Effect of the accuracy of pH control on hydrogen fermentation

The effect of a two-stage anaerobic digestion on digestates: Organic matter quality and microbial communities

The impact of a two-stage anaerobic digestion (AD) system integrating a dark fermentation (DF) step prior to AD, referred to as DF-AD2, on the organic matter (OM) quality and the microbial communities in anaerobic digestates was investigated. Two treatment routes (one-stage AD (AD1) and DF-AD2) were compared by advanced characterization using the same feedstock and treatment duration. The DF-AD2 improved the percentage of CH4 during AD2 by 8.3 % and the volatile solids removal by 6.8 % compared to AD1. The DF step increased the dissolved OM and mineralized nitrogen after AD despite similar OM complexity and predicted carbon mineralization in soils. Moreover, respirometry tests related the enhanced bioaccessibility of DF effluent to greater biological activity (126.3 ± 5.8 mg O2) compared to the substrate (51.1 ± 5.8 mg O2). Nonetheless, DF-AD2 did not impact the biological stability of digestates (32.09 ± 1.1 and 30.2 ± 1.5 mg O2 for AD1 and DF-AD2, respectively). Low-stress operational conditions of the tests might smooth the DF-AD2 effect on digestate biological stability and microbial communities. Archaea varied after DF but homogenized during AD2, with the genus Methanosarcina comprising 71-79 % of the relative abundance. Concurrently, the orders Bacteroidales, Spirochaetales and Cloacimonadales dominated Bacteria in both AD1 and AD2. Overall, this study evidence that a DF-AD2 system is a feasible way to improve both OM removal and the nitrogen fertilizing value of digestates, without hindering digestate biological stability or microbial communities. However, optimizing operational parameters and pre-treatment processes may be necessary to enhance the system's energy output.

Biomass-Based Hydrogen Extraction and Accompanying Hazards-Review

Nowadays, there is an increased demand for energy, the access to which, however, is limited due to the decreasing of fossil sources and the need to reduce emissions, especially carbon dioxide. One possible remedy for this situation is using hydrogen as a source of green energy. Hydrogen is usually bound to other chemical elements and can be separated via energy-intensive few-step conversion processes. A few methods are involved in separating H2 from biomass, including biological and thermochemical (TC) ones. Such methods and possible hazards related to them are reviewed in this study.

Presence of lactic acid bacteria in hydrogen production by dark fermentation: competition or synergy

Dark fermentation in mixed cultures has been extensively studied due to its great potential for sustainable hydrogen production from organic wastes. However, microbial composition, substrate competition, and inhibition by fermentation products can affect hydrogen yield and production rates. Lactic acid bacteria have been identified as the key organisms in this process. On one hand, lactic acid bacteria can efficiently compete for carbohydrate rich substrates, producing lactic acid and secreting bacteriocins that inhibit the growth of hydrogen-producing bacteria, thereby decreasing hydrogen production. On the other hand, due to their metabolic capacity and synergistic interactions with certain hydrogen-producing bacteria, they contribute positively in several ways, for example by providing lactic acid as a substrate for hydrogen generation. Analyzing different perspectives about the role of lactic acid bacteria in hydrogen production by dark fermentation, a literature review was done on this topic. This review article shows a comprehensive view to understand better the role of these bacteria and their influence on the process efficiency, either as competitors or as contributors to hydrogen production by dark fermentation.

Evaluation of individual and combined effect of lactic acid-consuming bacteria on mesophilic hydrogen production from lactic acid effluent from food waste treatment

Lactic acid has been applied as a precursor for hydrogen (H2) production from substrates rich in lactic acid bacteria (LAB), focusing on microbial interactions between producing and consuming LAB tested with model substrates. Therefore, this study evaluated the effect of single and combined lactic acid-consuming bacteria on mesophilic H2 production in batch tests from lactic acid from fermented food waste (FW). Megasphaera elsdenii, Clostridium beijerinckii, and Clostridium butyricum were inoculated at different ratios (v/v). Additionally, thermal pretreated sludge (TPS) was added to the strain mixtures. The highest production was obtained with M. elsdenii, C. beijerinckii, and C. butyricum (17:66:17 ratio), obtaining 1629.0 mL/Lreactor. The optimal mixture (68:32:0 of M. elsdenii and C. beijerinckii) enriched with TPS reached 1739.3 ± 98.6 mL H2/Lreactor, consuming 98 % of lactic acid added. M. elsdenii and Clostridium strains enhance H2 production from lactic acid as they persist in a microbial community initially dominated by LAB.

