Towards long-term operation of flow-electrode capacitive deionization (FCDI): Optimization of operating parameters and regeneration of flow-electrode

This study systematically optimized the key operating parameters and interpreted their effecting mechanisms in a flow-electrode capacitive deionization (FCDI) system. The optimal voltage, activated carbon electrode content, electrolyte concentration, feedwater flowrate, and electrode flowrate for desalinating low salinity feedwater (1.0 g L-1 NaCl) were determined to be 1.8 V, 2.0 wt%, 10.0 g L-1, 80 mL min-1, and 60 mL min-1, respectively. The variations of the above parameters can affect the system conductivity, the thickness and stability of the electric double layers, and/or the degree of concentration polarization, thereby influencing the desalination performance. Moreover, a sensitivity analysis identified the operating voltage as the dominant parameter with the most significant influence on the FCDI system. Subsequently, a long-term operation was carried out under single-pass mode. The results showed that the lab-scale FCDI system was able to constantly maintain the desalination efficiency of 1.0 g L-1 feedwater (NaCl) at 40-60 % for multiple operating cycles. Over 99.8 % of electrode material regeneration and desalination efficiency recovery was able to be obtained during a 60-h operation, demonstrating that the FCDI system showed strong stability and long-term operation potential.

Determination of effects of thermophilic and hyperthermophilic temperatures on anaerobic hydrolysis and acidogenesis of pig manure through a one-year study

Improving hydrolysis and acidogenesis through thermophilic and hyperthermophilic temperatures is critical for enhancing the anaerobic decomposition of organic waste like pig manure. However, whether higher temperatures can provide more enhanced performance has not been elucidated experimentally. This study, therefore, conducted a 375-day continuous operation experiment at 55 and 70 °C with a 5-day hydraulic retention time. The two temperature reactors entered a stable state after about 200 days and long-term microbial acclimation markedly changed their performances. In the thermophilic and hyperthermophilic reactor, the hydrolysis efficiencies were obtained at 29.7 % and 27.3 % respectively, whereas the acidogenesis efficiency was relatively low at 1.0 % and 3.1 %. Due to the occurrence of methanogenesis, the volatile fatty acid concentration in the thermophilic reactor was only 45 % of that in the hyperthermophilic reactor. The thermophilic reactor exhibited higher bacterial diversity; however, this difference between the two reactors apparently did not correlate with hydrolysis and acidogenesis performance.

Boosting the performance and durability of heterogeneous electrodes for solid oxide electrochemical cells utilizing a data-driven powder-to-power framework

Solid oxide electrochemical cells (SOCs) hold potential as a critical component in the future landscape of renewable energy storage and conversion systems. However, the commercialization of SOCs still requires further breakthroughs in new material development and engineering designs to achieve high performance and durability. In this study, a data-driven powder-to-power framework has been presented, fully digitizing the morphology evolution of heterogeneous electrodes from fabrication to long-term operation. This framework enables accurate performance prediction over the full life cycle. The intrinsic correlation between microstructural parameters and electrode durability is elucidated through parameter analysis. Rational control of the ion-conducting phase volume fraction can effectively suppress Ni coarsening and mitigate the excessive ohmic loss caused by Ni migration. The initial and degraded electrode performances are attributed to the interplay of multiple parameters. A practical optimization strategy to enhance the initial performance and durability of the electrode is proposed through the construction of the surrogate model and the application of the optimization algorithm. The optimal electrode parameters are determined to accommodate various maximum operation time requirements. By implementing the data-driven powder-to-power framework, it is possible to reduce the degradation rate of Ni-based electrodes from 2.132% to 0.703% kh^-1 with a required maximum operation time of over 50,000 h.

