Restoration of real sewage partial nitritation-anammox process from nitrate accumulation using free nitrous acid treatment

Intermittent hydroxylamine dosing to strengthen stability of partial nitrification and nitrogen removal efficiency through continuous-flow anaerobic-aerobic-anoxic reactor treating municipal wastewater

Intermittent hydroxylamine (NH2OH) dosing strategy was applied to enhance the stability of partial nitrification and total nitrogen (N) removal efficiency (TNRE) in a continuous-flow process. The results showed 2 mg/L of NH2OH dosing (once every 6 h) could maintain stably partial nitrification with nitrite accumulation rate (NAR) of 91.6 % and TNRE of 92.6 %. The typical cycle suggested NH2OH dosing could promote simultaneous nitrification-denitrification (SND) and endogenous denitrification (END) while inhibit exogenous denitrification (EXD). Nitrification characteristics indicated the NH2OH dosing enhanced stability of partial nitrification by suppressing specific nitrite oxidation rate (SNOR), Nitrospira and nitrite oxidoreductase enzyme (Nxr). The microbial community suggested the aerobic denitrfiers, denitrifying glycogen accumulating organisms (DGAOs) and traditional denitrfiers were the potential contributor for advanced N removal. Moreover, NH2OH dosage was positively associated with NAR, SND and END. Overall, this study offers a feasible strategy to maintain sustainably partial nitrification that has great application potential.

Feasibility of low-intensity ultrasound treatment with hydroxylamine to accelerate the initiation of partial nitrification and allow operation under intermittent aeration

Partial nitrification is a key aspect of efficient nitrogen removal, although practically it suffers from long start-up cycles and unstable long-term operational performance. To address these drawbacks, this study investigated the effect of low intensity ultrasound treatment combined with hydroxylamine (NH2OH) on the performance of partial nitrification. Results show that compared with the control group, low-intensity ultrasound treatment (0.10 W/mL, 15 min) combined with NH2OH (5 mg/L) reduced the time required for partial nitrification initiation by 6 days, increasing the nitrite accumulation rate (NAR) and ammonia nitrogen removal rate (NRR) by 20.4% and 6.7%, respectively, achieving 96.48% NRR. Mechanistic analysis showed that NH2OH enhanced ammonia oxidation, inhibited nitrite-oxidizing bacteria (NOB) activity and shortened the time required for partial nitrification initiation. Furthermore, ultrasonication combined with NH2OH dosing stimulated EPS (extracellular polymeric substances) secretion, increased carbonyl, hydroxyl and amine functional group abundances and enhanced mass transfer. In addition, 16S rRNA gene sequencing results showed that ultrasonication-sensitive Nitrospira disappeared from the ultrasound + NH2OH system, while Nitrosomonas gradually became the dominant group. Collectively, the results of this study provide valuable insight into the enhancement of partial nitrification start-up during the process of wastewater nitrogen removal.

Recurrent multi-stressor floc treatments with sulphide and free ammonia enabled mainstream partial nitritation/anammox

Selective suppression of nitrite-oxidising bacteria (NOB) over aerobic and anoxic ammonium-oxidising bacteria (AerAOB and AnAOB) remains a major challenge for mainstream partial nitritation/anammox implementation, a resource-efficient nitrogen removal pathway. A unique multi-stressor floc treatment was therefore designed and validated for the first time under lab-scale conditions while staying true to full-scale design principles. Two hybrid (suspended + biofilm growth) reactors were operated continuously at 20.2 ± 0.6 °C. Recurrent multi-stressor floc treatments were applied, consisting of a sulphide-spiked deoxygenated starvation followed by a free ammonia shock. A good microbial activity balance with high AnAOB (71 ± 21 mg N L-1 d-1) and low NOB (4 ± 17 % of AerAOB) activity was achieved by combining multiple operational strategies: recurrent multi-stressor floc treatments, hybrid sludge (flocs & biofilm), short floc age control, intermittent aeration, and residual ammonium control. The multi-stressor treatment was shown to be the most important control tool and should be continuously applied to maintain this balance. Excessive NOB growth on the biofilm was avoided despite only treating the flocs to safeguard the AnAOB activity on the biofilm. Additionally, no signs of NOB adaptation were observed over 142 days. Elevated effluent ammonium concentrations (25 ± 6 mg N L-1) limited the TN removal efficiency to 39 ± 9 %, complicating a future full-scale implementation. Operating at higher sludge concentrations or reducing the volumetric loading rate could overcome this issue. The obtained results ease the implementation of mainstream PN/A by providing and additional control tool to steer the microbial activity with the multi-stressor treatment, thus advancing the concept of energy neutrality in sewage treatment plants.

Comparison of typical nitrite oxidizing bacteria suppression strategies and the effect on nitrous oxide emissions in a biofilm reactor

In mainstream partial nitritation/anammox (PN/A), suppression of nitrite oxidizing bacteria (NOB) and mitigation of N2O emissions are two essential operational goals. The N2O emissions linked to three typical NOB suppression strategies were tested in a covered rotating biological contactor (RBC) biofilm system at 21 °C: (i) low dissolved oxygen (DO) concentrations, and treatments with (ii) free ammonia (FA), and (iii) free nitrous acids (FNA). Low emerged DO levels effectively minimized NOB activity and decreased N2O emissions, but NOB adaptation appeared after 200 days of operation. Further NOB suppression was successfully achieved by periodic (3 h per week) treatments with FA (29.3 ± 2.6 mg NH3-N L-1) or FNA (3.1 ± 0.3 mg HNO2-N L-1). FA treatment, however, promoted N2O emissions, while FNA did not affect these. Hence, biofilm PN/A should be operated at relatively low DO levels with periodic FNA treatment to maximize nitrogen removal efficiency while avoiding high greenhouse gas emissions.

