectively, and the maximum volumetric and areal productivities 3.57 g/L/day and 21.42 g m− 2/day, respectively. Intermittent mixing at a frequency of once every two hours (five min mixing each time) produced a cell growth profile similar to that of continuous mixing. Such intermittent mixing resulted in 98.38 % saving on mixing energy and 33.9 % reduction in water evaporation. It was observed that metabolic stress such as high pH and high nitrate concentration could inhibit cell division while causing cell size to enlarge abnormally. The results indicate that it is technically feasible to have high cell density culture of photoautotrophic microalgae with intermittent mixing and no culture circulation, opening the door towards the development of open algal farming systems of low capital, operational, and maintenance costs.

Biomass immobilization in hydrolyzed lignocellulosic material can enhance biohydrogen production from cassava residues?

This study evaluated cassava stems (CS) as support material and a potential co-substrate in dark fermentation. Pre-tests were carried out with cell immobilization in CS without hydrolysis and submitted to acid and steam hydrolysis. Subsequently, hydrogen production was evaluated in an anaerobic sequencing batch biofilm reactor inoculated with biomass immobilized in CS, using cassava starch wastewater as substrate (OLR of 11 and 15 gCarb L−1d−1). The reactor was run for 180 cycles with maximum volumetric hydrogen productivity and a yield of 1.48 LH2 L−1d−1 and 1.98 molH2 kg−1Carb (OLR 15 gCarb L−1d−1). The carbohydrate conversion remained above 97% in both assays, with a predominance of the acetate-ethanol route. During the assays, the Food/Microorganisms ratio remained between 0.8 and 1.0 gCarb gTVSd−1, promoting the biomass control in the reactor. The structural characterization of CS before and after fermentation indicates that the cellulose, hemicellulose, and lignin content in the stems were changed after hydrolysis and fermentation, confirming the material degradation. In addition, the hydrolysis increased the CS surface area and favored cell immobilization of hydrogen-producing microorganisms such as bacteria of the genus Clostridium and Hydrogenispora, demonstrating that CS can be an alternative support material and co-substrate to be explored in dark fermentation.

Optimization of Dilute Acid Pretreatment for Enhanced Release of Fermentable Sugars from Sugarcane Bagasse and Validation by Biophysical Characterization

Pretreatment of biomass is one of the most challenging steps in the process of second-generation (2G) ethanol and biochemical production. Dilute acid pretreatment is a widely adapted and convenient method to recover pentose (C5) as well as hexose (C6) sugars due to its featured solubilization of hemicellulose and cellulose before and after enzymatic saccharification, respectively. In the present study, dilute sulfuric acid (H2SO4) pretreatment of sugarcane bagasse (SCB) was statistically optimized using the face-centered composite design (FCCD) of response surface methodology (RSM) in terms of acid concentration (0.1–3% v/v), solid loading (5–20% w/v) and residence time (15–60 min) at constant temperature of 121 °C followed by enzymatic hydrolysis using commercial cellulase (Novozymes Cellic CTec2) for enhanced combined sugar yield (CSY) comprising of C5 and C6 sugars in pretreated as well as saccharified hydrolysates. Optimized process parameters found in the study were 2.18% (v/v) acid; 14.35% (w/v) solid loading; and 29.49 min residence time. CSY under optimized conditions was found to be 521.42 ± 7.2 g/kg raw SCB with 72.06 ± 1.0% sugars recovered out of the maximum theoretical sugars present in the raw biomass. Total reducing sugar yields in pretreated and saccharified hydrolysates were found to be 215.28 ± 2.4 and 306.14 ± 5.3 g/kg raw SCB, respectively. Morphological and structural changes in optimized pretreated and saccharified biomass further validated the efficiency of optimized pretreatment applied in the present study. The maximum ethanol concentration, volumetric productivity and yield from released sugars were calculated as 10.82 ± 2.2 g/L, 0.45 ± 0.9 g/L/h and 0.42 g/g-glucose consumed or 71.45 ± 2.5 g/kg raw SCB, respectively. Ethanol yield obtained from the fermentation of dilute H2SO4-pretreated SCB was corresponding to 82.4% of the theoretical ethanol yield.

