Simultaneous nutrient and carbon removal and electricity generation in self-buffered biocathode microbial fuel cell for high-salinity mustard tuber wastewater treatment

Development and operation of indigenous microalgal-bacterial consortium system treating eutrophic lake water: Consortium identification and system demonstration

Natural water bodies such as the inland lake suffers from eutrophication due to excessive nutrient, particularly nitrogen and phosphorus. This study demonstrated an indigenous microalgal-bacterial consortium (IMBC) system to treat eutrophic lake. Three IMBC were enriched from eutrophic lake water or/and sediments, exhibiting superior growth and complete nutrient removals compared to two commercial microalgal species. Particularly, the IMBC3 enriched from lake water and sediment (volume ratio of 1:1) were found to simultaneously achieve 91.0 % settling efficiency, attributed to its larger flocs and surface physical properties (e.g., higher surface hydrophobicity (78.0 %), protein/polysaccharide ratio (10.7) and zeta potential (-19.1 mV)). Subsequently, a long-term photobioreactor using IMBC3 further demonstrated stable nutrient removal and cold tolerance year-around. The microbial community's shift towards cold-tolerant genera and alleviated photoinhibition likely enhanced nitrogen cycling efficiency during colder months. These findings offer a feasible alternative using the IMBC with good environmental adaptation to eutrophication mitigation in nature water.

Achieving synchronous nitrogen and phosphorus removal by aerobic enrichment of electrotrophic/heterotrophic bacteria and denitrifying polyphosphate-accumulating organisms in repeatedly oxygen-rich microbial fuel cells

Realizing the enrichment of functional bacteria in microbial fuel cells (MFCs) for wastewater treatment holds substantial research significance. This study explored a novel method of repeatedly oxygen-rich anode environment to enrich electrotrophic/heterotrophic bacteria (EHB) and denitrifying polyphosphate-accumulating organisms (DPAOs) in membrane-less single-chamber air-cathode (AC) MFCs to treat household wastewater. Repeated accumulation of higher dissolved oxygen (DO) was conducive to enhancing the growth of EHB and DPAOs. The systems achieved the maximum removal of 99% of ammonium, 78% of total inorganic nitrogen and 55% of total phosphorus. Repeated oxygen-rich conditions favored the selection of nitrogen-oxidizing bacteria on both electrodes, such as unclassified_f_Xanthomonadaceae, unclassified_p_Bacteroidota, Nitrosomonas and Nitrospira, thereby increasing nitrate availability for DPAOs like Candidatus Contendobacter, unclassified_c_Actinomycetia as well as other denitrifiers such as Anaerolineales, unclassified_p_Chloroflexi, unclassified_o_Rhodospirillales. The genes nxrAB, narGH and nasC, associated with nitrification and denitrification, and the genes gcd, phoD, ugpQ, glpQ, involved in phosphate metabolism, were up-regulated in presence of repeated DO accumulation, thereby enhancing pollutants removal. This study presents a novel approach for the synchronous removal of nitrogen and phosphorus from domestic wastewater through the enrichment of functional bacteria in the repeatedly oxygen-rich ACMFCs.

Application of sponge iron-carbon to enrich anaerobic ammonia-oxidizing bacteria from sludge mixture and coupled denitrification for degradation of industrial wastewater

For the treatment of industrial wastewater, coupled iron‑carbon micro-electrolysis (ICME) with anaerobic ammonia oxidation (anammox) and denitrification was optimized under the following conditions: Fe/C = 2, C/N ≤ 2, and the temperature was 30 °C. The coupled ICME enriched ammonia-oxidizing bacteria (AnAOB) and denitrifying bacteria (DB) in the mixed sludge on the 76th day of the present experiment. Stable operation was achieved on the 78th day. The COD and TN removal rates during the operation were 86.20 % and 87.12 %, respectively, while the control group (without iron and carbon) had removal rates of 74.30 % and 60.31 % which were 11.9 % and 26.81 % higher, respectively. Notably, the abundance of AnAOB in the system increased from 0.44 % to 1.43 % during the operation from day 76 to day 100. High-throughput sequencing demonstrated that Candidatus_Kuenenia was a key anaerobic ammonia-oxidizing bacterium. Based on the experimental results, the ICME process could rapidly enrich anaerobic ammonia-oxidizing bacteria to change the microbial community structure of the sludge under the water quality conditions of industrial wastewater and increasing the tolerance of certain DB and Candidatus_Kuenenia to water quality. By combining with iron‑carbon, the rapid modification of mixed sludge was achieved, and the iron‑carbon micro-electrolysis coupled denitrification anaerobic ammonia oxidation process was established, which provides a certain reference value for treating industrial wastewater.

Augmented carbon utilization and ammonia assimilation in heterotrophic microorganism under magnetic field stimulation

Ammonia assimilation is crucial in microbial nitrogen metabolism, and researching the impact of magnetic field (MF) on heterotrophic ammonia assimilation (HAA) contributes to improving nitrogen utilization and environmental remediation. This study systematically investigated the profound effects of MF stimulation on carbon and ammonia assimilation mechanisms in heterotrophic microorganisms. The dynamic responses of microbial carbon source metabolic efficiency and nitrogen source assimilation rates were quantitatively analyzed by designing a multidimensional stimulation environment of different nutrient levels (C/N 20, 25, 30) and MF intensities (0, 1, 20 mT). The findings demonstrated that weak MF stimulation markedly enhanced carbon utilization efficiency and ammonia assimilation capacity, evidenced by accelerated metabolic flux, significant biomass augmentation and metabolic network reconfiguration. MF induced pronounced activation of key metabolic enzymes, uncovering coordinated regulatory mechanisms at the molecular level. Additionally, the MF stimulation enriched genera the Halomonas and Marinobacter, enhancing the stability of microbial community. These discoveries not only advance the theoretical understanding of MF-enhanced metabolic modulation but also offer cutting-edge insights and potential solutions for developing MF-assisted bioconversion and environmental remediation technologies.

A temperature-resilient anammox process for efficient treatment of rare earth element tailings wastewater via synergistic nitrite supply of partial nitritation and partial denitrification

Rare earth elements result in substantial tailings wastewater with high ammonium and nitrate during extraction. In this study, a temperature-resilient Anammox process was employed for efficient treatment of rare earth element tailings wastewater through implementing synergistic nitrite supply by partial nitritation (PN) and partial denitrification (PD). Enhancing temperature resilience of Anammox process relies on dynamic management of DO and COD inputs to shift the dominant nitrite supplier from PN to PD, stable PD (NAR ≥ 90 %) can boost nitrogen removal by Anammox to 97.8 %. The nitrogen removal rate and nitrogen removal efficiency at 10.6 °C could maintain at 0.12 kgN/m3·d-1 and 92.5 %, respectively. Microbial analysis reveals that Nitrosomonas, Thauera, and Candidatus_Kuenenia are the predominant genera responsible for nitrite supply and nitrogen removal, localized within the gas channels of granules, flocs, and micro-granules, respectively. Keeping the influent C/NO3--N ratio below 1.7 is ideal to prevent overgrowth of Thauera and maintain system stability.

Effect of sludge redistribution strategy on stability of partial nitrification-anammox process: Further exploration of the potential value of sludge

The stability of the two-stage partial nitrification-anammox (PN/A) system was compromised by the inappropriate conversion of insoluble organic matter. In response, a sludge redistribution strategy was implemented. Through the redistribution of PN sludge and anammox sludge in the two-stage PN/A system, a transition was made to the Anammox-single stage PN/A (A-PN/A) system. This specific functional reorganization, facilitated by the rapid reorganization of microbial communities, has the potential to significantly decrease the current risk of suppression. The results of the study showed that implementing the sludge redistribution strategy led to a substantial enhancement in the total nitrogen removal rate (TNRR) by 87.51%, accompanied by a significant improvement of 34.78% in the chemical oxygen demand removal rate (CRR). Additionally, this approach resulted in a remarkable two-thirds reduction in the aeration requirements. High-throughput sequencing revealed that the strategy enriched anammox and ammonia-oxidizing bacteria while limiting denitrifying bacteria, as confirmed by quantitative polymerase chain reaction analysis. Furthermore, the principal component analysis revealed that the location and duration of aeration had direct and indirect effects on functional gene expression and the evolution of microbial communities. This study emphasizes the potential benefits of restructuring microbial communities through a sludge redistribution strategy, especially in integrated systems that encounter challenges with suppression.

