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Dual selective pressure was applied as the driving condition to cultivate an enhanced aerobic granular sludge (AGS) with Fe(0)-based biochemical cycle galvanic-cells (BCGC) as the core. The BCGC-AGS coupled micro-electrolysis with synergistic autotrophic-heterotrophic denitrification to enhance nitrogen removal. COD and total nitrogen removal of 91.8% and 95.9% were achieved, respectively. The formation of circulation channel between Fe 3+ and Fe 2+ provided a solid foundation for the biochemical cycle of galvanic-cells with low consumption. The existence of micro-electrolysis selective pressure in BCGC-AGS was also confirmed. Facultative aerobic bacteria Methylocystis and Azospirillum were the most abundant genera. Facultative iron redox bacteria and autotrophic denitrifying bacteria Geobacter, Thiobacillus, Aquabacterium, Thauera and Azospirillum showed high abundance, affirming the success culture of EAGS system. Load shock test verified BCGC-AGS possessed excellent load shock resistance. Obtaining the advantages of fast-cultivation, high-efficiency and low galvanic-cells consumption, BCGC-AGS showed significant potential for practical application., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Oxygen has not been purposely introduced to the autotrophic denitrification systems and simultaneous nitrification/autotrophic denitrification (SNAD) has not been proposed. In this study, oxygen was introduced into a micro-electrolysis-enhanced Fe 0 -supported autotrophic denitrification (mFe 0 AD) system. The nitrogen removal performance was investigated and the application potential of iron-scraps-supported simultaneous nitrification/mFe 0 AD was evaluated. The results showed that Fe 0 AD was surprisingly enhanced by oxygen together with nitrification at average dissolved oxygen (DO) of 0.08-1.56 mg/L. The ammonia oxidizing bacterial, nitrite oxidizing bacteria, facultative autotrophic denitrificans, and iron compounds transformation bacteria were markedly enriched. Average denitrification rate shifted from 0.116 to 0.340 kg N/(m 3 ·d) with increase of average total nitrogen removal efficiency from 31.4% to 90.5%. Oxygen could enhance the biological conversion and storage of iron compounds, which was capable of reducing the coating of Fe 0 surface.The accelerating of oxygen on  Fe0 passivation appeared when increasing the average DO from 1.56 to 2.17 mg/L. Therefore, the SNAD was recommended to be operated at the DO range of 0.08-1.56 mg/L. ME significantly enhanced Fe 0 AD, and the utilization of iron-scraps reduced its cost. The denitrification rate is comparable with methanol supported heterotrophic denitrification with 58.9% reduction on the cost. The iron-scraps supported SNAD is competitive in both denitrification rate and costs in the ammonia contaminated low-carbon water treatment., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Nitrogen bioremediation in organic insufficient wastewater generally requires an extra carbon source. In this study, nitrate-contaminated wastewater was treated effectively through simultaneous autotrophic and heterotrophic denitrification based on micro-electrolysis carriers (MECs) and retinervus luffae fructus (RLF), respectively. The average nitrate and total nitrogen removal rates reached 96.3 and 94.0% in the MECs/RLF-based autotrophic and heterotrophic denitrification (MRAHD) system without ammonia and nitrite accumulation. The performance of MRAHD was better than that of MEC-based autotrophic denitrification for the wastewater treatment with low carbon nitrogen (COD/N) ratio. Real-time quantitative polymerase chain reaction (qPCR) revealed that the relative abundance of nirS-type denitrifiers attached to MECs (4.9%) and RLF (5.0%) was similar. Illumina sequencing suggested that the dominant genera were Thiobacillus (7.0%) and Denitratisoma (5.7%), which attached to MECs and RLF, respectively. Sulfuritalea was discovered as the dominant genus in the middle of the reactor. The synergistic interaction between autotrophic and heterotrophic denitrifiers played a vital role in the mixotrophic substrate environment.