Non-native Pathway Engineering with CRISPRi for Carbon Dioxide Assimilation and Valued 5-Aminolevulinic Acid Synthesis in Escherichia coli Nissle

Carbon dioxide emission and acidification during chemical biosynthesis are critical challenges toward microbial cell factories' sustainability and efficiency. Due to its acidophilic traits among workhorse lineages, the probiotic Escherichia coli Nissle (EcN) has emerged as a promising chemical bioproducer. However, EcN lacks a CO2-fixing system. Herein, EcN was equipped with a simultaneous CO2 fixation system and subsequently utilized to produce low-emission 5-aminolevulinic acid (5-ALA). Two different artificial CO2-assimilating pathways were reconstructed: the novel ribose-1,5-bisphosphate (R15P) route and the conventional ribulose-5-phosphate (Ru5P) route. CRISPRi was employed to target the pfkAB and zwf genes in order to redirect the carbon flux. As expected, the CRISPRi design successfully strengthened the CO2 fixation. The CO2-fixing route via R15P resulted in high biomass, while the engineered Ru5P route acquired the highest 5-ALA and suppressed the CO2 release by 77%. CO2 fixation during 5-ALA production in EcN was successfully synchronized through fine-tuning the non-native pathways with CRISPRi.

Enhancing and monitoring spore production in Clostridium butyricum using pH-based regulation strategy and a robust soft sensor based on back-propagation neural networks

Clostridium butyricum is a probiotic that forms anaerobic spores and plays a crucial role in regulating gut microbiota. However, the total viable cell count and spore yield of C. butyricum in industrial production are comparatively low. To this end, we investigated the metabolic characteristics of the strain and proposed three distinct pH regulation strategies for enhancing spore production. In addition, precise measurement of fermentation parameters such as substrate concentration, total viable cell count, and spore concentration is crucial for successful industrial probiotics production. Nevertheless, online measurement of these intricate parameters in the fermentation of C. butyricum poses a considerable challenge owing to the complex, nonlinear, multivariate, and strongly coupled characteristics of the production process. Therefore, we analyzed the capacitance and conductivity acquired from a viable cell sensor as the core parameters for the fermentation process. Subsequently, a robust soft sensor was developed using a seven-input back-propagation neural network model with input variables of fermentation time, capacitance, conductivity, pH, initial total sugar concentration, ammonium ion concentration, and calcium ion concentration. The model enables the online monitoring of total viable biomass count, substrate concentrations, and spore yield, and can be extended to similar fermentation processes with pH changes as a characteristic feature.

Unrevealing the role of metal oxide nanoparticles on biohydrogen production by Lactobacillus delbrueckii

The positive interaction between Clostridium sp. and lactic acid-producing bacteria (Lactobacillus sp) is commonly seen in various high-rate hydrogen production systems. However, the exact role of the hydrogen production ability of Lactobacillus sp in a dark fermentation production system is rarely studied. Lactobacillus delbrueckii was herein used for the first time, to the best of the author's knowledge, to demonstrate biohydrogen production under anaerobic conditions. At first, the pH condition was optimized, followed by the addition of nanoparticles for enhanced biohydrogen production. Under optimized conditions of pH 6.5, substrate concentration 10 g/L, and 100 mg/L of NiO/Fe₂O₃, the maximum hydrogen yield (HY) of 1.94 mol/mol hexose was obtained, which is 18 % more than the control. The enhanced H₂ production upon the addition of nanoparticles is supported via the external electron transfer (EET) mechanism, which regulates the metabolic pathway regulation with increased production of acetate and butyrate and reduced formation of lactate.

