Pilot-scale evaluation of anammox-based mainstream nitrogen removal from municipal wastewater

Comammox Nitrospira cooperate with anammox bacteria in a partial nitritation-anammox membrane bioreactor treating low-strength ammonium wastewater at high loadings

Research has revealed that comammox Nitrospira and anammox bacteria engage in dynamic interactions in partial nitritation-anammox reactors, where they compete for ammonium and nitrite or comammox Nitrospria supply nitrite to anammox bacteria. However, two gaps in the literature are present: the know-how to manipulate the interactions to foster a stable and symbiotic relationship and the assessment of how effective this partnership is for treating low-strength ammonium wastewater at high hydraulic loads. In this study, we employed a membrane bioreactor designed to treat synthetic ammonium wastewater at a concentration of 60 mg N/L, reaching a peak loading of 0.36 g N/L/day by gradually reducing the hydraulic retention time to 4 hr. Throughout the experiment, the reactor achieved an approximately 80 % nitrogen removal rate through strategically adjusting intermittent aeration at every stage. Notably, the genera Ca. Kuenena, Nitrosomonas, and Nitrospira collectively constituted approximately 40 % of the microbial community. Under superior intermittent aeration conditions, the expression of comammox amoA was consistently higher than that of Nitrospira nxrB and AOB amoA in the biofilm, despite the higher abundance of Nitrosomonas than comammox Nitrospira, implying that the biofilm environment is favorable for fostering cooperation between comammox and anammox bacteria. We then assessed the in situ activity of comammox Nitrospira in the reactor by selectively suppressing Nitrosomonas using 1-octyne, thereby confirming that comammox Nitrospira played the primary role in facilitating the nitritation (33.1 % of input ammonium) rather than complete nitrification (7.3 % of input ammonium). Kinetic analysis revealed a specific ammonia-oxidizing rate 5.3 times higher than the nitrite-oxidizing rate in the genus Nitrospira, underscoring their critical role in supplying nitrite. These findings provide novel insights into the cooperative interplay between comammox Nitrospira and anammox bacteria, potentially reshaping the management of nitrogen cycling in engineered environments, and aiding the development of microbial ecology-driven wastewater treatment technologies.

Comammox and unknown ammonia oxidizers contribute to nitrite accumulation in an integrated A-B stage process that incorporates side-stream EBPR (S2EBPR)

A novel integrated pilot-scale A-stage high rate activated sludge, B-stage short-cut biological nitrogen removal and side-stream enhanced biological phosphorus removal (A/B-shortcut N-S2EBPR) process for treating municipal wastewater was demonstrated with the aim to achieve simultaneous and carbon- and energy-efficient N and P removal. In this studied period, an average of 7.62 ± 2.17 mg-N/L nitrite accumulation was achieved through atypical partial nitrification without canonical known NOB out-selection. Network analysis confirms the central hub of microbial community as Nitrospira, which was one to two orders of magnitude higher than canonical aerobic oxidizing bacteria (AOB) in a B-stage nitrification tank. The contribution of comammox Nitrospira as AOB was evidenced by the increased amoB/nxr ratio and higher ammonia oxidation activity. Furthermore, oligotyping analysis of Nitrospira revealed two dominant sub-clusters (microdiveristy) within the Nitrospira. The relative abundance of oligotype II, which is phylogenetically close to Nitrospira_midas_s_31566, exhibited a positive correlation with nitrite accumulation in the same operational period, suggesting its role as comammox Nitrospira. Additionally, the phylogenetic investigation suggested that heterotrophic organisms from the family Comamonadacea and the order Rhodocyclaceae embedding ammonia monooxygenase and hydroxylamine oxidase may function as heterotrophic nitrifiers. This is the first study that elucidated the impact of integrating the S2EBPR on nitrifying populations with implications on short-cut N removal. The unique conditions in the side-stream reactor, such as low ORP, favorable VFA concentrations and composition, seemed to exert different selective forces on nitrifying populations from those in conventional biological nutrient removal processes. The results provide new insights for integrating EBPR with short-cut N removal process for mainstream wastewater treatment.

Granulation characteristics of anammox sludge in response to different signal-molecule-stimulants; mediated through programmed cell death

During the anammox process, mitigation of biomass washout to increase sludge retention is an important parameter of process efficiency. Signal molecular stimulants (SMS) initiate the sludge granulations controlled by programmed cell death (PCD) of microorganisms. In this study, the aerobic granular sludge (AGS), cell fragments, extracellular polymeric substances (EPS), and AGS process effluent were tested as SMS to identify their effect on anammox granulation. The results showed that the addition of SMS increased the nitrogen removal efficiency to varying degrees, whereas the addition of AGS process supernatant, as SMS, increased the ammonia removal efficiency up to 96%. The addition of SMS was also found to increase EPS production and contributed to sludge granulation. In this process, the proportion of PCD increased and both Gaiella and Denitratisoma abundance increased from 3.54% to 5.59%, and from 1.8% to 3.42%, respectively. In conclusion, PCD was found important to increase anaerobic ammonia oxidation performance through the granulation mechanism.

Iron-electrolysis assisted anammox/denitrification system for intensified nitrate removal and phosphorus recovery in low-strength wastewater treatment

Two iron-electrolysis assisted anammox/denitrification (EAD) systems, including the suspended sludge reactor (ESR) and biofilm reactor (EMR) were constructed for mainstream wastewater treatment, achieving 84.51±4.38 % and 87.23±3.31 % of TN removal efficiencies, respectively. Sludge extracellular polymeric substances (EPS) analysis, cell apoptosis detection and microbial analysis demonstrated that the strengthened cell lysate/apoptosis and EPS production acted as supplemental carbon sources to provide new ecological niches for heterotrophic bacteria. Therefore, NO3--N accumulated intrinsically during anammox reaction was reduced. The rising cell lysis and apoptosis in the ESR induced the decline of anammox and enzyme activities. In contrast, this inhibition was scavenged in EMR because of the more favorable environment and the significant increase in EPS. Moreover, ESR and EMR achieved efficient phosphorus removal (96.98±5.24 % and 96.98±4.35 %) due to the continued release of Fe2+ by the in-situ corrosion of iron anodes. The X-ray diffraction (XRD) indicated that vivianite was the dominant P recovery product in EAD systems. The anaerobic microenvironment and the abundant EPS in the biofilm system showed essential benefits in the mineralization of vivianite.

