Partial Nitritation-anammox Process Research Articles

Kinetic and elemental characterization of HAP-based high-rate partial nitritation/anammox system orienting stability and inorganic elemental requirements

Anammox-based processes are attractive for biological nitrogen removal, and the combination of anammox and hydroxyapatite (HAP) is promising for the simultaneous removal of nitrogen and phosphorus from wastewater. However, the kinetics of one-stage partial nitritation/anammox (PNA) in which ammonia-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) exist in a reactor are poorly understood. Moreover, inorganic elements are required to promote microbial cell synthesis and growth; therefore, monitoring of elements to prevent the limitation and inhibition of the process is critical. The minimum amounts of inorganic elements required for a one-stage PNA process and the elemental flow remain unknown. Therefore, in this study, kinetics, stoichiometry, and element flow in the long-term, high-rate, continuous, one-stage HAP-PNA process with microaerobic granular sludge at 25 °C were determined using process modeling, parameter estimation, and mass balance. The biomass elemental composition was determined to be CH2.2O0.89N0.18S0.0091, and the biomass yield (Yobs) was calculated to be 0.0805 g/g NH4+-N. Therefore, a stoichiometric reaction equation for the one-stage HAP-PNA system was also proposed. The maximum specific growth rate (μm) of AnAOB and AOB were 0.0360 and 0.0982 d−1 with doubling times of 19 and 7.1 d, respectively. Finally, the elemental requirements for stable and high-rate performance were determined using element flow analysis. These findings are essential for developing the anammox-based process in a stable and resource-efficient manner and determining engineering applicability.

Resource-efficient nitrogen and salinity removal in a synergistic partial nitritation-anammox bioelectrochemical system

We investigated the performance of a two-stage partial-nitritation-anammox biocathode integrated microbial desalination cell (TNiAmoxMDC) for concurrent nitrogen removal, desalination, and bioelectricity generation at different aeration rates in the nitritation chamber. The TNiAmoxMDC system achieved a maximum total nitrogen removal rate of 50.4 ± 8.2 g-TN/m3/d with 300 mW/m2 of maximum power production utilizing 500 mg/L of glucose chemical oxygen demand (COD) in the anode. The organic removal in the anode chamber showed a good fit to first order reaction model with a rate constant of 0.036 h−1. The biocatalytic activity of anammox bacteria in the biocathode produced a current density of 0.7 A/m2 in the absence of oxygen. More than 98% of the salinity was removed at a rate of 1.48 mg/h. The energy consumption values for nitrogen removal at aeration rates of 0.7, 2.2, and 10 ml/min were 0.022, 0.55, and 0.20 kWh/kg-N, respectively. The maximum energy produced by the TNiAmoxMDC was 0.0157 kWh/m3. Excluding the energy consumption by the system, the net energy recovery was 0.0023 kWh/m3 at an airflow rate of 10 ml/min. Thus, the integration of the partial nitritation-anammox process with microbial desalination cell (MDC) provides energy-and resource-efficient synergy for bioelectrochemical wastewater treatment.

Improved performance of single-stage partial nitritation-anammox membrane bioreactor (PN/A MBR) by adding biofilm carriers: start-up, membrane operation and microbial structure

Serious sludge washout is a bottleneck in the partial nitritation-anammox (PN/A) process that needs to be addressed urgently. A membrane bioreactor (MBR) is suitable for the PN/A process because of its 100 % biomass retention ability. Therefore, this study establishes a single-stage PN/A MBR and a PN/A moving bed membrane reactor (PN/A MB-MBR) for investigating the operation performance, membrane fouling behavior, and effect of biofilm carrier addition. The results demonstrate that the PN/A MB-MBR achieves a significantly shorter start-up time and substantially higher maximum nitrogen removal rate (NRR) of 17 days and 1.02 g N/L/d, respectively, compared to that of PN/A MBR (61 days and 0.19 g N/L/d, respectively). Further, the PN/A MB-MBR exhibits a lower membrane fouling of 0.023 kPa/L compared to that of 0.050 kPa/L in PN/A MBR because of the integration of the biofilm carrier. The biofilm in PN/A MB-MBR shows a higher relative abundance of Candidatus Kuenenia (18.90–25.10 %), which is a major AnAOB, compared to that observed in the suspended sludge of PN/A MB-MBR (10.30–14.32 %). Furthermore, the relative abundance of Candidatus Kuenenia absorbed on the membrane surface of PN/A MB-MBR (8.6 %) was lower than that on PN/A MBR (15.5 %). This study presented a promising method for enhancing the performance of PN/A MBR, which can serve as a valuable reference for its extensive implementation.

