Partial Nitrification-anammox Research Articles

Rapid transition from complete nitrification to partial nitrification-anammox at low temperatures via thermal inactivation of nitrite oxidoreductase

The stable and efficient biological nitrogen removal at low temperatures has always posed a challenge for the engineering application of the partial nitrification-anammox (PN/A) process. In this study, a complete nitrification system operating at 15 ± 1 °C was rapidly switched to PN/A mode and stably operated through thermal treatment of thickened sludge and anammox biofilm. Batch test results revealed that the ammonia uptake rate (AUR) and nitrite uptake rate (NUR) of activated sludge, after being treated at 35 °C for 2 days, decreased by 22.88 % and 96.10 %, respectively. Similarly, anammox biofilm exhibited no significant change in anammox activity (ram), but a significant decrease in NUR by 52.7 % after being treated at 40 °C for 2 days. Long-term coupling experiments demonstrated that the complete nitrification system at 15 °C was rapidly switched to PN/A mode with a TN removal rate (TNRR) of 87.91 ± 1.53 % through selective inactivation between ammonia monooxygenase (AMO) and nitrite oxidoreductase (NOR) at 35 °C for 2 days for activated sludge. Furthermore, the coexistence of Nitrospira in the seeding anammox biofilm and the proliferation of Nitrotoga in the biofilm led to the deterioration of the PN/A system. However, the collapsed TNRR could be rapidly restored to 96.42 % through selective inactivation between hydrazine synthase (HDH) and NOR at 40 °C for 2 days for anammox biofilm. The results of key enzymatic activity and Illumina MiSeq sequencing indicated a significant decrease in NOR activity but no significant change in community structure after thermal treatment for both thickened sludge and anammox biofilm. Therefore, the rapid transition of the PN/A system in a complete nitrification system at low temperatures was achieved through the selective inactivation of enzymes between NOR and AMO/HDH, rather than NOB washout.

Nitrogen removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions by simultaneous partial nitrification anammox and denitrification

In this study, a SNAD-SBBR process was implemented to achieve ammonia removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions (0.051 mg/L) for a continuous operation of 266 days. The process demonstrated excellent removal performance, with ammonia nitrogen removal efficiency reaching 100 %, total nitrogen removal efficiency reaching 87.56 %, and an average removal rate of 0.180 kg/(m3·d). The recalcitrant organic compound removal efficiency reached 34.96 %. Nitrogen mass balance analysis revealed that the Anammox process contributed to approximately 98.1 % of the nitrogen removal. Candidatus Kuenenia achieved a relative abundance of 1.49 % in the inner layer of the carrier. In the SNAD-SBBR system, the extremely low DO environment created by the highly efficient partial nitrification stage enabled the coexistence of AnAOB, denitrifying bacteria, and Nitrosomonas, synergistically achieving ammonia removal and carbon reduction. Overall, the SNAD-SBBR process exhibits low-cost and high-efficiency characteristics, holding tremendous potential for landfill leachate treatment.

Long-term operation of a two-reactor partial nitrification/anammox system with reverse sequence configuration and internal recirculation: Process performance and microbial community dynamics

Despite the higher investment costs of two-stage partial nitrification/anammox (PN/AMX) systems compared to single-reactor configurations, creating more favorable conditions for specific N-cycling microbial groups in different reactors can help more easily maintain the stability of the overall system. In this work, an unusual two-reactor PN/AMX system with brush-type carriers, reverse sequence configuration and internal recirculation was operated for 500 d using both high strength synthetic wastewater and real reject water. Feasibility of the proposed system was confirmed by relatively high (>70%) total N removal without strict control over the nitrification process due to synergistic activity of nitrifying, denitrifying and anammox bacteria. The limiting level of COD in the influent was ∼600 mg O2/L, exceeding which sharply deteriorated the process of nitrogen removal. The microbial community of the PN reactor naturally evolved into clearly distinguishable free-floating sludge and 2 types of biofilms (red and brown), which had different relative abundances of key N cycling groups in each phase of PN/AMX system operation. While the AMX community was dominated by the genus Candidatus Jettenia ecosi. Considering the ease of operation and relatively high N removal efficiency, the proposed PN/AMX system may merit further study for process optimization and practical use.