Robustness and stability of acetate-driven partial denitrification (PD) in response to high COD/NO3--N

Partial Denitrification (PD) producing nitrite for anammox may face the issue of relatively high chemical oxygen demand (COD) loading (i.e., COD/NO₃^--N) due to real wastewater being changed in substrate concentration and flowrate. In this study, three PD systems (R1, R2, R3) with sodium acetate providing electrons were developed to investigate the influence of the relatively high COD/NO₃^--N ratios (4.0, 6.0, and 8.0) on NO₂^--N production and the subsequent recoverability. It was found that a relatively high NO₂^--N production with nitrate-to-nitrite transformation ratio (NTR) of 74.0% could be still obtained despite COD/NO₃^--N even improving to 8.0 under limited reaction time (10 min) with small nitrate remaining. However, a deteriorated nitrite production was observed with sufficient reaction time (15 min) with NTR being lowered to 19.2%. Delightedly, when reducing influent COD/NO₃^--N to a normal level of 3.0, PD with high nitrite production was rapidly achieved after suffering from a relatively high COD/NO₃^--N (4.0-8.0) for 130 cycles. Besides, it was found the relatively high COD/NO₃^--N had a minor influence on the recoverability of PD, as evidenced by the close NTRs. Microbial analysis revealed the relative abundance of PD functional bacteria, Thauera, decreased under high COD/NO₃^--N, while it is still highly dominated in the systems, varying from 75.1% in R1 to 62.8% in R3 after around 110-cycles recovery. Furthermore, it appeared that the high pH (9.1-9.2) induced by sodium acetate also likely played a role in maintaining the excellent PD. Overall, this study demonstrated the robustness and stability of acetate-driven PD in response to high COD/NO₃^--N, further informing the technological superiority of PD in supplying stable and efficient nitrite, which provided solid technical support to apply it with anammox for high-efficient N removal.

In a quest for high-efficiency mainstream partial nitritation-anammox (PN/A) implementation: One-stage or two-stage?

Partial nitritation-anammox (PN/A) process is known as an energy-efficient technology for wastewater nitrogen removal, which possesses a great potential to bring wastewater treatment plants close to energy neutrality with reduced carbon footprint. To achieve this goal, various PN/A processes implemented in a single reactor configuration (one-stage system) or two separately dedicated reactors configurations (two-stage system) were explored over the past decades. Nevertheless, large-scale implementation of these PN/A processes for low-strength municipal wastewater treatment has a long way to go owing to the low efficiency and effectiveness in nitrogen removal. In this work, we provided a comprehensive analysis of one-stage and two-stage PN/A processes with a focus on evaluating their engineering application potential towards mainstream implementation. The difficulty for nitrite-oxidizing bacteria (NOB) out-selection was revealed as the critical operational challenge to achieve the desired effluent quality. Additionally, the operational strategies of low oxygen commonly adopted in one-stage systems for NOB suppression and facilitating anammox bacteria growth results in a low nitrogen removal rate (NRR). Introducing denitrification into anammox system was found to be necessary to improve the nitrogen removal efficiency (NRE) by reducing the produced nitrate with in-situ utilizing the organics from wastewater itself. However, this may lead to part of organics oxidized with additional oxygen consumed in one-stage system, further compromising the NRR. By applying a relatively high dissolved oxygen in PN reactor with residual ammonium control, and followed by a granules-based anammox reactor feeding with a small portion of raw municipal wastewater, it appeared that two-stage system could achieve a good effluent quality as well as a high NRR. In contrast to the widely studied one-stage system, this work provided a unique perspective that more effort should be devoted to developing a two-stage PN/A process to evaluate its application potential of high efficiency and economic benefits towards mainstream implementation.

Insights into the mechanism of the deterioration of mainstream partial nitritation/anammox under low residual ammonium

Residual ammonium is a critical parameter affecting the stability of mainstream partial nitritation/anammox (PN/A), but the underlying mechanism remains unclear. In this study, mainstream PN/A was established and operated with progressively decreasing residual ammonium. PN/A deteriorated as the residual ammonium decreased to below 5 mg/L, and this was paralleled by a significant loss in anammox activity in situ and an increasing nitrite oxidation rate. Further analysis revealed that the low-ammonium condition directly decreased anammox activity in situ via two distinct mechanisms. First, anammox bacteria were located in the inner layer of the granular sludge, and thus were disadvantageous when competing for ammonium with ammonium-oxidizing bacteria (AOB) in the outer layer. Second, the complete ammonia oxidizer (comammox) was enriched at low residual ammonium concentrations because of its high ammonium affinity. Both AOB and comammox presented kinetic advantages over anammox bacteria. At high residual ammonium concentrations, nitrite-oxidizing bacteria (NOB) were effectively suppressed, even when their maximum activity was high due to competition for nitrite with anammox bacteria. At low residual ammonium concentrations, the decrease in anammox activity in situ led to an increase in nitrite availability for nitrite oxidation, facilitating the activation of NOB despite the dissolved oxygen limitation (0.15-0.35 mg/L) for NOB persisting throughout the operation. Therefore, the deterioration of mainstream PN/A at low residual ammonium was primarily triggered by a decline in anammox activity in situ. This study provides novel insights into the optimized design of mainstream PN/As in engineering applications.