Partial nitritation-anammox for treatment of saline wastewater: Hydrazine-assisted salinity adaptation and nitrate control

In biological wastewater treatment, salinity can significantly restrict bacterial activity, and a long adaptation period to the saline condition is needed. This study investigated the feasibility of using partial nitritation-anammox (PNA) in an integrated fixed-film activated sludge (IFAS) reactor for enhanced biological nitrogen removal from saline municipal wastewater. The experimental results show that the salinity increase in the influent seriously affected the activity of anammox bacteria and disrupted the PNA process for autotrophic nitrogen removal when seawater was added to a 20% proportion with a resultant salinity of around 6.5‰. However, the addition of N2H4 (dosage: 5 mg/L for 3 weeks), an exogenous enhancer of both ammonia-oxidizing bacteria and anammox bacteria, effectively facilitated the rapid recovery and adaptation of PNA to salinity stress with a negligible additional cost for long-term operation. The PNA-IFAS reactor achieved a nitrogen removal capacity of 83.4 ± 5.9 mg N/(L‧d) in treating the saline municipal wastewater containing 40% seawater and an ammonia-N concentration of around 60 mg N/L. Salinity in the influent advantageously suppressed the activity and growth of the co-existing nitrite-oxidizing bacteria (NOB). The relative abundances of NOB in the suspended sludge (0.15%) and biofilms (0.33%) were kept at low levels, maintaining a low nitrate yield of 6.6 ± 1.5% in the bioreactor. The result suggests that salinity in the influent may offer potential benefits for reducing the risk of nitrate build-up and increasing the robustness of the PNA process for sustainable wastewater treatment.

Intensifying single-stage denitrogen by a dissolved oxygen-differentiated airlift internal circulation reactor under organic matter stress: Nitrogen removal pathways and microbial interactions

Current research focuses on efficient single-stage nitrogen removal from organic matter wastewater using the partial nitritation-anammox (PNA) process. In this study, we constructed a single-stage partial nitritation-anammox and denitrification (SPNAD) system using a dissolved oxygen-differentiated airlift internal circulation reactor. The system was operated continuously for 364 days at 250 mg/L NH4+–N. During the operation, the COD/NH4+–N ratio (C/N) was increased from 0.5 to 4 (0.5, 1, 2, 3, and 4), and the aeration rate (AR) gradually increased. The results showed that the SPNAD system maintained efficient and stable operation at C/N = 1–2 and AR = 1.4–1.6 L/min, with an average total nitrogen removal efficiency of 87.2%. The removal pathways of pollutants in the system and the interactions between microbes were revealed by analyzing the changes in sludge characteristics and microbial community structure at different phases. As the influent C/N increased, the relative abundance of Nitrosomonas and Candidatus Brocadia decreased, and that of denitrifying bacteria, such as Denitratisoma, increased to 44%. The nitrogen removal pathway of the system gradually changed from autotrophic nitrogen removal to nitrification-denitrification. At the optimum C/N, the SPNAD system synergistically removed nitrogen through PNA and nitrification-denitrification. Overall, the unique reactor configuration facilitated the formation of dissolved oxygen compartments, providing a suitable environment for different microbes. An appropriate organic matter concentration maintained the dynamic stability of microbial growth and interactions. These enhance microbial synergy and enable efficient single-stage nitrogen removal.

