Nitrogen removal performance and loading capacity of a novel single-stage nitritation-anammox system with syntrophic micro-granules

A high-rate A2O bioreactor with airlift-driven circulation and anoxic hybrid growth for enhanced carbon and nutrient removal from a nutrient rich wastewater

Within this research, a one-stage hybrid dual internal circulation airlift A2O (DCAL-A2O) bioreactor was designed and operated to simultaneously remove carbon, nitrogen and phosphorous (CNP) from milk processing wastewater (MPW) in different operational circumstances. The substantial operating variables monitored in this work were including hydraulic retention time (HRT), airflow rate (AFR) and aeration volume ratio (AVR) ranged from 7 to 15 h, 1-3 L/min and 0.324-0.464, respectively. From the view point of economics and process function, the optimum conditions were obtained at the HRT, AFR and AVR of 10 h, 2 L/min and 0.464, respectively. At the optimum conditions TCOD, TN, TP removal efficiencies and effluent turbidity were reported to be 97 %, 90 %, 92 % and 9 NTU, respectively. The impact of wastewater biodegradability (BOD5/COD) was evaluated on the bioreactor performance using two other wastewaters i.e. soft drink (SDW) and soybean oil plant wastewaters (SOW) in comparison with the MPW. Removal efficiencies for TCOD and TN exceeding 80 % were observed. The feeding location revealed a prominent impact on the TN and phosphorous removal efficiencies (both ≥80 %) related to the availability degree of the readily biodegradable organic substrate to denitrifiers and PAOs. The rise in HRT, AFR and AVR resulted in reducing microbial secretions as SMP present in sludge and bioreactor effluent as well as loosely bounded EPS (LB-EPS), reported to be 26, 28, 32.5 and 194.4 mg/L TOC, respectively. Different bacteria species were present at optimum conditions confirming concurrent CNP removal in a single body. Finally, the operating cost evaluation verified the effectiveness of the hybrid airlift A2O treating the MPW.

Anammox-based technologies: A review of recent advances, mechanism, and bottlenecks

The removal of nitrogen via the ANAMMOX process is a promising green wastewater treatment technology, with numerous benefits. The incessant studies on the ANAMMOX process over the years due to its long start-up and high operational cost has positively influenced its technological advancement, even though at a rather slow pace. At the moment, relatively new ANAMMOX technologies are being developed with the goal of treating low carbon wastewater at low temperatures, tackling nitrite and nitrate accumulation and methane utilization from digestates while also recovering resources (phosphorus) in a sustainable manner. This review compares and contrasts the handful of ANAMMOX -based processes developed thus far with plausible solutions for addressing their respective bottlenecks hindering full-scale implementation. Ultimately, future prospects for advancing understanding of mechanisms and engineering application of ANAMMOX process are posited. As a whole, technological advances in process design and patents have greatly contributed to better understanding of the ANAMMOX process, which has greatly aided in the optimization and industrialization of the ANAMMOX process. This review is intended to provide researchers with an overview of the present state of research and technological development of the ANAMMOX process, thus serving as a guide for realizing energy autarkic future practical applications.

Concurrent removal of carbon and nutrients in a one-stage dual internal circulation airlift A2O bioreactor from milk processing industrial wastewater: Process optimization, sludge characteristics and operating cost evaluation

In this work, a one-stage dual internal circulation airlift anaerobic/anoxic/aerobic (DCAL-A2O) bioreactor was continuously operated for concurrent removal of nutrients and organics from milk processing wastewater (MPW). Special configuration of the airlift A2O bioreactor created possibility of the formation of desired anaerobic, anoxic and aerobic zones in a single unit. The process functionality of the bioreactor was examined under three influential operating variables i.e. hydraulic retention time (HRT; 7-15 h), air flow rate (AFR; 1-3 L/min) and aerobic volume ratio (AVR; 0.324-0.464). The optimum region was identified at HRT of 13h, AFR of 2L/min and AVR of 0.437, leading to TCOD, TN and TP removal efficiency of 94.5 %, 59.6 %, and 62.2 %, respectively, and effluent turbidity of 8 NTU. The impact of feed biodegradability on the process performance of the bioreactor treating the MPW, soft drink wastewater (SDW) and soybean oil plant wastewater (SOW) was also assessed. From the results, the feed characteristics affected significantly the nutrients removal. Moreover, the feeding location played an effective role in the nutrient removal while treating the MPW at optimum operating conditions. In this study, the change in residual organic matters as soluble microbial products (SMP) was monitored at various operating conditions. In addition, the impact of SMP extracted from sludge, extracellular polymeric substances (EPS) comprising of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) was analyzed on sludge characteristics as bio-flocculation and settleability properties. According to the obtained data, the increase in operating variables led to the reduction in contents of effluent SMP, sludge SMP, LB-EPS, turbidity, and SVI, thereby, the enhancement in the sludge characteristics. Meanwhile, analysis of microbial communities verified the presence of various functional bacterial species. The cost operating evaluation confirmed the cost effectiveness of the airlift A2O bioreactor in reduction of energy consumption for the MPW treatment.

Effect of biochar on the SPNA system at ambient temperatures

Biochar has been extensively studied in wastewater treatment systems. However, the role of biochar in the single-stage partial nitritation anammox (SPNA) system remains not fully understood. This study explored the impact of biochar on the SPNA at ambient temperatures (20 °C and 15 °C). The nitrogen removal rate of the system raised from 0.43 to 0.50 g N/(L·d) as the biochar addition was raised from 2 to 4 g/L. Metagenomic analysis revealed that gene abundances of amino sugar metabolism and nucleotide sugar metabolism, amino acid metabolism, and quorum sensing were decreased after the addition of biochar. However, the gene abundance of enzymes synthesizing NADH and trehalose increased, indicating that biochar could stimulate electron transfer reactions in microbial metabolism and assist microorganisms in maintaining a steady state at lower temperatures. The findings of this study provide valuable insights into the mechanism behind the improved nitrogen removal facilitated by biochar in the single-stage partial nitritation anammox system.

