Detection of nitrifiers and evaluation of partial nitrification for wastewater treatment: A review

Anammox-based processes: How far have we come and what work remains? A review by bibliometric analysis

Nitrogen contamination remains a severe environmental problem and a major threat to sustainable development worldwide. A systematic analysis of the literature indicates that the partial nitritation-anammox (PN/AMX) process is still actively studied as a viable option for energy-efficient and feasible technology for the sustainable treatment of N- rich wastewaters, since its initial discovery in 1990. Notably, the mainstream PN/AMX process application remains the most challenging bottleneck in AMX technology and fascinates the world's attention in AMX studies. This paper discusses the recent trends and developments of PN/AMX research and analyzes the results of recent years of research on the PN/AMX from lab-to full-scale applications. The findings would deeply improve our understanding of the major challenges under mainstream conditions and next-stage research on the PN/AMX process. A great deal of efforts has been made in the process engineering, PN/AMX bacteria populations, predictive modeling, and the full-scale implementations during the past 22 years. A series of new and excellent experimental findings at lab, pilot and full-scale levels including good nitrogen removal performance even under low temperature (15-10 °C) around the world were achieved. To date, pilot- and full-scale PN/AMX have been successfully used to treat different types of industrial sewage, including black wastewater, sludge digester liquids, landfill leachate, monosodium glutamate wastewater, etc. Supplementing the qualitative analysis, this review also provides a quantitative bibliometrics study and evaluates global perspectives on PN/AMX research published during the past 22 years. Finally, general trends in the development of PN/AMX research are summarized with the aim of conveying potential future trajectories. The current review offers a valuable orientation and global overview for scientists, engineers, readers and decision makers presently focusing on PN/AMX processes.

Operational optimization of a three-stage nitrification moving bed biofilm reactor (NMBBR) by obtaining enriched nitrifying bacteria: Nitrifying performance, microbial community, and kinetic parameters

A two-sludge system consisting of A²/O (Anaerobic Anoxic Oxic) and NMBBR (Nitrification Moving Bed Biofilm Reactor) was developed. Stable and efficient denitrifying phosphorus removal can be realized by high-efficiency utilization of carbon sources in A²/O reactor with the electron acceptors of NO_x^--N in a three-stage NMBBR (consisting of N₁, N₂, N₃). The three-stage NMBBR was successfully started within 18 days without additional inoculation sludge. Then a long-term operation (22-120 d) for the optimization of nitrifying performance, microbial community, and kinetic parameters was investigated. The biofilm characteristics (MLSS and biofilm thickness) and real-time control parameters (DO and pH) initially revealed the differences of three stages, while FISH results confirmed the optimizing nitrifying bacteria populations including AOB, Nitrobacteria and Nitrospira (N₁: 5.94 ± 0.12%; N₂: 8.26 ± 0.42%; N₃: 10.06 ± 0.27% on day 50), basically consisting with the qPCR results (N₁: 4.05%; N₂: 8.04%; N₃: 14.14%). The specific ammonium oxidation rate (SAOR: 3.24-10.02 mg/(gMLSS·h)) and temperature coefficient (θ: 1.008-1.011) based on temperature variation (15-35 °C) exhibited a strong resistant ability to low temperature operation. Moreover, half-saturation constants (K_N,AOB, K_N,NOB, K_O,AOB and K_O,NOB) fitted by Monod equation proved that DO diffusion played a significant role than substrate utilization (NH₄⁺-N and NO₂^--N), but the diffusion resistance was negligible for flocs size smaller than 70 μm. Additionally, the dominant NOB (mainly Nitrospira) due to a higher K_N,NOB and K_O,NOB was more sensitive to mass transfer and diffusion resistance, which was helpful to understand the microbial competition for short-cut nitrification between AOB and NOB. Based on the above mechanism analysis, the MBBR optimization for the design and operation was put forward.

Enrichment of Denitrifying Bacterial Community Using Nitrite as an Electron Acceptor for Nitrogen Removal from Wastewater

This work aimed to enrich a denitrifying bacterial community for economical denitrification via nitrite to provide the basic objects for enhancing nitrogen removal from wastewater. A sequencing batch reactor (SBR) with continuous nitrite and acetate feeding was operated by reasonably adjusting the supply rate based on the reaction rate, and at a temperature of 20 ± 2 °C, pH of 7.5 ± 0.2, and dissolved oxygen (DO) of 0 mg/L. The results revealed that the expected nitrite concentration can be achieved during the whole anoxic reaction period. The nitrite denitrification rate of nitrogen removal from synthetic wastewater gradually increased from approximately 10 mg/(L h) to 275.35 mg/(L h) over 12 days (the specific rate increased from 3.83 mg/(g h) to 51.80 mg/(g h)). Correspondingly, the chemical oxygen demand/nitrogen (COD/N) ratio of reaction decreased from 7.9 to 2.7. Both nitrite and nitrate can be used as electron acceptors for denitrification. The mechanism of this operational mode was determined via material balance analysis of substrates in a typical cycle. High-throughput sequencing showed that the main bacterial community was related to denitrification, which accounted for 84.26% in the cultivated sludge, and was significantly higher than the 2.16% in the seed sludge.

Efficient nitrogen removal in separate coupled-system of anammox and sulfur autotrophic denitrification with a nitrification side-branch under substrate fluctuation

In order to achieve efficient nitrogen removal, a separate coupled-system of anaerobic ammonia oxidation (anammox) and sulfur autotrophic denitrification (S⁰-SADN) was established. In this study, the operational feasibility and stability of the coupled-system under substrate fluctuations were investigated. Results showed that the coupled-system improved the total nitrogen removal efficiency (TNRE) to 99.15 ± 0.68%. The tryptophan-like substances in anammox effluent positively impacted the growth of the S⁰-SADN biofilm. This positive cooperativity boosted the S⁰-SADN to achieve rapid 12-day startup and stable operation thereafter. The TNRE was determined at 95.27 ± 1.51% and 93.44 ± 0.96% under excessive nitrite and ammonium, respectively. The coupled-system recovered quickly after 21 days of starvation deterioration. To further treat the excessive ammonium, the nitrification side-branch of the coupled-system improved the TNRE to 99.08 ± 0.68%. Extracellular polymeric substances analysis revealed that the anammox and S⁰-SADN bacteria secreted protein-like substances to resist substrate fluctuation. Microbial community analysis indicated that the stability of bacterial community supported the stability of the coupled-system. These results collectively suggested that the separate coupled-system exhibited excellent performance and provided a platform for practical wastewater treatment in future.

Immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal

In this study, immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal was investigated. Three redox mediators (RMs), namely, 2-methyl-1,4-naphthoquinone (ME), anthraquinone (AQ) and 1-dichloroanthraquinone (1-AQ) were catalyzed to reduce nitrate to only nitrite by denitrification to integrate with the anammox process for nitrogen removal. First, our experimental results showed that there were 35.8, 42.2 and 53.0 mg-N L⁻¹ nitrite accumulation values with the addition of ME, AQ and 1-AQ, respectively, at the dose of 75 µM by the denitrification process at C/N = 2, which were 25.6%, 48.2% and 86.1% higher than that of the control without the addition of any RMs. Nitrate reductase activities were higher than that of nitrite reductase affected by RMs, which was the main reason for nitrite accumulation and further maintenance of the anammox process. Second, owing to the stable nitrite production by the partial denitrifying biomass with the addition of 1-AQ, the nitrogen removal rate of the reactor that integrated the partial denitrification and anammox process reached 1788.36 g-N m⁻³ d⁻¹ only using ammonia and nitrate as the influent nitrogen resource in the long-term operation. Third, the 16S rDNA sequencing results demonstrated that Yersinia frederiksenii and Thauera were the primary groups of the denitrifying biomass, which were considered the dominant partial denitrification species.

In this study, immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal was investigated.

