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With the increasing production of cow manure (CM) and the continuing decrease in the demand for manure compost, CM management has become an urgent and challenging task in Korea. In most cattle farms in Korea, CM mixed with bedding materials is left in pens exposed to the open air for several months before treatment, which makes CM an unsuitable feedstock for anaerobic digestion. This study examined the co-digestion of aged CM with a mixture of food waste and pig manure as the base substrate to assess the possibility of treating and valorizing CM using spare capacity in existing anaerobic digesters dealing with other wastes. The duplicate digesters initially fed with the base substrate were subjected to the addition of increasing amounts of CM (3-10% in the feed, w/v) over nine months. Co-feeding CM up to 5% in the feed (w/v) did not compromise the methanogenic degradation of the substrates, but adding more CM led to a significant performance deterioration likely related to the buildup of inhibitory free ammonia and H

Microbial electrosynthesis (MES) cells use renewable energy to convert carbon dioxide into valuable chemical products such as methane and acetate, but chemical production rates are low and pH changes can adversely impact biocathodes. To overcome these limitations, an MES reactor was designed with a zero-gap electrode configuration with a cation exchange membrane (CEM) to achieve a low internal resistance, and a vapor-fed electrode to minimize pH changes. Liquid catholyte was pumped through a carbon felt cathode inoculated with anaerobic digester sludge, with humidified N

Microbial fuel cells (MFCs) can generate electrical energy from the oxidation of the organic matter, but they must be demonstrated at large scales, treat real wastewaters, and show the required performance needed at a site to provide a path forward for this technology. Previous pilot-scale studies of MFC technology have relied on systems with aerated catholytes, which limited energy recovery due to the energy consumed by pumping air into the catholyte. In the present study, we developed, deployed, and tested an 850 L (1400 L total liquid volume) air-cathode MFC treating domestic-type wastewater at a centralized wastewater treatment facility. The wastewater was processed over a hydraulic retention time (HRT) of 12 h through a sequence of 17 brush anode modules (11 m

The anaerobic co-digestion of spent coffee grounds (SCG) and Ulva biomass, which are problematic wastes and unsuitable for mono-digestion, with food waste (FW) was investigated to widen the scope of feedstocks for biogas production. The effect of the feedstock mixing ratio on the methane yield and synergistic effect of co-digestion was analyzed by response surface analysis. The models for the methane yield and synergistic effect indicated different response patterns and predicted the maximum responses at different mixing ratios. As maximizing the conversion of individual feedstocks to methane is the primary focus in this study, the mixing ratio required for maximizing the synergy index is perceived to be more desirable than that for maximizing methane yield of the mixture. The experimental and modeling results demonstrated that FW, SCG, and Ulva biomass can be effectively co-digested with little antagonistic effect, regardless of their mixing ratio, and a synergistic effect in most cases. It is expected that co-digestion could be flexibly applied when managing the waste feedstocks to enhance their energy recovery potential. The findings of this study can help promote the valorization of underused waste feedstocks through co-digestion and increase the deployment of renewable energy.

The addition of electrically conductive materials may enhance anaerobic digestion (AD) efficiency by promoting direct interspecies electron transfer (DIET) between electroactive microorganisms, but an equivalent enhancement can also be achieved using non-conductive materials. Four high surface area brush materials were added to AD reactors: non-conductive horsehair (HB) and polyester (PB), and conductive carbon fiber (CB) and stainless steel (SB) brushes. Reactors with the polyester material showed lower methane production (68 ± 5 mL/g CODfed) than the other non-conductive material (horsehair) and the conductive (graphite or stainless steel) materials (83 ± 3 mL/g CODfed) (p

Two methods were examined to improve methane production efficiency in anaerobic digestion (AD) based on adding a large amount of surface area using a single electrically conductive carbon brush, or by adding electrodes as done in microbial electrolysis cells (MECs) to form a hybrid AD-MEC. To examine the impact of surface area relative to electrodes, AD reactors were fitted with a single large brush without electrodes (FB), half a large brush with two electrodes with an applied voltage (0.8 V) and operated in closed circuit (HB-CC) or open circuit (HB-OC) mode, or only two electrodes with a closed circuit and no large brush (NB-CC) (equivalent to an MEC). The three configurations with a half or full brush all had improved performance as shown by 57-82% higher methane generation rate parameters in the Gompertz model compared to NB-CC. The retained biomass was much higher in the reactors with large brush, which likely contributed to the rapid consumption of volatile fatty acids (VFAs) and therefore improved AD performance. A different microbial community structure was formed in the large-size brushes compared to the electrodes. Methanothrix was predominant in the biofilm of large-size carbon brush, while Geobacter (anode) and Methanobacterium (cathode) were highly abundant in the electrode biofilms. These results demonstrate that adding a high surface area carbon fiber brush will be a more effective method of improving AD performance than using MEC electrodes with an applied voltage.