Efficient partial nitrification with hybrid nitrifying granular sludge based on a simultaneous fill/draw SBR mode

In this study, a simultaneous fill/draw SBR was applied to investigate the feasibility of partial nitrification process with inoculation of matured aerobic granular sludge. The system operated stably over 120 days with the relatively high ammonium removal efficiency (≥ 98.83%) and nitrite accumulation rate (≥ 89.60%). Moreover, a hybrid flocs/granules system was formed stably after long-term operation. The nitrite-oxidizing bacteria (NOB) was suppressed effectively because of the combined effect of simultaneous fill/draw mode and intermittent aeration conditions. Furthermore, batch tests were separately tested with isolated granules (> 200 μm) and flocs (< 200 μm), showing that the specific ammonia oxidation rate of granules and flocs were 15.94 ± 2.85 and 66.77 ± 0.83 mg N/(g MLSS·h), respectively. Correspondingly, the abundance of Nitrosomonas as a typical AOB in granules (6.24%) and flocs (11.94%) was obtained via the microbial diversity analysis, while NOB was almost hardly detected in granules and flocs.

Insights into rapidly recovering the autotrophic nitrogen removal performance of single-stage partial nitritation-anammox systems: Reconstructing granular sludge and its functional microbes synergy

Partial nitritation-anammox (PNA) deteriorates easily and is difficult to recover. After an airlift inner-circulation partition bioreactor was impacted by low NH₄⁺-N wastewater containing organic matter, Nitrospira and Denitratisoma propagated rapidly, granular sludge disintegrated, and the total nitrogen removal efficiency (TNRE) decreased from 68.27 % to 5.97 %. This study used a unique strategy to recover deteriorated single-stage PNA systems and explored the mechanism of rapid performance recovery. The TNRE of the system recovered up to 61.77 % in 43 days. The high nitrogen loading rate and hydraulic shear force from the airlift caused the sludge in the reactor to granulate again. The microbial community structure recovered, with a decrease in the abundance of Nitrospira (0.05 %) and enrichment of Candidatus Brocadia (8.82 %). A favorable synergy among functional microbes in the reactor was thus re-established, promoting the rapid recovery of the nitrogen removal performance. This study provides a feasible recovery strategy for PNA processes.

Comparison of inhibitory roles on nitrite-oxidizing bacteria by hydroxylamine and hydrazine during the establishment of partial nitrification

The inhibitory roles of hydroxylamine (NH₂OH) and hydrazine (N₂H₄) on nitrite-oxidizing bacteria were investigated in a comparative study. The results showed that nitrite accumulation was achieved by adding 5 mg-N/L NH₂OH or N₂H₄ to two parallel sequencing batch reactors, with nitrite accumulation rate reaching 95.83% and 86.58% within 15 days after adopting aeration time control, respectively. Correspondingly, the maximum level of NO in typical cycles caused by NH₂OH addition was 0.18 mg-N/L, which was higher than obtained for N₂H₄. NH₂OH or N₂H₄ showed strong inhibition on Nitrospira and promoted the enrichment of Nitrosomonas, with the effects of NH₂OH being more significant. However, nitritation began to deteriorate after the cessation of inhibitors addition. In conclusion, NH₂OH was a better inhibitor than N₂H₄ for Nitrospira. The inhibitory role of NH₂OH was primarily related to NO toxicity, while for N₂H₄ it was attributed to its own toxicity, with NO playing a smaller role.

Start-up of mainstream anammox process through inoculating nitrification sludge and anammox biofilm: Shift in nitrogen transformation and microorganisms

The feasibility of starting up mainstream single-stage partial nitrification-anammox (SPNA) system by inoculating nitrification sludge and anammox biofilm was investigated. The SPNA system treating low-strength synthetic wastewater was rapidly started up with TN removal efficiency of 88.5 ± 1.8% and effluent nitrate concentration of 7.2 ± 1.2 mg/L. Both the abundance and maximum activity of nitrite oxidizing bacteria (NOB) in flocs decreased obviously. Interestingly, the abundance of anaerobic ammonium oxidizing bacteria (AnAOB) in flocs increased from 0.213% to 0.346% despite the sludge retention time (SRT) of flocs decreased to 60 days, the AnAOB in biofilm was 0.434%. That meant AnAOB gradually enriched in flocs and accounted for a fairly high proportion. The inhibition of NOB, partial denitrification and increased aerobic_chemoheterotrophy function in flocs might be the main reasons for AnAOB enrichment. The possibility of simultaneous fermentation, partial denitrification and anammox reaction was predicted in biofilm, further improving the stability of the system.

