Modeling nitrous oxide production by a denitrifying-enhanced biologically phosphorus removing (EBPR) activated sludge in the presence of different carbon sources and electron acceptors

Revealing the role of microalgae-bacteria niche for boosting wastewater treatment and energy reclamation in response to temperature

Conventional biological treatment usually cannot achieve the same high water quality as advanced treatment when conducted under varied temperatures. Here, satisfactory wastewater treatment efficiency was observed in a microalgae-bacteria consortia (MBC) over a wide temperature range because of the predominance of microalgae. Microalgae contributed more toward wastewater treatment at low temperature because of the unsatisfactory performance of the accompanying bacteria, which experienced cold stress (e.g., bacterial abundance below 3000 sequences) and executed defensive strategies (e.g., enrichment of cold-shock proteins). A low abundance of amoA-C and hao indicated that conventional nitrogen removal was replaced through the involvement of microalgae. Diverse heterotrophic bacteria for nitrogen removal were identified at medium and high temperatures, implying this microbial niche treatment contained diverse flexible consortia with temperature variation. Additionally, pathogenic bacteria were eliminated through microalgal photosynthesis. After fitting the neutral community model and calculating the ecological niche, microalgae achieved a maximum niche breadth of 5.21 and the lowest niche overlap of 0.38, while the accompanying bacterial community in the consortia were shaped through deterministic processes. Finally, the maximum energy yield of 87.4 kJ L^-1 and lipid production of 1.9 g L^-1 were achieved at medium temperature. Altogether, this study demonstrates that advanced treatment and energy reclamation can be achieved through microalgae-bacteria niche strategies.

Influences of carbon sources on N2O production during denitrification in freshwaters: Activity, isotopes and functional microbes

Denitrification is one of the major sources of N₂O in freshwaters. Diverse forms of organic compounds act as the electron donors for microbial denitrification. However, the influences of carbon sources on N₂O production, N₂O reduction, isotope fractionation and functional microbes during denitrification were largely unknown. In this study, five forms of carbon sources (i.e. acetate, citrate, glucose, cellobiose and leucine) were used to enrich denitrifiers in freshwater sediments. N₂O conversion in the enrichments was investigated by a combination of inhibition technique, natural stable isotope method and metagenomics. Acetylene was effective in inhibiting N₂O reduction without influencing the isotopic characteristics during N₂O production. Glucose led to the least N₂O production and reduction, in accordance with the lowest abundance of both NO and N₂O reductases in this enrichment. δ¹⁸O and site preference value (SP, =δ¹⁵N^α-δ¹⁵N^β) of N₂O were sensitive to discriminate the five carbon sources, except when comparing acetate and leucine. Isotopic values of N₂O were not significantly different in these two enrichments due to the similarity of NO reductases - Pseudomonas-type cNorB. Specifically, the enrichment with cellobiose produced N₂O with the lowest δ¹⁸O values (39.4‰±1.1‰), due to Alicycliphilus with both cNorB and qNorB. The enrichment with glucose led to the highest SP values (8.9‰±8.6‰), caused by Thiobacillus-type cNorB. Our results demonstrated the link between carbon sources, N₂O production and reduction, isotopic signatures, microbial populations and enzymes during denitrification in freshwaters.

A novel approach to estimate and control denitrification performance in activated sludge systems with respirogram technology

Respirogram technology has been widely applied for aerobic process, however, the response of respirogram to anoxic denitrification is still unclear. To reveal such response may help to design a new method for the evaluation of the performance of denitrification. The size distribution of flocs measured at different denitrification moments demonstrated a clear expansion of flocs triggered by denitrification, during which higher specific endogenous and quasi-endogenous respiration rates (SOUR_e and SOUR_q) were also observed. Furthermore, SOUR_q increases exponentially with the specific denitrification rate (SDNR), suggesting that there should be a maximum SDNR in conventional activated sludge systems. Based on these findings, an index R_q/t, defined as the ratio of quasi-endogenous (OUR_q) to maximum respiration rate (OUR_t), is proposed to estimate the denitrification capacity that higher R_q/t indicates higher denitrification potential, which can be readily obtained without complex measurement or analysis, and it offers a novel and promising respirogram-based approach for denitrification estimation and control by taking measures to extend anoxic time to maintain its value at a high level within a certain range.

Assessing Intermediate Formation and Electron Competition during Thiosulfate-Driven Denitrification: An Experimental and Modeling Study

There is increasing interest in thiosulfate-driven denitrification for low C/N wastewater treatment, but the denitrification performance varies with the thiosulfate oxidation pathways. Models have been developed to predict the products of denitrification, but few consider thiosulfate reduction to elemental sulfur (S⁰), an undesirable reaction that can intensify electron competition with denitrifying enzymes. In this study, the model using indirect coupling of electrons (ICE) was developed to predict S⁰ formation and electron competition during thiosulfate-driven denitrification. Kinetic data were obtained from sulfur-oxidizing bacteria (SOB) dominated by the branched pathway and were used to calibrate and validate the model. Electron competition was investigated under different operating conditions. Modeling results reveal that electrons produced in the first step of thiosulfate oxidation typically prioritize thiosulfate reduction, then nitrate reduction, and finally nitrite reduction. However, the electron consumption rate for S⁰ formation decreases sharply with the decline of thiosulfate concentration. Thus, a continuous feeding strategy was effective in alleviating the competition between thiosulfate reduction and denitrifying enzymes. Electron competition leads to nitrite accumulation, which could be a reliable substrate for anammox. The model was further evaluated with anammox integration. Results suggested that the branched pathway and continuous supply of thiosulfate are favorable to create a symbiotic relationship between SOB and anammox.

