Oxalate degradation in a bioelectrochemical system: reactor performance and microbial community characterization

Linking performance to dynamic migration of biofilm ecosystem reveals the role of voltage in the start-up of hybrid microbial electrolysis cell-anaerobic digestion

Applied voltage is a crucial parameter in hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) systems for enhancing methane production from waste activated sludge (WAS). This study explored the impact of applied voltage on the initial biofilm formation on electrodes during the MEC-AD startup using raw WAS (Rr) and heat-pretreated WAS (Rh). The findings indicated that the maximum methane productivity for Rr and Rh were 3.4 ± 0.5 and 3.4 ± 0.2 mL/gVSS/d, respectively, increasing 1.5 times and 2.6 times over the productivity at 0 V. The biomass on electrode biofilms for Rr and Rh at 0.8 V increased by 70 % and 100 % compared to 0 V. The core functional microorganisms in the cathode biofilm were Methanobacterium and Syntrophomonas, and Geobacter in the anode biofilm, enhancing methane production through syntrophism and direct interspecies electron transfer, respectively. These results offer academic insights into optimizing AD functional electrode biofilms by applying voltage.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial-electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Fabrication of the macro and micro-scale microbial fuel cells to monitor oxalate biodegradation in human urine

This study presented the fabrication of macro and micro-scale microbial fuel cells (MFCs) to generate bioelectricity from oxalate solution and monitor the biodegradation in a micro-scale MFC for the first time. The maximum generated power density of 44.16 W m^-3 in the micro-scale MFC elucidated its application as a micro-sized power generator for implantable medical devices (IMDs). It is also worthwhile noting that for the macro-scale MFC, the significant amounts of open circuit voltage, oxalate removal, and coulombic efficiency were about 935 mV, 99%, and 44.2%, respectively. These values compared to previously published studies indicate successful oxalate biodegradation in the macro-scale MFC. Regarding critical challenges to determine the substrate concentration in microfluidic outlets, sample collection in a suitable time and online data reporting, an analogy was made between macro and micro-scale MFCs to elicit correlations defining the output current density as the inlet and the outlet oxalate concentration. Another use of the system as an IMD is to be a platform to identify urolithiasis and hyperoxaluria diseases. As a versatile device for power generation and oxalate biodegradation monitoring, the use of facile and cheap materials (< $1.5 per device) and utilization of human excreta are exceptional features of the manufactured micro-scale MFC.

A review on approaches for hazardous organics removal from Bayer liquors

Alumina is a valuable raw material for the production of adsorbents, abrasives, polishing agents, refractory materials, and aluminum. It is generally produced from bauxites through the Bayer process. Several organic compounds such as humic matters and oxalates are introduced into the Bayer liquor during the digestion process, resulting in significant hazards to precipitation of aluminum hydroxide. Therefore, it is crucial to remove these organic compounds from Bayer liquor to enhance the production of alumina. It is difficult to remove these organic compounds. Various approaches for organics removal from Bayer liquors have been developed in the past few decades, including thermal treatment, chemical precipitation, membrane technology, photocatalytic degradation, biodegradation, and wet oxidation. This paper reviews the technologies for organics removal from Bayer liquor and the relative mechanisms proposed in the literature to identify its essential parameters. Chemicals dosage, temperature, pH value, reaction time, and solution concentration are essential factors in the process. Removal efficiency, green principle, and economic viability of various methods are discussed, and potential technologies are suggested. Wet oxidation appears to be a promising method for removing organic matters in Bayer liquors. Moreover, the combination of wet oxidation and electrooxidation shows excellent potential in organics removal. Various approaches for removing organic compounds and perspectives for further investigation are proposed.

