Distribution Of Volatile Fatty Acids Research Articles

Enhanced, continuous, liquid-liquid extraction and in-situ separation of volatile fatty acids from fermentation broth

In 2018 alone, the US landfilled 35.3 million tons of food waste, about 24% of the total landfilled mass. In addition to the negative impacts landfills have demonstrated on the environment and human health, some states have begun to outlaw or dissuade the disposal of food waste and sewage sludge into landfills altogether. An urgent need has thus been created for the development of digestion processes like anaerobic digestion (AD) and arrested methanogenesis (AM) to convert food waste into valuable chemical products. Unfortunately, the buildup of volatile fatty acids (VFAs) during these processes eventually halts the reaction, and energy efficient methodologies for VFA removal are critical for the operation of fermenters. Additionally, VFAs themselves can serve as valuable chemical precursors, and recently AD processes have been modified to increase VFA production during fermentation. However, even with significant research over the past three decades, the separation of VFAs from the fermenter broth has remained expensive. Moreover, the separation of these VFAs from the fermenter broth may cost up to 50% of the entire process budget, hindering the widespread commercial adoption of AD and AM. In this work, we present a novel liquid–liquid extraction process termed CLEANS (Continuous Liquid-liquid Extraction And iN-situ Separation) as a highly efficient method for continuously separating VFAs from a real fermentation broth solely under gravity. Our optimized process (using an aqueous broth feed pH of 2.5, tri-n-octylamine as an extractant, and a 10:1 ratio of aqueous broth to organic extractant), achieved a VFA distribution constant KD = 44.5 ± 7.9, a single-pass recovery = 81.3 ± 2.5%, and an extraction factor = 8.1 ± 0.3. These KD values are over an order of magnitude higher than what has been previously reported for comparable processes. A high aqueous-to-organic flowrate ratio, enabled for the first time by CLEANS, was found to be particularly crucial for achieving optimal extraction. Our separation process demonstrates excellent reproducibility and potential for scalability. The economic and environmental implications of this work are briefly discussed.

Enhancement of static magnetic field on biological hydrogen production via photo-fermentation of giant reed

The effect of static magnetic field (SMF) on the system of photo-fermentation biological hydrogen production remains dimness. The goal of this study was to clarify the correlation between external SMF addition and hydrogen production via photo-fermentation from giant reed. SMF with 20 mT improved the cumulative H2 yield by 26.1% and reduced the lag time of hydrogen production by 56.7% compared with that of without external magnetic field. Moreover, 20 mT of SMF not only enhanced the activity of nitrogenase by 94.52%, but also obtained the maximum energy conversion efficiency of 27.27%. The distribution of volatile fatty acids proved that the concentration of acetic acid and butyric acid were 137% and 81% higher than that of without SMF, respectively. The results would help to trigger the positive interaction between SMF and microorganism and to avoid the possible negative interaction.

Production of hydrogen from beverage wastewater by dark fermentation in an internal circulation reactor: Effect on pH and hydraulic retention time

• Interaction and the RSM should be performed to determine the optimal zone for H 2 . • Interaction between factors established that the pH was main factor in the operation. • Distribution of VFAs and multivariate statistics allow an integral analysis. Energy demand currently is mostly satisfied by the use of fossil fuels that present not only problems for the environment, but also are not renewable. Dark fermentation (DF) is a biological process that can provide an alternative to meet energy needs. The use of industrial effluents from carbon-rich waters in the production of hydrogen and volatile fatty acids (VFAs) through the DF is a promising strategy. However, determining the optimal operating conditions to increase the production of the sub-products has been not being thoroughly studied. This study aims at determining the optimal condition of hydraulic retention time (HRT) and pH to improve hydrogen production in an internal recirculation (IC) reactor treating beverage wastewater using Response Surface Methodology and interaction between factors. The study also assessed VFAs concentrations when operating at optimal conditions. The results showed that the combination of an 8.0 h HRT and 5.5 produced 30% of hydrogen and the interaction between factors established that the pH was the main factor influencing the results. Also, it was observed that the concentration of lactic acid did not inhibit the production of H 2 for the optimal value of pH. However, it is evident that more studies using HRT of less than 8 h and of VFAs distribution are needed. Future optimization technique considerations are necessary to reduce uncertainties for biological hydrogen production purposes.

