Partial Nitritation Reactor Research Articles

Dynamic simulation of N2O emissions from a full-scale partial nitritation reactor

This study deals with the potential and the limitations of dynamic models for describing and predicting nitrous oxide (N2O) emissions associated with biological nitrogen removal from wastewater. The results of a three-week monitoring campaign on a full-scale partial nitritation reactor were reproduced through a state-of-the-art model including different biological N2O formation pathways. The partial nitritation reactor under study was a SHARON reactor treating the effluent from a municipal wastewater treatment plant sludge digester. A qualitative and quantitative comparison between experimental data and simulation results was performed to identify N2O formation pathways as well as for model identification. Heterotrophic denitrifying bacteria and ammonium oxidizing bacteria (AOB) were responsible for N2O formation under anoxic conditions, whereas under aerated conditions the AOB were the most important N2O producers. Relative to previously proposed models, hydroxylamine (NH2OH) had to be included as a state variable in the AOB conversions in order to describe potential N2O formation by AOB under anoxic conditions. An oxygen inhibition term in the corresponding reaction kinetics was required to fairly represent the relative contribution of the different AOB pathways for N2O production. Nevertheless, quantitative prediction of N2O emissions with models remains a challenge, which is discussed.

Microbial ecology dynamics of a partial nitritation bioreactor with Polar Arctic Circle activated sludge operating at low temperature

A lab-scale partial nitritation SBR was operated at 11 °C for 300 days used for the treatment of high-ammonium wastewater, which was inoculated with activated sludge from Rovaniemi WWTP (located in Polar Arctic Circle) in order to evaluate the influence the temperature on the performance, stability and dynamics of its microbial community. The partial nitritation achieved steady-state long-term operation and granulation process was not affected despite the low temperature and high ammonia concentration. The steady conditions were reached after 60 days of operation where the granular biomass was fully-formed and the 50%–50% of ammonium-nitrite effluent was successful achieved. Inoculation with cold adapted inoculum showed to yield bigger, denser granules with faster start-up without necessity of low temperature adaptation period. Next-generation sequences techniques showed that Trichosporonaceae and Xanthomonadaceae were the dominant OTUs in the mature granules. Our study could be useful in the implementation of full-scale partial nitritation reactors in cold regions such as Nordic countries for treating wastewater with high concentration of ammonium.

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 CH1.61O0.61N0.17S0.01P0.03. 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.

Stability and control of a partial nitritation reactor with biomass retention

Partial nitritation–anammox process is an interesting process option for biological nitrogen removal from wastewater. Partial nitritation needs to provide about equimolar ammonium and nitrite concentrations as required for the Anammox conversion. This study analyzes the equilibrium points of a stand-alone partial nitritation process as part of a two-reactor partial nitritation-anammox configuration. Conditions for the feasibility and stability of the equilibrium points are obtained in the general case of a reactor with biomass retention and controlled oxygen concentration. This knowledge is subsequently applied to achieve the desired reactor behavior, namely suppressing nitrite oxidizing bacteria (NOB) and obtaining the desired effluent nitrite:ammonium ratio. The optimal oxygen concentration to achieve these two objectives is determined. Moreover, a control algorithm is proposed to drive the process to the desired operating point. Its steady state behavior is assessed for changing influent ammonium concentrations and biomass retention times. The dynamic performance of the controller is tested through simulation as well, prioritizing NOB suppression in case the two objectives are not compatible.

Critical analysis of decay rate measurement methodology

In modeling studies, estimation of microorganisms kinetic parameters set is a key element for proper model operation and predictability. Nitrification process is very often, a crucial element of the wastewater treatment systems as bacteria responsible for ammonium and nitrite oxidation are slow growing microorganisms, making whole nitrification process vulnerable to external factors i.e. temperature, inhibition and load fluctuations. Growth and decay rate of nitrifiers decide about amount nitrifying biomass in the wastewater treatment plants, thus the nitrification efficiency. Paper presents analysis of the decay rate (ba) estimation methodology based on respirometric assays measuring the oxygen uptake rate (OUR). Evaluation of this simple and cheap method was made based on decay estimation tests performed on sludge samples from side-stream partial nitritation reactor treating reject water from digested sludge dewatering. Database obtained from these tests were analyzed to evaluate the impact of respirometric assay duration on calculated decay rate values. 11 time ranges were selected for the performed analysis. Calculated ba values were compared showing the optimal test duration between 5–6 hours, while test shorter than 2 hours resulted in unsatisfactory ba outcome.

