Initial nitrite concentration promote nitrite-oxidizing bacteria activity recovery from transient anoxia: Experimental and modeling investigations

Effect of initial ammonium concentration on a one-stage partial nitrification/anammox biofilm system: Nitrogen removal performance and the microbial community

One-stage partial nitrification coupled with anammox (PN/A) technology effectively reduces the energy consumption of a biological nitrogen removal system. Inhibiting nitrite-oxidizing bacteria (NOB) is essential for this technology to maintain efficient nitrogen removal performance. Initial ammonium concentration (IAC) affects the degree of inhibited NOB. In this study, the effect of the IAC on a PN/A biofilm was investigated in a moving bed biofilm reactor. The results showed that nitrogen removal efficiency decreased from 82.49% ± 1.90% to 64.57% ± 3.96% after the IAC was reduced from 60 to 20 mg N/L, while the nitrate production ratio increased from 13.87% ± 0.90% to 26.50% ± 3.76%. NOB activity increased to 1,133.86 mg N/m2/day after the IAC decreased, approximately 4-fold, indicating that the IAC plays an important inhibitory role in NOB. The rate-limiting step in the mature biofilm of the PN/A system is the nitritation process and is not shifted by the IAC. The analysis of the microbial community structure in the biofilm indicates that the IAC was the dominant factor in changes in community structure. Ca. Brocadia and Ca. Jettenia were the main anammox bacteria, and Nitrosomonas and Nitrospira were the main AOB and NOB, respectively. IAC did not affect the difference in growth between Ca. Brocadia and Ca. Jettenia. Thus, modulating the IAC promoted the PN/A process with efficient nitrogen removal performance at medium to low ammonium concentrations.

Exploring Aeration Strategies for Enhanced Simultaneous Nitrification and Denitrification in Membrane Aerated Bioreactors: A Computational Approach

In this study we employ computational methods to investigate the influence of aeration strategies on simultaneous nitrification-denitrification processes. Specifically, we explore the impact of periodic and intermittent aeration on denitrification rates, which typically lag behind nitrification rates under identical environmental conditions. A two-dimensional deterministic multi-scale model is employed to elucidate the fundamental processes governing the behavior of membrane aerated biofilm reactors (MABRs). We aim to identify key factors that promote denitrification under varying aeration strategies. Our findings indicate that the concentration of oxygen during the off phase and the duration of the off interval play crucial roles in controlling denitrification. Complete discontinuation of oxygen is not advisable, as it inhibits the formation of anaerobic heterotrophic bacteria, thereby impeding denitrification. Extending the length of the off interval, however, enhances denitrification. Furthermore, we demonstrate that the initial inoculation of the substratum (membrane in this study) influences substrate degradation under periodic aeration, with implications for both nitrification and denitrification. Comparison between continuous and periodic/intermittent aeration scenarios reveals that the latter can extend the operational cycle of MABRs. This extension is attributed to relatively low biofilm growth rates associated with non-continuous aeration strategies. Consequently, our study provides a comprehensive understanding of the intricate interplay between aeration strategies and simultaneous nitrification-denitrification in MABRs. The insights presented herein can contribute significantly to the optimization of MABR performance in wastewater treatment applications.

Achieving ultra-high nitrogen and phosphorus removal from real municipal wastewater in a novel continuous-flow anaerobic/aerobic/anoxic process via partial nitrification, endogenous denitrification and nitrite-type denitrifying phosphorus removal

Achieving economic and efficient removal of nutrients in mainstream wastewater treatment plants (WWTPs) continues to be a challenging research topic. In this study, a continuous-flow anaerobic/aerobic/anoxic system with sludge double recirculation (AOA-SDR), which integrated partial nitrification (PN), endogenous denitrification (ED) and nitrite-type denitrifying phosphorus removal (nDNPR), was constructed to treat real carbon-limited municipal wastewater. The average effluent concentrations of total inorganic nitrogen (TIN) and PO43--P during the stable operation period were 1.8 and 0.3 mg/L, respectively. PN was achieved with an average nitrite accumulation ratio of 90.4 % by combined strategies. Adequate storage of polyhydroxyalkanoates and glycogen in the anaerobic zone promoted the subsequent nitrogen removal capacity. In the anoxic zone, nitrite served as the main electron acceptor for the denitrifying phosphorus removal process. Mass balance analysis revealed that nDNPR contributed to 23.6 % of TIN removal and 44.7 % of PO43--P removal. The enrichment of Nitrosomonas (0.45 %) and Ellin 6067 (1.31 %), along with the washout of Nitrospira (0.15 %) provided the bacterial basis for the successful implementation of PN. Other dominant endogenous heterotrophic bacteria, such as Dechlormonas (10.81 %) and Candidatus Accumulibacter (2.96 %), ensured simultaneous nitrogen and phosphorus removal performance. The successful validation of integrating PN, ED and nDNPR for advanced nutrient removal in the AOA-SDR process provides a transformative technology for WWTPs.