Addition of biomass ash as a promising strategy for high-value biohythane production from the organic fraction of municipal solid waste

In this work, the effect of adding ash from combustion of six locally available biomass types, namely common reed (Phragmites australis), corn stover, eucalyptus sawdust, hazelnut shells, pine chips, and rice husks at different concentrations (2.5, 5, 10, and 20 g/L) on H2 production from the organic fraction of municipal solid waste (OFMSW) was first investigated through batch experiments. The maximum cumulative H2 production of 1124 ± 3.21 mL was achieved using 5 g/L of rice husk ash (RHA), increasing by 644% compared to the control. Then, in a semi-continuous two-stage biohythane production process, the first reactor dedicated to H2 production was supplemented with 5 g/L of RHA and its performance was evaluated at three HRTs (3, 2, and 1 day). The results showed that the highest steady-state volumetric H2 production rate (554 ± 5.48 mL/L/d) and H2 yield (184.7 ± 1.83 mL/g-VSadded) were obtained at 2-day HRT. In the subsequent CH4 production stage, of the three HRTs tested (12, 8, and 4 days), the 12-day HRT resulted in a maximum volumetric CH4 production rate and CH4 yield of 321 ± 2.39 mL/L/d and 727.2 ± 5.46 mL/g-VSadded, respectively, under steady-state conditions. Throughout both stages, the pH remained within a suitable range without the addition of any commercial reagents. The overall energy yield of the two-stage process was found to be 31.2 kJ/g-VSadded, which corresponded to a total COD removal efficiency of 96%. Moreover, the composition of biohythane (H2 + CH4) obtained met the standards of a clean hythane fuel.

An integrated algal membrane photobioreactor as a green-transition technology for the carbon capture and utilization

The harvesting of microalgae strongly affects their effective application as a source of biomass for fuels and chemicals. The aim of this study was the evaluation of the performance of the recently developed Encapsulated Self-Forming Dynamic Membrane (ESFDM) module integrated into a cylindrical photobioreactor (PBR) for the efficient and simultaneous microalgae biomass cultivation and harvesting. The effect of different permeate fluxes was investigated (90–180 L m−2 h−1) obtaining excellent results in terms of harvesting rates (54.23–117.36 g m−2 h−1). Microalgal lipids content was determined under different nutrient regimes, including starvation periods for enhancing the content of lipids. A maximum volumetric biomass productivity of 257 mg L−1d−1 was achieved at a nitrogen content of 15.7 ± 10.6 mg L−1. On the other hand, the highest lipids productivity (42 mg L−1 d−1) was obtained during the starvation phase of nitrogen nutrients. The harvesting rate appeared to be strongly dependent on permeate flux, since the highest permeate flow corresponded in increase of the biomass harvested. The innovative SFDM enormously facilitates microalgae recovery. Indeed, the utilization of the SFDM has been demonstrated as an effective solution to increase the biomass harvesting rate while maintaining biomass concentration inside the photo-bioreactor below 1 g L−1 with a view at ensuring an efficient penetration of light into the membrane photobioreactor (mPBR) and consequently boosting the photosynthetic activity. The simultaneous implementation of the nitrogen starvation strategy can further increase the energy recovery potential from the biomass to address an algal biorefinery approach, besides reducing resources consumption.

Lactic acid production from food waste using the lactogenic Escherichia coli strain JU15: optimization of reducing sugar recovery