Revealing the comprehensive impact of organic compounds on the partial nitrification-anammox system during incineration leachate treatment: metabolic hierarchy and adaptation

Organics, as widespread pollutants in high-strength ammonia wastewater, typically exert adverse effects on the performance of partial nitrification-anammox (PNA) systems. However, the in-depth knowledge on how microbial consortia respond to these disturbances remains limited. In this study, we unveiled the evolution of complex organic matter flow and its impact on the metabolic hierarchy and adaptation of microbial consortia, employing multi-omics approaches, i.e., 16S amplicon sequencing, metagenomics, and metabolomics. In a two-stage PNA system sequentially treating synthetic wastewater and incineration leachate over 230 days, partial nitrification stayed stable (nitrite accumulation > 97%) while anammox efficiency dropped (nitrogen removal decreased from 86% to 78%). The phenomenon was revealed to be correlated with the evolution of dissolved organic matter (DOM) and xenobiotic organic compounds (XOCs). In the PN stage, ammonia-oxidizing bacteria (AOB) exhibited excellent adaptability through active metabolic regulation after treating leachate. Numerous heterotrophs proliferated to utilize DOM and XOCs, triggering a “boom” state evident in the glycerophospholipid metabolism. However, in the anammox stage, the competition between carbon fixation and central carbon metabolism within autotrophs and heterotrophs became evident. Increased biosynthesis costs inhibited the central metabolism (specific anammox activity decreased by 66%) and the Wood-Ljungdahl pathway of anammox bacteria (AnAOB) in the presence of recalcitrant organics. Additionally, the degradation of organics was limited, exhibiting a “bust” state. This study revealed the metabolic adaption and susceptibility of AOB and AnAOB in response to organics from the leachate, demonstrating the applicability of the two-stage configuration for treating high-strength wastewater containing abundant and diverse organics.

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

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

Achieving stable advanced nitrogen removal from mainstream municipal wastewater using integrated strategies in partial nitrification anammox (PNA) granular hybrid system

Partial nitrification anammox (PNA) is a promising technology for energy saving and nitrogen removal. The bottlenecks limiting the widespread application of PNA in mainstream municipal wastewater treatment include the difficulty in stably suppressing nitrite-oxidizing bacteria (NOB) and failure to meet discharge standards for total nitrogen (TN) due to high nitrate concentration in the effluent. To this end, this study aims to stably suppress NOB using integrated strategies (including the inoculated activated sludge pretreatment with FNA, low DO, residual ammonia nitrogen, and floc sludge discharge) while also investigating the feasibility and performance of coupling PNA with endogenous partial denitrification/anammox (EPDA) in a PNA granular hybrid system. During the 300 days of operation, the integrated strategies effectively stabilized the suppression of NOB, decreasing its abundance from 1.22 % to 0.55 %, and the final activity was reduced to 0.36 ± 0.12 mg-N/L/h. Meanwhile, the relative abundance of AnAOB as the dominant microbial community increased from 9.16 % to 24 %, with its activity reaching 2.84 ± 0.16 mg-N/L/h, limiting the supply of nitrite and ensuring the stable suppression of NOB. Typical cycle tests showed that EPDA and PNA were successfully coupled during the aeration stage with a nitrogen removal contribution rate of 80.55 %, ultimately achieving the average effluent TN and PO43--P were 4.22 mg/L and 0.09 mg/L, respectively. The metagenomic and metatranscriptomic sequencing results revealed that anammox bacteria (AnAOB) coexisted with ammonia-oxidizing bacteria (AOB), NOB, and endogenous denitrifying bacteria (EDB), and AnAOB played a primary role in the nitrogen removal process of mainstream PNA. This study brings mainstream PNA closer to widespread application and provides significant technical support for its engineering implementation.