Influence of key operational parameters on biohydrogen production from fruit and vegetable waste via lactate-driven dark fermentation

This study aims at investigating the influence of operational parameters on biohydrogen production from fruit-vegetable waste (FVW) via lactate-driven dark fermentation. Mesophilic batch fermentations were conducted at different pH (5.5, 6.0, 6.5, 7.0, and non-controlled), total solids (TS) contents (5, 7, and 9%) and initial cell biomass concentrations (18, 180, and 1800 mg VSS/L). Higher hydrogen yields and rates were attained with more neutral pH values and low TS concentrations, whereas higher biomass densities enabled higher production rates and avoided wide variations in hydrogen production. A marked lactate accumulation (still at neutral pH) in the fermentation broth was closely associated with hydrogen inhibition. In contrast, enhanced hydrogen productions matched with much lower lactate accumulations (even it was negligible in some fermentations) along with the acetate and butyrate co-production but not with carbohydrates removal. At pH 7, 5% TS, and 1800 mg VSS/L, 49.5 NmL-H2/g VSfed and 976.4 NmL-H2/L-h were attained.

Inhibitory effect of acetic acid on dark-fermentative hydrogen production

This study examined the working mechanisms of acetic acid inhibition on dark fermentative hydrogen production. It was found that undissociated acetic acid (UAA) concentration was the critical factor in acetic acid inhibition. Hydrogen production activity decreased by 50 % and 90 % when UAA concentrations was 76.3 mg/L (1.27 mmol/L) and 686.7 mg/L (11.44 mmol/L), respectively. Dominant microbes were changed from genus Clostridium_sensu_stricto_1 to genus Inhella, Aquabacterium and Caulobacter under the stress of acetic acid inhibition. Functional enzyme analysis showed that acetic acid inhibited the hydrogen production by activating the lactate formation pathway when UAA concentration was below the inhibition threshold, while by impairing most hydrogen-producing pathways when UAA concentration was over the inhibition threshold. In brief, acetic acid inhibited the hydrogen production by altering the dominant microbial community and regulating the metabolic pathways, controlling the UAA concentration would be a good strategy to alleviate the acetic acid inhibition.

Effect of pH dynamic control on ethanol-lactic type fermentation (ELTF) performance of glucose

This study proposed a new ethanol-lactic type fermentation (ELTF) and explored the optimal control strategy. Using batch experiments, the effects of pH, temperature and organic loading (OL) on ELTF were investigated. The sum of ethanol and lactic acid yield was highest at whole-control pH value of 4.0, 35°C temperature and OL of 33 gCOD/L. To improve ELTF, the dynamic pH control in the long-term CSTR was adjusted at 4.0 (1-28 days), 5.0 (29-44 days) and 4.0 (46-62 days) successively. The high concentration of ethanol and lactic acid was 8190.5 mg/L at 16th day of pH 4.0. At pH of 5.0, the average acidogenesis rate and total concentration of fermentation products increased 111.0% and 128.0%, respectively. Organisms of Lactobacillus and Bifidobacterium were the predominant bacteria in reactor. It can achieve the directional regulation of ELTF and provides parameter support for the application of two-phase anaerobic digestion.

Two-stage anaerobic digestion of food waste: Enhanced bioenergy production rate by steering lactate-type fermentation during hydrolysis-acidogenesis

This study proposed a lactate-based two-stage anaerobic digestion (AD) process to enhance bioenergy production rate from food waste (FW) and investigated the effect of inoculum addition and culture pH on hydrolysis-acidogenesis and further methanization. A series of batch fermentations were performed with an enriched lactate-producing consortium and without inoculum addition under controlled (5.7) and uncontrolled pH (initial 6.7) conditions. The interplay between the studied factors dictated the fate of lactate, particularly if it is produced and accumulated in the fermentation broth or is consumed by butyrogenic bacteria. Only the self-fermentation of FW with uncontrolled pH resulted in lactate accumulation (0.2 g/g volatile solid (VS) fed) with limited off-gas production (0.32 NL/L) and VS losses (≈16%). Such lactate-rich broth was successfully digested through biochemical methane potential tests, resulting in a maximum bioenergy production rate of 2891 MJ/ton-VS fed per day, which was two-fold higher compared to that achieved by one-stage AD.

