Simulation of simultaneous anammox and denitrification for kinetic and physiological characterization of microbial community in a granular biofilm system

An Advanced Synergy of Partial Denitrification-Anammox for Optimizing Nitrogen Removal from Wastewater: A Review

Anammox is a widely adopted process for energy-efficient removal of nitrogen from wastewater, but challenges with NOB suppression and NO₃^- accumulation have led to a deeper investigation of this process. To address these issues, the synergy of partial denitrification and anammox (PD-anammox) has emerged as a promising solution for sustainable nitrogen removal in wastewater. This paper presents a comprehensive review of recent developments in the PD-anammox system, including stable performance outcomes, operational parameters, and mathematical models. The review categorizes start-up and recovery strategies for PD-anammox and examines its contributions to sustainable development goals, such as reducing N₂O emissions and saving energy. Furthermore, it suggests future trends and perspectives for improving the efficiency and integration of PD-anammox into full-scale wastewater treatment system. Overall, this review provides valuable insights into optimizing PD-anammox in wastewater treatment, highlighting the potential of simultaneous processes and the importance of improving efficiency and integration into full-scale systems.

Sustainable nitrogen removal in anammox-mediated systems: Microbial metabolic pathways, operational conditions and mathematical modelling

Anammox-mediated systems have attracted considerable attention as alternative cost-effective technologies for sustainable nitrogen (N) removal from wastewater. This review comprehensively highlights the importance of understanding microbial metabolism in anammox-mediated systems under crucial operation parameters, indicating the potentially wide applications for the sustainable treatment of N-containing wastewater. The partial nitrification-anammox (PN-A), simultaneous PN-A and denitrification (SNAD) processes have demonstrated sustainable N removal from sidestream wastewater. The partial denitrification-anammox (PD-A) and denitrifying anaerobic methane oxidation-anammox (DAMO-A) processes have advanced sustainable N removal efficiency in mainstream wastewater treatment. Moreover, N2O production/emission hotspots are extensively discussed in anammox-based processes and are related to the dominant ammonia-oxidizing bacteria (AOB) and denitrifying heterotrophs. In contrast, N2O is not produced in the metabolism pathways of AnAOB and DAMO-archaea; Moreover, the actual contribution of N2O production by dissimilatory nitrate reduction to ammonium (DNRA) and DAMO-bacteria in their species remains uncertain. Thus, PD-A and DAMO-A processes would achieve reduction in greenhouse gas production, as well as energy consumption for the reliability of N removal efficiencies. In addition to reaction mechanisms, this review covers the mathematical models for simultaneous anammox, partial nitrification and/or denitrification (i.e., PN-A, PD-A, and SNAD). Promising NO3- reduction technologies by endogenous PD, sulfur-driven autotrophic denitrification, and DNRA by anammox are also discussed. In summary, this review provides a better understanding of sustainable N removal in anammox-mediated systems, thereby encouraging future investigation and exploration of the sustainable N bio-treatment from wastewater.

Model study on real-time aeration based on nitrite for effective operation of single-stage anammox

Anaerobic ammonia oxidation (Anammox) is an innovative technology for cost-efficient nitrogen removal without intensive aeration. However, effective control of the competition between nitrite oxidizing bacteria (X_NOB) and Anammox bacteria (X_ANA) for nitrite is a key challenge for broad applications of single-stage Anammox processes in real wastewater treatment. Therefore, a real-time aeration scheme was proposed to determine dissolved oxygen (DO) based on nitrite concentration for effective control of X_NOB growth while maintaining the X_ANA activity in a single-stage Anammox process. In this study, a non-steady state mathematical model was developed and calibrated using previously reported lab-scale Anammox results to investigate the efficiency of the proposed real-time aeration scheme in enhancing the Anammox process. Based on the calibrated model simulation results, DO of about 0.10 mg-O₂/L was found to be ideal for maintaining effective nitrite creation by ammonia oxidizing bacteria (X_AOB) while slowing down the growth of X_NOB. If DO is too low (e.g., 0.01 mg-O₂/L or lower), the overall rate of the ammonia removal is limited due to slow growth of X_AOB. On the other hand, high DO (e.g., 1.0 mg-O₂/L or higher) inhibits the growth of X_ANA, resulting in dominancy of X_AOB and X_NOB. According to the simulation results, nitrite concentration was found to be a rate-limiting parameter on effective nitrogen removal in single-stage Anammox processes. We also found that nitrite concentration can be used as a real-time switch for aeration in a single-stage Anammox process. A schematic aeration method based on real-time nitrite concentration was proposed and examined to control the competition between X_ANA and X_NOB. In the model simulation, the X_ANA activity was successfully maintained because the schematic aeration prevented an outgrowth of X_NOB, allowing energy-efficient nitrogen removal using single-stage Anammox processes.

