Volatile Fatty Acid Production Rates Research Articles

Improving dairy manure hydrolysis and acidification through microbial community restructuring by adaptation to hyperthermophilic conditions

Dairy manure (DM) contributes significantly to greenhouse gas emissions and ecosystem degradation, yet its resistance to biodegradation hinders widespread bioprocessing applications. Lignocellulosic materials in DM pose a particular challenge because of their recalcitrance. Bioprocessing under hyperthermophilic (≥70 °C) conditions potentially offers an advantage over traditional fermentation temperatures due to enhanced activity of enzymes and the kinetics of enzymatic reactions. This can lead to a higher conversion rate and a greater extent of biomass hydrolysis and acidification. To test the validity of this hypothesis, the current study evaluated the efficacy of anaerobic hydrolysis and acidogenic fermentation of DM under mesophilic, thermophilic, and hyperthermophilic conditions. All inocula were adapted to corresponding temperatures but were derived from the same mesophilic source. Hyperthermophilic conditions resulted in superior DM hydrolysis efficiency (53%) compared to mesophilic (34%) and thermophilic (42%) conditions. The hyperthermophilic environment was particularly favorable to the decomposition of crude proteins and hemicellulose, which were reduced by 64% and 54%, respectively. Furthermore, hyperthermophilic fermentation also yielded the highest volatile fatty acid (VFA) production rate of 460 mg/L/day during the first four days, representing improvements of 50% and 90% over mesophilic and thermophilic conditions. In part, this was attributed to the enhanced production of branched-chain VFAs, including an increase of 6–10% in isobutyric acid and 12–13% in isovaleric acid. At hyperthermophilic conditions, however, there was no accumulation of VFAs during the days 5–8 of fermentation, which could be due to acetate conversion by the syntrophic acetate-oxidizing bacteria. A considerable gain in hydrolysis efficiency and VFA production rate were accompanied by a reduction in microbial diversity, which suggests that hyperthermophilic temperature is a favorable environment for the selection of organisms with enhanced DM hydrolysis and fermentation capabilities. A significantly increased relative abundance of xylanolytic Caldicoprobacter (23% of population) and proteolytic Thermovirga (9% of population) could be the major contributors to improved decomposition of hemicellulose and protein. As revealed by the techno-economic analysis, acidogenic fermentation of DM at 70 °C and a retention period of 4 days provides the greatest positive net present value, highest internal rate of return of 9.2%, and shortest investment payback period of 9 years. This study demonstrates that hyperthermophilic conditions enable superior deconstruction and bioconversion of lignocellulose-containing biomass into VFAs under reduced retention times, offering a promising approach for improving DM management and generating bioproducts.

Enhancing biomethanation performance through co-digestion of diverse organic wastes: a comprehensive study on substrate optimization, inoculum selection, and microbial community analysis.

A blend of organic municipal solid waste, slaughterhouse waste, fecal sludge, and landfill leachate was selected in different mixing ratios to formulate the best substrate mixture for biomethanation. Individual substrates were characterized, and the mixing ratio was optimized with the help of a response surface methodology tool to a value of 1:1:1:1 (with a C/N ratio of 28±0.769 and total volatile fatty acid (VFA) concentration of 2500±10.53 mg/L) to improve the overall biomethanation. The optimized blend (C/N ratio: 28.6, VFA: 2538 mg/L) was characterized for physicochemical, biological, and microbial properties and subjected to anaerobic digestion in lab-scale reactors of 1000 mL capacity with and without the addition of inoculum. The biogas yield of individual substrates and blends was ascertained separately. The observed cumulative biogas yield over 21 days from the non-inoculated substrates varied between 142±1.95 mL (24.6±0.3 ml/gVS) and 1974.5±21.72 mL (270.4±3.1 ml/gVS). In comparison, the addition of external inoculation at a 5% rate (w/w) of the substrate uplifted the minimum and maximum cumulative gas yield values to 203±9.9 mL (35.0±1.6 mL/gVS) and 3394±13.4 mL (315.3±1.2 mL/gVS), respectively. The inoculum procured from the Defence Research and Development Organisation (DRDO) was screened in advance, considering factors such as maximizing VFA production and consumption rate, biogas yield, and digestate quality. A similar outcome regarding biogas yield and digestate quality was observed for the equivalent blend. The cumulative gas yield increased from 2673±14.5 mL (373.7±2.2 mL/gVS) to 4284±111.02 mL (391.47±20.02 mL/gVS) over 21 days post-application of a similar dosage of DRDO inoculum. The 16S rRNA genomic analysis revealed that the predominant bacterial population belonged to the phylum Firmicutes, with the majority falling within the orders Clostridiales and Lactobacillales. Ultimately, the study advocates the potential of the blend mentioned above for biomethanation and concomitant enrichment of both biogas yield and digestate quality.

