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The feasibility of a synergistic endogenous partial denitrification-phosphorus removal coupled anammox (SEPD-PR/A) system was investigated in a modified anaerobic baffled reactor (mABR) for synchronous carbon, nitrogen, and phosphorus removal. The mABR comprising four identical compartments (i.e., C1-C4) was inoculated with precultured denitrifying glycogen-accumulating organisms (DGAOs), denitrifying polyphosphate-accumulating organisms, and anammox bacteria. After 136 days of operation, the chemical oxygen demand (COD), total nitrogen, and phosphorus removal efficiencies reached 88.6 ± 1.0, 97.2 ± 1.5, and 89.1 ± 4.2%, respectively. Network-based analysis revealed that the biofilmed community demonstrated stable nutrient removal performance under oligotrophic conditions in C4. The metagenome-assembled genomes (MAGs) such as MAG106, MAG127, MAG52, and MAG37 annotated as denitrifying phosphorus-accumulating organisms (DPAOs) and MAG146 as a DGAO were dominated in C1 and C2 and contributed to 89.2% of COD consumption. MAG54 and MAG16 annotated as Candidatus_Brocadia (total relative abundance of 16.5% in C3 and 4.3% in C4) were responsible for 74.4% of the total nitrogen removal through the anammox-mediated pathway. Functional gene analysis based on metagenomic sequencing confirmed that different compartments of the mABR were capable of performing distinct functions with specific advantageous microbial groups, facilitating targeted nutrient removal. Additionally, under oligotrophic conditions, the activity of the anammox bacteria-related genes of hzs was higher compared to that of hdh . Thus, an innovative method for the treatment of low-strength municipal and nitrate-containing wastewaters without aeration was presented, mediated by an anammox process with less land area and excellent quality effluent.

Heterotrophic-sulfur autotrophic denitrification (HAD) has been proposed to be a prospective nitrogen removal process. In this work, the potential of fermentation liquid (FL) from waste-activated sludge (WAS) as the electron donor for denitrification in the HAD system was explored and compared with other conventional carbon sources. Results showed that when FL was used as a carbon source, over 99% of NO 3 - -N was removed and its removal rate exceeded 14.00 mg N/g MLSS/h, which was significantly higher than that of methanol and propionic acid. The produced sulfate was below the limit value and the emission of N 2 O was low (1.38% of the NO 3 - -N). Microbial community analysis showed that autotrophic denitrifiers were predominated in the HAD system, in which Thiobacillus (16.4%) was the dominant genus. The economic analysis showed the cost of the FL was 0.062 €/m 3 , which was 30% lower than that in the group dosed with methanol. Our results demonstrated the FL was a promising carbon source for the HAD system, which could reduce carbon emission and cost, and offer a creative approach for waste-activated sludge resource reuse., 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 © 2024 Elsevier Ltd. All rights reserved.)

Elemental sulfur autotrophic denitrification (S 0 AD) processes are temperature-sensitive, presenting a substantial challenge for the practical implementation of S 0 AD bioreactors. In this study, a comprehensive methodology for designing and operating S 0 AD bioreactors was developed, effectively managing fluctuations in nitrogen removal efficiency caused by seasonal temperature variations. Initially, the nitrate removal rate was correlated with simulated on-site temperature and nitrate loading, revealing correlation coefficients of k 1 , k 2 , k 3 , and A as 5.42×10 -4 , -0.41, 0.04, and 0.13, respectively, to establish a mathematical model for predicting S 0 AD efficiency. Subsequently, by considering influence factors such as dissolved oxygen and dynamic sulfur consumption, the model was employed to accurately design a pilot-scale S 0 AD bioreactor for a case study. By utilizing an alternative multi-subunit operation, a stable effluent nitrate concentration of less than 8 mg-N/L was maintained throughout the year. Importantly, this approach resulted in a substantial reduction of 76.8% in excessive nitrate removal, sulfur consumption, and sulfate production. This study aims to provide an optimal design and operation strategy for the practical application of S 0 AD bioreactors, thereby enhancing reliability and cost-effectiveness in the face of seasonal temperature changes., 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 © 2023 Elsevier Inc. All rights reserved.)

Sulfur-packed beds (SPBs) have been increasingly incorporated into constructed wetland systems to overcome limitations in achieving satisfactory nitrate removal efficiency. However, the underlying impact of hydraulic regimes on SPB performance remains understudied. This study investigated the performance of a pilot-scale SPB, encompassing sulfur autotrophic denitrification (SAD) and sulfur disproportionation (SDP) processes, under various horizontal flow (HF) and vertical flow (VF) regimes. The HF regime exhibited superior SAD efficiency, achieving 3.1-4.4 mg-N/L of nitrate removal compared to 0.9-2.8 mg-N/L under VF regimes. However, greater sulfide production of 3.8-5.6 mg/L was observed, in contrast to only 1.5-2.3 mg/L under VF regimes when SDP occurred. Employing current computational fluid dynamics simulations could predict general regimes but lacked precision in detailing sulfur layer dynamics. In contrast, determining the spatial distribution of SAD substrates and SDP products offered a viable solution, revealing stagnate, short-circuit, and back flows. Moreover, the feasibility of an aeration approach to reduce sulfide emissions below 0.5 mg/L in case of accidental SDP occurrence was confirmed. This study offers a method for assessing detailed hydraulic regimes within SPBs. Additionally, it provides guidance on optimizing the packing of sulfur-based materials when implementing SPBs in constructed wetland systems and presents a strategy for mitigating excessive sulfide emissions., 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 © 2023 Elsevier Inc. All rights reserved.)

Efficient recovery of nitrous oxide (N 2 O) through heterotrophic denitrification with the help of Fe(II)EDTA-NO as a chelating agent has been regarded as an ideal technology to treat nitric oxide (NO)-rich flue gas. In this study, an integrated NO-based biological denitrification model was developed to describe the sequential reduction of the NO fixed in Fe(II)EDTA-NO with organic carbon as the electron donor. With the inclusion of only the key pathways contributing to nitrogen transformation, the model was firstly developed and then calibrated/validated and evaluated using the data of batch tests mediated by the identified functional heterotrophic bacteria at various substrates concentrations and then used to explore the possibility of enhancing N 2 O recovery by altering the substrates condition and reactor setup. The results demonstrated that the optimal COD/N ratio decreased consistently from 1.5 g-COD/g-N at the initial NO concentration of 40 g-N/m 3 to 1.0 g-COD/g-N at the initial NO concentration of 420 g-N/m 3 . Furthermore, sufficiently increasing the headspace volume of the reactor was considered an ideal strategy to obtain ideal N 2 O production of 86.6 % under the studied conditions. The production of high-purity N 2 O (98 %) confirmed the practical application potential of this integrated treatment technology to recover a valuable energy resource from NO-rich flue gas., 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 © 2022 Elsevier B.V. All rights reserved.)