Nutrients recovery by coupled bioreactor of heterotrophic ammonia assimilation and microbial fuel cell in saline wastewater

Heterotrophic ammonia assimilation (HAA) process had been widely used in the treatment of high salt wastewater, but the electro enhanced coupling process and electron transfer process were rarely studied. In this study, a HAA process coupled microbial fuel cell (MFC) system was established to treat ammonia-containing wastewater under increasing salinity to achieve nitrogen recovery and electricity generation. Up to 95.4 % NH4+-N and 96.4 % COD removal efficiencies were achieved at 2 % salinity in HAA-MFC. The maximum power density and current density at 2 % salinity were 29.93 mW/m2 and 182.37 mA/m2, respectively. The residual organic matter in the cathode effluent was effectively removed by the anode. The increase of salinity not only enhanced the sludge settling performance and activity, but also promoted the enzyme activity and amino acid production of the ammonia assimilation pathway. Marinobacter and Halomonas were gradually enriched at the anode and cathode with increased salinity to promote ammonia assimilation and electron production. This research offered a promising solution to overcome salinity-related challenges in wastewater treatment and resource recovery.

Membrane aerated biofilm reactor system driven by pure oxygen for wastewater treatment

Pure oxygen is proposed for wastewater treatment due to its advantages over conventional air aeration. This study investigates a Pure Oxygen-based Membrane Aerated Biofilm Reactor (PO-MABR) for the first time under various operating conditions. The PO-MABR employs a gas-permeable membrane for direct diffusion of low-pressurized pure oxygen to the biofilm, ensuring exceptional carbon and nitrogen removal. The effectiveness of PO-MABR was investigated by varying operational conditions, including temperature, carbon-to-nitrogen ratio, gas pressure, and flow rate. Results indicate superior performance, with a 97% chemical oxygen demand removal and 19% higher total nitrogen removal than Air-Ventilated MABR (A-MABR) due to thicker biofilm and unique microbial structures in PO-MABR. Also, PO-MABR demonstrated resilience to low temperatures and effectively treated both high and low-strength wastewater. The findings emphasize the efficiency of PO-MABR in wastewater treatment, advocating for its adoption due to superior carbon and nitrogen removal across diverse operational conditions.

New attempts on acidic anaerobic digestion of poly (butylene adipate-co-terephthalate) wastewater in upflow anaerobic sludge blanket reactor

Poly (butylene adipate-co-terephthalate) (PBAT) wastewater is a highly concentrated, acidic, and toxic wastewater generated from biodegradable plastics production. Large amounts of alkali would be consumed when treating PBAT wastewater by anaerobic digestion due to the low pH value. This study employed acidic anaerobic digestion to treat PBAT wastewater and compared to neutral anaerobic digestion. The results indicated that the COD removal rates in the acidic upflow anaerobic sludge blanket (UASB) reactor were 65.5% and 59.9%, respectively at influent pH 6.0 and 5.0 with the COD concentration of around 11,000 mg L-1, and the methane conversion efficiency were 172.5 and 183.8 mLCH4/gCODr (gCODr: COD removed amount), respectively. Correspondingly, the average COD removal and methane conversion efficiency in the neutral UASB reactor were 63.2% and 188.0 mLCH4/gCODr, respectively. The treatment efficiency of acidic and neutral UASB reactors for PBAT wastewater was similar. The hydrogenotrophic methanogenic activity was further enhanced in the acidic UASB reactor compared to the neutral one. The increase of alkalinity in the acidic UASB reactor (2.4 mmol L-1) was higher than the neutral (2.0 mmol L-1). A higher level of syntrophic acetate oxidation bacteria and hydrogenotrophic methanogen was enriched in the acidic UASB reactor, ensuring efficient treatment and saving costs.

Full-scale upgrade activated sludge to continuous-flow aerobic granular sludge: Implementing microaerobic-aerobic configuration with internal separators

Aerobic granular sludge (AGS) has been successfully used in sequencing batch reactors. However, their application to existing continuous-flow systems remains challenging. In this study, a novel microaerobic-aerobic configuration with internal separators was implemented in a full-scale municipal wastewater treatment facility with a nominal capacity of 2.5 × 104 m3 d-1. Sludge characteristics, pollutant removal and associated pathways, shifts in the microbial community, and underlying granulation mechanisms were investigated. Following a two-month operation period, the transition from flocculent-activated sludge to well-defined AGS with distinct boundaries and compact structures was successfully achieved. The average size of sludge increased from 31.9 to 138.5 μm, with granules larger than 200 μm constituting 28.9 % of the total sludge and SVI30 averaging 51.4 ± 8.2 mL g-1. The 95th percentile effluent COD, NH4+-N, and TN concentrations were 35.0, 1.2, and 13.3 mg L-1, respectively. The primary pathways for pollutant removal were identified as simultaneous nitrification, denitrification, and phosphorus removal within the microaerobic tanks. The enrichment of denitrifying phosphorus-accumulating organisms, including Hydrogenophaga, Accumulibacter, Azospira, Dechloromonas, and Pseudomonas, provides an essential microbial foundation. Furthermore, computational fluid dynamics modeling revealed that the incorporation of internal separators in aerobic tanks induced shifts in the flow pattern, transitioning from a single-circulation cell to multiple vortical cells. This alteration amplified the local velocity gradients, generating the required shear forces to drive granulation. Moreover, mass balance analysis revealed that the microaerobic and aerobic tanks operated under feast and famine conditions, respectively, creating a microbial selection pressure that favored granulation. This process eliminates the need for external clarifiers, resulting in a footprint reduction of 38.2 % and one-third energy savings for sludge reflux. This study offers valuable insights into the application of continuous-flow AGS to upgrade existing activated sludge systems with limited retrofitting requirements.

Enhanced treatment of sludge drying condensate by A/O-MBR process: Microbial activity and community structure

With the rapid increase of sludge production from sewage treatment plants, the treatment of sludge drying condensate rich in a large amount of pollutants urgently needs to be addressed. Due to the unique characteristics of sludge drying condensate (high ammonia nitrogen and COD concentration), there are almost no reports on biological treatment methods specifically targeting sludge drying condensate. In this study, A/O-MBR process was proposed for sludge drying condensate treatment and the effects of ammonia nitrogen loads, alkalinity and aeration intensity were explored. Experimental results show that under the ammonia nitrogen load of 0.35 kg NH4+-N/(m3·d) and the aeration intensity of 0.5 m3/(m2·min), the removal rate of COD and NH4+-N could reach 94% and 99.86% with the addition of alkalinity (m(NaHCO3): m(NH4+-N) = 7:1), respectively. The distribution of living and dead microbial cells in the activated sludge of three reactors also proved that the supplement of alkalinity in the influent can ensure the feasible living conditions for microorganisms. In addition to traditional nitrifying bacteria, through the supplementation of alkalinity and the reduction of aeration intensity, the system had also domesticated high abundance heterogeneous nitrification aerobic denitrification (HN-AD) and aerobic denitrification bacteria (both more than 10% of the total bacterial count). The denitrification process of sludge drying condensate was simplified and the denitrification efficiency was greatly improved. The findings of this study could provide important theoretical guidance for the biological treatment process of sludge drying condensate.