Advanced nitrogen elimination from domestic sewage through two stage partial nitrification and denitrification (PND) coupled with simultaneous anaerobic ammonia oxidation and denitrification (SAD)

The start-up, efficient, and secure operation of Anammox treating low ammonia sewage, is an important research focus. In this study, a partial nitrification-denitrification coupled with simultaneous Anammox and denitrification (PND-SAD) process was achieved in sequencing batch reactor/up-flow anaerobic sludge bed (SBR-UASB). The key measures to maintain high efficiency PND were: (i) controlling dissolved oxygen in the SBR below 0.5 mg/L, which is not only conducive to PN, but also promotes the contribution of simultaneous nitrification and denitrification to nitrogen removal; (ii) monitoring the nitrate (NO₃^--N) of SBR effluent and discharging sludge to wash out nitrate oxidation bacteria when the NO₃^--N exceeds 1.0 mg/L. The nitrite accumulation rate reached 97.6%. SBR effluent and domestic sewage entered the UASB. Although Candidatus Brocadia only accounted for 0.8%, its contribution to nitrogen removal reached 76.8%. In PND-SAD system, the aerobic HRT was only 3.8 h, nitrogen removal efficiency up to 97.3%.

Oxygen control and stressor treatments for complete and long-term suppression of nitrite-oxidizing bacteria in biofilm-based partial nitritation/anammox

Mainstream nitrogen removal by partial nitritation/anammox (PN/A) can realize energy and cost savings for sewage treatment. Selective suppression of nitrite oxidizing bacteria (NOB) remains a key bottleneck for PN/A implementation. A rotating biological contactor was studied with an overhead cover and controlled air/N₂ inflow to regulate oxygen availability at 20 °C. Biofilm exposure to dissolved oxygen concentrations < 0.51 ± 0.04 mg O₂ L^-1 when submerged in the water and < 1.41 ± 0.31 mg O₂ L^-1 when emerged in the headspace (estimated), resulted in complete and long-term NOB suppression with a low relative nitrate production ratio of 10 ± 4%. Additionally, weekly biofilm stressor treatments with free ammonia (FA) (29 ± 1 mg NH₃-N L^-1 for 3 h) could improve the NOB suppression while free nitrous acid treatments had insufficient effect. This study demonstrated the potential of managing NOB suppression in biofilm-based systems by oxygen control and recurrent FA exposure, opening opportunities for resource efficient nitrogen removal.

Achieving stable and long-term partial nitrification of domestic wastewater by side-stream sludge treatment using a novel nitrite oxidation inhibitor chloroxylenol

Using inhibitors to selectively suppress the activity of nitrite-oxidizing bacteria (NOB) was an emerging way to rapidly achieve partial nitrification (PN). This study explored the feasibility of inactivating NOB by a novel inhibitor chloroxylenol (PCMX) in real domestic wastewater. Different frequencies (periodic strategy and concentrative time strategy) of PCMX side-stream sludge treatment were used to achieve and maintain PN during 250 days. PN was realized by PCMX treatment once a day about 20 days, due to the inhibition of Nitrospira. PN was completely destroyed after 212 days by periodic strategy, caused by the increase of Candidatus Nitrotoga. PN maintained without PCMX in following 201 days by concentrative time strategy. The risks of PCMX were assessed and almost no PCMX was detected in the effluent of mainstream sequencing batch reactors. These results meant PN realized by PCMX side-stream sludge treatment was feasible and concentrative time strategy was a better operating strategy.

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.

Stable nitritation of mature landfill leachate via in-situ selective inhibition by free nitrous acid

In-situ free nitrous acid (FNA) and free ammonia (FA) treatments are more feasible than side-stream methods to achieve nitritation. To assess the optimum conditions and long-term performance of in-situ inhibition by FNA, batch tests and a sequencing batch reactor (SBR) treating mature landfill leachate were conducted and established. As a result, the selective inhibition characteristic by FNA was more conspicuous than FA, and FNA (0.175 mg N/L, 6 h) treatment are more biocidal to nitrite oxidizing bacteria (NOB). Moreover, ammonia oxidizing bacteria (AOB) were more sensitive to the FA environment but its activity recovered preferentially compared to NOB. The SBR achieved a sustained nitrite accumulation rate above 90% for 200 days, with a significant decrease of NOB activity and microbial abundance according to qPCR and 16S rRNA gene sequencing results. In-situ selective inhibition by FNA (0.175 mg N/L, 6 h) has been proved to be effective to maintain stable nitritation.

Coupling anammox with denitrification in a full-scale combined biological nitrogen removal process for swine wastewater treatment

In order to enhance nitrogen removal through anammox process in the full-scale swine wastewater treatment plant, an innovative regulation strategy of nitrate-based carbon dosage and intermittent aeration was developed to apply the combined biological nitrogen removal process in a full scale anaerobic-anoxic-oxic (A²/O) system. TN removal efficiency reached at 65.5 ± 6.0% in Phase 1 with decreasing external carbon dosage in influent due to the reduction of return nitrate concentration, and it increased to 83.5 ± 6.7% when intermittent aeration was adopted in oxic zone and external carbon source was stopped adding into influent in Phase 2. As a result, the energy consumption for the swine wastewater treatment decreased from 1.93 to 0.9 kW h/m³ and 4.18 to 2.57 kW h/kg N, respectively. Microbial community analysis revealed that the average abundances of Candidatus Brocadia increased from 0.76% to 2.43% and removal of TN through anammox increased from 39% to 77%.