Optimization of Long-Chain Fatty Acid Synthesis from CO 2 Using Response Surface Methodology

A microbial electrosynthesis cell (MES) is an electrochemical technique in which electrolithoautotrophic electroactive microbes fix carbon dioxide (CO2) to longer-chain volatile fatty acids (VFAs). The synthesis of VFAs from CO2 requires optimization to improve the MES performance for industrial feasibility. This work studied the effect of different parameters, such as pH, headspace gas pressure, ethanol concentration, electrolyte, and trace element concentrations, on VFA synthesis from CO2 that used mixed anaerobic consortia in serum bottles. The operational parameters were varied according to a central composite design (CCD) and response surface methodology (RSM). A global optimum for the response variables was determined. The optimum values of different operating factors to maximize VFA production that was obtained from the optimizations were 1.12 × 105 Pa pressure, pH 7.149, ethanol = 2,318.7 mg/L, three times the standard Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ) 300 electrolyte concentration and five times the standard DSMZ 300 trace elements concentration. The experimental study was validated in triplicate, which considered the optimal values. The rate of total VFA production that was obtained from the operational parameters experimental study was 43.7 ± 5.9 mg/L/day under this optimized condition, which agreed with the predicted value of 30.39 mg/L/day. For the media optimization, validation of the experimental study was conducted at the optimal values, and their experimental response was 79.75 ± 19.26 mg/L/day, and the predicted response was 75.89 mg/L/day for total VFA production rates.

Effects of Different Combinations of Sugar and Starch Concentrations on Ruminal Fermentation and Bacterial-Community Composition in vitro.

High levels of starch is known to have positive effects on both energy supply and milk yield but increases the risk of rumen acidosis. The use of sugar as a non-structural carbohydrate could circumvent this risk while maintaining the benefits, but its effects and that of the simultaneous use of both sugar and starch are not as well-understood. This study aimed to evaluate the effects of different combinations of sugar and starch concentrations on ruminal fermentation and bacterial community composition in vitro in a 4 ×4 factorial experiment. Sixteen dietary treatments were formulated with 4 levels of sugar (6, 8, 10, and 12% of dietary dry matter), and 4 levels of starch (21, 23, 25, and 27% of dietary dry matter). Samples were taken at 0.5, 1, 3, 6, 12, and 24 h after cultivation to determine the disappearance rate of dry matter, rumen fermentation parameters and bacterial community composition. Butyric acid, gas production, and Treponema abundance were significantly influenced by the sugar level. The pH, acetic acid, and propionic acid levels were significantly influenced by starch levels. However, the interactive effect of sugar and starch was only observed on the rate of dry matter disappearance. Furthermore, different combinations of starch and sugar had different effects on volatile fatty acid production rate, gas production rate, and dry matter disappearance rate. The production rate of rumen fermentation parameters in the high sugar group was higher. Additionally, increasing the sugar content in the diet did not change the main phylum composition in the rumen, but significantly increased the relative abundance of Bacteroidetes and Firmicutes phyla, while the relative abundance of Proteobacteria was reduced. At the genus level, the high glucose group showed significantly higher relative abundance of Treponema (P < 0.05) and significantly lower relative abundance of Ruminobacter, Ruminococcus, and Streptococcus (P < 0.05). In conclusion, different combinations of sugar and starch concentrations have inconsistent effects on rumen fermentation characteristics, suggesting that the starch in diets cannot be simply replaced with sugar; the combined effects of sugar and starch should be considered to improve the feed utilization rate.