Pilot study of nitrogen removal from landfill leachate by stable nitritation-denitrification based on zeolite biological aerated filter

A pilot (about 1 m³/d) process consisting of pre-denitrification and zeolite biological aerated filter (ZBAF) was established and run for nitrogen removal of landfill leachate. The results showed that stable nitritation and denitrification was achieved for landfill leachate with removal efficiency of Chemical Oxygen Demand (COD_Cr), ammonium and total nitrogen (TN) of 53.2 ± 3.0%, 93.5 ± 2.4% and 74.7 ± 9.4%, respectively. Based on the ammonium adsorption equilibrium by zeolite, stable free ammonia could be maintained for inhibition of nitrite oxidizing bacteria (NOB) and dominance of ammonia oxidizing bacteria (AOB) in ZBAF, resulting in efficient nitritation with a nitrite accumulation ratio higher than 90.0% and an average nitrite production rate of 1.387 kg NO₂^--N m^-3 day^-1. High-throughput sequencing analysis further revealed enrichment of AOB and elimination of NOB in ZBAF. Compared to two-stage anoxic-oxic process, the pilot-scale process could save approximate 5000 mg/L glucose (about 3.10 US dollar/m³) with almost similar TN removal performance. All results obtained demonstrated the feasibility of the pilot process, which might be highly promising for the nitritation and denitrification of low C/N landfill leachate in the future.

Dissolved oxygen/free ammonia (DO/FA) ratio manipulation to gain distinct proportions of nitrogen species in effluent of entrapped-cell-based reactors

Oxygen-limiting and/or free ammonia (FA)-accumulating conditions are two common operating strategies for partial nitrification in wastewater. Controlling either bulk dissolved oxygen (DO) or free ammonia (FA) concentration to maintain partial nitrification can be challenging due to the strong interdependency between these two parameters as substrates for ammonia oxidation. In this study, DO/FA ratio is proposed as a controlling parameter for partial nitrification by entrapped-cell-based reactors. At DO/FA >1.5, both ammonia and nitrite oxidation proceeded without inhibition leading to complete oxidation of ammonia to nitrate. An effluent containing nitrate as the main nitrogen species can be produced at these ratios. At a DO/FA ratio range of 0.2-1.5, ammonia oxidation proceeded without efficiency deterioration, while nitrite oxidation decreased with decreasing DO/FA ratio. At the ratios of 0.2-0.6, an effluent containing mainly nitrite can be generated. At DO/FA <0.2, both ammonia oxidation and nitrite oxidation were inhibited and the effluent with nearly equal molar of ammonia and nitrite was obtained. By controlling DO/FA ratio, effluents with different proportions of nitrogen species can be produced allowing the entrapped-cell-based system to be adaptable as an initial reactor for various nitrogen removal approaches.

Enhancement in the partial nitrification of wastewater sludge via low-intensity ultrasound: Effects on rapid start-up and temperature resilience

The partial nitrification process can reduce the aeration energy consumption in bioreactors by 25%. Low-intensity ultrasound (0.25 W·mL^-1) was applied during the partial nitrification process to evaluate its effects on start-up and temperature resilience. Ultrasound application led to rapid start-up of the partial nitrification process (within 18 d) with a nitrite accumulation ratio of above 80% at 18 °C. Moreover, when the temperature was increased to 28 °C, the partial nitrification process was effectively maintained with a nitrite accumulation ratio of above 80%. Ultrasonic treatment for a long duration had a positive effect on ammonia oxidizing bacteria of the genus Nitrososphaera, whereas the population of nitrite oxidizing bacteria, Nitrospira, decreased. The temperature resilience of Nitrososphaera was also enhanced. These findings indicate that ultrasound induces rapid start-up of the partial nitrification process and enhances the temperature resilience of Nitrososphaera.

Performance of partial nitritation-anammox processes at mainstream conditions in an IFAS system

The partial nitritation-anammox processes implementation in the main line of wastewater treatment plants would lead them closer to the energy autarky. With this purpose, an integrated fixed film activated sludge (IFAS) reactor was operated at pilot scale. Efficient nitrogen removal (72 ± 11%) was achieved for anaerobically pre-treated municipal wastewater at low temperature (21 - 15 °C), with a nitrogen removal rate of 37 ± 3 g N/(m³·d) at 15 °C. The ammonium oxidizing bacteria were more abundant in the activated sludge, while anammox bacteria were primarily located in biofilm attached onto the carriers surface. Nitrite oxidizing bacteria (NOB) activity was similar between both fractions and its specific activity decreased more than that of other populations when the operating temperature was reduced. Furthermore, the IFAS operational strategy (aerobic/anoxic periods) allowed an efficient NOB activity suppression inside the reactor, which accounted only for the 10 - 20% of the maximum potential activity.

Nitrite and nitrate as electron acceptors for bioelectrochemical ammonium oxidation under electrostatic field

Bioelectrochemical ammonium oxidation with nitrite and nitrate as electron acceptors was investigated in bulk solution exposed to electrostatic field. In a bioelectrochemical reactor, electroactive nitrogen removal bacteria including ammonium oxidizing exoelectrogens (AOE) and denitrifying electrotrophs (DNE) were enriched by electrostatic field of 0.2 V/cm in a bulk solution containing nitrite, nitrate, and ammonium. Ammonium was oxidized simultaneously with decreases in nitrite and nitrate as electron acceptors due to direct interspecies electron transfer between AOE and DNE. The specific ammonium oxidation rate was 48 mg NH₄-N/g VSS.d when nitrate fraction was 1/3 in the electron acceptor composed of nitrite and nitrate. The specific ammonium oxidation rate gradually decreased with increasing nitrate fraction. However, it was still 24 mg NH₄-N/g VSS.d when nitrate was the only electron acceptor. This indicates that nitrate can be used as an electron acceptor for bioelectrochemical ammonium oxidation, although it is a less effective than nitrite. This finding provides an advantage that strict nitritation which selectively produces nitrite from ammonium can be avoided when treating ammonia-rich wastewater in a bioelectrochemical reactor.

Sustainable Approach to Eradicate the Inhibitory Effect of Free-Cyanide on Simultaneous Nitrification and Aerobic Denitrification during Wastewater Treatment

Simultaneous nitrification and aerobic denitrification (SNaD) is a preferred method for single stage total nitrogen (TN) removal, which was recently proposed to improve wastewater treatment plant design. However, SNaD processes are prone to inhibition by toxicant loading with free cyanide (FCN) possessing the highest inhibitory effect on such processes, rendering these processes ineffective. Despite the best efforts of regulators to limit toxicant disposal into municipal wastewater sewage systems (MWSSs), FCN still enters MWSSs through various pathways; hence, it has been suggested that FCN resistant or tolerant microorganisms be utilized for processes such as SNaD. To mitigate toxicant loading, organisms in SNaD have been observed to adopt a diauxic growth strategy to sequentially degrade FCN during primary growth and subsequently degrade TN during the secondary growth phase. However, FCN degrading microorganisms are not widely used for SNaD in MWSSs due to inadequate application of suitable microorganisms (Chromobacterium violaceum, Pseudomonas aeruginosa, Thiobacillus denitrificans, Rhodospirillum palustris, Klebsiella pneumoniae, and Alcaligenes faecalis) commonly used in single-stage SNaD. This review expatiates the biological remedial strategy to limit the inhibition of SNaD by FCN through the use of FCN degrading or resistant microorganisms. The use of FCN degrading or resistant microorganisms for SNaD is a cost-effective method compared to the use of other methods of FCN removal prior to TN removal, as they involve multi-stage systems (as currently observed in MWSSs). The use of FCN degrading microorganisms, particularly when used as a consortium, presents a promising and sustainable resolution to mitigate inhibitory effects of FCN in SNaD.