Revealing the response of community structure and metabolic pathway to varying organic matter stress in a dissolved oxygen-differentiated airlift internal circulation partial nitritation-anammox system

In practice, the influent organic matter is often pre-treated to reduce the impact on partial nitritation-anammox (PNA) process. However, the influent organics may also drive the denitrification process and improve total nitrogen removal efficiency of the PNA process. Thus, we designed and operated a novel dissolved oxygen-differentiated airlift internal circulation PNA (PNA-DOAIC) system in this study at various influent C/N ratios of 0–4.0. Nitrogen removal performance, microbial activity and community, and metabolic pathways in response to varying organic matter stress were investigated via the continuous experiment combined with batch test. The results showed that the optimum influent C/N ratio was 2.0 in this system, and the efficient and stable operation was still maintained at the C/N ratios of 0–3. At this time, the TN removal efficiency and removal rate could reach 95.1 % and 0.93 kg-N/m3/d, respectively, while COD efficiency remained at 95.4 %. Efficient removal performance was achieved via the PNA coupled with denitrification. However, the anammox bacteria (AnAOB) activity and abundance declined persistently as the influent C/N ratio was further raised, and heterotrophic bacteria gradually replaced AnAOB as dominate genus. Meanwhile, metabolic functions involving the material exchange and organic degradation were significantly enhanced. Nitrogen removal pathways changed from PNA to the nitrification-denitrification process. This study provides deep insights into effects of organic matter on the PNA process and can expand the application scope of this novel PNA-DOAIC bioreactor.

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.

Deciphering the effect of temperature reduction on the biofilm in partial nitritation-anammox system: Insights from microbial community to metabolic pathways

The concept of partial nitritation-anammox (PN/A) process has been proposed for many years, however seasonal temperatures and low ammonia concentrations under mainstream conditions always challenge the application in practical engineering. Thus, we operated a PN/A moving bed biofilm reactor (PN/A-MBBR) for treating 50 mg-NH4/L wastewater at various temperatures of 35 °C, 25 °C, and 15 °C. The effect of temperature reduction on biofilm system was investigated from process performance, microbial community and activity, to metabolic pathways. The results showed that total nitrogen removal efficiency declined from 76.6 % to 48.6 % as the temperature was reduced from 35 °C to 15 °C, while nitrogen removal rate decreased to 0.28 kg-N/m3/d. Low temperature stimulated the secretion of polysaccharide in loosely bound-EPS and protein in tightly bound-EPS. Meanwhile, specific activity of anammox declined from 1.53 to 0.56 g-N/g-VSS/d, and the relative abundance of Candidatus Brocadia dropped from 20.1 % to 9.4 %. Temperature reduction had significant effect on the expression of functional genes (hzsA and hdh) for anammox reaction. Nitrification-denitrification process was enhanced during low temperatures and recovery phases. Biofilm system might respond to temperature reduction by the decline of substance transport and energy consumption, as well as the enhancement of biofilm formation and diversity of nitrogen removal pathways. The findings of this study provide a deep insight into the effect of temperature reduction on PN/A process, also promote the practical applications of mainstream PN/A in the future.

Metagenomics reveals the microbial community and functional metabolism variation in the partial nitritation-anammox process: From collapse to recovery

Mainstream partial nitritation-anammox (PNA) process easily suffers from performance instability and even reactor collapse in application. Thus, it is of great significance to unveil the characteristic of performance recovery, understand the intrinsic mechanism and then propose operational strategy. In this study, we combined long-term reactor operation, batch tests, and metagenomics to reveal the succession of microbial community and functional metabolism variation from system collapse to recovery. Proper aeration control (0.10-0.25 mg O2/L) was critical for performance recovery. It was also found that Candidatus Brocadia became the dominant flora and its abundance increased from 3.5% to 11.0%. Significant enhancements in carbon metabolism and phospholipid biosynthesis were observed during system recovery, and the genes abundance related to signal transduction was dramatically increased. The up-regulation of sdh and suc genes showed the processes of succinate dehydrogenation and succinyl-CoA synthesis might stimulate the production of amino acids and the synthesis of proteins, thereby possibly improving the activity and abundance of AnAOB, which was conducive to the performance recovery. Moreover, the increase in abundance of hzs and hdh genes suggested the enhancement of the anammox process. Changes in the abundance of key genes involved in nitrogen metabolism indicated that nitrogen removal pathway was more diverse after system recovery. The achievement of performance recovery was driven by anammox, nitrification and denitrification coupled with dissimilatory nitrate reduction to ammonium. These results provide deeper insights into the recovery mechanism of PNA system and also provide a potential regulation strategy for the stable operation of the mainstream PNA process.

