Acceleration of electroactive anammox (electroammox) start-up by switching acetate pre-acclimated biofilms to electroammox biofilms

Anodic ammonium oxidation in microbial electrolysis cell: Towards nitrogen removal in low C/N environment

Biological nitrogen removal in low C/N environment is challenging in wastewater treatment for a long time. Autotrophic ammonium oxidation is promising due to the no need of carbon source addition, but alternative electron acceptors other than oxygen has to be widely investigated. Recently, microbial electrolysis cell (MEC), which applies a polarized inert electrode as the electron harvester, has been proved effective to oxidize ammonium with electroactive biofilm. That is, anodic microbes stimulated by exogenous low power can extract electron from ammonium and transfer electron to electrodes. This review aims to consolidate the recent advances in anodic ammonium oxidation in MEC. Various technologies based on different functional microbes and mechanisms of these processes are reviewed. Thereafter, the crucial factors influencing the ammonium oxidation technology are discussed. Challenges and prospects of anodic ammonium oxidation in ammonium-containing wastewater treatment are also proposed to provide valuable insights on the technologic reference and potential value of MEC in ammonium-containing wastewater treatment.

Comparative assessment of energy generation from ammonia oxidation by different functional bacterial communities

Bioelectrochemical ammonia oxidation (BEAO) in a microbial fuel cell (MFC) is a recently discovered process that has the potential to reduce energy consumption in wastewater treatment. However, level of energy and limiting factors of this process in different microbial groups are not fully understood. This study comparatively investigated the BEAO in wastewater treatment by MFCs enriched with different functional groups of bacteria (confirmed by 16S rRNA gene sequencing): electroactive bacteria (EAB), ammonia oxidizing bacteria (AOB), and anammox bacteria (AnAOB). Ammonia oxidation rates of 0.066, 0.083 and 0.082 g NH₄⁺-N L^-1 d^-1 were achieved by biofilms enriched with EAB, AOB, and AnAOB, respectively. With influent 444 ± 65 mg NH₄⁺-N d^-1, nitrite accumulation between 84 and 105 mg N d^-1 was observed independently of the biofilm type. The AnAOB-enriched biofilm released electrons at higher potential energy levels (anode potential of 0.253 V vs. SHE) but had high internal resistance (R_int) of 299 Ω, which limits its power density (0.2 W m^-3). For AnAOB enriched biofilm, accumulation of nitrite was a limiting factor for power output by allowing conventional anammox activity without current generation. AOB enriched biofilm had R_int of 18 ± 1 Ω and yielded power density of up to 1.4 W m^-3. The activity of the AOB-enriched biofilm was not dependent on the accumulation of dissolved oxygen and achieved 1.5 fold higher coulombic efficiency when sulfate was not available. The EAB-enriched biofilm adapted to oxidize ammonia without organic carbon, with R_int of 19 ± 1 Ω and achieved the highest power density of 11 W m^-3. Based on lab-scale experiments (scaling-up factors not considered) energy savings of up to 7 % (AnAOB), 44 % (AOB) and 475 % (EAB) (positive energy balance), compared to conventional nitrification, are projected from the applications of BEAO in wastewater treatment plants.

Mechanism of stable power generation and nitrogen removal in the ANAMMOX-MFC treating low C/N wastewater

This study investigated the mechanism of enhanced power generation and nitrogen removal in an ANAMMOX-MFC reactor through subsequent acetate addition. Data showed that nearly 99% total nitrogen removal (≤1 mg L^-1) and 1.41 W m^-3 power generation were achieved synchronously under low COD/N (∼1.5) after the subsequent addition of acetate (100 mgCOD·L^-1). The columbic efficiency of the system has increased by 15 times (from 0.64% to 9.48%) after adding acetate. Batch tests showed that the denitrification and ANAMMOX progress occurred synchronously before acetate addition the nitrogen removal rate was accelerated. A distinct shift of bacterial community driven by acetate addition was discovered. The high throughput sequencing analysis indicated acetate addition stimulated the enrichment of denitrifiers, such as Aquimonas, Bradyrhizobium, Thauera, and the potential exoelectrogens changing from Comamonas to Pseudomonas. Functional genes forecasts based on KEGG database and COG database showed that the expressions of TCA functional genes were highly promoted in ANAMMOX-MFC, which demonstrated the enhanced electron transfer pathway driven by acetate addition under low COD/N ratio.