Enhanced nitrogen removal from municipal wastewater via a novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) with an ultra-low hydraulic retention time (HRT)

Anaerobic ammonia oxidation (Anammox) is a highly productive research area in municipal wastewater treatment. A novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) was established in this paper using a sequencing batch reactor (SBR) and two up-flow sludge beds (USBs). Municipal wastewater after carbon removal pretreatment in SBR entered PN/A-USB. PN/A process was initiated and enhanced by optimizing the intermittent aeration mode under low dissolved oxygen (DO). After enhancing and stabilizing the PD/A process, PN/A effluent entered the PD/A-USB along with raw municipal wastewater at a ratio of 4:1 and the combined system was established. Through this, this study achieved a nitrogen removal efficiency (NRE) of 84.9 % from municipal wastewater at an ultra-low total hydraulic residence time (HRT) of 3.9 h. Candidatus Brocadia (1.8 % in PN/A, 1.0 % in PD/A) was the only functional anammox bacterium in the combined process.

Mathematical modelling of an intermittent anoxic/aerobic MBBR: Estimation of nitrification rates and energy savings

This study aimed at modelling the performance of a novel MBBR configuration, named A/O-MBBR, comprised of a pre-anoxic reactor, with an HRT of 4.5 h, coupled with an intermittent anoxic/aerobic MBBR (HRT = 6.8 h). The lab-scale system was fed with municipal wastewater with an average influent Total Ammonia Nitrogen (TAN) and total COD (TCOD) concentrations of 46 mg of TAN-N L^-1 and 310 mg TCOD L^-1. During the whole experimental period, TAN removal efficiency was always higher than 96%; denitrification was also very effective, achieving nitrate and nitrite concentrations in the effluent both lower than 5 mg NO_x-N L^-1 on average. Moreover, TCOD average removal efficiency was equal to 85%. Modelling was performed to investigate the nitrification efficacy enhancement; to this aim, a biofilm model was developed, adopting the equations for mixed-culture biofilms and the Activated Model Sludge n°1 (ASM1) for the biological processes rates. The model allowed to determine the maximum uptake rate for autotrophic growth (μ_A was 2.5 d^-1) and the semisaturation constant (K_OA was 0.2 mg O₂ L^-1), suggesting that the nitrification process was 3-fold faster than average and very effective at low oxygen concentrations. The model estimated that about 85% of TAN was removed by the biofilm and only the remaining part by suspended biomass in the bulk liquid. Finally, it was assessed that the A/O-MBBR configuration allowed for a 45-60% savings of the energy requirement compared to a Benchmark WWTP layout.

Simulation and experimental verification of nitrite-oxidizing bacteria inhibition by alternating aerobic/anoxic strategy

Anaerobic ammonium oxidation (ANAMMOX) is a promising technology for sewage treatment. Alternating aerobic/anoxic conditions have been widely adopted to achieve partial nitrification (PN), so as to provide substrates for ANAMMOX. In this study, the feasibility of PN with the strategy of intermittent aeration was investigated under mainstream conditions. At a low dissolved oxygen (DO) concentration, the nitrogen conversion characteristic under different intermittent aeration modes was evaluated by mathematical simulation and experimental method with (1) ordinary activated sludge, (2) mixed sludge with anaerobic ammonia-oxidizing bacteria (AnAOB), and (3) PN sludge, as seed sludge. The existence of functional microorganisms, such as AnAOB and denitrifying bacteria, which can utilize nitrites, was the prerequisite for NOB activity inhibition in the alternating aerobic/anoxic condition. Therefore, low nitrite may be an important factor in NOB activity inhibition under alternating aerobic/anoxic conditions. This study demonstrated a key controlling factor for NOB activity inhibition with alternating aerobic/anoxic condition.

