Multi-electrode continuous flow microbial electrolysis cell for biogas production from acetate

Most microbial electrolysis cells (MECs) contain only a single set of electrodes. In order to examine the scalability of a multiple-electrode design, we constructed a 2.5 L MEC containing 8 separate electrode pairs made of graphite fiber brush anodes pre-acclimated for current generation using acetate, and 304 stainless steel mesh cathodes (64 m²/m³). Under continuous flow conditions and a one day hydraulic retention time, the maximum current was 181 mA (1.18 A/m², cathode surface area; 74 A/m³) within three days of operation. The maximum hydrogen production (day 3) was 0.53 L/L-d, reaching an energy efficiency relative to electrical energy input of η_E = 144%. Current production remained relatively steady (days 3–18), but the gas composition dramatically shifted over time. By day 16, there was little H₂ gas recovered and methane production increased from 0.049 L/L-d (day 3) to 0.118 L/L-d. When considering the energy value of both hydrogen and methane, efficiency relative to electrical input remained above 100% until near the end of the experiment (day 17) when only methane gas was being produced. Our results show that MECs can be scaled up primarily based on cathode surface area, but that hydrogen can be completely consumed in a continuous flow system unless methanogens can be completely eliminated from the system.     

Impact of nano zero valent iron (NZVI) on methanogenic activity and population dynamics in anaerobic digestion

Nano zero valent iron (NZVI), although being increasingly used for environmental remediation, has potential negative impact on methanogenesis in anaerobic digestion. In this study, NZVI (average size = 55 ± 11 nm) showed inhibition of methanogenesis due to its disruption of cell integrity. The inhibition was coincident with the fast hydrogen production and accumulation due to NZVI dissolution under anaerobic conditions. At the concentrations of 1 mM and above, NZVI reduced methane production by more than 20%. At the concentration of 30 mM, NZVI led to a significant increase in soluble COD (an indication of cell disruption) and volatile fatty acids in the mixed liquor along with an accumulation of H₂, resulting in a reduction of methane production by 69% (±4% [standard deviation]). By adding a specific methanogenesis inhibitor-sodium 2-bromoethanesulfonate (BES) to the anaerobic sludge containing 30 mM NZVI, the amount of H₂ produced was only 79% (±1%) of that with heat-killed sludge, indicating the occurrence of bacterially controlled hydrogen utilization processes. Quantitative PCR data was in accordance with the result of methanogenesis inhibition, as the level of methanogenic population (dominated by Methanosaeta) in the presence of 30 mM NZVI decreased significantly compared to that of the control. On the contrary, ZVI powder (average size <212 μm) at the same concentration (30 mM) increased methane production presumably due to hydrogenotrophic methanogenesis of hydrogen gas that was slowly released from the NZVI powder. While it is a known fact that NZVI disrupts cell membranes, which inhibited methanogenesis described herein, the results suggest that the rapid hydrogen production due to NZVI dissolution also contribute to methanogenesis inhibition and lead to bacterially controlled hydrogenotrophic processes.     

Fate and effect of the antioxidant ethoxyquin on a mixed methanogenic culture

The potential inhibitory effect of ethoxyquin, an antioxidant commonly used as a preservative in the food processing industry (e.g., for stabilizing dissolved air flotation residuals), was evaluated at concentrations up to 300 mg/L using a mixed, mesophilic (35 degrees C) methanogenic culture and dextrin, peptone and methanol as the carbon source. A batch assay conducted with a range of ethoxyquin concentrations did not result in any inhibition up to an ethoxyquin concentration of 75 mg/L, but severe inhibition of methanogenesis was observed at concentrations higher than 150 mg/L. Ethoxyquin addition to a batch reactor with the same mixed, methanogenic culture, at ethoxyquin concentrations gradually increasing over 100 days, resulted in a transient and a complete inhibition of methanogenesis at ethoxyquin concentrations of 150 and 300 mg/L, respectively. Acidogens were not significantly impacted, whereas aceticlastic and methanol degrading methylotrophic methanogens were impacted the most. Acclimation of the methanogenic culture to ethoxyquin was not observed over an incubation period of more than 100 days. Long-term (>100 days) incubation at sub-inhibitory ethoxyquin concentrations did not result in ethoxyquin biotransformation. Similarly, ethoxyquin biotransformation was not evident over an 8-day aeration period in a laboratory-scale activated sludge reactor operated under fully aerobic conditions. Ethoxyquin phase distribution tests conducted with the mixed, methanogenic culture at 1.61 g/L volatile solids concentration and nominal ethoxyquin concentrations equal to or higher than 300 mg/L resulted in solid phase/liquid phase ethoxyquin ratios equal to or higher than 1.0. The combined effect of ethoxyquin recalcitrance under anaerobic conditions along with its phase distribution, which favors biosolids, will result in ethoxyquin accumulation in anaerobic treatment systems used by the food processing industry. Such accumulation may pose concerns relative to inhibitory effects in these treatment systems and the disposal of ethoxyquin-bearing biosolids.     