The management of cattle manure (CM) has become increasingly challenging because its production continues to rise, while the regulations on manure management have become increasingly stringent. In Korea, most farms produce CM as a dry mixture with lignocellulosic bedding materials (mainly sawdust), making it impractical to treat CM by anaerobic digestion. To address this problem, this study examined whether anaerobic co-digestion with food waste (FW) and pig manure (PM) could be an effective approach for the treatment of CM. The batch anaerobic digestion tests at different CM: FW: PM mixing ratios showed that more methane was produced as the FW fraction increased, and as the CM fraction decreased. The response surface models describing how the substrate mixing ratio affects the methane yield and synergistic effect (methane yield basis) were successfully generated. The models proved that the methane yield and synergistic effect respond differently to changes in the substrate mixing ratio. The maximum 30-day methane yield was predicted at 100% FW, whereas the maximum 30-day synergy index was estimated for the mixture of 47% CM, 6% FW, and 47% PM (total solids basis). The synergy index model showed that CM, FW, and PM could be co-digested without a substantial loss of their methane potential at any mixing ratio (30-day synergy index, 0.89&ndash, 1.22), and that a possible antagonistic effect could be avoided by keeping the FW proportion less than 50%. The results suggest that co-digestion with PM and FW could be flexibly applied for the treatment and valorization of CM in existing anaerobic digestion plants treating FW and PM.

A marine outfall can be a wastewater management system that discharges sewage and stormwater into the sea; hence, it is a source of microbial pollution on recreational beaches, including antibiotic resistant genes (ARGs), which lead to an increase in untreatable diseases. In this regard, a marine outfall must be efficiently located to mitigate these risks. This study aimed to 1) investigate the spatiotemporal variability of Escherichia coli (E. coli) and ARGs on a recreational beach and 2) design marine outfalls to reduce microbial risks. For this purpose, E. coli and ARGs with influential environmental variables were intensively monitored on Gwangalli beach, South Korea in this study. Environmental fluid dynamic code (EFDC) was used and calibrated using the monitoring data, and 12 outfall extension scenarios were explored (6 locations at 2 depths). The results revealed that repositioning the marine outfall can significantly reduce the concentrations of E. coli and ARGs on the beach by 46–99%. Offshore extended outfalls at the bottom of the sea reduced concentrations of E. coli and ARGs on the beach more effectively than onshore outfalls at the sea surface. These findings could be helpful in establishing microbial pollution management plans at recreational beaches in the future.

The integration of microbial electrolysis cells (MECs) and anaerobic digestion (AD) has recently been pursued to improve methane production through a combined system (AD-MEC). Analysis of 24 different experimental results published in 13 different studies showed that the only significant correlation between increased methane generation rate and other parameters was the surface area provided by the electrodes per volume of reactor, or the S/V ratio (R2 = 0.63, p-value

Quantifying the resistances that different components of microbial electrolysis cells (MECs) contribute to the total internal resistance is important for understanding how different reactor configurations affect overall performance. The impact of the reactor architecture was examined here by varying the relative sizes of the electrodes and quantifying the changes in resistances of the electrodes and limiting current densities as a function of the applied potential (Eap). The MECs with equal cathode:anode size ratios (SR=1), showed a steady increase in current up to 1.8 mA for Eap’s ≤ 0.9 V. However, lower limiting currents were obtained for configurations with smaller anodes as shown by a lack of an increase in current for Eap > 0.7 V (limiting current of 0.8 mA, SR=16, and 1.0 mA, SR=4). The largest component of the internal resistance changed with the relative sizes of the electrodes. For example, the cathode resistance was 58% of the internal resistance for the configuration of SR=1 and 51% for SR=2, but the anode was 57% of the internal resistance for SR=16. These results show how differences in reactor architectures can be quantified in terms of individual electrode resistances and limiting currents using polarization data obtained by varying the applied potentials.