Embedded Graphite and Carbon Nanofibers in a Polyurethane Matrix Used as Anodes in Microbial Fuel Cells for Wastewater Treatment

Composites of polyurethane and graphite and polyurethane and carbon nanofibers (PU/Graphite 0.5% and PU/CNF 1%) were synthesized and used as anodes in dual-compartment microbial fuel cells (MFCs) for municipal wastewater treatment; electrical energy generation and organic matter removal were assessed. The maximum power density, coulombic efficiency and chemical oxygen demand (COD) removal efficiency in the MFCs packed with the PU/Graphite 0.5% and PU/CNF 1% composites were 232.32 mW/m3 and 90.78 mW/m3, 5.87 and 4.41%, and 51.38 and 68.62%, respectively. In addition, the internal resistance of the MFCs with the best bioelectrochemical performance (PU/Graphite 0.5%) was 1051.11 Ω. The results obtained in this study demonstrate the feasibility of using these types of materials in dual-compartment MFCs for wastewater treatment with electric power generation.

Combination of partial nitrification and microbial fuel cell for simultaneous ammonia reduction, organic removal, and energy recovery

The chemical oxygen demand (COD) in municipal wastewater has become an obstacle for anammox in mainstream applications. In this study, the single chamber microbial fuel cell (MFC) was installed as an influent device for a partial nitrification-sequencing batch reactor (PN-SBR) to realize integrating COD removal and partial nitrification. After 80 days of operation, the nitrite accumulation rate reached 93%, while the COD removal efficiency was 56%. The output voltage and the power density of MFC were 66.62 mV and 2.40 W/m3, respectively. The content of EPS, especially polysaccharides in the stable phase, has increased compared with the seed sludge. The most dominant genus in MFC anode biofilm and SBR granular sludge was Thauera, which has organic compounds degradation capacity and could degrade nitrate. This study revealed the microbial interaction between MFC and partial nitrification and provided a new strategy for stable ammonia and nitrite supply for mainstream anammox plants.

Insight into the roles of microalgae on simultaneous nitrification and denitrification in microalgal-bacterial sequencing batch reactors: Nitrogen removal, extracellular polymeric substances, and microbial communities

This study explored the influence and mechanism of microalgae on simultaneous nitrification and denitrification (SND) in microalgal-bacterial sequencing batch reactors (MB-SBR). It particularly focused on nitrogen transformation in extracellular polymeric substances (EPS) and functional groups associated with nitrogen removal. The results showed that MB-SBR achieved more optimal performance than control, with an SND efficiency of 68.01% and total nitrogen removal efficiency of 66.74%. Further analyses revealed that microalgae changed compositions and properties of EPS by increasing EPS contents and improving transfer, conversion, and storage capacity of nitrogen in EPS. Microbial community analysis demonstrated that microalgae promoted the enrichment of functional groups and genes related to SND and introduced diverse nitrogen removal pathways. Moreover, co-occurrence network analysis elucidated the interactions between communities of bacteria and microalgae and the promotion of SND by microalgae as keystone connectors in the MB-SBR. This study provides insights into the roles of microalgae for enhanced SND.

Conductive carrier promotes synchronous biofilm formation and granulation of anammox bacteria

The extracellular electron transfer capability of some anaerobic ammonium oxidation (anammox) bacteria was confirmed in recent years. However, the effect of conductive carriers on the synchronous formation of anammox biofilm and granules is rarely reported. Anammox biofilm and granules with compact and stable structures accelerate the initiation and enhance the stability of the anammox process. In this study, we found that the conductive carbon fiber brush (CB) carrier promoted synchronous biofilm formation and granulation of anammox bacteria in the internal circulation immobilized blanket (ICIB) reactor. Compared with polyurethane sponge and zeolite carrier, the ICIB reactor packed with CB carrier can be operated under the highest total nitrogen loading rate of 6.53 kg-N/(m³·d) and maintain the effluents NH₄⁺-N and NO₂^--N at less than 1 mM. The volatile suspended solids concentration in the ICIB reactor packed with conductive carrier increased from 5.17 ± 0.40 g/L of inoculum sludge to 24.24 ± 1.20 g/L of biofilm, and the average particle size of granules increased from 222.09 µm to 879.80 µm in 150 days. Fluorescence in situ hybridization analysis showed that anammox bacteria prevailed in the biofilm and granules. The analysis of extracellular polymeric substances indicated that protein and humic acid-like substances played an important role in the formation of anammox biofilm and granules. Microbiome analysis showed that the relative abundance of Candidatus Jettenia was increased from 0.18% to 38.15% in the biofilm from CB carrier during start-up stage. This study provides a strategy for rapid anammox biofilm and granules enrichment and carrier selection of anammox process.

Aerobic electrotrophic denitrification coupled with biologically induced phosphate precipitation for nitrogen and phosphorus removal from high-salinity wastewater: Performance, mechanism, and microbial community

Electrotrophic denitrification (ED) is a promising nitrogen removal technique; however, the potential of ED coupled with biologically induced phosphate precipitation (BIPP) has not been fully explored. In this study, the performances, mechanisms, and microbial communities of the coupled system were investigated. The results showed that excellent nitrogen and phosphorus removal (both exceeding 92 %) was achieved in the salinity range of 20-60 g/L. ED contributed to approximately 83.4 % of nitrogen removal. BIPP removed approximately 63.5 % of the phosphorus. Batch activity tests confirmed that aerobic/anoxic bio-electrochemical and autotrophic/heterotrophic denitrification worked together for nitrate removal. Sulfate reduction had a negative impact on denitrification. Moreover, phosphorus removal was controlled by ED and calcium ions. The alkaline solution environment created by denitrification may greatly promote the formation of hydroxyapatite. Microbial community analyses indicated that the key bacteria involved in aerobic ED was Arcobacter. These findings will aid in the advanced treatment of high-salinity wastewater.

Hypoxia triggers the proliferation of antibiotic resistance genes in a marine aquaculture system

The transmission of antibiotic resistance genes (ARGs) affects the safety of aquaculture animals. Dissolved oxygen (DO) can affect the transmission of ARGs, but its mechanism of action in this process is unclear. We conducted laboratory breeding experiment with low and control DO groups. Combined quantitative PCR and 16S rRNA sequencing to study the effect of DO on the spread of ARGs. Hypoxia treatment significantly increased the accumulation of ammonium and nitrite in aquaculture water, and it increased the relative abundances of ARGs and mobile genetic elements (MGEs), especially the ARGs resistant to drugs in the categories of sulfonamide, (flor)/(chlor)/(am)phenicol, and MLSB (macrolide, lincosamide and streptogramin B) and the MGE intI-1(clinic), by 2.39-95.69 % in 28 days relative to the control DO treatment. Though the abundance of ARG carries, especially the Rhodocyclaceae, Caldilineaceae, Cyclobacteriaceae, Saprospiraceae, Enterobacteriaceae, Sphingomonadaceae families, showed higher abundance in low DO groups, relating to the vertical transmission of ARGs. Hypoxia treatment is more likely to promote the horizontal gene transfer (HGT)-related pathways, including ABC transporters, two component system, and quorum sensing, thus to induce the HGT of ARGs. The changed bacterial proliferation also altered the abundance of MGEs, especially intI-1(clinic), which induced HGT of ARGs as well. Additionally, pearson correlation results revealed that the succession of bacterial community function played the strongest role in ARG proliferation, followed by bacterial community structure and MGEs. Our results highlight the importance of suitable DO concentration in controlling the spread of ARGs especially the HGT of ARGs. In the context of global attention to food safety, our results provide important information for ensuring the safety of aquatic products and the sustainable development of aquaculture.

