Volatile Fatty Acids Degradation Research Articles

An integrated evaluation strategy for anaerobic digestion monitoring based on acid-base balance and thermodynamics of volatile fatty acid degradation

Process monitoring is a top priority for anaerobic digestion (AD), yet it poses a significant challenge due to its inherent inaccuracies and case-specific nature in diagnosing reactor status. Herein, a long-term instability simulation experiment using lignocellulosic biomass was conducted with varying organic loading rate (OLR) and hydraulic retention time (HRT) levels. The increase in OLR (0.75 to 20 kg VS m−3·d−1) and decrease in HRT (20 to 4 d) led to substantial total volatile fatty acids (TVFA) accumulation (>14 g/L) with a concomitant decrease in methane yield and organics removal. Full processing capacity with considerable conversion efficiency at four steps (hydrolysis: 60.43 ± 2.82 %, acidogenesis: 56.13 ± 3.27 %, acetogenesis: 56.03 ± 3.31 %, methanogenesis: 55.93 ± 3.32 %) and optimal methane yield (3.13 ± 0.17 L/(L·d)) was achieved at an OLR of 18 Kg VS·m−3·d−1 and HRT of 4 days. An integrated evaluation strategy for AD process stability is proposed based on the acid-base balance and thermodynamics of VFA degradation. Propionate, iso-valerate, TVFA/bicarbonate alkalinity (BA), and BA/total alkalinity (TA) were identified as potential auxiliary indicators for diagnosing AD imbalances, while ΔG for propionate and valerate degradation served as the early warning signs. To enhance the efficacy and universality of early warnings, relative variations for acid-base indicators based on steady-state (∼15 %) and the absolute threshold value for ΔGPropionate (-40 KJ/mol) and ΔGValerate (-55 KJ/mol) were adopted. This approach integrates considerations for microbial adaption, thereby enhancing an assessment of whether a given signal reflects irreversible acidification or is merely a temporary fluctuation induced by reversible acidification. The proposed strategy enables a reliable and accurate early warning, holding great promise for practical applications.

Harvest age of Urochloa hybrids regarding yield, chemical composition, and in vitro biogas production

Objective: To evaluate the chemical composition, fermentation, and in vitro biogas production of the Cayman and Cobra cultivars, at different cutting ages.
 Design/Methodology/Approach: Dry matter (DM), crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose (HEM), and cellulose (CEL) were determined at 28, 35, 42, and 49 cutting days. In vitro dry matter degradation (IVDMD), pH, and concentration of volatile fatty acids (VFA) were determined during fermentation. The biogas volume was estimated at 6, 12, 24, 48, and 72 hours; the volume of methane (CH4) and carbon dioxide (CO2) in the Cobra and Cayman forages was determined at 72 hours. A completely randomized design was used for the experiment.
 Results: There were no differences (P>0.05) in DM production during the different cutting ages. CP was higher (P<0.05) in both cultivars, at 28 and 35 days after the cutting. The NDF, ADF, HEM, and CEL percentages were different in both cultivars. IVDMD was higher (P<0.05) between day 28 and day 42. Finally, CH4 production was lower (P<0.05) at 28 and 35 days after the cutting.
 Study Limitations/Implications: The chemical composition of pastures is influenced by climate and, therefore, further analysis must be carried out during different periods or seasons of the year.
 Findings/Conclusions: The optimal cutting age of the Cobra and Cayman cultivars under drought conditions is between day 28 and day 35 of regrowth. During that period, they have the best chemical and fermentation characteristics.