Robustness of the anammox process at low temperatures and low dissolved oxygen for low C/N municipal wastewater treatment

Low water temperatures and ammonium concentrations pose challenges for anammox applications in the treatment of low C/N municipal wastewater. In this study, a 10 L-water bath sequencing batch reactor combing biofilm and suspended sludge was designed for low C/N municipal wastewater treatment. The nitrogen removal performance via partial nitrification anammox-(endogenous) denitrification anammox process was investigated with anaerobic-aerobic-anoxic mode at low temperatures and dissolved oxygen (DO). The results showed that with the decrease of temperature from 30 to 15℃, the influent and effluent nitrogen concentrations and nitrogen removal efficiencies were 73.7 ± 6.5 mg/L, 7.8 ± 2.8 mg/L, and 89.4 %, respectively, with aerobic hydraulic retention time of only 6 h and DO concentration of 0.2-0.5 mg/L. Among that, the stable anammox process compensated for the inhibitory effects of the low temperatures on the nitrification and denitrification processes. Notably, from 30 to 15℃, the anammox activity and relative abundance of the dominant Brocadia genus were increased from 39.7 to 45.5 mgN/gVSS/d and 7.3 to 12.0 %, respectively; the single gene expression level of the biofilm increased 9.0 times. The anammox bacteria showed a good adaptation to temperatures reduction. However, nitrogen removal by anammox was not improved by increasing DO (≥ 4 mg/L) at 8-4℃. Overall, the results of this study demonstrate the feasibility of the mainstream anammox process at low temperatures.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

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.

Sidestream bio-P and mainstream anammox in a BNR process with upstream carbon capture

The integration of biological phosphorus removal (bio-P) and shortcut nitrogen removal (SNR) processes is challenging because of the conflicting demands on influent carbon: SNR allows for upstream carbon diversion, but this reduction of influent carbon (especially volatile fatty acids [VFAs]) prevents or limits bio-P. The objective of this study was to achieve SNR, either via partial nitritation/anammox (PNA) or partial denitrification/anammox (PdNA), simultaneously with biological phosphorus removal in a process with upstream carbon capture. This study took place in a pilot scale A/B process with a sidestream bio-P reactor and tertiary anammox polishing. Despite low influent rbCOD concentrations from the A-stage effluent, bio-P occurred in the B-stage thanks to the addition of A-stage WAS fermentate to the sidestream reactor. Nitrite accumulation occurred in the B-stage via partial denitrification and partial nitritation (NOB out-selection), depending on operational conditions, and was removed along with ammonia by the tertiary anammox MBBR, with the ability to achieve effluent TIN less than 2 mg/L. PRACTITIONER POINTS: A sidestream reactor with sufficient fermentate addition enables biological phosphorus removal in a B-stage system with little-to-no influent VFA. Enhanced biological phosphorus removal is not inhibited by intermittent aeration and is stable at a wide range of process SRTs. Partial nitritation and partial denitrification are viable routes to produce nitrite within an A/B process with sidestream bio-P, for downstream anammox in a polishing MBBR.

Performance of vertical up-flow-constructed wetlands integrating with microbial fuel cell (VFCW-MFC) treating ammonium in domestic wastewater

Vertical up-flow-constructed wetlands integrating with microbial fuel cell (VFCW-MFC) were evaluated for NH4+-N removal and bioelectricity recovery. The experiments were carried out in lab-scale VFCW-MFC microcosms treating synthetic domestic wastewater under different operational conditions of pH, hydraulic retention time, and mass loading rate. Effects of wild ornamental grass (Cenchrus setaceus) on treatment performance and voltage output were investigated simultaneously. Experiments demonstrated that the neutral pH of influents favoured NH4+-N removal and power generation. Extended retention time improved treatment capacity and power output but likely depended on the substrate availability. COD removal and power output increased, while NH4+-N removal decreased with increasing mass loading rates. At the loading rate of 88.31 mg COD/L.day, planted VFCW-MFCs exhibited better NH4+-N treatment performance (36.9%) and higher voltage output (132%-143%) than unplanted systems. Plants did not affect the COD removal efficiency of VFCW-MFCs (>95%). Power density was in the range of 1.26-1.59 mW/m² in planted microcosms with a maximum CE of 13.6%. The anode layer accounted for a major proportion of NH4+-N removal in VFCW-MFCs. This study implies that NH4+-N in domestic wastewaters with relatively high COD:N ratios can be treated effectively in up-flow CW-MFCs via anaerobic processes, including anammox and heterotrophic denitrifying processes. The mass loading rate could be a critical parameter to balance different microbial processes, thus, coincidently determining the potential of power recovery from wastewaters.

One-year stable pilot-scale operation demonstrates high flexibility of mainstream anammox application

Mainstream nitrogen removal via anammox is widely recognized as a promising wastewater treatment process. However, its application is challenging at large scale due to unstable suppression of nitrite-oxidizing bacteria (NOB). In this study, a pilot-scale mainstream anammox process was implemented in an Integrated Fixed-film Activated Sludge (IFAS) configuration. Stable operation with robust NOB suppression was maintained for over one year. This was achieved through integration of three key control strategies: i) low dissolved oxygen (DO = 0.4 ± 0.2 mg O₂/L), ii) regular free nitrous acid (FNA)-based sludge treatment, and iii) residual ammonium concentration control (NH₄ ⁺ with a setpoint of ∼8 mg N/L). Activity tests and FISH demonstrated that NOB barely survived in sludge flocs and were inhibited in biofilms. Despite receiving organic-deficient wastewater from a pilot-scale High-Rate Activated Sludge (HRAS) system as the feed, the system maintained a stable effluent total nitrogen concentration mostly below 10 mg N/L, which was attributed to the successful retention of anammox bacteria. This study successfully demonstrated large-scale long-term mainstream anammox application and generated new practical knowledge for NOB control and anammox retention.

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.

Integration of EBPR with mainstream anammox process to treat real municipal wastewater: Process performance and microbiology

The mainstream application of anaerobic ammonium oxidation (anammox) for sustainable N removal remains a challenge. Similarly, with recent additional stringent regulations for P discharges, it is imperative to integrate N with P removal. This research studied integrated fixed film activated sludge (IFAS) technology to simultaneously remove N and P in real municipal wastewater by combining biofilm anammox with flocculent activated sludge for enhanced biological P removal (EBPR). This technology was assessed in a sequencing batch reactor (SBR) operated as a conventional A2O (anaerobic-anoxic-oxic) process with a hydraulic retention time of 8.8 h. After a steady state operation was reached, robust reactor performance was obtained with average TIN and P removal efficiencies of 91.3 ± 4.1% and 98.4 ± 2.4%, respectively. The average TIN removal rate recorded over the last 100 d of reactor operation was 118 mg/L·d, which is a reasonable number for mainstream applications. The activity of denitrifying polyphosphate accumulating organisms (DPAOs) accounted for nearly 15.9% of P-uptake during the anoxic phase. DPAOs and canonical denitrifiers removed approximately 5.9 mg TIN/L in the anoxic phase. Batch activity assays, which showed that nearly 44.5% of TIN were removed by the biofilms during the aerobic phase. The functional gene expression data also confirmed anammox activities. The IFAS configuration of the SBR allowed operation at a low solid retention time (SRT) of 5-d without washing out biofilm ammonium-oxidizing and anammox bacteria. The low SRT, combined with low dissolved oxygen and intermittent aeration, provided a selective pressure to washout nitrite-oxidizing bacteria and glycogen-accumulating organisms, as relative abundances of.