Microbial denitrification is a crucial biological process for the treatment of nitrogen-polluted water. Traditional denitrification process consumes external organic carbon leading to an increase in treatment costs. We developed a novel sulfide-driven denitrification methane oxidation (SDMO) system that integrates autotrophic denitrification (AD) and denitrification anaerobic methane oxidation (DAMO) for cost-effective denitrification and biogas utilization in situ. Two SDMO systems were operated for 735 days, with nitrate and nitrite serving as electron acceptors, to explore the performance of sewage denitrification and characterize metabolic mechanisms. Results showed SDMO system could reach as high as 100% efficiency of nitrogen removal and biogas desulfurization without an external carbon source when HRT was 10 days and inflow nitrogen concentrations were 50-100 mgN·L -1 . Besides, nitrate was a preferable electron acceptor for SDMO system. Biogas not only enhanced nitrogen removal but also intensified the DAMO, nitrogen removed through DAMO contribution doubled as original period from 2.9 mgN·(L·d) -1 to 6.2 mgN·(L·d) -1 , and the ratio of nitrate removal through AD to DAMO was 1.2:1 with nitrate as electron acceptor. While nitrogen removed almost all through AD contribution and DAMO was weaken as before, the ratio of nitrate removal through AD to DAMO was 21.2:1 with nitrite as electron acceptor. Biogas introduced into SDMO system with nitrate inspired the growth of DAMO bacteria Candidatus Methylomirabilis from 0.3% to 19.6% and motivated its potentiality to remove nitrate without ANME archaea participation accompanying with gene mfnE upregulating ∼100 times. According to the reconstructed genome from binning analysis, the dramatically upregulated gene mfnE was derived from Candidatus Methylomirabilis, which may represent a novel metabolism pathway for DAMO bacteria to replace the role of archaea for nitrate reduction., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

This work studied the impacts of key granule properties on the granular reactor performing partial nitritation/anammox from the modeling perspective. The results could guide not only future reliable modeling but also practical startup/operation of the reactor. To achieve high total nitrogen (TN) removal whilst avoiding significant N 2 O production, inoculated granules should be big and anammox-enriched. The optimum boundary layer thickness for maximum TN removal increased with the decreasing diffusivity of soluble components in the granule structure. Even though a thick boundary layer could protect anammox bacteria from elevated dissolved oxygen (DO) (e.g., 0.5 g-O 2 /m 3 ) and obtain high TN removal (>90.0%) and low N 2 O production (<1.8%), even complete removal of the boundary layer would fail to provide sufficient substrate for anammox and therefore couldn't increase TN removal to 90.0% and decrease N 2 O production to <2.4% at insufficient DO (e.g., 0.3 g-O 2 /m 3 in the presence of lifted influent NH 4 + concentration)., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

While sulfur-driven autotrophic denitrification (SDAD) occurring in the anoxic reactor of the sulfate reduction, autotrophic denitrification and nitrification integrated (SANI) system has been regarded as the main nitrogen removal bioprocess, little is known about the accompanying Anammox bacteria whose presence is made possible by the co-existence of NH 4 + and NO 2 - . Therefore, this work firstly developed an integrated SDAD-Anammox model to describe the interactions between sulfur-oxidizing bacteria and Anammox bacteria. The model was subsequently used to explore the impacts of influent conditions on the reactor performance and microbial community structure of the anoxic reactor. The results revealed that at a relatively low ratio of <1.5 mg S/mg N, Anammox bacteria could survive and even take a dominant position (up to 58.9%). Finally, a modified SANI system configuration based on the effective collaboration between SDAD and Anammox processes was proposed to improve the efficiency of the treatment of sulfate-rich saline sewage., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

To reclaim nitrous oxide (N 2 O) as an energy resource economically, this study developed an autotrophic denitrification-based system with thiosulfate (S 2 O 3 2- O production profile and NO conversion efficiency from thiosulfate-based denitrification under varied Fe (II)EDTA-NO conditions (5-20 mM). Up to 39% of NO was converted to gaseous N 2 O production profile and NO conversion efficiency from thiosulfate-based denitrification under varied Fe (II)EDTA-NO conditions (5-20 mM). Up to 39% of NO was converted to gaseous N 2 OR being suppressed harder than NOR. Microbial diversity was reduced in the continuous thiosulfate-driven Fe(II)EDTA-NO-based denitrification system. NO-resistant bacteria and sulfide-tolerant denitrifiers were enriched, facilitating NO conversion to N 2 OR being suppressed harder than NOR. Microbial diversity was reduced in the continuous thiosulfate-driven Fe(II)EDTA-NO-based denitrification system. NO-resistant bacteria and sulfide-tolerant denitrifiers were enriched, facilitating NO conversion to N 2 O thereafter., (Copyright © 2022. Published by Elsevier Ltd.)

Nitrous oxide (N 2 O) was previously deemed as a potent greenhouse gas but is actually an untapped energy source, which can accumulate during the microbial denitrification of nitric oxide (NO). Compared with the organic electron donor required in heterotrophic denitrification, elemental sulfur (S 0 ) is a promising electron donor alternative due to its cheap cost and low biomass yield in sulfur-driven autotrophic denitrification. However, no effort has been made to test N 2 O recovery from sulfur-driven denitrification of NO so far. Therefore, in this study, batch and continuous experiments were carried out to investigate the NO removal performance and N 2 O recovery potential via sulfur-driven NO-based denitrification under various Fe(II)EDTA-NO concentrations. Efficient energy recovery was achieved, as up to 35.5%-40.9% of NO was converted to N 2 O under various NO concentrations. N 2 O recovery from Fe(II)EDTA-NO could be enhanced by the low bioavailability of sulfur and the acid environment caused by sulfur oxidation. The NO reductase (NOR) and N 2 O reductase (N 2 OR) were inhibited distinctively at relatively low NO levels, leading to efficient N 2 O accumulation, but were suppressed irreversibly at NO level beyond 15 mM in continuous experiments. Such results indicated that the regulation of NO at a relatively low level would benefit the system stability and NO removal capacity during long-term system operation. The continuous operation of the sulfur-driven Fe(II)EDTA-NO-based denitrification reduced the overall microbial diversity but enriched several key microbial community. Thauera, Thermomonas, and Arenimonas that are able to carry out sulfur-driven autotrophic denitrification became the dominant organisms with their relative abundance increased from 25.8% to 68.3%, collectively., (© 2021 Wiley Periodicals LLC.)

The susceptibility of the completely autotrophic nitrogen removal over nitrite (CANON) process to high salinity limits its widespread application. The addition of glycine betaine (GB), a type of compatible solutes that could resist osmotic stress, could be an effective strategy to enhance the salt tolerance ability of aerobic and anaerobic ammonium oxidizing bacteria (AOB and anammox bacteria) involved in the CANON process. This study aims to make use of mathematical modeling to systematically investigate the effects of salt and GB addition on the activities of AOB and anammox bacteria and the treatment performance of the CANON process. To this end, a series of dedicated batch tests and long-term experiments for the CANON process with salt and GB additions were conducted and the data was used to calibrate and validate the model established to consider the relationships between salt and GB concentrations and bacterial growth in the CANON process. The calibrated/validated CANON process model was then applied to simulate the long-term impacts of GB addition concentration and sludge retention time (SRT) on the CANON process. The results showed that 1 mM GB addition and a SRT of 50 days would be sufficient to protect AOB and anammox bacteria under the high salinity (30 g/L NaCl) conditions studied and therefore reduce the time needed to recover the treatment performance of the CANON process from exposure to salt inhibition by 35%-40%., 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 © 2020 Elsevier Ltd. All rights reserved.)