Enhanced Fermentative Hydrogen Production from Food Waste in Continuous Reactor after Butyric Acid Treatment

End-product accumulation during dark fermentation leads to process instability and hydrogen production inhibition. To overcome this constraint, microbial community adaptation to butyric acid can induce acid tolerance and thus enhance the hydrogen yields; however, adaptation and selection of appropriate microbial communities remains uncertain when dealing with complex substrates in a continuous fermentation mode. To address this question, a reactor fed in continuous mode with food waste (organic loading rate of 60 gVS·L·d−1; 12 h hydraulic retention time) was first stressed for 48 h with increasing concentrations of butyric acid (up to 8.7 g·L−1). Performances were compared with a control reactor (unstressed) for 13 days. During 6 days in a steady-state, the pre-stressed reactor produced 2.2 ± 0.2 LH2·L·d−1, which was 48% higher than in the control reactor (1.5 ± 0.2 LH2·L·d−1). The pretreatment also affected the metabolites’ distribution. The pre-stressed reactor presented a higher production of butyric acid (+44%) achieving up to 3.8 ± 0.3 g·L−1, a lower production of lactic acid (−56%), and an enhancement of substrate conversion (+9%). The performance improvement was attributed to the promotion of Clostridium guangxiense, a hydrogen -producer, with a relative abundance increasing from 22% in the unstressed reactor to 52% in the stressed reactor.

Light stimulation strategy for promoting bio-hydrogen production: Microbial community, metabolic pathway and long-term application

Light stimulation strategy for promoting bio-H₂ production was firstly investigated with incandescent light. The light condition controlled by photon number (N_R, 0.63 × 10⁴-6.25 × 10⁴ μmol/(day∙L)) was applied to stimulate H₂ fermentation process. The optimal N_R of 3.75 × 10⁴ μmol/(day∙L) contributed to 1.4 folds H₂ yield of the dark reactor and promoted efficient H₂ producing pathway (acetate and nicotinamide adenine dinucleotide pathway) with increased microbial activities. Furthermore, the effect of light stimulation on microbial community was identified. Fervidobacterium, Coprothermobacter and OPB95 were the dominant genera that could be activated by light stimulation for promoting acetate pathway and contribute to higher H₂ production. Moreover, long-term operation showed more stable and higher H₂ production of light stimulated bioreactor than the dark one, which resulted from the light stimulated metabolic pathway, increased sludge conductance and promoted microbial immobilization. This novel light stimulation strategy is promising for future application on promoting bio-H₂ production.

A developed hybrid fixed-bed bioreactor with Fe-modified zeolite to enhance and sustain biohydrogen production

In this study, we describe the development of a hybrid bioreactor with integrated chlorinated polyethylene (CPE) fixed-bed and zeolite as a microorganism nutrition carrier (MNC), aiming at enhancing and sustaining biohydrogen production during the anaerobic digestion (AD) process. In the batch test, the hybrid bioreactor achieved a maximum biohydrogen production of 646.3 mL/L. Accordingly, the hybrid bioreactor significantly enhanced biohydrogen production and maintained a stable performance for 50 days of semi-continuous operation. This result should be attributed to the CPE providing roughness surface and high porosity for microorganism immobilization, resulting in the enhancement of microbial quantity, confirmed by our scanning electron microscope and immobilized biomass analyses. Moreover, the element ratio significantly decreased, indicating that zeolite could provide metal cations for stimulating microbial bioactivity and growth, as well as contributing to superior biohydrogen productivity during the 50-day operation. In order to further enhance and sustain long-term biohydrogen production, raw zeolite was modified with iron. The hybrid-Fe bioreactor (CPE with Fe-modified zeolite) operated mainly following the acetate pathway and exhibited higher sustainability in improving biohydrogen production with a peak value of 1893.0 mL/L during a 72-day-lasting operation. The synergistic mechanism of the Fe-modified zeolite and CPE fixed-bed revealed that it could effectively induce favorable pathways and contribute to the synthesis of essential enzymes, micronutrient supplementation, electoral conductivity, and microbial immobilization for biohydrogen production. Therefore, a hybrid-Fe bioreactor could provide a unique alternative for the enhancement of hydrogen production for practical applications.