Tratamiento de lodos activados de purga por explosión de vapor y acidogénesis alcalina

A process improvement of the anaerobic digestion of waste activated sludge (WAS) is needed to produce energy and chemicals to compensate the costs of the full wastewater treatment process. Alkaline steam explosion (A-SE) @160°C and 0.67 MPa absolute pressure, followed by thermophilic alkaline acidogenesis (50°C, pH 9) improves the WAS digestibility and the methane yields by breaking the cell walls and the extracellular polymers. This work studies the treatment time and alkalinity effects on the number of destroyed solids measured as cell damage and particle size reduction. To monitor the process a flow cytometer (FCM), through light scattering signals, proved to be an alternative to the measurement of total suspended solids by gravimetry. The effect of alkaline acidogenesis on the volatile fatty acids (VFA) yield is studied with the A-SE suspension which is cooled down to 50°C and fed to a continuous acidogenic reactor at pH 9 under several organic loads to find the optimum (6 g CODVSS/L.d) with the highest soluble COD and VFA production rates (1.4 and 0.8 g COD/L,d respectively). This effluent can be fed to a methanogenic reactor to produce 0.5 LCH4/L.d or the VFA can be separated for the chemical industry.

Metatranscriptomic insights of the metabolic process enhancement during food wastes fermentation driven by linear alkylbenzene sulphonates

In this study, the approach of stimulating the production of volatile fatty acids (VFAs) during food wastes (FW) fermentation by linear alkylbenzene sulphonates (LAS) is proposed and the underlying mechanisms from a metagenomic perspective are analyzed. The results show that with an appropriate level of LAS (50 mg/L), VFA production is greatly enhanced (11854 versus 1825 mg COD/L) with the increased proportions of high-value long-chain ones. Mechanistic explorations indicate that LAS contributes to the solubilization/hydrolysis of the main organic components in FW (i.e., carbohydrates, proteins and lipids) due to the amphipathic features. Besides, LAS would interact with the cell membranes and increase the cell permeability to promote the membrane transport of substrates into the functional microorganisms for further metabolisms. The metagenomic analysis demonstrates that the critical genes involved in hydrolase secretion (i.e., bglX, pepD and lip), ABC transporter (i.e., rfbA and lolD), main substrate metabolism (especially the proteins and lipids, i.e., glsA and glpK) and fatty acid biosynthesis (i.e., accA, accC and accD) all highly expressed in LAS-conditioned reactors. The upregulation of these encoding genes stimulates microbial metabolic activities and improves the production rates of VFAs. Besides, the distribution of functional microbial populations and their associations with genetic expressions in different reactors were investigated. This study provides profound information on the stimulating effects of LAS on FW fermentation for VFA production from genetic and functional perspectives.

Mathematical modeling of volatile fatty acids production from cheese whey: Evaluation of pH and substrate-inoculum ratio effects

In this work, the pH and substrate-inoculum (S/I) ratio effects on the volatile fatty acids (VFA) production from raw cheese whey were studied. An easily applied mathematical model was proposed to analyze the dynamical behavior of key process variables, allowing the identification of operating conditions that increase the VFA production. The proposed model was developed by considering the hydrolysis and acid-forming stages, and it was adjusted and validated using experimental data obtained in a batch fermenter at different initial pH and S/I ratio conditions. The model parameters were estimated by using the Levenberg-Marquardt algorithm, finding that the estimated kinetic parameters were physical and statistically consistent. Numerical simulations provide the operating conditions where VFA production can be favored, which correspond with those found experimentally, initial pH = 7.0 and S/I ratio of 32.62 gCOD gVSS−1. Therefore, the proposed model might be very useful in predicting the maximum VFA production rate and the maximum organic loading rate (OLR) that could be used without risking the fermentation process stability.