Achieving advanced nitrogen removal from mature landfill leachate in continuous flow system involving partial nitrification-anammox and denitrification

Considering the challenges associated with nitrogen removal from mature landfill leachate, a novel combined continuous-flow process integrating denitrification and partial nitrification-Anammox (PN/A) was developed using an internal circulation (IC) system and a biological aerated filter (BAF) biofilm reactor (IBBR). In this study, IBBR successfully operated for 343 days, and when influent NH4+-N concentration of mature landfill leachate reached 1258.1 mg/L, an impressive total nitrogen removal efficiency (TNRE) of 93.3 % was achieved, along with a nitrogen removal rate (NRR) of 1.13 kg N/(m3·d). The analysis of the microbial community revealed that Candidatus Kuenenia, the dominant genus responsible for anammox, accounted for 1.7 % (day 265). Additionally, Nitrosomonas, Thauera and Truepera were identified as key contributors to the efficient removal of nitrogen from mature landfill. As a novel nitrogen removal strategy, the practical application of the IBBR system offers novel perspectives on addressing mature landfill leachate.

Research progress of anaerobic ammonium oxidation (Anammox) process based on integrated fixed-film activated sludge (IFAS).

The integrated fixed-film activated sludge (IFAS) process is considered one of the cutting-edge solutions to the traditional wastewater treatment challenges, allowing suspended sludge and attached biofilm to grow in the same system. In addition, the coupling of IFAS with anaerobic ammonium oxidation (Anammox) can further improve the efficiency of biological denitrification. This paper summarises the research progress of IFAS coupled with the anammox process, including partial nitrification anammox, simultaneous partial nitrification anammox and denitrification, and partial denitrification anammox technologies, and describes the factors that limit the development of related processes. The effects of dissolved oxygen, influent carbon source, sludge retention time, temperature, microbial community, and nitrite-oxidising bacteria inhibition methods on the anammox of IFAS are presented. At the same time, this paper gives an outlook on future research focus and engineering practice direction of the process.

Advanced nitrogen removal from municipal wastewater by autotrophy-heterotrophy coupled anammox system in a novel simultaneous microaerobic/limited-oxygen SBR: Interspecific correlation network

Nowadays, autotrophic nitrogen removal of mainstream municipal wastewater by anammox is highly promising. However, due to the lack of a long-term stable nitrite-oxidizing bacteria (NOB) inhibition strategy, the accumulation of NO3−-N byproducts severely affects the nitrogen removal efficiency (NRE). In this study, a simultaneous autotrophy-heterotrophy coupled anammox system was developed in a microaerobic/oxygen-limited SBR through the co-induction of the regulation of aeration intensity and limited organics to achieve the optimization of the mainstream Partial nitrification-anammox (PN/A) processes. The nitrogen removal system constructed by anammox and denitrification achieved the high NRE of 90.77 ± 1.82 %. Under the autotrophic-heterotrophic multiple induction of nitrite supply (PN, partial denitrification and endogenous partial denitrification), the contribution of the nitrogen removal by anammox increased to 57.93 %. The high-throughput sequencing analysis showed that the relative abundance of AnAOB increased from 0.40 % to 0.67 %. In-situ tests assay further demonstrated that the realization of multi-channel nitrite supply facilitated the enrichment of anammox bacteria (AnAOB). The endogenous denitrifying bacterial genus Candidatus (Ca.) Competibacter was significantly enriched in the system (7.98 %, P ≤ 0.001), which enhanced the deep removal of NOX−-N and organics. Correlation network analysis indicated that closer and more balanced interspecies interactions were the key to the high efficiency and stability of the autotrophy-heterotrophy coupled anammox system. Based on the nitrite recirculation and denitrification performance, the simultaneous autotrophy-heterotrophy coupled anammox system established by the novel SBR provides a potential strategy for mainstream anammox applications.