Versatility of nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO): First demonstration with real wastewater

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) processes have been proven effective for nitrogen removal from synthetic wastewater. However, the demonstration using real wastewater has not been achieved yet. To this end, this study investigated the versatile applications of n-DAMO process in real wastewater treatment for the first time. Two methane-based membrane biofilm reactors (MBfRs) were employed to combine anammox and n-DAMO microorganisms, targeting nitrogen removal in mainstream (i.e., domestic sewage) and sidestream (i.e., anaerobic digestion liquor), respectively. Considering various technologies in sewage treatment, three different technical routes, including nitritation + methane-based MBfR, partial nitritation + methane-based MBfR and partial nitritation + anammox + methane-based MBfR, were investigated comprehensively, all producing effluent quality with total nitrogen (TN) at 5 mg N/L or less. Regarding the sidestream treatment, the methane-based MBfR also removed up to 96% TN from the partially nitrified anaerobic digestion liquor at a practically useful rate of 0.5 kg N/m³/d. Microbial communities revealed by 16S rRNA gene amplicon sequencing indicated the dominance of n-DAMO archaea in both reactors, along with the existence of anammox bacteria and n-DAMO bacteria. As the first demonstration of n-DAMO process in real wastewater, this study comprehensively confirmed the applicability of using methane as carbon source to remove nitrogen from both mainstream and sidestream wastewater, supporting their adoption by industries in practice.

A review of partial nitrification in biological nitrogen removal processes: from development to application

To further reduce the energy consumption in the wastewater biological nitrogen removal process, partial nitrification and its integrated processes have attracted increasing attentions owing to their economy and efficiency. Shortening the steps of ammonia oxidation to nitrate saves a large amount of aeration, and the accumulated nitrite could be reduced by denitritation or anammox, which requires less electron donors compared with denitrification. Therefore, the strategies through mainstream suppression and sidestream inhibition for the achievement of partial nitrification in recent years are reviewed. Specifically, the enrichment strategies of functional microorganisms are obtained on the basis of their growth and metabolic characteristics under different selective pressures. Furthermore, the promising developments, current application bottlenecks and possible future trends of some biological nitrogen removal processes integrating partial nitrification are discussed. The obtained knowledge would provide a new idea for the fast realization of economic, efficient and long-term stable partial nitrification and biological nitrogen removal process.

The role of hydroxylamine in promoting conversion from complete nitrification to partial nitrification: NO toxicity inhibition and its characteristics

This study investigated a strategy for hydroxylamine (NH₂OH) addition for promoting the conversion of complete nitrification to partial nitrification in a sequencing batch reactor (SBR). The results showed that continuous dosing of 5 mg-N/L NH₂OH into a complete nitrification reactor for 16 days led to an increase in the nitrite accumulation ratio (NAR) from 0.22% to 95.08% and a significant enhancement in the accumulation of NO and N₂O in the liquid. The maximum concentration of NO in each cycle rose with the increase of NAR during NH₂OH addition. With the stopping of NH₂OH addition, the partial nitrification disappeared progressively in 21 days. The analysis for microbial community showed that Nitrospira was the main NOB and its relative abundance decreased with NH₂OH addition and recovered after the cessation of NH₂OH addition. Accordingly, NH₂OH has a significant and reversible inhibition on Nitrospira and its essence might be related to NO toxicity.

Deammonification process in municipal wastewater treatment: Challenges and perspectives

The deammonification process has been proved to be an efficient nitrogen removal process in treating high NH₄⁺-N concentration wastewater (sidestream deammonification). It is very hopeful to bring WWTP close to energy autarky. However, the feasibility of applying mainstream deammonification to sewage treatment need to be further explored. Therefore, this review attempts to give an overview of challenges in applying mainstream deammonification and to discuss the impacts of unfavorable conditions on main functional species. In addition, some novel control strategies to maintain the dominant position of desired species were summarized. Efficient solution to the conflict between AnAOB (Anaerobic ammonium-oxidizing bacteria) biomass retention and NOB (Nitrite oxidizing bacteria) wash out was also reviewed. Ultimately, we suggested further studies including effective improved process that achieve combination of autotrophy and organotrophy species based on the metabolic diversity of AnAOB.

Fast start-up of PN/A process in a single-stage packed bed and mechanism of nitrogen removal

The single-stage partial nitritation-anammox (PN/A) process is severely limited by a long start-up time and unstable removal efficiency. In this study, PN/A was developed in 67 days in a novel packed bed equipped with porous bio-carriers by gradually increasing the influent nitrogen loading rate (0.15-0.73 kg-N m^-3·d^-1) and controlling the dissolved oxygen (< 1.2 mg L^-1). An average ammonium nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (TNR) of 87.01 and 72.41%, respectively, were obtained. This represents a reliable alternative method of achieving rapid PN/A start-up. The results of 16S rRNA sequencing showed that Proteobacteria and Planctomycetes, with which ammonia-oxidizing bacteria and anammox bacteria were affiliated, accounted for 38.8%, representing the dominant phylum in the system after acclimation. The abundance of Nitrosomonas and Candidatus Brocadia increased by 16 and 1.79%, respectively. The results of metagenomics and metatranscriptomics revealed that the nitrite oxidation process was blocked by the transcriptional suppression of nitrite oxidoreductase and the entire nitrogen metabolism process was dominated by the partial nitritation and anammox process. Moreover, a high abundance of heterotrophic bacteria with potential for nitrogen removal was detected. In the nitrogen cycle, a widespread nitrite-accumulated denitrification helps to form a nitrite loop, which promotes the efficiency of total nitrogen removal. This is crucial for further improving the nitrogen removal mechanism in the PN/A system.