Synergistic degradation of sulfamethoxazole in an oxalate-enhanced Fered-Fenton system: The critical heterogeneous solid-liquid interfacial mechanism and an insight in practical application

It was demonstrated in this study that appropriate concentrations of oxalate (Ox) would lead to greatly accelerated electro-generation of Fe²⁺ but obviously lower power consumption in the Fered-Fenton system. Depending on the Ti electrode with pristine TiO₂ layer, effects of important parameters on the SMX degradation were investigated in the Fered-Fenton-Ox system. It was found that the heterogeneous interfacial electrochemically reduction of Fe^III was critical in the Fered-Fenton-Ox system relying on the surface hydroxyl bonding Fe^III-Ox and formation of FeOTi bonds. A heterogeneous-homogeneous reaction mechanism was therefore proposed. It included the heterogeneous interfacial electrochemical generation of Fe^II-Ox and the heterogeneous-homogenous Fenton oxidation of pollutants. The promotional role of Ox would be also homogenous and heterogeneous, i.e. maintaining ferric at higher pH and forming specific Fe^III-Ox complex as well as accelerating the solid-liquid interfacial heterogeneous iron cycle. Furthermore, a continuous-flow pilot study was conducted in treating a printing and dyeing industrial wastewater. As compared to conventional Fenton and Fered-Fenton systems, the Fered-Fenton-Ox system could achieve more efficient COD removal with a relative low cost/△COD, suggesting great advantages in its practical applications for treating real industrial complex wastewaters.

Extracellular polymeric substance decomposition linked to hydrogen recovery from waste activated sludge: Role of peracetic acid and free nitrous acid co-pretreatment in a prefermentation-bioelectrolysis cascading system

Free nitrous acid (FNA) has been recently reported to be an effective and eco-friendly inactivator for waste activated sludge (WAS), while the limited decomposition of the extracellular polymeric substance (EPS) matrix hampers resource recovery from WAS. This work employed peracetic acid (PAA) to assist FNA and explored the contribution of co-pretreatment to hydrogen recovery in a prefermentation-bioelectrolysis cascading system. The results showed that co-pretreatment led to approximately 8.8% and 20.4% increases in the exfoliation of particulate proteins and carbohydrates, respectively, from tightly bound EPS (TB-EPS) over that of sole FNA pretreatment. Electron paramagnetic resonance analysis verified that the synergistic effect of FNA, PAA and various generated free radicals was the essential process. This effect further promoted the accumulation of volatile fatty acids (VFAs) after 96 h of prefermentation, and the peak concentration in co-pretreated WAS (AD-FPWAS) was approximately 2.5-fold that in sole FNA-pretreated WAS (AD-FWAS). Subsequently, the cascading utilization of organics in the bioelectrolysis step contributed to efficient hydrogen generation. A total of 10.8 ± 0.3 mg H₂/g VSS was harvested in microbial electrolysis cells (MECs) fed with AD-FPWAS, while 6.2 ± 0.1 mg H₂/g VSS was obtained from AD-FWAS. X-ray photoelectron spectroscopy (XPS) revealed the effective decomposition of the phospholipid bilayer in the cytomembrane and the transformation of macromolecular organics into VFAs and hydrogen in the cascading system. Further microbial community analysis demonstrated that co-pretreatment enhanced the accumulation of functional consortia, including anaerobic fermentative bacteria (AFB, 28.1%), e.g., Macellibacteroides (6.3%) and Sedimentibacter (6.9%), and electrochemically active bacteria (EAB, 57.0%), e.g., Geobacter (39.0%) and Pseudomonas (13.6%), in the prefermentation and MEC steps, respectively. The possible synergetic and competitive relationships among AFB, EAB, homo-acetogens, nitrate-reducing bacteria and methanogens were explored by molecular ecological network analysis. From an environmental and economic perspective, this promising FNA and PAA co-pretreatment approach provides new insight for energy recovery from WAS biorefineries.