Dynamic evolution of electrochemical and biological features in microbial fuel cells upon chronic exposure to increasing oxytetracycline dosage

Dynamic changes in power generation and electrochemical properties were compared between the control microbial fuel cells (C-MFC) and an oxytetracycline (OTC)-treated MFC (O-MFC) on days 84, 139, 174, 224, 295, 307 and 353. The results showed that a high concentration of OTC (>5 mg·L^-1) could inhibit microbial activity and result in a decline of voltage output and power density compared with the same C-MFC. However, with the prolongation of incubation time, the inhibitory effect was gradually weakened. Electrochemical analyses demonstrated that long-term OTC acclimation reduced the ohmic and polarisation resistance of the anode, which was conducive to the recovery of electrochemical performance. More than 99% of 10 mg·L^-1 OTC could be removed within 48 h, and the antibacterial activity of the MFC effluent on Escherichia coli DH5α was conclusively eliminated. High-throughput sequencing analysis revealed that the diversity and richness of the microbial community decreased significantly after long-term OTC enrichment. Acinetobacter, Petrimonas, Spirochaetaceae and Delftia were enriched and played a dominant role in C-MFC stability and power generation. The promotion by Cupriavidus, Geobacter and Stenotrophomonas in simultaneous OTC degradation and bioelectricity generation in the O-MFC was demonstrated.

Longer persistence of quorum quenching bacteria over quorum sensing bacteria in aerobic granules

Involvements of quorum sensing (QS) in the formation of aerobic granules for wastewater treatment have been well recognized. In previous studies the evolution of the QS-related activities and communities during bioreactor start-up period has been extensively studied, while the variation of QS in long-term reactor operation remains unrevealed. Furthermore, information about the roles of quorum quenching (QQ) in bioreactors is very limited. In this work, both QS and QQ during the start-up and successive long-term operation period of an aerobic granule bioreactor were explored. The QS activity and communities increased in the start-up but gradually decreased in the long-term operation, while the QQ activity and communities remained stable. These results indicate the longer persistence of QQ than QS in the granules and the minor contribution of QS in the long-term operation. This work provides a new insight into the roles of QQ and QS in wastewater treatment bioreactors.

Long-term bio-power of ceramic microbial fuel cells in individual and stacked configurations

In order to improve the potential of Microbial Fuel Cells (MFCs) as an applicable technology, the main challenge is to engineer practical systems for bioenergy production at larger scales and to test how the prototypes withstand the challenges occurring during the prolonged operation under constant feeding regime with real waste stream. This work presents the performance assessment of low-cost ceramic MFCs in the individual, stacked (modular) and modular cascade (3 modules) configurations during long term operation up to 19 months, utilising neat human urine as feedstock. During 1 year, the performance of the individual MFC units reached up to 1.56 mW (22.3 W/m3), exhibiting only 20% power loss on day 350 which was significantly smaller in comparison to conventional proton or cation exchange membranes. The stack module comprising 22 MFCs reached up to 21.4 mW (11.9 W/m3) showing power recovery to the initial output levels after 580 days, whereas the 3-module cascade reached up to 75 mW (13.9 W/m3) of power, showing 20% power loss on day 446. In terms of chemical oxygen demand (COD) removal, the 3-module cascade configuration achieved a cumulative reduction of >92%, which is higher than that observed in the single module (56%).

Microbial Fuel Cell stack performance enhancement through carbon veil anode modification with activated carbon powder

The chemical energy contained in urine can be efficiently extracted into direct electricity by Microbial Fuel Cell stacks to reach usable power levels for practical implementation and a decentralised power source in remote locations. Herein, a novel type of the anode electrode was developed using powdered activated carbon (PAC) applied onto the carbon fibre scaffold in the ceramic MFC stack to achieve superior electrochemical performance during 500 days of operation. The stack equipped with modified anodes (MF-CV) produced up to 37.9 mW (21.1 W m-3) in comparison to the control (CV) that reached 21.4 mW (11.9 W m-3) showing 77% increase in power production. The novel combination of highly porous activated carbon particles applied onto the conductive network of carbon fibres promoted simultaneously electrocatalytic activity and increased surface area, resulting in excellent power output from the MFC stack as well as higher treatment rate. Considering the low cost and simplicity of the material preparation, as well as the outstanding electrochemical activity during long term operation, the resulting modification provides a promising anode electrocatalyst for high-performance MFC stacks to enhance urine and waste treatment for the purpose of future scale-up and technology implementation as an applied off-grid energy source.