Development of a novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) to simultaneous treatment of mature landfill leachate and waste activated sludge

Introducing fermentation technology into sewage treatment is a sustainable development concept, but future application still faces many challenges. A novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) was achieved in three separated SBR type reactors, simultaneously treating high ammonia (1766.6 mg/L) mature landfill leachate and external waste activated sludge (WAS, MLSS = 20.6 g/L). Firstly, NH₄⁺-N was oxidized to NO₂^--N in partial nitrification reactor (PN-SBR), with nitrite accumulation ratio (NAR) of 96.5%. Next, the PN-SBR effluent (NO₂^--N = 1529.8 mg/L) coupled with the WAS were introduced to an anoxic reactor for integrated fermentation-denitrification (IFD-SBR). The occurrence of fermentation was mainly attributed to free nitrous acid (FNA, nitrite protonate form) promoting the splitting decomposition of sludge spatial configuration and interfacial forces. The released volatile fatty acids (VFAs) were utilized in situ during the denitrification process (NO₂^--N→N₂), obtaining 0.6 kg/m³•d nitrogen removal rate and 3.3 kg/m³•d sludge reduction rate. Finally, undesirable fermentation by-products from IFD-SBR (NH₄⁺-N = 119.2 mg/L) were further removed in the endogenous post-denitrification reactor (EPD-SBR) through operational strategy of anaerobic/aerobic/anoxic by residual VFAs as the carbon source. In the EPD-SBR, Defluviicoccus (0.9%) and Candidatus Competibacter (5.8%) dominated carbon source storage and nitrogen removal, acting as a typical denitrifying glycogen-accumulating organism (DGAO), with an intracellular carbon storage efficiency of 83.1% and nitrogen removal contribution of 93.7%. After 200 days of operation, the PN-F-Double/DN process provided effluent containing, on average, 1.86 mg/L NH₄⁺-N and 5.5 mg/L NO_x^--N, with 98.5% TN removal. Compared with traditional bioprocesses, PN-F-Double/DN allowed up to 25% saving in aeration energy consumption, 100% decrease in carbon source demand, and achieve 46.1% external WAS reduction.

Anammox-Based Processes for Mature Leachate Treatment in SBR: A Modelling Study

Mature landfill leachates are characterized by high levels of ammoniacal nitrogen which must be reduced for discharge in the sewer system and further treatment in municipal wastewater treatment plants. The use of anammox-based processes can allow for an efficient treatment of ammonium-rich leachates. In this work, two real scale sequencing batch reactors (SBRs), designed to initially perform partial nitritation/anammox (PN/A) and simultaneous partial nitrification and denitrification (SPND) for the treatment of ammonium-rich urban landfill leachate, were modelled using BioWin 6.0 in order to enable plant-wide modelling and optimizing. The constructed models were calibrated and validated using data from long- and short-term (one cycle) SBR operation and fit well to the main physical-chemical parameters (i.e., ammonium, nitrite and nitrate concentrations) measured during short-term (one cycle) operations. Despite the different strategies in terms of dissolved oxygen (DO) concentrations and aeration and mixing patterns applied for SBR operation, the models allowed for understanding that in both reactors the PN/A process was shown as the main contributor to nitrogen removal when the availability of organic carbon was low. Indeed, in both SBRs, the activity of nitrite oxidizing bacteria was inhibited due to high levels of free ammonia, whereas anammox bacteria were active due to the simultaneous presence of ammonium and nitrite and their ability to recover from DO inhibition. Increasing the external carbon addition, a prompt decrease of the anammox biomass was observed, with SPND becoming the main nitrogen removal mechanism. Models were also applied to estimate the production rates of nitrous oxide by aerobic ammonia oxidizing bacteria and heterotrophic denitrifiers. The models were found to be a robust tool for understanding the effects of different operating conditions (i.e, temperature, cycle phases, DO concentration, external carbon addition) on the nitrogen removal performances of the two reactors, assessing the contribution of the different bacterial groups involved.