Conductive carrier promotes synchronous biofilm formation and granulation of anammox bacteria

The extracellular electron transfer capability of some anaerobic ammonium oxidation (anammox) bacteria was confirmed in recent years. However, the effect of conductive carriers on the synchronous formation of anammox biofilm and granules is rarely reported. Anammox biofilm and granules with compact and stable structures accelerate the initiation and enhance the stability of the anammox process. In this study, we found that the conductive carbon fiber brush (CB) carrier promoted synchronous biofilm formation and granulation of anammox bacteria in the internal circulation immobilized blanket (ICIB) reactor. Compared with polyurethane sponge and zeolite carrier, the ICIB reactor packed with CB carrier can be operated under the highest total nitrogen loading rate of 6.53 kg-N/(m³·d) and maintain the effluents NH₄⁺-N and NO₂^--N at less than 1 mM. The volatile suspended solids concentration in the ICIB reactor packed with conductive carrier increased from 5.17 ± 0.40 g/L of inoculum sludge to 24.24 ± 1.20 g/L of biofilm, and the average particle size of granules increased from 222.09 µm to 879.80 µm in 150 days. Fluorescence in situ hybridization analysis showed that anammox bacteria prevailed in the biofilm and granules. The analysis of extracellular polymeric substances indicated that protein and humic acid-like substances played an important role in the formation of anammox biofilm and granules. Microbiome analysis showed that the relative abundance of Candidatus Jettenia was increased from 0.18% to 38.15% in the biofilm from CB carrier during start-up stage. This study provides a strategy for rapid anammox biofilm and granules enrichment and carrier selection of anammox process.

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.

Design strategy and mechanism of nitrite oxidation suppression of elevated loading rate partial nitritation system

There is a current need for a low operational intensity, effective and small footprint system to achieve stable partial nitritation for subsequent anammox treatment at mainstream municipal wastewaters. This research identifies a unique design strategy using an elevated total ammonia nitrogen (TAN) surface area loading rate (SALR) of 5 g TAN/m2.d to achieve cost-effective, stable, and elevated rates of partial nitritation in a moving bed biofilm reactor (MBBR) system under mainstream conditions. The elevated loaded partial nitritation MBBR system achieves a TAN surface area removal rate (SARR) of 2.01 ± 0.07 g TAN/m2.d and NO2−-N: NH4+-N stoichiometric ratio of 1.15:1, which is appropriate for downstream anammox treatment. The elevated TAN SALR design strategy promotes nitrite-oxidizing bacteria (NOB) activity suppression rather than a reduction in NOB population as the reason for the suppression of nitrite oxidation in the mainstream elevated loaded partial nitritation MBBR system. NOB activity is limited at an elevated TAN SALR likely due to thick biofilm embedding the NOB population and competition for dissolved oxygen (DO) with ammonia-oxidizing bacteria for TAN oxidation to nitrite within the biofilm structure, which ultimately limits the uptake of DO by NOB in the system. Therefore, this design strategy offers a cost-effective and efficient alternative for mainstream partial nitritation MBBR systems at water resource recovery facilities.

Genome-centered metagenomics illuminates adaptations of core members to a partial Nitritation-Anammox bioreactor under periodic microaeration

The partial nitritation-anaerobic ammonium oxidation (anammox; PN-A) process has been considered a sustainable method for wastewater ammonium removal, with recent attempts to treat low-strength wastewater. However, how microbes adapt to the alternate microaerobic-anoxic operation of the process when treating low ammonium concentrations remains poorly understood. In this study, we applied a metagenomic approach to determine the genomic contents of core members in a PN-A reactor treating inorganic ammonium wastewater at loading as low as 0.0192 kg-N/m³/day. The metabolic traits of metagenome-assembled genomes from 18 core species were analyzed. Taxonomically diverse ammonia oxidizers, including two Nitrosomonas species, a comammox Nitrospira species, a novel Chloroflexota-related species, and two anammox bacteria, Ca. Brocadia and Ca. Jettenia, accounted for the PN-A reactions. The characteristics of a series of genes encoding class II ribonucleotide reductase, high-affinity bd-type terminal oxidase, and diverse antioxidant enzymes revealed that comammox Nitrospira has a superior adaptation ability over the competitors, which may confer the privileged partnership with anammox bacteria in the PN-A reactor. This finding is supported by the long-term monitoring experiment, showing the predominance of the comammox Nitrospira in the ammonia-oxidizing community. Metagenomic analysis of seven heterotrophs suggested that nitrate reduction is a common capability in potentially using endogenous carbohydrates and peptides to enhance nitrogen removals. The prevalence of class II ribonucleotide reductase and antioxidant enzymes genes may grant the adaptation to cyclically microaerobic/anoxic environments. The predominant heterotroph is affiliated with Chloroflexota; its genome encodes complete pathways for synthesizing vitamin B6 and methionine. By contrast, other than the two growth factors, Nitrospira and anammox bacteria are complementary to produce various vitamins and amino acids. Besides, the novel Chloroflexota-related ammonia oxidizer lacks corresponding genes for detoxifying the reactive oxygen species and thus requires the aid of co-existing members to alleviate oxidative stress. The analysis results forecast the exchanges of substrates and nutrients as well as the collective alleviation of oxidative stress among the core populations. The new findings of the genomic features and predicted microbial interplay shed light on microbial adaptation to intermittent microaeration specific to the PN-A reactor, which may aid in improving its application to low-strength ammonium wastewater.

Anammox biofilter with denitrification sludge as seed in treating low nitrogen strength wastewater

Deficient seed sludge, low substrate concentrations are recognized as the major barriers for the application of anaerobic ammonia oxidation (Anammox) to treat mainstream wastewater. In this work, anammox biofilter (A-BF) was started up by inoculating denitrification sludge at low nitrogen strength at 25 °C. The total nitrogen removal efficiency (TNRE) and nitrogen removal rate (NRR) reached 74.8 ± 3.4% and 0.81 kg-N m^-3 d^-1 under nitrogen loading rate (NLR) of 1.20 kg-N m^-3 d^-1 with 7.00 mg-NH₄⁺-N L^-1 and 10.00 mg-NO₂^--N L^-1 as influent. 1.00-2.00 mg-DO L^-1 negatively impacted effluent, but the total nitrogen of effluent (TN_eff) was 10.65 ± 2.76 mg L^-1, in limit of the standard of Class 1A for municipal WWTP discharge (GB18918-2002). The abundance of Planctomycetes increased from 0.6% to 1.4-2.6%, in which, Candidatus_Brocadia was the dominant genera. The results establish the application feasibility of A-BFs as advanced nitrogen removal technique in treating mainstream wastewater.

