Specific Hydrogen Production Rate Research Articles

Responses of syntrophic microbial communities and their interactions with polystyrene nanoplastics in a microbial electrolysis cell

Microbial electrochemical technologies are promising for simultaneous energy recovery and wastewater treatment. Although the inhibitory effects of emerging pollutants, particularly micro/nanoplastics (MPs/NPs), on conventional wastewater systems have been extensively studied, the current understanding of their impact on microbial electrochemical systems is still quite limited. Microplastics are plastic particles ranging from 1 μm to 5 mm. However, nanoplastics are smaller plastic particles ranging from 1 to 100 nm. Due to their smaller size and greater surface area, they can penetrate deeper into biofilm structures and cell membranes, potentially disrupting their integrity and leading to changes in biofilm composition and function. This study first reports the impact of polystyrene nanoplastics (PsNPs) on syntrophic anode microbial communities in a microbial electrolysis cell. Low concentrations of PsNPs (50 and 250 μg/L) had a minimal impact on current density and hydrogen production. However, 500 μg/L of PsNPs decreased the maximum current density and specific hydrogen production rate by ∼43 % and ∼48 %, respectively. Exposure to PsNPs increased extracellular polymeric substance (EPS) levels, with a higher ratio of carbohydrates to proteins, suggesting a potential defense mechanism through EPS secretion. The downregulation of genes associated with extracellular electron transfer was observed at 500 μg/L of PsNPs. Furthermore, the detrimental impact of 500 μg/L PsNPs on the microbiome was evident from the decrease in 16S rRNA gene copies, microbial diversity, richness, and relative abundances of key electroactive and fermentative bacteria. For the first time, this study presents the inhibitory threshold of any NPs on syntrophic electroactive biofilms within a microbial electrochemical system.

Optimum Conditions for Enhanced Biohydrogen Production from a Mixture of Food Waste and Sewage Sludge with Alkali Pretreatment

Given the increasing demand for hydrogen, owing to its environmentally friendly nature, it is important to explore efficient methods for hydrogen production. This study investigates dark-fermentative hydrogen production by the co-digestion of food waste and sewage sludge. Both wastes were subjected to alkali pretreatment (at pH 13) to enhance biodegradability. Batch tests were conducted to enhance hydrogen production from food waste and sewage sludge under various volatile solid (VS) concentrations of 1.5–5% and food waste to sewage sludge mixing ratios of 0:100–100:0. We found that alkali pretreatment was effective in increasing hydrogen yields. The maximum specific hydrogen production rate of 163.8 mL H2/g volatile suspended solid/h was obtained at a VS concentration of 5.0% and food waste composition of 62.5%. Additionally, VS concentration of 2.8% and food waste composition of 100% yielded a maximum hydrogen production potential of 152.1 mL H2/g VS. Our findings indicate that food waste and sewage sludge with alkali pretreatment are potential substrates to produce biohydrogen.

A Thermosiphon Photobioreactor for Photofermentative Hydrogen Production by Rhodopseudomonas palustris.

A thermosiphon photobioreactor (TPBR) can potentially be used for biohydrogen production, circumventing the requirement for external mixing energy inputs. In this study, a TPBR is evaluated for photofermentative hydrogen production by Rhodopseudomonas palustris (R. palustris). Experiments were conducted in a TPBR, and response surface methodology (RSM), varying biomass concentration, and light intensity and temperature were employed to determine the operating conditions for the enhancement of both hydrogen production as well as biomass suspension. Biomass concentration was found to have had the most pronounced effect on both hydrogen production as well as biomass suspension. RSM models predicted maximum specific hydrogen production rates of 0.17 mol m−3h−1 and 0.21 mmol gCDW−1h−1 at R. palustris concentrations of 1.21 and 0.4 g L−1, respectively. The experimentally measured hydrogen yield was in the range of 45 to 77% (±3.8%), and the glycerol consumption was 8 to 19% (±0.48). At a biomass concentration of 0.40 g L−1, the highest percentage of biomass (72.3%), was predicted to remain in suspension in the TPBR. Collectively, the proposed novel photobioreactor was shown to produce hydrogen as well as passively circulate biomass.