Extracellular polymeric substances and microbial community shift during the start-up of a single-stage partial nitritation/anammox process

A sequencing batch reactor (SBR) was operated to investigate variations of extracellular polymeric substances (EPS) and microbial community during the start-up of the single-stage partial nitritation/anammox (SPN/A) process at intermittent aeration mode. The SPN/A system was successfully started on day 34, and the nitrogen removal efficiency and total nitrogen loading rate were 82.29% and 0.31 kg N/(m3 ·day), respectively. Furthermore, the relationship between the protein secondary structures and microbial aggregation was strongly related. The α-helix/ (β-sheet + random coil) ratios increased obviously from 0.20 ± 0.03 to 0.23 ± 0.01, with the sludge aggregation mean size increased from 56 to 107 μm during the start-up of SPN/A. During the start-up of SPN/A, Candidatus Kuenenia was the primary anammox bacteria, whereas Nitrospira was the main functional bacteria of nitrite-oxidizing bacteria. Correlation between the microbial community and EPS components was performed. The EPS and microbial community played important roles in keeping stable nitrogen removal and the formation of sludge granules. PRACTITIONER POINTS: Intermittent aeration strategy promoted SPN/A system start-up. EPS composition and protein secondary structure were related with the sludge disintegration and aggregation. Microbial community shift existed and promoted the stability of sludge and reactor performance during SPN/A start-up.

Improved Properties and Enhancement Strategies of Hydroxyapatite-Based Functional Granular Sludge for a High-Rate Partial Nitritation/Anammox System

Retaining sufficient anammox bacteria (AnAOB) while keeping the anammox-based process stable is the focus of the study of anammox technology, especially in a one-stage partial nitritation/anammox (PNA) process. The use of hydroxyapatite (HAP) granules in an anammox-based process is innovative for its potential to improve the nitrogen removal rate and achieve simultaneous removal of phosphorus. In this study, the HAP-based granular sludge was employed using enhancement strategies for an excellent nitrogen removal performance in a one-stage PNA process. Compared to those of other granular sludge PNA systems, a remarkable sludge volume index of 7.8 mL/g and an extremely high mixed liquor volatile suspended solids of 15 g/L were achieved under a low hydraulic retention time of 2 h. Consequently, an unprecedented nitrogen removal rate as high as 4.8 kg N/m3/d at 25 °C was obtained under a nitrogen loading rate of 6 kg N/m3/d. After a long-term operation of 870 days, the enhancement strategies underlying the superior performance of the granular sludge were identified. These findings clearly demonstrate that the enhancement strategies are crucial for the superior operating performance of the PNA process, and they can promote the application of the anammox-based process.

Resilience of anammox application from sidestream to mainstream: A combined system coupling denitrification, partial nitritation and partial denitrification with anammox

Anaerobic ammonium oxidation (anammox) is a potential process to achieve the neutralization of energy and carbon. Due to the low temperature and variation of municipal sewage, the application of mainstream anammox is hard to be implemented. For spreading mainstream anammox in practice, several key issues and bottlenecks including the start-up, stable NO₂^--N supply, maintenance and dominance of AnAOB with high activity, prevention of NO₃^--N buildup, reduction of sludge loss, adaption to the seasonal temperature and alleviation of COD impacts on AnAOB are discussed and summarized in this review in order to improve its startup, stable operation and resilience of mainstream anammox. Hence a combined biological nitrogen removal (CBNR) system based on conventional denitrification, shortcut nitrification-denitrification, Partial Nitritation and partial Denitrification combined Anammox (PANDA) process through the management of organic matter and nitrate is proposed correspondingly aiming at adaptation to the variations of seasonal temperature and pollutants in influent.

Improving mechanical stability of anammox granules with organic stress by limited filamentous bulking

Under organic stress, the limited filamentous bulking (FB) was demonstrated to improve anammox capability by inhibiting granule disintegration and washout. The accumulation of internal stress played a more important role than the adverse physicochemical properties (low viscoelasticity and hydrophobicity) of granules in limiting granular strength by consuming the granular elastic energy. Different from the floc-forming heterotrophic bacteria (HB) that stored its growth stress as internal stress by pushing the surrounded anammox micro-colonies outwards under the spatial constraint of elastic anammox "shell", the filamentous HB grew into a uniform network structure within granules, endowed granules low internal stress and acted as the granular skeleton due to its rich amyloid substance, which was benefited from the elimination of inhomogeneous growth and the consequent expansion competition for living space. Combined with the mechanical instability and sticking-spring models, controlling FB at limited level was effective for improving granular strength without affecting sludge-water separation.

Recent developments in anammox-based membrane bioreactors: A review

The anammox-based process has been considered a promising biological nitrogen elimination method for the treatment of nitrogen-rich wastewater ever since its discovery 40 years ago. However, the slow growth rate of anammox bacteria and severe sludge washout result in a long startup period and limit its widespread industrial application. A membrane bioreactor (MBR) is considered an ideal reactor for the operation of the anammox-based process because the membranes allow for 100 % biomass retention. According to a systematic review of the literature, anammox-based MBR is becoming a research hotspot in the field of nitrogen wastewater treatment. The fundamental understanding of anammox-based MBR and its membrane fouling situation is essential for the development and application of anammox-based MBR. In this paper, the application of MBR in different kinds of anammox process are reviewed. The membrane fouling mechanism and strategies to control membrane fouling are also proposed. It is expected that this review will serve as an invaluable guide for future research and in the engineering applications of anammox-based MBR process.

A novel bioscrubber for the treatment of high loads of ammonia from polluted gas

This work presents a novel bioscrubber configuration for the treatment of high ammonia loads at short contact times. The biological reactor was designed to work as a moving-bed biofilm rector (MBBR) increasing biomass retention time. This configuration is still unexplored for the treatment of waste gases. Long-term operation of a lab-scale bioscrubber under different inlet concentrations of ammonia (60-570 ppm_v) and a gas contact time of 4 s was performed to study the system operational limits during 250 days. The effect of the dissolved oxygen concentration on the nitrification rate was also evaluated. Under these conditions, a critical elimination capacity (EC) of 250 NH₃·m^-3·h^-1 and a maximum EC of 300 g NH₃·m^-3·h^-1 were obtained. The maximum nitrification rate obtained was 0.5 kg N·m^-3·day^-1. However, this nitrification rate only was possible to be achieved under partial nitrification. For complete nitrification, the critical nitrification rate was 0.3 kg N·m^-3·day^-1. These results confirm that bioscrubber coupled to a MBBR is a good alternative to treat high ammonia loads with remarkable advantages, such as the retention of properly biomass concentration without auxiliary equipment.