A review on the factors influencing biohydrogen production from lactate: The key to unlocking enhanced dark fermentative processes

Dark fermentation (DF) is one of the most promising biological methods to produce bio-hydrogen and other value added bio-products from carbohydrate-rich wastes and wastewater. However, process instability and low hydrogen production yields and rates have been highlighted as the major bottlenecks preventing further development. Numerous studies have associated such concerns with the inhibitory activity of lactate-producing bacteria (LAB) against hydrogen producers. However, an increasing number of studies have also shown lactate-based metabolic pathways as the prevailing platform for hydrogen production. This opens a vast potential to develop new strategies to deal with the "Achilles heel" of DF - LAB overgrowth - while untapping high-performance DF. This review discusses the key factors influencing the lactate-driven hydrogen production, paying particular attention to substrate composition, the operating conditions, as well as the microbiota involved in the process and its potential functionality and related biochemical routes. The current limitations and future perspectives in the field are also presented.

Enhanced Anaerobic Digestion of Long Chain Fatty Acid by Adding Magnetite and Carbon Nanotubes

This study investigated the impact of stimulating direct interspecies electron transfer (DIET), by supplementing nano-sized magnetite (nFe3O4, 0.5 g Fe/g VSS) and carbon nanotubes (CNT, 1 g/L), in anaerobic digestion of oleic acid (OA) at various concentrations (0.10 - 4.00 g chemical oxygen demand(COD)/L). Both supplementations could enhance CH4 production, and its beneficial impact increased with increased OA concentration. The biggest improvements of 114% and 165% compared to the control were achieved by nFe3O4 and CNT, respectively, at OA of 4 g COD/L. The enhancement can be attributed to the increased sludge conductivity: 7.1 ± 0.5 (control), 12.5 ± 0.8 (nFe3O4-added), and 15.7 ± 1.1 µS/cm (CNT-supplemented). Dissolved iron concentration, released from nFe3O4, seemed to have a negligible role in improving CH4 production. The excretion of electron shuttles, i.e., humic-like substances and protein-like substances, were found to be stimulated by supplementing nFe3O4 and CNT. Microbial diversity was found to be simplified under DIET-stimulating conditions, whereby five genera accounted for 88% of the total sequences in the control, while more than 82% were represented by only two genera (Methanotrix concilli and Methanosarcina flavescens) by supplementing nFe3O4 and CNT. In addition, the abudance of electro-active bacteria such as Syntrophomonas zehnderi was significantly increased from 17% to around 45%.

Influence of the pH control strategy and reactor volume on batch fermentative hydrogen production from the organic fraction of municipal solid waste

Three different experimental sets of runs involving batch fermentation assays were performed to evaluate the influence of the experimental conditions on biological hydrogen production from the source-separated organic fraction of municipal solid waste collected through a door-to-door system. The fermentation process was operated with and without automatic pH control, at a pH of 5.5 and 6.5, food-to-microorganism ratios of 1/3 and 1/1 (wet weight basis) and with different working volumes (0.5 and 3 L). The experimental results showed that the pH control strategy and the reactor volume did not affect the final hydrogen production yield but played an important role in determining the time evolution of the process. Indeed, although the different experimental conditions tested yielded comparable hydrogen productions (with maximum average values ranging from 68.5 to 88.5 NLH₂ (kgTVS_OF)^-1), the automatic pH control strategy improved the process from the kinetic viewpoint resulting in a t₉₅ reduction from an average of 34.9 h without automatic pH control to an average of 19.5 h.

Effect of process parameters on enhanced biohydrogen production from tequila vinasse via the lactate-acetate pathway

In this study, a lactate-type fermentation entailing the consumption of lactate and acetate (lactate-acetate pathway) is proposed to deal with lactic acid bacteria (LAB) inhibition during the production of biohydrogen (bioH₂) from tequila vinasse. The effects of total solids content, substrate concentration, nutrient formulation and inoculum addition on bioH₂ production performance were investigated. Batch experiments were performed in a 3-L completely mixed reactor at 35 °C and pH 6.5-5.8. The lactate-acetate pathway mediated consistent bioH₂ production which was influenced by inoculum addition followed by substrate concentration, nutrient formulation and solids content. Maximum bioH₂ production rate (225 NmL/L-h) and yield (124 NmL/g VS_added) were achieved by removing suspended solids and enhancing nutrient content, respectively. Illumina sequencing-based analysis revealed a dominance of Clostridium in the inoculum, which together with LAB and acetic acid bacteria shaped a keystone cluster for avoiding LAB inhibition while ensuring consistent bioH₂ production performance.