Cooperation between partial-nitrification, complete ammonia oxidation (comammox), and anaerobic ammonia oxidation (anammox) in sludge digestion liquid for nitrogen removal

The challenge of sludge digester liquor treatment is its high ammonium nitrogen (NH₄⁺-N) concentration. Early reports found that complete ammonia oxidation (comammox) was not present and anaerobic ammonia oxidation (anammox) was difficult to achieve in most sludge digester liquor treatments. In this study, NH₄⁺-N removal by cooperation between partial-nitrification, comammox, and anammox processes was achieved in a sequencing batch reactor (SBR) for sludge digester liquor treatment. The results showed that 2100-2200 mg/L of NH₄⁺-N was removed in the SBR with 98.82% removal efficiency. In addition, 55.11% of NH₄⁺-N was converted to nitrite nitrogen (NO₂^--N) by partial-nitrification, 25.43% of NH₄⁺-N was converted to nitrate nitrogen (NO₃^--N) by comammox, and 18.28% of NH₄⁺-N was removed by anammox. During the operation, in the SBR, the relative abundance of the dominant ammonia-oxidizing bacteria (Chitinophagaceae) was 18.89%, that of the dominant anammox bacteria (Candidatus Kuenenia) was 0.10%, and that of the dominant comammox bacteria (Nitrospira) was 0.20%. Therefore, the high nitrogen removal efficiency in this system was considered the result of the combination of the three processes. These results showed that comammox and anammox could play very important roles in nitrogen transformation and energy-saving in nitrogen removal systems.

Enhanced microbial metabolism in one stage partial nitritation-anammox system treating low strength wastewater by novel composite carrier

One stage partial nitritation-anammox (PN-A) process has attracted more and more attention due to the low investment cost but the instability in treating low strength wastewater. In this study, for producing a novel composite carrier that could provide high ammonia microenvironment in low strength wastewater, the zeolites and floating materials were combined in the spherical shell and distributed evenly by the spherical polyhedron. And a moving bed biofilm reactor (MBBR) with the composite carriers and ordinary carriers without zeolites as control group was operated for nearly 120 days. The PN-A process were realized in 53 days, and the total nitrogen removal efficiency reached around 85% at influent ammonium concentration of 50 mg/L finally. Analysis of 16S rRNA gene sequencing revealed that the composite carriers showed significant promotion on the proliferation of ammonium oxidizing bacteria (AOB) and enrichment of anaerobic ammonium oxidizing bacteria (AnAOB), accounting for 19.14% and 41.65% on the surface, respectively. Moreover, the existence of relative higher abundance of ammonia on the composite carrier surface was validated by the metabolite biomarker of glutamate and especially spermidine. The metabolomics analysis and 16S rRNA function prediction showed that the protein synthesis pathway was obviously upregulated on the composite carriers surface compared with that on the ordinary carriers surface. The higher abundance of glutamate and putrescine indicated that the composite carrier could stimulate the metabolism and growth of bacteria. The present study provided a functional carrier to realize the transformation of activated sludge system into PN-A system treating low strength wastewater, which is significant to the application of the process in mainstream.

Metagenomic insights into functional traits variation and coupling effects on the anammox community during reactor start-up

Anammox technology is an energy-efficient wastewater treatment process and anammox community structure has gained extensive attention. However, the dynamics of community functional traits are still elusive. Here, we combined the long-term reactor operation and metagenomic, multiple bioinformatic and network analyses to reveal the succession of anammox community and function traits during reactor start-up. We found the cooperation of denitrifiers that affiliated to the phylum Proteobacteria could reduce nitrite to dinitrogen gas. These organisms and genes had higher abundance after the inhibition phase, which could contribute to nitrite consuming and reactor performance recovery. Importantly, the Terrimonas and Anaerolinea organisms had ability of extracellular polymers secretion or aggregate formation. They had the highest abundance at the end of the lag phase, which could benefit for promoting the nitrogen removal rate (NRR). Meanwhile, Terrimonas and Anaerolinea bacteria could cooperate with methanogenic and nitrite-denitrifying methanotrophic organisms based on H₂ and CH₄, respectively. Since these organisms also had higher abundance after the inhibition phase, their cooperation could prevent anammox bacteria from nitrite inhibiting when the influent nitrite concentration was higher. The analysis of community and function shift is expected to emphasize the importance of functional bacteria in anammox process and provides a potential control strategy for nitrogen-containing wastewater treatment process.

Isolation and identification of a salt-tolerant aerobic denitrifying bacterial strain and its application to saline wastewater treatment in constructed wetlands

A salt-tolerant aerobic denitrifying bacterium, Zobellella denitrificans strain A63, was isolated, and its effects on the efficiency of denitrification of saline wastewater and the denitrifying microbial community structure in the matrix were studied in vertical-flow constructed wetlands (VFCWs). In a VFCW system with strain A63, the removal efficiencies of NH₄⁺-N, NO₃^--N, and total nitrogen reached 79.2%, 95.7%, and 89.9%, respectively. Quantitative PCR analysis indicated that the amoA gene from ammonia-oxidizing archaea (AOA) was highly abundant, whereas amoA from ammonia-oxidizing bacteria and nxrA from nitrite-oxidizing bacteria were lowly abundant because of the influent salinity, irrespective of whether strain A63 was added. However, the addition of strain A63 significantly increased the abundance of nirK in the top layer of the VFCW. Therefore, AOA-driven partial nitrification and aerobic denitrification by strain A63 occurred in VFCWs. Our findings suggest that adding salt-tolerant denitrifying strains to constructed wetlands can enhance denitrification for saline wastewater treatment.

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

Transient anoxia due to the periodic anoxic/aerobic operation is beneficial for the nitrite-oxidizing bacteria (NOB) suppression. A continuous reactor of modified University of Cape Town process treating municipal wastewater was equipped with alternating anoxic/aerobic zones to maintain nitritation. Higher nitrite accumulation ratio in the oxic zones was achieved through transient anoxia and shorter aerobic actual hydraulic retention time (15 min), but it steeply deteriorated from above 95.0% to 21.0% after elevated temperature (25 °C). Batch experiments indicated that the existence of initial nitrite at the starting of aerobic phase promoted the recovery of NOB activity from transient anoxia and inhibited the activity of ammonium-oxidizing bacteria. Furthermore, a supplemental modeling further confirmed that the specific growth rates of NOB (μ_NOB) decreased at the anoxic phase and the recovery extent of μ_NOB after anoxic exposure have a positive correlation with the initial concentrations of nitrite, leading to the failure of maintaining nitritation.

Aerobic bacterial pretreatment to overcome algal growth inhibition on high-strength anaerobic digestates

Coupling anaerobic digestion and algae cultivation has attracted attention as a sustainable means of treating high-strength wastewaters. In such a scenario, nutrients from the liquid anaerobic digestate are used by algae to produce biomass. However, use of full-strength digestate results in poor algal growth and nutrient removal. Most researchers have overcome this challenge by diluting digestate 10-30 fold prior to algae growth but such dilution rates demand large amounts of fresh water, posing challenges for scale-up. The objectives of this study were to 1) assess whether ammonium, turbidity, and heavy metals in digestate were the primary sources of inhibition for a highly-nutrient tolerant strain of Chlorella sorokiniana, and, 2) develop a biological pretreatment strategy to overcome algal growth inhibition on full strength digestate. Ammonia toxicity, turbidity, and heavy metals have been commonly hypothesized as the source of algal growth inhibition, but our results showed that these factors were not critical inhibitors of C. sorokiniana. Dose response studies showed that C. sorokiniana could grow robustly on 3,500 mg/L ammonium. Regardless, full strength digestates of wastewater sludge and food waste were very inhibitory to C. sorokiniana. We utilized a pretreatment approach using activated sludge which led to robust algal growth on full-strength digestate. High growth rates of 250-500 mg/L/d were achievable on pretreated digestates despite very high ammonium levels of ∼2,000 mg/L. Pretreating digestate also led to significantly faster algal nutrient uptake compared to untreated digestate (p < 0.001).