Continuous Production of Volatile Fatty Acids (VFAs) from Swine Manure: Determination of Process Conditions, VFAs Composition Distribution and Fermentation Broth Availability Analysis

For pollution control and waste utilization, a promising future direction is to obtain high-value carbon sources from organic waste. In this experiment, swine manure was efficiently converted into high concentration volatile fatty acids through continuous hydrolysis-acidification bioreactors. This study determined the process conditions, the composition distribution of volatile fatty acids and the availability of fermentation broth. The results showed that the reactor with a hydraulic retention time of 1.5 days had the optimal production performance of volatile fatty acids. The highest hydrolysis degree (62.2%) and acidification degree (42.5%) were realized in this reactor at the influent soluble chemical oxygen demand of 5460 mg/L. Furthermore, when the influent soluble chemical oxygen demand was 7660 mg/L, volatile fatty acids of 6065 mg-COD/L could be produced stably, and the proportion of volatile fatty acids in soluble chemical oxygen demand was the largest (75%). Additionally, the fermentation broth rich in volatile fatty acids could be applied to deep nitrogen and phosphorus removal. This work provides a productive approach to resource recovery from swine manure.

Effect of the solid retention time in the obtention of polyhydroxyalkanoates

<ns3:p><ns3:bold>Background:</ns3:bold><ns3:bold> </ns3:bold>The effect of solid retention time (SRT) over cheese whey substrates in a fermentation process drives changes in the composition of polyhydroxyalkanoates (PHAs) obtained. Volatile fatty acids produced in the first step of an anaerobic sequencing batch reactor were used as substrates to produce PHA using mixed microbial cultures under aerobic dynamic feeding conditions.</ns3:p><ns3:p> <ns3:bold>Methods: </ns3:bold>Analytical methods were used for the standard analysis of parameters of interest including measuring the amount of ammonium and phosphate, chemical oxygen demand, among others.</ns3:p><ns3:p> <ns3:bold>Results</ns3:bold>: The SRT increasing from 4 to 6 and 10 days produced changes in the distribution of volatile fatty acids produced. The polyhydroxybutyrate-hydroxyvalerate copolymers formed in the accumulation stage gave the following results: 58:42, 68:32 and 81:19 (%), referred to SRTs of 10, 6 and 4 days, respectively. The maximum PHA accumulation obtained at 10 days of SRT was 52% of the dry cell weight within 7 h, reaching a PHA productivity of 0.62 g L<ns3:sup>−1</ns3:sup> h<ns3:sup>−1</ns3:sup> and a storage yield of 0.37.</ns3:p><ns3:p> <ns3:bold>Conclusion:</ns3:bold><ns3:bold> </ns3:bold>The SRT variation impact on the distribution of volatile fatty acids in the acidogenic reactor and consequently on the PHA production and composition formed in the accumulation stage.</ns3:p>

Three-dimensional CFD simulation of anaerobic reactions in a continuous packed-bed bioreactor

In this study, a lab-scale continuous packed-bed bioreactor with multi-sized distributed particles has been simulated in a structured layout using computational fluid dynamics. The goal was to achieve a well-estimated simple model which can describe the hydrodynamic regime of flow as well as mass transfer of species within the bioreactor. A MATLAB program was coded to create the packed-bed geometry by generating non-overlapping spherical particles. The flow hydrodynamics was assumed to be single-phase creeping flow regime (Stokes flow). The kinetic data of batch experiments at similar operating conditions were used to formulate the rate of anaerobic reactions. The anaerobic methanogenesis reactions were assumed to occur at the particle surfaces (on the biofilm). The bioreactor temperature was fixed at 37°C. The commercial software COMSOL Multiphysics 5.0 was employed to solve the governing equations. Velocity profiles and concentration distribution of volatile fatty acids (VFAs) are shown inside the packed-bed reactor. Simulation results were found to be in good agreement with the experimental data at various influent flow rates (23.8 – 43.5 mL/h) and various influent concentrations of VFAs (0.001 – 0.042 M). In all cases, the relative error was less than 15% which is acceptable. This proposed model is applicable for scale-up and optimization purposes as well as evaluation of other kinetic models.