Thiocyanate (SCN - )-based autotrophic denitrification (AD) has recently been demonstrated as a promising technology that could be integrated with anaerobic ammonium oxidation (Anammox) to achieve simultaneous removal of nitrogen and SCN - . However, there is still a lack of a complete SCN - -based AD model, and the potential microbial competition/synergy between AD bacteria and Anammox bacteria under different operating conditions remains unknown, which significantly hinders the possible application of coupling SCN - -based AD with Anammox. To this end, a complete SCN - -based AD model was firstly developed and reliably calibrated/validated using experimental datasets. The obtained SCN - -based AD model was then integrated with the well-established Anammox model and satisfactorily verified with experimental data from a system coupling AD with Anammox. The integrated model was lastly applied to investigate the impacts of influent NH 4 + -N/NO 2 - -N ratio and SCN - concentration on the steady-state microbial composition as well as the removal of nitrogen and SCN - . The results showed that the NH 4 + -N/NO 2 - -N ratio in the presence of a certain SCN - level should be controlled at a proper value so that the maximum synergy between AD bacteria and Anammox bacteria could be achieved while their competition for NO 2 - would be minimized. For the simultaneous maximum removal (>95%) of nitrogen and SCN - , there existed a negative relationship between the influent SCN - concentration and the optimal NH 4 + -N/NO 2 - -N ratio needed., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The sustainability of autotrophic granular system performing partial nitritation and anaerobic ammonium oxidation (anammox) for complete nitrogen removal is impaired by the production of nitrous oxide (N 2 O). A systematic analysis of the pathways and affecting parameters is, therefore, required for developing N 2 O mitigation strategies. To this end, a mathematical model capable of describing different N 2 O production pathways was defined in this study by synthesizing relevant mechanisms of ammonium-oxidizing bacteria (AOB), nitrite-oxidizing bacteria, heterotrophic bacteria (HB), and anammox bacteria. With the model validity reliably tested and verified using two independent sets of experimental data from two different autotrophic nitrogen removal biofilm/granular systems, the defined model was applied to reveal the underlying mechanisms of N 2 O production in the granular structure as well as the impacts of operating conditions on N 2 O production. The results show that: (a) in the aerobic zone close to the granule surface where AOB contribute to N 2 O production through both the AOB denitrification pathway and the NH 2 OH pathway, the co-occurring HB consume N 2 O produced by AOB but indirectly enhance the N 2 O production by providing NO from NO 2 - reduction for the NH 2 OH pathway, (b) the inner anoxic zone of granules with the dominance of anammox bacteria acts as a sink for NO 2 O production from the AOB denitrification pathway, (c) operating parameters including bulk DO, influent NH - diffusing from the outer aerobic zone and, therefore, reduces N 2 O production from the AOB denitrification pathway, (c) operating parameters including bulk DO, influent NH 4 + , and granule size affect the N 2 OH pathway and heterotrophic denitrification for nitric oxide leads to the positive role of HB in reducing N 2 OH pathway of AOB, accounting for 34-58% of N 2 O turnover, and (d) the competition between the NH 2 OH pathway and heterotrophic denitrification for nitric oxide leads to the positive role of HB in reducing N 2 O production in the autotrophic nitrogen removal granules, which could be further enhanced in the presence of a proper level of influent organics., (© 2019 Wiley Periodicals, Inc.)

A common and long-existing operational issue of wastewater denitrification is the unexpected accumulation of nitrite (NO 2 - O) that could emit as a potent greenhouse gas. Recently, it has been confirmed that the accumulation of these denitrification intermediates in biological wastewater treatment process is greatly influenced by the electron competition between the four denitrification steps. However, little is known about this in biofilm systems. In this work, we applied a mathematical model that links carbon oxidation and nitrogen reduction processes through a pool of electron carriers, to assess electron competition in denitrifying biofilms. Simulations were performed comprehensively at seven combinations of electron acceptor addition scheme (i.e., simultaneous addition of one, two or three among nitrate (NO 2 O) that could emit as a potent greenhouse gas. Recently, it has been confirmed that the accumulation of these denitrification intermediates in biological wastewater treatment process is greatly influenced by the electron competition between the four denitrification steps. However, little is known about this in biofilm systems. In this work, we applied a mathematical model that links carbon oxidation and nitrogen reduction processes through a pool of electron carriers, to assess electron competition in denitrifying biofilms. Simulations were performed comprehensively at seven combinations of electron acceptor addition scheme (i.e., simultaneous addition of one, two or three among nitrate (NO 3 - ), NO 2 - , and N 2 O) to compare the effect of electron competition on NO 3 - , NO 2 - and N 2 O reduction. Overall, the effects of substrate loading, biofilm thickness and effective diffusion coefficients on electron competition are not always intuitive. Model simulations show that electron competition was intensified due to the substrate load limitation (from 120 to 20 mg COD/L) and increasing biofilm thicknesses (from 0.1 to 1.6 mm) in most cases, where electrons were prioritized to nitrite reductase because of the insufficient electron donor availability in the biofilm. In contrast, increasing effective diffusion coefficients did not pose a significant effect on electron competition and only increased electrons distributed to nitrite reductase when both NO 2 - and N 2 O are added.

Hydrogenotrophic denitrification is a novel and sustainable process for nitrogen removal, which utilizes hydrogen as electron donor, and carbon dioxide as carbon source. Recent studies have shown that nitrous oxide (N 2 O), a highly undesirable intermediate and potent greenhouse gas, can accumulate during this process. In this work, a new mathematical model is developed to describe nitrogen oxides dynamics, especially N 2 O, during hydrogenotrophic denitrification for the first time. The model describes electron competition among the four steps of hydrogenotrophic denitrification through decoupling hydrogen oxidation and nitrogen reduction processes using electron carriers, in contrast to the existing models that couple these two processes and also do not consider N 2 O accumulation. The developed model satisfactorily describes experimental data on nitrogen oxides dynamics obtained from two independent hydrogenotrophic denitrifying cultures under various hydrogen and nitrogen oxides supplying conditions, suggesting the validity and applicability of the model. The results indicated that N 2 O accumulation would not be intensified under hydrogen limiting conditions, due to the higher electron competition capacity of N 2 O reduction in comparison to nitrate and nitrite reduction during hydrogenotrophic denitrification. The model is expected to enhance our understanding of the process during hydrogenotrophic denitrification and the ability to predict N 2 O accumulation., (© 2017 Wiley Periodicals, Inc.)

Biological nitrogen removal through the nitrite pathway (NH 4 + → NO 2 - → N 2 ) is favorable for wastewater treatment plants without sufficient carbon sources. This study demonstrates an innovative approach for attaining the nitrite pathway based on sludge treatment using free ammonia (FA, i.e., NH 3 ). This approach is based on our innovative discovery in this study that FA at 210 mg NH 3 -N/L is far less biocidal to ammonium-oxidizing bacteria (AOB) than to nitrite-oxidizing bacteria (NOB). A total of 22% of the activated sludge from the sequencing batch reactor (SBR) receiving synthetic domestic wastewater was treated in an FA treatment unit at 210 mg NH 3 -N/L for 1 day. The FA-treated sludge was afterward recirculated back to the SBR. A nitrite accumulation ratio of above 90% was quickly achieved (in 40 days) and maintained stably in the SBR, indicating the establishment of the nitrite pathway. The NOB population and activity after implementing FA treatment was less than 5% of those without FA treatment, suggesting the washout of NOB. In contrast, the AOB population and activity in the SBR were not affected. The nitrogen-removal performance was significantly improved after incorporating the FA approach. The FA approach is a closed-loop approach and is economically and environmentally attractive.

Recent studies have shown that sulfide- and sulfur-based autotrophic denitrification (AD) processes play an important role in contributing to nitrous oxide (N2O) production and emissions. However, N2O production is not recognized in the current AD models, limiting their ability to predict N2O accumulation during AD. In this work, a mathematical model is developed to describe N2O dynamics during sulfide- and sulfur-based AD processes for the first time. The model is successfully calibrated and validated using N2O data from two independent experimental systems with sulfide or sulfur as electron donors for AD. The model satisfactorily describes nitrogen reductions, sulfide/sulfur oxidation, and N2O accumulation in both systems. Modeling results revealed substantial N2O accumulation due to the relatively low N2O reduction rate during both sulfide- and sulfur-based AD processes. Application of the model to simulate long-term operations of activated sludge systems performing sulfide- and sulfur-based AD processes indicates longer sludge retention time reduced N2O emission. For sulfide-based AD process, higher initial S/N ratio also decreased N2O emission but with a higher operational cost. This model can be a useful tool to support process operation optimization for N2O mitigation during AD with sulfide or sulfur as electron donor.