A revised representation of ruminal pH and digestive reparameterization of the Molly cow model

Ruminal pH is a critical factor to regulate nutrient degradation and fermentation. However, it has been poorly predicted in the Molly cow model, and recent improvements in the representation of nitrogen cycling across the rumen wall altered some of the modeled responses to feed nutrients, resulting in some model bias. The objectives of this study were to further improve the representation of pH and to refit parameters related to ruminal metabolism and nutrient digestion in the model to resolve this bias, and to use the improved model to estimate nitrogen and energy fluxes with varying rumen-degradable protein (RDP; 40 vs. 60%) and ruminally degraded starch (RDSt; 50 vs. 75%). A meta data set containing 284 peer reviewed studies with 1,223 treatment means was used to derive parameter estimates for ruminal metabolism and nutrient digestions. Refitting the parameters significantly improved the accuracy and precision of the model predictions for ruminal nutrient outflow [acid detergent fiber (ADF), neutral detergent fiber (NDF), total N, microbial N, nonammonia N, and nonammonia nonmicrobial N], ammonia and blood urea concentrations, and fecal nutrient outflow (protein, ADF, and NDF). The prediction error for body weight was decreased from 19.3 to 6.2% with decreased mean bias (from 76.0 to 11.5%) and slope bias (from 17.2 to 7.7%), primarily due to improved representations of ruminal dry matter and liquid pool size. Adding ammonia concentration as a driver to the pH equation increased the precision of predicted ruminal pH and, thereby, the precision of predicted volatile fatty acid (VFA) concentrations, due to improved representation of pH regulation of VFA production rates. Although minor mean and slope bias were observed for ruminal pH and VFA concentrations, the concordance correlation coefficients indicated that much of the observed variation in these variables remains unexplained. Overall, the biological functions of nutrient degradation and digestion appear to be represented without bias. Simulated results indicated that decreasing RDP and RDSt proportions in an isonitrogenous and isocaloric diet can slightly improve N efficiency, and increasing RDSt proportions can increase energy efficiency.

Enhancing acidogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from physical structure and interfacial thermodynamics

The poor biodegradability of waste activated sludge (WAS) is widely regarded as one of the main bottlenecks in the fermentation of sludge and is attributed mainly to the complex nature of sludge. In this study, the physical structure and interfacial thermodynamics of sludge, which reflect its complex nature, were explored to reveal the effects of isoelectric-point (pI) pretreatment on enhancing the production of volatile fatty acids (VFA). It was observed that the maximum VFA production and the initial VFA production rate increased by 151.2% and 46.6%, respectively, after pI pretreatment, which indicates that pI pretreatment significantly improved the generation efficiency of VFA. The experimental results of 12-day acidogenic fermentation assays following pI pretreatment show that the maximum concentrations of soluble total organic carbon, soluble protein and soluble polysaccharide increased by 209.8%, 148.9% and 84.5%, respectively, and the maximal proportion of low molecular weight (<1 kDa) soluble organic substances increased by 92.4%, thus confirming that pI pretreatment can promote organic solubilisation and hydrolysis in sludge. The analyses of changes in the fractal dimension (Df), the spatial configuration of extracellular polymeric substances, and the interfacial non-covalent interaction energy of sludge during the fermentation process reveal that pI pretreatment can loosen the physical structure, promote the spatial extension of biopolymer molecular chains, and increase the driving forces of solid-liquid interfacial enzymatic reactions. It is thus hypothesised that these changes could be responsible for the high degree of organic solubilisation, hydrolysis and acidification of WAS, which is further confirmed by correlation analyses of the Df and interfacial free energy versus VFA production. These findings are expected to provide a possible means to improve the biodegradability of sludge via its pI to trigger dismantling of the sludge structure and increase the driving forces of interfacial enzymatic reactions.