Anodic microbial community analysis of microbial fuel cells based on enriched inoculum from freshwater sediment

The characterization of anodic microbial communities is of great importance in the study of microbial fuel cells (MFCs). These kinds of devices mainly require a high abundance of anode respiring bacteria (ARB) in the anode chamber for optimal performance. This study evaluated the effect of different enrichments of environmental freshwater sediment samples used as inocula on microbial community structures in MFCs. Two enrichment media were compared: ferric citrate (FeC) enrichment, with the purpose of increasing the ARB percentage, and general enrichment (Gen). The microbial community dynamics were evaluated by polymerase chain reaction followed by denaturing gradient gel electrophoresis (PCR-DGGE) and real time polymerase chain reaction (qPCR). The enrichment effect was visible on the microbial community composition both during precultures and in anode MFCs. Both enrichment approaches affected microbial communities. Shannon diversity as well as β-Proteobacteria and γ-Proteobacteria percentages decreased during the enrichment steps, especially for FeC (p < 0.01). Our data suggest that FeC enrichment excessively reduced the diversity of the anode community, rather than promoting the proliferation of ARB, causing a condition that did not produce advantages in terms of system performance.

Rapid start-up of a bioelectrochemical system under alkaline and saline conditions for efficient oxalate removal

This study examined a new approach for starting up a bioelectrochemical system (BES) for oxalate removal from an alkaline (pH > 12) and saline (NaCl 25 g/L) liquor. An oxalotrophic biofilm pre-grown aerobically onto granular graphite carriers was used directly as both the microbial inoculum and the BES anode. At anode potential of +200 mV (Ag/AgCl) the biofilm readily switched from using oxygen to graphite as sole electron acceptor for oxalate oxidation. BES performance was characterised at various hydraulic retention times (HRTs, 3-24 h), anode potentials (-600 to +200 mV vs. Ag/AgCl) and influent oxalate (25 mM) to acetate (0-30 mM) ratios. Maximum current density recorded was 363 A/m³ at 3 h HRT with a high coulombic efficiency (CE) of 70%. The biofilm could concurrently degrade acetate and oxalate (CE 80%) without apparent preference towards acetate. Pyro-sequencing analysis revealed that known oxalate degraders Oxalobacteraceae became abundant signifying their role in this novel bioprocess.

Bioelectrochemical oxidation of organics by alkali-halotolerant anodophilic biofilm under nitrogen-deficient, alkaline and saline conditions

This work aimed to study the feasibility of using bioelectrochemical systems (BES) for organics removal under alkaline-saline and nitrogen (N) deficient conditions. Two BES inoculated with activated sludge were examined for organics (oxalate, acetate, formate) oxidation under alkaline-saline (pH 9.5, 25g/L NaCl) and N deficient conditions. One reactor (R1) received ammonium chloride as an N-source, while the other (R2) without. The reactors were initially loaded with only oxalate (25mM), but start-up was achieved only when acetate was added as co-substrate (5mM). Maximum current were R1: 908A/m³ (organic removal rate (ORR) 4.61kgCOD/m³·d) and R2: 540A/m³ (ORR 2.06kgCOD/m³·d). Formate was utilised by both anodic biofilms, but the inefficient oxalate removal was likely due to the paucity of microorganisms that catalyse decarboxylation of oxalate into formate. Further development of this promising technology for the treatment of alkaline-saline wastewater is warranted.

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.

Unravelling the active microbial community in a thermophilic anaerobic digester-microbial electrolysis cell coupled system under different conditions