The role of magnetite (Fe3O4) particles for enhancing the performance and granulation of anammox

In this study, two lab-scale sequencing batch reactors each with an effective volume of 2.3 L were operated as C-AMX (no carrier addition) and M-AMX (magnetite carrier added) for 147 days with synthetic wastewater at an NLR range of 0.19-0.47 kgN/m³/d. The long-term effect of magnetite on the granulation and performance of anammox bacteria in terms of nitrogen removal and other essential parameters were confirmed. In phase I (1-24 days), M-AMX took approximately 12 days to obtain a nitrogen removal rate (NRR) above 80 % of the initial input nitrogen. Although free nitrous acid inhibited the reactor at a high concentration at the onset of phase III, the NRR of M-AMX recovered about 3.7 times faster than that of C-AMX. In addition, it was confirmed that the M-AMX granules had a dense and compact structure compared to C-AMX, and the presence of the carrier promoted the development of these resilient granules. While the measured microbial stress gradually increased in C-AMX reactor, a vice versa was observed in the M-AMX reactor as granulation proceeded. Compared to other alternative iron-based carrier particles, the stable crystal structure of magnetite as a carrier created a mechanism where filamentous bacteria groups were repelled from the granulation hence the microbial stress in the M-AMX in the final phase was 61.54 % lower than that in the C-AMX. The iron rich environment created by the magnetite addition led to Ignavibacteria, (a Feammox bacteria) increasing significantly in the M-AMX bioreactor.

Control strategy and performance of simultaneous removal of nitrogen and organic matter in treating swine manure digestate using one reactor with airlift and micro-granule

A simultaneous partial nitritation, anammox, denitrification, and COD oxidation (SNADCO) process was used to evaluate the nitrogen and biodegradable organic matter removal of swine manure digestate based on a nitrite limitation and ammonium surplus strategy. As influent ammonium concentration increased from 500 mg/L to 2100 mg/L, the 5 day biochemical oxygen demand (BOD₅) maintained at a high removal efficiency of 95.4%. However, nitrogen removal efficiency (NRE) decreased from 90.9% to 68.2% due to the inhibition of AnAOB caused by an ammonium concentration of 2100 mg/L. The contribution of AnAOB to nitrogen removal was 75.6-86.5%, while that of denitrifying bacteria was 4.6-7.0%. In the case of COD removal, the contributions were from ordinary heterotrophic organisms and denitrifying bacteria, at 27.1-64.9% and 11.2-22.1%, respectively. The results of specific bacteria activity tests and microbial analysis showed that a highly efficient synergism between functional microorganisms is essential for the stability of the SNADCO process.

Low-strength influence on nitrogen removal performance and bacterial community structure of the anammox process

The effect of low strength on anaerobic ammonium oxidation (anammox) was investigated in an anaerobic moving bed biofilm reactor (AMBBR) treating artificial wastewater. Influent NH4+-N concentration with 10.74 ± 2.73 mg L^-1 adversely impacted nitrogen removal permanence, the total nitrogen removal efficiency was significantly increased from 61.4% to 80.0%, when influent nitrogen increased to 22.36 ± 5.83 mg·L^-1. NH4+-N removal efficiency decreased obviously while that of NO2--N was basically unaffected by the influent nitrogen concentration decrease. Illumina high-throughput sequencing results revealed that the predominant bacterial (64.71%) phylum was Proteobacteria and the dominant functional microorganisms were Nitrosospira, Nitrospira, and Candidatus Brocadia. Simple model simulation results showed that the inhibition effect of the low substrate was most likely due to the increase of bulk DO, which comes from influent and gas-liquid transfer. The reversible inhibition effect of low strength on nitrogen removal performance in an anammox reactor was demonstrated, and strictly regulation of the bulk DO was presumed to be critical to achieve a successful and stable operating performance under low strength.

Biofilm growth characterization and treatment performance in a single stage partial nitritation/anammox process with a biofilm carrier

Biofilm carriers can avoid microorganism washout while maintaining a high amount of biomass, but are also associated with a long biofilm formation period and biofilm aging. A single stage partial nitritation/anammox process (single stage PN/A) reactor was setup to study the biofilm growth characterization and treatment performance under an NLR of 0.53 to 0.90 gN/L/d over one year. Biofilm growth was divided into three stages: the formation stage, maturation stage and aging stage. The initial biofilm was observed at day 84. A nitrogen removal efficiency of 83.4% was achieved at an NLR of 0.90 gN/L/d during the mature biofilm stage. Starvation, nitrogen gas accumulation and hydroxyapatite formation resulted in biofilm aging. After mechanical stirring treatment, biofilm reactivation was achieved by biofilm re-formation within one month. There is clear potential for phosphorus recovery, as indicated by the 5.24% - 6.29% phosphorus content in the biofilm (similar to the 5%-7% phosphorus content in enhanced biological phosphate removal sludge). The AnAOB genera abundance in the biofilm maintained at a high level of 18.25%-32.31%, while the abundance of AnAOB increased from the initial 4.10% to 13.78% after mechanical stirring treatment in the suspended sludge ensured biofilm reactivation. The results of this study clearly show that mechanical stirring treatment can be used to achieve the biofilm reactivation as the biofilm fills with the hollow cylindrical carrier. This study has potential as a useful reference for the realization of the wide application of the biofilm single stage PN/A process in the future.

Treating the filtrate of mainstream anaerobic membrane bioreactor with the pilot-scale sludge-type one-stage partial nitritation/anammox process operated from 25 to 15 °C

At ambient temperature condition, the one-stage partial nitritation/anammox (PNA) process has been successfully adopted to treat the filtrate from the mainstream anaerobic membrane bioreactor (AnMBR). However, there is no investigation of the performance of this process at low-temperature condition. In this study, the nitrogen removal performance of a pilot-scale PNA reactor at the temperature of 15 °C for treating the filtrate of a mainstream AnMBR was investigated. The nitrogen removal rate of 0.09 kg/m³/d and the nitrogen removal efficiency of 37.6% were achieved. The anammox reaction was the rate-limiting step of the nitrogen removal. Nitrogen removal was attributed in part to denitrification activity. The microbial community analysis confirmed that the main functional bacteria comprised of genus Nitrosomonas and genus Kuenenia. In sum, this research demonstrated the applicability of PNA process for mainstream AnMBR filtrate treatment to some extent and enriched the related knowledge.

Insights into the synergy between functional microbes and dissolved oxygen partition in the single-stage partial nitritation-anammox granules system

The rapid start-up and stable operation of the single-stage partial nitritation-anammox (PNA) process remains a challenge in practical applications. An integrated investigation of nitrogen removal performance, sludge characteristics, activity and abundance, and microbial dynamics was implemented for 360 days via an airlift internal circulation reactor. During long-term operation, the reactor realized a stable dissolved oxygen (DO) partition and cultivated granular sludge. The nitrogen removal rate increased from 0.15 kg-N/m³/d to 1.24 kg-N/m³/d, and a high nitrogen removal efficiency of 82.6% was obtained. A stable DO partition further accelerated the bioreaction rates and enhanced the activity of functional microbes. The activities of ammonia oxidation and anammox reached 1.21 g-N/g-VSS/d and 1.43 g-N/g-VSS/d, respectively. Sludge granulation efficiently enriched the abundances of Candidatus Brocadia (7.4%) and Nitrosomonas (5.2%). These results demonstrated that efficient DO partition and stable culture of granular sludge could enhance the synergy of functional microbes for autotrophic nitrogen removal.

