Incorporation of the complete ammonia oxidation (comammox) process for modeling nitrification in suspended growth wastewater treatment systems

Role of comammox bacteria in granular bioreactor for nitrogen removal via partial nitritation/anammox

In this study, two bioprocess models were first constructed with the newly-discovered comammox process described as one-step and two-step nitrification and evaluated against relevant experimental data. The validated models were then applied to reveal the potential effect of comammox bacteria on the granular bioreactor particularly suitable for undertaking partial nitritation/anammox (PN/A) under different operating conditions of bulk dissolved oxygen (DO) and influent NH4+. The results showed although comammox bacteria-based PN/A could achieve > 80.0 % total nitrogen (TN) removal over a relatively wider range of bulk DO and influent NH4+ (i.e., 0.25-0.40 g-O2/m3 and 470-870 g-N/m3, respectively) without significant nitrous oxide (N2O) production (< 0.1 %), the bulk DO should be finely controlled based on the influent NH4+ to avoid the undesired full nitrification by comammox bacteria. Comparatively, conventional ammonium-oxidizing bacteria (AOB)-based PN/A not only required higher bulk DO to achieve > 80.0 % TN removal but also suffered from 1.7 %∼2.8 % N2O production.

Mathematical modeling and experimental validation of a novel Circulating Oxygenation Biofilm Equipment (COBE) for the management of decentralized wastewater treatment

The difficulties of management were the key barriers to the promotion of decentralized wastewater treatment in remote areas. In this study, a novel decentralized Circulating Oxygenation Biofilm Equipment (COBE) and its remote management potential based on mathematical modeling were investigated. The COBE is an integrated biofilm reactor that employs drippage aeration and enables oxygenation, filtration, and effluent processes to be controlled, thus providing convenience for controlling. The model for the COBE describing drippage aeration, comprehensive ammonia-related microbes, and corncob carbon source release process was studied to uncover the impacts of operational conditions on decentralized wastewater treatment in the COBE system. The equipment regulation parameter (circulating oxygenation ratio) was found to be linearly correlated with the oxygen mass transfer coefficient. This discovery enabled highly accurate prediction of COD, NH4-N, and TN concentrations in the equipment effluent at various scenarios. The comprehensive ammonia oxidation biological model indicated that the model could duplicate the actual situation of the succession of ammonia metabolizing related microorganisms. Comammox and ammonia-oxidizing archaea (AOA) dominated ammonia metabolism in this equipment rather than conventional ammonium-oxidizing bacteria (AOB). This study could contribute to the Internet of Things system construction of decentralized wastewater treatment equipment, and provide a solution for timely decentralized equipment management in remote areas.

The ASM2d model with two-step nitrification can better simulate biological nutrient removal systems enriched with complete ammonia oxidizing bacteria (comammox Nitrospira)

The discovery of comammox Nitrospira, a complete ammonia-oxidizing microorganism belonging to the genus Nitrospira, has brought new insights into the nitrification process in wastewater treatment plants (WWTPs). The applicability of Activated Sludge Model No. 2 d with one-step nitrification (ASM2d-OSN) or two-step nitrification (ASM2d-TSN) for the simulation of the biological nutrient removal (BNR) processes of a full-scale WWTP in the presence of comammox Nitrospira was studied. Microbial analysis and kinetic parameter measurements showed comammox Nitrospira was enriched in the BNR system operated under low dissolved oxygen (DO) and long sludge retention time (SRT). The relative abundance of Nitrospira under the conditions of stage I (DO = 0.5 mg/L, SRT = 60 d) was about twice of that under stage II conditions (DO = 4.0 mg/L, SRT = 26 d), and the copy number of the comammox amoA gene for stage I was 33 times higher than that for stage II. Compared to the ASM2d-OSN model, the ASM2d-TSN model simulated the performance of the WWTP under stage I conditions better, and the Theil inequality coefficient values of all the tested water quality parameters were lower than using ASM2d-OSN. These results indicate that an ASM2d model with two-step nitrification is a better choice for the simulation of WWTPs with the presence of comammox.