Fate and effect of quaternary ammonium compounds on a mixed methanogenic culture

The potential inhibitory effect of four quaternary ammonium compounds (QACs) and Vigilquat, a commercial sanitizer which is a mixture of the four QACs, was investigated at concentrations up to 100 mg/L using a mixed, mesophilic (35 degrees C) methanogenic culture. Dextrin and peptone were used as the carbon and energy sources. A batch assay conducted at a range of QAC concentrations showed that QACs were inhibitory to methanogens at and above 25 mg/L. Methanogenesis was more susceptible to QAC inhibition than acidogenesis. Adsorption of QACs on biomass was successfully simulated with the Freundlich isotherm equation. The inhibitory effect of Vigilquat on the mixed methanogenic culture was also investigated in a batch reactor fed with dextrin and peptone. Methanogens were inhibited when the total QAC concentration reached 30 mg/L and volatile fatty acids (VFAs) accumulated. However, methane production recovered in 57 days of incubation, and all VFAs were consumed, suggesting that a prolonged incubation period is necessary for the methanogens to overcome the transient inhibition at a relatively low QAC concentration. None of the QACs tested in this study was biodegraded under methanogenic conditions.     

Biogas Production: New Trends for Alternative Energy Sources in Rural and Urban Zones

Biogas is a biofuel with a high energy value and basically consisting of methane, which can be used as a renewable energy source as a substitute for natural gas or liquefied petroleum gas. It can be produced by anaerobic digestion of agricultural organic waste or manure in rural areas, where it can be used to generate electric, thermal or mechanical energy. It can also be generated in landfills from the organic fraction of municipal solid wastes and used as an alternative energy source in urban areas. Industrialized and urbanized areas are afflicted by serious environmental problems associated with the generation of organic residues. Anaerobic microorganisms can degrade pollutants resulting in two kinds of products, i.e., digested sludge and biogas, which can be exploited as a soil fertilizer and a renewable energy source, respectively. The correct management of residual waste involves high costs, and inadequate treatment and storage can compromise its quality. Environmental agencies have been encouraging the dissemination of anaerobic digesters to produce biogas from organic residues and the use of the resulting sludge as fertilizer since it is able to destroy pathogenic agents and reduce the humidity level. This review aims to evaluate the production capability of biogas and its application as an alternative energy source in rural and urban areas.     

Effect of ammonia concentration on hythane (H2 and CH4) production in two-phase anaerobic digestion

Co-production of hydrogen and methane by two-phase anaerobic digestion (AD) may offer a sustainable solution for the centralized treatment of food waste (FW), while ammonia accumulation is potentially encountered. A mesophilic two-phase AD was investigated for hydrogen and methane production from FW at varying ammonia concentrations. The process achieved a hydrogen yield of 47.7 mL/g VS and a methane yield of 335 mL/g VS by optimizing the organic loading rate (OLR) and recirculation ratio. Total ammonia nitrogen (TAN) concentration of 4044 mg/L corresponded to a threshold in the hydrogen reactor, above which ammonia would initiate inhibition of hydrogenogenesis and acidogenesis. Methane yield was recovered in the methane reactor after acute inhibiting effects of TAN below 5800 mg/L, while TAN above 6200 mg/L caused chronic inhibition of methanogens. Adjusting hydraulic retention time (HRT) and recirculation ratio in hydrogen and methane reactors reduced TAN to 960 and 2105 mg/L respectively, resulting in successful recovery was achieved in the hydrogen reactor but not in the methane reactor. The two-phase AD for methane and hydrogen production can be a promising solution for ammonia accumulation in AD from FW.     

Bioelectrochemical regulation accelerates facultatively syntrophic proteolysis

Bioelectrochemical systems can affect microbial metabolism by controlling the redox potential. We constructed bioelectrochemical cultures of the proteolytic bacterium, Coprothermobacter proteolyticus strain CT-1, both as a single-culture and as a co-culture with the hydrogenotrophic methanogen, Methanothermobacter thermautotrophicus strain ?H, to investigate the influences of bioelectrochemical regulation on facultatively syntrophic proteolysis. The co-culture and single-culture were cultivated at 55°C with an anaerobic medium containing casein as the carbon source. The working electrode potential of the bioelectrochemical system was controlled at -0.8V (vs. Ag/AgCl) for bioelectrochemical cultures and was not controlled for non-bioelectrochemical cultures. The cell densities of hydrogenotrophic methanogen and methane production in the bioelectrochemical co-culture were 3.6 and 1.5 times higher than those in the non-bioelectrochemical co-culture after 7 days of cultivation, respectively. Contrastingly, the cell density of Coprothermobacter sp. in the bioelectrochemical co-culture was only 1.3 times higher than that in the non-bioelectrochemical co-culture. The protein decomposition rates were nearly proportional to the cell density of Coprothermobacter sp. in the all types of cultures. These results indicate that bioelectrochemical regulation, particularly, affected the carbon fixation of the hydrogenotrophic methanogen and that facultatively syntrophic proteolysis was accelerated as a result of hydrogen consumption by the methanogens growing well in bioelectrochemical co-cultures.     

