Dynamics of propionic acid degradation in a two-phase anaerobic system

Influence of microwave temperature and power on the biomethanation of food waste under mesophilic anaerobic conditions

Food waste is an attractive feedstock for Anaerobic Digestion due to its high biodegradability and moisture content. Nevertheless, due to its complex structure and composition, methane yield is typically compromised with 50-60% of the theoretical maximum obtained. The well-known limitation of the hydrolysis step can be circumvented by adopting feedstock pre-treatments, such as microwave irradiation. It improves solubilization of various FW components making them more readily available for the microorganisms and reducing AD process duration. In this work different heating rates (7.8, 3.9 and 1.9 °C/min) and temperatures (85, 115, 145, 175 °C) were applied when pre-treating food waste as a substrate for AD. Increase in the solubilization of organic matter in the form of Soluble Chemical Oxygen Demand was the most significative change in FW characteristics after pre-treatment, with final temperature of 175 °C and heating rate of 3.9 °C showing a 73.19% increment. Nevertheless, process performance of AD of MW FW was optimum at 85 °C 7.8 ramp, showing no intermediate products accumulation, up to 77% more methane produced in the first week of digestion compared to the other conditions tested and reduction of 96.36% on the lag phase duration, compared to the control. On the other hand, samples treated at 175 °C, regardless of heating rate, consistently showed poor process performance, with low methane yield, possibly due to the formation of hard-to-digest compounds. This work underlines the importance of adjusting microwave temperature and power when pre-treating FW for biomethane production so the process is optimized.

GO/iron series systems enhancing the pH shock resistance of anaerobic systems for sulfate-containing organic wastewater treatment

In this study, the effect of pH shock during the treatment of sulfate-containing organic wastewater was investigated using an anaerobic fermentation system reinforced with graphene oxide (GO)/iron series systems. The results show that the anaerobic system with the GO/iron series systems exhibited enhanced resistance to pH shock. Among them, the GO/Fe⁰ system had the strongest resistance to pH shock, the systems of GO/Fe₃O₄ and GO/Fe₂O₃ followed close behind, while the blank system performed the worst. After pH shock, the COD_Cr removal rate, SO₄²⁻ removal rate, and gas production of the GO/Fe⁰ group were significantly improved compared with those of the control group by 51.0%, 65.3%, and 34.6%, respectively, while the accumulation of propionic acid was the lowest. Further, detailed microbial characterization revealed that the introduction of the GO/iron series systems was beneficial to the formation of more stable anaerobic co-metabolic flora in the system, and the relative abundance of Geobacter, Clostridium, Desulfobulbus and Desulfovibrio increased after acidic and alkaline shock.

In this paper, we studied the pH shock resistance mechanism of GO/iron series from the perspectives of the treatment effect, changes in effluent pH and VFA, and microbial co-metabolic stability, providing a reference for the practical application.

Start-up phase optimization of two-phase anaerobic digestion of food waste: Effects of organic loading rate and hydraulic retention time

The effects of organic loading rate (OLR) and hydraulic retention time (HRT) on the dynamics of acidogenic and methanogenic processes in two-phase anaerobic digestion (TPAD) of food waste (FW) were investigated to determine the start-up operational conditions. Seven arrangements of TPAD systems under mesophilic conditions were evaluated, each containing one acidogenic reactor and one methanogenic reactor. The work analyzed two HRTs (2 and 3 days) and four OLRs (2, 3, 4 and 5 kgVS.m^-3.d^-1). The 2D5KG system obtained VS and COD removal of 68% and 72%; SMP of 273 L_methane.kg.VS^-1. The 3D4KG system obtained VS and COD removal of 70 and 66%; SMP of 252 L_methane.kg.VS^-1. Valeric acid predominated in the acidogenic reactor in both HRT and OLR evaluated, followed by butyric acid. In the methanogenic reactor, the main methane production route was the butyric acid conversion into acetic acid and finally methane. Higher OLR benefits the methane production. The microbiological profile indicated the pathway of methanogenesis by acetoclastic methanogenesis. The canonical correlation analysis allowed to verify that the groups are independent and, therefore, the variables analyzed in the acidogenic reactor have an influence on the methanogenic reactor.