Effect of poised cathodic potential on anodic ammonium nitrogen removal from domestic wastewater by air-cathode microbial fuel cells

Performances of anodic ammonia oxidation have been investigated for various bioelectrochemical systems at a wide range of poised anodic potentials in the literature. The effect of poised cathodic potential on ammonium nitrogen (NH₄⁺-N) and total nitrogen (TN, sum of NH₄⁺-N, NO₂^--N, and NO₃^--N) removal from domestic wastewater by single chamber air-cathode microbial fuel cells (MFCs) was investigated. Poising the air-cathode potential at +0.7 V vs. SHE significantly increased current generation (from 11 ± 1 mA to 22.8 ± 5 mA) and oxygen permeation into the MFC through the air-cathode (from 75.4 ± 1.2 g-O₂/m³/d to 151 ± 3.7 g-O₂/m³/d), which consequently resulted in a high NH₄⁺-N removal rate of 150 ± 13 g-NH₄⁺-N/m³/d and TN removal rate of 63 ± 16 g-TN/m³/d. These high NH₄⁺-N and TN removal rates could be attributed to the enhancement of dual respiratory pathways: the electrode-assisted anodic and aerobic NH₄⁺ oxidation.

Bioelectrochemical anoxic ammonium nitrogen removal by an MFC driven single chamber microbial electrolysis cell

Nitrogen removal from wastewater is an indispensable but highly energy-demanding process, and thus more energy-saving treatment processes are required. Here, we investigated the performance of bioelectrochemical ammonium nitrogen (NH₄⁺-N) removal from real domestic wastewater without energy-intensive aeration by a single chamber microbial electrolysis cell (MEC) that was electrically powered by a double chamber microbial fuel cell (MFC). Anoxic NH₄⁺-N oxidation and total nitrogen (TN) removal rates were determined at various applied voltages (0-1.2 V), provided by the MFC. The MEC achieved a NH₄⁺-N oxidation rate of 151 ± 42 g NH₄⁺-N m^-3 d^-1 and TN removal rate of 95 ± 42 g-TN m^-3 d^-1 without aeration at the applied voltage of 0.8 V (the anode potential E_anode = +0.633 ± 0.218 V vs. SHE). These removal rates were much higher than the previously reported values and conventional biological nitrogen removal processes. Open and closed-circuit MEC batch experiments confirmed that anoxic NH₄⁺-N oxidation was an electrochemically mediated biological process (that is, an anode acted as an electron acceptor) and denitrification occurred simultaneously without NO₂^- and NO₃^- accumulation. Moreover, ex-situ¹⁵N tracer experiment and microbial community analysis revealed that anammox and heterotrophic denitrification mainly contributed to the TN removal. Thus, the bioelectrochemical anodic NH₄⁺-N oxidation was coupled with anammox and denitrification in this MFC-assisted MEC system. Taken together, our MFC-driven single chamber MEC could be a high rate energy-saving nitrogen removal process without external carbon and energy input and high energy-demanding aeration.

Large-scale switchable potentiostatically controlled/microbial fuel cell bioelectrochemical wastewater treatment system

The treatment of municipal wastewater is an energy-intensive process, with the delivery of oxygen as an electron acceptor accounting for a significant share of the total energy consumption. Microbial communities growing on polarized electrodes can facilitate wastewater treatment processes by exchanging electrons with the electrodes. As a new approach, we combined the use of polarized electrodes with microbial fuel cells (MFCs) to develop a switchable dual-mode bioelectrochemical wastewater treatment system. In this system, we first enriched microbial communities on polarized anodes and cathodes. After enrichment, the system was switched to either a self-powered MFC (SP-MFC) mode or a potentiostatically controlled (PC) mode. The system was evaluated at the laboratory scale (260 L, 4 anode and cathode pairs) and the pilot scale (1200 L, 16 anode and cathode pairs). PC and SP-MFC systems showed improved COD removal relative to control (41.6 ± 3.5 and 38.4 ± 3.1 vs 28.8 ± 2.1 mg L^-1 d^-1, respectively). The laboratory-scale system was operated without biological or electrical interruption for one year. Finally, specific enrichment of active microbial communities was observed on PC anodes in comparison to mixed-operation and non-polarized control anodes. The combined PC and SP-MFC systems allowed us to develop a sustainable and failure-free bioelectrochemical wastewater treatment system.