Response of partial nitritation-anammox process to substrate concentration and temperature variations in a single-stage airlift circulation system: Performance and microbial community dynamics

Low-strength ammonia and temperature challenge the application of partial nitritation-anammox (PNA) process in the treatment of mainstream wastewater. In this study, we operated a single-stage PNA process under various substrate concentrations (250, 150, and 50 mg/L) and temperatures (32, 25, 15 °C). Changes in nitrogen removal performance, biochemical reaction rate, and specific activity were investigated via the long-term continuous experiment and batch tests. Microbial diversity, community structure, and metabolic pathways in response to the various conditions were explored. When the temperature declined to 15 °C, the nitrogen removal efficiency decreased from 83.9 % to 51.5 % with the influent of 50 mg-NH4/L, while the nitrogen removal rate dropped to 0.33 kg N/m3/d. Meanwhile, the relative abundances of Candidatus Brocadia and Nitrosomonas reached 14.1 % and 1.5 %, respectively. The results showed that the mechanisms of performance degradation caused by low ammonia concentration and low temperature were significantly different. Moreover, the substrate concentration mainly affected the reaction rate and activity of anammox bacteria and hardly affected the relative abundance of functional bacteria. But temperature reduction resulted in a significant decline in the relative abundance of anammox bacteria. Changes in metabolic pathways suggested that this PNA system may respond to low substrate concentrations and low temperatures by facilitating substance transport and reducing energy consumption. These results provide a deep insight into the microbial community dynamics and interactions in the treatment of mainstream wastewater by PNA process.

Endogenous partial denitritation as an efficient remediation to unstable partial nitritation-anammox (PNA) process: Bacteria enrichment and superior robustness

The application of partial nitritation-anammox (PNA) process suffers severe obstacles due to the instability of partial nitritation (PN) process. This study sought to evaluate the feasibility of endogenous partial denitratation (EPD) as a remediation or alternative to supply nitrite for the unstable PNA process. A novel strategy of optimal organics utilization through pre-anaerobic carbon storage and post-anoxic endogenous denitrification coupled with anammox was developed in a single-stage bioreactor treating actual municipal wastewater with low C/N (∼3.2). Specifically, the undesired NO2−/NH4+ ratio (2.4 to 0.04) and nitrate accumulation were obtained by increasing the aeration rate (0.6 to 1.8 L/min) to simulate the PN instability. Delightedly, advanced nitrogen removal efficiency (92.1%) was maintained despite a dramatic decrease in nitrite accumulation ratio from 97.6% to 2.6%. This was attributed to the significant increase in anammox contribution to total nitrogen removal from 30.2% to 80.5%. The steady nitrite flux supplied from EPD coupled with PN (EPD contribution increased from 0 to 97.0%) was assumed to be the main reason for the continually increasing abundance and bioactivity of anammox bacteria. Both the anammox bacteria (1.5%, Ca. Brocadia) and glycogen accumulating organisms (6.0%, responsible for EPD) were enriched and coexisted stably in the single reactor. Our study confirms that coupling EPD with anammox has great potential as a remediation for the unstable mainstream PNA process.