Stratification of nitrifier guilds in granular sludge in relation to nitritation

A lab-scale partial nitritation granular sludge air-lift reactor was operated in continuous mode treating low strength synthetic medium (influent ca. 50 mg-N-NH4+/L). Granules were initially stratified with AOB in the external shell and NOB in the inner core at 20 °C. Once temperature was decreased progressively from 20 °C to 15 °C, nitrate production was initially observed during several weeks. However, by maintaining relatively high ammonium concentrations in the liquid (ca. 28 mg-N-NH4+/L), effluent nitrate concentrations in the reactor decreased in time and process performance was recovered. Batch tests were performed in the reactor at different conditions. To understand the experimental results an existing one-dimensional biofilm model was used to simulate batch tests and theoretically assess the impact of stratification, dissolved oxygen (DO) and short-term effects of temperature on time course concentrations of ammonium, nitrite and nitrate. This theoretical assessment served to develop an experimental methodology for the evaluation of in-situ batch tests in the partial nitritation reactor. These batch tests proved to be a powerful tool to easily monitor the extent of stratification of nitrifier guilds in granular sludge and to determine the required bulk ammonium concentration to minimize nitrite oxidation. When nitrifier guilds were stratified in the granular sludge, a higher bulk ammonium concentration was required to efficiently repress NOB at lower temperature (ca. 19 versus 7 mg-N-NH4+/L at 15 and 20 °C, respectively).

Coupling of partial nitritation and anammox in two- and one-stage systems: Process operation, N2O emission and microbial community

Advances in research and technological development in the field of wastewater treatment encourage the implementation of engineered autotrophic nitrogen removal (ANR) systems based on the coupling of partial nitritation (PN) and anaerobic ammonium oxidation (anammox). Such processes can be conducted in two independent dedicated reactors (i.e., two-stage system) or, alternatively, in the same reactor under limited aeration (i.e., one-stage system). In this investigation, both configurations were successfully tested using the sequencing batch reactor (SBR) technology. Processed wastewater was supernatant from a sewage sludge anaerobic digester containing about 1 g NH4+-N/L and 0.3 g PO4-P/L. Pre-conditioning of the supernatant through dilution and magnesium phosphates (e.g., struvite) precipitation favored the anammox process performance under both configurations. The N loading rate (NLR) applied in the PN reactor was ≤1.3 g N/(L·d) with nitrite production efficiencies of about 48%, whereas the N removal rate (NRR) in the anammox reactor was 0.43–0.56 g N/(L·d). On the other hand, in the one-stage reactor, the NRR was approximately 0.27 g N/(L·d). Estimated emissions of nitrous oxide (N2O) in such bioreactors ranged from 0.4 to 4.6% of the N loaded. Comparatively, similar NRRs were achieved for both reactor configurations but the one-stage system used a smaller reaction volume, ran at a lower NLR, and emitted N2O at a lower rate than the two-stage system. The microbial community in both systems was dominated by aerobic ammonium-oxidizing bacteria of the genus Nitrosomonas and the anammox species Ca. Brocadia sinica.

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

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

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

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

Two-stage partial nitritation-anammox process for high-rate mainstream deammonification.