Microalgae-based anaerobic fermentation as a promising technology for producing biogas and microbial oils

Abstract This study assessed the potential of using microalgae anaerobic fermentation as a novel choice for producing microbial oils and biogas. Microalgae biomass (Chlorella vulgaris) was used for volatile fatty acids (VFAs) production at three organic loading rates (OLR = 6, 9, 12 g COD/Ld). Regardless of the OLR, anaerobic systems displayed similar organic matter conversion yields into VFAs (VFAs-COD/CODin = 38%) and VFAs distribution profiling. Obtained digestates were further used with a two-fold purpose, producing microbial oils from the VFA rich liquid fraction, and generating methane from the remaining solid spent. The yeast Yarrowia lipolytica was able to grow on VFAs concentration up to 17.6 g/L while the highest VFAs concentration (24.2 g/L, digestate obtained at 12 g COD/Ld) resulted inhibitory. Despite of that, Y. lipolytica exhibited remarkably high lipid content (15–23%) when grown in digestates. The three spent solid fractions (corresponding to the 3 OLRs) reached the same methane potential.

Zero-valent iron enhanced anaerobic digestion of pre-concentrated domestic wastewater for bioenergy recovery: Characteristics and mechanisms

Pre-concentrated domestic wastewater (PDWW) rich in organic matters can be a suitable substrate for anaerobic digestion (AD) towards holistic resource and bioenergy recovery. Micron zero-valent iron (ZVI) was applied in designed batch experiments during anaerobic treatment of PDWW to verify its roles in performance enhancement and associated mechanisms. In the selected range of food to microorganism (F/M) ratio, 0.5 gCOD/gMLVSS was most appropriate as biomethane production potential (BMP) of 0.275 L CH4/gCOD was obtained. The optimal ZVI dosage at fixed F/M of 0.5 was 6 g/L, further enhancing the BMP by 15.2%. Furthermore, ZVI improved the hydrolysis process (producing more soluble organics) and regulated acidification process (affecting volatile fatty acids distribution). No obvious impact on acetoclastic and hydrogenotrophic methanogenesis processes was noted with ZVI addition. ZVI based AD of the PDWW is promising for promoting the practical application of advanced domestic wastewater treatment strategy (pre-concentration plus anaerobic digestion).

Pertraction of volatile fatty acids through biodiesel-based liquid membranes

Volatile fatty acids (VFAs) are ideal substrates for mixed microbial cultures to synthesize polyhydroxyalkanoates. Therefore, VFA pertraction from anaerobic diluted fermentation broths to slightly alkaline aerobic medium becomes a crucial step to develop a closed loop concept in which VFAs are used for polyhydroxyalkanoate production. New liquid membranes (LMs) based on lipophilic amines and biodiesel were proposed for this purpose. The performance of each LM was tested through a VFA distribution coefficient test and pertraction kinetic experiments. The transfer of VFAs from LM to receiving solution was the main limiting step of the process, with flux rates of 0.001–0.04 cm3 cm−2 h−1 (best LM with 10 wt% of trioctylamine). The biocompatibility of trioctylamine -biodiesel LM towards anaerobic bacteria was confirmed through a 15-d acidogenic fermentation test. Trioctylamine-biodiesel LM was finally tested in a specifically designed continuous pertraction module, where VFA transfer rates of 1–3 h−1 were achieved.

Volatile fatty acids production from cheese whey: influence of pH, solid retention time and organic loading rate

AbstractBACKGROUNDThe influence of pH, sludge retention time (SRT) and organic loading rate (OLR) were evaluated in the fermentation of cheese whey to produce volatile fatty acids (VFA) in a sequencing batch reactor (SBR). The reactor performance was studied at SRT of 4, 6 and 10 days and the influence of pH (5, 5.5 and 6) was determined at each SRT. The OLR effect was also studied in the range 3 to 12 g COD L‐1 d‐1, while keeping the other parameters mentioned above constant.RESULTSA more acidic pH , improved the degree of acidification at short SRT (4 and 6 days), reaching 76 and 84%, respectively, meanwhile at pH 6 the degree of acidification (DA) improved and reached 85% at longer SRT (10 d). Regarding the VFA profile, a pH shift from 5 to 6 stimulated the production of acetic and propionic acids, whereas the production of butyric and valeric acids decreased irrespective of the SRT applied. Moreover, the increase of SRT from 4 to 10 d boosted propionic and valeric acids production regardless of the pH applied. Lower OLR favoured the production of propionic and valeric acids, while higher OLR increased the production of butyric acid. Lactose degradation and VFA production followed the Michaelis–Menten model.CONCLUSIONA selective VFA distribution can be reached by modifying parameters such as pH, SRT and OLR in acidogenic reactors, therefore, they could be suitable for further valorisation purposes in the production of polyhydroxyalkanoates from such VFA. © 2017 Society of Chemical Industry