Response of mixed community anammox biomass against sulfide, nitrite and recalcitrant carbon in terms of inhibition coefficients and functional gene expressions

Anaerobic ammonium oxidation (anammox) has evolved as a carbon and energy-efficient nitrogen management bioprocess. However, factors such as inhibitory chemicals still challenge the easy operation of this powerful bioprocess. This research systematically evaluated the inhibition kinetics of sulfide, nitrite, and recalcitrant carbon under a genomic framework. The inhibition at the substrate and genetic levels of sulfide, nitrite and recalcitrant carbon on anammox activity was studied using batch tests. Nitrite inhibition of anammox followed substrate inhibition and was best described by the Aiba model with an inhibition coefficient [Formula: see text] of 324.04 mg N/L. Hydrazine synthase (hzsB) gene (anammox biomarker) expression was increased over time when incubated with nitrite up to 400 mg N/L. However, despite having the highest specific nitrite removal (SNR), the expression of hzsB at 100 and 200 mg N/L of nitrite was more muted than in most other samples with lower SNRs. Sulfide severely inhibited anammox activities. The inhibition was fitted with a Monod-based model with a [Formula: see text] of 4.39 mg S/L. At a sulfide concentration of 5 mg/L, the hzsB expression decreased throughout the experiment from its original value at he beginning. Recalcitrant carbon of filtrate from thermal hydrolysis process pretreated anaerobic digester had a minimal effect on maximum specific anammox activity (MSAA), and thus the value of the inhibition coefficient could not be calculated. At the same time, its hzsB expression profile was similar to that in the control. Resiliency and recovery tests indicated that the inhibition of nitrite (up to 400 mg N/L) and recalcitrant carbon (in 100% filtrate) were reversible. About 32% of MSAA was recovered after repeated exposures to sulfide at 2.5 mg/L, while at 5 mg/L, the inhibition was irreversible. Findings from this study will be helpful for the successful design and implementation of anammox in full-scale applications.

Biomass retention and microbial segregation to offset the impacts of seasonal temperatures for a pilot-scale integrated fixed-film activated sludge partial nitritation-anammox (IFAS-PN/A) treating anaerobically pretreated municipal wastewater

Partial nitritation-anammox (PN/A) is a promising deammonification process to develop energy-neutral wastewater treatment plants. However, the mainstream application of PN/A still faces the challenges of low nitrogen concentration and low temperatures, and has not been studied under a realistic condition of large-scale reactor (kiloliter level), real municipal wastewater (MWW) and seasonal temperatures. In this research, a pilot-scale one-stage PN/A, with integrated fixed-film activated sludge (IFAS) configuration, was operated to treat the real MWW pretreated by anaerobic membrane bioreactor. The removal efficiency of total nitrogen (TN) was 79.4%, 75.7% and 65.9% at 25, 20 and 15°C, corresponding to the effluent TN of 7.3, 9.7 and 12.0 mg/L, respectively. The suppression of ammonium-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) occurred at lower temperatures, and the significant decrease in AOB treatment capacity was the reason for the poorer nitrogen removal at 15°C. Biomass retention and microbial segregation were successfully achieved. Specifically, Candidatus_Brocadia and Candidatus_Kuenenia were main AnAOB genera and mainly enriched on carriers, Nitrosomonas and uncultured f_Chitinophagaceae were main AOB genera and mainly distributed in suspended sludge and retained by sedimentation tank. Moreover, nitrite-oxidizing bacteria (NOB) were sufficiently suppressed by intermittent aeration and low dissolved oxygen, the presence of heterotrophic bacteria upgraded the PN/A to a simultaneous partial nitritation, anammox, denitrification, and COD oxidation (SNADCO) system, which improved the overall removal of TN and COD. The results of this investigation clearly evidence the strong feasibility of PN/A as a mainstream nitrogen removal process in temperate climates.

Enhancing nitrogen removal through directly integrating anammox into mainstream wastewater treatment: Advantageous, issues and future study

Anaerobic ammonium oxidation (anammox) has great potential to be applied to the process of nitrogen removal from mainstream wastewater. However, directly applying complete anammox to the mainstream is typically hindered by low temperatures, a low ammonia concentration, and high organic matter concentrations. Directly integrating anammox into mainstream treatment by enhancing the in-situ enrichment of anammox bacteria in wastewater treatment plants (WWTPs) could effectively improve the nitrogen removal efficiency and reduce the treatment cost. A certain anammox bacteria abundance in full-scale WWTPs provides the feasibility of directly integrating anammox into mainstream treatment and realizing partial mainstream anammox. The technical development status of partial anammox and the mechanisms of achieving partial mainstream anammox by aeration and organic control are summarized. This review provides an enhanced understanding of this novel technical route of partial mainstream anammox treatment for improving the quality, performance, and prospects for this technology to be used in upgrading WWTPs.

Design methodologies to determine optimal staging of membrane-aerated biofilm reactors for mainstream treatment with anammox

Partial nitritation anammox (PNA) membrane-aerated biofilm reactors (MABRs) can be used in mainstream nitrogen removal to help facilities reduce their energy consumption. Previous PNA MABR research has not investigated the impacts of staging, i.e. arraying MABRs in series, on their nitrogen removal performance, operation, and ability to suppress nitrite oxidizing bacteria. In this paper, a mathematical model simulated PNA MABR performance at different influent total ammonia concentrations and loadings. A design methodology for staging PNA MABRs was created and found that the amount of membrane surface area is dependent upon the total ammonia-nitrogen concentration and loading, and the air loading to the membrane must be proportional to the total ammonia-nitrogen loading to maximize the total inorganic nitrogen (TIN) removal rate. This led to approximately equal-sized stages that each had a TIN removal percentage of 71% of the influent total ammonia nitrogen. Staging a treatment train resulted in 9.8% larger total ammonia and 9.3% larger total nitrogen removal rates when compared with an un-staged reactor. The un-staged reactor also was not able to produce an effluent total ammonia concentration below 5 mg N/L which would be necessary for many facilities' permits.

Effect of exogenous hydrazine on metabolic process of anammox bacteria

The effect of N₂H₄ (hydrazine) on AnAOB (anaerobic ammonia oxidizing bacteria) metabolic pattern is unknown. Therefore, the main purpose of this paper was to explore the effects of exogenous N₂H₄ on the SAA (specific anammox activity), characteristics and metabolic pathway of AnAOB. The results showed that low N₂H₄ concentration (1-5 mg/L) continuous dosing can promote SAA. The promoting effect was found to be more obvious within the dosage of 3-5 mg/L N₂H₄. It was also indicated that high N₂H₄ concentration dosing (5-10 mg/L) can trigger the self-protection mechanism of AnAOB granular sludge by secreting a large amount of B-PN (binding polymeric protein). Intermittent addition of N₂H₄ at low concentration is conducive to the long-term stable operation of anammox process. Exogenous N₂H₄ can be directly oxidized by AnAOB to promote the consumption of NO₂^--N and NH₄⁺-N. In addition, excess electrons can also drive the process of NO₃^--N reduction and NO₂^--N disproportionation. Theoretically, these reaction processes need two and ten extra electrons respectively, which is not easy to occur compared with the anammox process.