Increasing information supported that achieving high-rate mainstream deammonification through two-stage partial nitritation (PN)-anammox process should be a better option than through single-stage process. However, direct experimental evidence was limited so far. Herein, a two-stage PN-anammox process was successfully operated for nitrogen removal from low-strength wastewater in winter. Influent shift from synthetic wastewater to actual anaerobically pretreated sewage had little impact on the process performance. Promising nitrogen removal rates (NRRs) of 0.28-0.07kgNm-3d-1 with an average effluent concentration of 5.2mg TNL-1 were achieved for the anaerobically pretreated sewage treatment at 15-7°C. Moreover, nearly all the degradable COD in the pretreated sewage was steadily removed in the first-stage PN reactor, despite the varied influent COD concentrations of 22-78mgL-1 and the operating temperature decrease, suggesting the positive role of the first-stage PN in protecting anammox bacteria. The low temperature seemingly was the only deterministic factor inhibiting the anammox activity, and hence made the anammox reaction to be the rate-limiting step for nitrogen removal in the two-stage PN-anammox process. Unexpectedly, nearly all the anammox bacteria remained active at low temperatures with the process actual anammox activity reached about 76-85% of their maximum potential, implying that higher NRRs would be easily realized through bioaugmentation or enrichment of anammox bacteria. Overall, the present investigation provides direct and valuable information for implementing the two-stage PN-anammox process to treat mainstream municipal wastewater. A control strategy was also proposed to optimize the operation of the two-stage mainstream deammonification process.

High nitrite concentration accelerates nitrite oxidising organism's death.

High nitrite is a known operation parameter to inhibit the biological oxidation of nitrite to nitrate. The phenomenon is traditionally expressed using a Monod-type equation with non-competitive inhibition, in which the reaction associated with the biomass growth is reduced when high nitrite is present. On the other hand, very high nitrite is also known to slay nitrifiers. To clarify the difference between the growth inhibition and the poisoning, cell counting for living microorganisms in the nitrite oxidiser-enriched activated sludge was conducted in batch conditions under various nitrite concentrations together with measurements of biomass chemical oxygen demand (COD) concentration and oxygen uptake rate. The experiments demonstrated that these measureable parameters were all decayed when nitrite concentration exceeded 100-500 mgN/L at pH 7.0 in the system, indicating that nitrite poisoning took place. Biomass growth was recognised in lower range of nitrite which was expressed with growth inhibition only. Based on the response, a kinetic model for the biological nitrite oxidation was developed with a modification of IWA ASM1. The model was further utilised to calculate a possibility to wash out nitrite oxidiser in the aeration tank where a part of the return activated sludge was exposed to high nitrite liquor in a side-stream partial nitritation reactor.

Predictions of the Influent and Operational Conditions for Partial Nitritation with a Model Incorporating pH Dynamics.

Ammonium partial oxidation to nitrite (i.e., partial nitritation) is required in a two-stage autotrophic nitrogen removal system, to provide effluent suitable for the anammox reaction. This study aims to establish influent (ammonium and bicarbonate concentrations) and operational (dissolved oxygen (DO) concentration and solids retention time (SRT)) conditions that favor partial nitritation. This is achieved through extending the nitritation and nitratation models to predict pH variation as well as the effects of pH, free ammonia (NH3), and free nitrous acid (HNO2) on the two reactions. Experiments were performed on a lab-scale sequencing batch reactor (SBR) operated for over 500 days to provide dynamic data for the calibration of model parameters, particularly those related to the NH3 and HNO2 inhibition on nitrite-oxidizing bacteria (NOB). The influent ammonium (19-84 mM) and bicarbonate (23-72 mM) were varied, which led to dynamic ammonium, nitrite, and nitrate data suitable for model calibration and validation. The model was able to well-describe pH dynamics as well as the inhibitory effects of NH3 and HNO2 on NOB. Model-based scenario analysis was then undertaken to establish the joint regions of influent ammonium and bicarbonate concentrations and the operational DO, temperature, and SRT conditions that favor partial nitritation. The results provide support to the design and optimization of partial nitritation reactors.