AbstractBACKGROUNDd‐Lactic acid (DLA) has numerous industrial applications, including production of biodegradable polylactic acid, which is one of the most promising biodegradable polymers. Looking for alternative feedstocks is a high priority and recent focus has been on its biological synthesis and biotechnological valorization from food waste (FW) materials. In this study, a response surface methodology (RSM) approach and central composite design (CCD) were performed to hydrolyze FW, obtaining released reducing sugars (RRS) for DLA production using the lactogenic Escherichia coli strain JU15.RESULTSAfter an optimized sulfuric acid‐based pretreatment methodology using RSM and CCD, FW was utilized to obtain the maximum concentration of RRS, obtaining liquors containing glucose (85.59%), fructose (5.48%) and arabinose (8.92%) with an efficiency of ∼98% total sugar conversion from hydrolysis of FW. As the total sugar sources obtained titers of 26.85 ± 1.28 g L−1 (DNS) and DLA production was 22.66 ± 1.01g L−1, the DLA yield was 0.99 g g −1 total sugar. While arabinose was discretely consumed, with a remaining concentration of 0.36 ± 0.02 g L−1 after culture, the production of DLA through bacterial fermentation of the FW hydrolysates by the JU15 strain showed total depletion of glucose and fructose, achieving a maximum DLA production of 22.66 g L−1, with a maximum volumetric productivity of 0.73 g L−1 h−1 at 36 h.CONCLUSIONThis study provides an experimental optimization of a pretreatment to obtain released reducing sugars to lactic acid production from food waste as feedstock using the lactogenic E. coli strain JU15. © 2021 Society of Chemical Industry (SCI).

Saccharification of agave bagasse with Cellulase 50 XL is an effective alternative to highly specialized lignocellulosic enzymes for continuous hydrogen production

Agave bagasse is the lignocellulosic residue of the tequila industry and a potential feedstock for hydrogen production (H2) via dark fermentation after proper saccharification. Highly specialized enzymes for hydrolysis of lignocellulosic biomass are costly, hindering their application in continuous processes. This work investigated the use of a generic cellulase (Cellulase 50 XL), to prepare the hydrolysates from agave bagasse, either alone or combined with a highly specialized hemicellulase (Viscozyme L). The combination of Cellulase 50 XL and Viscozyme L increased the release of both total carbohydrates and chemical oxygen demand (COD) in 28.5% and 22.2%, respectively compared with Cellulase 50 XL. Conversely, the maximum volumetric H2 production rate (VHPR, 9.9 L H2/L-d) reached with the Cellulase 50 XL hydrolysates in continuous stirred tank reactors at an organic loading rate of 100 g COD/L-d, is the highest VHPR reported so far with enzymatic agave bagasse hydrolysates obtained with non-specialized enzyme. The estimated cost of H2 production using Cellulase 50 XL at lab-scale was 0.7 USD/L H2. From a systematic comparison of H2 productivities and lab-scale costs with similar studies it was concluded that: i) the composition, and not only the concentration of total carbohydrates plays a critical role in the performance of H2 production, ii) Cellulase 50 XL was one of the best cost-effective alternatives despite being designed for food sectors and not for the hydrolysis of lignocellulosic biomass. Overall, considering H2 productivities and cost of H2 production, Cellulase 50 XL hydrolysates resulted an appropriate option for cost-effective continuous H2 production.

Low cost and sustainable hyaluronic acid production in a manufacturing platform based on Bacillus subtilis 3NA strain.

Hyaluronic acid (HA) is a high value glycosaminoglycan mostly used in health and cosmetic applications. Commercial HA is produced from animal tissues or in toxigenic bacteria of the genus Streptococcus grown in complex media, which are expensive and raise environmental concerns due to the disposal of large amounts of broth with high organic loads. Other microorganisms were proposed as hosts for the heterologous production of HA, but the methods are still costly. The extraordinary capacity of this biopolymer to bind and retain water attracts interest for large-scale applications where biodegradable materials are needed, but its high cost and safety concerns are barriers for its adoption. Bacillus subtilis 3NA strain is prototrophic, amenable for genetic manipulation, GRAS, and can rapidly reach high cell densities in salt-based media. These phenotypic traits were exploited to create a platform for biomolecule production using HA as a proof of concept. First, the 3NA strain was engineered to produce HA; second, a chemically defined medium was formulated using commodity-priced inorganic salts combined at the stoichiometric ratios needed to build the necessary quantities of biomass and HA; and third, a scalable fermentation process, where HA can be produced at the maximum volumetric productivity (VP), was designed. A comparative economic analysis against other methods indicates that the new process may increase the operating profit of a manufacturing plant by more than 100%. The host, the culture medium, and the rationale employed to develop the fermentation process described here, introduce an IP-free platform that could be adaptable for production of other biomolecules. KEY POINTS: • A biomolecule production platform based on B. subtilis 3NA strain and a synthetic medium was tested for hyaluronic acid biosynthesis • A fermentation process with the maximum volumetric productivity was designed • A techno-economic analysis forecasts a significant reduction in the manufacturing cost compared to the current methods.