Developing single‐substrate steady‐state model for biohydrogen production in continuous anaerobic activated sludge–rotating biological contactor system: Novel insights on the process

Kinetic-modeling studies are a key step to attain economically practical technology in terms of efficient scheme and bioreactor scale-up to describe the substrate utilization, biomass growth, and product formation in the biohydrogen production process. Kinetic-modeling study of biohydrogen production from molasses using mixed culture was investigated in an anaerobic activated sludge-rotating biological contactor. Single-substrate with endogenous metabolism and Luedeking–Piret model was used to describe the kinetic parameters at different dilution rate (0.032-0.167 h−1) with influent concentration of 6, 15, and 24 g COD/L. The maximum experimentally estimated value of μmax, kS, and Yx/s was found to be 0.053 h−1, 9.98 g/L, and 0.11 g/g in dilution 0.167 h−1, respectively. It can be concluded that increase in dilution rate is a suitable for microbial activities in the biological hydrogen production process. Accordingly, maximum specific growth rate and hydrogen production rate will be increased. Contrariwise, decrease in dilution rate is associated with lower biomass concentration and higher endogenous metabolism. Therefore, dilution rate is an important parameter in production of biohydrogen in this bioreactor and should be kept in an appropriate range.

Characterization and evaluation of the photocatalytic activity of oxides based on TiO2 synthesized by hydrolysis controlled by the use of water/acetone mixtures

New photocatalysts based on TiO2 were synthesized and characterized. The synthesis involved the controlled hydrolysis of titanium tetraisopropoxide using water containing different proportions of acetone. X-ray diffraction analyses combined with Raman spectroscopy revealed crystalline oxides characterized by the coexistence of the anatase and brookite phases. The Rietveld refinement of diffractograms showed that the presence of acetone in the synthesis process influenced the composition of these crystalline phases, with the proportion of brookite growing from 13% to 22% with the addition of this solvent in the synthesis process. The BET isotherms revealed that these materials are mesoporous with surface area approximately 12% higher than that of the oxide prepared from hydrolysis using pure water. The photocatalytic potential of these oxides was evaluated by means degradation tests using the dyes Ponceau 4R and Reactive Red 120 as oxidizable substrates. The values achieved using the most efficient photocatalyst among the synthesized oxides were, respectively, 83% and 79% for mineralization, and 100% for discoloration of these dyes. This same oxide loaded with 0.5% of platinum and suspended in a 5:1 v/v water/methanol mixture, produced 56 mmol of gaseous hydrogen in 5 h of reaction, a specific hydrogen production rate of 138.5 mmol h−1g−1, a value 60% higher than that achieved using TiO2 P25 under similar conditions.

Hydrogen production by immobilized Rhodopseudomonas palustris in packed or fluidized bed photobioreactor systems

The production of biohydrogen via photofermentation has been shown to have a low environmental impact and can often be integrated into wastewater treatment systems. However, currently, photofermentation has low production rates in comparison to industrial hydrogen production processes, and therefore requires improvement. One route for enhancing hydrogen productivity is the development of improved photobioreactor (PBR) systems. The aim of this study was to compare the hydrogen productivity of Rhodopseudomonas palustris under planktonic, and immobilized cell conditions, with the reactor operating either as a packed bed or a fluidized bed. The fluidized bed PBR achieved a maximum specific hydrogen production rate and substrate conversion efficiency of 15.74 ± 2.2 mL/g/h and 43% respectively, outperforming the conventional planktonic culture and the packed bed PBR. This work demonstrates a significant improvement in productivity over planktonic photofermentation, as well as demonstrating the use of immobilized cells under reactor conditions not usually associated with photosynthetic systems.

Dark fermentation of starch factory wastewater with acid- and base-treated mixed microorganisms for biohydrogen production

Biohydrogen (Bio-H2) can be produced from starch factory wastewater and mixed microorganisms using dark fermentation. Acidic and basic chemicals were used to treat the microorganisms to select the hydrogen (H2)-producing culture. The experiment used a 120 mL bioreactor at 35 °C and the operation commenced with the initial pH level of wastewater in the pH range 4–7 in batch mode. The bacteria:chemical oxygen demand (COD) ratio was 0.2. The initial pH level of the wastewater in the fermentation process affected the H2 yield and the specific hydrogen production rate (SHPR). For acid-treated bacteria, the maximum H2 yield and SHPR were produced at an initial pH of 6.5. The maximum H2 yield and SHPR were 138 mL/g COD degraded and 7.42 mL/g cells‧h, respectively. For the base-treated bacteria, the maximum H2 yield and SHPR were produced at initial pH of 6.5 and pH 7, respectively. The maximum H2 yield and SHPR were 182 mL/g COD degraded and 25.60 mL/g cells‧h, respectively. The COD degradation efficiency levels were 16 and 20% for acid- and base-treated bacteria, respectively. The digested wastewater remained acidic at pH 4.79–4.83. Throughout the study, no methane gas was observed in the gas mixture produced.