Recovering partial nitritation in a PN/A system during mainstream wastewater treatment by reviving AOB activity after thoroughly inhibiting AOB and NOB with free nitrous acid

Starting up or recovering partial nitritation is a major challenge for achieving or maintaining stable partial nitritation/anammox (PN/A) during mainstream wastewater treatment. This study presents a novel strategy for recovering the nitrite pathway by selectively reviving ammonium oxidizing bacteria (AOB) after thoroughly inhibiting AOB and nitrite oxidizing bacteria (NOB) using free nitrous acid (FNA). A sequencing batch reactor was operated for PN/A to treat real domestic wastewater for 423 days, during which twice FNA treatment was temporarily implemented. Results showed that with a single 0.45 mg/L FNA treatment on flocculent sludge, the NO₃^--N concentration during the aerobic period showed an uptrend again and the partial nitritation performance was deteriorated. In contrast, 1.35 mg/L FNA treatment induced the inhibition of both AOB and NOB leading to regressive ammonium oxidation, but a subsequently higher DO (1.5 mg/L) and longer aeration duration recovered partial nitritation. For the relative abundances of the acquired biomass related to nitrogen conversion, Nitrosomonas, Nitrospira and Nitrolancea increased to 9.65%, 10.27% and 4.35%, respectively, at the beginning of the 1.35 mg/L FNA treatment, and Nitrospira and Nitrolancea decreased to 2.80% and 0.03% whereas Nitrosomonas declined to 8.71% after 76 days. Ca. Brocadia showed less resilience after the 1.35 mg/L FNA treatment, with the relative abundance decreasing from 13.38% to 0.62% due to insufficient nitrite. Molecular ecological network analysis indicates that among anammox taxa, Ca. Kuenenia and Ca. Brocadia formed important links with other N cycle processes. Moreover, the proposed strategy shows operational flexibility because it can be easily used to control NOB in mainstream PN/A applications offered by flocculent sludge systems.

Advanced nitrogen removal from mature landfill leachate via partial nitrification-Anammox biofilm reactor (PNABR) driven by high dissolved oxygen (DO): Protection mechanism of aerobic biofilm

A novel partial nitrification-Anammox biofilm reactor (PNABR) operated under high dissolved oxygen (DO) with pre-anoxic - aerobic - anoxic operational mode was developed for efficient denitrogenation from mature landfill leachate. With DO concentration gradually increasing to 4.03 ± 0.03 mg/L, the ammonia oxidation rate (AOR) was enhanced to 25.8 mgNH4+-N/(L h), while nitrite oxidation bacteria (NOB) was inhibited effectively by alternating free ammonia (FA) and oxygen starvation. DO micro-distribution revealed that estimated 1900 μm of aerobic biofilm could protect anammox biofilm underneath from being inhibited by high DO. qPCR analysis further suggested that ammonia oxidation bacteria (AOB) abundance in whole biofilm was 6.12 × 109 gene copies/(g dry sludge), which was twice than found in the floc. Anammox bacteria accounted for 2.39% of total bacteria in whole biofilm, contributing 90.0% to nitrogen removal. Nitrogen removal rate (NRR) and nitrogen removal efficiency (NRE) finally reached 396.6 gN/(m3 d) and 96.1%, respectively.

Return-Sludge Treatment with Endogenous Free Nitrous Acid Limits Nitrate Production and N2O Emission for Mainstream Partial Nitritation/Anammox

Nitrite oxidizing bacteria (NOB) and nitrous oxide (N₂O) hinder the development of mainstream partial nitritation/anammox. To overcome these, endogenous free ammonia (FA) and free nitrous acid (FNA), which can be produced in the sidestream, were used for return-sludge treatment for two integrated-film activated sludge reactors containing biomass in flocs and on carriers. The repeated exposure of biomass from one reactor to FA shocks had a limited impact on NOB suppression but inhibited anammox bacteria (AnAOB). In the other reactor, repeated FNA shocks to the separated flocs failed to limit the system's nitrate production since NOB activity was still high on the biofilms attached to the unexposed carriers. In contrast, the repeated FNA treatment of flocs and carriers favored aerobic ammonium-oxidizing bacteria (AerAOB) over NOB activity with AnAOB negligibly affected. It was further revealed that return-sludge treatment with higher FNA levels led to lower N₂O emissions under similar effluent nitrite concentrations. On this basis, weekly 4 h FNA shocks of 2.0 mg of HNO₂-N/L were identified as an optimal and realistic treatment, which not only enabled nitrogen removal efficiencies of ∼65% at nitrogen removal rates of ∼130 mg of N/L/d (20 °C) but also yielded the lowest cost and carbon footprint.