Long-term performance, microbial evolution and spatial microstructural characteristics of anammox granules in an upflow blanket filter (UBF) treating high-strength nitrogen wastewater

Granule formation, microstructure and microbial spatial distribution are crucial to granule stability and nitrogen removal. Here, an upflow blanket filter (UBF) reactor with porous fixed cylinder carriers was fabricated and operated for 234 days to investigate overall performance and the formation mechanism of anammox granules. Results showed that the UBF performed the highest nitrogen removal efficiency of 93.19 ± 3.39% under nitrogen loading rate of 3.6 kg-N/m³/d and HRT of 2 h. The tryptophan-like proteins as the key component in EPS were vital for granules formation. Further 16 s rRNA analysis indicated that SBR1031 with a relative abundance of 40.5% played an important role in cell aggregation. Thus, anammox granules were developed successfully with a two-layered spatial structure where outer-layer was ammonia oxidizing bacteria and inner-core was anaerobic ammonia oxidizing bacteria. Together, introduction of porous fixed cylinder carriers is a valid method to avoid biomass loss and floatation.

Recent progress in microbial fuel cells using substrates from diverse sources

Increasing untreated environmental outputs from industry and the rising human population have increased the burden of wastewater and other waste streams on the environment. The most prevalent wastewater treatment methods include the activated sludge process, which requires aeration and is, therefore, energy and cost-intensive. The current trend towards a circular economy facilitates the recovery of waste materials as a resource. Along with the amount, the complexity of wastewater is increasing day by day. Therefore, wastewater treatment processes must be transformed into cost-effective and sustainable methods. Microbial fuel cells (MFCs) use electroactive microbes to extract chemical energy from waste organic molecules to generate electricity via waste treatment. This review focuses use of MFCs as an energy converter using wastewater from various sources. The different substrate sources that are evaluated include industrial, agricultural, domestic, and pharmaceutical types. The article also highlights the effect of operational parameters such as organic load, pH, current, and concentration on the MFC output. The article also covers MFC functioning with respect to the substrate, and the associated performance parameters, such as power generation and wastewater treatment matrices, are given. The review also illustrates the success stories of various MFC configurations. We emphasize the significant measures required to fill in the gaps related to the effect of substrate type on different MFC configurations, identification of microbes for use as biocatalysts, and development of biocathodes for the further improvement of the system. Finally, we shortlisted the best performing substrates based on the maximum current and power, Coulombic efficiency, and chemical oxygen demand removal upon the treatment of substrates in MFCs. This information will guide industries that wish to use MFC technology to treat generated effluent from various processes.

Recovery of anammox process performance after substrate inhibition: Reactor performance, sludge morphology, and microbial community

Most of the current studies have focused on the inhibition of anaerobic ammonium oxidation (anammox) by substrates, however, little attention has been paid to the recovery process of the reactor after inhibition. Therefore, we investigated the changes in reactor performance, granular sludge structure, and microbial community during the recovery phase after being inhibited by a high nitrogen load for 15 d. The nitrogen removal rate of the reactorwasrestored to pre-inhibition levels after 75 d of recovery, and the stoichiometric ratio converged to the theoretical value. The surface of the granular sludge developed into a broccoli-like structure, and the Ca and P contents of the granules increased from 6.88% and 4.39% to 24.42% and 13.88%, respectively. The abundance of the anammox bacterium Candidatus brocadia increased from 5.86% to 12.10%, and network analysis indicated that SMA102 and SBR1031 were positively correlated with the occurrence of Candidatus brocadia.

Membrane penetration of nitrogen and its effects on nitrogen removal in dual-chambered microbial fuel cells

Owing to membrane penetration, a novel route of nitrogen removal was proposed in a dual-chamber microbial fuel cell with a proton exchange membrane (PEM). The results showed that NH₄⁺-N rapidly migrated across PEM with a mass transfer coefficient (K_A) of 1.79 ± 0.51 × 10^-4 cm s^-1, 50% of which was oxidized to NO₃^--N in the cathode chamber, then the remainder being eliminated by short-cut nitrification/denitrification. Meanwhile, NO₃^--N went across the PEM again with a low K_A of 5.50 ± 0.24 × 10^-6 cm s^-1, and was subsequently reduced via anodic denitrification. In the anode, the functional microorganisms were divided into exoelectrogenic bacteria (46.2%) and denitrifying bacteria (37.3%), while the dominated bacteria were mainly affiliated with nitrifying bacteria (19.6%) and aerobic denitrifying bacteria (52.9%) in the cathode. These findings provide a new insight into nitrogen removal during bioelectrochemical treatment of actual wastewater.

Simultaneous removal of NOX and SO2 from flue gas in an integrated FGD-CABR system by sulfur cycling-mediated Fe(II)EDTA regeneration

Chemical absorption-biological reduction (CABR) process is an attractive method for NO_X removal and Fe(II)EDTA regeneration is important to sustain high NO_X removal. In this study a sustainable and eco-friendly sulfur cycling-mediated Fe(II)EDTA regeneration method was incorporated in the integrated biological flue gas desulfurization (FGD)-CABR system. Here, we investigated the NO_X and SO₂ removal efficiency of the system under three different flue gas flows (100 mL/min, 500 mL/min, and 1000 mL/min) and evaluated the feasibility of chemical Fe(III)EDTA reduction by sulfide in series of batch tests. Our results showed that complete SO₂ removal was achieved at all the tested scenarios with sulfide, thiosulfate and S⁰ accumulation in the solution. Meanwhile, the total removal efficiency of NO_X achieved ∼100% in the system, of which 3.2%-23.3% was removed in spray scrubber and 76.7%-96.5% in EGSB reactor along with no N₂O emission. The optimal pH and S^2-/Fe(III)EDTA for Fe(II)EDTA regeneration and S⁰ recovery was 8.0 and 1:2. The microbial community analysis results showed that the cooperation of heterotrophic denitrifier (Saprospiraceae_uncultured and Dechloromonas) and iron-reducing bacteria (Klebsiella and Petrimonas) in EGSB reactor and sulfide-oxidizing, nitrate-reducing bacteria (Azoarcus and Pseudarcobacter) in spray scrubber contributed to the efficient removal of NO_X in flue gas.

Advanced bioelectrochemical system for nitrogen removal in wastewater

Nitrogen (N) pollution in water has become a serious issue that cannot be ignored due to the harm posed by excessive nitrogen to environmental safety and human health; as such, N concentrations in water are strictly limited. The bioelectrochemical system (BES) is a new method to remove excessive N from water, and has attracted considerable attention. Compared with other methods, it is highly efficient and has low energy consumption. However, the BES has not been applied for N removal in practice due to lack of in-depth research on the mechanism and construction of high-performance electrodes, separators, and reactor configurations; this highlights a need to review and examine the efforts in this field. This paper provides a comprehensive review on the current BES research for N removal focusing on the reaction principles, reactor configurations, electrodes and separators, and treatment of actual wastewater; the corresponding performances in these realms are also discussed. Finally, the prospects for N removal in water using the BES are presented.

Food waste composting based on patented compost bins: Carbon dioxide and nitrous oxide emissions and the denitrifying community analysis

Mature compost and rice bran were used as bulking agents to perform Food Waste Rapid Composting (FWRC) in a patented composting bin. The characteristics of CO₂ and N₂O emission and the denitrifying community were investigated. The release of CO₂ and N₂O concentrated in the early composting stage and reduced greatly after 28 h, and the N₂O emission peak of the treatment with mature compost was 8.5 times higher than that of rice bran. The high N₂O generation resulted from massive denitrifying bacteria and NO_x^--N in the composting material. The relative abundances of denitrifiers, correspondingly genes of narG and nirK were much higher in the treatment with mature compost, which contributed to the N₂O emission. Moreover, the correlation matrices revealed that N₂O fluxes correlated well with moisture, pH, temperature, and the abundances of nirK and nosZ genes during FWRC.