Volatile fatty acids production during mixed culture fermentation – The impact of substrate complexity and pH

Volatile fatty acids (VFAs) are obtained during chemical routes from non-renewable petrochemicals. Intensive exploitation of oil resources have renewed the idea of VFAs production during biological routes, mainly throughout anaerobic mixed culture fermentation (MCF). We carried out MCF trials at initial acidic (5.0), neutral (7.0, with addition of specific methanogenesis inhibitor) and alkaline (11.0) pH conditions for four different substrates i.e. maize silage, cheese whey, microalgae biomass and glucose. The goal of the study was to investigate the impact of the substrate complexity on the produced VFAs’ quality and quantity. The highest VFAs concentration occurred in neutral pH proceeded by initial alkaline pH (0.83gVFAs/gSCOD for microalgae biomass, 0.78gVFAs/gSCOD for maize silage and 0.71gVFAs/gSCOD for cheese whey, respectively). In the fermentation of glucose, the highest VFAs concentration was achieved in neutral pH. We demonstrated that the alkaline pH was favorable for hydrolysis of complex organic matter (acidification yield over 71% for maize silage fermentation), while the neutral pH was beneficial for the acidogenesis and the overall VFAs production. Our findings showed that it was possible to carry out efficient and stable MCF process without using a large amounts of acid or base for pH adjustment and that the distribution of VFAs only merely depended on the substrate type.

Novel insight into the relationship between organic substrate composition and volatile fatty acids distribution in acidogenic co-fermentation

BackgroundCo-fermentation is an attractive technology for improving volatile fatty acids (VFAs) production by treatment of solid organic wastes. However, it remains unclear how the composition of different organic matters in solid waste influences the VFAs distribution, microbial community structure, and metabolic pathway during acidogenic co-fermentation. In this study, different organic wastes were added into waste activated sludge (WAS) as co-fermentation substrates to explore the impact of organic matter composition on VFAs pattern and the microbiological mechanism .ResultsAcetate was the most dominant VFA produced in all fermentation groups, making up 41.3–57.6% of the total VFAs produced during acidogenic co-fermentation under alkaline condition. With the increased addition of potato peel waste, the concentrations of propionate and valerate decreased dramatically, while ethanol and butyrate concentrations increased. The addition of food waste caused gradual decreases of valerate and propionate, but ethanol increased and butyrate was relatively stable. Some inconsistency was observed between hydrolysis efficiency and acidification efficiency. Our results revealed that starch was mainly responsible for butyrate and ethanol formation, while lipids and protein favored the synthesis of valerate and propionate. Microbial community analysis by high-throughput sequencing showed that Firmicutes had the highest relative abundance at phylum level in all fermentation groups. With 75% potato peel waste or 75% food waste addition to WAS, Bacilli (72.2%) and Clostridia (56.2%) were the dominant respective classes. In fermentation using only potato peel waste, the Bacilli content was 64.1%, while the Clostridia content was 53.6% in the food-only waste fermentation.ConclusionsAcetate was always the dominant product in acidogenic co-fermentation, regardless of the substrate composition. The addition of carbon-rich substrates significantly enhanced butyrate and ethanol accumulation, while protein-rich substrate substantially benefited propionate and valerate generation. Potato peel waste substantially favored the enrichment of Bacilli, while food waste dramatically increased Clostridia content in the sludge.

Mass Loss Controlled Thermal Pretreatment System to Assess the Effects of Pretreatment Temperature on Organic Matter Solubilization and Methane Yield from Food Waste

The effects of thermal pretreatment (TP) on the main characteristics of food waste (FW) and its biochemical methane potential (BMP) and distribution of volatile fatty acids (VFAs) under mesophilic condition (35 ⁰C) were investigated. The TP experiments were carried out at 80 °C, 100 °C, 120 °C for 2 hour and 140 °C for 1 hour. The designed TP set-up was able to minimize the organic matter loss during the course of the pretreatments. Soluble organic fractions evaluated in terms of chemical oxygen demand (COD) and soluble protein increased linearly with pretreatment temperature. In contrast, the carbohydrate solubilization was more enhanced (30 % higher solubilization) by the TP at lower temperature (80 °C). A slight increment of soluble phenols was found, particularly for temperatures exceeding 100 °C. Thermally pretreated FW under all conditions exhibited an improved methane yield than the untreated FW, due to the increased organic matter solubilization. The highest cumulative methane yield of 442 (± 8.6) mL/gVSadded, corresponding to a 28.1 % enhancement compared to the untreated FW, was obtained with a TP at 80 °C. No significant variation in the VFAs trends were observed during the BMP tests under all investigated conditions.