A process combining micro-electrolysis and autotrophic denitrification (CEAD) with iron-carbon micro-electrolysis carriers was developed for nitrate removal. The process was performed using organic-free influent with a NO3(-)-N concentration of 40.0±3.0mg/L and provided an average nitrate removal efficiency of 95% in stable stages. The total nitrogen removal efficiency reached 75%, with 21% of NO3(-)-N converted into NH4(+)-N. The corresponding hydraulic retention time was 8-10h, and the optimal pH ranged from 8.5 to 9.5. Microbial analysis with high-throughput sequencing revealed that dominant microorganisms in the reactor belonged to the classes of β-, γ-, and α-Proteobacteria. The abundance of the genera Thermomonas significantly increased during the operation, comprising 21.4% and 24.1% in sludge attached to the carriers in the middle and at the bottom of the reactor, respectively. The developed CEAD achieved efficient nitrate removal from water without organics, which is suitable for practical application., (Copyright © 2016. Published by Elsevier Ltd.)

A process combining catalyzed Fe(0)-carbon microelectrolysis (IC-ME) with activated carbon (AC) adsorption was developed for advanced reclaimed water treatment. Simultaneous nitrate reduction and chemical oxygen demand (COD) removal were achieved, and the effects of composite catalyst (CC) addition, AC addition, and initial pH were investigated. The reaction kinetics and reaction mechanisms were calculated and analyzed. The results showed that CC addition could enhance the reduction rate of nitrate and effectively inhibit the production of ammonia. Moreover, AC addition increased the adsorption capacity of biorefractory organic compounds (BROs) and enhanced the degradation of BRO. The reduction of NO3−-N at different pH values was consistently greater than 96.9%, and NH4+-N was suppressed by high pH. The presence of CC ensured the reaction rate of IC-ME at high pH. The reaction kinetics orders and constants were calculated. Catalyzed iron scrap (IS)-AC showed much better nitrate reduction and BRO degradation performances than IS-AC and AC. The IC-ME showed great potential for application to nitrate and BRO reduction in reclaimed water. [ABSTRACT FROM AUTHOR]

• Novel characteristics of Fe(0)-ADN with aeration were revealed. • Opportune DO (1.6 ± 0.1 mg/L) enhanced Fe(0)-ADN and reduced N2O/NO 2 − accumulation. • Bioconversion of Fe3O4 into FeO(OH) was enhanced and benefitted H2/Fe2+ generation. • Genes napA , nirK & nosZ, and enzymes NAP, NIR & NosZ maintained aerobic Fe(0)-ADN. • Iron-scraps is potential as the Fe(0)-source under the mediation of micro-electrolysis. This study revealed novel characteristics of the Fe(0)-oxidizing autotrophic denitrification [Fe(0)-ADN] process with aeration, including nitrogen removal efficiency, electron donor transformation, N 2 O/NO 2 − accumulation, and variation of microbial communities and functional genes/enzymes. Iron-scraps (ISs) was utilized as Fe(0) source under the mediation of micro-electrolysis for ISs-ADN process establishment. The anaerobic ISs-ADN process achieved an initial denitrification load of 0.12 ± 0.02 kgN/(m3·d), and it subsequently shifted to 0.34 ± 0.01 kgN/(m3·d) when dissolved oxygen (DO) was increased from 0.1 ± 0.1 mg/L to 1.6 ± 0.1 mg/L. Significant accumulations of N 2 O (0.9 ± 0.2 mgN 2 O-N/L) and NO 2 − (5.5 ± 0.7 mgNO 2 −-N/L) were observed in the anaerobic ISs-ADN system, they were reduced to 0.07 ± 0.02 mgN/L and 0.2 ± 0.2 mgN/L, respectively, at DO of 1.6 ± 0.1 mg/L. X-ray diffraction spectra showed that oxygen could enhance bioconversion of Fe 3 O 4 into FeO(OH) and reduce the Fe(0)-coating by Fe 3 O 4 , and hence enhance the ME process for H 2 /Fe2+ production. High-throughput sequencing results showed that iron-compound transformation bacteria (e.g. Sphaerotillus) and facultative autotrophic denitrificans (e.g. Hydrogenophaga) were enriched at higher DO of 1.6 ± 0.1 mg/L. Genes napA (encoding NAP), nirK (encoding NIR) and nosZ (encoding NosZ) exhibited higher oxygen tolerance. Enzymes NAP, NIR and NosZ maintained the aerobic denitrification, and NIR and NosZ inhibited the accumulation of N 2 O and NO 2 −, respectively. Exorbitant aeration (DO ≥ 2.2 ± 0.2 mg/L) accelerated iron oxidation to Fe 3 O 4 on the ISs surface and restrained ISs-ADN. This study provides an in-depth understanding on the impact of oxygen on the Fe(0)-ADN. The micro-electrolysis mediated ISs-ADN is competitive in both efficiency and cost with the utilization of ISs. [ABSTRACT FROM AUTHOR]