Mechanistic understanding of the NOB suppression by free ammonia inhibition in continuous flow aerobic granulation bioreactors

A partial nitritation continuous flow reactor (CFR) was operated for eight months demonstrating that partial nitritation granular sludge can remain stable under continuous flow conditions. The ammonia oxidizing bacteria (AOB)-to-nitrite oxidizing bacteria (NOB) activity ratios were determined for a series of granule sizes to understand the impact of mass diffusion limitation on the free ammonia (FA) inhibition of NOB. When dissolved oxygen (DO) limitation is the only mechanism for NOB suppression, the AOB:NOB ratio was usually found to increase with the granule size. However, the trend is reversed when FA has an inhibitory effect on NOB, as was observed in this study. The decrease in AOB:NOB ratio indicates that smaller granules, e.g. diameter <150 μm, are preferred for nitrite accumulation when high FA concentration is present, as in the partial nitritation process. The trend was further verified by observing the increase in the apparent inhibition coefficient as granule size increased. Indeed, this study for the first time quantified the effect of diffusion limitation on the apparent inhibition coefficient of NOB in aerobic granules. A mathematical model was then utilized to interpret the observed suppression of NOB and predicted that NOB suppression was only complete at the granule surface. The NOB that did survive in larger granules was forced to dwell within the granule interior, where the AOB growth declines due to DO diffusion limitation. This means FA inhibition can be taken advantage of as an effective means for NOB suppression in small granules or thin biofilms. Further, both FA inhibition and DO limitation were found to be required for the suppression of NOB in mainstream aerobic granules.

Operational pattern affects nitritation, microbial community and quorum sensing in nitrifying wastewater treatment systems

Achievement of nitrite accumulation is critical for the application of advanced nitrogen removal processes. Two lab-scale sequencing batch biofilm reactors (SBBRs) and two sequencing batch reactors (SBRs) were operated under intermittent aeration with different feeding patterns. The nitrite accumulation ratio was 56.6% (pulse feeding) and 68.9% (constant feeding) in SBBRs with nitritation. Nitrate accounted for 98% of the effluent nitrogen in SBRs with complete nitrification. The dominant nitrifier was Nitrosomonas in SBBRs and Nitrospira in SBRs. Four types of acyl homoserine lactones (AHLs) were detected. N‑[(RS)‑3‑Hydroxybutyryl]‑l‑homoserine lactone and N‑octanoyl‑l‑homoserine lactone had a high concentration in the extracellular polymeric substance phase, and had an obvious relationship with nitrite accumulation and ammonia removal. Various microbial communities coexisted in nitrifying systems, with diverse microbial interactions. Microorganisms harboring AHLs-related genes had more interactions with each other, suggesting that nitritation could be regulated by AHLs based quorum sensing.

Nitrifying moving bed biofilm reactor: Performance at low temperatures and response to cold-shock

In contrast with suspended growth systems, attached growth technologies such as the moving bed biofilm reactors (MBBR) have recently demonstrated significant nitrification rates at temperatures as low as 1 °C. The purpose of this study was to investigate the performance of the nitrifying MBBR system at elevated municipal concentrations with exposures to low temperatures and cold-shock conditions down to 1 °C using an enhanced temperature-controlled room. A removal rate of 98.44 ± 4.69 gN·m^-3·d^-1 was identified as the intrinsic rate of nitrifying MBBR systems at 1 °C and was proposed as the conservative rate for low temperature design. A temperature threshold at which attached growth nitrification displayed a significant decrease in kinetics was identified between 2 °C and 4 °C. Arrhenius correction coefficients of 1.086 and 1.09 previously applied for low temperature nitrifying MBBR systems resulted in conservative modeled removal rates on average 21% lower than the measured rates. Thus, an Arrhenius correction coefficient of 1.049 is proposed between the temperatures of 10 °C and 4 °C and another correction coefficient of 1.149 to model rates at 1 °C. For the transition from 4 °C to 1 °C, the adjustment of a previously reported Theta model is proposed in this study to account for exposure time at low temperatures; with the modified model showing strong correlation with measured rates (R² = 0.88). Finally, a comparison of nitrification kinetics between MBBR systems acclimatized to 1 °C and systems that are cold-shocked to 1 °C demonstrated that shocked removal rates are 21% lower.

Acute and chronic toxicity of nickel to nitrifiers at different temperatures

This study investigated the acute nickel toxicity on nitrification of low ammonia synthetic wastewater at 10, 23, and 35°C. The nickel inhibition half-velocity constants (K_I,Ni) for ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) based on Ni/MLSS ratio at 10, 23, and 35°C were 5.4 and 5.6 mg Ni/g MLSS, 4.6 and 3.5 mg Ni/g MLSS, and 9.1 and 2.7 mg Ni/g MLSS, respectively. In addition, chronic toxicity of nickel to nitrification of low ammonia synthetic wastewater was investigated at 10°C in two sequencing batch reactors (SBRs). Long-term SBRs operation and short-term batch tests were comparable with respect to the extent of inhibition and corresponding Ni/MLSS ratio. The μ_max, b, and K_o of AOB were 0.16 day^-1, 0.098 day^-1 and 2.08 mg O₂/L after long-term acclimatization to nickel of 1 mg/L at 10°C, high dissolved oxygen (DO) (7 mg/L) and long solids retention time (SRT) of 63-70 days. Acute nickel toxicity of nitrifying bacteria was completely reversible.

Diagnosis of soil contamination using microbiological indices: A review on heavy metal pollution

Heavy metal contamination of soil has become a serious global issue because of their persistence in the environment and the non-biodegradable nature leading to their accumulation to toxic levels. In order to achieve early warning and prevent soil quality from deteriorating, it is necessary to select suitable indices to diagnose heavy metal pollution. Microbiological indices for monitoring soil pollution by heavy metals are gaining attention. However, the related researches are scattered, and critical review is imperative. This review is mainly to provide readers with an in-depth understanding of the merits and limitations of microbiological indices for heavy metals contaminated and remediated soils. Microbiological indicators include microbial abundance, community diversity and structure, functional activity. The changes of different microbiological indices and the mechanism of microbial response to heavy metal stress in soils are comprehensively summarized. Furthermore, research gaps and future directions of the microbial ecotoxicological diagnosis of soil contamination by heavy metals are also proposed and discussed.

Characterization of partial-denitrification (PD) granular sludge producing nitrite: Effect of loading rates and particle size

The granule-based partial-denitrification (PD) reactor can achieve an efficient nitrite production for anammox process. In this study, the PD granules were successfully cultivated in a sequencing batch reactor (SBR), the physicochemical properties and microbial activities were characterized at increasing nitrate loading rate (NLR). Results indicated that high NLR benefited the PD for nitrite production, and a more dense and compact granule can be developed at the NLR of 0.24-0.48 Kg N/m³/d. Whereas the settling ability decreased when the NLR increased to 0.96 Kg N/m³/d, which was likely caused by the decrease of proteins (PN) in extracellular polymeric substances (EPS) content and the increase of loosely bound EPS (L-EPS) fraction. Besides, the characterization of size-fractionated PD granules revealed that a better settling ability was generally obtained with larger granules except for the size of d > 3.35 mm, which the settling velocity was reduced, likely attributed to the excess L-EPS fraction (56.3%) and presence of empty capsule inside granules. The smaller granules exhibited higher microbial activities due to the favorable mass transfer, the nitrate reduction rate was as high as 152.6 mg N/h/g VSS with granules of d < 1.0 mm. Results obtained in this study provided a better understanding of the properties of PD granules and would be helpful for the future development of granule-based PD reactor in achieving an efficient and stable nitrite production.

Engineering feasibility, economic viability and environmental sustainability of energy recovery from nitrous oxide in biological wastewater treatment plant

Currently, the biological wastewater treatment has been challenged by their high energy consumption. An increasing effort has been devoted to exploring energy recovery from nitrous oxide (N₂O) as a powerful fuel additive rather than as an unwanted byproduct during biological nitrogen removal. This review aims to offer a holistic and critical analysis of the ideas for N₂O production and energy recovery in terms of engineering feasibility, economic viability and environmental sustainability. It turns out that the recoverable energy from N₂O produced in municipal wastewater is below 0.03 kWh/m³, which is insignificant compared with the in-plant energy consumption, while complicated process configuration and high cost associated with harvesting and post-purification of N₂O will be incurred. An environmental risk related to global climate change due to the emission of residual dissolved N₂O is also concerned. Further effort on N₂O production and recovery technologies is indeed required to improve the overall energy balance.