The influence of combined low-strength ultrasonics and micro-aerobic pretreatment process on methane generation and sludge digestion: Lipase enzyme, microbial activation, and energy yield

Low-frequency ultrasonics is a potential technology to reduce the hydrolysis phase period in anaerobic digestion process. In this study, theinfluence of combined low frequency ultrasonics and micro-aerobic (MA) pretreatment on sewage sludge solubilization, enzyme activity and anaerobic digestion were assessed. Initially, the effect of ultrasonic density (0.012, 0.014, 0.016, 0.018, 0.1, 0.12 and 0.14 W/mL) and irradiation time (1, 3, 5, 8, 9, 10 and 12 min) of 20 kHz frequency waves were investigated. Accordingly, the effect of micro-aerobic pretreatment (Air flow rate (AFR) = 0.1, 0.2, 0.3 and 0.5 VVM) within 20, 30, 40.48 and 60 h were examined. In addition, the effect of combined pretreatment on COD solubilization, lipase enzyme activation, ATP, percentage of live bacteria and methane gas production during the anaerobic process were examined. The results showed that the highest lipase activity (14.9 Umol/mL) was obtained under the effect of ultrasonic density of 0.1 W/ml within 9 min. The highest solubilization (65%) was observed under optimal micro-aerobic conditions: AFR = 0.2 (VVM) and micro-aerobic time: 40 h. Combined ultrasonic and micro-aerobic (US + MA) pretreatment increases the solubilization (70%), microbial activity (2080%) and lipase enzymatic activity (129%) compared to individual pretreatment. The Biogas production during anaerobic digestion pretreated with combined methods increased by 193% compared to the control, while the elevated values of biogas production in reactors pretreated by ultrasonic and micro-aerobic pretreatment alone were observed to be 101% and 165%, respectively. The net energy in reactor with the combined pre-treatment methods was calculated to be 1.26 kWh, while this value for control, pretreated ultrasonic and micro-aerobic reactors were obtained to be 0.56, 0.67 and 1.2 kWh, respectively.

Synergistic effects of rice straw and rice bran on enhanced methane production and process stability of anaerobic digestion of food waste

This study investigated the synergistic effects of rice straw (RS) and rice bran (RB) addition on methane production and process stability of anaerobic digestion of food waste (FW). Positive synergistic effect (Synergy index (SI) = 1.03-1.24 > 1) was noticed in all the co-digestion reactors. The optimum mixing ratio of FW:RS:RB (volatile solid (VS) basis) was 60:10:30 with the maximum SI (1.24), achieving 27.4% increase in methane yield (235.4 mL/g-VS) and around 5 days shorter of λ (3.7 days) compared to the mono-digestion of FW (184.8 mL/g-VS and 8.2 days). Remarkably high concentration of volatile fatty acids (VFAs) was also accumulated in the mono-digestion of FW, especially propionic acid, which to a great extent caused the methane production to stagnate. Results from this study demonstrate that co-digestion of FW and RS with RB has high potentials for energy recovery from AD of the mixed feedstocks and its stable operation.

Effect of micro-aerobic process on improvement of anaerobic digestion sewage sludge treatment: flow cytometry and ATP assessment