Microbial community response to influent shift and lowering temperature in a two-stage mainstream deammonification process

The effects of influent shift from synthetic wastewater to anaerobically pretreated actual sewage coupling with lowering temperature on microbial community of a two-stage partial nitritation (PN)-anammox process were evaluated through high-throughput sequencing. Venn diagrams and Hill numbers showed the significantly increased bacterial diversity both in the PN and anammox reactor. However, taxonomic analysis indicated that outstanding enrichment of heterotrophic bacteria and reduction of autotrophic species mainly occurred in the PN reactor, while nearly all of the dominant bacteria in the anammox reactor only slightly decreased in abundance. Moreover, immigrant bacteria from the PN reactor to the following anammox reactor had no negative effect on the anammox function. These results implied the positive role of the first-stage PN in maintaining the stability of the following anammox community. Nitrosomonas europaea (17.9–52.9%) and one cluster (19.2–27.7%) within Candidatus Brocadia remained as the dominant functional species in the PN and anammox reactor, respectively.

Pilot-scale ELAN ® process applied to treat primary settled urban wastewater at low temperature via partial nitritation-anammox processes

A single stage partial nitritation and anammox granular pilot scale reactor (600 L) was operated to treat primary settled sewage in an urban wastewater treatment plant. The fed wastewater contained low total nitrogen concentrations of 6–25 mg TN/L and the system operated without temperature control ranging from 18 to 12 °C. A control strategy, based on the pH value, was applied to stop the aeration supply. The pH set-point was fixed at 6.0 and allowed obtaining a total nitrogen removal efficiency approximately of 50% treating a load of 67 mg TN/(L·d) without the addition of any chemicals. Although nitrite oxidizing bacteria were present in the inoculated sludge, when the pH-based control was implemented (day 30) the ammonium oxidation was favored compared to the nitrite oxidation activity. Then, the system operated stable the rest of the operational period (days 30–94) despite the presence of organic matter in the wastewater and the high variability of nitrogen load and temperature during the operation. Nitrogen was autotrophically removed accomplishing the stringent discharge limits (10 mg TN/L) and nitrate concentrations in the effluent lower than 3 mg NO3−-N/L. Both biomass concentration and granules size increased during the operational period indicating the growth of the biomass inside the reactor and therefore the potential treatment capacity.

Evaluation of nitrous oxide gaseous emissions from a partial nitritation reactor operating under different conditions

Evaluation of nitrous oxide gaseous emissions from a partial nitritation reactor operating under different conditions

Understanding bacterial communities of partial nitritation and nitratation reactors at ambient and low temperature

This study focused on comparing bacterial communities from two lab-scale sequencing batch reactors (SBR) operated in nitritation and nitratation mode, respectively. Four samples were collected until the bioreactors had been run at a stable and efficient nitrogen conversion at different operational temperatures of 22 °C and 12 °C. The bacterial community compositions were profiled by Illumina MiSeq sequencing and quantitative real-time (q-PCR). Sequencing results indicated that bacterial compositions in the two reactors at the phylum level showed a similar structure with the decreased temperature, while the disparity between respective heterotrophic communities was clearly demonstrated at the genus level except autotrophic nitrogen-oxidizing species, Nitrosomonas and Nitrobacter highly enriched inside the bioreactors. In addition, the dominated Nitrosomonas and Nitrobacter were further checked through q-PCR and high concentration heterotrophic denitrifiers using nirK as gene markers were also found, consistent with a variety of potential heterotrophs analyzed by high-throughput sequencing. Due to the weak effect of temperature on bacterial communities of different functional nitrifying sludge, heterotrophic communities possibly depended on the influent substrate. Taking into account these results, heterotrophic bacterial communities coexisting with ammonia-oxidizing bacteria (AOB) or nitrite-oxidizing bacteria (NOB) were mutually different and distinct in the diversity and stability.