Interspecies electron transfer (IET) between microbial populations with different functions is critical to stable anaerobic digestion. This study, in an attempt to facilitate IET, investigated the effect of magnetite supplementation on the biomethanation of dairy effluent in continuous mode. The magnetite-added reactor (RM) was significantly more resistant and resilient to process imbalance than the reactor run without magnetite addition (RC). RC showed unstable performance with repeated process upsets, but its performance improved to be comparable to that of RM after applying magnetite supplementation. Magnetite was particularly effective in stabilizing a build-up of propionic acid and therefore improving the process robustness and reliability. The enhanced biomethanation in terms of productivity and stability was attributed to the facilitated direct IET (DIET) between exoelectrogens and methanogens via magnetite particles. Methanosaeta was the predominant methanogen group in the experimental reactors and likely played a key role in both DIET-mediated carbon dioxide-reducing and aceticlastic methanogenesis.

While most microbial electrolysis cell (MEC) tests and other bioelectrochemical tests use applied voltages (Eap) of 1 V or less, higher voltages are used in some tests that could lead to the generation of free chlorine species (FCS), from chloride ions, and hydroxyl radicals. To examine the impact of high Eap on bioanodes, MECs were acclimated at Eap = 1 V, tested for one cycle at an Eap of 3 or 4 V until the same total coulombs were achieved as Eap = 1 V, and then returned to cycles of Eap = 1 V. All biotic MECs with chloride ions showed severe biofilm damage based on the absence of current production, lack of acetate oxidation, and the absence of hydrogen gas production in subsequent cycles at 1 V. Abiotic tests conducted at Eap = 4 V, with same amount of total coulombs transferred as that which occurred in biotic tests at Eap = 4 V, showed 1.8-fold higher acetate removal than biotic cells at 4 V, suggesting 43% of generated coulombs could have contributed to microbial inactivation. FCS generation, rather than hydroxyl radical production, was concluded to be the major contribution to oxidation of organics due to small changes in acetate oxidation in the presence of a hydroxyl radical scavenger, and the measurement of FCS. These results demonstrated that high applied voltages should be avoided if bioanodes are needed in bioelectrochemical systems when chloride species are present in the solution.

The effects of mild-temperature thermochemical pretreatments with HCl or NaOH on the solubilization and biomethanation of Ulva biomass were assessed. Within the explored region (0-0.2M HCl/NaOH, 60-90°C), both methods were effective for solubilization (about 2-fold increase in the proportion of soluble organics), particularly under high-temperature and high-chemical-dose conditions. However, increased solubilization was not translated into enhanced biogas production for both methods. Response surface analysis statistically revealed that HCl or NaOH addition enhances the solubilization degree while adversely affects the methanation. The thermal-only treatment at the upper-limit temperature (90°C) was estimated to maximize the biogas production for both methods, suggesting limited potential of HCl/NaOH treatment for enhanced Ulva biomethanation. Compared to HCl, NaOH had much stronger positive and negative effects on the solubilization and methanation, respectively. Methanosaeta was likely the dominant methanogen group in all trials. Bacterial community structure varied among the trials according primarily to HCl/NaOH addition.

Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid-oxidizing bacteria and electrotrophic methanogens has recently been discovered, and studies have suggested that promoting DIET by adding electrically conductive material can effectively enhance the methanogenic performance and stability of anaerobic digestion (AD). This study investigated the effect of conductive magnetite (Fe3O4) addition on the AD of a sulfur-rich organic waste mixture, with an emphasis on the fate of sulfur and on H2S production. In contrast to previous findings, methanogenic performance under magnetite-added conditions was not significantly enhanced within the tested dose range of up to 20 mM Fe. When magnetite was added, H2S production decreased remarkably along with the extracellular accumulation of S0. Moreover, the H2S content in biogas was below 100 ppmv at magnetite doses of 8 mM Fe or higher (>6000 ppmv under control conditions, i.e., without magnetite). The reduced H2S production appears to be due to the anaerobic oxidation of sulfide to S0 because sulfate reduction remained active, whereas FeS was not produced under magnetite-added conditions. Based on microbial community analysis results and thermodynamic calculations, the electric syntrophy via DIET between exoelectrogenic anaerobic sulfide-oxidizing bacteria and electrotrophic methanogens is suggested to have been promoted by magnetite. To the best of our knowledge, this study is the first to propose an electro-syntrophic association that couples the oxidation of sulfide to S0 with the reduction of CO2 to CH4 in methanogenic environments. The present findings open a new possibility for in situ H2S control and sulfur recovery in AD processes for sulfur-rich waste treatment.

Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid oxidizers and electrotrophic methanogens plays an important role in keeping the overall anaerobic digestion (AD) process well-balanced. This study examined the individual and combined effects of two different DIET-promoting strategies, i.e., magnetite addition (20 mM Fe) and external voltage application (0.6 V), in continuous digesters treating dairy wastewater. Although the strategies were both effective in enhancing the process performance and stability, adding magnetite had a much greater stimulatory effect. External voltage contributed little to the methane yield, and the digester with magnetite addition alone achieved stable performance, comparable to that of the digester where both strategies were combined, at short hydraulic retention times (down to 7.5 days). Diverse (putative) electroactive microorganisms were significantly enriched under DIET-promoting conditions, particularly with magnetite addition. The overall results suggest that magnetite addition could effectively enhance AD performance and stability by promoting DIET-based electro-syntrophic microbial interactions.

The effect of biostimulation with ferric oxides, semiconductive ferric oxyhydroxide, and conductive magnetite on the anaerobic digestion of dairy wastewater was examined in a batch mode. The reactors supplemented with ferric oxyhydroxide (R2) and magnetite (R3) showed significantly enhanced biomethanation performance compared with the control (R1). The removal of chemical oxygen demand (COD) after 30 days was 31.9, 59.3, and 82.5% in R1, R2, and R3, respectively. The consumed COD was almost fully recovered as biogas in R2 and R3, while only 79% was recovered in R1. The total energy production as biogas was accordingly 32.2, 71.0, and 97.7 kJ in R1, R2, and R3, respectively. The reactors also differed in the acid formation profile with more propionate and butyrate found in R1 and more acetate found in R3. The enhanced biomethanation seems to be associated with variations in the bacterial community structure supposedly induced by the ferric oxides added. In contrast, no evident variation was observed in the archaeal community structure among the reactors. The potential electric syntrophy formed between Methanosaeta concilii-like methanogens and electroactive iron-reducing bacteria, particularly Trichococcus, was likely responsible for the enhanced performance. The stimulated growth of fermentative iron reducers may also have contributed by altering the metabolic characteristics of the bacterial communities to produce more favorable acidogenic products for methanogenesis. The overall results suggest the potential of biostimulation with (semi)conductive ferric oxides to enhance the rate and efficiency of the biomethanation of organic wastes. This seems to be potentially attractive, as increasing attention is being paid to the energy self-sufficiency of waste/wastewater treatment processes today.

Functioning biocathodes are essential for electromethanogenesis. This study investigated the development of a biocathode from non-acclimated anaerobic sludge in an electromethanogenesis cell at a cathode potential of -0.7V (vs. standard hydrogen electrode) over four cycles of repeated batch operations. The CO2-to-CH4 conversion rate increased (to 97.7%) while the length of the lag phase decreased as the number of cycles increased, suggesting that a functioning biocathode developed during the repeated subculturing cycles. CO2-resupply test results suggested that the biocathode catalyzed the formation of CH4 via both direct and indirect (H2-mediated) electron transfer mechanisms. The biocathode archaeal community was dominated by the genus Methanobacterium, and most archaeal sequences (>89%) were affiliated with Methanobacterium palustre. The bacterial community was dominated by putative electroactive bacteria, with Arcobacter, which is rarely observed in biocathodes, forming the largest population. These electroactive bacteria were likely involved in electron transfer between the cathode and the methanogens.

Promotion of direct interspecies electron transfer (DIET) between exoelectrogenic bacteria and electron-utilizing methanogens has recently been discussed as a new method for enhanced biomethanation. This study evaluated the effect of magnetite-promoted DIET in continuous anaerobic digestion of dairy effluent and tested the magnetic separation and recycling of magnetite to avoid continuous magnetite addition. The applied magnetite recycling method effectively supported enhanced DIET activity and biomethanation performance over a long period (>250days) without adding extra magnetite. DIET via magnetite particles as electrical conduits was likely the main mechanism for the enhanced biomethanation. Magnetite formed complex aggregate structures with microbes, and magnetite recycling also helped retain more biomass in the process. Methanosaeta was likely the major methanogen group responsible for DIET-based methanogenesis, in association with Proteobacteria and Chloroflexi populations as syntrophic partners. The recycling approach proved robust and effective, highlighting the potential of magnetite recycling for high-rate biomethanation.