Long term analysis of N2O emission from partial nitritation anammox process under oxygen limitation

Partial nitritation with anammox (PNA) process was achieved during 300 days to investigate the fate of N2O and NO. N2O emission factor increased from 0.4 ± 0.1 % up to 6.1 ± 0.3 % during intensification phase of 5 months and progressively stabilized at 2.0 ± 0.1 % after 10 months without any significant emission of NO. The emission of N2O correlated with the ammonium consumption rate and the oxygen transfer rate. Experiment carried out in anoxic condition suggested that heterotrophic denitrification poorly contributed. Measurements of δ15N, δ18O and SP of N2O demonstrated that its production resulted from the reduction of nitrite. The stimulating effect of nitrite on N2O production was demonstrated with specific tests and observed during transient nitrite accumulation periods. This study highlighted the importance of nitrite and oxygen transfer rate control for future control strategies aiming to mitigate N2O emission in PNA system.

Key factors improving the stability and the loading capacity of nitrogen removal in a hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox process

Anammox is attracting attention as the most promising nitrogen removal technology. Recently, the combination of the anammox process and the hydroxyapatite (HAP) granules comes with the advantages, including simultaneous removals of nitrogen and phosphorus and better settleability of the sludge. In this study, the key factors for a stable and efficient nitrogen removal in a one-stage partial nitritation/anammox (PNA) process with HAP-based granules was explored. Under a fixed hydraulic retention time of 12 h, in the case that HAP was not properly formed and phosphorus removal efficiency (PRE) was low, the highest nitrogen removal rate (NRR) achieved stably was only 1.3 ± 0.07 kg N/m3/d, while a stable nitrogen removal efficiency of 81 ± 3.3 % and a higher NRR of 2.0 ± 0.1 kg N/m3/d were obtained with the proper formation of HAP and higher PRE. The achievement of the high NRR benefited by a high microorganism concentration with improved settleability due to the formation of HAP-based granules. To achieve a high NRR in a long-term operation, a necessary phosphorus removal rate/nitrogen removal rate ratio above 0.02 was confirmed, which allowed for the desired sludge settleability and concentration. The average diameter of the formed micro-granules was 92–159 μm. The spatial distribution of the functional bacteria in the granules was analyzed with fluorescence in situ hybridization. Anammox bacteria and the ammonium oxidizing bacteria the two most abundant bacteria during the whole operation. The results of this study are evidence of the necessity and potential of proportionable HAP formation for highly-efficient nitrogen removal in the one-stage PNA process.

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

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

Revealing the effect of biofilm formation in partial nitritation-anammox systems: Start-up, performance stability, and recovery

Successful application of partial nitritation-anammox (PNA) processes is currently and primarily associated with biofilm systems. Biofilm characteristics significantly influence start-up, performance stability, and recovery. Here, two PNA systems with and without carriers were implemented simultaneously for treating wastewater containing 50 mg-NH4/L. The performance characteristics of these two PNA systems were compared. Stable nitrogen removal efficiencies of 76.3 ± 2.8% and 72.9 ± 1.6% were obtained for suspended sludge and biofilm systems, respectively. The slow process of biofilm colonization resulted in a long start-up time in the biofilm system. Biofilm enrichment and protection conferred stable performance when exposed to aeration shock. The suspended sludge system displayed good elasticity during performance recovery after shock compared to the slow recovery in the biofilm system. Moreover, suitable control of dissolved oxygen could improve the activity and abundance of the functional microbes. This study provides new insights into the operation and control of PNA systems for treating mainstream wastewater.

Rapid initiation of a single-stage partial nitritation-anammox process treating low-strength ammonia wastewater: Novel insights into biofilm development on porous polyurethane hydrogel carrier

Media-supported biofilm is a powerful strategy for growth and enrichment of slow-growing microorganisms. In this study, a single-stage nitritation-anammox process treating low-strength wastewater was successfully started to investigate the biofilm development on porous polyurethane hydrogel carrier. Suspended biomass migration into the carrier and being entrapment by its internal interconnected micropores dominated the fast initial colonization stage. Both surface-attached growth and embedded growth of microbes occurred during the following accumulation stage. Fluorescence in situ hybridization analysis of mature biofilm indicated that ammonium-oxidizing bacteria located at the outer layers featured a surface-attached growth, while anammox microcolonies housed in the inner layers proliferated as an embedded-like growth. In this way, the growth rate of anammox bacteria (predominated by Candidatus Kuenenia) could be 0.079 d-1. The anammox potential of the biofilm reactor reached 1.65 ± 0.3 kg/m3/d within two months. This study provides novel insights into nitritation-anammox biofilm formation on the porous polyurethane hydrogel carrier.