Microeukaryote community in a partial nitritation reactor prior to anammox and an insight into the potential of ciliates as performance bioindicators

An in-depth, long-term, multidisciplinary study was conducted in order to study the microeukaryote community in a partial nitritation (PN) reactor prior to anammox. The PN reactor operated with moving bed biofilm reactor (MBBR) technology, using plastic supports (carriers) for biofilm development. The microeukaryote community from the biofilm (BF) and the surrounding media (mixed liquor or ML) were analysed separately. Despite the physicochemical conditions under which the PN-MBBR operated (an average of 305.9±117mg TAN l−1 and 328.4±131.9mg N-NO2− l−1), up to 24 microeukaryotic taxa were observed by microscope. Microeukaryote species showed an uneven distribution in the PN-MBBR, thus suggesting the existence of two habitats: the BF, preferred by species with specific structures for adhering to a substrate, such as the stalked Peritrichia, and the ML, preferred by free-swimming or non-substrate dependent species. The results indicated that most ciliate population dynamics mainly responded to the nitrous acid and free ammonia concentrations and, to a lesser extent, to sCOD values. In the BF, variations in the population of Epistylis camprubii and Opercularia coarctata suggest the existence of competition between these species due to niche overlap. A V4 18S rDNA molecular survey (Illumina) was carried out for some samples with the aim of obtaining maximum coverage of the main eukaryote species that were microscopically detected throughout the study. The diversity and abundance data provided by both detection methods were compared. The study helped identify broader tolerance ranges of the microeukaryote taxa to the physicochemical parameters analysed.

Nitrous Oxide Production in a Granule-based Partial Nitritation Reactor: A Model-based Evaluation

Sustainable wastewater treatment has been attracting increasing attentions over the past decades. However, the production of nitrous oxide (N2O), a potent GHG, from the energy-efficient granule-based autotrophic nitrogen removal is largely unknown. This study applied a previously established N2O model, which incorporated two N2O production pathways by ammonia-oxidizing bacteria (AOB) (AOB denitrification and the hydroxylamine (NH2OH) oxidation). The two-pathway model was used to describe N2O production from a granule-based partial nitritation (PN) reactor and provide insights into the N2O distribution inside granules. The model was evaluated by comparing simulation results with N2O monitoring profiles as well as isotopic measurement data from the PN reactor. The model demonstrated its good predictive ability against N2O dynamics and provided useful information about the shift of N2O production pathways inside granules for the first time. The simulation results indicated that the increase of oxygen concentration and granule size would significantly enhance N2O production. The results further revealed a linear relationship between N2O production and ammonia oxidation rate (AOR) (R2 = 0.99) under the conditions of varying oxygen levels and granule diameters, suggesting that bulk oxygen and granule size may exert an indirect effect on N2O production by causing a change in AOR.

High rate nitrogen removal by ANAMMOX internal circulation reactor (IC) for old landfill leachate treatment

This study aimed to evaluate the performance of a high rate nitrogen removal lab-scale ANAMMOX reactor, namely Internal Circulation (IC) reactor, for old landfill leachate treatment. The reactor was operated with pre-treated leachate from a pilot Partial Nitritation Reactor (PNR) using a high nitrogen loading rate ranging from 2 to 10kgNm−3d−1. High rate removal of nitrogen (9.52±1.11kgNm−3d−1) was observed at an influent nitrogen concentration of 1500mgNL−1. The specific ANAMMOX activity was found to be 0.598±0.026gN2-NgVSS−1d−1. Analysis of ANAMMOX granules suggested that 0.5–1.0mm size granular sludge was the dominant group. The results of DNA analysis revealed that Candidatus Kueneniastuttgartiensis was the dominant species (37.45%) in the IC reactor, whereas other species like uncultured Bacteroidetes bacterium only constituted 5.37% in the system, but they were still responsible for removing recalcitrant organic matter.

Shortcut nitrification/partial nitritation start-up for reject water treatment in a SBR

For many wastewater treatment plants (WWTPs), side-stream treatment of reject water from digested sludge dewatering is a feasible opportunity to improve N-removal efficiency without costly plant expansion. Biological nitrogen removal over nitrite or combined partial nitritation (PN)-Anammox process has recently become a popular treatment method for such ammonium-rich streams. Shortcut nitrification and PN start-ups were successfully performed in a pilot-scale SBR treating real reject water. In all performed experiments, effective nitrate production inhibition occurred in less than 20 days due to operational conditions selection and without advanced control system. pH adjustment in the PN reactor allowed to achieve NO 2 -N /NH 4 -N ratio suitable for Anammox process (1.24±0.07).