Total nitrogen (TN) removal from low organic carbon wastewater is challenging. Traditional heterotrophic denitrification requires additional organics as electron donors, potentially increasing the operational challenges and costs of wastewater treatment. Especially for decentralized domestic sewage in rural areas, it is essential to develop an economical and efficient integrated reactor to satisfy high TN removal demand. This study presents an integrated O/A two-stage packed-bed reactor (INT-PBR) composed of two concentric zones, with total effective volume of 30.6 L. Influent is pumped into the inner oxic (O) zone for nitrification, then flows through the outer anoxic (A) zone for denitrification. Iron–carbon micro–electrolysis carriers (IC-MECs), which provide H 2 /[H] and Fe 2+ as inorganic electron donors for autotrophic denitrifiers, and luffa cylindrical sponge (LCS), which provides slow–release organic carbon for heterotrophic denitrifiers, were placed into the A zone to achieve combined heterotrophic and autotrophic denitrification (HAD) without requiring organic carbon in influent. Thus, the INT-PBR involves no return sludge and internal recycle. Using organic-free synthetic wastewater with 39.36 ± 0.74 mg/L NH 4 + -N, approximately 98% of ammonia in influent was transferred to nitrate in the O zone, and approximately 93% of TN was removed by HAD process at 30 ± 2 °C in the A zone. Microbial community analysis showed that autotrophic denitrifiers (e.g., Thiobacillu s) and heterotrophic/mixotrophic denitrifiers (e.g., Hydrogenophaga ) were combined in the reactor, consistent with the performance findings. The results suggest a promising technology for total nitrogen removal from wastewater with low organic carbon. [ABSTRACT FROM AUTHOR]

Hydrogenotrophic denitrification is promising for tertiary nitrogen removal from municipal wastewater. To reveal the influence of residual organics in municipal wastewater on hydrogenotrophic denitrifiers, we adopted high-throughput 16S rRNA gene amplicon sequencing to examine microbial communities in hydrogenotrophic denitrification enrichments. Using effluent from a municipal wastewater treatment plant as water source, COD, nitrate and pH were controlled the same except for a gradient of biodegradable carbon ( i.e. , primary effluent (PE), secondary effluent (SE), or combined primary and secondary effluent (CE)). Inorganic synthetic water (IW) was used as a control. Hydrogenophaga , a major facultative autotroph, accounted for 17.1%, 5.3%, 32.7% and 12.9% of the sequences in PE, CE, SE and IW, respectively, implicating that Hydrogenophaga grew well with or without organics. Thauera , which contains likely obligate autotrophic denitrifiers, appeared to be the most dominant genera (23.6%) in IW and accounted for 2.5%, 4.6% and 8.9% in PE, CE and SE, respectively. Thermomonas , which is related to heterotrophic denitrification, accounted for 4.2% and 7.9% in PE and CE fed with a higher content of labile organics, respectively. In contrast, Thermomonas was not detected in IW and accounted for only 0.6% in SE. Our results suggest that Thermomonas are more competitive than Thauera in hydrogenotrophic denitrification with biodegradable organics. Moreover, facultative autotrophic denitrifiers, Hydrogenophaga , are accommodating to residual organic in effluent wastewater, thus we propose that hydrogenotrophic denitrification is amenable for tertiary nitrogen removal. [ABSTRACT FROM AUTHOR]