Partial nitrification response to dissolved oxygen variation and aerobic starvation: Kinetics and microbial community analyses

Partial nitrification (PN) has great significance in promoting both anaerobic ammonium oxidation (anammox) and advanced nitrogen removal. However, the kinetics of PN have not been sufficiently studied. Deterioration of PN often occurs due to nitrite-oxidizing bacteria (NOB) outgrowth, for which the dissolved oxygen (DO) is a critical factor. This study investigated the kinetic parameters of ammonia-oxidizing bacteria (AOB) and NOB by varying the DO concentration. The partial nitrification-anammox (PNA) process was operated under sequentially low (0.3–0.8 mg/L) and high (4.0–5.0 mg/L) DO. The results show that AOB could not adapt to low DO, resulting in a significant decrease in the maximum AOB activity (Vmax,AOB) and a slightly altered oxygen half-saturation constant of AOB (KO,AOB). Conversely, KO,NOB decreased and Vmax,NOB maintained, indicating that the main NOB genus, Nitrospira, was more adaptable to DO changes. Under high DO, AOB growth was weaker than NOB, leading to a low Vmax,AOB – Vmax,NOB value; KO,AOB increased and was larger than KO,NOB and PNA was eventually destroyed. The aerobic starvation was then adopted to restore PN. AOB recovered faster and dominated, with Vmax,AOB greatly exceeding Vmax,NOB. Finally, NOB were inhibited and PN was recovered. During this process, KO,NOB increased and was higher than KO,AOB. Changes in the kinetic parameters, especially the Vmax,AOB -Vmax,NOB value, have specific guiding significance for DO regulation during PNA. This work helps better understand the effect of DO variations on the nitrifying microbial community, which influences oxygen affinity and maximum activity.

Nitrogen removal and performance deterioration in digested effluent treatment by partial nitrification-anammox (PNA) process based on aeration sedimentation integrated microaerobic reactor (ASIMR)

The existing microaerobic reactors for PN/A process have the drawbacks of complex structure and hard to scale-up in full scale application. Aiming at these problems, we have designed new aeration sedimentation integrated microaerobic reactor (ASIMR). The feasibility of ASIMR for PN/A processes and the optimal HRT were explored by gradually reducing the hydraulic retention time (HRT). When the HRT was 4.45 h and the NLR was 3.98 kg·(m3·d)-1, ASIMR achieved the highest nitrogen removal performance, with the ammonia nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (NRE) of 90.5 % and 79.2 %, respectively, and the total nitrogen removal rate (NRR) could reach 3.15 kg·(m3·d)-1. When HRT decreased to 3.60 h with the NLR of 4.76 kg·(m3·d)-1, the performance deterioration was observed, with ARE and NRE decreasing to 35.9 % and 12.4 %, and NRR decreasing to 0.59 kg·(m3·d)-1. The main reasons for the decrease in nitrogen removal performance were overhigh hydraulic load in the sedimentation zone which led to the loss of nitrifying sludge. The loss of nitrifying sludge in turn reduced the efficiency of ammonia nitrogen removal resulting in the accumulation of free ammonia (FA) which inhibited the activity of nitrogen removal functional bacteria. Although reactor performance deteriorated at too low HRT, when the HRT was much than 4.45 h, ASIMR could achieve a good performance for nitrogen removal owe to the combined action of reactor design, enriched nitrified and anammox sludge as inoculum, and dissolved oxygen control. Except good nitrogen removal performance, ASIMR has another advantage, such as simple structure, low cost for manufacture, and more suitable for engineering scale-up in full scale application.

Attaining superior nitrogen removal from integrated mature landfill leachate and kitchen waste digestion liquid via a two-stage partial nitrification/anammox (PN/A) process