Ammonia oxidation and denitrification in a bio-anode single-chambered microbial electrolysis cell

In this study, anodic ammonia oxidation and denitrification were performed in single-chamber bioelectrochemical systems at a wide range of anodic potentials (-400 to +400 mV) versus Ag/AgCl. The low coulombic efficiencies (~30.84%) in reactors were mainly due to electrons being transferred to atmospheric oxygen through the electrode and reversal of the electrode. The removal efficiencies of acetate, ammonia, and total nitrogen were 100%, 100%, and 40.44% at +200 mV and 100%, 100%, and 50.24% at -200 mV, respectively. The nitrogen-removal mechanisms were nitrogen respiration/nitrate reduction at +200 mV and denitrification at -200 mV, and ammonia oxidation occurred by coupling with sulfate-reducing at -300 and -400 mV. Thauera, Comamonas, Alicycliphilus, Nitrosomonas, Desulforhabdus, Dethiosulfatibacter, and Desulfomicrobium were the dominant genera at the anode which participated in the nitrification/denitrification or sulfate-reducing processes. In summary, ammonia oxidation and denitrification could be coupled with carbon-removal or sulfur-reduction using a bio-anode with a suitable anodic potential.

System performance and microbial community in ethanol-fed anaerobic reactors acclimated with different organic carbon to sulfate ratios

Sulfate influences the organics removal and methanogenic performance during anaerobic wastewater treatment. System performance, microbial community and metabolic pathways in ethanol-fed anaerobic reactors were investigated under different COD/SO₄^2- ratios (2, 1 and 0.67) and control without sulfate addition. The sulfate removal percentages declined (99%, 60% and 49%) with decreasing COD/SO₄^2- ratios, and methanogenesis was completely inhibited. Acetate accumulated to 903-734 mg/L, though propionate was constantly lower than 30 mg/L. Without sulfate, acetate and propionate did not accumulate, despite the extended time for propionate degradation. Incomplete oxidizing sulfate reducing bacteria (Desulfobulbus and Desulfomicrobium) and hydrolysis-acidification genera (Treponema and Bacteroidales) predominated but could not degrade acetate. Desulfobulbus was the key genus for propionate degradation through the pyruvate & propanoate metabolism pathway. Pseudomonas and Desulfobulbus, possessing genes encoding Type IV pili and cytochrome c6 OmcF, respectively, potentially participated in the direct interspecies electron transfer in sulfate-rich conditions.

Bidirectional extracellular electron transfers of electrode-biofilm: Mechanism and application

The extracellular electron transfer (EET) between microorganisms and electrodes forms the basis for microbial electrochemical technology (MET), which recently have advanced as a flexible platform for applications in energy and environmental science. This review, for the first time, focuses on the electrode-biofilm capable of bidirectional EET, where the electrochemically active bacteria (EAB) can conduct both the outward EET (from EAB to electrodes) and the inward EET (from electrodes to EAB). Only few microorganisms are tested in pure culture with the capability of bidirectional EET, however, the mixed culture based bidirectional EET offers great prospects for biocathode enrichment, pollutant complete mineralization, biotemplated material development, pH stabilization, and bioelectronic device design. Future efforts are necessary to identify more EAB capable of the bidirectional EET, to balance the current density, to evaluate the effectiveness of polarity reversal for biocathode enrichment, and to boost the future research endeavors of such a novel function.

Metabolomics Uncovers the Regulatory Pathway of Acyl-homoserine Lactones Based Quorum Sensing in Anammox Consortia

Acyl-homoserine lactones (AHLs)-mediated quorum sensing in bacterial communities have been extensively observed. However, the metabolic pathways regulated by AHLs in bacteria remain elusive. Here, we combined long-term reactor operation with microbiological and metabolomics analyses to explore the regulatory pathways for different AHLs in anammox consortia, which perform promising nitrogen removal for wastewater treatment. The results showed that no obvious shifts induced by exogenous AHLs occurred in the microbial community and, mainly, dosing AHLs induced changes in the metabolites. 3OC6-HSL, C6-HSL, and C8-HSL controlled the electron transport carriers that influence the bacterial activity. In contrast, only 3OC6-HSL regulated LysoPC(20:0) metabolism, which affected bacterial growth. AHLs mainly regulated the synthesis of the amino acids Ala, Val, and Glu and selectively regulated Asp and Leu to affect extracellular proteins. Simultaneously, all the AHLs regulated the ManNAc biosynthetic pathways, while OC6-HSL, OC8-HSL, and C6-HSL particularly enriched the UDP-GlcNAc pathway to promote exopolysaccharides, resulting in different aggregation levels of the anammox consortia. Our results not only provide the first metabolic insights into the means by which AHLs affect anammox consortia but also hint at potential strategies for overcoming the limitations of the long start-up period required for wastewater treatment by anammox processing.