Biogas Production from a Highly Organic Loaded Winery Effluent Through a Two-Stage Process

The application of a two-stage process to effectively treat winery effluents containing a high concentration of organic matter, measured as chemical oxygen demand (COD), was studied in laboratory scale reactors. The effect of the organic loading rate on either the acidogenic or the methanogenic reactors performance was evaluated. The first stage reached an organic load of 220 kg COD/m3/d with 12% of organics removed and volatile fatty acid (VFA) production rate of 27 ± 1.5 kg COD/m3/d, where acetate was the main VFA. The importance of this investigation results in an efficient conversion of the organic matter to VFA obtained in the acidogenic phase avoiding the acidification in the methanogenic stage. Hydrolysis of the particulate material was conducted regardless of the high rate of VFA production. During the methanogenic stage, organic loading rates up to 26 kg COD/m3/d were efficiently treated, and methane production rates were up to 5.5 m3CH4/m3/d. Maximal methane yields reached 326 mL CH4/g COD. It was demonstrated that it was feasible to treat winery effluents containing high initial organic matter (up to 220 g COD/L) by using a two-stage (acidogenic followed by methanogenic) process. Such configuration reduced the initial organic matter by up to 97% and generated methane with yields close to the theoretical value.

Benefit of solid-liquid separation on volatile fatty acid production from grass clipping with ultrasound-calcium hydroxide pretreatment

Ultrasound-calcium hydroxide (US-Ca(OH)2) pretreatment effectively enhanced volatile fatty acid (VFA) production from lignocellulosic biomass. In this paper, solid and liquid fraction of pretreated grass clipping was for the first time separately fermented in order to improve organic recovery from liquid fraction and reduce inhibition due to alkaline pretreatment. The total VFA yield and VS removal reached 515 mg/g TS and 59.7% after solid-liquid separation, exhibiting an increase of 116.7% and 91.9% comparing to that of mixture sample. The dominate components of VFAs are acetic and propionate acid, accounting for 80–90% of total VFAs. Kinetic analysis showed that the highest maximum VFA production rate of 690 mg/L·d and the highest cumulative VFA production potential of 3299 mg/L were achieved in the fermentation of solid fraction. Microbial analysis showed that the dominate genera for VFA production were Halocella and Ruminiclostridium, both with a relative abundance of 20.1% in fermentation of solid fraction.

Mixed culture fermentation of synthesis gas in the microfiltration and ultrafiltration hollow-fiber membrane biofilm reactors

The effects of pore sizes on the in-situ utilization of synthesis gas (syngas, H2 and CO) mixed culture fermentation (MCF) in the hollow-fiber membrane biofilm reactor (HfMBR) are not clear. Thus, the ultrafiltration (R1) and microfiltration (R2) HfMBRs were constructed. Syngas was totally consumed within the formed biofilm in R1; contrarily, it accumulated notably in R2. In the batch mode of R1 and R2, volatile fatty acids (VFAs) of acetate, butyrate and caproate were the main metabolites, but the production rate of total VFA in R1 (61.9 mmol-C/(L·d)) was higher than that of R2 (27.6 mmol-C/(L·d)). In the continuous mode, the R1 performance was much better than that of R2, and the biofilm in R2 was even washed out. Furthermore, Clostridium (30.0%) was the main genus in the enriched biofilm of R1, which converted syngas to VFAs. Thus, the ultrafiltration membrane shall be the suitable candidate for syngas MCF.

Production of volatile fatty acids from slaughterhouse blood by mixed-culture fermentation

The volatile fatty acid (VFA) production potential from animal blood and the factors affecting this process were investigated in this study. In order to simulate an industrial process different operation modes, batch, fed batch and semi-continuous, were also evaluated. Due to high ammonia concentration in fermentation broth, VFA concentration up to 100 g L−1 was achieved without addition of buffer and methanogen inhibitor. In general, acetic, n-butyric and iso-valeric acids were the most predominant species, although different operational conditions affected the VFA concentration, profile, production rate and yield. The microbial community analysis was conducted on the reactors with the best performance, revealing that 70–90% of the microbial population was from the Clostridiales order with a strong presence from the Sporanaerobacter genus. These results demonstrated the feasibility of a VFA platform bio-refinery using high-protein wastes as substrate via mixed-culture fermentation under non-sterilised conditions.