Enhanced co-production of biohydrogen and algal lipids from agricultural biomass residues in long-term operation

This study evaluates a two-stage process by dark fermentation and microalgal cultivation for hydrogen and lipid production from different agricultural biomass residues, such as corn cob, corn stalk, rice straw and wheat straw. Results indicate that corn stalk was the best fermentation feedstock for hydrogen production and the highest accumulative hydrogen volume of 762.3 mL L^-1 was obtained in batch mode. Dark fermentative effluent from corn stalk was rich in acetate and butyrate, and was favorable to lipid production. The maximum algal biomass and total lipid content reached 1461.1 mg L^-1 and 35.2%, respectively. Meanwhile, the energy conversion efficiency in two-stage cultivation significantly increased from 5.78% to 16.96%. The system was stable and effective in long-term operation, and the average hydrogen production rate of 811.1 mL L^-1 d^-1 and lipid concentration of 588.5 mg L^-1 were achieved. This study provides a promising process for enhancing energy production from agricultural wastes.

Characterization of electricity generation and microbial community structure over long-term operation of a microbial fuel cell

In this study, a continuous-flow microbial fuel cell (MFC) system was constructed to treat azo dye wastewater for 400 days. The electrical properties of the MFC after 400-day operation and the removal efficiencies of the MFC after long- and short-term operation were analyzed with respect to co-substrate concentrations. The power output of the MFC system decreased from 586 to 330 mV with increasing operating time, and the removal efficiencies of the MFC remained stable after long-term operation in comparison to those after short-term operation, even when the co-substrate concentration was reduced. Analysis of the degradation products showed that products generated from long-term operation of the MFC were present at low concentrations. The microbial community analysis revealed that the relative abundance of microorganisms related to the degradation of organics in the MFC increased after long-term operation, and microorganisms related to electricity generation decreased.

Effects of carbon source on the formation, stability, bioactivity and biodiversity of the aerobic granule sludge

Three aerobic granular sludge systems were operated as sequencing batch reactors (SBR) with acetate, ethanol and glucose as carbon source. The SBR cycle was 6 h, with an anaerobic phase followed by an aerobic phase. The acetate granules (>1.5 mm) had the greatest microbial diversity and better results in terms of removal efficiency for carbon and nutrients (TN ≈ 72% and TP ≈ 42%) and also in the resistance tests. However, partial disintegration was observed. On the other hand, when ethanol was the substrate, the granules were stable, good nitrogen removal was achieved (TN ≈ 53%), but phosphorus removal was not favored (TP ≈ 31%). Glucose presented the lowest efficiency values for nitrogen (TN ≈ 44%) and phosphorous removal (TP ≈ 21%), and the granules formed (<1 mm) had the lowest microbial diversity. Therefore, the carbon source had a high impact on the characteristics of the granules.

Performance evaluation of mesophilic semi-continuous anaerobic digestion of high-temperature thermally pre-treated olive mill solid waste

The aim of the present work was to evaluate the effects of a thermal pre-treatment of olive mill solid waste (OMSW) and phenol extraction process on the semi-continuous anaerobic digestion of this pre-treated waste during a prolonged operational period (275 days) in order to assess the organic loading rates (OLR) of 1 ad 2 g Volatile Solids (VS)/(L·d). The anaerobic digestion of thermally pre-treated and de-phenolized OMSW was stable at an OLR of 1 g VS/(L·d), which permitted a specific production rate of 172 ± 60 mL CH₄/(g VS·d). However, the system was not able to operate at an OLR of 2 g VS/(L·d), which resulted in the total failure of the process. Regardless of the applied OLR, the phenolic compounds were effectively degraded and the inhibition thresholds were not reached. The inhibition of the anaerobic digestion process at an OLR of 2 g VS/(L·d) was probably due to the overloading of the system, indicated by the accumulation of organic matter and volatile fatty acids. The operation of the anaerobic digester under stable conditions allowed for high profitability for the proposed bio-refinery concept, which would still be profitable at a phenol extract price above 51.8 €/kg, which is 90% lower than the current price of 520 €/kg.