A new strategy to simultaneous removal and recovery of nitrogen from wastewater without N2O emission by heterotrophic nitrogen-assimilating bacterium

Biological assimilation that recovery the nitrogen from wastewater in the form of biomass offers a more environmentally friendly solution for the limitations of the conventional wastewater treatments. This study reported the simultaneous removal and recovery of nitrogen from wastewater without N₂O emission by a heterotrophic nitrogen-assimilating Acinetobacter sp. DN1 strain. Nitrogen balance, biomass qualitative analysis, genome and enzyme studies have been performed to illustrate the mechanism of nitrogen conversion by strain DN1. Results showed that the ammonium removal followed one direct pathway (GOGAT/GDH) and three indirect pathways (NH₄⁺ → NH₂OH → NO → NO₂^- → NH₄⁺ → GOGAT/GDH; NH₄⁺ → NH₂OH → NO → NO₂^- → NO₃^- → NO₂^- → NH₄⁺ → GOGAT/GDH; NH₄⁺ → NH₂OH → NO → NO₃^- → NO₂^- → NH₄⁺ → GOGAT/GDH). Nitrogen balance and biomass qualitative analysis showed that over 70 % of the ammonium in the wastewater was converted into intracellular nitrogen-containing compounds and stored in the cells of strain DN1. Traditional denitrification pathway was not detected and the ammonium was removed through assimilation, which makes it more energy-saving for nitrogen recovery when compared with Haber-Bosch process. This study provides a new direction for simultaneous nitrogen removal and recovery without N₂O emission by the heterotrophic nitrogen-assimilating bacterium.

The coexistence and competition of canonical and comammox nitrite oxidizing bacteria in a nitrifying activated sludge system - Experimental observations and simulation studies

The second step of nitrification can be mediated by nitrite oxidizing bacteria (NOB), i.e. Nitrospira and Nitrobacter, with different characteristics in terms of the r/K theory. In this study, an activated sludge model was developed to account for competition between two groups of canonical NOB and comammox bacteria. Heterotrophic denitrification on soluble microbial products was also incorporated into the model. Four 5-week washout trials were carried out at dissolved oxygen-limited conditions for different temperatures (12 °C vs. 20 °C) and main substrates (NH₄⁺-N vs. NO₂^--N). Due to the aggressive reduction of solids retention time (from 4 to 1 d), the biomass concentrations were continuously decreased and stabilized after two weeks at a level below 400 mg/L. The collected experimental data (N species, biomass concentrations, and microbiological analyses) were used for model calibration and validation. In addition to the standard predictions (N species and biomass), the newly developed model also accurately predicted two microbiological indicators, including the relative abundance of comammox bacteria as well as nitrifiers to heterotrophs ratio. Sankey diagrams revealed that the relative contributions of specific microbial groups to N conversion pathways were significantly shifted during the trial. The contribution of comammox did not exceed 5 % in the experiments with both NH₄⁺-N and NO₂^--N substrates. This study contributes to a better understanding of the novel autotrophic N removal processes (e.g. deammonification) with nitrite as a central intermediate product.

New insights into modeling two-step nitrification in activated sludge systems - The effects of initial biomass concentrations, comammox and heterotrophic activities

In this study, the conventional two-step nitrification model was extended with complete ammonia oxidation (comammox) and heterotrophic denitrification on soluble microbial products. The data for model calibration/validation were collected at four long-term washout experiments when the solid retention time (SRT) and hydraulic retention time (HRT) were progressively reduced from 4 d to 1 d, with mixed liquor suspended solids (MLSS) of approximately 2000 mg/L at the start of each trial. A new calibration protocol was proposed by including a systematic calculation of the initial biomass concentrations and microbial relationships as the calibration targets. Moreover, the impact assessment of initial biomass concentrations (X) and maximum growth rates (μ) for ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), comammox Nitrospira, and heterotrophs on the calibration accuracy were investigated using the response surface methodology (RSM). The RSM results revealed the strongest interaction of X_AOB and μ_AOB on the model calibration accuracy. All the examined model efficiency measures confirmed that the extended model was accurately calibrated and validated. The estimated μ values were as follows: μ_AOB = 0.38 ± 0.005 d^-1, μ_NOB = 0.20 ± 0.01 d^-1, μ_CMX = 0.20 ± 0.01 d^-1, μ_HET = 1.0 ± 0.03 d^-1. For comparison, when using the conventional model, μ_AOB and μ_NOB increased respectively by 26 and 15 % (μ_AOB = 0.48 ± 0.02 d^-1 and μ_NOB = 0.23 ± 0.005 d^-1). This study provides better understanding of the effects of the initial biomass composition and the accompanying processes (comammox and heterotrophic denitrification) on modeling two-step nitrification.