Anammox Granule Enlargement by Heterogenous Granule Self-assembly

Expansion in the size is an indispensable stage in the granular sludge life cycle, but little attention has been payed to the enlargement mechanism of granular sludge. Here, we propose a novel anammox granule enlargement mechanism by the self-assembly of heterogenous granules. Two different colors of anammox granules, dark-red granules (DR-Granules) and bright-red granules (BR-Granules) were found in an expanded granular sludge bed reactor. These two heterogenous granules were not isolated but were assembled into granules with a larger DR-Granule in the center and many smaller BR-Granules aggregated on the surface, increasing the overall granular size. Their physiochemical characteristics in terms of EPS, adherence, rheological properties, and microbial compositions, were identified and compared to elucidate the interaction between the different colors of granules. The BR-Granules created 92% more extracellular polymeric substances than the DR-Granules. This material blocked the passage of gas and substrate, leading to BR-Granules smaller size and a yield stress approximately 48% lower than that of the DR-Granules. Nevertheless, the BR-Granules had compact extracellular protein secondary structures and a high adherence rate to the surface of the DR-Granules, upon which they formed a compact adhered layer. These unique features enabled them to directionally adhere to DR-Granules in the core, that is, two heterogenous colors of granules self-assembled into large anammox granules. The enlargement mechanism was further supported by the abundance of K-strategy Ca. Kuenenia in the DR-Granules (inner layer) being higher than in the BR-Granules (outer layer; 2.9 ± 0.4% vs. 0.4 ± 0.1%; p = 0.0003) and by visualized confirmation that the larger BR-Granules wrapped around smaller DR-Granules inside. This demonstrates that heterogenous anammox granules actively self-assemble into large granules, which is an important step in the lifecycle of anammox granules.

The effect of the COD: N ratio on mainstream deammonification in an integrated fixed-film activated sludge sequencing batch reactor

For eight months, a sequencing batch reactor (SBR) with integrated fixed-film activated sludge (IFAS) was operated in ambient temperature to study engineering and practical aspects of application of deammonification for mainstream conditions. For biofilm formation, K3 Kaldnes carriers were used, where the anaerobic ammonium oxidation (anammox) process can occur in deep layers of biofilm, while partial nitritation occurs in oxygen-rich outer layers. After the initial running phase of the reactor (Phase 1) to provide time for microorganisms to adapt, the COD: N ratio increased to around 2.6 in Phase 2 through reducing the ammonium concentration and increasing COD in synthetic wastewater to get closer to mainstream conditions. The total reaction time in each half-day batch cycle was kept 625 min throughout various phases, but the duration of intermittent aeration was regulated at 4 ± 1 min. While final nitrogen removal efficiency (NRE) for Phase 1 was 43%, at the end of Phase 2, it decreased to 37%. However, a maximum NRE at 90% was achieved during Phase 2. The identification of the responsible microorganisms was made through Fluorescence in situ hybridization (FISH), while Mixed Liquor Suspended Solid (MLSS) and Mixed Liquor Volatile Suspended Solid (MLVSS) was used to estimate the physical presence of bacteria. Ammonium oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing bacteria (AnAOB) were dominant bacteria, respectively. The adverse effects of a gradual increase of COD: N ratio from 0.17 to more than 2.0 caused a decline in NRE to around 15%.

Partial nitritation performance and microbial community in sequencing batch biofilm reactor filled with zeolite under organics oppression and its recovery strategy

Influences of organics on partial nitritation performance were investigated in a lab-scale sequencing batch biofilm reactor filled with zeolite. Significant differences in nitrite production rate (NPR) were observed between different dosages of glucose. With influent COD/N ratio from 0 to 1.5, NPR declined from 0.4 to 0.05 kg/(m3·d). Meanwhile, an appropriate NO2--N/NH4+-N ratio (1.4 ± 0.5) could be obtained for simultaneous anammox denitrification at COD/N ratio of 0.5. Increasing airflow rate was found as an effective recovery strategy. Other than competition of heterotrophs with nitrifiers for dissolved oxygen, it has been verified that addition of organics generated higher free ammonia, and then further inhibitedammonium oxidizing bacteria (AOB). Moreover, three-dimensional excitation-emission matrix (3D-EEM) results revealed that protein-like and humic acid-like substances were the main components in extracellularpolymericsubstances (EPS). And high-throughput sequencing analysis demonstrated that the relative abundance of AOB decreased.