Transparent polyvinyl-alcohol cryogel as immobilisation matrix for continuous biohydrogen production by phototrophic bacteria

BackgroundPhototrophic purple non-sulfur bacteria (PNSB) have gained attention for their ability to produce a valuable clean energy source in the form biohydrogen via photofermentation of a wide variety of organic wastes. For maturation of these phototrophic bioprocesses towards commercial feasibility, development of suitable immobilisation materials is required to allow continuous production from a stable pool of catalytic biomass in which energy is not diverted towards biomass accumulation, and optimal hydrogen production rates are realised. Here, the application of transparent polyvinyl-alcohol (PVA) cryogel beads to immobilisation of Rhodopseudomonas palustris for long-term hydrogen production is described. PVA cryogel properties are characterised and demonstrated to be well suited to the purpose of continuous photofermentation. Finally, analysis of the long-term biocompatibility of the material is illustrated.ResultsThe addition of glycerol co-solvent induces favourable light transmission properties in normally opaque PVA cryogels, especially well-suited to the near-infrared light requirements of PNSB. Material characterisation showed high mechanical resilience, low resistance to diffusion of substrates and high biocompatibility of the material and immobilisation process. The glycerol co-solvent in transparent cryogels offered additional benefit by reinforcing physical interactions to the extent that only a single freeze–thaw cycle was required to form durable cryogels, extending utility beyond only phototrophic bioprocesses. In contrast, conventional PVA cryogels require multiple cycles which compromise viability of entrapped organisms. Hydrogen production studies of immobilised Rhodopseudomonas palustris in batch photobioreactors showed higher specific hydrogen production rates which continued longer than planktonic cultures. Continuous cultivation yielded hydrogen production for at least 67 days from immobilised bacteria, demonstrating the suitability of PVA cryogel immobilisation for long-term phototrophic bioprocesses. Imaged organisms immobilised in cryogels showed a monolithic structure to PVA cryogels, and demonstrated a living, stable, photofermentative population after long-term immobilisation.ConclusionTransparent PVA cryogels offer ideal properties as an immobilisation matrix for phototrophic bacteria and present a low-cost photobioreactor technology for the further advancement of biohydrogen from waste as a sustainable energy source, as well as development of alternative photo-bioprocesses exploiting the unique capabilities of purple non-sulfur bacteria.

A Pd/Al2O3-based micro-reformer unit fully integrated in silicon technology for H-rich gas production

This work reports the design, manufacturing and catalytic activity characterization of a micro-reformer for hydrogen-rich gas generation integrated in portable-solid oxide fuel cells (μ-SOFCs). The reformer has been designed as a silicon micro monolithic substrate compatible with the mainstream microelectronics fabrication technologies ensuring a cost-effective high reproducibility and reliability. Design and geometry of the system have been optimized comparing with the previous design, consisting in an array of more than 7x103 vertical through-silicon micro channels perfectly aligned (50 μm diameter) and a 5 W integrated serpentine heater consisting of three stacked metallic layers (TiW, W and Au) for perfect adhesion and passivation. Traditional fuels for SOFCs, such as ethanol or methanol, have been replaced by dimethyl ether (DME) and the chosen catalyst for DME conversion consists of Pd nanoparticles grafted on an alumina active support. The micro-channels have been coated by atomic layer deposition (ALD) with amorphous Al2O3 and the influence of rapid thermal processing (RTP) on such film has been studied. A customized ceramic 3D-printed holder has been designed to measure the specific hydrogen production rates, DME conversion and selectivity profiles of such catalyst at different temperatures.