Sulfur-based autotrophic denitrification and heterotrophic denitrification have been demonstrated to be promising technological processes for simultaneous removal of nitrate NO3(-) and chromate (Cr (VI)), two common contaminants in surface and ground waters. In this work, a mathematical model was developed to describe and evaluate the microbial and substrate interactions among sulfur oxidizing denitrifying organism, methanol-based heterotrophic denitrifiers and chromate reducing bacteria in the biofilm systems for simultaneous nitrate and chromate removal. The concomitant multiple chromate reduction pathways by these microbes were taken into account in this model. The validity of the model was tested using experimental data from three independent biofilm reactors under autotrophic, heterotrophic and mixotrophic conditions. The model sufficiently described the nitrate, chromate, methanol, and sulfate dynamics under varying conditions. The modeling results demonstrated the coexistence of sulfur-oxidizing denitrifying bacteria and heterotrophic denitrifying bacteria in the biofilm under mixotrophic conditions, with chromate reducing bacteria being outcompeted. The sulfur-oxidizing denitrifying bacteria substantially contributed to both nitrate and chromate reductions although heterotrophic denitrifying bacteria dominated in the biofilm. The mixotrophic denitrification could improve the tolerance of autotrophic denitrifying bacteria to Cr (VI) toxicity. Furthermore, HRT would play an important role in affecting the microbial distribution and system performance, with HRT of higher than 0.15 day being critical for a high level removal of nitrate and chromate (over 90%)., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Biological wastewater treatment processes under a dynamic regime with respect to carbon substrate can result in microbial storage of internal polymers (e.g., polyhydroxybutyrate (PHB)) and their subsequent utilizations. These storage turnovers play important roles in nitrous oxide (N2O) accumulation during heterotrophic denitrification in biological wastewater treatment. In this work, a mathematical model is developed to evaluate the key role of PHB storage turnovers on N2O accumulation during denitrification for the first time, aiming to establish the key relationship between N2O accumulation and PHB storage production. The model is successfully calibrated and validated using N2O data from two independent experimental systems with PHB storage turnovers. The model satisfactorily describes nitrogen reductions, PHB storage/utilization, and N2O accumulation from both systems. The results reveal a linear relationship between N2O accumulation and PHB production, suggesting a substantial effect of PHB storage on N2O accumulation during denitrification. Application of the model to simulate long-term operations of a denitrifying sequencing batch reactor and a denitrifying continuous system indicates the feeding pattern and sludge retention time would alter PHB turnovers and thus affect N2O accumulation. Increasing PHB utilization could substantially raise N2O accumulation due to the relatively low N2O reduction rate when using PHB as carbon source.

Nanoscale zero valent iron (NZVI) based microbial denitrification has been demonstrated to be a promising technology for nitrate removal from groundwater. In this work, a mathematical model is developed to evaluate the performance of this new technology and to provide insights into the chemical and microbial interactions in the system in terms of nitrate reduction, ammonium accumulation and hydrogen turnover. The developed model integrates NZVI-based abiotic reduction of nitrate, NZVI corrosion for hydrogen production and hydrogen-based microbial denitrification and satisfactorily describes all of the nitrate and ammonium dynamics from two systems with highly different conditions. The high NZVI corrosion rate revealed by the model indicates the high reaction rate of NZVI with water due to their large specific surface area and high surface reactivity, leading to an effective microbial nitrate reduction by utilizing the produced hydrogen. The simulation results further suggest a NZVI dosing strategy (3-6 mmol/L in temperature range of 30-40 °C, 6-10 mmol/L in temperature range of 15-30 °C and 10-14 mmol/L in temperature range of 5-15 °C) during groundwater remediation to make sure a low ammonium yield and a high nitrogen removal efficiency.

A denitrifying phosphorus removal process undergoes frequent alternating anaerobic/anoxic conditions to achieve phosphate release and uptake, during which microbial internal storage polymers (e.g., Polyhydroxyalkanoate (PHA)) could be produced and consumed dynamically. The PHA turnovers play important roles in nitrous oxide (N2O) accumulation during the denitrifying phosphorus removal process. In this work, a mathematical model is developed to describe N2O dynamics and the key role of PHA consumption on N2O accumulation during the denitrifying phosphorus removal process for the first time. In this model, the four-step anoxic storage of polyphosphate and four-step anoxic growth on PHA using nitrate, nitrite, nitric oxide (NO), and N2O consecutively by denitrifying polyphosphate accumulating organisms (DPAOs) are taken into account for describing all potential N2O accumulation steps in the denitrifying phosphorus removal process. The developed model is successfully applied to reproduce experimental data on N2O production obtained from four independent denitrifying phosphorus removal study reports with different experimental conditions. The model satisfactorily describes the N2O accumulation, nitrogen reduction, phosphate release and uptake, and PHA dynamics for all systems, suggesting the validity and applicability of the model. The results indicated a substantial role of PHA consumption in N2O accumulation due to the relatively low N2O reduction rate by using PHA during denitrifying phosphorus removal.

The accumulation of the denitrification intermediates in wastewater treatment systems is highly undesirable, since both nitrite and nitric oxide (NO) are known to be toxic to bacteria, and nitrous oxide (N2O) is a potent greenhouse gas and an ozone depleting substance. To date, two distinct concepts for the modelling of denitrification have been proposed, which are represented by the Activated Sludge Model for Nitrogen (ASMN) and the Activated Sludge Model with Indirect Coupling of Electrons (ASM-ICE), respectively. The two models are fundamentally different in describing the electron allocation among different steps of denitrification. In this study, the two models were examined and compared in their ability to predict the accumulation of denitrification intermediates reported in four different experimental datasets in literature. The N-oxide accumulation predicted by the ASM-ICE model was in good agreement with values measured in all four cases, while the ASMN model was only able to reproduce one of the four cases. The better performance of the ASM-ICE model is due to that it adopts an "indirect coupling" modelling concept through electron carriers to link the carbon oxidation and the nitrogen reduction processes, which describes the electron competition well. The ASMN model, on the other hand, is inherently limited by its structural deficiency in assuming that carbon oxidation is always able to meet the electron demand by all denitrification steps, therefore discounting electron competition among these steps. ASM-ICE therefore offers a better tool for predicting and understanding intermediates accumulation in biological denitrification., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Competition for electrons among different steps of denitrification has previously been shown to occur, and to play an important role in the accumulation and emission of N2O in wastewater treatment. However, this electron competition is not recognized in the current denitrification models, limiting their ability to predict N2O accumulation during denitrification. In this work, a new denitrification model is developed for wastewater treatment processes. It describes electron competition among the four steps of denitrification, through modeling the carbon oxidation and nitrogen reduction processes separately, in contrast to the existing models that directly couple these two types of processes. Electron carriers are introduced to link carbon oxidation, which donates electrons to carriers, and nitrogen oxides reduction, which receives electrons from these carriers. The relative ability of each denitrification step to compete for electrons is modeled through the use of different affinity constants with reduced carriers. Model calibration and validation results demonstrate that the developed model is able to reasonably describe the nitrate, nitrite, and N2O reduction rates of a methanol-utilizing denitrifying culture under various carbon and nitrogen oxides supplying conditions. The model proposed, while subject to further validation, is expected to enhance our ability to predict N2O accumulation in denitrification.