Polyhydroxyalkanoate Production on Waste Water Treatment Plants: Process Scheme, Operating Conditions and Potential Analysis for German and European Municipal Waste Water Treatment Plants.

This work describes the production of polyhydroxyalkanoates (PHA) as a side stream process on a municipal waste water treatment plant (WWTP) and a subsequent analysis of the production potential in Germany and the European Union (EU). Therefore, tests with different types of sludge from a WWTP were investigated regarding their volatile fatty acids (VFA) production-potential. Afterwards, primary sludge was used as substrate to test a series of operating conditions (temperature, pH, retention time (RT) and withdrawal (WD)) in order to find suitable settings for a high and stable VFA production. In a second step, various tests regarding a high PHA production and stable PHA composition to determine the influence of substrate concentration, temperature, pH and cycle time of an installed feast/famine-regime were conducted. Experiments with a semi-continuous reactor operation showed that a short RT of 4 days and a small WD of 25% at pH = 6 and around 30 °C is preferable for a high VFA production rate (PR) of 1913 mgVFA/(L×d) and a stable VFA composition. A high PHA production up to 28.4% of cell dry weight (CDW) was reached at lower substrate concentration, 20 °C, neutral pH-value and a 24 h cycle time. A final step a potential analysis, based on the results and detailed data from German waste water treatment plants, showed that the theoretically possible production of biopolymers in Germany amounts to more than 19% of the 2016 worldwide biopolymer production. In addition, a profound estimation regarding the EU showed that in theory about 120% of the worldwide biopolymer production (in 2016) could be produced on European waste water treatment plants.

Representing interconversions among volatile fatty acids in the Molly cow model

The Molly cow model uses fixed stoichiometric coefficients for predicting volatile fatty acid (VFA) production from the fermented individual dietary nutrient fractions of forage and concentrate. We previously showed that predictions of VFA production had large errors and hypothesized that it was due to a lack of representation of carbon exchange among VFA. The objectives of the present study were to add VFA interconversion equations based on thermodynamics to the Molly cow model and evaluate the effect of these additions on model accuracy and precision of VFA predictions. Previously described thermodynamic equations were introduced to represent interconversions among VFA. The model was further modified to predict de novo acetate, propionate, and butyrate production coefficients based on forage-to-concentrate ratios rather than discrete, fixed sets of coefficients for forage-based, concentrate-based, and mixed diets. Both the original model and the modified one were reparameterized and evaluated against a common data set containing 8 studies reporting pH, VFA concentration, and VFA production rates using isotope dilution techniques and 62 studies reporting VFA concentrations and pH. Evaluations after parameter estimation revealed that predictions of VFA production rates were not improved, with root mean squared prediction errors (RMSPE) of 77, 60, and 51% for acetate, propionate, and butyrate, respectively, for the revised model versus 75, 63, and 55, respectively, for the original model. The RMSPE for predictions of VFA concentrations were reduced from 28, 46, and 40% to 22, 31, and 26% for acetate, propionate, and butyrate, respectively, simply by rederiving the VFA coefficients, but minimal further improvement was achieved with the addition of thermodynamically driven interconversion equations (RMSPE of 21, 32, and 27% for acetate, propionate, and butyrate, respectively). Thus, the results indicate that thermodynamically driven interchanges among VFA, as represented in this study, may not be a primary determinant for the accuracy of predictions of net production rates. Including the effect of pH on VFA absorption reduced the mean bias of propionate production and slope bias of acetate production, but not the overall RMSPE. The larger prediction errors for VFA production as compared with concentrations suggest the data quality may not be high, or that our representation of VFA production and absorption as well as ruminal digestion is inadequate. Additional data are required to discriminate among these hypotheses.