Nitrite accumulation stability evaluation for low-strength ammonium wastewater by adsorption and biological desorption of zeolite under different operational temperature

How to achieve stable nitrite accumulation was still a huge challenge for low-carbon and energy-saving biological nitrogen removal of low-strength ammonium wastewater. This study proposed a new way to solve this problem with zeolite biological fixed bed (ZBFB) by cycle operation of adsorption and biological desorption. In order to evaluate nitritation performance of this reactor, the influence of operational temperature on nitrite accumulation stability was investigated by 126 cycles operation in four parallel ZBFB reactors for low-strength ammonium wastewater (50 mg/L NH₄⁺-N). It was found that higher operational temperature (i.e., 36.0 °C), rather than other temperature (i.e., 27.0 °C, 30.0 °C, 33.0 °C), could maintain stable nitrite accumulation with nitrite production rate of 0.312 kg NO₂^--N·m^-3 zeolite·day^-1 and nitrite accumulation ratio higher than 95.0% after biological desorption. High-throughput sequencing analysis results showed that bacterial structure significantly changed in ZBFB under different operational temperature, and obvious enrichment of genus Nitrosomonas (AOB) and gradually enhanced free ammonia (FA) inhibition on genus Nitrospira and Nitrobacter (NOB) were found by elevation of operational temperature, leading to different nitrite accumulation performance in ZBFB reactors. The mechanism for stable nitrite accumulation performance by ZBFB might be attributed to overwhelming growth rate of AOB than NOB, faster ammonium desorption and enhanced FA inhibition on NOB under operational temperature (i.e., 36.0 °C). All in all, keeping high temperature for biological desorption step should be extremely crucial for stable nitrite accumulation by ZBFB, which could facilitate further low-carbon and energy-saving biological nitrogen removal for low-strength ammonium wastewater treatment.

An improved start-up strategy for mainstream anammox process through inoculating ordinary nitrification sludge and a small amount of anammox sludge

The difficulties in enriching anammox bacteria and maintaining stable partial nitrification during start-up phase limit the application of mainstream anammox process. In this study, the feasibility of starting up simultaneous partial nitrification, anammox and denitrification (SNAD) reactor treating municipal wastewater by inoculating ordinary nitrification sludge (96.2%) and a small amount of anammox sludge (3.8%) was investigated. A sequencing batch reactor with intermittent aeration was used for the SNAD process. The SNAD reactor was started up in 75 days with a nitrogen removal efficiency of 85.4% at ambient temperature. The nitrogen removal performance maintained stable despite the fluctuating inflow. Anammox bacterial activity exponentially increased although nitrite oxidizing bacteria (NOB) activity in seeding sludge was high. The enhanced ammonium oxidizing bacterial activity and partial denitrification provided sufficient nitrite for anammox bacteria. Moreover, NOB was inhibited by intermittent aeration, anammox bacteria had competitive advantage on nitrite. The improved particle size and settleability of activated sludge also favored the anammox bacterial enrichment. This study provided an improved and easily-implemented start-up strategy for mainstream anammox. The seeding sludge was easily obtained and the operation strategy was simple. These findings were meaningful to the engineering application of mainstream anammox.

Nitrogen-associated niche characteristics and bacterial community estimated by 15N-DNA-stable isotope probing in one-stage partial nitritation/anammox process with different ammonium loading

Anaerobic ammonium oxidation coupled with partial nitritation is critical for cleaner production in sewage treatment. The long-term effects of high- and low-strength influent ammonium (NH₄⁺-N) on the anammox activity, ecological niche characteristics and active microbial community were investigated in a one-stage partial nitritation/anammox (PN/A) process. The total nitrogen (TN) removal efficiency was up to 90% with influent NH₄⁺-N of 192 mg/L. The ¹⁵N-isotope pairing technique illustrated that the potential anammox rate could reach to 3507.8 nmoL/g-sludge/h, accounting for 73.2% of dinitrogen production. As the influent NH₄⁺-N decreased to 63 mg/L, the anammox population significantly decreased and the Nitrospira became the dominant specialized species in the PN/A system. The Nitrobacter had the smallest niche overlap value and the furthest ecological distance to the anammox bacteria among the seven investigated nitrogen conversion-related genes along the influent NH₄⁺-N concentration gradient, indicating different ecological similarities. The redundancy analysis showed that the rise of dissolved oxygen caused by low NH₄⁺-N might be the main cause of the excessive proliferation of the Nitrospira. The ¹⁵N-DNA-stable isotope probing illustrated that both the class Anaerolineae and Proteobacteria had closely symbiotic relations with the Planctomycetacia in this in situ surveys. This study provides a deep understanding of PN/A process treating low-ammonium mainstream wastewater from the viewpoint of microecology.

Inhibition of nitrite oxidizing bacterial activity based on low nitrite concentration exposure in an auto-recycling PN-Anammox process under mainstream conditions

For municipal wastewater with low temperature and ammonium, conventional oxygen-limited have difficulty achieving long-term stable inhibition of nitrite oxidizing bacteria (NOB) and stable nitritation. So a partial nitrification-anaerobic ammonium oxidation integrated reactor with independent partitions was used to investigate the feasibility of adding an auto-recycling system to promote low exposure of nitrite in the aerobic zone and to inhibit the NOB activity. The results showed that nitrite produced in the aerobic zone could be timely transported to the anaerobic zone for Anammox utilization, and the nitrite nitrogen concentration was diluted to keep within 1 mg/L in the aerobic zone by the effluent recycling. NOB growth was inhibited by nitrite deficiency. The maximum nitrogen removal rate of the reactor was 0.29 kg/(m³·d), and the nitrate nitrogen production rate of NOB was controlled within 0.04 kg/(m³·d). Nitrosomonas and Candidatus Kuenenia were found as functional species of ammonia-oxidizing bacteria and Anammox bacteria, respectively.