Development of long-term dynamic BioWin® model simulation for ANAMMOX UASB micro-granular process

Three different innovative mathematical models were established to assess the volumetric nitrogen conversion rates of a lab-scale ANAMMOX upflow anaerobic sludge blanket reactor. Despite the vast technological and economical advantages of ANAMMOX, major challenges in process implementation call for mathematic simulations of the process, optimization of operating conditions, and kinetic/statistical analysis of the entire process. In this study, all developed mathematical models implemented via BioWin®, were calibrated and validated, with adequate representations of a bench-scale micro-granular ANAMMOX process, to understand the potential setbacks of ANAMMOX process start-up and stabilization. Fundamental calculations of the kinetic and stoichiometric constants were integrated in the BioWin® software, and the adjusted parameters based on experimental analysis were applied for the assessments. Based on the results from the statistical approach, one of the models (Model III) exhibited a precise prognosis of the effluent data for the entire operational phases with a mean relative error (MRE) of approximately 1.96, 4.36 and 2.54% for nitrogen removal efficiency, removal rate and loading rate, respectively. Evaluating alkalinity and pH during the operation, led to identifying an acceptable fit between the experiment and Model III results, with a MRE of -7.19 and -0.35%, correspondingly. This study confirms the reliability of ANAMMOX-based process modeling and high predictive ability with BioWin®. The presented simulation constants and modeling outline, can be further employed in full-scale applications design and development.

Application of electric fields to mitigate inhibition on anammox consortia under long-term tetracycline stress

The feasibility of applying electric fields to mitigate inhibition of tetracycline (TC) on anammox process and improve system stability was evaluated in this study. Three electric field intensities of 1, 3 and a variable intensity of 1-6 V (VEF) were used to optimize electric field intensity under gradually increasing addition of TC (0.5, 2 and 10 mg L^-1). Results showed that the application of electric fields (3 V and VEF) could improve TC tolerance and keep relatively high-efficiency nitrogen removal performance, especially at TC ≥ 2 mg L^-1. Furthermore, applying electric fields contributed to mitigate irreversible inhibition and improve the stability of community structure. Underlying mechanism analysis indicated that the main mechanism of applying electric fields to mitigate inhibition relies on sludge structure strengthening. This study explored a novel strategy to reduce the inhibition of antibiotics on microbial denitrification and broaden the application of anammox in industrial water treatment.

Enhancing autotrophic nitrogen removal with a novel dissolved oxygen-differentiated airlift internal circulation reactor: Long-term operational performance and microbial characteristics

Autotrophic nitrogen removal (ANR) processes have not been widely applied in wastewater treatment due to their long start-up time and unstable performance. In this study, a novel dissolved oxygen-differentiated airlift internal circulation reactor was developed to enhance ANR from wastewater. During 200 days of continuous operation, the reactor start-up was achieved within 30 days; a high total nitrogen removal efficiency of 80% was achieved and stably maintained under an aeration rate of 0.90 L/min and hydraulic retention time of 6 h. Additionally, the color of sludge went from a light yellow to dark red, and the amount and size of the micro-granules increased obviously. Medium-sized (1.0-2.5 mm) micro-granules accounted for 72.4% on day 190. The specific anammox activity increased from 0.53 to 1.43 g-N/g-VSS/d, while the SNOA decreased from 0.93 to 0.08 g-N/g-VSS/d. Furthermore, the microbial analysis showed that the Nitrosomonas (4.2%) and Candidatus Brocadia (22.6%) were enriched and formed the micro-granules after the reactor's long-term operation. The results indicate that novel configuration realizes the partitioning of dissolved oxygen (DO), optimizes nitritation and anammox reactions, and accelerates biochemical reactions, thereby enhancing ANR performance. This study provides a practical alternative to enhance ANR performance and a scientific basis for the development and application of novel nitrogen removal reactors.

High nitrogen removal performance of anaerobically treated fish processing wastewater by one-stage partial nitritation and anammox process with hydroxyapatite (HAP)-based syntrophic granules and granule structure

The one-stage partial nitritation and anammox process with the hydroxyapatite (HAP)-based syntrophic granules was studied for the ammonium nitrogen removal from the effluents of a self-agitated anaerobic baffled reactor treating the fish processing wastewater. When the ammonium in the influent was 1140 mg N·L^-1, a high nitrogen removal rate and nitrogen removal efficiency of 1.51 ± 0.10 kg N·m^-3·d^-1 and 88.2% were obtained, respectively. Anammox bacteria of Candidatus Kuenenia stuttgartiensis and ammonium oxidizing bacteria of Nitrosomonas were the two most predominant bacteria, while nitrite oxidizing bacteria activity was low and could be neglected during the treatment. The inorganic element properties of the sludge were analyzed by several methods to confirm the existence of HAP granules. Optical microscopic observation and scanning electron microscopy analysis revealed the structure of the granular sludge.This study supports the feasibility and potential of this process for high-efficiency nitrogen removal from fish processing wastewater.

High-rate nitrogen removal in a continuous biofilter anammox reactor for treating low-concentration nitrogen wastewater at moderate temperature

The high-rate nitrogen removal in a continuous biofilter anammox reactor (CBAR) was investigated to treat low-concentration nitrogen wastewater. Shortening hydraulic retention time (HRT) gradually could restart CBAR and accumulate anammox bacteria effectively in the reactor, where the carmine anammox granular sludge and biofilm were coexisted well. It spent 21 days to restart CBAR completely after it had been idle for 116 days. Meanwhile, the total nitrogen removal rate remained stable at 86.42% accompanied with a total biomass concentration of 26.02 g-SS/L in 0 ~ 20 cm zone under nitrogen loading rate of 4.25 ± 0.10 kg-N/(m³·day), HRT of 20 min and 25 ℃. In addition, the specific anammox activity of biomass exceeded 0.28 g-N/(g-VSS·day) in 0 ~ 20 cm zone, which was related with a high relative abundance of Candidatus Brocadia (>30%) in the same zone. Thus, it is a feasible approach to adopt CBAR to treat low-concentration nitrogen wastewater efficiently.