Conversion of nitrogen and carbon in enriched paddy soil by denitrification coupled with anammox in a bioelectrochemical system

The aim of this study is to investigate conversion of nitrogen and COD in enriched paddy soil by nitrification coupled with anammox process in a dual chamber bioelectrochemical system. The paddy soil was enriched for denitrification coupled with anammox by microbial consortia and was acclimatized in the cathodic chamber of microbial fuel cells (MFCs). The bioelectrochemical systems were treated with different ammonium concentrations in the cathodic chamber: the MFC with low concentration ammonium (LA-MFC, 50 mg/L ammonium), the MFC with medium concentration ammonium (MA-MFC, 500 mg/L ammonium), and MFC with high concentration ammonium (HA-MFC, 1000 mg/L ammonium), and the initial COD in the anodic chamber was 1200 mg/L. The CK treatments were conducted with 1000 mg/L ammonium under the same conditions, except without inoculum in the cathode chamber. The consumption rate of ammonium in the cathodic chambers of CK, LA-MFC, MA-MFC, and HA-MFC were 9%, 64%, 84%, and 84%, respectively. The degradation rate for COD achieved in the anode chambers of CK, LA-MFC, MA-MFC, and HA-MFC were 70%, 86%, 93%, and 93%, respectively. The analysis of the microbial community of three treated MFCs in the cathode chamber indicated that the nitrification-denitrification process occurs in the cathode chamber. The dominant species for nitrification was Nitrospira, and the dominant species for denitrification were Denitratisoma, Dechloromonas, and Candidatus_Competibacter. Moreover, anammox process also observed in the cathode chamber. The functional genes nirS/K, hzsB, and 16S rDNA were assessed by qPCR analysis, and the results confirmed the presence of denitrification-coupled anammox in the cathodic chamber.

Effects of salinity shock on simultaneous nitrification and denitrification by a membrane bioreactor: Performance, sludge activity, and functional microflora

Physical and chemical treatments of Tungsten smelting wastewater, with high salt content and low C/N ratio, are often tedious. As a solution, this study suggested a simultaneous nitrification and denitrification membrane bioreactor (SND-MBR) for salinity gradient domestication. During the salinity acclimation period, we observed 20% and 11% removal of NH₄⁺-N and Chemical Oxygen Demand (COD), respectively. However, the SND efficiency reached 95.55% after stable operation at 3.0% salinity. Through stoichiometric and kinetic analyses, we confirmed that increased salinity significantly inhibited electron transport system activity, nitrification, and denitrification, evidenced by the extremely low ammonia monooxygenase and nitrite reductase activities. Further high-throughput sequencing showed that Nitrosomonas dominated the functional microbial flora succession and denitrification in high salinity environments. In comparison with a control, the Kyoto Encyclopedia of Genes and Genomes analysis showed that wastewater salinity weakened the functional gene level of MBR microbial flora, and the enzyme key to the assimilation nitrate reduction changed from nitrate reductase to assimilation nitrate reductase.

Metagenomics reveals functional species and microbial mechanisms of an enriched thiosulfate-driven denitratation consortia

In this study, thiosulfate-driven denitratation (TDD) system was successfully established under optimal S/N molar ratio of 1.00, with nitrite accumulation efficiency (NAE) of 82.24 ± 17.09%. This work highlighted that thiosulfate significantly preferred the reduction of nitrate than nitrite. However, after the depletion of thiosulfate, the in-situ formed intermediate product element sulfur (S⁰) served as the main electron donor, and significantly favored the reduction of nitrite than nitrate, which constrained nitrite accumulation and nitrate removal. In addition, metagenomic sequencing revealed that the functional denitratation species might be Thiobacillus_sp._65-29, but the occurrence of Nir-annotated species would decrease nitrite accumulation. Under S/N ratio of 1.00, the decreased abundant Nir-annotated species (e.g., Thiobacillus_sp.), as well as the down-regulated quorum sensing interactions between Nar- and Nir-annotated species were key microbial metabolisms of high NAE in the TDD system. Overall, this work provides new sight into the metagenome-base functional species and metabolic potential of thiosulfate-driven denitratation.

Performance and microbial community dynamics relationship within a step-feed anoxic/oxic/anoxic/oxic process (SF-A/O/A/O) for coking wastewater treatment

A step-feed anoxic/oxic/anoxic/oxic (SF-A/O/A/O) was developed and successfully applied to full-scale coking wastewater treatment. The performance and microbial community were evaluated and systematically compared with the anoxic/oxic/oxic (A/O/O) process. SF-A/OA/O process exhibited efficient removal of COD, NH₄⁺-N, TN, phenols, and cyanide with corresponding average effluent concentrations of 317.9, 1.8, 46.2, 1.1, and 0.2 mg·L^-1, respectively. In particular, the TN removal efficiency of A/O/O process was only 7.8%, with an effluent concentration of 300.6 mg·L^-1. Furthermore, polycyclic aromatic hydrocarbons with high molecular weight were the dominant compounds in raw coking wastewater, which were degraded to a greater extent in SF-A/OA/O. The abundance in Thiobacillus, SM1A02, and Thauera could be the main reason why SF-A/O/A/O was superior to A/O/O in treating TN. The microbial community structure of SF-A/O/A/O was similar among stages in system (P ≥ 0.05, Welch's t-test) and was less affected by environmental factors, which may have been one of the important factors in the system's strong stability.

Carbon Source Applied in Enrichment Stage of Mixed Microbial Cultures Limits the Substrate Adaptability for PHA Fermentation Using the Renewable Carbon

Suitability of different substrates for enriched mixed microbial cultures (MMCs) is of importance to the polyhydroxyalkanoate (PHA) fermentation using renewable carbon. In this study, three enriched MMCs were evaluated for their fermentation features and kinetics with different carbon sources (sodium acetate, glucose, or starch). The results showed that the highly specific bacterial community composition was developed depending on the applied carbon source. Correspondence analysis suggested that the genus affiliated in Gammaproteobacteria_unclassified was related to 3-hydroxybutyrate (HB) synthesis in acetate-fed MMC (relative abundance of 38%) and glucose-fed MMC (relative abundance of 76.7%), whereas Vibrio genus was related to 3-hydroxyvalerate (HV) production in glucose-fed MMC (relative abundance of 0.4%) and starch-fed MMC (relative abundance of 94.6%). The acetate-fed MMC could not use glucose and starch as fermentation carbon sources, showing the limitation of microbial species developed with the specific metabolic substrate. Glucose-fed MMC produced the highest PHA cell content of 64.2% cell dry weight when using sodium acetate as the fermentation carbon. Glucose-fed MMC showed wide resilience and adaptation to various carbon sources. When actual landfill leachate was used for fermentation by glucose-fed MMC, maximum PHA cell content of 45.5% cell dry weight and the PHA volumetric productivity of 0.265 g PHA/(L·h) were obtained. This study suggested carbon sources applied in the MMC enrichment stage had a significant influence on utilization of carbon in the fermentation stage.

Performance and mechanism of a novel woodchip embedded biofilm electrochemical reactor (WBER) for nitrate-contaminated wastewater treatment

In this study, a woodchip biofilm electrode reactor (WBER) with woodchips embedded anode and cathode was developed, and its denitrification mechanism was analyzed by investigating the denitrification performance, organic matter change, redox environment and microbial community. The results show that the WBER with a carbon rod as anode (C-WBER) had a higher denitrification efficiency (2.58 mg NO- 3-N/(L·h)) and lower energy consumption (0.012 kWh/g NO- 3-N) at 350 mA/m². By reducing the hydroxyl radical and dissolved oxygen concentrations, anode embedding technology effectively decreased the inhibition on microorganisms. Lignin decomposition, nitrification and aerobic denitrification were carried out in anode. Additionally, hydrogen autotrophic denitrification and heterotrophic denitrification were occurred in cathode. The WBER effectively removed nitrate and reduced the cost, providing a theoretical basis and direction for further develop BERs.