Autotrophic denitrification has attracted increasing attention for wastewater with insufficient organic carbon sources. Nevertheless, in situ identification of autotrophic denitrifying communities in reactors remains challenging. Here, a process combining micro-electrolysis and autotrophic denitrification with high nitrate removal efficiency was presented. Two batch reactors were fed organic-free nitrate influent, with H 13 CO 3 − and H 12 CO 3 − as inorganic carbon sources. DNA-based stable-isotope probing (DNA-SIP) was used to obtain molecular evidence for autotrophic denitrifying communities. The results showed that the nirS gene was strongly labeled by H 13 CO 3 − , demonstrating that the inorganic carbon source was assimilated by autotrophic denitrifiers. High-throughput sequencing and clone library analysis identified Thiobacillus -like bacteria as the most dominant autotrophic denitrifiers. However, 88% of nirS genes cloned from the 13 C-labeled “heavy” DNA fraction showed low similarity with all culturable denitrifiers. These findings provided functional and taxonomical identification of autotrophic denitrifying communities, facilitating application of autotrophic denitrification process for wastewater treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • NO 2 –/N 2 O accumulation in Fe(0)-/Fe(II)-ADN processes are systematically evaluated. • Fe(II)-ADN aggravated but H-ADN reduced NO 2 –/N 2 O accumulation in Fe(0)-ADN process. • Influence of pH, temperature and carbon source on NO 2 –/N 2 O accumulation was explored. • Thiobacillus and Thauera are capable of reducing NO 3 − and NO 2 –/N 2 O, respectively. • NIR and NosZ encoded by nirK and nosZ catalyze NO 2 – and N 2 O reduction, respectively. The iron (Fe(0))-/ferrous iron (Fe(II))-driven autotrophic denitrification processes have been alternative methods for nitrogen removal from low organic carbon (OC) wastewater, but the accumulation of nitrous oxide (N 2 O) and nitrite (NO 2 −) along with these processes remains unclear. This research aimed to systematically characterize the N 2 O/NO 2 − accumulation in Fe(0)-/Fe(II)-ADN processes through investigating the mechanisms, impact factors, and molecular biological characteristics. Results showed that Fe(II)-ADN was effective in NO 3 − reduction but was less efficient in N 2 O reduction (k = 0.50 h−1) than Fe(0)-ADN (k = 1.82 h−1). NO 2 −/N 2 O accumulation in Fe(II)-ADN (28.6%/30.7%) was much higher than that in Fe(0)-ADN (12.6%/1.5%). Introducing hydrogenotrophic denitrification (H-ADN) into Fe(II)-ADN system significantly (p < 0.05) reduced NO 2 −/N 2 O accumulation. Fe(0)-ADN was proved a coupled process of Fe(II)- and H-ADN by in-situ generating Fe(II)/H 2 , and Fe(II)-ADN and H-ADN mainly contributed to NO 3 − and NO 2 −/N 2 O reduction, respectively. Optimum pH (7.5) and temperature (30–35 °C) were confirmed with controlled NO 2 –/N 2 O accumulation and effective denitrification. Dosing inorganic carbon (IC) and OC enhanced denitrification and reduced NO 2 –/N 2 O accumulation, where OC was more efficient with an optimum dosage of 0.25 mmol C/mmol N. 16S rRNA high-throughput sequencing and Pearson Correlation Coefficients verified that Thiobacillus was the main contributor to NO 3 − reduction, whereas Thauera and Acidovorax possessed high NO 2 −/N 2 O reduction capability. Real-time quantitative polymerase chain reaction and enzyme activity assay demonstrated that the nitrite reductase encoded by gene nirK and the nitrous oxide reductase encoded by gene nosZ were efficient in catalyzing the further reduction of NO 2 − and N 2 O, respectively. This study could provide an in-depth understanding of NO 2 −/N 2 O accumulation in Fe(II)-/Fe(0)-ADN processes and contribute to their application, optimization and secondary pollutants control. [ABSTRACT FROM AUTHOR]