Effects of triclosan on performance, microbial community and antibiotic resistance genes during partial denitrification in a sequencing moving bed biofilm reactor

Effects of triclosan (TCS) on performance, microbial community and antibiotic resistance genes (ARGs) during partial denitrification (PD) were investigated in a sequencing moving bed biofilm reactor (SMBBR). TCS inhibited nitrite accumulation; inhibition effect was more obvious as TCS concentration increased from 1 to 5 mg/L, but it could recover. Extracellular polymeric substances contents increased with 1 mg/L TCS addition and decreased a lot at 5 mg/L TCS. Community structure in biofilm was different from that in floccular sludge, but it was similar at 5 mg/L TCS. Illumina sequencing showed that Pseudomonas, Aeromonas, Shewanella and Thauera became dominant genera. Abundance of nirS was stable and higher than that of narG and nosZ. High-throughput qPCR showed that mexF, acrA-02, fabK, etc. were screened at 5 mg/L TCS. IntI1 and tnpA-04 were abundant mobile genetic elements. The study furthers understanding of effects of TCS on PD, bacterial communities and ARGs in SMBBR.

Long-term microbial community dynamics at two full-scale biotrickling filters treating pig house exhaust air

In this study, the microbial community structure of two full‐scale biotrickling filters treating exhaust air from a pig housing facility were evaluated using 16S metabarcoding. The effect of inoculation with activated sludge of a nearby domestic waste water treatment plant was investigated, which is a cheap procedure and easy to apply in practice. The study was performed at a three‐stage and a two‐stage full‐scale biotrickling filter; of which, only the latter was inoculated. Both biotrickling filters evolved towards a rather similar community over time, which differed from the one in the activated sludge used for inoculation. However, the bacterial population at both biotrickling filters showed small differences on the family level. A large population of heterotrophic bacteria, including denitrifying bacteria, was present in both biotrickling filters. In the non‐inoculated biotrickling filter, nitrite‐oxidizing bacteria (NOB) could not be detected, which corresponded with the incomplete nitrification leading to high nitrite accumulation observed in this system. Inoculation with the wide spectrum inoculum activated sludge had in this study a positive effect on the biotrickling filter performance (higher ammonia removal and lower nitrous oxide production). It could thus be beneficial to inoculate biotrickling filters in order to enrich NOB at the start‐up, making it easier to keep the free nitrous acid concentration low enough to not be inhibited by it.

New framework for automated article selection applied to a literature review of Enhanced Biological Phosphorus Removal

AIMS

Enhanced Biological Phosphorus Removal (EBPR) is a technology widely used in wastewater treatment to remove phosphorus (P) and prevent eutrophication. Establishing its operating efficiency and stability is an active research field that has generated almost 3000 publications in the last 40 years. Due to its size, including over 119 review articles, it is an example of a field where it becomes increasingly difficult to manually recognize its key research contributions, especially for non-experts or newcomers. Therefore, this work included two distinct but complementary objectives. First, to assemble for the first time a collection of bibliometric techniques into a framework for automating the article selection process when preparing a literature review (section 2). Second, to demonstrate it by applying it to the field of EBPR, producing a bibliometric analysis and a review of the key findings of EBPR research over time (section 3).

FINDINGS

The joint analysis of citation networks, keywords, citation profiles, as well as of specific benchmarks for the identification of highly-cited publications revealed 12 research topics. Their content and evolution could be manually reviewed using a selection of articles consisting of approximately only 5% of the original set of publications. The largest topics addressed the identification of relevant microorganisms, the characterization of their metabolism, including denitrification and the competition between them (Clusters A-D). Emerging and influential topics, as determined by different citation indicators and temporal analysis, were related to volatile fatty acid production, P-recovery from waste activated sludge and aerobic granules for better process efficiency and stability (Clusters F-H).

CONCLUSIONS

The framework enabled key contributions in each of the constituent topics to be highlighted in a way that may have otherwise been biased by conventional citation-based ranking. Further, it reduced the need for manual input and a priori expertise compared to a traditional literature review. Hence, in an era of accelerated production of information and publications, this work contributed to the way that we are able to use computer-aided approaches to curate information and manage knowledge.

Influence of recirculation over COD and N-NH4 removals from landfill leachate by horizontal flow constructed treatment wetland

Treatment of landfill leachate is a challenge due to its complex chemical composition and high recalcitrance and because of high costs for conventional wastewater treatment. In our study, leachate from the Quitaúna Landfill, Sao Paulo Metropolitan Region, Brazil, was treated at a laboratory scale with a horizontal subsurface flow constructed treatment wetland (HF-CTW) operating under a recirculation regime. Two units planted with Heliconia psittacorum (HP) and Cyperus papyrus (CP), and one unplanted control unit were assessed. With a recirculation regime over 21 days, the planted units removed 40% of chemical oxygen demand (COD) while the control unit removed only 29%. True color removal efficiencies were 2, 22, and 23% for the control, HP, and CP HF-CTWs, respectively. The ammonium nitrogen removal efficiencies for a 21-day hydraulic retention time (HRT) were 63-81% for planted units and 72% for the control. The increase of the HRT from 7 to 21 days led to the enhancement of ammonium nitrogen removal but did not affect the COD and total nitrogen removals. This phenomenon is a consequence of leachate's low biodegradability. The present study shows the importance of the HRT and plant presence for landfill leachate treatment using HF-CTWs.

Partial nitrification in entrapped-cell-based reactors with two different cell-to-matrix ratios: performance, microenvironment, and microbial community

In this study, we investigated the effect of different cell-to-matrix ratios (1% and 4%) on the partial nitrification of phosphorylated polyvinyl alcohol-entrapped-cell-based reactors and evaluated the microenvironment, microbial community, and microbial localization within the gel matrices. The results indicated that the reactor with a 1% cell-to-matrix ratio required 184 days of operation to reach partial nitrification that produced anaerobic ammonium oxidation-suitable effluent. In contrast, partial nitrification was achieved from the beginning of the operation of the reactor with the 4% cell-to-matrix ratio. The oxygen-limiting zone (dissolved oxygen = 0.5-1.5 mg L^-1), where nitrite-oxidizing activity has been suggested to be suppressed and ammonia-oxidizing activity was reported to be maintained, occurred at 10-230 µm from the gel matrices surface. In addition, the layer of ammonia-oxidizing bacteria observed in this zone is likely to have played a role in obstructing oxygen penetration into the inner region of the gel matrices. The next-generation sequencing results indicated that members of the family Nitrosomonadaceae accounted for 16.4-20.7% of the relative abundance of bacteria at the family level, while members of the family Bradyrhizobiaceae, to which the genus Nitrobacter belongs, accounted for approximately 10% of the relative abundance of bacteria at the genus level in the gel matrices.

Impact of free nitrous acid shock and dissolved oxygen limitation on nitritation maintenance and nitrous oxide emission in a membrane bioreactor

This study investigated the initiation and maintenance of nitritation in a membrane bioreactor (MBR) with long solids retention time (SRT) of 43.8 days. Nitritation was initiated within 65 days in the MBR via dissolved oxygen (DO) limitation (<0.5 mg/L). However, nitrite oxidizing bacteria (NOB) (Nitrospira and Nitrobacter) acclimated to the low DO environment and proliferated from day 81, leading to nitrate accumulation. Thereafter, the combined strategy of DO limitation and in-situ generated free nitrous acid (FNA) shock successfully restored and maintained stable nitritation for >70 days. Quantitative polymerase chain reaction (qPCR) results showed that cell abundances of Nitrospira and Nitrobacter decreased by between 50.0 to 68.9% and 60.6 to 96.4%, respectively following the FNA shocks. The maximum ammonium loading rate achieved was 1.81 kg N/(m³ day) with ammonium removal ratio and nitrite accumulation ratio of over 0.97 and 0.96, respectively. Average emission rate of N₂O from the MBR was 2.1 ± 0.72% of ammonium removed. FNA shock on day 195 reduced the N₂O emission by 13.6%. The strategy developed in this study verified that spiked FNA shock together with DO limitation can be used for maintaining nitritation in MBRs with long SRTs. This method can potentially allow for maintaining nitritation at relatively low capital and operating expenditure when treating high concentration ammonium wastewater.