The advanced nitrogen removal of mature landfill leachate (MLL) with low carbon to nitrogen ratio (C/N) is a topic worthy of further research. A two-stage partial nitrification/anammox (PN/A) system for integrated MLL and kitchen waste digestion liquid (KWDL) treatment was established in this study. The nitrogen removal efficiency (NRE) of this process was 98.1 ± 1.3 %, with an effluent TIN concentration of 15.2 ± 1.8 mg/L. In the partial nitrification-sequencing batch reactor (PN-SBR), aerobic hydraulic retention time (HRT) accounted for just 36.4 % of the whole process with dissolved oxygen (DO) < 0.3 mg/L, which significantly reduced energy consumption. Furthermore, the introduction of KWDL heightened anoxic denitrification and aerobic simultaneous nitrification and denitrification (SND) processes, contributing to 74.2 ± 4.3 % and 25.8 ± 4.3 % of the total NRE, respectively. This greatly strengthened the nitrogen removal capacity of PN-SBR. Batch tests indicated that the capacity of KWDL as a carbon source to drive the process of partial denitrification coupled with anammox (PDA) was equal to that of sodium acetate. Adding KWDL in the later phase of each cycle in the anammox-sequencing batch reactor (A-SBR) facilitated achieving stable PDA and advanced nitrogen removal. The nitrate accumulation problem in the traditional two-stage PN/A process has been resolved without the addition of carbon sources. Nitrosomonas (3.7 %→7.9 %), Thauera (0.04 %→40.7 %) and a salt-tolerant AnAOB microbe, unclassified_f_Brocadiaceae (0 %→5.3 %) were dominant in the combined system. This study revealed a pioneering avenue for efficient high-strength wastewater treatment.

Evaluate the partial nitrification/anammox biofilm system treating the anaerobic biogas slurry of multisource organic solid waste

Anaerobic digestion used for treating organic solid waste (OSW) usually produces much biogas slurry with high ammonia levels and low C/N ratio, suitable for partial nitrification/anammox (PN/A) process. However, the multisource OSW makes the biogas slurry complex, which may affect the stability of PN/A system. Therefore, this study explored the performance of a PN/A biofilm system treating biogas slurry with different water qualities. Different biogas slurry slightly affected nitrogen removal efficiency of the system (maintained at 90 %), whereas the contribution of anammox decreased gradually. The organic compounds in the biogas slurry were complex and difficult to degrade, but changed the microbial structure of the system. Complex biogas slurry promoted extracellular polymeric substances (EPS) secretion, helped maintain the stability of the biofilm and reduce the impact of the external environment. The relative abundance of Candidatus_Kuenenia, which was the dominant anammox genus, decreased from 9.02 % to 1.26 %. In addition, the relative abundance of Denitratisoma and Thauera decreased sharply, however, that of Truepera increased from 0.07 % to 6.52 %, making it the dominant denitrifying bacterial genus in the system. This study provides a reference for application of PN/A systems in the treatment of actual biogas slurry.

Single-stage partial nitrification/anammox process performance restoration by bio-augmenting nitrifiers with or without nitrite oxidizing bacteria suppression

The use of low dissolved oxygen (DO) is common to suppress the growth of nitrite oxidizing bacteria (NOB) in the partial nitrification/anammox (PN/A) process. However, this approach can negatively impact the activity of ammonium oxidizing bacteria (AOB) and reduce the oxygen consumption rate, disrupting the essential low DO environment required for anammox bacteria (AMX) and leading to a decline in PN/A performance. We innovatively harnessed the strategy of common nitrifiers (AOB + NOB) bioaugmentation in an impaired single-stage PN/A reactor after operation for a period of time. The results demonstrated an immediate improvement, with the total nitrogen removal efficiency increasing from approximately 60 % to 80 %. This enhancement was attributed to decreased NOB activity and increased AMX activity, which occurred at reduced DO level (0.24 to 0.12 mg O2/L) following nitrifiers bioaugmentation. Partial nitrification (PN) and PN/A batch experiments with nitrifiers bioaugmentation further supported these findings. This strategy pioneered a fresh perspective that the diversion of nitrite from NOB to AMX was the pivotal underlying mechanism. Mathematical simulations complemented these results by demonstrating that nitrifiers bioaugmentation broadened the optimal oxygen supply window.