Identity, Abundance, and Reactivation Kinetics of Thermophilic Fermentative Endospores in Cold Marine Sediment and Seawater.

Cold marine sediments harbor endospores of fermentative and sulfate-reducing, thermophilic bacteria. These dormant populations of endospores are believed to accumulate in the seabed via passive dispersal by ocean currents followed by sedimentation from the water column. However, the magnitude of this process is poorly understood because the endospores present in seawater were so far not identified, and only the abundance of thermophilic sulfate-reducing endospores in the seabed has been quantified. We investigated the distribution of thermophilic fermentative endospores (TFEs) in water column and sediment of Aarhus Bay, Denmark, to test the role of suspended dispersal and determine the rate of endospore deposition and the endospore abundance in the sediment. We furthermore aimed to determine the time course of reactivation of the germinating TFEs. TFEs were induced to germinate and grow by incubating pasteurized sediment and water samples anaerobically at 50°C. We observed a sudden release of the endospore component dipicolinic acid immediately upon incubation suggesting fast endospore reactivation in response to heating. Volatile fatty acids (VFAs) and H2 began to accumulate exponentially after 3.5 h of incubation showing that reactivation was followed by a short phase of outgrowth before germinated cells began to divide. Thermophilic fermenters were mainly present in the sediment as endospores because the rate of VFA accumulation was identical in pasteurized and non-pasteurized samples. Germinating TFEs were identified taxonomically by reverse transcription, PCR amplification and sequencing of 16S rRNA. The water column and sediment shared the same phylotypes, thereby confirming the potential for seawater dispersal. The abundance of TFEs was estimated by most probable number enumeration, rates of VFA production, and released amounts of dipicolinic acid during germination. The surface sediment contained ∼105–106 inducible TFEs cm-3. TFEs thus outnumber thermophilic sulfate-reducing endospores by an order of magnitude. The abundance of cultivable TFEs decreased exponentially with sediment depth with a half-life of 350 years. We estimate that 6 × 109 anaerobic thermophilic endospores are deposited on the seafloor per m2 per year in Aarhus Bay, and that these thermophiles represent >10% of the total endospore community in the surface sediment.

Increased biohydrogen yields, volatile fatty acid production and substrate utilisation rates via the electrodialysis of a continually fed sucrose fermenter

Electrodialysis (ED) removed volatile fatty acids (VFAs) from a continually-fed, hydrogen-producing fermenter. Simultaneously, electrochemical removal and adsorption removed gaseous H2 and CO2, respectively. Removing VFAs via ED in this novel process increased H2 yields by a factor of 3.75 from 0.24molH2mol−1hexose to 0.90molH2mol−1hexose. VFA production and substrate utilisation rates were consistent with the hypothesis that end product inhibition arrests H2 production. The methodology facilitated the recovery of 37g of VFAs, and 30L H2 that was more than 99% pure, both of which are valuable, energy dense chemicals. Typically, short hydraulic and solid retention times, and depressed pH levels are used to suppress methanogenesis, but this limits H2 production. To produce H2 from real world, low grade biomass containing complex carbohydrates, longer hydraulic retention times (HRTs) are required. The proposed system increased H2 yields via increased substrate utilisation over longer HRTs.

Operating strategies for acid phase digestion: an industrial case study

AbstractAcid phase digestion was investigated for enhanced operation in an industrial wastewater treatment plant. In particular, sludge retention time (SRT), temperature and pH were assessed for determining optimal conditions under operating constraints. Volatile fatty acid (VFA) production and soluble chemical oxygen demand (SCOD) were the key process parameters used to assess system performance. Increase in SRT from 0.8 to 1.4 days (at 22°C) had moderate effect on VFA production (approximately 15% increase), achieving a maximum VFA production of 3600 mg/L. High VFA production rate (0.101 ± 0.035 mg/mgVS.d) was obtained at the highest operating retention time during summer (27°C). The degree of solubilisation of particulate organic matter increased with temperature and retention time. Despite an increase in SCOD (6472 ± 873 mg/L max.), a corresponding increase in VFA was not observed. SCOD showed a linear correlation with decrease in pH, while acidogenesis (SCOD conversion to VFA) was found to be favourable at a pH of 4.5.