Fast formation of anammox granules using a nitrification-denitrification sludge and transformation of microbial community

A lengthy start-up period has been one of the key obstacles limiting the application of the anammox process. In this investigation, a nitrification-denitrification sludge was used to start-up the anammox EGSB process. The transformation process from nitrification-denitrification sludge to anammox granule sludge was explored through the aspects of nitrogen removal performance, granule properties, microbial community structure, and evolution route. A successful start-up of the anammox process was achieved after 94 days of reactor operation. The highest nitrogen removal rate (NRR) obtained was 7.25±0.16 gN/L/d at a nitrogen loading rate (NLR) of 8.0 gN/L/d, and the corresponding nitrogen removal efficiency was a high 90.61±1.99%. The results of the microbial analysis revealed significant changes in anammox bacteria, nitrifying bacteria, and denitrifying bacteria in the sludge. Notably, the anammox bacteria abundance increased from 2.5% to 29.0% during the operation, and Candidatus Kuenenia and Candidatus Brocadia were the dominant genera. Distinct-different successions on Candidatus Brocadia and Candidatus Kuenenia were also observed over the long-term period. In addition, the settling performance, anammox activity and biomass retention capacity of the granules were significantly enhanced during this process, and the corresponding granule evolution route was also proposed. The results in this study indicate the feasibility of using available seed sludge source for the fast-transformation of anammox granules, it is beneficial to the large-scale application of anammox process and the utilization of excess sludge.

A 20-Year Journey of Partial Nitritation and Anammox (PN/A): from Sidestream toward Mainstream

Anaerobic ammonium oxidation (anammox) was discovered as a new microbial reaction in the late 1990s, which led to the development of an innovative energy- and carbon-efficient technology─partial nitritation and anammox (PN/A)─for nitrogen removal. PN/A was first applied to remove the nitrogen from high-strength wastewaters, e.g., anaerobic digestion liquor (i.e., sidestream), and further expanded to the main line of wastewater treatment plants (i.e., mainstream). While sidestream PN/A has been well-established with extensive full-scale installations worldwide, practical application of PN/A in mainstream treatment has been proven extremely challenging to date. A key challenge is achieving stable suppression of nitrite-oxidizing bacteria (NOB). This study examines the progress of NOB suppression in both sidestream- and mainstream PN/A over the past two decades. The successful NOB suppression in sidestream PN/A was reviewed, and these successes were evaluated in terms of their transferability into mainstream PN/A. Drawing on the learning over the past decades, we anticipate that a hybrid process, comprised of biofilm and floccular sludge, bears great potential to achieve efficient mainstream PN/A, while a combination of strategies is entailed for stable NOB suppression. Furthermore, the recent discovery of novel nitrifiers would trigger new opportunities and new challenges for mainstream PN/A.

Mainstream short-cut N removal modelling: current status and perspectives

This work gives an overview of the state-of-the-art in modelling of short-cut processes for nitrogen removal in mainstream wastewater treatment and presents future perspectives for directing research efforts in line with the needs of practice. The modelling status for deammonification (i.e., anammox-based) and nitrite-shunt processes is presented with its challenges and limitations. The importance of mathematical models for considering N₂O emissions in the design and operation of short-cut nitrogen removal processes is considered as well. Modelling goals and potential benefits are presented and the needs for new and more advanced approaches are identified. Overall, this contribution presents how existing and future mathematical models can accelerate successful full-scale mainstream short-cut nitrogen removal applications.

Ex-situ manometric activity test for evaluation of mainstream anammox in Moving Bed Biofilm Reactors

Implementing anammox based processes in the mainstream at municipal wastewater treatment plants is challenging and requires carefully constructed control strategies in order to be successful. A method to measure the specific anammox activity was further developed and implemented in the mainstream anammox pilot plant at Sjölunda Wastewater Treatment Plant. The method was found to be reliable and reproducible. The influence of initial nitrite and ammonium concentrations on the activity as well as temperature dependency were analysed. The continuously monitored manometric batch tests were performed on Moving Bed Biofilm Reactor carriers type K1^®. The activity showed a dependency of initial nitrite concentrations below 75 mg N L^-1 whereas the activity was independent of initial nitrite concentrations in the interval of 75-125 mg N L^-1. Temperature dependency of the specific anammox activity as expressed in activation energy Ea increased at lower temperatures (10-20°C) compared to higher temperatures (20-30°C). Decreasing temperature from 30°C to 10°C resulted in 95-98% loss of the anammox activity.

Mainstream nitrogen removal in membrane aerated biofilm reactor at minimal lumen pressure

Nitrogen removal via anammox is a promising and sustainable solution in mainstream wastewater treatment. To maintain stable anammox process, competitors of anammox bacteria should be suppressed while cooperators need to be favoured. This study demonstrated a synchronous aerobic and anaerobic ammonium removal process in a membrane aerated biofilm reactor (MABR) under minimal lumen pressure. By adjusting the lumen pressure, aerobic and anaerobic ammonium oxidation rate can be synchronized to minimize interference of nitrite oxidizing bacteria (NOB) by limiting NOB's access to both oxygen and nitrite. Long-term performance indicated that PN/A in MABR could be achieved at zero positive aeration pressure. Furthermore, by connecting two MABRs in series, high total nitrogen (TN) removal efficiency of 71.1% ± 5.3% was attained with a TN removal rate of 30.1 ± 3.2 mg-N/L/d. The organic carbon present in the wastewater reduced the nitrate concentration in the effluent while not affecting the overall nitrogen removal efficiency and rate. Real-time qPCR analysis suggested that the abundance of amoA gene was relatively stable while K-strategist Nitrospira 16S rRNA gene did not surge in the long-term operation. High throughput sequencing showed that Candidatus Brocadia and uncultured anaerobic ammonium oxidizing bacteria from Chloroflexi were the most abundant anammox taxa. Denitrifiers, such as Denitratisoma may be responsible to reduce the nitrate in the effluent.

Towards mainstream partial nitritation/anammox in four seasons: Feasibility of bioaugmentation with stored summer sludge for winter anammox assistance

The strong effect of low temperatures on anammox challenges its mainstream application over the winter in temperate climates. Winter bioaugmentation with stored summer surplus sludge is a potential solution to guarantee sufficient nitrogen removal in winter. Firstly, the systems for which nitrogen removal deteriorated by the temperature decrease (25 °C → 20 °C) could be fully restored bioaugmenting with granules resp. flocs stored for 6 months at 118 resp. 220% of the initial biomass levels. Secondly, the reactivation of these stored sludges was tested in lower temperature systems (15.3 ± 0.4/10.4 ± 0.4 °C). Compared to the activity before storage, between 56% and 41% of the activity of granules was restored within one month, and 41%-32% for flocs. Additionally, 85-87% of granules and 50-53% of flocs were retained in the systems. After reactivation (15.3 ± 0.4/10.4 ± 0.4 °C), a more specialized community was formed (diversity decreased) with Candidatus Brocadia still dominant in terms of relative abundance. Capital and operating expenditures (CAPEX, OPEX) were negligible, representing only 0.19-0.36% of sewage treatment costs.