N2 yields from monochloramine conversion by granular activated carbons are decisive for effective swimming pool water treatment

Inorganic chloramines (mono-, di- and trichloramine) are formed in swimming pool water from the unintended reaction of free chlorine with ammonia that is introduced by bathers. Monochloramine is of particular interest as it is known to react further in pool water forming harmful DBPs, such carcinogenic N-nitrosodimethylamine (NDMA). During pool water treatment with granular activated carbon (GAC) filters, monochloramine is transformed by chemical reactions on the carbon surface to N₂ and ammonia. As ammonia is led back into the pool where it is chlorinated again under the renewed formation of inorganic chloramines, it is recommended to use GACs with a high N₂ yield for monochloramine transformation in pool water treatment. In this study, yields of N₂ and ammonia from monochloramine conversion by commercially available GACs were determined using a fixed-bed reactor system under conditions that are typical for swimming pool water treatment. The N₂ yields remained constant with on-going exposure of the GAC to monochloramine and ranged from 0.5% to 21.3%, depending on the type of GAC used. Correlation analyses were conducted to identify carbon properties that can determine the N₂ yield for monochloramine conversion, such as the amount of oxygen groups, the elemental composition and the trace metal content. It was found that the N₂ yield significantly correlates with the copper content of the tested carbons. Model calculations combining pool hydraulics with formation/abatement of inorganic chloramines and NDMA as well as chloramine transformations in GAC filters showed that the concentration of inorganic chloramines and carcinogenic NDMA can be decreased by a factor of ∼2, if the tested GACs could be modified to convert up to ∼50% of the monochloramine to N₂.

Recent advances in nitrogen removal from landfill leachate using biological treatments - A review

Landfill leachate, generated from the wastes in a landfill, is a type of wastewater with high concentrations of ammonia and organics, causing a serious environmental pollution. Because of its complex and changing characteristics, it is difficult to remove nitrogen from landfill leachate economically and effectively. Hence, nitrogen removal is a significant research priority of landfill leachate treatment in recent years. Biological processes are known to be effective in nitrogen removal. In this work, the biological nitrogen removal treatments were divided into the following processes: conventional nitrification-denitrification process, nitritation-denitritation process, endogenous denitritation process, and anaerobic ammonium oxidation (Anammox) process. This manuscript summarized the theories and applications of these approaches in detail, and concluded that appropriate processes should be selected in accordance with different characteristics of landfill leachate, in order to effectively remove nitrogen from all stages of landfill leachate and reduce disposal costs. Finally, perspective on the challenges and opportunities of biological nitrogen removal from landfill leachate was also presented.

Effectiveness of Nitrification and Denitrification Processes in Biofilters Treating Wastewater from De-Icing Airport Runways

The basic factors determining the efficiency of the removal of nitrogen and carbon compounds from airport wastewater containing de-icing agents are low temperature and the C/N ratio (carbon to nitrogen ratio). Biofilm reactors (biofilters) create better conditions for nitrification and denitrification than suspended biomass reactors. The scope of the study included determination of the influence of the C/N ratio in the wastewater on nitrification, denitrification and organic compound removal in biofilm reactors depending on the temperature. The experiment was performed in 24 circular laboratory biofilters with LECA (Light Expanded Clay Aggregates) filling. The study was divided into three series differing in organic carbon loading. In each series, carried out at the same hydraulic retention time, biofilters were operated at 25, 8, 4 or 0 °C. The study showed the effective removal of nitrogen compounds across a very wide temperature range. The applied filling and properly selected operating parameters of the reactors resulted in effective simultaneous nitrification and denitrification. The highest efficiency of nitrogen removal at 0 °C (34.57 ± 4.54%) was obtained at the C/N ratio of 0.5 gC/gN. The efficiency of denitrification (the lowest at the temperature of 0 °C) increased as the temperature and C/N ratio increased in the wastewater.

Application of metabolic division of labor in simultaneous removal of nitrogen and thiocyanate from wastewater

Metabolic division of labor is a key ecological strategy in bacteria to allow concurrent execution of multiple tasks through functional differentiation and metabolite exchange. While it is prevalent in nature, a lot of novel interactions remain to be further explored for improved wastewater biological treatment. Here, we present a combined experimental and modeling study on the simultaneous removal of nitrogen and thiocyanate from wastewater by using a syntrophic microbial community. The syntrophic division of labor was achieved by coupling autotrophic denitrification (AD) and anaerobic ammonium oxidation (AN) through both cooperative and competitive interactions. We demonstrated that the syntrophic community can achieve almost complete removal of all pollutants under certain initial conditions. We then perturbed the initial condition by varying the concentration ratio between ammonium to thiocyanate as well as the biomass ratio between AD and AN. Our observations show that adding ammonium negatively impacts the thiocyanate removal efficiency and adding anammox bacteria have opposite effects on the removal efficiency of thiocyanate and ammonium. Using a mathematical model, we simultaneously varied these two initial conditions and identified the parameter regime where our syntrophic ecosystem can be most efficient in removing total nitrogen. By highlighting the utility of syntrophic pair of functional bacteria in removing pollutants, our study will facilitate the rational design of more complex microbial consortia for the removal of toxic and hazardous compounds from industrial wastewater.

Improved performance of simultaneous nitrification and denitrification via nitrite in an oxygen-limited SBR by alternating the DO

In this study, the performance of simultaneous nitrification and denitrification via nitrite was investigated by alternating the dissolved oxygen (DO) concentration in a sequencing batch reactor with the DO-control area and the non-control area. In addition, bacterial communities and their metabolic functions were analyzed by high-throughput sequencing technology and phylogenetic investigation of the communities by reconstruction of unobserved states (PICRUSt). The removal efficiencies of NH₄⁺-N and total nitrogen via the nitrite pathway were 97.91 ± 2.04% and 72.28 ± 2.23%, respectively, by maintaining low DO levels (0.7 ± 0.1 mg/L) in the DO-control area. PICRUSt analysis showed that the metabolic potential of the bacterial community for amino acids, nucleotides, coenzymes and inorganic ions decreased, while the relative abundance of key enzymes involved in nitrification and denitrification, and the relative population of denitrifying bacteria increased when the DO decreased from 1.2 ± 0.2 mg/L to 0.7 ± 0.1 mg/L.

Achieving energy-efficient nitrogen removal and excess sludge reutilization by partial nitritation and simultaneous anammox denitrification and sludge fermentation process

Energy savings via achieving the reduction of aeration and excess sludge is required to realize energy self-sufficiency in wastewater treatment plants. A novel partial nitritation + simultaneous anammox denitrification and sludge fermentation (PN + SADF) process was operated for nearly two years, during which simultaneous energy-efficient nitrogen removal and waste activated sludge (WAS) reduction was achieved, with a stable nitrogen removal efficiency of 80% and external WAS reduction of 40%-50%. In the PN reactor, presence of ammonia oxidizing bacteria and absence of nitrite oxidizing bacteria ensured the stable nitritation. In the SADF reactor, nitrogen was removed via denitrification and anammox by using nutrients and organics released from WAS solubilization. Comparable performance of the SADF reactor at ambient temperature (12-32 °C) to that at 30 °C indicated a practical application potential for the PN + SADF process. An initial estimation of a full-scale PN + SADF process serving a population of 100000 showed that it could save economy and energy in comparison with conventional nitrification-denitrification process. Despite some challenges in implementation, this paper highlights the potential implication for sustaining mainstream nitritation-anammox towards energy-efficient operation with excess sludge reutilization.