Effect of modified anode on bioenergy harvesting and nutrients removal in a microbial nutrient recovery cell

The microbial nutrient recovery cell i.e. modified microbial fuel cell containing a middle recovery chamber can be used to purify wastewater and remove valuable nutrients, while simultaneously generating electricity. The study investigated nutrient removal and microorganism interactions with carbon (CB- HT and CB- APTES) and stainless steel (SSB-HT) modified anodes used in microbial nutrient recovery cells. The removal efficiencies of ammonium ions were found higher in carbon-based CB-APTES (~98%) and CB-HT (~98.27%) systems in comparison to SSB-HT (~87.16%) system. On comparing further, the removal efficiencies of chemical oxygen demand (~99.5%) and total phosphorus (~99%) in CB- APTES system were superior to the cases of CB- HT, and SSB- HT systems. Besides, the CB-APTES based microbial fuel cell (MFC) displayed an average stable voltage of 0.5 V and a maximum power density of ~ 850 mW/m².

Valorisation and emerging perspective of biomass based waste-to-energy technologies and their socio-environmental impact: A review

The economic developments around the globe resulted in the increased demand of energy, which overburdened the supply chain sources of energy. Fossil fuel reserves are exploited to meet the high demand of energy and their combustion is becoming the main source of environmental pollution. So there is dire need to find safe, renewable and sustainable energy resources. Waste to energy (WtE) may be viewed as a possible alternate source of energy, which is economically and environmentally sustainable. Municipal solid waste (MSW) is a major contributor to the development of renewable energy and sustainable environment. At present the scarcity of renewable energy resources and disposal of MSW is a challenging problem for the developing countries, which has generated a wide ranging socioeconomic and environmental problems. This situation stimulates the researchers to develop alternatives for converting WtE under a variety of scenarios. Herein, the present scenario in developing the WtE technologies such as, thermal conversion methods (Incineration, Gasification, Pyrolysis, Torrefaction), Plasma technology, Biochemical methods, Chemical and Mechanical methods, Bio-electrochemical process, Mechanical biological treatment (MBT), Photo-biological processes for efficacious energy recovery and the challenges confronted by developing and developed countries. In this review, a framework for the evaluation of WtE technologies has been presented for the ease of researchers working in the field. Furthermore, this review concluded that WtE is a potential renewable energy source that will partially satisfy the demand for energy and ensure an efficient MSW management to overcome the environmental pollution.

Porous graphitic carbon from mangosteen peel as efficient electrocatalyst in microbial fuel cells

In this study, a low-cost and efficient strategy to synthesize nitrogen self-doped porous graphitic carbon was proposed by using mangosteen peel as both the carbon and nitrogen source, combined with molten KOH activation and Co²⁺ catalytic graphitization. The mangosteen peel carbon catalyst prepared at 800 °C (referred to as MPC-800) possessed a large specific surface area (1168 m²/g), appropriate porous structure, high graphitization degree, and high pyridinic and graphitic nitrogen content. Further, electrochemical measurements indicated that the MPC-800 catalyst showed good oxygen reduction reaction activity. Moreover, MPC-800 as cathode catalyst displays an onset potential of 0.150 V (vs. Ag/AgCl) and half-wave potential of -0.091 V (vs. Ag/AgCl) in neutral medium, which is more positive than commercial Pt/C (0.121 V and -0.113 V, respectively). The maximum power density of microbial fuel cells using MPC-800 was 240 mW/m², which was slightly superior to that of the Pt/C cathode (220 mW/m²). This work proposed a novel method, based on the low cost and wide availability of waste mangosteen peel, to synthesize an excellent oxygen reduction reaction catalyst for microbial fuel cells.

Substrate salinity: A critical factor regulating the performance of microbial fuel cells, a review

Substrate salinity is a critical factor influencing microbial fuel cells (MFCs) performance and various studies have suggested that increasing substrate salinity first improves MFC performance. However, a further increase in salinity that exceeds the salinity tolerance of exoelectrogens shows negative effects because of inhibited bacterial activity and increased activation losses. In this review, electricity generation and contaminant removal from saline substrates using MFCs are summarized, and results show different optimal salinities for obtaining maximum performance. Then, electroactive bacteria capable of tolerating saline environments and strategies for improving salinity tolerance are discussed. In addition to ohmic resistance and bacterial activity, membrane resistance and catalyst performance will also be affected by substrate salinity, all of which jointly contribute the final overall MFC performance. Therefore, the combined effect of salinity is analyzed to illustrate how the MFC performance changes with increasing salinity. Finally, the challenges and perspectives of MFCs operated in saline environments are discussed.

Iron-carbon could enhance nitrogen removal in Sesuvium portulacastrum constructed wetlands for treating mariculture effluents

This study investigated an Iron-carbon (Fe-C) micro-electrolysis method to enhance nitrogen removal of Sesuvium portulacastrum constructed wetlands (CWs) when treating mariculture effluents. The main objective was to investigate the effects of Fe-C on nitrogen purification performance and microbial characteristics of Sesuvium portulacastrum CWs. Results showed that the presence of Fe-C and Sesuvium portulacastrum could improve nitrogen removal efficiency by 20-30% and 15-30%, respectively. CWs with 33% v/v Fe-C addition performed well on nitrogen removal: TAN, 41.49 ± 13.64%; NO₂^--N, 13.32%; NO₃^--N, 60.02 ± 6.17%; TIN, 63.40 ± 12.11%. Microbial analysis revealed that Fe-C altered the microbial communities, and improved the abundance of denitrification related genera. Based on microbial enzyme activities and genes abundance, the anammox and denitrification processes were promoted by Fe-C in CWs. These findings indicate that Sesuvium portulacastrum CWs with 33% v/v Fe-C represents an effective nitrogen removal for mariculture wastewater with insufficient carbon source.

Long-Term Fertilization Shapes the Putative Electrotrophic Microbial Community in Paddy Soils Revealed by Microbial Electrosynthesis Systems

Electrotrophs play an important role in biogeochemical cycles, but the effects of long-term fertilization on electrotrophic communities in paddy soils remain unclear. Here, we explored the responses of electrotrophic communities in paddy soil-based microcosms to different long-term fertilization practices using microbial electrosynthesis systems (MESs), high-throughput quantitative PCR, and 16s rRNA gene-based Illumina sequencing techniques. Compared to the case in the unfertilized soil (CK), applications of only manure (M); only chemical nitrogen, phosphorous, and potassium fertilizers (NPK); and M plus NPK (MNPK) clearly changed the electrotrophic bacterial community structure. The Streptomyces genus of the Actinobacteria phylum was the dominant electrotroph in the CK, M, and MNPK soils. The latter two soils also favored Truepera of Deinococcus-Thermus or Arenimonas and Thioalkalispira of Proteobacteria. Furthermore, Pseudomonas of Proteobacteria and Bacillus of Firmicutes were major electrotrophs in the NPK soil. These electrotrophs consumed biocathodic currents coupled with nitrate reduction and recovered 18-38% of electrons via dissimilatory nitrate reduction to ammonium (DNRA). The increased abundances of the nrfA gene for DNRA induced by electrical potential further supported that the electrotrophs enhanced DNRA for all soils. These expand our knowledge about the diversity of electrotrophs and their roles in N cycle in paddy soils and highlight the importance of fertilization in shaping electrotrophic communities.