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.

Reaction of the anammox granules to various antibiotics and operating the anammox coupled denitrifying reactor for oxytetracycline wastetwater treatment

The anaerobic ammonium oxidation (anammox) basedtechnology has been considered as an economic and efficient way to remove nitrogen. However, the anammox bacteria could be strongly inhibited by antibiotics. In present research, inhibiting properties of oxytetracycline, penicillin and polymyxin sulfate upon the anammox activity were investigated through batch experiment. The results implied that anammox activity was significantly inhibited by oxytetracycline and polymyxin sulfate. The non-competitive inhibiting model showed that the inhibiting constants (Ki) of oxytetracycline and polymyxin sulfate were 188.5 and 17.7 mg/L, respectively. Meanwhile, the anammox process was not suppressed while the concentration of penicillin reached 3000 mg/L. In long-run experiment, the influent oxytetracycline concentration of the anammox coupled denitrifying reactor was operated at 20 mg/L. It was observed that the anammox performance completely deteriorated, while the NO₂^--N removing efficiency reached 15.8%. The obtained findings could provide important instruction for the treatment of antibiotic contaminated wastewater.

The first pilot-scale demonstration of the partial nitritation/anammox-hydroxyapatite process to treat the effluent of the anaerobic membrane bioreactor fed with the actual municipal wastewater

Recently, it was reported that for synthetic low-strength wastewater, the excellent nitrogen removal rate (NRR) accompanied with phosphorus removal could be achieved through the partial nitritation/anammox (PNA)-hydroxyapatite (HAP) process. Thus, this research further investigated the performance of the pilot-scale PNA-HAP process treating the effluent of an anaerobic membrane bioreactor (AnMBR) fed with the actual municipal wastewater. The results showed that with the hydraulic retention time of 4.0 h, the influent ammonium concentration ranging from 36.0 to 41.0 mg/L, and the BOD₅ ranging from 6.3 to 12.7 mg/L, the average NRR and the nitrogen removal efficiency was 0.13 kg/m³/d and 63.38%, respectively. The specific activity test of sludge confirmed that the PNA process was the main nitrogen metabolism pathway. The effluent nitrate and the BOD₅ were almost zero, indicating the existence of denitrification activity in reactor. Given that the oxygenation condition, the heterotrophic organic matter oxidization activity also occurred in reactor. The sludge analysis confirmed the phosphate formation in sludge. Thus, in the reactor, four kinds of biological activities and chemical crystallization occurred harmoniously in sludge. From the mixed liquid volatile suspended solid of 2.4 g/L and the low distribution range of granule size, it was obvious that the sludge had a high dispersity. Based on the well settling ability of sludge during the operation, it was inferred that there was a close bond between biomass and HAP in sludge, which was helpful to enhance the settleability of sludge granule. Besides, the phosphorus-containing sludge was suitable as the fertilizer. In all, this study demonstrated that the PNA-HAP process is an ideal alternative treating the effluent of the AnMBR process in the municipal wastewater treatment.

Storage without nitrite or nitrate enables the long-term preservation of full-scale partial nitritation/anammox sludge

Bioaugmentation with summer harvested sludge during winter could compensate for bacterial activity loss but requires that sludge activity can be restored after storage. This study assesses the effect of temperature and redox adjustment during the storage over 180 days of partial nitritation/anammox (PN/A) granular resp. floccular sludge from potato processing resp. sludge reject water treatment. Anoxic storage conditions (in the presence of nitrite or nitrate and the absence of oxygen) resulted in a loss of 80-100% of the anammox bacteria (AnAOB) activity capacity at 20 °C and 4 °C, while anaerobic conditions (without oxygen, nitrite, and nitrate) lost only 45-63%. Storage at 20 °C was more cost-effective compared to 4 °C, and this was confirmed in the sludge reactivation experiment (20 °C). Furthermore, AnAOB activity correlated negatively with the electrical conductivity level (R² > 0.85, p < 0.05), so strong salinity increases should be avoided. No significant differences were found in the activity capacity of aerobic ammonia-oxidizing bacteria (AerAOB) under different storage conditions (p > 0.1). The relative abundance of dominant AnAOB (Candidatus Brocadia) and AerAOB genera (Nitrosomonas) remained constant in both sludges. In conclusion, preserving PN/A biomass without cooling and nitrite or nitrate addition proved to be a cost-effective strategy.

In-situ fermentation coupling with partial-denitrification/anammox process for enhanced nitrogen removal in an integrated three-stage anoxic/oxic (A/O) biofilm reactor treating low COD/N real wastewater

Mainstream partial-denitrification with anammox (PD-anammox) process faced the challenge of complex organics involved in real sewage. Herein, PD-anammox coupled with in-situ fermentation was successfully achieved in a full biofilm system formed by three-stage anoxic/oxic reactor to treat real wastewater with low COD/N of 3.6. The total nitrogen (TN) removal efficiency was enhanced to 78.4% ± 3.6% with average TN and ammonium concentrations in effluent of 10.6 and 0.5 mg N/L, respectively. Batch tests confirmed that partial-denitrification was the major nitrite provider for anammox in the anoxic biofilm, while in-situ fermentation could decompose the complex organics to readily-biodegradable organics for full- or partial-denitrification. Additionally, a significant anammox bacteria (Candidatus Brocadia) population was detected in the second (3.53%) and third (4.46%) anoxic zones, while denitrifiers and fermentative bacteria were mainly enriched in the first anoxic zone. This study presents a feasible approach for PD-anammox process in practical application under mainstream condition.

Assessing the stability of one-stage PN/A process through experimental and modelling investigations

The mainstream partial nitritation/anammox (PN/A) process has been intensively studied but its stability remains a key challenge. It is shown here that biofilm thickness can exhibit a critical role in controlling the process stability of mainstream PN/A against dissolved oxygen (DO) variation. In a laboratory moving bed biofilm reactor (MBBR), PN/A performance was initially established in 200 days by controlling a low DO of 0.13 ± 0.07 mg O₂/L in the bulk liquid, which deteriorated with an increase of DO (0.35 ± 0.13 mg O₂/L) for over two months, and then rapidly recovered in a month with the initial low DO level re-applied. Biofilm thickness of PN/A carriers was measured during the experiment, which became significantly thinner (367 ± 146 μm) at mainstream conditions. The thin thickness primarily decreased the in-situ consumption rate of nitrite, rather than ammonium, when DO increased from 0.1 to 0.4 mg O₂/L, due to that the thin thickness can only restrict anammox capacity. These results illustrated the role of biofilm thickness in regulating PN/A performance and microbial activities. Further investigation using an established model revealed the joint contribution of biofilm thickness and DO concentration to PN/A process, while particularly, the biofilm thickness can determine the optimal DO level for maximizing the nitrogen removal efficiency and system robustness against DO variation. These results highlight the need of considering biofilm thickness in PN/A process optimization and stability improvement in low-strength wastewater treatment.