Micro-aeration as a pretreatment method improves the efficiency of anaerobic digestion of municipal sewage sludge and consequently promotes the methane production. In this study, adenosine triphosphate (ATP) and flow cytometry (FCM) were employed to monitor the performance of the micro-aerobic process and investigate the survival of bacterial cells within the process. At first, the effect of air flow rate (AFR) (0.1, 0.2, 0.3 and 0.5 vvm) on hydrolysis of mixed sludge in 5 aeration cycles (20, 30, 40, 48 and 60 hours) was examined. Then, the effects of the micro aerobic process on methane (CH₄) production in anaerobic digestion were surveyed. The highest VSS reduction was 30.6% and 10.4% for 40 hours in the reactor and control, respectively. Soluble COD also fluctuated between 40.87 and 65.14% in micro-aerobic conditions; the highest SCOD was achieved at the time of 40 h. Microbial activities were increased by 597%, 170% and 79.4% for 20, 30 and 40 h pretreatment with the micro-aerobic process, respectively. Apoptosis assay showed that micro-aerobic pre-treatment at 20, 30 and 40 h increased the percentage of living cells by 57.4, 62.8 and 67.9%, respectively. On the other hand, FCM results showed that the highest percentage of viable bacteria (i.e., 67.9%) was observed at 40 h pretreating which was approximately 40% higher the ones for the control. Variation in cumulative methane production shows that methane production was increased by 221% compared to anaerobic digestion (control group). Therefore, ATP and FCM can be employed as two appropriate, accurate, relatively specific indicators for monitoring the process and bacteria viability.

Micro-aeration as a pretreatment method improves the efficiency of anaerobic digestion of municipal sewage sludge and consequently promotes the methane production.

Co-digestion of microalgae with potato processing waste and glycerol: effect of glycerol addition on methane production and the microbial community

The production of methane-rich biogas from the anaerobic digestion (AD) of microalgae is limited by an unfavorable biomass carbon-to-nitrogen (C/N) ratio; however, this may be ameliorated using a co-digestion strategy with carbon-rich feedstocks. For reliable plant operation, and to improve the economics of the process, secure co-feedstock supply (ideally as a waste-stream) is important. To this end, this study investigated the feasibility of co-digesting microalgae (Chlorella vulgaris) with potato processing waste (potato discarded parts, PPW_dp; potato peel, PPW_p) and glycerol, while monitoring the response of the methanogenic community. In this semi-continuous study, glycerol (1 and 2% v/v) added to mixtures of C. vulgaris : PPW_dp enhanced the specific methane yields the most, by 53–128%, whilst co-digestion with mixtures of C. vulgaris : PPW_p enhanced the methane yields by 62–74%. The microbial communities diverged markedly over operational time, and to a lesser extent in response to glycerol addition. The acetoclast Methanosaeta was abundant in all treatments but was replaced by Methanosarcina in the potato peel with glycerol treatment due to volatile fatty acid (VFA) accumulation. Our findings demonstrate that the performance of microalgae co-digestion is substantially improved by the addition of glycerol as an additional co-feedstock. This should improve the economic case for anaerobically digesting microalgae as part of wastewater treatment processes and/or the terminal step of a microalgae biorefinery.

Glycerol as an additional co-substrate enhanced methane yields by up to 128% when co-digestion with microalgae and potato waste.

Effects of substrate concentration, hydraulic retention time and headspace pressure on acid production of protein by anaerobic fermentation

Protein is a potential resource for fermentation, which is one of the rate limiting factors of fermentation. This study investigated some of the regularities of protein on dark fermentation for acid production, whey (82% protein) liquid was used to study the effect of varying headspace pressure (HP; 0.02-0.10 MPa), protein concentration (6.25-18.75 g/L) and hydraulic retention time (HRT; 24-48 h) on VFA production. The main fermentation product is acetate and butyrate. Low HP, low substrate concentration and short HRT were favorable for the generation of acid. When the protein concentration was 12.5 g/L and HRT was 24 h, the VFA concentration was up to 8924.190 mg/L at 0.02 MPa, compared to a 61.55% increase under 0.10 MPa. While the HRT was 48 h or the protein concentration was 18.75 g/L, VFA concentration was low and no more n-butyric acid or valeric acid. After variance analysis, we observed a significant effect of the three factors on acid.