Effect of elastic filler on the treatment effect and archaea community structure of ABR

This research endeavored to elucidate the influence of elastic fillers on the performance and archaeal community configuration of the Anaerobic Baffled Reactor (ABR). Utilizing high-throughput sequencing technology, the archaeal community structure was assessed in two distinct ABR sets. The findings revealed that the introduction of elastic fillers led to an enhancement in the average COD removal rate, BOD5 removal rate, effluent biodegradability, and fecal coliform removal rate of the ABR by 3.55%, 3.02%, 7.45%, and 96.85% correspondingly. Concurrently, it facilitated the transformation and degradation of organic matter within the ABR, thus resulting in an increased conversion of organic matter into Volatile Fatty Acids (VFA) within the initial compartment, and enhanced VFA degradation in the subsequent compartments. Indices such as Chao1, Observed species, Simpson, and Shannon concurred that the abundance and heterogeneity of the archaeal community within the ABR were augmented. The average relative abundance of dominant archaea (Methanosaeta and Methanobacterium) in the first compartment of ABR diminished by 3.44%, whereas in the second, third, and fourth compartments, the abundance escalated by 10.50%, 4.18%, and 2.78% respectively. The inclusion of elastic fillers not only amplified the hydrolysis and acid production process in the first compartment of the ABR but also fostered the methane production process in the subsequent compartments.

Functionalized polyethyleneimine grafted magnetic biochar for facilitating in-situ CO2 conversion to improve methane production in thermophilic anaerobic digestion

Redundant CO2 produced during wet anaerobic digestion (AD) needs proper in-situ minimization, in which the amino groups in polyethyleneimine (PEI) can react with CO2 to form bicarbonate by diffusing into the liquid phase. Hence, the addition of PEI-grafted material may provide a novel way to in-situ supplement bicarbonate in the AD system, which could further improve the buffer capacity and promote syntrophic methanogenesis. In this study, different CO2 adsorbents prepared by grafting PEI or epoxide-functionalized PEI (FPEI) on alkali-magnetic biochar were used to investigate the in-situ application feasibility. A series of weight loadings of the FPEI to alkali-magnetic biochar (10BFP = 1:10, 5BFP = 1:5, 2BFP = 1:2, and BFP = 1:1) were evaluated in batch AD tests. Fumed silica was also used as an unconducive solid support material for FPEI grafting. After adding 5BFP, the methane production significantly increased by 17.8 %, along with an increased CH4/CO2 ratio from 181.7 % to 219.6 %. The bicarbonate generated by PEI/FPEI facilitated the degradation of volatile fatty acids by enhancing syntrophic reactions. The increased relative abundance of the genera Acetomicrobium, Thermovirga, and Methanosarcina indicates that FPEI improved the stability of the AD system. The 5BFP adsorbent could play a similar role as the carbonic anhydrase enzyme, with lower dosages favouring methane production.

Performance and Energy Utilization Efficiency of an Expanded Granular Sludge Bed Reactor in the Treatment of Cassava Alcohol Wastewater

In recent years, expanded granular sludge blanket (EGSB) reactor has been widely used in the treatment of high-concentration organic wastewater, but its research mainly focused on treatment efficiency and microbial community composition. There were few studies on the relationship of operation conditions and energy utilization efficiency. Therefore, the methanogenic characteristics and energy utilization efficiency of EGSB reactor were studied by using cassava alcohol wastewater (CAW) as a raw material at (36 ± 1) °C. The results show that the degradation of volatile fatty acids (VFAs) is an important step affecting methane generation compared to the hydrolysis stage. When organic load rate (OLR) was 12.73 gCOD/L·d, the chemical oxygen demand (COD) removal rate was above 95%, the methane production efficiency of raw material was 202.73 mLCH4/ gCOD·d, the four-stage conversion efficiency was the highest, and the energy utilization efficiency was 62.26%, which was the optimal stage for EGSB reactor to treat CAW. These findings support high-efficiency bioenergy recovery from CAW in practice and highlight the potential wide application of high-performance anaerobic reactors for CAW.

Zero-Valent Iron and Activated Carbon Coupled to Enhance Anaerobic Digestion of Food Waste: Alleviating Acid Inhibition at High Loads