Long-term evaluation of methane production in a bio-electrochemical anaerobic digestion reactor according to the organic loading rate

In this study, the effects of differing organic loading rates (OLRs) on methane production were evaluated via long-term operation of a bio-electrochemical anaerobic digestion (BEAD) reactor and an anaerobic digestion (AD) reactor. In the AD reactor, the maximum OLR was 4 kg/m³·d whereas the electro-active microbial community in bulk and on the biofilm of the BEAD reactor produced methane even at 10 kg/m³·d. The BEAD reactor rapidly removed volatile fatty acids (VFAs) and reduced H⁺ to H₂ at high OLRs, thereby preventing VFA accumulation and pH decrease. After the steady state was achieved, both the AD and BEAD reactors exhibited similar organic matter removal and methane production at a low OLR. Thus, a BEAD reactor can stably produce methane under conditions of high OLR and severe OLR fluctuation and can even shorten the stabilization period over the long term.

Aerobic granular sludge: Cultivation parameters and removal mechanisms

Aerobic granular sludge (AGS) has been the focus of many investigations, and the main parameters responsible for AGS formation are hydrodynamic shear force, short periods and feast-famine cycles. However, some other parameters are associated with AGS maintenance after long periods of operation. This review evaluates the parameters responsible for AGS formation and maintenance and some reference values are proposed. In addition, some discussions are addressed about the main metabolic pathways that AGS uses for the removal of some compounds, such as nutrients, organic matter, dyes, recalcitrant compounds, among others. Finally, the main microbial groups present in the AGS and their respective functions are discussed. It is also highlighted that many parameters that are taken as reference currently for AGS cultivation and maintenance can be optimized for energy savings, implementation costs, among others, as well as a greater recovery of resources during wastewater treatment, within the scope of the biorefinery concept.

One-year operation of 1000-L modularized microbial fuel cell for municipal wastewater treatment

This study constructed a 1000 L modularized MFC system, the largest volume so far, to treat practical municipal wastewater. This MFC system was operated under two different water flow connections in two municipal wastewater treatment plants (MWTP) for more than one year to test their treating abilities for wastewater with both low (average 80 mg L^-1) and high initial COD concentration (average 250 mg L^-1). The COD concentration in the effluent from the MFC system remained below 50 mg L^-1 with a removal rate of 70-90%, which stably met the level A of the first class in discharge standard of pollutants for MWTP of China. A maximum power density of 125 W m^-3 (7.58 W m^-2) was generated when the MFC system was fed with artificial wastewater, while it lay in a range of 7-60 W m^-3 (0.42-3.64 W m^-2) when treating municipal wastewater. The energy recovery of 0.033 ± 0.005 kWh per m³ of municipal wastewater was achieved, with a hydraulic retention time (HRT) of 2 h.

Recovery of Cu(II) from aqueous solution by induced crystallization in a long-term operation

The feasibility of copper recovery by induced crystallization in a long period (174days) was investigated in a seeded fluidized bed reactor (FBR). The process was divided into 3 periods according to different influent conditions, and the period III was separated into III-a and III-b due to the adjustment of the molar ratio of CO₃^2- concentration to copper concentration ([C_T]/[Cu²⁺]). The removal efficiency could exceed 95% and the average effluent copper concentration decreased to 3.0mg/L. The mean particle size of seed grains with copper crystals coating on, raised to 0.36mm from initial 0.18mm. During period III-a, the supersaturation exceeded 2.88×10⁴, the removal efficiency decreased to 60%-80% and the particle size dropped to 0.30mm, due to the generation of fines by homogeneous crystallization and seeds breaking. And the morphology of the crystals on the seed grains changed from rod-like to spherical which lead to the particle size decreasing. In period III-b, the supersaturation was modified by adjusting the molar ratio of [C_T]/[Cu²⁺] to 1.2 from 2. The efficiency was back to 95% and the mean particle size grew to 0.36mm at the end of III-b, the crystals coating on the seeds turned back to rod-like products of good stability. This study illustrated that the copper salt crystal could keep on growing on the seed grains for over 150days, the feasibility and controllability of copper recovery by induced crystallization process in FBR were satisfactory, even under the dramatic changes in influent conditions.