A novel control strategy to strengthen nitrogen removal from domestic wastewater through eliminating nitrite oxidizing bacteria in a plug-flow process

In this study, intermittent aeration strategy was investigated in a plug-flow reactor on real municipal wastewater. Over 200 days of operation, the total inorganic nitrogen (TIN) removal efficiency of 72.43 ± 7.56% was achieved with a total aerobic hydraulic retention time in the range 2.4-3.0 h under a low C/N ratio of 3.19. The batch tests showed that the activity of nitrite oxidizing bacteria (NOB) was effectively inhibited, and simultaneous nitrification and denitrification via nitrite were observed under double intermittent aeration mode. The Illumina MiSep sequencing revealed that the relative abundance of the Nitrospira as the only detected NOB, decreased from 2.22% (day 0) to 0.91% (day 207) at the genus level. Overall, this study provides a new strategy for NOB suppression to strengthen nitrogen removal from low C/N domestic wastewater through the continuous process.

Enhanced nitrogen removal from low COD/TIN mainstream wastewater in a continuous plug-flow reactor via partial nitrification, simultaneous anammox and endogenous denitrification (PN-SAED) process

A continuous plug-flow reactor with anaerobic/front-aerobic/anoxic/post-aerobic zones, where partial nitrification occurred in the front-aerobic zone, followed by simultaneous anammox and endogenous denitrification in the anoxic zone (PN-SAED), was built up to treat municipal wastewater. Alternating anoxic/aerobic conditions and longer anoxic duration facilitated stable partial nitrification. The nitrite accumulation ratio (NAR) was maintained at 97.4 ± 1.2%, with temperatures between 13.3℃ to 19.8℃. Candidatus Brocadia were naturally enriched in-situ from the anoxic zone with relative abundances of 31.93% and 6.67% on the agitator blade and carriers, respectively. High removal efficiencies of total inorganic nitrogen (TIN) (95.1 ± 1.9%) and effluent TIN (2.6 ± 1.1 mg N/L) were acquired from low COD/TIN (3.4 ± 0.4) municipal wastewater with anammox contribution of 13.5%±5.8% to TIN removal. The PN-SAED process is a promising mainstream nitrogen removal method.

Inhibition of nitrite-oxidizing bacteria in automatic recycling PN/ANAMMOX under mainstream conditions

At present, sustainable and stable partial nitrification has not been widely achieved in the mainstream PN/ANAMMOX process. Here, the feasibility of sustainable and stable partial nitrification was demonstrated in automatic recycling PN/ANAMMOX reactor under mainstream conditions using both simulation and experimental methods. Stable nitrite accumulation in the aerobic zone could be achieved via regulating dissolved oxygen (DO) concentrations and sludge retention time (SRT). The DO concentrations required for the repression of nitrite-oxidizing bacteria (NOB) were lower at longer SRTs. The DO concentrations and SRTs required for NOB repression were lower at lower temperatures. However, NOB repression was diminished by a persistent low DO and short SRT under mainstream conditions. With the introduction of automatic recycling, sustainable and stable partial nitrification was achieved. Effluent recycling could limit the nitrite-nitrogen required for NOB growth. Collectively, effluent recycling may serve as a feasible and useful strategy for NOB inhibition during the PN/ANAMMOX process.

Achieving stable mainstream nitrogen and phosphorus removal assisted by hydroxylamine addition in a continuous partial nitritation/anammox process from real sewage

Hydroxylamine (NH₂OH) as the putative intermediate for anammox ensures the robustness of partial nitritation/anammox (PN/A) process; however, the feasible for NH₂OH addition to improve the stability of PN/A process under low-strength ammonia (NH₄⁺-N) condition need to be further investigated. In this study, the restoration and steady operation of mainstream PN/A process were investigated to treat real sewage with in situ NH₂OH added in a continuous alternating anoxic/aerobic with integrated fixed-film activated sludge (A³-IFAS) reactor. Results showed that the deteriorated PN/A process caused by nitrate (NO₃^--N) built-up was rapidly restored with a distinct decrease of the NO₃^--N_produced/NH₄⁺-N_consumed ratio from 28.7% to <10.0% within 20 days, after 5 mg N/L of NH₂OH was added daily into the aerobic zone of A³-IFAS reactor. After 230 days of operation, the average total nitrogen (TN) and phosphate (PO₄^3--P) removal efficiencies of 80.8% and 91.5%, respectively were stably achieved, with average effluent sCOD, NH₄⁺-N, TN and PO₄^3--P concentrations reaching 23.1, 2.3, 7.7 and 0.4 mg/L, respectively. Microbial community characterization revealed Candidatus Brocadia (3.60% and 2.92%) and Ignavibacteriae (1.56% and 2.66%) as the dominant anammox bacteria and denitrifying bacteria, respectively, jointly attached in the biofilm_1 and biofilm_2, while Candidatus Microthrix (5.17%) dominant in floc sludge was main responsible for phosphorus removal. This study confirmed that NH₂OH addition is an effective strategy for nitrite-oxidizing bacteria suppression, contributing to the in situ restoration of PN/A process and high stable mainstream nitrogen and phosphorus removal in a continuous PN/A process from real sewage.