Nitrogen removal and microbial community diversity in single-chamber electroactive biofilm reactors with different ratios of the cathode surface area to reactor volume

Removal of nitrogen compounds is particularly important domestic wastewater treatment. Our recent study reported the successful removal of nitrogen in single-chamber electroactive biofilm reactors (EBRs) under aeration-free conditions. We hypothesized that the oxygen diffused from the air-cathode is a key factor in the removal of nitrogen in the EBR. If so, the effect of the penetrated oxygen would vary according to the ratio of the air-cathode surface area to the reactor volume (AV ratio) and the hydraulic retention time (HRT). In this study, single-chamber EBRs with three different AV ratios: 125 m²/m³ (EBR-125), 250 m²/m³ (EBR-250), and 500 m²/m³ (EBR-500) were evaluated for the removal of nitrogen under different HRTs of 0.5-6 h. The higher the AV ratio, the greater the increase in nitrification. The total nitrogen (TN) removal efficiency of EBR-125 and EBR-250 decreased as the HRT decreased, while that of EBR-500 increased. EBR-250 showed the highest TN removal (62.0%) with well-balanced nitrification (83.9%) and denitrification (75.1%) at an HRT of 6 h. However, EBR-500 appeared to be superior for practical application because it showed a comparable TN removal (59%) at a substantially short HRT of 1 h. The microbial communities that were involved in the nitrogen cycle varied according to whether the biofilms were located on the anodes, separators, and cathodes but were similar among EBRs with different AV ratios. Nitrifying bacteria were detected in the biofilms that were presented on the cathodes (approximately 7.8% of the total phylotypes), while denitrifying bacteria were mainly found in biofilm that were located on the anodes (approximately 23.3%). Anammox bacteria were also detected on the anode (approximately 3.7%) and in the separator biofilms (approximately 1.9%) of all the EBRs. These results suggest that both the A/V ratio and the HRT could affect the counter diffusion of substrates (NH₄⁺ and organic compounds) and oxygen in the biofilms and allow interactions between a diversity of microorganisms for the successful removal of nitrogen in EBRs. These findings are expected to aid in the development of new applications using EBR for energy-saving wastewater treatment.

Advanced microbial fuel cell for waste water treatment-a review

Petroleum, coal, and natural gas reservoir were depleting continuously due to an increase in industrialization, which enforced study to identify alternative sources. The next option is the renewable resources which are most important for energy purpose coupled with environmental problem reduction. Microbial fuel cells (MFCs) have become a promising approach to generate cleaner and more sustainable electrical energy. The involvement of various disciplines had been contributing to enhancing the performance of the MFCs. This review covers the performance of MFC along with different wastewater as a substrate in terms of treatment efficiencies as well as for energy generation. Apart from this, effect of various parameters and use of different nanomaterials for performance of MFC were also studied. From the current study, it proves that the use of microbial fuel cell along with the use of nanomaterials could be the waste and energy-related problem-solving approach. MFC could be better in performances based on optimized process parameters for handling any wastewater from industrial process.

Long-term stable and energy-neutral mixed biofilm electrode for complete nitrogen removal from high-salinity wastewater: Mechanism and microbial community

The steady mixed biofilm electrode (MBE) was investigated for the removal of nitrogen from mustard tuber wastewater. Results showed that complete nitrogen removal occurred over a wide initial chemical oxygen demand (COD)/total nitrogen (TN) ratio ranging from 2.8 to 9.8 using MBE. MBE revealed broad-spectrum applicability for the treatment of high-salinity wastewater containing different forms of nitrogen. Bio-electrochemical process, in-situ heterotrophic nitrogen reduction, ammonia stripping, nitrogen assimilation, and endogenous denitrification coexisted for the removal of nitrogen. Batch activity tests and functional microorganism analysis confirmed that autotrophic/heterotrophic nitrification, anoxic/aerobic denitrification, and nitrogen bio-electrochemical reduction cooperated to achieve efficient nitrogen conversion. More importantly, the analysis of the preliminary energy balance demonstrated that MBE was self-sustaining. The long-term operation stability of MBE was of great importance for its practical application. The results provided herein offer new insights into bioelectrochemical nitrogen removal and resource treatment of high-salinity wastewater.

Performance evaluation of microbial fuel cell for landfill leachate treatment: Research updates and synergistic effects of hybrid systems

Over half of century, sanitary landfill was and is still the most economical treatment strategy for solid waste disposal, but the environmental risks associated with the leachate have brought attention of scientists for its proper treatment to avoid surface and ground water deterioration. Most of the treatment technologies are energy-negative and cost intensive processes, which are unable to meet current environmental regulations. There are continuous demands of alternatives concomitant with positive energy and high effluent quality. Microbial fuel cells (MFCs) were launched in the last two decades as a potential treatment technology with bioelectricity generation accompanied with simultaneous carbon and nutrient removal. This study reviews capability and mechanisms of carbon, nitrogen and phosphorous removal from landfill leachate through MFC technology, as well as summarizes and discusses the recent advances of standalone and hybrid MFCs performances in landfill leachate (LFL) treatment. Recent improvements and synergetic effect of hybrid MFC technology upon the increasing of power densities, organic and nutrient removal, and future challenges were discussed in details.

Carbamazepine removal from wastewater and the degradation mechanism in a submerged forward osmotic membrane bioreactor

A submerged forward osmotic membrane bioreactor (FOMBR) was used to reveal the removal and degradation mechanism of carbamazepine (CBZ) from wastewater. The results showed that the removal mechanism consisted of the rejection of the forward osmotic (FO) membrane and biodegradation of the activated sludge. The removal efficiencies of COD, NH₄⁺-N, and CBZ by the FOMBR were approximately 94.77%-97.45%, 93.56%-99.28%, and 88.20%-94.45%, respectively. Moreover, the COD and NH₄⁺-N removal efficiencies were positively correlated with the increased CBZ concentrations. The results of the soluble microbial products (SMP) and extracellular polymeric substances (EPS) tests illustrated that the membrane fouling potential of EPS may be higher than that of SMP. According to the identified 14 degradation products, oxidation, hydroxylation, and decarboxylation were defined as the primary CBZ degradation mechanism. In addition, the RNA results showed that Delftia could be the characteristic bacteria in the CBZ degradation process.

Electricity Generation, Salt and Nitrogen Removal and Microbial Community in Aircathode Microbial Desalination Cell for Saline-Alkaline Soil-Washing Water Treatment

An aircathode microbial desalination cell (AMDC) was successfully started by inoculating anaerobic sludge into the anode of a microbial desalination cell and then used to study the effects of salinity on performance of AMDC and effect of treatment of coastal saline-alkaline soil-washing water. The results showed that the desalination cycle and rate gradually shorten, but salt removal gradually increased when the salinity was decreased, and the highest salt removal was 98.00 ± 0.12% at a salinity of 5 g/L. COD removal efficiency was increased with the extension of operation cycle and largest removal efficiency difference was not significant, but the average coulomb efficiency had significant differences under the condition of each salinity. This indicates that salinity conditions have significant influence on salt removal and coulomb efficiency under the combined action of osmotic pressure, electric field action, running time and microbial activity, etc. On the contrary, COD removal effect has no significant differences under the condition of inoculation of the same substrate in the anode chamber. The salt removal reached 99.13 ± 2.1% when the AMDC experiment ended under the condition of washing water of coastal saline-alkaline soil was inserted in the desalination chamber. Under the action of osmotic pressure, ion migration, nitrification and denitrification, NH4+-N and NO3−-N in the washing water of the desalination chamber were removed, and this indicates that the microbial desalination cell can be used to treatment the washing water of coastal saline-alkaline soil. The microbial community and function of the anode electrode biofilm and desalination chamber were analyzed through high-throughput sequencing, and the power generation characteristics, organics degradation and migration and transformation pathways of nitrogen of the aircathode microbial desalination cell were further explained.