Temperature-phased anaerobic digestion of the hydromechanically separated organic fraction of municipal solid waste with sewage sludge

This paper examines the temperature-phased anaerobic digestion (TPAD) process treating the mixture of municipal sewage sludge with the hydromechanically separated organic fraction of municipal solid waste (HS-OFMSW). The experiments showed that the performance of the TPAD process strongly depends on the conditions applied in the thermophilic stage. The TPAD system operated at a solids retention time (SRT) of 1 and 14 days in the first and second steps, respectively, achieved the overall methane yield of 333 l CH4 kgVS−1 and the volatile solids reduction of 52.1%, whereas the corresponding values reported for a control single-stage mesophilic process were 230 l CH4 kgVS−1 and 37.23%, respectively. However, when an SRT of the thermophilic reactor was extended to 2 days, the methane production in the subsequent mesophilic stage significantly decreased. It was therefore concluded that the prolonged exposure of feedstock to the thermophilic temperatures can lead to greater intensity of protein degradation. Consequently, higher amounts of ammonia are liberated to the liquid phase, which results in the inhibition of methanogenesis in the subsequent mesophilic step. Moreover, ammonia affects the release and distribution of volatile fatty acids, which also influences the performance of the whole TPAD system.

Fast Acidogenic Fermentation of Corn Stover Through a Two-Step Method: Nitric Acid Hydrolysis Combined with the Fermentation of Hydrolysate

This study investigated a novel route for fast acidogenic fermentation of corn stover through a two-step method. Corn stover was first hydrolyzed by nitric acid, and the hydrolysate was then used for acidogenic fermentation. The results showed that the yields of soluble xylose, arabinose, and glucose in hydrolysate obtained from HNO3 hydrolysis of corn stover at the optimum condition achieved 96.86% of initial xylose, 98.54% of initial arabinose, and 18.58% of initial glucose, respectively. Acidogenic fermentation of hydrolysate proved to be effective, compared to acidogenic fermentation of raw corn stover; the production of volatile fatty acids (VFAs) increased by 54.8%, fermentation time decreased by 50%, and the distribution of VFAs was more suitable for subsequent methanogenic fermentation. This research provided an effective, suitable, and economical method for biogas production from corn stover.

Comparative study of biohydrogen production by four dark fermentative bacteria

Dark fermentation is a promising biological method for hydrogen production because of its high production rate in the absence of light source and variety of the substrates. In this study, hydrogen production potential of four dark fermentative bacteria (Clostridium butyricum, Clostridium pasteurianum, Clostridium beijerinckii, and Enterobacter aerogenes) using glucose as substrate was investigated under anaerobic conditions. Batch experiments were conducted to study the effects of initial glucose concentration on hydrogen yield, hydrogen production rate and concentration of volatile fatty acids (VFA) in the effluents. Among the four different fermentative bacteria, C. butyricum showed great performance at 10 g/L of glucose with hydrogen production rate of 18.29 mL-H2/L-medium/hand specific hydrogen production rate of 3.90 mL-H2/g-biomass/h. In addition, it was found that the distribution of volatile fatty acids was different among the fermentative bacteria. C. butyricum and C. pasteurianum had higher ratio of acetate to butyrate compared to the other two species, which favored hydrogen generation.