Unveiling performance stability and its recovery mechanisms of one-stage partial nitritation-anammox process with airlift enhanced micro-granules

The performance stability and its recovery mechanisms of a partial nitritation-anammox process were investigated. A one-stage airlift enhanced micro-granules (AEM) system was operated for 650 days continuously to treat 50 mg-NH₄/L wastewater. During the stable stage, a high nitrogen removal efficiency of 72.7 ± 8.4% lasting for 230 days was successfully achieved under 0.28 L/min aeration rate and 0.10-0.20 mg/L dissolved oxygen (DO) concentration. A microbial consortium with good granularity appeared in red. The specific activity of anammox and ammonia oxidation increased to 1.02 and 0.93 g-N/g-VSS/d, respectively. Meanwhile, the microbial analysis showed the AEM system shifted the dominant microflora from Proteobacteria to Planctomycetes in which Candidatus Brocadia abundance reached a high of 35.0%. The results reveal that the long-term airlift-aeration promoted granulation and further enhanced activities, the abundances of anammox bacteria, and suppressed nitrite-oxidizing bacteria. Optimizing the DO control is also critical for stability increment and process recovery.

Achieving single-stage partial nitritation and anammox (PN/A) using a submerged dynamic membrane sequencing batch reactor (DM-SBR)

Single-stage partial nitration and anammox (PN/A) process was achieved in a sequencing batch reactor (SBR) using a submerged dynamic membrane (DM) in this study. The reactor was stably operated for 200 days, and the nitrogen removal efficiency (NRE) was sustained at 70.3 ± 7.2% at a nitrogen loading rate (NLR) ranging from 0.1 to 0.3 kgNm^-3  day^-1 with a hydraulic retention time (HRT) of 24 hr. When the NLR was 0.2 kgN m^-3 day^-1 , the NRE achieved was high as 80% with a low concentration of dissolved oxygen (DO) of 0.13 mg/L. In addition, the specific activity of anammox bacteria and ammonia-oxidizing bacteria (AOB) reached was 2.72 and 16.80 gN gVSS^-1 day^-1 , respectively. The DM intercepted the biomass due to the lamellar, intact, dense biofilm self-generated on the surface of the supporting material, which had an effluent turbidity of 10 NTU. The enriched anammox functional bacteria were Candidatus Jettenia (11.06%) and the AOB-like functional bacteria consisted primarily of Nitrosomonas, with a relative abundance of 2.76%, which ensured the PN/A process proceeding. This study provides a novel reactor configuration of the single-stage PN/A process in the view of practical applications. PRACTITIONER POINTS: Single-stage partial nitration and anammox (PN/A) process was achieved using a submerged dynamic membrane (DM) in this study. The reactor was stably operated for 200 days, and the nitrogen removal efficiency was sustained at 70.3 ± 7.2%. The feasibility of the PN/A system with DM is evaluated. The main objective is to provide a control strategy of the DM-SBRs for practical applications.

Saline conditions effect on the performance and stress index of anaerobic ammonium oxidizing (anammox) bacteria

In this study, a lab-scale sequencing batch reactor dominated by freshwater anammox bacteria (FAB) was used to study the performance and stress index of the anammox bacteria at various saline conditions. The reactor with an effective volume of 1.8 L was operated for about 160 days. The nitrogen-loading rate was maintained at 0.364 kg-N m^-3d^-1 throughout the operational period. At the start-up phase, the seed biomass acclimation to the lab bioreactor showed an inconsistent performance. However, a stable performance was observed after day 38. The average substrate removal efficiency was 92% during most of the operational period. Anammox stress index; a ratio of dissolved Adenosine Triphosphate (dATP_amx) to total Adenosine Triphosphate (tATP_amx) showed an irrefutable correlation between NaCl concentration, anammox stress and microbial community. A drop in the biomass cellular ATP at 5 g L^-1 salinity led to a significant decrease in the Specific Anammox activity. Candidatus Brocadia was identified as the main anammox species and its relative abundance reduced along the stepwise salinity increment.

Rapid enrichment of anammox bacteria linked to floc aggregates in a single-stage partial nitritation-anammox process: Providing the initial carrier and anaerobic microenvironment

Rapid enrichment of anaerobic ammonia oxidation bacteria (AnAOB) is highly associated with the granulation process; however, the interactive mechanism remains unclear, especially for the initial granulation process. A single-stage partial nitritation-anammox (PN/A) bioreactor combined with granular/floc sludge was operated for 400 days. During the experimental period, the nitrogen removal rate increased from 0.60 to 1.21 kg N m^-3d^-1, and the nitrogen removal capability improved primarily during a transition period (days 200-250), which was accompanied by a particle size increase and AnAOB proliferation (4.9 ± 1.7 days). Moreover, as observed by the biomass physio-morphology, the size distribution, and the microbial community shift, small flocs (< 200 μm) aggregated due to the addition of excess sodium acetate. The emerging floc aggregates represented an early form of granules, providing the initial biological carrier and necessary anaerobic microenvironment for the growth of attached AnAOB, resulting in a high AnAOB growth-rate. These results are the first direct evidence that floc aggregates are essential to AnAOB enrichment, and that they can be affected by operational conditions. This study provides an in-depth understanding of the link between floc aggregations and AnAOB enrichment and broadens the feasibility of optimizing PN/A applications.

Hydroxyapatite crystallization-based phosphorus recovery coupling with the nitrogen removal through partial nitritation/anammox in a single reactor

For digestion effluent treatment, while the anammox-based process has been successfully applied for nitrogen removal, in most cases, phosphorus (P) represents another major concern. In this study, a novel process, integrating the partial nitritation/anammox and hydroxyapatite crystallization (PNA-HAP) in a single airlift reactor, was developed for the simultaneous nitrogen removal and P recovery from synthetic digestion effluent. With a stable influent P concentration of 20.0 mg/L, an HRT of 6 h, and alternating increases of influent calcium and ammonium, the final achieved nitrogen removal rate was 1.2 kg/m³/d and the P removal efficiency was 83.0%. The settleability of sludge was desirably enhanced with the calcium addition and a high biomass concentration was achieved in reactor. Quantitative and qualitative analyses confirmed that HAP was the main inorganic content in sludge, which could be harvested for P recovery. According to the Scanning Electron Microscope observation and the Energy Dispersive X-ray spectrometry analysis, the microbes were mainly distributed on the outer layer of the sludge aggregate, while the HAP mainly in the interior. The relevant theoretical calculation also revealed that the sludge discharge manipulation has direct effect on the sludge composition and aggregate structure. In sum, the results are evidence of the feasibility of simultaneous nitrogen removal and P recovery through one-stage PNA-HAP process for digestion effluent.