Anaerobic digestion (AD) has the advantages of utilizing complex substrates and producing renewable energy and is currently one of the mainstream technologies for food waste (FW) resourcing. However, at high organic loads and low inoculum-to-substrate ratios (ISRs), AD with FW as substrate is prone to acid accumulation, resulting in a drastic decrease in gas production and system collapse. This study investigated the effect of the coupled addition of zero-valent iron (ZVI) and activated carbon (AC) on the AD of FW at three low ISRs of 0.715, 0.625, and 0.5. The results showed that the control group acidified and stopped producing biogas when the ISR decreased to 0.625 and 0.5, but ZVI coupled with AC alleviated the acidification and increased the cumulative biogas yield. Especially at ISR = 0.5, the cumulative biogas yield for the ZVI + AC group was 31.5%, 99.5%, and 11.43 times higher than that of the ZVI, AC, and control groups, respectively. ZVI coupled with AC also increased the degradation of volatile fatty acids (70.5–84.4%) and soluble chemical oxygen demand (50.0–72.9%) while decreasing propionate concentration and improving the stability of the AD system. COD mass balance analyses indicated that the coupled addition of ZVI and AC promoted the conversion of particulate organic matter to soluble organic matter and increased the conversion of carbon sources to methane.

Enhanced food waste anaerobic digestion by simultaneously controlling sludge loads and iron oxide addition

The influence of the combination of sludge load control and iron oxide (IO) addition strategies on food waste anaerobic digestion (AD) was explored. The results showed that low sludge loads or IO addition could significantly enhance AD performance. Low sludge loads promoted substrate hydrolysis and volatile fatty acids degradation, which enhanced the biochemical methane potential (BMP) and production rate, respectively. IO addition promoted the CH4 yield and maximum CH4 production rate (Rm) by 65.50% and 206.79%, respectively, by promoting hydrolysis and interspecies electron transfer among syntrophic microbes. Moreover, although the presence of IO could prevent the failure of AD in extremely high sludge load conditions, it could not ensure complete BMP recovery. In an IO-assisted reactor, low sludge loads decreased the methanogenic lag phase by 53.15% and increased the BMP and Rm by 49.93% and 120.11%, respectively. Although adequate IO dosage could effectively improve the digester performance, excessive dosage may limit the economic benefits. Overall, the AD performance could be optimized by simultaneously controlling the sludge load and IO addition.

Integrated genome-centric metagenomic and metaproteomic analyses unravel the responses of the microbial community to ammonia stress

Ammonia is a major inhibitor in anaerobic digestion of nitrogen-rich organic wastes. In this study, integrated genome-centric metagenomic and metaproteomic analyses were used to identify the key microorganisms and metabolic links causing instability by characterizing the process performance, microbial community, and metabolic responses of key microorganisms during endogenous ammonia accumulation. The identification of 89 metagenome-assembled genomes and analysis of their abundance profile in different operational phases permitted the identification of key taxa (Firmicutes and Proteobacteria) causing poor performance. Metabolic reconstruction indicated that the key taxa had the genetic potential to participate in the metabolism of C2C5 volatile fatty acids (VFAs). Further investigation suggested that during Phase I, the total ammonia nitrogen (TAN) level was maintained below 2000 mg N/L, and the reactor showed a high methane yield (478.30 ± 33.35 mL/g VS) and low VFAs concentration. When the TAN accumulated to > 2000 mg N/L, acid accumulation, mainly of acetate, began to occur, and the methane yield gradually decreased to 330.44 mL/g VS (Phase II). During this phase, the VFA degradation functions of the community were mainly mediated by Firmicutes. Approximately 61.54% of significant differentially expressed proteins (DEPs) related to acetate metabolism in Firmicutes were down-regulated, which led to an increase in acetate concentration to 4897.91 ± 1558.96 mg/L. However, the reactor performance showed spontaneous recovery without any interference (Phase III), during which Firmicutes gradually adapted to the high ammonia conditions. Approximately 75% of the significant DEPs related to acetate metabolism of Proteobacteria were also up-regulated in Phase III compared with Phase II; thus, VFA-related metabolic functions of the community were enhanced, which resulted in a decrease in the total VFA concentration to 195.39 mg/L. When the TAN increased above 4000 mg N/L, the system gradually showed acid accumulation dominated by propionate, accompanied by a second decrease in methane yield (Phase IV). During this phase, the number of up-regulated and down-regulated proteins related to acetate metabolism of Firmicutes and butyrate/valerate metabolism of Proteobacteria was comparable with that of Phase III, indicating that the metabolic functions related to acetate, butyrate, and valerate of the microbial community were not significantly affected. However, for propionate metabolism, the expression activity of fumarate hydratase from Firmicutes and Proteobacteria was severely inhibited by ammonia, as shown by down-regulation ratios of 63.64% and 85.71%, respectively. No protein with the same function that was not inhibited by ammonia could be detected, and the fumarate degradation function of the microbial community was severely damaged, leading to blocked propionate metabolism and irreversible deterioration of reactor performance. This study has provided a new perspective on the microecological mechanisms of ammonia inhibition.