Acidic pH has previously been found to increase nitrous oxide (N₂O) accumulation during heterotrophic denitrification in biological wastewater treatment. However, the mechanism of this phenomenon still needs to be clarified. By using an enriched methanol utilizing denitrifying culture as an example, this paper presents a comprehensive study on the effect of pH (6.0-9.0) on N₂O reduction kinetics with N₂O as the sole electron acceptor, as well as the effect of pH on N₂O accumulation with N₂O as an intermediate of nitrate reduction. The pH dependency of nitrate and nitrite reduction was also investigated. The maximum biomass-specificN₂O reduction rate is higher than the corresponding maximum nitrate and nitrite reduction rates in the entire pH range studied. However, the maximum biomass-specific N₂O reduction rate is much more sensitive to pH variation outside of the optimal range (pH 7.5 to pH 8.0) than the maximum biomass-specific nitrate and nitrite reduction rates. The half-saturation coefficient of the N₂O reductase increased from 0.10 mg N₂O-N/L to 0.92 mg N₂O-N/L as pH increased from pH 6.0 to 9.0. At pH 6.0, approximately 20% and 40% of the denitrified nitrate accumulated as N₂O in the presence and absence of methanol (as an exogenous carbon source), respectively. However, at pH 6.5, these fractions decreased to 0% and 30%, respectively. No N₂O accumulation occurred at pH 7.0 to 9.0 independent of the availability of methanol. These results suggest that the competition for electrons among different nitrogen oxides reductases likely plays a role in N₂O accumulation at low pH conditions., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Nitrous oxide (N(2)O) can be formed during biological nitrogen (N) removal processes. In this work, a mathematical model is developed that describes N(2)O production and consumption during activated sludge nitrification and denitrification. The well-known ASM process models are extended to capture N(2)O dynamics during both nitrification and denitrification in biological N removal. Six additional processes and three additional reactants, all involved in known biochemical reactions, have been added. The validity and applicability of the model is demonstrated by comparing simulations with experimental data on N(2)O production from four different mixed culture nitrification and denitrification reactor study reports. Modeling results confirm that hydroxylamine oxidation by ammonium oxidizers (AOB) occurs 10 times slower when NO(2)(-) participates as final electron acceptor compared to the oxic pathway. Among the four denitrification steps, the last one (N(2)O reduction to N(2)) seems to be inhibited first when O(2) is present. Overall, N(2)O production can account for 0.1-25% of the consumed N in different nitrification and denitrification systems, which can be well simulated by the proposed model. In conclusion, we provide a modeling structure, which adequately captures N(2)O dynamics in autotrophic nitrification and heterotrophic denitrification driven biological N removal processes and which can form the basis for ongoing refinements.

Abstract: Limited availability of carbon sources has been regarded as an important factor leading to N2O accumulation during denitrification in wastewater treatment. By varying the carbon (methanol) loading rate to a methanol utilizing denitrifying culture in the presence of various electron acceptors (nitrate, nitrite, N2O and their combinations), this study quantitatively investigated the electron distribution among different nitrogen oxide reductases during denitrification. The results showed that electron competition occurs under not only carbon limiting but also carbon abundant conditions. The electron distribution among the nitrogen oxide reductases is affected by the carbon loading rate, with a lower fraction of electrons distributed to the N2O reductase with reduced carbon loading rate. N2O accumulation occurs when the electron flux going to nitrite reduction is higher than that going to N2O reduction. The study also showed that, for the culture investigated, the carbon to nitrogen ratio is not a key factor leading to N2O accumulation. [Copyright &y& Elsevier]

Abstract: A new kinetic model is developed to describe the microbial storage process under anoxic conditions in activated sludge. The accumulation of nitrite in denitrification is taken into account in this model. The formation of storage polymers under anoxic conditions is coupled with energy generation and anoxic respiration, and their formation rate is found to be proportional to the substrate utilization rate. The sensitivity of the nitrogen uptake rate toward the stoichiometric and kinetic coefficients of the established model is analyzed by using an “absolute–relative” sensitivity function implemented in software AQUASIM. Batch denitrifying experiments were conducted to calibrate and validate the established model. The model is compared with ASM1 and ASM3, the two commonly used activated sludge models. Results show that the kinetic model established in this work is able to better describe the microbial storage processes under anoxic conditions. [Copyright &y& Elsevier]

Abstract: In the widely used Activated Sludge Models (ASMs) for biological wastewater treatment, denitrification is assumed as a single-step process, which is not true for some cases where a considerable amount of nitrite is build-up in system. This causes limitation to the application of ASMs. In this work, with a consideration of denitrification on both nitrite and nitrate, and an introduction of the simultaneous storage and growth concept, a two-step ASM3-based denitrification model is established. The sensitivity of the effluent chemical oxygen demand, and concentrations toward the stoichiometric and kinetic coefficients is analyzed. Model calibration is performed by comparing the measured and predicted values for model components. Thereafter, this model is evaluated with the experimental results of four different case studies on denitrification and compared with the original ASM3. The evaluation and comparison results demonstrate that the developed model is able to better and more mechanistically describe the denitrification process in the activated sludge systems. [Copyright &y& Elsevier]

Abstract: Acidic pH has previously been found to increase nitrous oxide (N2O) accumulation during heterotrophic denitrification in biological wastewater treatment. However, the mechanism of this phenomenon still needs to be clarified. By using an enriched methanol utilizing denitrifying culture as an example, this paper presents a comprehensive study on the effect of pH (6.0–9.0) on N2O reduction kinetics with N2O as the sole electron acceptor, as well as the effect of pH on N2O accumulation with N2O as an intermediate of nitrate reduction. The pH dependency of nitrate and nitrite reduction was also investigated. The maximum biomass-specific N2O reduction rate is higher than the corresponding maximum nitrate and nitrite reduction rates in the entire pH range studied. However, the maximum biomass-specific N2O reduction rate is much more sensitive to pH variation outside of the optimal range (pH 7.5 to pH 8.0) than the maximum biomass-specific nitrate and nitrite reduction rates. The half-saturation coefficient of the N2O reductase increased from 0.10 mg N2O-N/L to 0.92 mg N2O-N/L as pH increased from pH 6.0 to 9.0. At pH 6.0, approximately 20% and 40% of the denitrified nitrate accumulated as N2O in the presence and absence of methanol (as an exogenous carbon source), respectively. However, at pH 6.5, these fractions decreased to 0% and 30%, respectively. No N2O accumulation occurred at pH 7.0 to 9.0 independent of the availability of methanol. These results suggest that the competition for electrons among different nitrogen oxides reductases likely plays a role in N2O accumulation at low pH conditions. [Copyright &y& Elsevier]