Iron compounds are abundantly present in both natural and engineered anaerobic environments where various biological processes including methanogenesis take place. Iron is an essential trace element for methanogens, and also other microorganisms involved in anaerobic digestion (AD). Dissolved iron can be readily taken up and used as a micronutrient by microorganisms; however, different iron species and compounds can influence methanogenesis in significantly different ways, both positive and negative, according to their physicochemical properties. This suggests that controlling methanogenic activity (i.e., stimulation or inhibition) in an AD system may be possible by adding a suitable type and amount of iron compound. The different effects of iron compounds on methanogenesis in anaerobic environments have not been systematically reviewed, and more comprehensive information is needed to look into the possible applications of iron compounds in biogas production. This review summarizes recent findings on the effects of different iron compounds on methanogenesis and discusses the underlying mechanisms and implications. Previous studies on the effects of iron addition on AD have reported contradictory observations for different iron sources, and the solubility, crystallinity, conductivity, and redox activity of iron compounds are the key factors that determine the direction and extent of effect on methanogenesis. These physicochemical properties are directly related to the bioavailability and/or electron-mediating capability of an iron compound, which affect the energy metabolism of methanogens and their syntrophic partners. This review will help deepen our understanding of the role and function of iron in AD and provide a reference for the control or promotion of methanogenesis, which is of particular interest from the perspective of energy production.

Proper management of spent coffee grounds has become a challenging problem as the production of this waste residue has increased rapidly worldwide. This study investigated the feasibility of the anaerobic co-digestion of spent coffee ground with various organic wastes, i.e., food waste, Ulva, waste activated sludge, and whey, for biomethanation. The effect of co-digestion was evaluated for each tested co-substrate in batch biochemical methane potential tests by varying the substrate mixing ratio. Co-digestion with waste activated sludge had an apparent negative effect on both the yield and production rate of methane. Meanwhile, the other co-substrates enhanced the reaction rate while maintaining methane production at a comparable or higher level to that of the mono-digestion of spent coffee ground. The reaction rate increased with the proportion of co-substrates without a significant loss in methanation potential. These results suggest the potential to reduce the reaction time and thus the reactor capacity without compromising methane production.

Although anaerobic digestion (AD) is a widely used option to manage waste activated sludge (WAS), there are some drawbacks related to its slow reaction rate and low energy productivity. This study examined an anaerobic WAS digester, augmented with an iron-reducing microbial consortium, relative to changes in microbial community structure and process performance at decreasing hydraulic retention times (HRTs) of 20 to 10 days. The enhanced methanation performance (approximately 40 % increase in methane yield) by the bioaugmentation was sustained until the HRT was decreased to 12.5 days, under Fe(3+)-rich conditions (ferric oxyhydroxide, 20 mM Fe). Enhanced iron-reducing activity was evidenced by the increased Fe(2+) to total Fe ratio maintained above 50 % during the stable operational phases. A further decrease in HRT to 10 days resulted in a significant performance deterioration, along with a drop in the Fe(2+) to total Fe ratio to35 %, after four turnovers of operation. Prevailing existence of putative iron-reducing bacteria (IRBs) was identified by denaturing gradient gel electrophoresis (DGGE), with Spirochaetaceae- and Thauera-related organisms being dominant members, and clear dominance shifts among them with respect to decrease in HRT were observed. Lowering HRT led to evident shifts in bacterial community structure likely associated with washout of IRBs, leading to decreases in iron respiration activity and AD performance at a lower HRT. The bacterial community structure shifted dynamically over phases, and the community transitions correlated well with the changes in process performance. Overall, the combined biostimulation and bioaugmentation investigated in this study proved effective for enhanced methane recovery from anaerobic WAS digestion, which suggests an interesting potential for high-rate AD.

This study investigated the potential of enhancing the activity of iron-reducing bacteria (IRBs) to increase the biomethanation rate of waste activate sludge (WAS). The effects of biostimulation by ferric oxyhydroxide (Phase 2) and bioaugmentation with an enriched IRB consortium (Phase 3) were examined in a continuous anaerobic reactor treating WAS. Compared to the control operation (Phase 1), significant rises in methane yield (10.8–59.4%) and production rate (24.5–52.9%) were demonstrated by the biostimulation and bioaugmentation treatments. Visible structural changes were observed in bacterial community with the phases while not in archaeal community. Acinetobacter- and Spirochaetales-related populations were likely the major players driving anaerobic iron respiration and thus leading to enhanced biomethanation performance, in Phases 2 and 3, respectively. Our results suggest an interesting new potential for enhancing biomethanation of WAS.