Boosting biogas production from recalcitrant lignin-based feedstock by adding lignin-derived carbonaceous materials within the anaerobic digestion process

Lignocellulosic biomass can add to the worldwide resource of biogas; however, the aromatic structure of lignin is recalcitrant which impairs biodegradation. Direct interspecies electron transfer (DIET) may overcome limitations in the biodegradation of lignin derivatives. Within a circular bioeconomy system, lignin-derived biochar and activated carbon were assessed for their ability to enhance the digestion of a typical lignin monomer – syringaldehyde. Biochar at 5–10 g/L significantly reduced the lag-phase time by 33–42% possibly due to the enhancement of syntrophic hydrogenotrophic methanogenesis. In comparison, activated carbon at 1–10 g/L reduced the lag-phase time by 46–85% and significantly accelerated the degradation of volatile fatty acids, due to a combinational effect of enhanced syntrophic oxidation and DIET. When activated carbon was added at a higher dosage of 20 g/L, the highest biomethane yield (426.6 ml/g) was achieved; an increase of 33% compared to the digestion of syringaldehyde alone. The enhancement was ascribed to the metabolic shift from the hydrogenotrophic to the DIET pathway, which could be implied from the microbial community dominated by Methanosaeta. The superior function of activated carbon over biochar was speculated to be associated with its larger surface area and higher abundance of the CO group.

Calcium cyanamide reduces methane and other trace gases during long-term storage of dairy cattle and fattening pig slurry

Calcium cyanamide (CaCN2) has been used in agriculture for more than a century as a nitrogen fertilizer with nitrification inhibiting and pest-controlling characteristics. However, in this study, a completely new application area was investigated, as CaCN2 was used as a slurry additive to evaluate its effect on the emission of ammonia and greenhouse gases (GHG) consisting of methane, carbon dioxide, and nitrous oxide. Efficiently reducing these emissions is a key challenge facing the agriculture sector, as stored slurry is a major contributor to global GHG and ammonia emissions. Therefore, dairy cattle and fattening pig slurry was treated with either 300 mg kg−1 or 500 mg kg−1 cyanamide formulated in a low-nitrate CaCN2 product (Eminex®). The slurry was stripped with nitrogen gas to remove dissolved gases and then stored for 26 weeks, during which gas volume and concentration were measured. Suppression of methane production by CaCN2 began within 45 min after application and persisted until the storage end in all variants, except in the fattening pig slurry treated with 300 mg kg−1, in which the effect faded after 12 weeks, indicating that the effect is reversible. Furthermore, total GHG emissions decreased by 99% for dairy cattle treated with 300 and 500 mg kg−1 and by 81% and 99% for fattening pig, respectively. The underlying mechanism is related to CaCN2-induced inhibition of microbial degradation of volatile fatty acids (VFA) and its conversion to methane during methanogenesis. This increases the VFA concentration in the slurry, lowering its pH and thereby reducing ammonia emissions.

Enhanced stability of food waste anaerobic digestion under low inoculum to substrate ratio by using biochar