Water hyacinth producing bio-hydrogen by Klebsiella oxytoca ATCC 13182 and their optimization

Our energy requirements are almost totally provided by carbon containing fossil sources such as oil, coal and nature gas, but they cause serious environmental problems during combustion such as CO2 emission and climate changes. Bio-hydrogen production from Klebsiella oxytoca ATCC 13182 and water hyacinth was taken as a substrate. Water hyacinth are good source of cellulose and hemicelluloses content used for bio-hydrogen production. This research paper includes the effect of age of inoculation, volume of inoculation and acid pre-treatment (concentrated sulfhuric acid) on bio-hydrogen production along with their specific hydrogen production rate (SHPR), carbon consuming efficiency (CCE) and pH.

Shift of biofilm and suspended bacterial communities with changes in anode potential in a microbial electrolysis cell treating primary sludge

This study, for the first time, documented microbial community shifts in response to the changes in anode potential in a microbial electrolysis cell (MEC) operated with primary sludge. At an anode potential of −0.4 V vs. Ag/AgCl, the MEC showed COD and VSS removal efficiencies of 73 ± 1% and 75 ± 2%, respectively. The volumetric current density and specific hydrogen production rate were 23 ± 1.2 A/m3, and 145 ± 4.1 L/m3-d, respectively. The anodic microbial community was consisted of various fermentative/hydrolytic bacteria (e.g., Bacteroides and Dysgonomonas) and anode-respiring bacteria (Geobacter), while different hydrolytic/fermentative bacteria were abundant in suspension. The MEC showed substantially inferior performance along with a higher accumulation of various volatile fatty acids when the anode potential was switched to more positive values (0 V and +0.4 V). Both biofilms and suspended communities were also shifted when the anode potential was changed. Notably, at +0.4 V, Geobacter genus entirely disappeared from the biofilms, while Paludibacter species (known fermentative bacteria) were selectively enriched in biofilms. Also, the relative abundance of genus Bacteroides (known hydrolytic bacteria) substantially decreased in both biofilms and suspension, which was correlated with the inferior hydrolysis of VSS. Quantitative comparison of biofilms and suspended microbial communities at different anode potentials revealed a sharp decrease in bacterial cell numbers in anode biofilms after changing anode potential from −0.4 V to +0.4 V. By contrast, bacterial cell numbers in suspension were slightly decreased. Collectively, these results provide new insights into the role of anode potential in shaping key microbial players associated with hydrolysis/fermentation and anodic respiration processes when MECs are operated with real biowastes.

Experimental Calibration of a Biohydrogen Production Estimation Model

Abstract A dynamic physics-based model developed for the prediction of biohydrogen production in a compact tubular photobioreactor (PBR) was calibrated experimentally. The spatial domain in the model was discretized with lumped control volumes and the principles of classical thermodynamics, mass, species, and heat transfer were combined to derive a system of ordinary differential equations, whose solution was the temperature and mass fraction distributions across the entire system. Two microalgae species, namely, Acutodesmus obliquus and Chlamydomonas reinhardtii strain cc125, were cultured in triplicate with different culture media via indirect biophotolysis. Measured biomass and hydrogen concentrations were then used to adjust the specific microalgae growth and hydrogen production rates in the model based on residual sum of squares (RSS) and the direct search method. Despite its simplicity, the presented volume element model was verified to well predict both hydrogen and biomass concentration over time. The microalgae growth rate for each species was determined as 2.16 μalga,0 s−1 and 0.91 μalga,0 s−1 for A. obliquus and C. reinhardtii strain cc125, respectively, where μalga,0 is the specific growth rate of Scenedesmus almeriensis for given temperature and irradiance. The adjusted maximum hydrogen production rates for the local nonmutant A. obliquus and for C. reinhardtii strain cc125 were 1.3 × 10−7 s−1 and 4.1 × 10−7 s−1. Consequently, these hydrogen production rates were 59 times and 19 times smaller, respectively, than that for the mutant C. reinhardtii strain cc849.

Enterobacter ludwigii strain IF2SW-B4 isolated for bio-hydrogen production from rice bran and de-oiled rice bran

Abstract This research is aimed to study fermentative bio-hydrogen producing bacteria isolated from rice bran (RB) and de-oiled rice bran (DORB) wastes of oil extracting industry where RB used as renewable biomass. Therefore, the present research was carried out to isolate different bacterial species from RB and DORB wastes and their along with microscopic and biochemical characterization. Secondary screening was performed to analyse their efficiency of bio-hydrogen production (BHP) under optimum temperature of 37 ± 2 °C and at initial pH 6. Total 115 bacteria were isolated seasonally from RB and DORB wastes. Highest production of 545 ± 5 ml/l H 2 bio-hydrogen was obtained in second season by RB II-6 was confirmed as Enterobacter ludwigii strain IF2SW-B4 based on 16S rRNA gene sequencing. Its specific hydrogen production rate (SHPR) is 3.29 ml/g substrate/h in 3rd day and 1.89 ml/g substrate/h after 6th day of BHP whereas DORB II-4 gave 295 ± 5 ml/l H 2 production and SHPR of 1.82 ml/g substrate/h in 3rd day and 1.02 ml/g substrate/h in 6th day of production.