Black water collected from the septic tank treated with a living machine system: HRT effect and microbial community structure

In this study, the performance of a living machine (LM) system was evaluated for use in the treatment of black water collected from septic tanks with hydraulic retention times (HRTs) of 6, 5, and 4 days. We found that the HRT had little effect on the removal efficiency of chemical oxygen demand (COD). However, the removal rates of total nitrogen (TN) and ammonium nitrogen (NH₄⁺-N) decreased with the reduction of HRT, whereas the removal efficiency of total phosphate (TP) was consistently low because of the long sludge retention time. The working conditions of #1 achieved the highest removal efficiency of COD (85%), NH₄⁺-N (75%), and TN (47%), although the removal efficiency of TP (11%) was slightly lower than that of #2 (12%). The microbial communities in each tank of the LM system were characterized by high-throughput sequencing, which showed that the LM system successfully created more favorable conditions for fermentative bacteria than traditional systems, with relative abundances of 13% (#1), 13% (#2), and 15% (#3) compared to that of the anaerobic/anoxic/oxic (A²O) system (<3%). Smithella was the dominant fermentative bacteria, accounting for 9% (#1), 7% (#2), and 10% (#3) of total bacteria in the LM system. The relative abundances of ammonia oxidizing bacteria (AOB) (12%) and anaerobic ammonium oxidizing bacteria (AnAOB) (7%) in the LM system were much higher than that in the A²O system. Overall, the LM system offered a more sustainable and economical solution for treating black water.

New insights into enhanced anaerobic degradation of Fischer-Tropsch wastewater with the assistance of magnetite

In this study, magnetite (Fe₃O₄), as the typical conductive material, was supplemented in anaerobic sequential batch reactor (ASBR) with the attempt to enhance pollutants removal and methane production during Fischer-Tropsch wastewater treatment. The results showed that COD removal efficiency and cumulative methane production with the addition of optimum magnetite dosage (0.4 g) were as high as 84.3 ± 2.0% and 7.46 ± 0.24 L, which were higher than other test groups (0, 0.2 and 0.6 g). Furthermore, the combination of high-throughput 16S rRNA gene pyrosequencing and metagenomic analysis in this study further confirmed that the Geobacter and Methanosaeta species were specially enriched in bacterial and archaeal community at the optimum magnetite dosage, suggesting that magnetite-mediated direct interspecies electron transfer (DIET) between Geobacter and Methanosaeta species was likely a crucial reason to promote syntrophic metabolism of propionic acid and butyric acid, and further enhance final methanogenesis.

Targeting Bacteria and Methanogens To Understand the Role of Residual Slurry as an Inoculant in Stored Liquid Dairy Manure

Microbial communities in residual slurry left after removal of stored liquid dairy manure have been presumed to increase methane emission during new storage, but these microbes have not been studied. While actual manure storage tanks are filled gradually, pilot- and farm-scale studies on methane emissions from such systems often use a batch approach. In this study, six pilot-scale outdoor storage tanks with (10% and 20%) and without residual slurry were filled (gradually or in batch) with fresh dairy manure, and methane and methanogenic and bacterial communities were studied during 120 days of storage. Regardless of filling type, increased residual slurry levels resulted in higher abundance of methanogens and bacteria after 65 days of storage. However, stronger correlation between methanogen abundance and methane flux was observed in gradually filled tanks. Despite some variations in the diversity of methanogens or bacteria with the presence of residual slurry, core phylotypes were not impacted. In all samples, the phylum Firmicutes predominated (∼57 to 70%) bacteria: >90% were members of ClostridiaMethanocorpusculum dominated (∼57 to 88%) archaeal phylotypes, while Methanosarcina gradually increased with storage time. During peak flux of methane, Methanosarcina was the major player in methane production. The results suggest that increased levels of residual slurry have little impact on the dominant methanogenic or bacterial phylotypes, but large population sizes of these organisms may result in increased methane flux during the initial phases of storage.IMPORTANCE Methane is the major greenhouse gas emitted from stored liquid dairy manure. Residual slurry left after removal of stored manure from tanks has been implicated in increasing methane emissions in new storages, and well-adapted microbial communities in it are the drivers of the increase. Linking methane flux to the abundance, diversity, and activity of microbial communities in stored slurries with different levels of residual slurry can help to improve the mitigation strategy. Mesoscale and lab-scale studies conducted so far on methane flux from manure storage systems used batch-filled tanks, while the actual condition in many farms involves gradual filling. Hence, this study provides important information toward determining levels of residual slurry that result in significant reduction of well-adapted microbial communities prior to storage, thereby reducing methane emissions from manure storage tanks filled under farm conditions.