• Oxic/anoxic and micro-electrolysis integrated constructed wetland was developed. • Decentralized low-carbon greywater was treated at ambient temperature −11.5 to 8.0 °C. • High NH 4 +-N (94%), TN (86%), TP (98%) and COD (93%) removals were achieved. • Micro-electrolysis-produced H 2 /Fe2+ mediated denitrification and dephosphorization. • AOB, NOB, autotrophic denitrifier and FeII-oxidizing bacteria dominated the system. The treatment of decentralized low-carbon greywater in rural area, particularly in cold weather, remains a challenge. Oxic/anoxic process and Fe/C micro-electrolysis were incorporated into vertical constructed wetland to develop ME-(O/A)CW for practical decentralized low-carbon greywater treatment. ME-(O/A)CW provided NH 4 +-N, TN, TP and COD removal of 94.3%, 86.2%, 98.0% and 92.7%, respectively, at hydraulic loading rate of 0.9 m3/(m2·d) under low ambient temperature of −11.5 to 8.0 °C. Effective nitrification, phosphorus-accumulating and organic-degradation were proceeded in the aerobic layers and efficient H 2 -/Fe2+-mediated autotrophic denitrification and Fe3+-based phosphorus immobilization were developed in the anaerobic layers through in-situ H 2 -/Fe2+-supply by Fe/C micro-electrolysis. AOB (e.g. Nitrosomonadales), NOB/PAOs (e.g. Nitrospira), autotrophic denitrificans (e.g. Thiobacillus , Hydrogenophaga and Sulfurimonas), heterotrophic denitrificans (e.g. Denitratisoma) and Fe(II)-oxidizing bacteria (e.g. Ferritrophicum) dominated ME-(O/A)CW and confirmed the reaction mechanisms. The developed ME-(O/A)CW presented significant potential in the practical application for decentralized low-carbon greywater treatment under low ambient temperature. [ABSTRACT FROM AUTHOR]

• Novel characteristics and mechanism of Fe(0)-AD with organic carbon were revealed. • Optimum dosage 1.0 mgCOD/mgTN enhanced Fe(0)-AD and reduced N 2 O/NO 2 − accumulation. • Bioconversion of iron-compounds was enhanced and Fe(0)-passivation of was reduced. • H 2 /Fe2+-based autotrophic denitrifiers and genes napA, nirK & nosZ were enriched. • NaAC was more efficient in enhancing the Fe(0)-AD process than glucose and EtOH. Iron [Fe(0)]-oxidizing supported autotrophic denitrification [Fe(0)-AD] is a promising process for low-carbon water treatment. Organic-carbon (OC) has been a common pollutant associated with nitrogen pollution. However, its interaction with Fe(0)-AD remains unclear. In this study, influence characteristics and mechanism of OC on Fe(0)-AD were investigated. Fe(0)-AD was established based on iron-scraps (ISs) and iron-carbon micro-electrolysis (ME) (ME-ISs-AD). The results showed that the OC-free ME-ISs-AD obtained denitrification load of 0.191 ± 0.028 kgN/(m3·d), which shifted to 0.445 ± 0.011 kgN/(m3·d) at OC-dosage of 1.0 mgCOD/mgTN. N 2 O (1.52 ± 0.16 mgN/L) and NO 2 − (6.86 ± 1.11 mgN/L) accumulated markedly in Fe(0)-AD and were reduced to 0.03 ± 0.01 mgN/L and 0.31 ± 0.20 mgN/L at OC-dosage of 1.0 mgCOD/mgTN, respectively. XRD analysis demonstrated that OC could reduce the Fe(0)-passivation, accelerating biological transformation and storage of iron-compounds, enhancing H 2 (from 0.06 ± 0.01 to 0.22 ± 0.04 mg/L) and Fe2+ (from 7.4 ± 1.0 to 13.9 ± 1.5 mg/L) production. High-throughput sequencing revealed the enrichment of iron-oxidizing bacteria (e.g. Sphaerotilus and Ferriphaselus) and autotrophic denitrifiers (e.g. Hydrogenophaga and Hyphomicrobium). Real-time qPCR and enzyme activity analysis indicated that the nosZ -encoded NosZ and nirK -encoded NIR controlled N 2 O and NO 2 − accumulation in OC-dosed Fe(0)-AD, respectively. Overdose of OC (≥1.5 mgCOD/mgTN) accelerated the bio-organics-Fe complexes (BioFe) production and blocked the H 2 /Fe2+ transfer. NaAC was more efficient and cost-effective than Glucose and EtOH for Fe(0)-AD enhancement. The utilization of ISs under the enhancement of ME reduced the cost of Fe(0)-AD by 3.87-times comparing to the in-situ ZVI-AD process, and the dose of OC further improved the denitrification load by 1.33-times. This study provided an in-depth understanding of the influence of OC on Fe(0)-AD and could contribute to the application, optimization and secondary pollutants control of Fe(0)-AD process in low-carbon water treatment. [ABSTRACT FROM AUTHOR]