High-calorific bio-hydrogen production under self-generated high-pressure condition

For the use of biologically produced H₂, removal of CO₂ is an indispensable process. Unlike conventional CO₂ removal methods, this study proposed a self-generated high-pressure dark fermentation (HPDF) process as a novel strategy for directly producing high-calorific bio-H₂. The pressure was automatically increased by self-generated gas, while the maximum pressure inside fermenter was restricted to 1, 3, 5, 7, and 10 bar in a batch operation. As the pressure increased from 1 to 10 bar, the H₂ content increased from 55% to 80%, whereas the H₂ yield decreased from 1.5 to 0.9 mol H₂/mol hexose_added. The highest H₂ content of 80% was obtained at both of 7 and 10 bars. Increased lactate production with increased abundance of lactic acid bacteria was observed at high-pressure. Despite the lower H₂ yields at high-pressure conditions, HPDF was found to be economically beneficial for obtaining high-calorific bio-H₂ owing to the low CO₂ removal cost.

Hydrogen production by dark fermentation from pre-fermented depackaging food wastes

In this study, a specific fraction of food waste, i.e. depackaging waste, was studied as substrate for hydrogen production by dark fermentation. During storage and transport of this liquid mixture, inhibitory compounds like acids or alcohol might be produced by endogenous flora. A factorial fractional design based on the composition of the substrate was used to determine the best condition to convert this substrate into hydrogen. First results indicated that the consortium used might convert high quantity of lactate into hydrogen. A batch culture confirmed that lactate was used as the main carbon source and a global yield of 0.4mol_H2·mol_lactate^-1 was obtained. This study demonstrated the ability of the consortium tested to convert different carbon sources (carbohydrates or lactate) with good efficiency. These data represented an important parameter in the prospect of using an industrial substrate whose composition is liable to vary according to the conditions of storage and transport.

Biohydrogen production from food waste: Current status, limitations, and future perspectives

Among the various biological routes for H₂ production, dark fermentation is considered the most practically applicable owing to its capability to degrade organic wastes and high H₂ production rate. Food waste (FW) has high carbohydrate content and easily hydrolysable in nature, exhibiting higher H₂ production potential than that of other organic wastes. In this review article, first, the current status of H₂ production from FW by dark fermentation and the strategies applied for enhanced performance are briefly summarized. Then, the technical and economic limitations of dark fermentation of FW are thoroughly discussed. Economic assessment revealed that the economic feasibility of H₂ production from FW by dark fermentation is questionable. Current efforts to further increase H₂ yield and waste removal efficiency are also introduced. Finally, future perspectives along with possible routes converting dark fermentation effluent to valuable fuels and chemicals are discussed.

A parametric response surface study of fermentative hydrogen production from cheese whey

Batch factorial experiments were performed on cheese whey+wastewater sludge mixtures to evaluate the influence of pH and the inoculum-to-substrate ratio (ISR) on fermentative H₂ production and build a related predictive model. ISR and pH affected H₂ potential and rate, and the fermentation pathways. The specific H₂ yield varied from 61 (ISR=0, pH=7.0) to 371L H₂/kg TOC_whey (ISR=1.44gVS/g TOC, pH=5.5). The process duration range was 5.3 (ISR=1.44gVS/g TOC, pH=7.5) - 183h (ISR=0, pH=5.5). The metabolic products included mainly acetate and butyrate followed by ethanol, while propionate was only observed once H₂ production had significantly decreased. The multiple metabolic products suggested that the process was governed by several fermentation pathways, presumably overlapping and mutually competing, reducing the conversion yield into H₂ compared to that expected with clostridial fermentation.

Influence of pH and neutralizing agent on anaerobic succinic acid production by a Corynebacterium crenatum strain

Environmental conditions, particularly pH, have significant effects on the efficiency and final titers of bio-based products. Therefore, these factors need to be identified to ensure the fermentation process is economically attractive. In this study, strategies for controlling pH were optimized to enhance succinic acid production by Corynebacterium crenatum J-2. The results indicate that pH 6.8 is the optimal value for anaerobic succinic acid production by C. crenatum J-2 in terms of productivity and titer. The use of Mg(OH)₂ as the neutralizing agent for pH control resulted in the highest levels of succinic acid concentration, yield, and productivity; superior to the levels obtained with Ca(OH)₂, KOH, and NaOH. Under conditions of pH 6.8 and Mg(OH)₂ as the neutralizing agent, 45.7 g/L succinic acid was produced within 12 h during the prophase of anaerobic fermentation, resulting in a succinic acid productivity of 3.8 g/(L·h). Succinic acid concentration reached 53.8 g/L at 22 h, with a productivity of 2.45 g/(L·h). The results of this study will be useful for the development of highly efficient succinic acid production processes utilizing industrial Corynebacterium spp. strains.