Microbial community composition and electricity generation in cattle manure slurry treatment using microbial fuel cells: effects of inoculum addition

Microbial fuel cell (MFC) is a sustainable technology to treat cattle manure slurry (CMS) for converting chemical energy to bioelectricity. In this work, two types of allochthonous inoculum including activated sludge (AS) and domestic sewage (DS) were added into the MFC systems to enhance anode biofilm formation and electricity generation. Results indicated that MFCs (AS + CMS) obtained the maximum electricity output with voltage approaching 577 ± 7 mV (~ 196 h), followed by MFCs (DS + CMS) (520 ± 21 mV, ~ 236 h) and then MFCs with autochthonous inoculum (429 ± 62 mV, ~ 263.5 h). Though the raw cattle manure slurry (RCMS) could facilitate electricity production in MFCs, the addition of allochthonous inoculum (AS/DS) significantly reduced the startup time and enhanced the output voltage. Moreover, the maximum power (1.259 ± 0.015 W/m²) and the highest COD removal (84.72 ± 0.48%) were obtained in MFCs (AS + CMS). With regard to microbial community, Illumina HiSeq of the 16S rRNA gene was employed in this work and the exoelectrogens (Geobacter and Shewanella) were identified as the dominant members on all anode biofilms in MFCs. For anode microbial diversity, the MFCs (AS + CMS) outperformed MFCs (DS + CMS) and MFCs (RCMS), allowing the occurrence of the fermentative (e.g., Bacteroides) and nitrogen fixation bacteria (e.g., Azoarcus and Sterolibacterium) which enabled the efficient degradation of the slurry. This study provided a feasible strategy to analyze the anode biofilm formation by adding allochthonous inoculum and some implications for quick startup of MFC reactors for CMS treatment.

Effect of volatile fatty acids in anaerobic conditions on viability of helminth ova (Ascaris suum) in sanitization of municipal sludge

The present work aimed at evaluating the effect of four different mixtures of diverse volatile fatty acids (VFAs) on the viability of helminth ova (Ascaris suum), under mesophilic (35°C) anaerobic conditions and at different incubation times, in order to reproduce the process of two-phase anaerobic digestion. The mixtures of VFAs contained acetic, propionic, butyric, valeric, and isovaleric acids, used at concentrations normally found in acidogenic anaerobic digesters. The four treatments all showed a reduction in Ascaris suum ova viability, among which Treatment III (4.2 g-acetic acid L^-1 +  2.2 g-propionic acid L^-1 + 0.6 g-valeric acid L^-1 + 0.6 g-isovaleric acid L^-1) resulted the most efficient. We found that the full effect of VFAs on the viability loss of Ascaris suum ova in mesophilic conditions requires a minimum incubation time of 3 days. The highest efficiency in the loss of viability was observed with Treatment III and 4-day incubation. Interestingly, the proportion of acetic acid was three times as much in this treatment than in the other ones and resulted in an effect in a minimum time of 3 days. The mesophilic condition, however, was not sufficient to induce a complete loss of viability.

Thermodynamically enhancing propionic acid degradation by using sulfate as an external electron acceptor in a thermophilic anaerobic membrane reactor

In this study, sulfate was employed as an external electron acceptor for enhancing the degradation of propionate in a thermophilic anaerobic membrane reactor (AnMBR). The organic loading rate (OLR) was increased gradually from the initial 3.9 kg-COD/m³d to the inhibiting OLR of 14.6 kg-COD/m³d. Feeding was stopped for 98 days but the process did not recover until 500 mg/L of sulfate was added into the AnMBR. After that, the enhanced propionate degradation was achieved up to an OLR of 15 kg-COD/m³d with a reduced sulfate addition of 300 mg/L. However, the thermodynamic calculation indicated that the syntrophic propionic acid degradation, coupled with methanogenesis, was unfavorable with a △G of +3 kJ/mol under the enhanced conditions. Conversely, the utilization of propionic acid by sulfate reduction bacterial (SRB) would be more favourable by having a much lower △G of -180 kJ/mol. The hydrogen conversion was presumed to go through the methanogenesis pathway according to the thermodynamic results. The mechanism of the propionic and hydrogen metabolism was supported as well by comparing the microbial communities with and without sulfate addition. As a result, the role of the sulfate enhancing propionic degradation can be concluded by combining the process performance, thermodynamic, and microbiology results.