Changes in ruminal volatile fatty acid production and absorption rate during the dry period and early lactation as affected by rate of increase of concentrate allowance

The aim of the present experiment was to study changes in volatile fatty acid (VFA) production using an isotope dilution technique, and changes in VFA fractional absorption rate (kaVFA) using a buffer incubation technique (BIT) during the dry period and early lactation, as affected by the postpartum (pp) rate of increase of concentrate allowance. The current results are complementary to previously reported changes on rumen papillae morphology from the same experiment. From 50 d antepartum to 80 d pp, VFA production rate was measured 5 times and kaVFA was measured 10 times in 12 rumen-cannulated Holstein Friesian cows. Cows had free access to a mixed ration, consisting of grass and corn silage, soybean meal, and (dry period only) chopped straw. Treatment consisted of either a rapid (RAP; 1.0 kg of DM/d; n=6) or gradual (GRAD; 0.25 kg of DM/d; n=6) increase of concentrate allowance (up to 10.9 kg of DM/d), starting at 4 d pp, aimed at creating a contrast in rumen-fermentable organic matter intake. For the BIT, rumen contents were evacuated, the rumen washed, and a standardized buffer fluid introduced [120 mM VFA, 60% acetic (Ac), 25% propionic (Pr), and 15% butyric (Bu) acid; pH 5.9 and Co-EDTA as fluid passage marker]. For the isotope dilution technique, a pulse-dose of 13C-labeled Ac, Pr, and Bu and Co-EDTA as fluid passage marker was infused. The rate of total VFA production was similar between treatments and was 2 times higher during the lactation (114 mol/d) than the dry period (53 mol/d). Although papillae surface area at 16, 30, and 44 d pp was greater in RAP than GRAD, Bu and Ac production at these days did not differ between RAP and GRAD, whereas at 16 d pp RAP produced more Pr than GRAD. These results provide little support for the particular proliferative effects of Bu on papillae surface area. Similar to developments in papillae surface area in the dry period and early lactation, the kaVFA (per hour), measured using the BIT, decreased from 0.45 (Ac), 0.53 (Pr) and 0.56 (Bu) at 50 d antepartum to 0.28 (Ac), 0.34 (Pr) and 0.38 (Bu) at 3 d pp. Thereafter, kaVFA (/h) rapidly increased up to 0.67 (Ac), 0.79 (Pr), and 0.79 (Bu) at 80 d pp. Although papillae surface area was greater at 16, 30, and 44 d pp in RAP than GRAD, no differences in kaVFA between RAP and GRAD were observed during these days showing papillae surface area is not the limiting factor for kaVFA during early pp adaptation.

Enhancing Food Waste Hydrolysis and the Production Rate of Volatile Fatty Acids by Prefermentation and Hydrothermal Pretreatments

The effects of prefermentation pretreatment (PF) alone and prefermentation combined with hydrothermal (HT) pretreatments (PF-HT) on the production of volatile fatty acids (VFAs) by the anaerobic fermentation of food waste (FW) were investigated. The results showed that PF could enhance the dissolution and hydrolysis of organic materials. The greater levels of soluble organic materials (soluble sugar, 10.8 g/L; and soluble protein, 1.61 g/L) were achieved after the PF-HT pretreatment. In the subsequent anaerobic fermentation tests, for the PF and PF-HT groups, the production rates of VFAs increased 1.3 times and 2.0 times, respectively, as compared to the control over the first 5 days. The maximum concentrations of VFAs for the PF and PF-HT groups on the 15th day of fermentation were 32.9 and 35.5 g COD (chemical oxygen demand)/L, respectively, with corresponding increases in the yields of VFAs of 12% and 21% over the control group. It is suggested that the high content of lipid and cellulose in the FW pre...