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.

Comparison of Different Carriers to Maintain a Stable Partial Nitrification Process for Low-Strength Wastewater Treatment.

Practical application of the partial nitritation–anaerobic ammonium oxidation (anammox) process has attracted increasing attention because of its low operational costs. However, the nitritation process, as a promising way to supply nitrite for anammox, is sensitive to the variations in substrate concentration and dissolved oxygen (DO) concentration. Therefore, a stable supply of nitrite becomes a real bottleneck in partial nitritation–anammox process, limiting their potential for application in mainstream wastewater treatment. In this study, five 18-L sequencing batch reactors were operated in parallel at room temperature (22°C ± 4°C) to explore the nitritation performance with different carrier materials, including sepiolite-nonwoven carrier (R1), zeolite-nonwoven carrier (R2), brucite-nonwoven carrier (R3), polyurethane carrier (R4), and nonwoven carrier (R5). The ammonia oxidation rate (AOR) in R1 reached the highest level of 0.174 g-N L−1 d−1 in phase II, which was 1.4-fold higher than the control reactor (R4). To guarantee a stable supply of nitrite for anammox process, the nitrite accumulation efficiency (NAE) was always higher than 77%, even though the free ammonia (FA) decreases to 0.08 mg-N/L, and the pH decreases to 6.8 ± 0.3. In phase V, the AOR in R1 reached 0.206 g-N L−1 d−1 after the DO content increase from 0.7 ± 0.3 mg/L to 1.7 ± 0.3 mg/L. The NAE in R1 was consistently higher than 68.6%, which was much higher than the other reactor systems (R2: 43.8%, R3: 46.6%, R4: 23.7%, R5: 22.7%). Analysis of 16S rRNA gene sequencing revealed that the relative abundance of Nitrobacter and Nitrospira in R1 was significantly lower than other reactors, indicating that the sepiolite carrier plays an important role in the inhibition of nitrite-oxidizing bacteria. These results indicate that the sepiolite nonwoven composite carrier can effectively improve the nitritation process, which is highly beneficial for the application of partial nitritation–anammox for mainstream wastewater treatment.

Nitrogen removal by a Hydroxyapatite-enhanced Micro-granule type One-stage partial Nitritation/anammox process following anaerobic membrane bioreactor treating municipal wastewater

Nitrogen removal from wastewater by the partial nitritation/anammox (PNA) technology is promising from both economic and environmental perspectives. However, this technology has not been popularized in the mainstream because of low biomass retention and the growth of the nitrite oxidizing bacteria. In this study, a one-stage PNA process with hydroxyapatite (HAP)-enhanced granules was used to treat effluent from a mainstream anaerobic membrane bioreactor. The HAP-enhanced reactor allowed an enriched high biomass of 6.9 ± 0.2 g/L at a low hydraulic retention time of 2 h. A nitrogen removal efficiency of 80 ± 6.0 %, a nitrogen removal rate of 0.36 ± 0.05 kg/m3/d and a COD removal efficiency of 54 ± 15 % were achieved stably, leading to a low total nitrogen concentration of 8.5 ± 2.7 mg/L and a low COD concentration of 19.7 ± 5.9 mg/L in the effluent. Anammox bacteria of Candidatus Kuenenia stuttgartiensis and ammonium oxidizing bacteria of Nitrosomonas were found to be the two most predominant bacteria.

Nitrogen Removal from Real Sewage Pretreated with Anaerobic Membrane Bioreactors Using One-Stage Partial Nitritation-Anammox Process with Hydroxyapatite Crystallization and Partial Nitritation-Anammox Granule

Nitrogen Removal from Real Sewage Pretreated with Anaerobic Membrane Bioreactors Using One-Stage Partial Nitritation-Anammox Process with Hydroxyapatite Crystallization and Partial Nitritation-Anammox Granule