Effect of temperature on the gas-phase photocatalytic H2 generation using microreactors under UVA and sunlight irradiation

The effect of temperature on the photocatalytic hydrogen generation from a gaseous water-ethanol mixture has been tested in a silicone microreactor containing nine microchannels of 500 μm (width) × 1 mm (depth) × 47 mm (length) coated with Au/TiO2 photocatalyst under UVA irradiation. Kinetic analyses have indicated that the hydrogen production rate follows the Langmuir-Hinshelwood model. The effect of temperature from 298 to 348 K has been determined by thermodynamic parameters, such as enthalpy (ΔH≠), entropy (ΔS≠) and Gibbs free energy (ΔG≠) of activation, using the transition state theory (TST). The apparent rate constants (kapp) are higher by increasing the temperature, and the activation energy has been determined to be 24 ± 1 kJ·mol−1. In order to evaluate if solar concentration could be used to enhance the photoproduction of hydrogen, the reaction has also been conducted under direct sunlight using a solar concentrator of about 1 m in diameter. Finally, the microreactor has been scaled out by a factor of ca. 10 to a device containing thirty-two microchannels of 500 μm (width) × 1 mm (depth) × 117.5 mm (length). The specific (i.e. per irradiated area of catalyst) hydrogen production rates of both microreactors using sunlight are very similar suggesting that this technology could lead to viable solar hydrogen production.

Catalytic reforming of dimethyl ether in microchannels

The steam reforming and oxidative steam reforming of dimethyl ether (DME) were tested at 573–773 K over a CuZn/ZrO2 catalyst in microreactors with three different types of channels: ceramic square channels with side lengths of 900 and 400 μm, and silicon microchannels of 2 μm of diameter. The channels were first coated with ZrOCl2 (ceramic channels) or Zr(i-PrO)4 (silicon microchannels) and calcined at 773 K for 2 h to obtain a homogeneous and well-adhered ZrO2 layer, as determined by SEM, and then Cu and Zn (Cu:Zn = 1:1 M, 20 wt% total metal) were co-impregnated. Operation at highly reduced residence time (10−3 s) while achieving hydrogen yields similar to those recorded over the ceramic channels was possible for the silicon microchannels due to the three orders of magnitude increased contact area. In addition, the amount of catalyst used for coating the silicon microchannels was two orders of magnitude lower with respect to the conventional ceramic channels. Outstanding specific hydrogen production rates of 0.9 LN of H2 per min and cm3 of reactor volume were achieved as well as stable operation for 80 h, which demonstrates the feasibility of using on-site, on-demand hydrogen generation from DME for portable fuel cell applications.

Amylolysis is predominated by cell-surface-bound hydrolase during anaerobic fermentation under mesophilic conditions

While knowing the amylolysis mechanism is important to effectively decompose corn starch fed into an anaerobic digestor, the objective of this study was to detect the activities and locations of α-amylase in a continuous reactor and batch cultures. In the continuous reactor operated at 35°C, the greatest cell-bound α-amylase activity was found to be 4.7 CU mL-1at hydraulic retention time (HRT)=9h, while the greatest volumetric hydrogen production rate (rH2) was observed at HRT=3h as 61mmolL-1 day-1. In the batch tests, the cell-bound α-amylase activities increased when the carbohydrate concentration decreased, and no significant reducing sugar accumulation was found in the serum bottles. By examining the specific hydrogen production rate (qH2) against different corn starch concentrations, the half-saturation constant (KSta) and the maximum qH2 were regressed to be 0.47gL-1 and 6mmol g-VSS-1d-1, respectively. The electronic microscopic images showed that the microbes could colonize on the starch granules without the disturbance of any floc-like materials. Conclusively, by excluding the methanogens and floc matrix, the secreted α-amylases are predominately bound on the cell surfaces and enabled the microbial cells favorably attach on large substrates for hydrolysis under the mesophilic condition.