Statistical optimization of conditions for minimum H2 consumption in mixed anaerobic cultures: Effect on homoacetogenesis and methanogenesis

Hydrogen (H₂) production using mixed anaerobic cultures often suffers severe yield reduction due to the syntrophic association between H₂ consumers (methanogens and homoacetogens) and H₂ producers (acidogens). The objective of this study was to uncouple the syntrophic association between H₂ producers and consumers by optimizing conditions for minimum H₂ consumption using a Box–Behnken design approach. The factors investigated in this study include temperature, pH and linoleic acid (LA) concentration. A quadratic response surface model was developed to predict the H₂ consumed by mixed anaerobic cultures and the optimum conditions for minimum H₂ consumption were 38 °C, pH 5.5 and 2 g L⁻¹ LA. Methanogenesis was inhibited in cultures fed 2 g L⁻¹ LA and maintained at pH 6.0 and 53 °C. In comparison, both methanogenesis and homoacetogenesis were inhibited in cultures fed 1–2 g L⁻¹ LA and maintained at a pH of 4.5 (Fig. 2B and 2E and Table 2 Expt. # 1, 2 and 11). Microbial diversity analysis revealed that LA fed cultures was dominated by spore forming Clostridium sp. in addition to Syntrophus aciditrophus. In comparison, control cultures were dominated by Eubacterium sp., Methanocalculus halotolerans and Methanococcoides alaskense. This study described an approach for regulating H₂ consumption in mixed cultures by optimizing process and environmental factors. Understanding the effects of these individual factors and their interaction is important in the full-scale operation of H₂ production facilities.     

Occurrence of methanogenesis during start-up of a full-scale synthesis gas-fed reactor treating sulfate and metal-rich wastewater

The start-up of a full-scale synthesis gas-fed gas-lift reactor treating metal and sulfate-rich wastewater was investigated. Sludge from a pilot-scale reactor was used to seed the full-scale reactor. The main difference in design between the pilot- and full-scale reactor was that metal precipitation and sulfate reduction occurred in the same reactor. After 7 weeks the full-scale reactor achieved the sulfate conversion design rate of 15 kg/m3day. Zinc sulfide precipitation inside the reactor did not interfere with obtaining a high rate of sulfate reduction. 16S rRNA gene analysis demonstrated that the bacterial communities in both reactors were dominated by the sulfate-reducing genus Desulfomicrobium. Archaeal communities of both reactors were dominated by the methanogenic genus Methanobacterium. Most Probable Number (MPN) counts confirmed that heterotrophic Sulfate-Reducing Bacteria (SRB) were dominant (10(11) -10(12) cells/g VSS) compared to homoacetogens (10(5) -10(6) cells/g VSS) and methanogens (10(8) -10(9) cells/g VSS). Methanogenesis was not suppressed during start-up of the full scale-reactor, despite the predominance of SRB, which have a lower hydrogen threshold. Due to the short sludge retention time (4-7 days) competition for hydrogen is determined by Monod kinetics, not hydrogen thresholds. As the kinetic parameters for SRB and methanogens are similar, methanogenesis may persist which results in a loss of hydrogen.     

Continuous biotransformation and removal of nitrophenols under denitrifying conditions

The effect of COD/NO(3)(-)-N ratio on the biotransformation and removal of 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), and 2,4-dinitrophenol (2,4-DNP) was studied in bench scale upflow anaerobic sludge blanket (UASB) reactors. Sodium acetate and sodium nitrate were used as electron donor (substrate) and electron acceptor, respectively. Nitrate nitrogen loading was increased from 0.098 to 0.6 kg/m(3)d in order to keep COD/NO(3)(-)-N ratio as 20.8, 14.3, 9.8, 5.0, 4.0 and 3.33. Throughout the study, input nitrophenolic concentration and hydraulic retention time (HRT) were kept constant as 30 mg/l and 24h, respectively. 2-Aminophenol (2-AP), 4-aminophenol (4-AP) and 2-amino,4-nitrophenol (2-A,4-NP) were found as the major intermediate metabolite of 2-NP, 4-NP and 2,4-DNP, respectively. Removal of all the three nitrophenols increased with lowering of COD/NO(3)(-)-N ratio. However, nitrophenols removal got adversely affected when COD/NO(3)(-)-N ratio was reduced below 5. Maximum removal achieved were 91.63%, 90.17% and 86.10% for 2-NP, 4-NP and 2,4-DNP, respectively at a COD/NO(3)(-)-N ratio of 5. Simultaneous denitrification and methanogenesis was observed in all the reactors throughout the study.