Performance evaluation and model-based optimization of the mainstream deammonification in an integrated fixed-film activated sludge reactor

This study aimed to model and optimize mainstream deammonification in an integrated fixed-film activated sludge (IFAS) pilot plant under natural seasonal temperature variations. The effect of gradually decreasing temperature on the performance was evaluated during a winter season and a transition period to summer conditions, and the correlation of the performance parameters was investigated using principal component analysis (PCA). The optimization of intermittent aeration in the long-term (30 days) dynamic conditions with on/off ratio and dissolved oxygen (DO) set-point control was used to maximize the N-removal rate (NRR) and N-removal efficiency (NRE). Optimization results (DO set-point of 0.2-0.25 mgO₂/L, and on/off ratio of 0.05) increased the NRE and NRR of total inorganic N (daily average) from 30% to > 50% and 15 gN/m³d to 25 gN/m³d, respectively. This novel long-term optimization strategy is a powerful tool for enhancing the efficiency in mainstream deammonification.

Bacterial Dynamics and Their Influence on the Biogeochemical Cycles in a Subtropical Hypereutrophic Lake During the Rainy Season

Lakes in subtropical regions are highly susceptible to eutrophication due to the heavy rainfall, which causes significant runoff of pollutants (e.g., nutrients) to reach surface waters, altering the water quality and influencing the microbial communities that regulate the biogeochemical cycles within these ecosystems. Lake Cajititlán is a shallow, subtropical, and endorheic lake in western Mexico. Nutrient pollution from agricultural activity and wastewater discharge have affected the lake’s water quality, leading the reservoir to a hypereutrophic state, resulting in episodes of fish mortality during the rainy season. This study investigated the temporal dynamics of bacterial communities within Lake Cajititlán and their genes associated with the nitrogen, phosphorus, sulfur, and carbon biogeochemical cycles during the rainy season, as well as the influences of physicochemical and environmental variables on such dynamics. Significant temporal variations were observed in the composition of bacterial communities, of which Flavobacterium and Pseudomonas were the dominant genera. The climatological parameters that were most correlated with the bacterial communities and their functional profiles were pH, DO, ORP, turbidity, TN, EC, NH₄⁺, and NO₃^–. The bacterial communities displayed variations in their functional composition for nitrogen, phosphorus, and sulfur metabolisms during the sampling months. The bacterial communities within the lake are highly susceptible to nutrient loads and low DO levels during the rainy season. Bacterial communities had a higher relative abundance of genes associated with denitrification, nitrogen fixation, assimilatory sulfate reduction, cysteine, SOX system, and all phosphorus metabolic pathways. The results obtained here enrich our understanding of the bidirectional interactions between bacterial communities and major biogeochemical processes in eutrophic subtropical lakes.

Response of substrate kinetics and biological mechanisms to various pH constrains for cultured Nitrobacter and Nitrospira in nitrifying bioreactor

Nitrite (NO₂^-) oxidation is an essential step of biological nitrogen cycling in natural ecosystems, and is performed by chemolithoautotrophic nitrite-oxidizing bacteria (NOB). Although Nitrobacter and Nitrospira are regarded as representative NOB in nitrification systems, little attention has focused on kinetic characterisation of the coexistence of Nitrobacter and Nitrospira at various pH values. Here, we evaluate the substrate kinetics, biological mechanism and microbial community dynamics of an enrichment culture including Nitrobacter (17.5 ± 0.9%) and Nitrospira (7.2 ± 0.6%) in response to various pH constrains. Evaluation of the Monod equation at pH 6.0, 6.5, 7.0, 7.5, 8.0 and 8.5 showed that the enrichment had maximum rate (r_max) and maximum substrate affinity (K_S) for NO₂^- oxidation at pH 7.0, which was also supported by the largest absolute abundance of Nitrobacter nxrA (5.26 × 10⁷ copies per g wet sludge) and Nitrospira nxrB (1.975 × 10⁹ copies per g wet sludge) genes. Moreover, the predominant species for the Nitrobacter-like nxrA were N. vulgaris and N. winogradskyi, while for the Nitrospira-like nxrB, the predominant species were N. japonica, N. calida and Ca. N. bockiana. Furthermore, the r_max was strongly and positively correlated with the abundance of the Nitrobacter nxrA or Nitrospira nxrB genes, or N. winogradsk, whereas K_S was positively correlated with the abundance of Nitrobacter nxrA or Nitrospira nxrB genes or Ca. N. bockiana. Overall, this study could improve basis kinetic parameters and biological mechanism of NO₂^- oxidation in WWTPs.