Stimulating extracellular polymeric substances production in integrated fixed-film activated sludge reactor for advanced nitrogen removal from mature landfill leachate via one-stage double anammox

Introducing carbon sources to achieve nitrogen removal from mature landfill leachate not only increases the costs and carbon emissions but also inhibits the activity of autotrophic bacteria. Thus, this study constructed a double anammox system that combines partial nitrification-anammox (PNA) and endogenous partial denitrification-anammox (EPDA) within an integrated fixed-film activated sludge (IFAS) reactor. In this system, PNA primarily contributes to nitrogen removal pathways, achieving a nitrite accumulation rate of 98.23%. The production of extracellular polymer substances (EPS) in the IFAS reactor is stimulated by introducing co-fermentation liquid. Through the utilization of EPS, the system effectively achieves EPDA with the nitrite transformation rate of 97.20%. Under the intermittent aeration operation strategy, EPDA combined with PNA and anammox in the oxic and anoxic stages enhanced the nitrogen removal efficiency of the system to 99.70 ± 0.12%. The functional genus Candidatus kuenenia became enriched in biofilm sludge, while Thauera and Nitrosomonas predominated in floc sludge.

Advanced nitrogen removal from municipal wastewater through partial nitrification-denitrification coupled with anammox in step-feed continuous system

Combined partial nitrification-denitrification/anammox (PN-PD/A) processes have attracted great attention from researchers in recent years to achieve high nitrogen removal from low carbon /nitrogen (C/N) municipal wastewater. In this context, a step-feed anoxic/oxic (A/O) process was conducted in this study through the combination of the partial nitrification-anammox (PN/A) and partial denitrification-anammox (PD/A) to remove N from municipal wastewater with low C/N. The enhancement of the PN-PD/A process resulted in N removal efficiency of 85.6% at C/N of 2.8. The contributions of the anammox reached 36.4 and 8.8% in the anoxic and oxic chambers, respectively. The biocarriers added to the anoxic and oxic chambers increased the relative abundance of the anammox bacteria in biofilms from 0.61% to 1.51% and 1.02%, respectively. This study demonstrated that employing the step-feed A/O process can create optimal conditions for the anammox bacteria growth, thereby ensuring advanced N removal from low C/N municipal wastewater.

Successive exposure to traditional disinfectants and quaternary ammonium compounds enhances resistance genes expression and co-selection in biofilm-based partial nitrification-anammox systems

Quaternary ammonium compounds (QACs) are recommended disinfectants with surfactant properties, surpassing triclosan (TCS) and chloroxylenol (PCMX). Given the transition from traditional disinfectants, it is essential to investigate their impacts on biological nitrogen removal systems and the fate of resistance genes (RGs). In this study, three biofilm-based partial nitrification-anammox (PN/A) systems were established. A reactor named PD was successively exposed to 1 mg/L PCMX and 3 mg/L dioctadecyldimethylammonium chloride (DODMAC, a common QACs). A reactor named TD was successively exposed to 1 mg/L TCS and 3 mg/L DODMAC. A reactor named CD served as a control with only 3 mg/L DODMAC exposure. Results indicated that the total nitrogen removal performance of CD deteriorated markedly with DODMAC exposure compared to that of PD and TD. This phenomenon correlated closely with variations in RGs and their co-selection patterns. Pre-exposure to PCMX or TCS increased the abundance of RGs in the extracellular DNA of the PN/A biofilm, but reduced RGs abundances in the extracellular DNA of water. The tolerance of the PN/A system to successive exposure to the two disinfectants may be strengthened through co-selection of QACs RGs (qacEdelta1–01, qacEdelta1–02, qacH-01 and qacH-02) and mobile genetic elements (intI1 and tnpA-04). Furthermore, potential hosts of RGs are crucial for maintaining PN/A performance. Accumulation of extracellular polymeric substances, reactive oxygen species, and lactate dehydrogenase plays vital roles in the accumulation and transmission of RGs within the PN/A system.