The role of methanogens in acetic acid production under different salinity conditions

In this study, a fed-batch acidogenic reactor was operated at a 3 d hydraulic retention time (HRT) and fed with alkaline pre-treated sludge to investigate salinity effects on methanogens' abundance, activities and their consumption of produced acetic acid (HAc) and total volatile fatty acids (VFAs). The salinity concentration was increased step-wise by adding sodium chloride. At 3‰ (parts per thousand) salinity, the average produced volatile fatty acids (VFAs) concentration was 2410.16 ± 637.62 mg COD L(-1) and 2.70 ± 0.36 L methane was produced daily in the acidogenic reactor. Further batch tests indicated methanogens showed a HAc degradation rate of 3.81 mg COD g(-1) VSS h(-1) at initial HAc concentration of 1150 mg COD L(-1), and showed tolerance up to 16‰ salinity (3.76 g Na(+) L(-1)) as indicated by a constant HAc degradation rate. The microbiological study indicated this can be related to the predominance of acetate-utilizing Methanosarcinaceae and Methanomicrobiales in the reactor. However, with salinity increased to 20‰ and 40‰, increases in VFAs and HAc production and decreases in methane production, methanogens population, acidogenic bacteria population and acidification extent were observed. This study demonstrated presence of acetate-utilizing methanogens in an acidogenic reactor and their high tolerance to salinity, as well as their negative impacts on net VFAs production. The results would suggest the presence of methanogens in the acidogenic reactor should not be ignored and the recovery of methane from the acidogenic reactor needs to be considered to avoid carbon loss.

Impact of undissociated volatile fatty acids on acidogenesis in a two-phase anaerobic system

This study investigated the degradation and production of volatile fatty acids (VFAs) in the acidogenic phase reactor of a two-phase anaerobic system. 20 mmol/L bromoethanesulfonic acid (BESA) was used to inhibit acidogenic methanogens (which were present in the acidogenic phase reactor) from degrading VFAs. The impact of undissociated volatile fatty acids (unVFAs) on "net" VFAs production in the acidogenic phase reactor was then evaluated, with the exclusion of concurrent VFAs degradation. "Net" VFAs production from glucose degradation was partially inhibited at high unVFAs concentrations, with 59%, 37% and 60% reduction in production rates at 2190 mg chemical oxygen demand (COD)/L undissociated acetic acid (unHAc), 2130 mg COD/L undissociated propionic acid (unHPr) and 2280 mg COD/L undissociated n-butyric acid (unHBu), respectively. The profile of VFAs produced further indicated that while an unVFA can primarily affect its own formation, there were also unVFAs that affected the formation of other VFAs.

Acetic acid effects on methanogens in the second stage of a two-stage anaerobic system

This study reports on biomass tolerance towards high concentrations of acetic acid (HAc) within the system. Biomass from the second stage of a two-stage anaerobic sludge digestion system was used for this study. Microbial community analysis by 454 pyrosequencing highlighted hydrogenotrophic Methanomicrobiales was the predominant archaeal population in the second stage (>99% of the total archaeal community). Second stage biomass degraded HAc up to 4200 mg HAc L(-1) without observable lag phase. However, at HAc-shock loading of 7400 mg HAc L(-1), it showed a one day lag phase associated with decreased biomass activity. After stepwise HAc-acclimation over 27 d, the biomass degraded HAc of up to 8200 mg HAc L(-1) without observable lag phase. The dominance of Methanomicrobiales had remained unchanged in proportion - while the total archaeal population increased during acclimation. This study showed stepwise acclimation could be an approach to accommodate HAc accumulation and hence higher concentrations resulting from an enhanced first stage.