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

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

The performance of freshwater one-stage partial nitritation/anammox process with the increase of salinity up to 3.0

The freshwater anammox-based process would usually experience performance fluctuation and need adoption period when subjected to saline wastewater according to previous studies. In this study, the performance of nitrite-limited freshwater one-stage partial nitritation/anammox (PNA) process subjected to saline wastewater was investigated. The results showed that the nitrite-limited freshwater system could smoothly adapt to the salinity of 0.25%. The stable nitrogen removal could be achieved until the salinity of 2.5%, at which the desirable average NRR of 0.74 ± 0.1 kg/m³/d was achieved. The microbial community analysis showed that during the whole experiment, the main functional microbes were from genus Nitrosomonas and genus Kuenenia, which were through to be the crucial factors for achieving the excellent performance. This study indicates the nitrite-limited strategy is admissible for stabilizing the performance of freshwater one-stage PNA process subjected to saline wastewater in actual application.

System Performance Corresponding to Bacterial Community Succession after a Disturbance in an Autotrophic Nitrogen Removal Bioreactor

Performance of a bioreactor is affected by complex microbial consortia that regulate system functional processes. Studies so far, however, have mainly emphasized the selective pressures imposed by operational conditions (i.e., deterministic external physicochemical variables) on the microbial community as well as system performance, but have overlooked direct effects of the microbial community on system functioning. Here, using a bioreactor with ammonium as the sole substrate under controlled operational settings as a model system, we investigated succession of the bacterial community after a disturbance and its impact on nitrification and anammox (anaerobic ammonium oxidation) processes with fine-resolution time series data. System performance was quantified as the ratio of the fed ammonium converted to anammox-derived nitrogen gas (N₂) versus nitrification-derived nitrate (npNO₃ ^-). After the disturbance, the N₂/npNO₃ ^- ratio first decreased, then recovered, and finally stabilized until the end. Importantly, the dynamics of N₂/npNO₃ ^- could not be fully explained by physicochemical variables of the system. In comparison, the proportion of variation that could be explained substantially increased (tripled) when the changes in bacterial composition were taken into account. Specifically, distinct bacterial taxa tended to dominate at different successional stages, and their relative abundances could explain up to 46% of the variation in nitrogen removal efficiency. These findings add baseline knowledge of microbial succession and emphasize the importance of monitoring the dynamics of microbial consortia for understanding the variability of system performance.IMPORTANCE Dynamics of microbial communities are believed to be associated with system functional processes in bioreactors. However, few studies have provided quantitative evidence. The difficulty of evaluating direct microbe-system relationships arises from the fact that system performance is affected by convolved effects of microbiota and bioreactor operational parameters (i.e., deterministic external physicochemical forcing). Here, using fine-resolution time series data (daily sampling for 2 months) under controlled operational settings, we performed an in-depth analysis of system performance as a function of the microbial community in the context of bioreactor physicochemical conditions. We obtained statistically evaluated results supporting the idea that monitoring microbial community dynamics could improve the ability to predict system functioning, beyond what could be explained by operational physicochemical variables. Moreover, our results suggested that considering the succession of multiple bacterial taxa would account for more system variation than focusing on any particular taxon, highlighting the need to integrate microbial community ecology for understanding system functioning.

Start-up of single-stage partial nitritation-anammox micro-granules system: Performance and microbial community dynamics

By manipulating influent nitrogen load and DO concentration in bulk liquid, the start-up and performance of a new micro-granule based partial nitritation-anammox process was investigated in a continuous stirred tank reactor (CSTR). Under the condition of nitrogen loadings from 0.3 to 1.4 kgN /m³/d and DO <0.21mg/L, the single-stage partial nitritation-anammox (SPNA) system was successfully started, with a nitrogen removal of 76.2%. Meanwhile, the oxygen utilization efficiency by ammonium oxidizing bacteria (AOB) increased in the system with the increase of influent ammonia loading rate. Micro-granules with an average diameter of 0.25 mm were formed. Sludge granulation was promoted by increasing influent nitrogen load, and there was a positive correlation between nitrogen load, extracellular polymeric substances (EPS) content and sludge particle size. Ca. Kuenenia became the dominant anaerobic ammonium oxidizing bacteria (AnAOB) in the SPNA system. As the dominant AOB genera, Nitrosomonas coexist with Ca. Kuenenia in the micro-granules.

Determination of the response characteristics of anaerobic ammonium oxidation bioreactor disturbed by temperature change with the spectral fingerprint

Anaerobic ammonium oxidation (anammox) bacteria are sensitive and susceptible to operating condition fluctuations that can lead to the instability of a bioreactor. Through multivariate spectral analysis, the dynamic changes of intracellular and extracellular metabolites of anammox sludge under the declined temperature stress were characterized. It was found that effluent fluorescence components were positively related to the bacterial activity, and the response of the protein-like substances to the temperature change was more sensitive than that of humic substances. Under the transient disturbance during temperature change from 35 to 15 °C, anammox system tended to considerably excrete extracellular polymeric substances to resist the low temperature inhibition. However, the long-term exposure of the sludge at 10 °C resulted in the considerably inhibition of sludge activity, granular disintegration and heterotrophic denitrification bacteria increase. The two-dimensional correlation analysis further revealed that the humic acid in extracellular polymeric substances was preferentially responded to the temperature change than protein. Anammox bacteria tended to increase the intracellular protein and electron transfer-related reactive substance excretion to counteract the low temperature inhibition. Herein, both the intra- and extra-cellular response characteristics of anammox sludge to temperature variation were successfully resolved via the combined spectra. This work provides a comprehensive understanding on the mechanism of anammox sludge to temperature variation and may be valuable for the development of bioreactor monitoring techniques.

Biodegradable organic matter-containing ammonium wastewater treatment through simultaneous partial nitritation, anammox, denitrification and COD oxidization process

For both nitrogen and COD removal from biodegradable organic matter (BOM)-containing ammonium wastewater, the simultaneous partial nitritation, anammox, denitrification and COD oxidization (SNADCO) process is a promising solution. In this study, with the stable influent ammonium concentration of 250.0 mg/L (nitrogen loading rate of 0.5 kg/m³/d) and the variation of influent COD/NH₄⁺-N (C/N) ratio from 0.0 to 1.6, the performance of the SNADCO process in a one-stage carrier-packing airlift reactor with continuous mode was investigated for the first time. The results showed that until the C/N ratio of 0.8, both the well nitrogen and COD removal targets could be reached. Mass balance calculations indicated that the average nitrogen removal efficiency (NRE) of 80.9% achieved at the C/N ratio of 0.8 were due to both the anammox and denitrification pathways. Correspondingly, the achieved average COD removal efficiency of 94.6% was attributed to both the denitrification and COD oxidization pathways. Based on the specific sludge activity tests and Fluorescence in Situ Hybridization observation, anammox and denitrification bacteria were mainly distributed in the biofilm sludge, while ammonium oxidizing bacteria and ordinary heterotrophic organisms were mainly in the suspended sludge. At the C/N ratio of 1.6, the washout of suspended sludge became serious while the biofilm sludge was well retained, resulting in inefficient nitritation and a subsequent decrease in NRE. The microbial interaction analysis provided a clear explanation of the performance change of the SNADCO process under different C/N ratios. This research enriches the knowledge of the SNADCO process in BOM-containing ammonium wastewater treatment.