Enhanced Photoelectrochemical Activity by Autologous Cd/CdO/CdS Heterojunction Photoanodes with High Conductivity and Separation Efficiency.

The development for hydrogen from solar energy has attracted great attention due to the global demand for clean, environmentally friendly energy. Herein, autologous Cd/CdO/CdS heterojunctions were prepared in a carefully controlled process with metallic Cd as the inner layer and CdO as the interlayer. Further research revealed that the transportation and separation of photogenerated pairs were enhanced due to low resistance of the Cd inner layer and the type II CdO/CdS heterojunction. As a result, the optimized Cd/CdO/CdS heterojunction photoanode showed outstanding and long-term photoelectrochemical activity for water splitting, with a current density of 3.52 mA cm-2 , or a benchmark specific hydrogen production rate of 1.65 μmol cm-2 min-1 at -0.3 V versus Ag/AgCl, by using the environmental pollutants of sulfide and sulfite as sacrificial agents.

Study on Measurement Method of Anaerobic Sludge Activity: Hydrogen Production

In this research finding, a new method is derived by adopting Maximum Specific Hydrogen Production Rate (Umax•H2) to measure the activities of anaerobic sludge for hydrogen production. With batch fermentation instrument under mesophilic (35°C ± 1°C) condition the research has been implemented and concluded. In this study, during the demonstration of experiments, for hydrogen fermentation as for the inoculum there three different active sludges are used and as for the feeding materials there kitchen wastes are tested. After resulting analysis it’s been showed that using the Maximum Specific Hydrogen Production Rate (Umax•H2) as a means to be an index or a method of sludge activity measurement-is reported as satisfactorily feasible and applicable. The Umax•H2 values of experimental groups for this research methodology are recorded as the following: Umax•H2(A)=30.24mL/gVSS•h, Umax•H2(B)=10.80mL/gVSS•h, Umax•H2(C)=18.05mL/gVSS•h. The Correlations between Umax•H2 and other parameters, such as cumulative hydrogen yield, fermentation period and degradation rate of TS are all remain Significant throughout the research. During experimental implementation, Pearson Correlation between Umax•H2 and fermentation period is reported as 0.997 achieving statistical Significance 0.047 (<0.05). Pearson Correlation between Umax•H2 and cumulative hydrogen yield is reported as 0.999 achieving a considerable trend toward Significance 0.022 (<0.05). Pearson Correlation between Umax•H2 and degradation rate of TS is reported as 0.999 Signifying a marginal trend toward Significance 0.027 (<0.05).

Kinetic modeling of hydrogen production rate by photoautotrophic cyanobacterium A. variabilis ATCC 29413 as a function of both CO2 concentration and oxygen production rate

ABSTRACTHydrogen production by cyanobacteria could be one of the promising energy resources in the future. However, there is very limited information regarding the kinetic modeling of hydrogen production by cyanobacteria available in the literature. To provide an in-depth understanding of the biological system involved during the process, the Haldane’s noncompetitive inhibition equation has been modified to determine the specific hydrogen production rate (HPR) as a function of both dissolved CO2 concentration (CTOT) and oxygen production rate (OPR). The highest HPR of 15 was found at xCO2 of 5% vol/vol and the rate consequently decreased when the CTOT and OPR were 0.015 k mol m−3 and 0.55 mL h−1, respectively. The model provided a fairly good estimation of the HPR with respect to the experimental data collected.

음식물류 폐기물의 혐기성 소화 시 식종 방법에 따른 start-up 특성 평가

This study was performed to evaluate the characteristics of start-up of anaerobic digestion from food waste with different inoculum ratios. The hydrogen yield was similar with different inoculum ratios. The hydrogen production rate increased with increasing inoculum ratio. But the specific hydrogen production rate decreased with increasing inoculum ratio. Total volatile fatty acids composition analysis showed that butyrate and acetate were the prevalent products in all reactors, followed by lactate and propionate. The acetate was most prevalent product in reactors at <TEX>$X_0/S_0=0.080$</TEX> and 0.159. But in reactors at <TEX>$X_0/S_0=0.239$</TEX> and 0.318, butyrate accounted for greater than 50% of the total volatile fatty acids.