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 nitrogen removal performance of mainstream PN-ANAMMOX by combining high-temperature shock for selective recovery of AOB activity

This paper describes the new concept of the mainstream partial nitritation (PN)-anaerobic ammonium oxidation (ANAMMOX) combined with a high-temperature shock strategy for the selective recovery of ammonia-oxidizing bacteria (AOB) activity. In the preliminary test, the temperature shock condition for PN was optimized (60 °C and > 20 min). Based on this, the implementation strategy in a continuous stirred tank reactor (CSTR) system was studied further, and the polyvinyl alcohol (PVA)/sodium alginate carrier exposure ratio (ER) and dissolved oxygen (DO) concentration were considered as primary variables. The AOB activity was recovered selectively when the ER of the carrier ranged from 20 to 40%, and the DO was higher than 2.3 mg O₂/L. This was not the case for nitrite-oxidizing bacteria (NOB) (AOB: 1.17±0.1 gNH₄⁺-N/L_Carrier/d, NOB: 0.34±0.1 gNO₃^--N/L_Carrier/d). As a result, the activity of AOB was recovered selectively with a decrease in Nitrospira spp., which was verified by kinetic and microbial analyses for the AOB (K_{S, DO} = 3.89 mgO₂/L) and NOB (K_{S, DO} = 1.14 mgO₂/L). Eventually, the mainstream PN-ANAMMOX was achieved with a nitrogen removal efficiency of 81.5±3.3% for 95 days. The findings provide insight to establishing a stable mainstream PN-ANAMMOX process using a high-temperature shock 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.

Autotrophic nitrogen removal for decentralized treatment of ammonia-rich industrial textile wastewater: process assessment, stabilization and modelling

Digital textile printing (DTP) is a game-changer technology that is rapidly expanding worldwide. On the other hand, process wastewater is rich in ammoniacal and organic nitrogen, resulting in relevant issues for discharge into sewer system and treatment in centralized plants. The present research is focused on the assessment of the partial nitritation/anammox process in a single-stage granular sequencing batch reactor for on-site decentralized treatment. The technical feasibility of the process was assessed by treating wastewater from five DTP industries in a laboratory-scale reactor, in one case investigating long-term process stabilization. While experimental results indicated nitrogen removal efficiencies up to about 70%, complying with regulations on discharge in sewer system, these data were used as input for process modelling, whose successful parameter calibration was carried out. The model was applied to the simulation of two scenarios: (i) the current situation of a DTP company, in which wastewater is discharged into the sewer system and treated in a centralized plant, (ii) the modified situation in which on-site decentralized treatment for DTP wastewater is implemented. The second scenario resulted in significant improvements, including reduced energy consumption (- 15%), reduced greenhouse gases emission, elimination of external carbon source for completing denitrification at centralized WWTP and reduced sludge production (- 25%).

Response of wastewater treatment performance, microbial composition and functional genes to different C/N ratios and carrier types in MBBR inoculated with heterotrophic nitrification-aerobic denitrification bacteria

To operate the moving bed biofilm reactor inoculated with HN-AD bacteria (B-MBBR) instead of activated sludge for livestock and poultry breeding wastewater (LPBW) disposal in most efficient manner, nitrogen removal (NR) efficiency and microbial composition of two MBBRs with different carrier types under various C/N ratios were explored. Results indicated that the performance on NR greatly various in different carrier types under various C/N ratios. Attributing to the bacterial protection provided by the porous structure of polyvinyl alcohol (PVA) gel, MBBR using PVA gel as the carrier exhibited a more stable NR performance (range from 78.05% to 83.76%) versus that using Kaldnes (K1) as the carrier (range from 78.05% to 83.76%). Besides, microbial analysis indicated that MBBR with PVA gel as the carrier is conducive to the growth of oligotrophic and HN-AD bacteria (Paracoccus and Acinetobacter), and the highest relative abundance was 16.37% at C/N ratio of 6.

Mainstream-sidestream wastewater switching promotes anammox nitrogen removal rate in organic-rich, low-temperature streams

The main issues with mainstream anammox application are loss of bacterial activity by low temperatures and by a high organic content of wastewater. We demonstrate a novel switching method between sidestream and mainstream wastewater. The wastewater flow was switched between sidestream (reject water at >22°C) and mainstream (municipal wastewater at 16.5°C), so that the anammox biomass activity and biomass growth could benefit from sidestream conditions. Real sidestream wastewater (biogas plant effluent) (≈1000 mg NH⁺ ₄-N L^-1) and synthetic mainstream (municipal wastewater-like source) (≈100 mg NH⁺ ₄-N) wastewater were used for 20 L biofilm reactor feeding. The highest total nitrogen removal rate (TNRR) of 527 g N m^-3 d^-1 (average TNRR 180 (±140) g N m^-3 d^-1) was achieved with sidestream wastewater at a low chemical oxygen demand (COD)/TN ratio of 1.1/1. For reactor feeding with mainstream, the highest TNRR achieved was 61 g N m^-3 d^-1. Average TNRR for mainstream of 20 (±15) g N m^-3 d^-1 was low due to a higher COD/N ratio of 3.2/1. The highest TNRR in a batch test was achieved at the COD concentration of 480 mg L^-1, reflecting a TNRR of ≈5 mg N g^-1 TSS h^-1. With a high COD concentration of 2600 mg L^-1 (TOC/TN = 8/1), TNRR decreased similarly in both feeds to 1.6 mg N g^-1 TSS h^-1. The anammox microorganism's genus Candidatus Brocadia enrichment in deammonification biofilm reactor was higher in the mainstream operation (7.6% of all bacteria) than in sidestream operation period (<0.7% of all bacteria).

Adaptation of flocculent anammox culture to low temperature by cold shock: long-term response of the microbial population

Partial nitritation-anammox (PN/A) process will substantially reduce the costs for the removal of nitrogen in the mainstream of municipal sewage. However, one of the mainstream PN/A challenges is to reduce the time necessary for the adaptation of anammox bacteria to lower temperatures in mild climates. In this study, we exposed anammox flocculent culture to cold shocks [35°C → 5°C (8 h) → 15°C] and evaluated long-term cold shock response. Over a post-shock period of 40 d at 15°C, the nitrogen removal rates in the shocked culture were significantly higher compared to control, with maximum rates up to 0.082 and 0.033 kg-N/kg-VSS/d or 0.164 and 0.076 kg-N/m3/d, for shocked culture and control, respectively. In the corresponding semi-batch cycles, the shocked culture was on average 136 ± 101% more active than the control, due to the negative effect of cold shock on side populations and more active anammox cells. Per FISH, Ca. Brocadia anammoxidans and Ca. Scalindua survived the shock and remained present throughout. Thus, both anammox microorganisms seem to respond favourably to cold shocks. In sum, we provide further evidence that cold shocks accelerate the adaptation of anammox to the mainstream of municipal WWTPs. Further, for the first time, we report the long-term adaptive response of anammox to cold shocks.