Developing microalgal oil production for an outdoor photobioreactor

In this paper the preparations are described to develop a production of oil rich microalgal biomass under south European conditions. Ten microalgal species were compared in shake flasks in an incubator for potential for oil production. Potential oil production capacity was assayed as maximum total fatty acid (TFA) concentration and volumetric TFA productivity. TFA concentration ranged from 5 to 40% DW while TFA productivity rate ranged from 0 to 204 mg TFA L−1 day−1. To control the oil enrichment process in the outdoor microalgal batch culture, a quadratic equation was proposed, predicting the TFA concentration based on biomass inverse nitrogen quota. A concentrated substrate was developed to add to sea water, made from natural sea-salt and tap water.

C/N Ratio and Specific Growth Rate Plays Important Role on Enhancing Isoprene Production in Recombinant Escherichia coli.

Effect of fermentation parameters such as C/N ratio, specific growth rate, phosphate limitation, and plasmid instability on enhancing isoprene production is the focus of the current study. Isoprene productivity in the recombinant Escherichia coli K12_MVA strain showed a bell-shaped relationship with specific growth rate in bioreactor studies with isoprene volumetric productivity peaking at 0.35/h. This behavior was depicted by a production inhibition kinetic model which envisaged a serious competition between the cellular growth, acetic acid production, and isoprene biosynthesis. The model equation derived showed a reasonable fit with the experimental values. Judicious control of the growth rates and acetate accumulation by optimizing C/N ratio, phosphate concentration, and intermittent feeding strategy resulted in maximizing the carbon flux towards isoprene. Plasmid instability caused by metabolic burden posed by the presence of dual plasmids on the bacteria was simulated using first-order degradation kinetics. The experimental plasmid loss trend was in accordance with the model simulated trend, where higher plasmid loss correlated with higher specific growth rates. Modulating the growth rate, acetate accumulation, and plasmid instability resulted in achieving maximum isoprene volumetric productivity of 1.125 g/l/h with 46.67% of carbon flux towards isoprene and a isoprene titre of 18 g/l in 16 h fermentation run.

Assessment of a novel single-stage integrated dark fermentation-microbial fuel cell system coupled to proton-exchange membrane fuel cell to generate bio-hydrogen and recover electricity from wastewater

The production of bio-hydrogen and bio-electricity from biological processes has become a relevant scientific subject in recent years, given that renewable energy can be obtained from organic substrate. Dark fermentation (DF), and microbial electrolysis cells (MECs) have been used to produce bio-hydrogen. In this study, a novel integrated single-stage DF-Microbial fuel cells (MFCs) system was developed to generate bio-hydrogen and electricity simultaneously from wastewater treatment. A proton-exchange membrane fuel cell (PEMFC) was used to produce electricity from the bio-hydrogen gas generated by the integrated system. The effect of hydraulic retention time (HRT) on the bio-hydrogen production and electricity generation was examined. The maximum volumetric bio-hydrogen production rate (VHPR) was 0.44 L H2/L.d (0.66 L H2/g CODremoved), simultaneously obtaining an electricity production of 530 mV (100 mW/m2) at a HRT of 8 d. According to the 16S rRNA gene-based analysis, the microorganisms identified on the anode of the integrated system were: Chryseobacterium, Azotobacter, Bacillus, Enterococcus, Citrobacter, and Methanobacterium. The PEMFC employed to generate voltage using the bio-hydrogen generated from the integrated system was able to produce a maximum voltage of 459 mV (367 mW), with a maximum cell efficiency of 44% (fuel usage of 1.5 × 10−5 mol/h).