A continuous plug-flow anaerobic/aerobic/anoxic/aerobic (AOAO) process treating low COD/TIN domestic sewage: Realization of partial nitrification and extremely advanced nitrogen removal

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

Achieving simultaneous nitritation, anammox and denitrification (SNAD) in an integrated fixed-biofilm activated sludge (IFAS) reactor: Quickly culturing self-generated anammox bacteria

In this study, by inoculating nitritation suspended sludge, simultaneous nitritation, anammox and denitrification (SNAD) was established quickly in an integrated fixed-biofilm activated sludge (IFAS) reactor to treat high-ammonia municipal wastewater. Results showed that, deep-level total nitrogen and chemical oxygen demand removal efficiencies (92.8% and 78.8%, respectively) were achieved, and their effluent concentrations were 13.2 and 39.3 mg/L, respectively. Excess generation of nitrate was once occurred under continuous aerobic condition, but it could be solved by suppressing nitrite oxidizing bacteria activity stably via switching to intermittent aeration mode (alternate 7 min of aerobic and 21 min of anoxic) and rising influent ammonium concentration temporarily (lasted 31 days). High-throughput sequencing analysis revealed that, Candidatus_Brocadia, as dominant anammox bacteria, was self-generated in flocs (2.93%) but mainly biofilm (7.67%), whereas uncultured_f_Nitrosomonadaceae as ammonia oxidizing bacteria was mainly found in flocs (2.4%). This work not only demonstrated that anammox bacteria could be self-generated and retained in the SNAD-IFAS system, but also suggested a promising application of the SNAD-IFAS in wastewater treatment plants.

An integrated mainstream and sidestream strategy for overcoming nitrite oxidizing bacteria adaptation in a continuous plug-flow nutrient removal process

An integrated mainstream aeration and sidestream sludge treatment was demonstrated to be effective in overcoming the adaptationof nitrite oxidizing bacteria (NOB) in an anoxic/oxic process. Results showed that by employing the alternating free nitrous acid and free ammonia (FNA/FA) sidestream sludge treatment alone, nitritation was established but varied, which was addressed by integrating alternating aeration with step feeding (ALASF) in reactor. Two critical considerations contributed to stable effluent nitrite accumulation (>83.8 %)and nitrogen removal (>83.0 %): 1) aerobic sludge rather than return sludge should be taken for FNA/FA treatment to avoid anoxic starvation which facilitated NOB recovery; 2) ALASF ensured timely denitritation and created constant anoxic disturbance for NOB inhibition. Nitrospira and Nitrobacter after 540-day operation were 0.38 % of seed sludge.A20 % reduction of operating cost was obtained in this nitritation process. This study moved nitritation one step closer to application in continuous plug-flow process from municipal wastewater.

Multiple strategies for maintaining stable partial nitritation of low-strength ammonia wastewater

Stable production of nitrite is an essential technical challenge for mainstream anaerobic ammonia oxidation (Anammox). Due to difficulties in the stable inhibition of nitrite oxidizing bacteria (NOB) and maintenance of long-term partial nitritation (PN), integrated multiple, rather than a single, controlling strategies were preferred especially in a continuous-flow treatment system. A mathematically model was developed to evaluate effects of integrated multiple-strategies on ammonia oxidizing bacteria (AOB) and NOB. Through experimental study and model simulation, intermittent aeration and low SRT (3.5 d) resulted in unstable nitrite accumulation. Integrated multiple-strategies of intermittent aeration, low SRT (3.5 d) and bioaugmentation achieved nitrite accumulation rate of 81% and NO₂^--N/NH₄⁺-N ratio in effluent of 1.29, which was preferable for further anammox process. Meanwhile, the richness and diversity of microbial community increased due to the bioaugmentation. The AOB/NOB ratio increased from 13.8 to 34.1 which facilitated nitrite accumulation. In combination with bioaugmentation, the observed growth rates of AOB and NOB increased from -0.0835 and -0.0282 to 0.0434 and 0.0127 d^-1, respectively, which promoted AOB outcompeting NOB in the mixed liquid.