[Display omitted] • Low concentrations (less than 5 mM) of EDTA improved nitrate removal rate. • High concentrations of EDTA hindered TN removal, leading to N 2 O accumulation. • EDTA increased the redox potential of pyrite, resulting in the improvement of pyrite oxidation. • Ferric iron as product of denitrification could facilitate the conversion of solid pyrite into S0. • Fe(II)/Fe(III) cycle was a critical biological process driving the bioleaching of pyrite. Pyrite autotrophic denitrification (PAD) is crucial for removing nitrate in groundwater and advanced wastewater treatment. While disulfide is typically seen as the electron donor, the role of iron in pyrite oxidation is often ignored. In this study, Ethylene Diamine Tetraacetic Acid (EDTA) had been proven to have the potential to facilitate pyrite oxidation by promoting the solubility of iron in solution, thereby enhancing the kinetics of pyrite autotrophic denitrification. Firstly, EDTA could improve PAD performance, i.e. the nitrate removal rate at EDTA addition of 5 mM was 0.69 mg N/L•h and twice that of the control. However, high concentrations of EDTA inhibited the activity of the nitrous oxide reductase, resulting in the maximum accumulation of N 2 O reaching 13.26 mg N/L with an EDTA concentration of 15 mM. Furthermore, the X-ray Photoelectron Spectroscopy (XPS) result indicated that the EDTA addition promoted the production of bioavailable sulfur, with the concentration of S0 at 15 mM EDTA being more than twice higher than that of the control. Transmission electron microscopy (TEM) analysis indicated that EDTA could relieve the damage of iron encrustation on cell activity because it was difficult for EDTA-chelated ferrous iron to penetrate the cell membrane. Microbial community analyses showed an increase abundance of iron oxidizing bacteria, suggesting that EDTA addition promoted the bioavailability of iron for Fe(II) autotrophic denitrification. Overall, this study will provide insights into the understanding of oxidation mechanism of PAD process and the practical application for efficient nitrogen removal and N 2 O mitigation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Various nitrogen removal pathways are exhibited by different species of GAOs. • The denitrification capacity of GAOs enables their potential synergy with anammox. • EPD offers a dependable and effective alternative for a nitrite supply. • EPDA process extends anammox-based application combined with PN and DPR processes. • Control strategies and functional microbial interaction mechanisms are imperative. The recently proposed glycogen accumulating organisms (GAOs)-driven anammox process, which terminates nitrate (NO 3 –) reduction to nitrite (NO 2 –) without external carbon source addition, has emerged as a promising alternative for supplying NO 2 – to anammox bacteria. The endogenous partial denitrification anammox (EPDA) process demonstrates superior nitrogen elimination efficiency, cost-effectiveness, and operational stability in low C/N wastewater treatment, concurrently mitigating greenhouse gas releases. This comprehensive review presents and discusses an in-depth analysis of the notable progress in EPDA, covering the physiology and biochemistry of diverse GAO-lineages under specified conditions, intricate microbial interactions and metabolic pathways shared between GAOs and anammox bacteria, and appropriate process modifications. Strategies for sludge morphology, carbon content, and spatial distribution of functional bacteria are discussed in detail to regulate the metabolic activity of EPDA. Future research needs to focus on balancing the competitiveness between functional consortia such as GAOs, phosphorus accumulating organisms (PAOs) and anammox bacteria in the synergetic system, and countermeasures to deal with the effects of adverse conditions on nutrient removal performance of the EPDA-based processes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Impacts of NH 2 OH and N 2 H 4 on the SDAD process were studied. • NH 2 OH and N 2 H 4 showed different potential for NO 2 − and N 2 O accumulation. • NIR/N 2 OR were sensitive to NH 2 OH inhibition, while N 2 H 4 primarily suppressed N 2 OR. Hydroxylamine (NH 2 OH) and hydrazine (N 2 H 4) are crucial intermediates in biological nitrogen removal processes, but their potential effects on the nitrogen-related metabolic pathway during the sulfur-driven autotrophic denitrification (SDAD) process remain unclear and were thus explored in this work. The results of dedicated batch tests indicated that the accumulation ratio of NO 2 − was positively correlated with the NH 2 OH concentration, reaching 86.2 ± 2.6% at the NH 2 OH concentration of 2.0 mg-N/L, while it was barely affected by the N 2 H 4 concentration. Furthermore, the joint inhibition of NH 2 OH and free nitrous acid led to the substantial accumulation of N 2 O (up to 32.9 ± 1.3%) during the NO 2 − reduction phase. In contrast, N 2 H 4 exhibited great potential for facilitating N 2 O accumulation in both the NO 3 − reduction and NO 2 − reduction phases with an accumulation ratio of 13.0 ± 0.3% and 27.5 ± 1.5%, respectively, at the N 2 H 4 concentration of 2.0 mg-N/L. Total enzyme activity analyses revealed that the presence of NH 2 OH inhibited the activity expression of all denitrifying enzymes, with the downstream enzymes (such as NO 2 − reductase and N 2 O reductase) being more susceptible to NH 2 OH. The addition of N 2 H 4 did not cause a significant decrease in the activity of NO 3 − reductase and NO 2 − reductase, but notably curbed the activity of N 2 O reductase. Based on the responses of the SDAD-related nitrogen metabolic pathway to NH 2 OH/N 2 H 4 , this work provided effective technical means for i) the promotion of NO 2 − accumulation which might enable the coupling of SDAD with Anammox and ii) the efficient recovery of N 2 O via the SDAD process. [ABSTRACT FROM AUTHOR]

In this work, a model framework was constructed to assess and predict nitrous oxide (N 2 O) production, substrate and microbe interactions in an anammox biofilm bioreactor. The anammox kinetics were extended by including kinetics of autotrophic soluble microbial products (SMP) formation, which consisted of utilization-associated products (UAP) and biomass-associated products (BAP). Heterotrophic bacteria growing on UAP, BAP and decay released substance (SS) were modelled to perform four-step sequential reductions from nitrate to dinitrogen gas. N 2 O was modelled as an intermidiate of heterotrophic denitrification via three pathways with UAP, BAP and SS as the electron donors. The developed model framework was evaluated using long-term operational data from an anammox biofilm reactor and satisfactorily reproduced effluent nitrogen and SMP as well as N 2 O emission factors under different operational conditions. The modeling results revealed that N 2 O was mainly produced with UAP as the electron donor while BAP and SS play minor roles. Heterotrophic denitrifiers growing on UAP would significantly contribute to N 2 O emission from anammox biofilm reactor even though heterotrophs only account for a relatively small fraction of active biomass in the anammox biofilm. Comprehensive simulations were conducted to investigate the effects of N loading rate and biofilm thickness, which indicated that maintaining a low N loading rate and a thick biofilm thickness were essential for high total nitrogen removal efficiency and low N 2 O emission. Image 1 • The model describes SMP production and three N 2 O pathways in anammox biofilm. • N 2 O via UAP serves as the main pathway, while BAP and SS pathways play minor roles. • SMP increased with decreasing N loading and increasing biofilm thickness. • A low N loading and a thick biofilm are crictial for high TN removal and low N 2 O. [ABSTRACT FROM AUTHOR]

Abstract Simultaneous sludge fermentation and nitrite removal is an effective approach to enhance nutrient removal from low carbon-wastewater. It was found in this work that the presence of nitrite largely promoted hydrogen production from acidic fermentation of waste activated sludge (WAS). The results showed that with an increase of nitrite from 0 to 250 mg/L, the maximal hydrogen yield increased from 8.5 to 15.0 mL/g VSS at pH 5.5 fermentation and 8.1–13.0 mL/g VSS at pH 6 fermentation. However, the maximal hydrogen yield from WAS fermentation at pH 8 remained almost constant (2.9–3.7 mL/g VSS) when nitrite was in the range of 0–250 mg/L. Further analyses revealed that free nitrous acid (FNA) rather than nitrite was the major contributor to the promotion of hydrogen yield. The mechanism investigations showed that FNA not only accelerated the disruption of sludge cells but also promoted the biodegradability of organics released, thereby provided more biodegradable substrates for subsequent hydrogen production. Although FNA inhibited activities of all microbes involved in the anaerobic fermentation, its inhibitions to hydrogen consumers were much severer than those to hydrolytic microorganisms and hydrogen producers. Further investigations with microbial community showed that FNA increased the abundances of hydrogen producers (e.g., Citrobacter sp.) and denitrifiers (e.g., Dechloromonas sp.), but reduced the abundances of hydrogen consumers (e.g., Clostridium_aceticum). This work demonstrated for the first time that FNA in WAS fermentation systems enhanced hydrogen production. The findings obtained expand the application field of FNA and may provide supports for sustainable operation of wastewater treatment plants. Graphical abstract Image 1 Highlights • Nitrate addition promoted hydrogen production from anaerobic fermentation of sludge. • Free nitrous acid (FNA) rather than nitrite was the major contributor to the promotion of hydrogen yield. • FNA not only accelerated the disruption of sludge cells but also promoted the biodegradability of organics released. • FNA increased the abundances of hydrogen producers and denitrifiers, but reduced the abundances of hydrogen consumers. [ABSTRACT FROM AUTHOR]

Complete nitrogen removal has recently been demonstrated by integrating anaerobic ammonium oxidation (anammox) and denitrifying anaerobic methane oxidation (DAMO) processes. In this work, the effect of methane partial pressure on the performance of a membrane biofilm reactor (MBfR) consisting of DAMO and anammox microorganisms was evaluated. The activities of DAMO archaea and DAMO bacteria in the biofilm increased significantly with increased methane partial pressure, from 367 ± 9 and 58 ± 22 mg-N L −1 d −1 to 580 ± 12 and 222 ± 22 mg-N L −1 d −1 , respectively, while the activity of anammox bacteria only increased slightly, when the methane partial pressure was elevated from 0.24 to 1.39 atm in the short-term batch tests. The results were supported by a long-term (seven weeks) continuous test, when the methane partial pressure was dropped from 1.39 to 0.78 atm. The methane utilization efficiency was always above 96% during both short-term and long-term tests. Taken together, nitrogen removal rate (especially the nitrate reduction rate by DAMO archaea) and methane utilization efficiency could be maintained at high levels in a broad range of methane partial pressure (0.24–1.39 atm in this study). In addition, a previously established DAMO/anammox biofilm model was used to analyze the experimental data. The observed impacts of methane partial pressure on biofilm activity were well explained by the modeling results. These results suggest that methane partial pressure can potentially be used as a manipulated variable to control reaction rates, ultimately to maintain high nitrogen removal efficiency, according to nitrogen loading rate. [ABSTRACT FROM AUTHOR]