Towards more efficient nitrogen removal and phosphorus recovery from digestion effluent: Latest developments in the anammox-based process from the application perspective

Over the past forty years, anammox-based processes have been extensively researched and applied to some extent. However, some of the long-standing problems present serious impediments to wide application of these processes, and knowledge gap between lab-scale research and full-scale operations is still considerable. In recent years, anammox-based research has developed rapidly and some emerging concepts have been proposed. The focus of this review is on the critical problems facing actual application of anammox processes. The latest developments in anammox-based processes are summarized, and particular consideration is given to the following aspects: (1) the evolution of the chemical stoichiometry of anammox reaction; (2) the status of several main anammox-based processes; (3) the critical problems and countermeasures; (4) the emerging anammox-based processes; and (5) the suggested optimal process integrating partial nitritation, anammox, hydroxyapatite crystallization and denitratation for digestion effluent treatment towards more efficient nitrogen removal and phosphorus recovery in the future.

Modeling the performance of Single-stage Nitrogen removal using Anammox and Partial nitritation (SNAP) process with backpropagation neural network and response surface methodology

Two novel feedforward backpropagation Artificial Neural Networks (ANN)-based-models (8:N_H:1 and 7:N_H:1) combined with Box-Behnken design of experiments methodology was proposed and developed to model NH₄⁺ and Total Nitrogen (TN) removal within an upflow-sludge-bed (USB) reactor treating nitrogen-rich wastewater via Single-stage Nitrogen removal using Anammox and Partial nitritation (SNAP) process. ANN were developed by optimizing network architecture parameters via response surface methodology. Based on the goodness-of-fit standards, the proposed three-layered NH₄⁺ and TN removal ANN-based-models trained with Levenberg-Marquardt-algorithm demonstrated high-performance as computations exhibited smaller deviations-(±2.1%) as well as satisfactory coefficient of determination (R²), fractional variance-(FV), and index of agreement-(IA) ranging 0.989-0.997, 0.003-0.031 and 0.993-0.998, respectively. The computational results affirmed that the ANN architecture which was optimized with response surface methodology enhanced the efficiency of the ANN-based-models. Furthermore, the overall performance of the developed ANN-based models revealed that modeling intricate biological systems (such as SNAP) using ANN-based models with the view to improve removal efficiencies, establish process control strategies and optimize performance is highly feasible. Microbial community analysis conducted with 16S rRNA high-throughput approach revealed that Candidatus Kuenenia was the most pronounced genera which accounted for 13.11% followed by Nitrosomonas-(6.23%) and Proteocatella-(3.1%), an indication that nitrogen removal pathway within the USB was mainly via partial-nitritation/anammox process.

Autotrophic nitrogen removal in an integrated fixed-biofilm activated sludge (IFAS) reactor: Anammox bacteria enriched in the flocs have been overlooked

In this study, an autotrophic nitrogen removal process was established using an integrated fixed-biofilm activated sludge (IFAS) reactor treated with high ammonium wastewater. A nitrogen removal rate (NRR) of 2.78 kg N/(m³·d) was obtained during the 206-day operation. Moreover, during the stable period, the large flocs (D > 0.2 mm) had a significantly higher abundance of anammox bacteria than the small flocs (D < 0.2 mm) and biofilm, resulting in 51% of the anammox bacteria being located in the flocs. The result indicates that anammox bacteria can be enriched in the flocs and in the biofilm, which has been rarely reported for IFAS reactors. In addition, the large flocs are likely formed through biofilm detachment since the microbial community was similar for the two kinds of biomass. Overall, the role of flocs in IFAS reactors are complicated and their contribution to the anammox reaction have been overlooked thus far.

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

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

Probing operational conditions of mixing and oxygen deficiency using HSV color space

In this work, the relationship between sludge color and operational conditions was studied. It was found that the coordinates H and S of the HSV color space well correlated with biological status and the operational conditions of mixing and oxygen deficiency, and a coefficient of variation (CV_H/S) of the ratio of H to S in sludge cake images was derived. A smaller CV_H/S indicated better mixing conditions based on the observations of four laboratory-scale experiments and two full scale WWTPs, which can be used as a promising index for the monitoring of mixing condition. The coordinate oxygen uptake rate (OUR_q) of the respirogram space showed similar trend as CV_H/S, and analysis of microbial community also showed that CV_H/S could indicate changes of biological community including species and richness. These findings suggested that CV_H/S is a biological meaningful index for detecting the effect of changing operational conditions, which gives a key to quantify a large number of empirical rules accumulated in the past. Furthermore, it promotes the Internet of Things (IoT) application to the management of WWTPs, as color is readily available with MEMS (Micro-Electro-Mechanical Systems) sensors such as smart phones.

Influence of temperature on an Anammox sequencing batch reactor (SBR) system under lower nitrogen load

The nitrogen removal performance and microbial communities of an Anammox sequencing batch reactor (SBR) was studied under varied temperatures with a lower nitrogen loading rate (NLR) about 0.28 kgN/m³/d. Results showed that the temperature could influence the nitrogen removal performance and the community structure in the Anammox SBR system. Under lower temperatures, both the nitrogen removal efficiencies and Anammox activity were in lower levels. When temperature was raised again, the Anammox activity recovered accordingly. When the temperature dropped from 33 ± 1 °C to15 °C, the dominant Anammox bacteria shifted from Ca. Brocadia to Ca. Kuenenia in the sludge. When the temperature returned over, the abundance of Ca. Brocadia recovered, while the Ca. Kuenenia was still the dominant Anammox bacteria. This indicated that Ca. Kuenenia is more adaptable to low temperature environment than Ca. Brocadia under low NLR with temperature variation.