ABSTRACT The influence of biochar on anaerobic digestion (AD) of organic waste have been widely studied. However, the effect of biochar on the mitigation of acidification and subsequently the stimulation of methanogenesis recovery during mono food waste (FW) digestion process under a low inoculum to substrate (I/S) ratio (i.e. a high organic loading) is rarely investigated. In this study, the benefit of biochar with respect to methane production from FW was explored in a mono FW AD system with four different additional amounts of biochar, i.e. 0, 5, 10 and 15 g/L. Results revealed that biochar boosted methane production in AD at a low I/S ratio by 390–530% through stimulating methanogenic activity, improving organics removal and enhancing process stability. The biochar dosage of 10 g/L demonstrated the highest biodegradability of 92.3% and the highest specific methane production of 553.0 mL/g VSremoved among all groups. Without biochar addition, volatile fatty acids (VFAs) accumulated to 20 g/L and the highest total ammonium-N (TAN) was > 1200 mg/L. The suppression of methanogenesis was significantly correlated with VFA and TAN (p < 0.05). Therefore, biochar addition presented a positive effect on VFAs degradation and buffering capacity which could be an effective approach to enhance methane production from FW digestion at a low inoculum to substrate ratio without the fear of system failure.

Biochar-assisted anaerobic membrane bioreactor towards high-efficient energy recovery from swine wastewater: Performances and the potential mechanisms

A high-efficient energy recovery system of biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was established for swine wastewater treatment. Comparing with a conventional AnMBR, biochar addition accelerated volatile fatty acids (VFA) degradation during start-up stage, thereby shortened start-up duration by 44.0 %. Under a high organic loading rate (OLR) of 21.1 gCOD/L/d, BC-AnMBR promoted COD removal efficiency from 90.1 % to 95.2 %, and maintained a high methane production rate of 4.8L CH4/L/d. The relative abundance of Methanosaeta declined from 53.9 % in conventional AnMBR to 21.0 % in BC-AnMBR, whereas that of Methanobrevibacter dramatically increased from 10.3 % to 70.9 %, respectively. Metabolic pathway analysis revealed that biochar not only strengthened hydrogenotrophic methanogenesis pathway, but also upregulated the genes encoding electron transfer carriers and riboflavin metabolism, suggesting the role of biochar facilitating direct interspecies electron transfer for syntrophic methanogenesis. The excellent energy yield performances under high OLR confirmed BC-AnMBR as an advanced system for high-strength swine wastewater treatment.

Bio-Electrochemical Performance of a Ceramic Microbial Fuel Cell Treating Kitchen Waste Leachate: Effect of Organic Loading Rate and Anode Electrode Surface Area

Performance evaluation of a ceramic microbial fuel cell (CMFC) by varying organic strength, hydraulic retention time (HRT) and anode electrode surface area (AESA) to treat leachate generated from acidogenesis of kitchen waste (KW) was studied by the central composite design of experiment. The increase in organic loading rate (OLR) positively affected power density (PD) while negatively influencing organic removal and coulombic efficiency (CE). This behavior is possible due to substrate inhibition and the coercive effect of low HRT, i.e., substrate washout, biofilm abrasion, and reduced contact period, while at high HRT, the volatile fatty acid (VFA) degradation improved. Since acetic acid is the final product of long-chain VFAs degradation, a pseudo consumption order for VFAs was obtained: butyric &gt; propionic &gt; acetic. The AESA aided organics removal and PD but had a negligible effect on CE. According to ANOVA, the COD removal was linearly modeled, while PD and CE were quadratic. The validation runs (VR) proved efficient as the highest COD removal was for VR2 (83.7 ± 3.6%), while maximum PD and CE values obtained were 0.224 ± 0.02 W/m3 and 2.62 ± 0.33%, respectively, for VR3, supported by the lower anode potential.

Enhanced direct interspecies electron transfer and methane production during anaerobic digestion of fat, oil, and grease by coupling carbon-based conductive materials and exogenous hydrogen

To investigate the combination of carbon-based conductive materials and exogenous hydrogen (EH2) on methane recovery from fat, oil, and grease (FOG), granular activated carbon (GAC) and carbon cloth (CC) were chosen to collaborate with EH2, resulting in increased methane production by 59 % and 84 %, respectively. Further digestion of long chain fatty acids (LCFAs) confirms that enhanced direct interspecies electron transfer (DIET) was achieved in the reactors with GAC/CC + EH2 than those with GAC/CC only. Other evidences (such as increased microbial population and rapid degradation of volatile fatty acids) were found to support the role of GAC/CC + EH2 in promotion of DIET. Significant change of microbial community was observed using GAC/CC + EH2, which was mainly attributed to the enrichment of electrogenic species (such as Spirochaetaceae, Syntrophomonas palmitatica, and Methanosaeta), leading to some changes in metabolic pathways during acidogenesis and methanogenesis. Together, enhanced DIET was achieved by GAC/CC + EH2, thus improving the methane recovery from FOG.