Exploring community evolutionary characteristics of microbial populations with supplementation of Camellia green tea extracts in microbial fuel cells

This first-attempt study deciphered combined characteristics of species evolution and bioelectricity generation of microbial community in microbial fuel cells (MFCs) supplemented with Camellia green tea (GT) extracts for biomass energy extraction. Prior studies indicated that polyphenols-rich extracts as effective redox mediators (RMs) could exhibit significant electrochemical activities to enhance power generation in MFCs. However, the supplementation of Camellia GT extract obtained at room temperature with significant redox capabilities into MFCs unexpectedly exhibited obvious inhibitory effect towards power generation. This systematic study indicated that the presence of antimicrobial components (especially catechins) in GT extract might significantly alter the distribution of microbial community, in particular a decrease of microbial diversity and evenness. For practical applications to different microbial systems, pre-screening criteria of selecting biocompatible RMs should not only consider their promising redox capabilities (abiotic), but also possible inhibitory potency (biotic) to receptor microbes. Although Camellia tea extract was well-characterized as GRAS energy drink, some contents (e.g., catechins) may still express inhibition towards organisms and further assessment upon biotoxicity may be inevitably required for practice.

Microbial community structures and functions of hypersaline heterotrophic denitrifying process: Lab-scale and pilot-scale studies

High-nitrate wastewaters are known pose substantial risks to human and environmental health, while their effective treatment remains difficult. The denitrification of saline, high-NO₃^- wastewaters was investigated at the laboratory- and pilot-scale experiment. Complete denitrification was achieved for three different realistic wastewaters, and the maximum influent [NO₃^-]₀ and salinity were as high as 20,500 mg/L and 7.8%, respectively. The results of microbial community structure analyses revealed that the sequences of denitrifying functional bacteria accounted for 96.2% of all sequences, and the functional genes for denitrification in bacteria were enriched with elevated salinity and [NO₃^-]₀. A significant difference was observed in the dominant bacterial genus between synthetic and realistic wastewaters. Thauera and Halomonas species evolved to be the most common dominant genera contributing to the processes of nitrate, nitrite, and nitrous oxide reductase. This study is practically valuable for the treatment of realistic, saline, high-NO₃^- wastewaters via denitrification by heterotrophic bacteria.

Comparison of the effects of salinity on microbial community structures and functions in sequencing batch reactors with and without carriers

This study investigated and compared the microbial communities between a sequencing batch reactor (SBR) without carriers and a hybrid SBR with addition of carriers for the treatment of saline wastewater. The two systems were operated over 292 days with alternating aerobic/anoxic mode (temperature: 28℃, salinity: 0.0-3.0%). High removal efficiency of chemical oxygen demand (COD) and total inorganic nitrogen (TIN) was achieved in both the SBR (above 86.7 and 95.4% respectively) and hybrid SBR (above 84.4 and 94.0%) at 0.0-2.5% salinity. Further increasing salinity to 3.0% decreased TIN removal efficiency to 78.4% in the hybrid SBR. Steep decline of biodiversity and relative abundance of ammonia-oxidizing bacteria (AOB) contributed to the worse performance. More genera related to sulfide-oxidizing and sulfate-reducing bacteria were detected in the hybrid SBR than the SBR at 3.0% salinity. The abundance of halotolerant bacteria increased with the salinity increase for both reactors, summing up to 25.5% in the suspended sludge (S-sludge) from the SBR, 28.9 and 22.9% in the S-sludge and biofilm taken from the hybrid SBR, respectively. Nitrification and denitrification via nitrate was the main nitrogen removal pathway in the SBR and hybrid SBR at 0.0 and 0.5% salinity, while partial nitrification and denitrification via nitrite became the key process for nitrogen removal in the two reactors when the salinity was increased to 1.0-3.0%. Higher abundance of anaerobic ammonium-oxidizing (ANAMMOX) and sulfide-oxidizing autotrophic denitrification (SOAD) bacteria were found in the hybrid SBR at 3.0% salinity.

Simultaneous wastewater treatment and energy harvesting in microbial fuel cells: an update on the biocatalysts

The development of microbial fuel cell (MFC) makes it possible to generate clean electricity as well as remove pollutants from wastewater. Extensive studies on MFC have focused on structural design and performance optimization, and tremendous advances have been made in these fields. However, there is still a lack of systematic analysis on biocatalysts used in MFCs, especially when it comes to pollutant removal and simultaneous energy recovery. In this review, we aim to provide an update on MFC-based wastewater treatment and energy harvesting research, and analyze various biocatalysts used in MFCs and their underlying mechanisms in pollutant removal as well as energy recovery from wastewater. Lastly, we highlight key future research areas that will further our understanding in improving MFC performance for simultaneous wastewater treatment and sustainable energy harvesting.

Bifunctional cathode using a biofilm and Pt/C catalyst for simultaneous electricity generation and nitrification in microbial fuel cells

Biofouling frequently causes catalyst deterioration at the cathode of microbial fuel cells (MFCs). A biofilm-covered Pt/C cathode (BPC) was fabricated via in situ cultivation of a biofilm on a Pt/C cathode (PC) in a dual-chambered MFC, which enables effective removal of NH4+-N and copious generation of electricity. Experimental results show 99% NH4+-N removal by the nitrifying bacteria that constitute 35.7% of all microorganisms on the BPC and a maximum BPC-MFC power density of 0.97 W/m2, which is comparable to that of PC-MFCs (0.99 W/m2). BPC biofilm size is restricted by the limited amount of organic material in the cathode chamber, which constrains the biomass to less than 0.3 g protein /m2. The bifunctional-cathode equipped MFC shows great promise as an energy-saving technology for wastewater treatment in the future.

Enhanced degradation of quinoline by coupling microbial electrolysis cell with anaerobic digestion simultaneous

In this study, the feasibility of quinoline-wastewater treatment was investigated in a coupled microbial electrolysis cell and anaerobic digestion system (MEC-AD). Improved degradation and enhanced mineralization of quinoline were obtained, and the optimal voltage was determined to be 1.0 V. Effective removal of quinoline at relative high concentration, and a 1.5-fold increase in methane production were achieved. The results indicated that the MEC-AD could simultaneously remove carbon and nitrogen from quinoline. Gas chromatography-mass spectrometry analysis identified 2-hydroxyquinoline and 8-hydroxycoumarin as the intermediates of quinoline. The formation and degradation of metabolites were rapid, and they did not accumulate in the MEC-AD. The results of microbial community structure analysis demonstrated that the functional species were enriched and coexisted, and that the dominant bacterial genera were SM1A02, Comamonas, Desulfovibrio, Geobacter, and Actinomarinales_norank; the dominant archaeal genera were Methanocorpusculum and Nitrosoarchaeum. Furthermore, the applied current played a selective role in the enrichment of microorganisms.

Simultaneous removal of organics, sulfide and ammonium coupled with electricity generation in a loop microbial fuel cell system

A loop microbial fuel cell (MFC) system consisting of a denitrifying organics/sulfide removal MFC and a nitrification chamber was developed, and its performance at different feeding ratios of total organic carbon to sulfide (TOC/S) and dissolved oxygen (DO) levels of cathodic feeding were investigated. High feeding TOC/S ratio favored elemental sulfur production and anodic electron recovery. Introducing oxygen into the cathode enhanced nitrogen removal and electricity generation but hindered elemental sulfur production. At the optimal feeding TOC/S mass ratio of 4.69 and cathodic feeding DO of 4.2 mg/L, 100% of TOC, 100% of sulfide and 82.6 ± 0.9% of total nitrogen were removed, achieving a sulfur production percentage of 35.1 ± 4.4% and a coulombic efficiency of 53.0 ± 2.2%. Cathodic nitrogen removal was catalyzed by denitrifiers, nitrifiers and anammox bacteria. This work provided a novel approach for simultaneously removing organics, sulfide and ammonium coupled with electricity generation from wastewater.