Nitrate contamination of groundwater has been recognized as a significant environmental problem world widely. Sulfur-driven mixotrophic denitrification has been demonstrated as a promising groundwater treatment process, which though plays an important role in nitrous oxide (N 2 O) emissions, significantly contributing to the overall carbon footprint of the system. However, the current process optimizations only focus on nitrate removal and excess sulfate control, with the N 2 O emission being ignored. In this work, an integrated mathematical model was proposed to evaluate the N 2 O emission as well as the excess sulfate production and carbon source utilization in sulfur-driven mixotrophic denitrification process. In this model, autotrophic and heterotrophic denitrifiers use their corresponding electron donors (sulfur and organic matter, respectively) to reduce nitrate to nitrogen gas, with each modeled as three-step denitrification (NO 3 − to N 2 via NO 2 − and N 2 O) driven by sulfur or organic matter to describe all potential N 2 O accumulation steps. The developed model, employing model parameters previously reported in literature, was successfully validated using N 2 O and sulfate data from two mixotrophic denitrification systems with different initial conditions. Modeling results revealed substantial N 2 O accumulation due to the relatively low autotrophic N 2 O reduction activity as compared to heterotrophic N 2 O reduction activity, explaining the observation that higher carbon source addition resulted in lower N 2 O accumulation in sulfur-driven mixotrophic denitrifying system. Based on the validated model, optimizations of the overall system performance were carried out. Application of the model to simulate long-term operations of sulfur-driven mixotrophic denitrification process indicates that longer sludge retention time reduces N 2 O emission due to better retention of active biomass. High-level total nitrogen removal with significant N 2 O emission mitigation, appropriate excess sulfate control and maximized COD utilization can be achieved simultaneously through controlling the influent nitrate and COD concentrations. [ABSTRACT FROM AUTHOR]

The mobilization of arsenic (As) in aquatic environment (groundwater) can cause severe environmental and healthy issues. To develop remediation strategies, we proposed a comprehensive mathematical model to describe the As removal in a arsenite (As (III)) oxidizing and ferrous iron (Fe (II)) oxidizing denitrifying granular biofilm system. In the model framework, the growth-linked microbial oxidation of As (III) and Fe (II) was coupled to chemolithotrophic denitrification of one-step reduction of nitrate to nitrogen gas. Meanwhile, the precipitation of ferric iron (Fe (III)) and adsorption of arsenate (As (V)) onto the biogenic Fe (III) (hydr)oxides were also considered. The model was calibrated by comparing the model predictions against experimental data from batch experiments. The validity of the model was further demonstrated through testing against long-term experimental results from five independent bioreactors with different reactor configurations and operational conditions. Modeling results revealed that the granule size would exert a limited impact on arsenic and iron removal. Nevertheless, their removal efficiencies increased rapidly with the increase of hydraulic retention time (HRT) from 1 h to 12 h, but became independent of HRT as it further increased. The established model framework enables interpretation of a range of experimental observations on As and Fe removal and helps to identify the optimal conditions for enhanced arsenic remediation. [ABSTRACT FROM AUTHOR]

Perchlorate (ClO 4 − ) is recognized as an important contaminant in surface water and groundwater, which would pose health risks at very low concentrations. A methane-based membrane biofilm reactor (MBfR) has been successfully demonstrated for perchlorate reduction, which provided an alternative solution for perchlorate remediation with low cost. In this work, a multispecies biofilm model was developed to evaluate perchlorate reduction in the methane-based MBfR under different operational conditions. The model was calibrated and validated using the experimental data from the long-term operation of the MBfR at seven distinct stages. The results suggested that the developed model could satisfactorily describe perchlorate reduction and denitrification performances in the MBfR (R 2 > 0.9). The modeling results provided insight into the microbial community distribution in the biofilm, with aerobic methanotrophs and perchlorate reduction bacteria being mainly located at the membrane side (∼60%) and heterotrophic bacteria being situated near the liquid side (∼50%). The model simulations indicated that over 80% of perchlorate removal efficiency could be achieved through controlling the optimal combinations of methane pressure (P CH4 ) and perchlorate loading (L ClO4 ) (e.g., applying a P CH4 of 30 kPa at a L ClO4 of 0.08 g Cl/m 2 /d). In addition, the perchlorate reduction would be inhibited by the presence of nitrate and nitrite in the MBfR, which should be appropriately controlled during the future practical application of the promising process. [ABSTRACT FROM AUTHOR]

The mainstream anaerobic ammonium oxidation (anammox) has attracted extensive attention recently, particularly due to its potential of transforming current wastewater treatment plants from energy consuming to energy neutral or positive. However, the presence of biodegradable chemical oxygen demanding (COD, 20–80 mg COD L −1 ) in the mainstream anammox reactor stimulates the growth of heterotrophic bacteria, which would compete for oxygen with ammonia-oxidizing bacteria (AOB) and for nitrite with anammox bacteria, thus interfering with the autotrophic nitrogen removal process. In the present work, with consideration of granule size distribution, a one-dimensional model describing the mainstream simultaneous partial nitrification, anammox and denitrification (SNAD) in a granule-based reactor was established, calibrated and validated, based on the long-term experimental results. Through applying the verified model, simulation studies were conducted and the results showed that the effluent total nitrogen concentration of <5 mg N L −1 could be achieved at C/N ratio of 0.2–0.6, DO concentration of 0.2–0.4 mg L −1 and granule radius of 300–600 μm. The combined effects indicated that the SNAD process with TN removal efficiency >90% was obtained at C/N ratio and DO concentration of 0.2–1.0 and 0.2–0.4 mg O 2 L −1 respectively. Finally, the various granule size distribution patterns were simulated, which confirmed that the size distribution needed to be incorporated in the model to accurately describe the granular anammox system, considering a model based on a uniform granule size does not reflect the real situations. These results provide guides to optimize the operation of mainstream granular autotrophic nitrogen removal process. [ABSTRACT FROM AUTHOR]

[Display omitted] • A SNAD model aimed to recover N 2 O from Fe(II)EDTA-NO was developed and evaluated. • Impacts of key operational conditions on N 2 O recovery were studied. • S/N mass ratio of ∼ 1.0 and high gas/liquid volumetric ratios favoured N 2 O recovery. • N 2 O recovery efficiency of up to 80.2%−84.9% was reached within 3.1 h–3.5 h. Instead of the conventional perception of nitrous oxide (N 2 O) as a potent greenhouse gas whose production should be minimized, this work aimed to assess N 2 O recovery as a potential energy source from nitric oxide (NO) in the form of Fe(II)EDTA-NO through element sulfur (S0) or thiosulfate (S 2 O 3 2−)-driven NO-based autotrophic denitrification (SNAD S0 or SNAD S2O3). A mathematical model was proposed to describe substrate dynamics related to N 2 O production and reduction and was successfully calibrated and validated using batch experimental data from lab-scale SNAD S0 and SNAD S2O3 systems under different substrates conditions. The model was subsequently employed to assess the potential of N 2 O accumulation and recovery by altering the S/N mass ratio and the ratio of gas volume to liquid volume of the system. The simulation results suggested that with a S/N mass ratio of nearly 1.0, high-purity N 2 O could be more rapidly and efficiently recovered from Fe(II)EDTA-NO in the SNAD S0 and SNAD S2O3 systems with a higher ratio of gas volume to liquid volume (i.e., a N 2 O recovery efficiency of up to 80.2%−84.9% reached within 3.1 h−3.5 h under the studied conditions). Comparatively, the SNAD S0 process showed an economic and viable advantage for practical applications to the efficient treatment and resource utilization of NO-containing flue gas. [ABSTRACT FROM AUTHOR]