Comparing two start up strategies and the effect of temperature fluctuations on the performance of mainstream anammox reactors

Anammox cultivated with high substrate concentration (NH₄⁺-N, 150 mg/L; NO₂^--N, 200 mg/L) at 35 °C was first used as seed sludge to start up reactors at 35 (Ra), 20 (Rb) and 15 °C (Rc) with low substrate concentration (NH₄⁺-N 30 mg/L, NO₂^--N 40 mg/L). The results showed that anammox activity initially decreased in the three reactors, but that activity levels and nitrogen loading rate (NLR) increased as the bacteria gradually adapted to the new conditions (12-30 days). Temperature and concentration shift affected anammox activity jointly. In the process, the abundance of mRNA of the key functional genes of hdh and nirS, changed with time but this change did not reflect the change of anammox activity. When the reactors reached a stable state after 40 d, the effect of temperature fluctuations was tested. The results showed that anammox adapted to low temperatures as soon as temperature decreased (i.e., decreased from 35 °C to 15 °C). When temperature increased, 2-3 days were needed for activity recovery. From this result, it may be concluded that reactors with low temperatures and low substrate (mainstream) concentrations can be started up using anammox cultivated at a higher temperature (35 °C) with low substrate. Then anammox in Ra was used to start up a mainstream reactor at 15 °C and it was operated for 60 days. The results showed that the activity in Ra decreased sharply to the level as that of Rc at the stable state. After the experiment, microbiological analysis showed that the anammox was stable and that Candidatus Kuenenia was the dominant species.

Fast start-up of the CANON process with a SABF and the effects of pH and temperature on nitrogen removal and microbial activity

The long start-up time of the completely autotrophic nitrogen removal over nitrite (CANON) process is one of the main disadvantages of this system. In this paper, the CANON process with a submerged aerated biological filter (SABF) was rapidly started up within 26 days. It gave an average ammonium nitrogen removal rate (ANR) and a total nitrogen removal rate (TNR) of 94.2% and 81.3%, respectively. The phyla Proteobacteria and Planctomycetes were confirmed as the ammonia oxidizing bacteria (AOB) and anaerobic ammonium oxidation bacteria (AnAOB). The genus Candidatus Brocadia was the major contributor of nitrogen removal. pH and temperature affect the performance of the CANON process. This experimental results showed that the optimum pH and temperature were 8.0 and 30 °C, respectively, which gave the highest average ANR and TNR values of 94.6% and 85.1%, respectively. This research could promote the nitrogen removal ability of CANON process in the future.

The transformation from anammox granules to deammonification granules in micro-aerobic system by facilitating indigenous ammonia oxidizing bacteria

Granular deammonification process is a good way to retain aerobic and anaerobic ammonia oxidizing bacteria (AOB and anammox bacteria) and exhaust flocculent nitrite oxidizing bacteria (NOB). In this study, to facilitate indigenous AOB growth on anammox granules, by stepwise reducing influent nitrite, anammox granules were effectively transformed into deammonification granules in a micro-aerobic EGSB in 100 days. Total nitrogen removal efficiency of 90% and nitrogen removal rate of 2.3 g N/L/d were reached at stable deammonification stage. High influent FA and limited oxygen supply contributed suppression for Nitrospira-like NOB. In transition stages, Proteobacteria and Chloroflexi were always dominated. Anammox abundance decreased, while AOB abundance grew fast. Anammox bacteria and AOB were dominated by Brocadia fulgida and Nitrosomonas europaea, respectively. Denitrification activity and bacteria existed although without influent organic. The final AOB abundance was about 4.55-13.8 times more than anammox bacteria abundance, with almost equal potential activities.

Effects of salinity on the denitrification efficiency and community structure of a combined partial nitritation- anaerobic ammonium oxidation process

The effects of salinity changes on nitrogen transformation efficiency and recoverability were studied by using a partial nitration (PN)- anaerobic ammonium oxidation (Anammox) integrated reactor. The changes of microbial community structure and population abundance during the increase and decrease of salinity were also analyzed by 16S rRNA gene high-throughput sequencing. The results showed that when the salinity was increased to 1.35%, the combined PN-Anammox process achieved the maximum stimulated and total nitrogen removal rate (TNRR) arrived at 1.1kg/(m³·d). When the salinity was higher than 1.35%, the activities of AOB and Anammox bacteria began to be inhibited. When the salinity reached 2.4%, the TNRR decreased to 60%. TNRR was fast restored, when salinity was reduced to 0.11%. The genes of AOB and Anammox bacteria indicated that the TNRR of the reactor was restored after salinity inhibition, but the functional microbial community structure and abundance had relatively large, irreversible changes.

Single-stage autotrophic nitrogen removal process at high loading rate: granular reactor performance, kinetics, and microbial characterization

For the possible highest performance of single-stage combined partial nitritation/anammox (PNA) process, a continuous complete-mix granular reactor was operated at progressively higher nitrogen loading rate. The variations in bacterial community structure of granules were also characterized using high-throughput pyrosequencing, to give a detail insight to the relationship between reactor performance and functional organism abundance within completely autotrophic nitrogen removal system. In 172 days of operation, a superior total nitrogen (TN) removal rate over 3.9 kg N/(m³/day) was stable implemented at a fixed dissolved oxygen concentration of 1.9 mg/L, corresponding to the maximum specific substrate utilization rate of 0.36/day for TN based on the related kinetics modeling. Pyrosequencing results revealed that the genus Nitrosomonas responsible for aerobic ammonium oxidation was dominated on the granule surface, which was essential to offer the required niche for the selective enrichment of anammox bacteria (genus Candidatus Kuenenia) in the inner layer. And the present of various heterotrophic organisms with general functions, known as fermentation and denitrification, could not be overlooked. In addition, it was believed that an adequate excess of ammonium in the bulk liquid played a key role in maintaining process stability, by suppressing the growth of nitrite-oxidizing bacteria through dual-substrate competitions.

Upgrading of the symbiosis of Nitrosomanas and anammox bacteria in a novel single-stage partial nitritation-anammox system: Nitrogen removal potential and Microbial characterization

A novel single-stage partial nitritation-anammox process equipped with porous functional suspended carriers was developed at 25°C in a CSTR by controlling dissolved oxygen <0.3mg/L. The nitrogen removal performance was almost unchanged over a nitrogen loading rate ranging from 0.5 to 2.5kgNH₄⁺-N/m³/d with a high nitrogen removal efficiency of 81.1%. The specific activity of AOB and anammox bacteria was of 3.00g-N/g-MLVSS/d (the suspended sludge), 3.56g-N/g-MLVSS/d (the biofilm sludge), respectively. The results of pyrosequencing revealed that Nitrosomonas (5.66%) and Candidatus_Kuenenia (4.95%) were symbiotic in carriers while Nitrosomonas (40.70%) was predominant in the suspended flocs. Besides, two specific types of heterotrophic filamentous bacteria in the suspended flocs (Haliscomenobacter) and the functional carrier biofilm (Longilinea) were shown to confer structural integrity to the aggregates. The novel single-stage partial nitritation-anammox process equipped with functional suspended carriers was shown to have good potential for the nitrogen-rich wastewater treatment.