Thermophilic anaerobic digestion (AD) of pig slurry coupled to a microbial electrolysis cell (MEC) with a recirculation loop was studied at lab-scale as a strategy to increase AD stability when submitted to organic and nitrogen overloads. The system performance was studied, with the recirculation loop both connected and disconnected, in terms of AD methane production, chemical oxygen demand removal (COD) and volatile fatty acid (VFA) concentrations. Furthermore, the microbial population was quantitatively and qualitatively assessed through DNA and RNA-based qPCR and high throughput sequencing (MiSeq), respectively to identify the RNA-based active microbial populations from the total DNA-based microbial community composition both in the AD and MEC reactors under different operational conditions. Suppression of the recirculation loop reduced the AD COD removal efficiency (from 40% to 22%) and the methane production (from 0.32 to 0.03 m³ m^-3 d^-1). Restoring the recirculation loop led to a methane production of 0.55 m³ m^-3 d^-1 concomitant with maximum MEC COD and ammonium removal efficiencies of 29% and 34%, respectively. Regarding microbial analysis, the composition of the AD and MEC anode populations differed from really active microorganisms. Desulfuromonadaceae was revealed as the most active family in the MEC (18%-19% of the RNA relative abundance), while hydrogenotrophic methanogens (Methanobacteriaceae) dominated the AD biomass.

Overcoming organic and nitrogen overload in thermophilic anaerobic digestion of pig slurry by coupling a microbial electrolysis cell

The combination of the anaerobic digestion (AD) process with a microbial electrolysis cell (MEC) coupled to an ammonia stripping unit as a post-treatment was assessed both in series operation, to improve the quality of the effluent, and in loop configuration recirculating the effluent, to increase the AD robustness. The MEC allowed maintaining the chemical oxygen demand removal of the whole system of 46±5% despite the AD destabilization after doubling the organic and nitrogen loads, while recovering 40±3% of ammonia. The AD-MEC system, in loop configuration, helped to recover the AD (55% increase in methane productivity) and attained a more stable and robust operation. The microbial population assessment revealed an enhancement of AD methanogenic archaea numbers and a shift in eubacterial population. The AD-MEC combined system is a promising strategy for stabilizing AD against organic and nitrogen overloads, while improving the quality of the effluent and recovering nutrients for their reutilization.

Coupling of a bioelectrochemical system for p-nitrophenol removal in an upflow anaerobic sludge blanket reactor

Coupling of a bioelectrochemical system (BES) into the upflow anaerobic sludge blanket (UASB) was developed for enhanced p-nitrophenol (PNP) removal in this study. Compared to the control UASB reactor, both PNP removal and the formation of its final reductive product p-aminophenol (PAP) were notably improved in the UASB-BES system. With the increase of current density from 0 to 4.71 A m(-3), the rates of PNP removal and PAP formation increased from 6.16 ± 0.11 and 4.21 ± 0.29 to 6.77 ± 0.00 and 6.11 ± 0.28 mol m(-3) d(-1), respectively. More importantly, the required dosage of organic cosubstrate was significantly reduced in the UASB-BES system than that in the UASB reactor. Organic carbon flux analysis suggested that biogas production from organic cosubstrate was seriously suppressed while direct anaerobic reduction of PNP was not remarkably affected by current input in the UASB-BES system. This study demonstrated that the UASB-BES coupling system had a promising potential for the removal of nitrophenol-containing wastewaters especially without adequate organic cosubstrates inside.

Effective reduction of enteric methane production by a combination of nitrate and saponin without adverse effect on feed degradability, fermentation, or bacterial and archaeal communities of the rumen

This study evaluated the effects of Quillaja saponin (0.6 and 1.2g/L), propynoate (4 and 8mM), and nitrate (5 and 10mM), alone or in combinations, on methanogenesis, fermentation, bacterial and archaeal communities, and abundances of select ruminal microbial populations. All treatment decreased methane production, but combination of all three inhibitors at high dose achieved the greatest inhibition (85%). Propynoate, alone or in combination with nitrate or saponin, decreased feed degradability and total volatile fatty acid (TVFA) concentrations. However, saponin and nitrate alone at high dose and in combination at low dose inhibited methanogenesis substantially while increasing feed degradability and TVFA concentrations. The abundances of methanogens were lowered by all inhibitors except saponin alone. Fibrobacter succinogenes and Ruminococcus flavefaciens were increased by saponin, both alone and in combination with nitrate, but inhibited by propynoate. Combination of saponin and nitrate may have practical application in mitigating methane emission from ruminants.