Electrode and azo dye decolorization performance in microbial-fuel-cell-coupled constructed wetlands with different electrode size during long-term wastewater treatment

Microbial-fuel-cell-coupled constructed wetlands (CW-MFCs) with various cathode layers were used for long-term azo dye wastewater treatment. Their performance was assessed using cathode diameters ranging from 20 to 27.5cm and the influence of plants at the cathode was also examined. Bioelectricity generation, ABRX3 decolorization, and chemical oxygen demand (COD) removal performances first increased and then decreased with increasing cathode diameter. The CW-MFCs with larger cathodes had an anoxic region at the cathode where ABRX3 was decolorized. This phenomenon has not been reported in previous research on MFCs using traditional air cathodes. Anode performance was influenced by the cathode. The CW-MFC with a cathode diameter of 25cm showed the best electrode performance, and the highest voltage and power density were 560mV and 0.88W/m³, respectively. The highest ABRX3 decolorization and COD removal volumes were 271.53mg/L and 312.17mg/L, respectively.

The long-term effects of wall attached microalgal biofilm on algae-based wastewater treatment

The influence of the reactor wall attached biofilm on the nutrient removal performance was investigated in an open photobioreactor during long-term operation. Total nitrogen and phosphorus removal efficiencies were statistically similar between reactor with (reactor A) and without (reactor B) biofilm at the Hydraulic Retention Time (HRT) of 18, 13.5 and 9days. When the HRT reduced to 8days, total nitrogen and phosphorus removal efficiencies in the reactor A were 42.95±5.11% and 97.97±1.12%, respectively, while significant lower removal efficiencies (38.06±5.80% for total nitrogen and 83.14±8.16% for phosphorus) were obtained in the reactor B. The VSS concentrations throughout the test were statistically similar for the two reactors, with a mean value of 0.63±0.25g/l for reactor A and 0.69±0.20g/l for reactor B. This study indicated that the reactor wall attached biofilm supported high phosphorus and nitrogen removal, which may provide insight into the practical implementation of microalgae-based wastewater treatment.

Aerobic granules cultivated and operated in continuous-flow bioreactor under particle-size selective pressure

A novel method based on the selective pressure of particle size (particle-size cultivation method, PSCM) was developed for the cultivation and operation of aerobic granular sludge in a continuous-flow reactor, and compared with the conventional method based on the selective pressure of settling velocity (settling-velocity cultivation method, SVCM). Results indicated that aerobic granules could be cultivated in continuous operation mode by this developed method within 14days. Although in the granulation process, under particle-size selective pressure, mixed liquor suspended solids (MLSS) in the reactor fluctuated greatly and filamentous bacteria dominated the sludge system during the initial operation days, no obvious difference in profile was found between the aerobic granules cultivated by PSCM and SVCM. Moreover, aerobic granules cultivated by PSCM presented larger diameter, lower water content and higher specific rates of nitrification, denitrification and phosphorus removal, but lower settling velocity. Under long term operation of more than 30days, aerobic granules in the continuous-flow reactor could remain stable and obtain good chemical oxygen demand (COD), NH4(+)-N, total nitrogen (TN) and total phosphorus (TP) removal. The results indicate that PSCM was dependent on the cultivation and maintenance of the stability of aerobic granules in continuous-flow bioreactors.