Research progress on enhancing the performance of autotrophic nitrogen removal systems using microbial immobilization technology

The autotrophic nitrogen removal process has great potential to be applied to the biological removal of nitrogen from wastewater, but its application is hindered by its unstable operation under adverse environmental conditions, such as those presented by low temperatures, high organic matter concentrations, or the presence of toxic substances. Granules and microbial entrapment technology can effectively retain and enrich microbial assemblages in reactors to improve operating efficiency and reactor stability. The carriers can also protect the reactor's internal microorganisms from interference from the external environment. This article critically reviews the existing literature on autotrophic nitrogen removal systems using immobilization technology. We focus our discussion on the natural aggregation process (granulation) and entrapment technology. The selection of carrier materials and entrapment methods are identified and described in detail and the mechanisms through which entrapment technology protects microorganisms are analyzed. This review will provide a better understanding of the mechanisms through which immobilization operates and the prospects for immobilization technology to be applied in autotrophic nitrogen removal systems.

Highly enriched anammox within anoxic biofilms by reducing suspended sludge biomass in a real-sewage A2/O process

This study proposes a novel strategy of stably enriching anammox in mainstream, based on the competitive difference to NO₂^- between anoxic biofilms and suspended sludge. A modified anaerobic-anoxic-oxic (A²/O) process run for 500 days with actual municipal wastewater. Microbial analysis revealed that anoxic-carrier biofilms had a significantly higher abundance of anammox (qPCR: 0.74% - 4.34%) than suspended sludge (P< 0.001). Batch tests showed that anammox within anoxic-carrier biofilms contributed to significant nitrogen removal, coupled with partial-denitrification (NO₃^- → NO₂^-). The anammox genus, Ca. Brocadia, was highly enriched when suspended sludge was accidentally lost. Further batch tests found that reducing suspended biomass helped anammox enrichment in anoxic-carrier biofilms, because the suspended sludge had strong NO₂^- competition (NO₂^- → N₂) with anammox (increased nirK). Metagenomic sequencing revealed that Ca. Brocadia dominates in the anoxic-carrier biofilms, and is the most important narG contributor to NO₃^- → NO₂^-, which could have promoted the competition of NO₂^- with heterotrophic bacteria. For this A²/O process, the low effluent total nitrogen (8.9 mg ± 1.0 mg N/L) was attributed to partial-denitrification coupling with anammox, demonstrating that this process is applicable to the general influent N-concentration range (30 mg - 50 mg NH₄⁺-N/L) of municipal wastewater treatment plants (WWTPs). Based on the special competitive preference of anammox for NO₂^-, this study provides a promising and practical alternative for enriching anammox bacteria in municipal WWTPs.

Comammox Nitrospira Species Dominate in an Efficient Partial Nitrification-Anammox Bioreactor for Treating Ammonium at Low Loadings

Bacteria capable of complete ammonia oxidation (comammox) are widespread and contribute to nitrification in wastewater treatment facilities. However, their roles in partial nitrification-anaerobic ammonium oxidation (anammox) systems remain unclear. In this study, a bench-scale bioreactor with continuous stirring was operated for more than 1000 days with limited oxygen supply to achieve efficient nitrogen removal (70.1 ± 2.7%) at a low ammonium loading of 35.2 mg-N/L/day. High-throughput amplicon sequencing analysis of the comammox ammonia monooxygenase subunit A (amoA) gene revealed seven sequence types from two clusters in clade A of comammox Nitrospira. Quantitative polymerase chain reaction analyses suggested that the comammox species dominated the ammonia-oxidizing community, with an abundance as high as 89.2 ± 7.9% in total prokaryotic amoA copies. Multiple linear regression further revealed the substantial contribution of the comammox Nitrospira to ammonia oxidation in the bioreactor. The investigation with bioreactor and batch experiments consistently showed that activities of comammox Nitrospira were inhibited by free ammonia far more severely than other ammonia-oxidizing microbes. Overall, this study provided new insight into the ecology of comammox Nitrospira under hypoxic conditions and suggested comammox-associated partial nitrification-anammox as a potential method for treating low-strength ammonium-containing wastewater.

Towards mainstream deammonification: Comprehensive review on potential mainstream applications and developed sidestream technologies

Deammonification (partial nitritation-anammox) process is a favorable and innovative process, for treatment of nitrogen-rich wastewater due to decreased oxygen and carbon requirements at very high nitrogen loadings. The bacterial groups responsible for this process are anaerobic ammonium oxidation (anammox) bacteria in symbiosis with ammonium oxidizing bacteria (AOB) which have an active role in development of nitrogen removal biotechnology in wastewater. Development and operation of sidestream deammonification processes has augmented since the initial full-scale systems, yet there are several aspects which mandate additional investigation and deliberation by the practitioners, to reach the operating perspective, set for the facility. Process technologies for treatment of streams with high ammonia concentrations continue to emerge, correspondingly, further investigation towards feasibility of applying the deammonification concept, in the mainstream treatment process is required. Mainstream deammonification can potentially improve the process of achieving more sustainable and energy-neutral municipal wastewater treatment, however feasible applications are not accessible yet. This critical review focuses on a comprehensive assessment of the worldwide lab-scale, pilot-scale and full-scale sidestream applications as well as identifying the major issues obstructing the implementation of mainstream processes, in addition to the designs, operational factors and technology advancements at both novel and/or conventional levels. This review aims to provide a novel and broad overview of the status and challenges of both sidestream and mainstream deammonification technologies and installations worldwide to assess the global perspectives on deammonification research in the recent years. The different configurations, crucial factors and overall trends in the development of deammonification research are discussed and conclusively, the future needs for feasible applications are critically reviewed.

Extracellular biopolymers recovered as raw biomaterials from waste granular sludge and potential applications: A critical review

Granular sludge (GS) is a special self-aggregation biofilm. Extracellular polymeric substances (EPS) are mainly associated with the architectural structure, rheological behaviour and functional stability of fine granules, given that their significance to the physicochemical features of the biomass catalysing the biological purification process. This review targets the EPS excretion from GS and introduces newly identified EPS components, EPS distribution in different granules, how to effectively extract and recover EPS from granules, key parameters affecting EPS production, and the potential applications of EPS-based biomaterials. GS-based EPS components are highly diverse and a series of new contents are highlighted. Due to high diversity, emerging extraction standards are proposed and recovery process is capturing particular attention. The major components of EPS are found to be polysaccharides and proteins, which manifest a larger diversity of relative abundance, structures, physical and chemical characteristics, leading to the possibility to sustainably recover raw materials. EPS-based biomaterials not only act as alternatives to synthetic polymers in several applications but also figure in innovative industrial/environmental applications, including gel-forming materials for paper industry, biosorbents, cement curing materials, and flame retardant materials. In the upcoming years, it is foreseen that productions of EPS-based biomaterials from renewable origins would make a significant contribution to the advancement of the circular economy.