Regulating effects of Fe/C materials on thermophilic anaerobic digestion of kitchen waste: Digestive performances and methanogenic metabolism pathways

This study aimed to evaluate the effects of typical exogenous materials (Fe and C) addition on the performances of kitchen waste thermophilic (55 °C) anaerobic digestion in semi-continuous system. The expression of key enzymes in methanogenesis were demonstrated by microbiological approach, and the regulating effectsf Fe and C addition on methanogenic metabolism pathway were specially explored. Four anaerobic digestion systems (TAD, TAD + Fe, TAD + C and TAD + Fe + C) were carried out, and the running period lasted for 120 days, which was divided into four stages according to organic loading rates (OLRs) from 1.05 to 4.22 gVS/L/day. From this study, C addition improved soluble polysaccharide (sPolysaccharide), soluble protein (sProtein), soluble total organic carbon (sTOC) concentrations and hydrolase (amylase, protease and lipase) activities, while Fe addition facilitated the subsequent degradation of volatile fatty acids (VFAs). As a result, the average methane yields were promoted from 478.0 (TAD) to 487 (TAD + Fe), 597 (TAD + C) and 647.3 mL/gVS/day (TAD + Fe + C), respectively. For microbial communities, C addition promoted the relative abundances of hydrolytic bacteria (Defluviitoga), while Fe addition realized the directional enrichment of Methanothermobacter, and improved hydrogenotrophic methanogenesis ratios from 30.81 % (TAD) to 52.27 % (TAD + Fe) and 64.40 % (TAD + Fe + C). According to the expression of key enzymes in methanogenic metabolism pathways, Fe addition enhanced the syntrophic acetic acid oxidation and hydrogenotrophic methanogenesis pathway, realizing the conversion of dominated methanogenic metabolism pathway from aceticlastic methanogenesis to hydrogenotrophic methanogenesis. This study is of great significance to exposit the behaviors of conductive materials during thermophilic digestion of kitchen waste.

Independent and combined effects of biochar and microbial agents on physicochemical parameters and microbial community succession during food waste composting

This study evaluated the independent and combined effects of biochar and microbial agents on food waste composting. The results indicated that combined addition increased the peak temperature to 63.5 °C and extended the thermophilic periods to 8 days, resulting in the highest organic matter degradation rate (12.7%). Analysis of enzymatic activity indicated that combined addition increased urease and dehydrogenase activity by 22.9% and 26.5%. Furthermore, the degradation of volatile fatty acids also increased by 37.4% with combined addition. Microbial analysis demonstrated that combined addition effectively increased the relative abundances of Enterobacter, Sphingobacterium and Aspergillus, which could be attributed to the optimal environment provided by biochar and stimulation of microbial agents. Moreover, correlation analysis showed a strong interaction between the microbial community and environment with combined addition. In general, combined addition could be beneficial for composting based on the synergistic effects of biochar and inoculation on microorganism.

Start-up of the mesophilic anaerobic co-digestion of two-phase olive-mill waste and cattle manure using volatile fatty acids as process control parameter

In this work, the start-up and stabilization stages of mesophilic anaerobic co-digestion of 2POMW and CM in a semi-continuously fed stirred tank reactor (SSTR) were analyzed. Volatile fatty acids (VFAs) were monitored and used as the main control parameter for the start-up and stabilization stages, as well as to evaluate the potential inhibition episodes. The results showed that accumulation of propionic acid was the key factor in the inhibition of the methanogenic phase, leading to process imbalance. To avoid the problems associated with inhibition by high VFA concentrations, several reinoculations were performed using a suitable inoculum adapted to VFA degradation. The start-up phase was carried out in batch conditions for 97 days, reaching a final concentration of propionic acid of 12.77 mg/L. From that moment, the reactor was fed in a semi-continuous mode with a hydraulic retention time (HRT) of 40 days. A total period of 140 days was required to achieve a stable performance of the reactor with a methane productivity of 0.34 LCH4/LRd.