Using sulfide as nitrite oxidizing bacteria inhibitor for the successful coupling of partial nitrification-anammox and sulfur autotrophic denitrification in one reactor

The partial nitrification-anammox (PNA) process, as a new “green” biological nitrogen removal technology, is completely autotrophic and has great economic and environmental benefits. However, the applied bottleneck of this process is the inability to inhibit nitrite oxidizing bacteria (NOB) activity and the low total nitrogen removal efficiency (TNRE) due to nitrate accumulation. In this study, the addition of sulfide was utilized to selectively screen NOB while promoting the sulfur autotrophic denitrification (SAD) process in one sequencing batch biofilter granular reactor. The results showed that when the influent concentration of NH4+-N and S2− were 184.6 ± 4.4 mg/L and 153.9 ± 3.1 mg/L, the average TNRE reached 89.6 ± 3.6 %. At this stage, aerobic ammonia oxidizing bacteria (AOB) and anaerobic ammonium oxidation bacteria (AnAOB) showed good activities (specific oxygen uptake rate of AOB and specific anammox activity could reach to 0.09 ± 0.00 mg O2/g SS/h and 0.47 ± 0.06 mg TN/g SS/h, respectively), while NOB activity was significantly suppressed by sulfide, with specific oxygen uptake rate of 0.003 ± 0.00 mg O2/g SS/h, which further demonstrating the feasibility and long-term application of this sustainable and efficient process. Anammox was the predominant nitrogen removal pathway contributing approximately 89.0 % of nitrogen removal. Typical AOB (Nitrosomonas), AnAOB (Candidatus Kuenenia), and SAD bacteria (Thiobacillus, Pseudoxanthomonas, and Limobacter) coexisted in the PNA-SAD system. The syntrophic interactions among sulfur-oxidizing, sulfate-reducing, and anammox bacterial populations of this system were revealed by PICRUST2 and network analysis. Sulfides were most likely oxidized to biologically produced elemental sulfur first and then converted to sulfate via the Sox system. Our study provided a new reaction scheme and theoretical reference for efficient nitrogen and sulfur removal in a single reactor.

Anammox granule destruction and reconstruction in a partial nitrification/anammox system under hydroxylamine stress

Nitrite oxidizing bacteria (NOB) outcompeting anammox bacteria (AnAOB) poses a challenge to the practical implementation of the partial nitrification/anammox (PN/A) process for municipal wastewater. A granules-based PN/A bioreactor was operated for 260 d with hydroxylamine (NH2OH) added halfway through. qPCR results detected the different amounts of NOB among granules and flocs and the dynamic succession during operation. CLSM images revealed a unique layered structure of granules that NOB located inside led to the inhibition effect of NH2OH delayed. Besides, the physical and morphological characteristics revealed that anammox granules experienced destruction. AnAOB took the broken granules as an initial biofilm aggregate to reconstruct new granules. RT-qPCR and high throughput sequencing results suggested that functional gene expression and community structure were regulated for the AnAOB metabolism process. Correspondingly, the rapid proliferation (0.52 → 1.99%) of AnAOB was realized, and the nitrogen removal rate achieved a nearly quadruple improvement (0.21 → 0.83 kg-N/m3·d). This study revealed that anammox granules can self-reconstruct in the PN/A system when granules are disintegrated under NH2OH stress, broadening the feasibility of applying PN/A process.

Efficient nitrogen removal and robustness enhancement of a two-stage partial nitrification-anammox (PN/A) process with low sludge concentration for mature landfill leachate

The biological treatment system for high-strength wastewater, particularly landfill leachate, typically requires high sludge concentration to maintain nitrogen removal performance. However, it frequently causes an excessive accumulation of toxins in microbial metabolism, resulting in unstable performance during long-term operation. An efficient two-stage partial nitrification/anammox (PN/A) mature landfill leachate process with low sludge concentration was constructed by settling time reduction and Ca2+ addition. The ammonia removal rate reached 46.7 mg N/(L·h) in PN-SBR. Nitrosomonas (2.0%) was the sole genus responsible for partial nitrification. The influent NO2−-N/NH4+-N of A-SBR was kept at 1.39, leading to a dynamic equilibrium of anammox and denitrification. Ca. Brocadia recovered fastest (0.32% → 1.8%) among the detected AnAOB genera. The process achieved NRE of 95.0% with effluent TIN of 37.6 mg/L (<40 mg/L). This research offered recommendations for the favorable operation of the two-stage PN/A mature landfill leachate treatment system with low sludge concentration.