More value from food waste: Lactic acid and biogas recovery

Anaerobic digestion (AD) is one of the traditional technologies for treating organic solid wastes, but its economic benefit is sometimes questioned. To increase the economic feasibility of the treatment process, the aim of this study was to recover not only biogas from food waste but lactic acid (LA) as well. At first, LA fermentation of food waste (FW) was conducted using an indigenous mixed culture. During the operation, temperature was gradually increased from 35 °C to 55 °C, with the highest performance attained at 50 °C. At 50 °C and hydraulic retention time (HRT) of 1.0 d, LA concentration in the broth was 40 kg LA/m(3), corresponding to a yield of 1.6 mol LA/mol hexoseadded. Pyrosequencing results showed that Lactobacillus (97.6% of the total number of sequences) was the predominant species performing LA fermentation of FW. The fermented broth was then centrifuged and LA was extracted from the supernatant by the combined process of nanofiltration and water-splitting electrodialysis. The process could recover highly purified LA by removing 85% of mineral ions such as Na(+), K(+), Mg(2+), and Ca(2+) and 90% of residual carbohydrates. Meanwhile, the solid residue remained after centrifugation was further fermented to biogas by AD. At HRT 40 d (organic loading rate of 7 kg COD/m(3)/d), the highest volumetric biogas production rate of 3.5 m(3)/m(3)/d was achieved with a CH4 yield of 0.25 m(3) CH4/kg COD. The mass flow showed that 47 kg of LA and 54 m(3) of biogas could be recovered by the developed process from 1 ton of FW with COD removal efficiency of 70%. These products have a higher economic value 60 USD/ton FW compared to that of conventional AD (27 USD/ton FW).

Ratiometric fluorescent pH-sensitive polymers for high-throughput monitoring of extracellular pH

Extracellular pH has a strong effect on cell metabolism and growth. Precisely detecting extracellular pH with high throughput is critical for cell metabolism research and fermentation applications. In this research, a series of ratiometric fluorescent pH sensitive polymers are developed and the ps-pH-neutral is characterized as the best one for exculsive detection of extracellular pH. Poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA) is used as the host polymer to increase the water solubility of the pH sensitive polymer without introducing cell toxicity. The fluorescent emission spectra from the polymeric sensor under excitation at the isosbestic point 455 nm possess two fluorescence peaks at 475 nm and 505 nm, which have different responding trends to pH. This enables the polymer to detect pH using fluorescent maxima at 475 nm and 505 nm (I475nm /I505nm ) ratiometrically. The cell impermeability ensures the sensor can solely detect the environmental pH. The sensor is tested to detect the extracellular pH of bacteria or eukaryotic cells in high throughput assays using a microplate reader. Results showed that the pH sensor can be used for high throughput detection of extracellular pH with high repeatability and low photobleaching effect.

Hydrogen fermentation of food waste by alkali-shock pretreatment: microbial community analysis and limitation of continuous operation

In the study, at first, batch tests were performed to investigate the effect of alkali-shock on H2 production from food waste (FW). After alkali-pretreatment of FW at pH 9.0-13.0, the FW was cultivated under mesophilic condition at pH 6.0 for 30 h without external inoculum addition. The amount of H2 production from FW pretreated at pH 11.0 and 12.0 was higher than that achieved in other pretreatment pH. The main metabolite was butyrate, and Clostridium were dominant at pH 11.0 and 12.0. Meanwhile, lactate was the main metabolite with Enterococcus and Streptococcus being the dominant genus at other pretreatment pH. When the batch process was switched to a continuous mode, H2 production was significantly dropped due to the increased activity of H2-consumers. The reliability of alkali-pretreatment at pH 11.0 was proven by repeating the scale-up batch process, recording 1.57±0.11 mol H2/mol hexose(added) (17±2LH2/kg FW) and 4.39±0.32LH2/L/d.