Volatile fatty acids distribution during acidogenesis of algal residues with pH control

The anaerobic acidification of protein-rich algal residues with pH control (4, 6, 8, 10) was studied in batch reactors, which was operated at mesophilic(35°C) condition. The distribution of major volatile fatty acids (VFAs) during acidogenesis was emphasized in this paper. The results showed that the acidification efficiency and VFAs distribution in the acid reactor strongly depended on the pH. The main product for all the runs involved acetic acid except that the proportion of butyric acid acidified at pH 6 was relatively higher. The other organic acids remained at lower levels. The VFAs yield reached the maximum value with about 0.6g VFAs/g volatile solid (VS) added as pH was 8, and also the content of total ammonia nitrogen (TAN) reached the highest values of 9,629mg/l. Low acidification degrees were obtained under the conditions at pH 4 and 10, which was not suitable for the metabolism of acidogens. Hydralic retention time (HRT) required for different conditions varied. As a consequence, it was indicated that pH was crucial to the acidification efficiency and products distribution. The investigation of acidogenesis process, which was producing the major substrates, short-chain fatty acids, would play the primary role in the efficient operation of methanogenesis.

Hydrogenase activity monitoring in the fermentative hydrogen production using heat pretreated sludge: A useful approach to evaluate bacterial communities performance

The conversion of organic compounds into H 2 has received increasing attention. Enrichment of inocula by heat pretreatment eliminates non-spore forming hydrogen consuming microorganisms and promotes spore germination in genus Clostridium, which is known as one of the key hydrogen producers. Useful information about metabolic pathway is provided by some intermediate metabolites, such as: acetic, propionic, butyric and formic acids. The increase of acetic/butyric acids ratio indicates H 2 production in heat pretreated inoculum when compared to untreated inoculum in the same cultivation conditions. The effect of heat pretreatment on inocula and consequently on the performance of bacterial communities responsible for H 2 production was monitored through the measurement of the level of hydrogenase gene expression, as well as through the content and distribution of volatile fatty acids. The acetic acid type fermentation was followed by the microorganisms presented in untreated and heat pretreated sludge. The medium containing untreated sludge presented a ratio of acetic/butyric acid of approximately 4, the same parameter was 7 when heat pretreated sludge was employed. The level of hydrogenase gene expression tripled when heat pretreated inoculum was used, indicating a higher production of H 2.

Effects of temperature and initial pH on biohydrogen production from food-processing wastewater using anaerobic mixed cultures

This study attempted to determine the optimal temperature and initial cultivation pH by conducting a series of batch tests in stirred-tank bioreactor using fructose-producing wastewater as an organic substrate. The bioreactor temperature was controlled at 35-55°C with an initial pH of 4-8. Hydrogen production efficiency was assessed using specific hydrogen production potential (SHPP) and the maximum specific hydrogen production rate (SHPR(m)). Experimental results indicated that temperature and initial pH markedly affected SHPP and SHPR(m), volatile fatty acids distribution as well as the ratio of butyrate/acetate (BHu/HAc). Two-fold higher SHPP and SHPR(m) were obtained at thermophilic condition (55°C) than those at mesophilic condition (35°C). The optimal initial pH was 6 for hydrogen production with peak values of SHPP of 166.8 ml-H(2)/g-COD and SHPR(m) of 26.7 ml-H(2)/g-VSS-h for fructose-processing wastewater. Molasses-processing wastewater had a higher SHPP (187.0 ml-H(2)/g-COD) and SHPR(m) (42.7 ml-H(2)/gVSS-h) than fructose-processing wastewater at pH 6. The DGGE profiles indicated that molasses-processing wastewater is a better substrate than fructose-processing wastewater for growth of hydrogen-producing bacteria due to the high staining intensity of bands.

Fermentative hydrogen production by new marine Clostridium amygdalinum strain C9 isolated from offshore crude oil pipeline

The present study investigated hydrogen production potential of novel marine Clostridium amygdalinum strain C9 isolated from oil water mixtures. Batch fermentations were carried out to determine the optimal conditions for the maximum hydrogen production on xylan, xylose, arabinose and starch. Maximum hydrogen production was pH and substrate dependant. The strain C9 favored optimum pH 7.5 (40 mmol H 2/g xylan) from xylan, pH 7.5–8.5 from xylose (2.2–2.5 mol H 2/mol xylose), pH 8.5 from arabinose (1.78 mol H 2/mol arabinose) and pH 7.5 from starch (390 ml H 2/g starch). But the strain C9 exhibited mixed type fermentation was exhibited during xylose fermentation. NaCl is required for the growth and hydrogen production. Distribution of volatile fatty acids was initial pH dependant and substrate dependant. Optimum NaCl requirement for maximum hydrogen production is substrate dependant (10 g NaCl/L for xylose and arabinose, and 7.5 g NaCl/L for xylan and starch).