A novel anammox reactor with a nitrogen gas circulation: performance, granule size, activity, and microbial community

Anammox process was regarded to be one of the vital links to achieve energy-saving or energy-producing wastewater treatment plant. In the study, an anammox reactor with the nitrogen gas circulation was constructed to culture anammox granules, and the performance, granule size distribution, and microbial community were investigated. Dissolved oxygen loading is found to be an important factor for the start-up of the anammox process, and the nitrogen removal rate of 2.12 kg N m^-3 day^-1 was achieved under the average nitrogen loading rate of 2.6 kg N m^-3 day^-1. The activity test showed that the highest specific anammox activity of 345.9 mg N gVSS^-1 day^-1 was achieved for granules with size of 0.5-1.0 mm. The Illumina high-throughput sequencing analysis revealed the consistent variation of Candidatus Brocadia and Denitratisoma abundance in granues of all sizes, suggesting possible synergistic mechanism between heterotrophic bacteria Denitratisoma and anammox bacteria Ca. Brocadia. Furthermore, the results indicated the reactor with the nitrogen gas circulation is an efficient strategy to start-up anammox.

Simultaneous Ammonium oxidation denitrifying (SAD) in an innovative three-stage process for energy-efficient mature landfill leachate treatment with external sludge reduction

High-loaded ammonia and low-strength organics mature landfill leachate is not effectively treated by conventional biological processes. Herein, an innovative solution was proposed using a three-stage Simultaneous Ammonium oxidation Denitrifying (SAD) process. Firstly, ammonia (1760 ± 126 mg N/L) in wastewater was oxidized to nitrite in a partial nitrification sequencing batch reactor (PN-SBR). Next, 93% PN-SBR effluent and concentrated external waste activated sludge (WAS; MLSS = 23057 ± 6014 mg/L) were introduced to an anoxic reactor for integrated fermentation and denitrification (IFD-SBR). Finally, ammonia (101.4 ± 13.8 mg N/L) released by fermentation in the IFD-SBR and residual 7% nitrite in the PN-SBR were removed through the anaerobic ammonium oxidation (anammox) process in the SAD up-flow anaerobic sludge bed (SAD-UASB). In addition, NO₃^--N generation during the anammox process could be reduced to nitrite by partial denitrification (PD) and reused as substrate for anammox. A satisfactory total nitrogen (TN) removal efficiency (98.3%), external sludge reduction rate (2.5 kg/m³ d) and effluent TN concentration (16.7 mg/L) were achieved after long-term operation (280 days). The IFD-SBR and SAD-UASB contributed to 81.9% and 12.3% nitrogen removal, respectively. Microbial analysis showed that anammox bacteria (1.5% Candidatus Brocadia) cooperated well with partial denitrifying bacteria (4.3% Thauera) in SAD-UASB, and average nitrogen removal contribution were 83.1% during significant stability of anammox and 9.2% during the denitrification process, respectively. The three-stage SAD process provides an environmental and economic approach for landfill leachate treatment since it has the advantage of 25.4% less oxygen, 100% organic matter savings and 47.9% less external sludge than traditional biological processes.

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

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

Model-based analysis of microbial consortia and microbial products in an anammox biofilm reactor

A mathematical model for a granular biofilm reactor for leachate treatment was validated by long-term measured data to investigate the mechanisms and drivers influencing biological nitrogen removal and microbial consortia dynamics. The proposed model, based on Activated Sludge Model (ASM1), included anaerobic ammonium oxidation (anammox), nitrifying and heterotrophic denitrifying bacteria which can attach and grow on granular activated carbon (GAC) particles. Two kinetic descriptions for the model were proposed: with and without soluble microbial products (SMP) and extracellular polymeric substance (EPS). The model accuracy was checked using recorded total inorganic nitrogen concentrations in the effluent and estimated relative abundance of active bacteria using quantitative fluorescence in-situ hybridization (qFISH). Results suggested that the model with EPS kinetics fits better for the relative abundance of anammox bacteria and nitrifying bacteria compared to the model without EPS. The model with EPS and SMP confirms that the growth and existence of heterotrophs in anammox biofilm systems slightly increased due to including the kinetics of SMP production in the model. During the one-year simulation period, the fractions of autotrophs and EPS in the biomass were almost stable but the fraction of heterotrophs decreased which is correlated with the reduction in nitrogen surface loading on the biofilm.