Electroactive microorganisms enriched from activated sludge remove nitrogen in bioelectrochemical reactor

The bioelectrochemical anaerobic nitrogen removal was demonstrated in an anaerobic batch reactor equipped with a pair of polarized bioelectrodes. The bioelectrochemical reactor was operated in sequential batch mode after inoculating activated sludge and polarizing the electrode to 0.6 V. The medium contains ammonium, nitrite, alkalinity and trace minerals, but no organic carbon source. By the repetitive sequential operation, simultaneous removals of ammonium, nitrite and alkalinity were improved, and the electrochemical activity of the bulk sludge was confirmed from the redox peaks of the cyclic voltammogram. This indicates that ammonia oxidizing exoelectrogens (AOE) and denitritating electrotrophs (DNE) were enriched more in the bulk solution. Biogas production that mainly consisted of nitrogen was observed from the bioelectrochemical reactor, and the minor components in the biogas were methane and carbon dioxide. This demonstrates that AOE use nitrite as an electron acceptor to oxidize ammonia. The requirements of nitrite and alkalinity for the removal of ammonia nitrogen are around 0.72 mg NO₂-N/mg NH₄-N and 1.73 mg as CaCO₃/mg NH₄-N, respectively, and nitrate was not produced as a by-product. The bacterial groups involved in the bioelectrochemical nitrogen removal are electroactive autotrophs and can be enriched from activated sludge by polarized electrode. This bioelectrochemical ammonia oxidation is a novel approach recommended for treatment of nitrogen-rich wastewater.

How biomass growth mode affects ammonium oxidation start-up and NOB inhibition in the partial nitritation of cold and diluted reject water

The inhibition of undesirable nitrite oxidizing bacteria (NOB) and desirable ammonium oxidizing bacteria (AOB) by free ammonia (FA) and free nitrous acid (FNA) in partial nitritation (PN) is crucially affected by the biomass growth mode (suspended sludge, biofilm, encapsulation). But, the limitations of these modes towards less concentrated reject waters (≤600 mg-N L^-1) are unclear. Therefore, this work compares the start-up and stability of three PN sequencing batch reactors (SBRs) with biomass grown in one of the three modes: suspended sludge, biofilm and biomass encapsulated in polyvinyl alcohol (PVA) pellets. The SBRs were operated at 15°C with influent total ammonium nitrogen (TAN) concentrations of 75-600 mg-TAN L^-1. PN start-up was twice as fast in the biofilm and encapsulated biomass SBRs than in the suspended sludge SBR. After start-up, PN in the biofilm and suspended sludge SBRs was stable at 150-600 mg-TAN L^-1. But, at 75 mg-TAN L^-1, full nitrification gradually developed. In the encapsulated biomass SBR, full nitrification occurred even at 600 mg-TAN L^-1, showing that NOB in this set-up can adapt even to 4.3 mg-FA L^-1 and 0.27 mg-FNA L^-1. Thus, PN in the biofilm was best for the treatment of an influent containing 150-600 mg-TAN L^-1.

Nitritation, nitrous oxide emission pathways and in situ microbial community in a modified University of Cape Town process

Achieving nitritation is a prerequisite to promote nutrients removal and save energy, but emission of nitrous oxide as a greenhouse gas cannot be ignored. This study established the nitritation in a continuous-flow MUCT process and investigated the mechanism of N₂O generation. The nitrite accumulation ratio (NAR) reached 95% by controlling the low DO of 0.3-0.5 mg/L and short HRT of 8 h. The ¹⁵N-isotope tracer experiment indicated that the percentage of nitrifier-denitrification (ND) pathway increased by 12.7% under the limited-aeration mode, improving the stable operating of nitritation. Meanwhile, the autotrophic anammox pathway increased with the contribution ratio of 14.7% to N₂ emission under the nitritation mode. The ¹⁵N-DNA-SIP revealed that the Nitrosomonas executed the ND pathway and the Planctomycetes conducted the anammox process, respectively. The integration of autotrophic and heterotrophic process based on nitritation technique has potential to solve the carbon-limited issue for total nitrogen removal in mainstream WWTPs.

Effective partial nitrification of ammonia in a fluidized bed bioreactor

A lab-scale fluidized bed bioreactor with high-density polyethylene as biofilm carrier media was operated to study partial nitrification (PN) performance with high ammonia concentrations. The system was run at nitrogen loading rates (NLRs) from 1.2 to 4.8 kg N/(m³ d) with empty bed contact time of 2.0 and 2.7 h and four different influent ammonia concentrations of 100, 200, 300 and 400 mg/L. Dissolved oxygen concentration and temperature were maintained around 1.3 mg/L and 35°C, respectively. Stable PN was successfully achieved during the whole period with low effluent NO₃-N concentration at less than 15 mg/L, due to effective suppression of nitrite-oxidizing bacteria activity at high concentrations of free ammonia (5.3-27.3 mg N/L) and low alkalinity-to-ammonia ratio. At the NLR of 3.6 kg N/(m³ d), NH₄-N conversion and NO₂-N accumulation ratios were 57.8% and 53.9%, respectively, which could be further used in the anaerobic ammonium oxidation process (ANAMMOX) as the effluent NO₂-N/NH₄-N ratio was 1.27.

Effects of free nitrous acid treatment conditions on the nitrite pathway performance in mainstream wastewater treatment

Inline sludge treatment using free nitrous acid (FNA) was recently shown to be effective in establishing the nitrite pathway in a biological nitrogen removal system. However, the effects of FNA treatment conditions on the nitrite pathway performance remained to be investigated. In this study, three different FNA treatment frequencies (daily sludge treatment ratios of 0.22, 0.31 and 0.38, respectively), two FNA concentrations (1.35 mgN/L and 4.23 mgN/L, respectively) and two influent feeding regimes (one- and two-step feeding) were investigated in four laboratory-scale sequencing batch reactors. The nitrite accumulation ratio was positively correlated to the FNA treatment frequency. However, when a high treatment frequency was used e.g., daily sludge treatment ratio of 0.38, a significant reduction in ammonia oxidizing bacteria (AOB) activity occurred, leading to poor ammonium oxidation. AOB were able to acclimatise to FNA concentrations up to of 4.23 mgN/L, whereas nitrite oxidizing bacteria (NOB) were limited by an FNA concentration of 1.35 mgN/L over the duration of the study (up to 120 days). This difference in sensitivity to FNA could be used to further enhance nitrite accumulation, with 90% accumulation achieved at an FNA concentration of 4.23 mgN/L and a daily sludge treatment ratio of 0.31 in this study. However, this high level of nitrite accumulation led to increased N₂O emission, with emission factors of up to 3.9% observed. The N₂O emission was mitigated (reduced to 1.3%) by applying two-step feeding resulting in a nitrite accumulation ratio of 45.1%. Economic analysis showed that choosing the optimal FNA treatment conditions depends on a combination of the wastewater characteristics, the nitrogen discharge standards, and the operational costs. This study provides important information for the optimisation and practical application of FNA-based sludge treatment technology for achieving the mainstream stable nitrite pathway.

Upflow packed bed Anammox reactor used in two-stage deammonification of sludge digester effluent

The sludge digester effluent taken from a full scale municipal wastewater treatment plant (WWTP) in Istanbul, Turkey, was successfully deammonified using a laboratory scale two-stage partial nitritation (PN)/Anammox (A) process and a maximum nitrogen removal rate of 1.02 kg N/m³/d was achieved. In the PN reactor, 56.8 ± 4% of the influent NH₄-N was oxidized to NO₂-N and the effluent nitrate concentration was kept below 1 mg/L with 0.5-0.7 mg/L of dissolved oxygen and pH of 7.12 ± 12 at 24 ± 4°C. The effluent of the PN reactor was fed to an upflow packed bed Anammox reactor where high removal efficiency was achieved with NO₂-N:NH₄-N and NO₃-N:NH₄-N ratios of 1.32 ± 0.19:1 and 0.22 ± 0.10:1, respectively. The results show that NH₄-N removal efficiency up to 98.7 ± 2.4% and total nitrogen removal of 87.7 ± 6.5% were achieved.