Understanding the mechanisms behind enhanced anaerobic digestion of corn straw by humic acids

Humic acids (HAs) are abundant on earth, yet their effects on anaerobic digestion (AD) of cellulosic substrate are not fully uncovered. The effects of HAs on AD of corn straw and the mechanisms behind were analyzed in this study. Results showed that the effects of HAs on methane yield were closely related to the total solids (TS) content. At relative high TS content of 5.0%, HAs benefited AD process by increasing 13.8% of methane yield, accelerating methane production rate by 43% and shortening lag phase time by 37.5%. Microbial community analysis indicated that HAs increased the relative abundance of syntrophic bacteria (Syntrophomonadaceae and Synergistaceae), facilitating the degradation of volatile fatty acids. HAs might act as electron shuttles to directly transfer electrons to hydrogenotrophic methanogens for CO2 reduction to CH4. This study provides a simple and efficient strategy to facilitate the AD of cellulosic substrate by HAs addition.

Recovery of microbial fuel cells with high COD molasses wastewater and analysis of the microbial community

The microbial fuel cell (MFC) is a green technology that can be used to simultaneously degrade organics and generate energy. In this work, a double-chamber microbial fuel cell (DCMFC) was used to treat molasses wastewater with different concentrations, in batch tests. The results demonstrated that when the COD of the influent wastewater was low (3630 mg/L), the MFC system ran stably for 12 h, the maximum power density reached 3.28 W/m3, the internal resistance was 94 Ω, and a total COD removal efficiency of 88.5% was obtained at the end of the cycle. However, as the influent increased to 9060 mg/L, the performance of the MFC significantly declined due to the accumulation of volatile fatty acids (VFAs) and low pH. Interestingly, 10 days later the MFC system began to recover: output voltage increased from 209 to 535 mV, VFA declined from 2428.25 to 0 mg/L, and pH increased from 4.83 to 6.98. The microbial diversity of the biofilm on the anode was also investigated using high-throughput sequencing technology in order to clarify the effect of microbials on the treatment efficiency. The dominant microbial genera, such as Azospirillum, Sulfuricurvum, Syntrophomonas and Curvibacter, are highly related to the electron transfer and VFA degradation. In addition, Geobacter found on the carbon felt was responsible for electricity generation. These results demonstrate that MFCs could be an efficient way to treat molasses wastewater, because of its great self-recovery capability.

Thermodynamics of volatile fatty acid degradation during anaerobic digestion under organic overload stress: The potential to better identify process stability

Anaerobic digestion (AD) operating under organic overload stress usually increases the potential for process instability, leading to significant economic and ecological consequences. Volatile fatty acids (VFAs) accumulation is regularly considered a major factor during AD and their degradation is subject to thermodynamic constraints. To date, no study has systematically investigated the mechanisms of VFA degradation on process stability from the perspective of thermodynamics. Hence, increased substrate-to-inoculum ratio was applied in this study to simulate organic overload stress using batch tests with Hybrid Pennisetum. As a result, VFAs accumulation increased, accompanied by decreased methane yield, slower methane production kinetics and even severe process instability. Metagenomic analysis demonstrated that the accumulated propionate and butyrate were degraded by methyl-malonyl-CoA and the β-oxidation pathway while syntrophic acetate oxidation was preferred during acetate degradation. The deviation of stability parameters to varying degrees from the recommended threshold values was observed. However, a subsequent thermodynamic analysis revealed that moderate organic overload stress merely retarded the syntrophic oxidation of propionate, butyrate, and acetate. As a result, the methanogenic activity decreased, and the lag phase of AD was extended, but no adverse thermodynamic effects actually occurred. Changes in the Gibbs free energy for syntrophic propionate and acetate oxidation have the potential to better identify process stability. This study provided novel insights into the underlying thermodynamic mechanisms of VFA degradation and may have important implications for improving the current diagnostic mode for AD process stability.