The presence of a high level of dissolved methane (e.g., 20–26 g m −3 ) in the anaerobic sludge digestion liquor represents a major challenge to the treatment of this stream, as its emission to the atmosphere contributes significantly to the carbon footprint of wastewater treatment. Here we propose a new approach to simultaneous ammonium and dissolved methane removal from the anaerobic digestion liquor through integrating partial nitritation-Anammox and denitrifying anaerobic methane oxidation (DAMO) processes in a single-stage membrane biofilm reactor (MBfR). In such an MBfR, the anaerobic digestion liquor is provided in the bulk liquid, while oxygen is supplied through gas-permeable membranes to avoid dissolved methane stripping. A previously developed model with appropriate extensions was applied to assess the system performance under different operational conditions and the corresponding microbial interactions. Both influent surface loading (or hydraulic retention time) and oxygen surface loading are found to significantly influence the total nitrogen (TN) and dissolved methane removal, which jointly determine the overall system performance. The counter diffusion and concentration gradients of substrates cause microbial stratification in the biofilm, where ammonia-oxidizing bacteria (AOB) attach close to the membrane surface (biofilm base) where oxygen and ammonium are available, while Anammox and DAMO microorganisms jointly grow in the biofilm layer close to the bulk liquid where methane, ammonium, and nitrite are available with the latter produced by AOB. These results provide first insights and useful information for the design and operation of this new technology for simultaneous ammonium and dissolved methane removal in its potential future applications. [ABSTRACT FROM AUTHOR]

• A novel iGB-A/PD reactor was developed for treating mainstream wastewaters. • An effluent TN of ∼3 mg•L−1 was achieved at a NRR of 0.8 ± 0.1 kg-N•m−3•d−1. • Multilevel A/PD is responsible for the exceptional performance in the single reactor. • PD and DNRA were responsible for 50% and 25% of nitrate reduction, respectively. This study describes an integrated granular sludge and fixed-biofilm (iGB) reactor innovatively designed to carry out the anammox/partial-denitrification (A/PD) process for nitrogen removal with mainstream municipal wastewater. The iGB-A/PD reactor consists of anammox granules inoculated in the lower region of reactor and an acclimated fixed-biofilm positioned in the upper region. Compared to the other reported A/PD systems for mainstream wastewater treatment, this iGB-A/PD reactor is notable due to its higher quality effluent with a total inorganic nitrogen (TIN) of ∼3 mg•L−1 and operation at a high nitrogen removal rate (NRR) of 0.8 ± 0.1 kg-N•m−3•d−1. Reads-based metatranscriptomic analysis found that the expression values of hzsA and hdh , key genes associated with anammox, were much higher than other functional genes on nitrogen conversion, confirming the major roles of the anammox bacteria in nitrogen bio-removal. In both regions of the reactor, the nitrate reduction genes (napA / narG) had expression values of 56–99 RPM, which were similar to that of the nitrite reduction genes (nirS / nirK). The expression reads from genes for dissimilatory nitrate reduction to ammonium (DNRA), nrfA and nirB , were unexpectedly high, and were over the half of the levels of reads from genes required for nitrate reduction. Kinetic assays confirmed that the granules had an anammox activity of 16.2 g-NH 4 +-N•kg−1-VSS•d−1 and a nitrate reduction activity of 4.1 g-N•kg−1-VSS•d−1. While these values were changed to be 4.9 g- NH 4 +-N•kg−1-VSS•d−1and 4.3 g-N•kg−1-VSS•d−1 respectively in the fixed-biofilm. Mass flux determination found that PD and DNRA was responsible for ∼50% and ∼25% of nitrate reduction, respectively, in the whole reactor, consistent with high effluent quality and treatment efficiency via a nitrite loop. Metagenomic binning analysis revealed that new and unidentified anammox species, affiliated with Candidatus Brocadia, were the dominant anammox organisms. Myxococcota and Planctomycetota were the principal organisms associated with the PD and DNRA processes, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

Using solar energy to catalytically convert nitrate into ammonia is attractive for waste recycling and sustainable development. However, the rapid recombination of electron-hole pairs and the poor selectivity are obstructing photocatalytic nitrate reduction to ammonia to be mass applied currently. In this work, we reported a facile synthesis of carbon/bismuth/bismuth oxide photocatalyst via a one-pot hydrothermal reaction without using reducing reagent. Compared with α-bismuth oxide (α-Bi 2 O 3), the photocatalytic ammonia yield of the optimum sample increased 3.65 times. In addition, the ammonia selectivity increased from 65.21% to 95.00%. The highly enhanced photocatalytic performance was attributed to the surface plasmon resonance of metallic bismuth. Meanwhile, the formation of carbon enables to boost the transfer of electrons significantly. Under light irradiation, electrons can be accumulated on metallic bismuth, effectively boosting the reduction of nitrate. The findings of this work will contribute to the recycling of nitrate for ammonia synthesis and sustainable environmental development. [Display omitted] • A carbon/bismuth/Bi 2 O 3 photocatalyst prepared via a one-pot hydrothermal reaction. • The composites exhibited efficient NO 3 − reduction to NH 3 with high selectivity. • The mechanism for the enhanced photocatalytic activity was studied. [ABSTRACT FROM AUTHOR]

The increasing use of perfluorooctanoic acid (PFOA) raises concerns about its potential toxicity to the environment. However, the interaction between PFOA and aerobic granular sludge has never been documented. This work therefore aims to provide such support through investigating the fate of PFOA at environmentally relevant levels in aerobic granular sludge systems and its impact on aerobic granular sludge. Experimental results showed that 32.0%∼36.4% of wastewater PFOA was removed by aerobic granular sludge in stable operation when PFOA concentration was ranged from 0.1 to 1.0 mg/L. Mass balance analyses and X-ray photoelectron spectroscopy survey scan revealed that the removal of PFOA was dominated by adsorption rather than biodegradation, and sorption kinetic analysis indicated that inhomogeneous multilayer adsorption was responsible for this removal. The adsorbed PFOA deteriorated the settleability of granular sludge and biological nitrogen and phosphorus removal significantly. Experimental results showed that 1.0 mg/L PFOA inhibited anaerobic phosphate release, aerobic phosphate uptake, nitrate reduction, and nitrite reduction processes by 60%, 50%, 13.1%, and 5.8%, respectively. It was observed that PFOA induced large amounts of filamentous villus growing on the surface and increased the extracellular polymeric substances of granular sludge. Fourier-transform infrared spectra and X-ray photoelectron spectroscopy spectrum showed that several function groups in extracellular polymeric substances such as hydroxyl groups, amides and polysaccharides were affected by PFOA. It was also found that PFOA inhibited the cyclic transformations of polyhydroxyalkanoates and glycogen. Microbial community analyses showed that PFOA decreased the abundances of Nitrosomonas , Nitrospira , Accumulibacter , and other function microbes such as Rhodospirillaceae , Thauera , and Azoarcus. Image 1 • PFOA was removed by aerobic granular sludge mainly through adsorption rather than biodegradation. • PFOA deteriorated the settleability and biological nitrogen and phosphorus removal of aerobic granular sludge. • Several function groups such as hydroxyl groups, amides and polysaccharides were affected by PFOA. • PFOA inhibited the transformations of polyhydroxyalkanoates and glycogen. [ABSTRACT FROM AUTHOR]