Successful operation performance and syntrophic micro-granule in partial nitritation and anammox reactor treating low-strength ammonia wastewater

The stable operation of the partial nitritation and anammox (PN/A) process is a challenge in the treatment of low-strength ammonia wastewater like sewage mainstream. This study demonstrated the feasibility of achieving stable operation in the treatment of 50 mg/L ammonia wastewater with a micro granule-based PN/A reactor. The long-term operation results showed nitrogen removal efficiencies of 71.8 ± 9.9% were stably obtained under a relatively short hydraulic retention time (HRT) of 2 h. The analysis on the physicochemical properties of the granules indicated most of the granules were in a size in a range of 265-536 μm, and the elementary composition of the granules was determined to be CH_1.61O_0.61N_0.17S_0.01P_0.03. The microbial analysis revealed Candidatus Kuenenia stuttgartiensis anammox bacteria and Nitrosomonas-like AOB were the two most dominant bacteria with 27.6% and 10.5% abundance, respectively, both of which formed spatially syntrophic co-immobilization within the micro-granules. The ex-situ activity tests showed the activity of NOB was well limited through DO regulation in the reactor. These results provide an alternative PN/A process configuration for low-strength wastewater treatment by sustaining microstate granules. Optimization of the nitrogen sludge loading rate and DO regulation are important for the successful performance.

Rapid start-up and stable maintenance of domestic wastewater nitritation through short-term hydroxylamine addition

This study investigated the nitritation of domestic wastewater through the short-term addition of hydroxylamine (NH₂OH). Sequencing batch reactor (SBR) was used and the NH₂OH solution with an initial concentration of 5.0 mg/L was injected at each cycle. With NH₂OH addition, the nitritation was quickly established in 5 d with nitrite accumulation ratio above 95%. Further, stable nitritation could be maintained without NH₂OH addition in the following 53 days, even under unfavorable conditions (DO = 3 mg/L). According to qPCR results, NH₂OH significantly reduced and continuously suppressed nitrite oxidizing bacteria (NOB), while the abundance of ammonia oxidizing bacteria was stable, induced the start-up and maintenance of nitritation. Among NOB, NH₂OH significantly suppressed Nitrospira, while did not affect Nitrobactor. Nitrobactor gradually increased during the operation, which could induce the final deterioration of nitritation. Overall, this research provided the fundamental knowledge required to optimize the NH₂OH addition strategy for operating a stable nitritation in domestic wastewater.

Stable partial nitrification of domestic sewage achieved through activated sludge on exposure to nitrite

Partial nitrification is crucial for application of autotrophic nitrogen removal which is beneficial for treating carbon-limited wastewater. This study presents an alternative strategy for achieving partial nitrification of domestic wastewater treatment, by treating activated sludge of a nitrifying reactor on long-term nitrite exposure. Initially, the nitrifying reactor obtained complete nitrification. After the sludge of the reactor was treated with nitrite at concentration of 5-30 mg·L^-1 without feeding for 32 days, the nitrite accumulation ratio (NAR) above 90% was achieved in 30 days, and then the NAR stabled at 97.9% under low temperature of 16.6 °C. Further analysis showed that the activity decay rate of ammonium-oxidizing bacteria (AOB) (0.020 d^-1) was lower than that of nitrite-oxidizing bacteria (NOB) (0.035 d^-1) under nitrite stress. Meanwhile, the NOB were effectively suppressed while AOB were dominant. These observations supported the feasibility of achieving stable partial nitrification by treating sludge on long-term exposure to nitrite.

Nitritation of real sewage: start-up and maintenance by the side-stream heat-shock treatment

In this study, the side-stream heat-shock treatment was used to start up and maintain the nitritation of real sewage. Complete nitrification was obtained when the real sewage was treated in a sequencing batch reactor (SBR). Then, about 50% of the mixed sludge was collected from the SBR and treated with the heat-shock treatment at 60 °C for 40 min in another reactor every 2 weeks. After providing the heat-shock treatment for four times, the effluent nitrate in the SBR gradually decreased from 22.5 to 3.2 mg/L, while the nitrite accumulation rate increased from 4.4% to 81.8%, indicating a successful start-up of nitritation. Further, the sewage nitritation was stable with the regular side-steam heat-shock treatment for 91 days, and the ammonium removal efficiency of 80.6% and nitrite accumulation rate of 91.2% were achieved. This study suggests that the side-stream heat-shock treatment could be used to start up sewage nitritation and maintain stability for a long-term operation.

A critical review of one-stage anammox processes for treating industrial wastewater: Optimization strategies based on key functional microorganisms

The one-stage nitritation/anammox (anaerobic ammonium oxidation) process is an energy-saving technology, which has been successfully developed and widely applied to treat industrial wastewaters. For the one-stage nitritation/anammox process, key functional microbes generally include anaerobic ammonia oxidation bacteria (AnAOB), ammonia-oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), and heterotrophic bacteria (HB). Cooperation and competition among the key functional microbes are critical to the stability and performance of anammox process. Based upon key functional microorganisms, this review summarizes and discusses the optimized strategies that promote the operation of one-stage nitritation/anammox process. In particular, the review focuses on strategies related to: (1) the retention of anammox biomass through granular sludge or biofilm, (2) the balanced relationship between AOB and AnAOB, (3) the NOB suppression and (4) the HB management by controlling the influent organic matter. In addition, the review proposes further research to address the existing challenges.