The effect of temperature shifts on N2O and NO emissions from a partial nitritation reactor treating reject wastewater

Temperature has a known effect on ammonia oxidizing bacteria (AOB) activities, reducing its ammonia oxidizing rate (AOR) when temperature is lowered. However, little is known concerning its effect on N₂O and NO emissions which are produced during ammonia oxidation having a greenhouse effect. To study this, an AOB enriched partial nitrification sequencing batch reactor (PN-SBR) was operated within a two step-wise feed under 5 different temperatures (30-25-20-15-10 °C). A decrease on the specific AOR (sAOR) was detected when decreasing the temperature. N₂O emissions were also affected by the temperature but only the ones produced during the first aeration of the cycle, when AOBs shifted from a period of low activity to a period of high activity. N₂O emission factors (%) detected during the second aerobic phase were similar among all temperatures tested and lower than the emissions detected during the first aerated phase. The average N₂O emission factor was in the range of 0.15-0.70% N₂O-N/NH₄⁺-N oxidized in the first aeration phase and 0.14-0.15% N₂O-N/NH₄⁺-N-oxidized in the second aeration phase at 10 to 30 °C, respectively. On the other hand, NO emissions were very similar under all temperatures resulting in 0.03-0.06% of NH₄⁺-N oxidized.

Stable and efficient partial nitritation granular sludge reactor treating domestic sewage at low temperature

The success of combined partial nitritation (PN) and anammox process treating low-strength domestic wastewater depends on achieving a stable and efficient PN. In this study, desirable PN for domestic sewage with low temperature of 11.8-16.9 °C was achieved in a granular sludge reactor operated in anaerobic/aerobic (A/O) mode. Average nitrite accumulation ratio of 97.3% was obtained with an effluent nitrite/ammonium ratio of 1.2 for influent ammonium of 39.3-78.7 mg·L^-1. Quantitative microbial analysis and activity batch test showed that nitrite oxidizing bacteria (NOB) were effectively suppressed, while ammonium oxidizing bacteria (AOB) were dominant. For the efficient suppression of NOB, A/O mode, aerobic phosphorus uptake and granular sludge could play important roles. Furthermore, high AOB activity was obtained with an average ammonium oxidation rate of 11.6 mg N·L^-1·h^-1, which could be due to the abundant psychrotolerant microorganisms, increased content of extracellular polymeric substances and relatively high dissolved oxygen condition of the reactor.

Treatment of municipal sewage with low carbon-to-nitrogen ratio via simultaneous partial nitrification, anaerobic ammonia oxidation, and denitrification (SNAD) in a non-woven rotating biological contactor

In this study, a non-woven rotating biological contactor was evaluated for the treatment of municipal sewage via simultaneous partial nitrification, anaerobic ammonia oxidation (anammox), and denitrification (SNAD). Fluorescence in situ hybridization analysis showed that the dominant bacterial group in the aerobic outer layer of the biofilm was ammonia-oxidizing bacteria (65.13%), whereas anammox (47.17%) and denitrifying (38.91%) bacteria were present in the anaerobic inner layer. Response surface methodology was applied to develop mathematical models for the interaction between C/N and dissolved oxygen (DO) for chemical oxygen demand (COD) and total nitrogen (TN) removal. Results showed that the optimum region for SNAD was at C/N = 1.4-2.3 and DO = 0.2-0.8 mg/L. The most optimal operating condition was determined at C/N = 2.3 and DO = 0.2 mg/L, with actual removal rates of COD and TN were 83.12% and 79.13%, respectively, which are in close model consistency with model prediction (84% and 80%).

Fast start-up of the single-stage nitrogen removal using anammox and partial nitritation (SNAP) from conventional activated sludge in a membrane-aerated biofilm reactor

The single-stage nitrogen removal using anammox and partial nitritation (SNAP) is a promising alternative for low-cost ammonium removal from wastewaters. This study aimed to evaluate the anammox biomass enrichment and SNAP process start-up in a laboratory-scale membrane-aerated biofilm reactor (MABR) at nitrogen loading rates of 50 g N.m^-3.d^-1 (period 1) and 100 g N.m^-3.d^-1 (period 2). Anammox activity was observed after 48 days, and the SNAP process was stable after 80 days. In period 1, the average total nitrogen (TN) removal was 78 ± 6%, and the maximum removal was 84%. In period 2, the average TN removal was 61 ± 5%, and the maximum was 69%. Higher dissolved oxygen levels may have caused imbalances in the microbial community in period 2, decreasing the reactor performance. These results demonstrated the potential of the MABR for the fast implementation of the single-stage partial nitritation and anammox processes.

Rapid start-up and performance of denitrifying granular sludge in an upflow sludge blanket (USB) reactor treating high concentration nitrite wastewater

Denitrifying granular sludge reactor holds better nitrogen removal efficiency than other kinds of denitrifying reactors, while this reactor commonly needs seeding anaerobic granular sludge and longer period for start-up in practice, which restricted the application of denitrifying granular sludge reactor. This study presented a rapid and stable start-up method for denitrifying granular sludge. An upflow sludge blanket (USB) reactor with packings was established with flocculent activated sludge for treatment of high concentration nitrite wastewater. Results showed mature denitrifying granular sludge appeared only after 15 days with highest nitrogen removal rate of 5.844 kg N/(m³ day), which was much higher than that of compared anoxic sequencing batch reactor (ASBR). No significant nitrite inhibition occurred in USB and denitrification performance was mainly influenced by hydraulic retention time, influent C/N ratio and internal reflux ratio. Hydraulic shear force created by upflow fluid, shearing of gaseous products and stable microorganisms adhesion on the packings might be the reasons for rapid achievement of granular sludge. Compared to inoculated sludge and ASBR, remarkable microbial communitiy variations were detected in USB. The dominance of Proteobacteria and Bacteroidetes and enrichment of species Pseudomonas_stutzeri should be responsible for the excellent denitrification performance, which further verified the feasibility of start-up method.

Control of partial nitrification using pulse aeration for treating digested effluent of swine wastewater

Three sequencing batch reactors (SBRs) were used to investigate the influence of pulse frequencies on the partial nitrification (PN) process in this study. At a total aeration time of 6 min each hour, the aerated frequencies of R1, R2 and R3 were 6, 3 and 2 time h^-1. During the steady period (117-143d), the nitrite accumulation rates (NARs) were 90.80%, 90.71% and 90.23% in R1, R2 and R3, respectively, indicating a steady nitritation was acquired. Activity measurements of the sludge samples taken at day 138 showed the activity of nitrite oxidating bacteria (NOB) was 0, indicating NOBs were successfully suppressed. The ratio of NO₂^--N to NH₄⁺-N in the effluent of R3 was 1.35, which most closely matched the influent of Anammox process. However, the energy efficiency evaluation showed that R1 had the highest actual oxygen transfer efficiency (AOTE) and dynamical efficiency (DE).

Advances in algal-prokaryotic wastewater treatment: A review of nitrogen transformations, reactor configurations and molecular tools

The synergistic activity of algae and prokaryotic microorganisms can be used to improve the efficiency of biological wastewater treatment, particularly with regards to nitrogen removal. For example, algae can provide oxygen through photosynthesis needed for aerobic degradation of organic carbon and nitrification and harvested algal-prokaryotic biomass can be used to produce high value chemicals or biogas. Algal-prokaryotic consortia have been used to treat wastewater in different types of reactors, including waste stabilization ponds, high rate algal ponds and closed photobioreactors. This review addresses the current literature and identifies research gaps related to the following topics: 1) the complex interactions between algae and prokaryotes in wastewater treatment; 2) advances in bioreactor technologies that can achieve high nitrogen removal efficiencies in small reactor volumes, such as algal-prokaryotic biofilm reactors and enhanced algal-prokaryotic treatment systems (EAPS); 3) molecular tools that have expanded our understanding of the activities of algal and prokaryotic communities in wastewater treatment processes.