N2O reduction during denitrifying phosphorus removal with propionate as carbon source

Denitrifying phosphorus removal was realized in sequencing batch reactors using different carbon sources (acetate, propionate, and a mixture of acetate/propionate). Nutrient removal and nitrous oxide (N₂O) production were investigated, and the factors affecting N₂O production were explored. Nitrogen removal was 40.6% lower when propionate was used as the carbon source instead of acetate, while phosphorus removal was not significantly different. N₂O production was greatly reduced when propionate was used as the carbon source instead of acetate. The emission factor in the propionate system was only 0.43%, while those in the acetate and mixed-carbon source system were 16.3% and 1.9%, respectively. Compared to the propionate system, ordinary heterotrophic organisms (i.e., glycogen-accumulating organisms) were enriched in the acetate system, explaining the higher N₂O production in the acetate system. The lower nitrite accumulation in the propionate system compared to the acetate system was the dominant factor leading to the lower N₂O production.

Integrated omics analyses reveal differential gene expression and potential for cooperation between denitrifying polyphosphate and glycogen accumulating organisms

Unusually high accumulation of the potent greenhouse gas nitrous oxide (N₂ O) has previously been documented in denitrifying biological phosphorus (P) removal bioprocesses, but the roles of differential denitrification gene expression patterns and ecological interactions between key functional groups in driving these emissions are not well understood. To address these knowledge gaps, we applied genome-resolved metagenomics and metatranscriptomics to a denitrifying bioprocess enriched in as-yet-uncultivated denitrifying polyphosphate accumulating organisms (PAOs) affiliated with Candidatus Accumulibacter. The six transcriptionally most active populations in the community included three co-occurring Accumulibacter strains affiliated with clades IF (a novel clade identified in this study), IA and IC, a competing glycogen accumulating organism (GAO) affiliated with Competibacteraceae (GAO1), a Gammaproteobacteria PR6 and an Anaerolineae CH7. Strongly elevated expression of nitrite reductase genes compared to nitrous oxide reductase genes was observed in the overall community and in Accumulibacter populations, suggesting a strong role for differential gene expression in driving N₂ O accumulation. Surprisingly, while ~90% of the nirS gene transcripts were expressed by the three co-occurring PAO populations, ~93% of the norB gene transcripts were expressed by GAO1 and ~75% of the norZ gene transcripts were mapped to PR6 and several other non-PAO flanking populations. This suggests the potential for cooperation between flanking populations and PAOs in reducing denitrification intermediates. Such cooperation may benefit the community by reducing the accumulation of toxic nitric oxide.

Groundwater Nitrate Removal Performance of Selected Pseudomonas Strains Carrying nosZ Gene in Aerobic Granular Sequential Batch Reactors

Four granular sequencing batch reactors (GSBRs) were inoculated with four denitrifying Pseudomonas strains carrying nosZ to study the process of granule formation, the operational conditions of the bioreactors, and the carbon concentration needed for nitrate removal. The selected Pseudomonas strains were P. stutzeri I1, P. fluorescens 376, P. denitrificans Z1, and P. fluorescens PSC26, previously reported as denitrifying microorganisms carrying the nosZ gene. Pseudomonas denitrificans Z1 produced fluffy, low-density granules, with a decantation speed below 10 m h−1. However, P. fluorescens PSC26, P. stutzeri I1, and P. fluorescens 376 formed stable granules, with mean size from 7 to 15 mm, related to the strain and carbon concentration. P. stutzeri I1 and P. fluorescens 376 removed nitrate efficiently with a ratio in the range of 96%, depending on the source and concentration of organic matter. Therefore, the findings suggest that the inoculation of GSBR systems with denitrifying strains of Pseudomonas spp. containing the nosZ gene enables the formation of stable granules, the efficient removal of nitrate, and the transformation of nitrate into nitrogen gas, a result of considerable environmental interest to avoid the generation of nitrous oxide.

Nitrous oxide emission during denitrifying phosphorus removal process: A review on the mechanisms and influencing factors

Excessive emissions of nitrogen (N) and phosphorus (P) pollutants are leading to increased eutrophication of water bodies. Biological N and P removal processes have become a research priority in the field of sewage treatment with the aim of improving sewage discharge standards in countries worldwide. Denitrifying P removal processes are more efficient for solving problems related to carbon source competition, sludge age conflict, and high aeration energy consumption compared to traditional biological N and P removal processes, but they are easy to produce nitrous oxide (N₂O) in the process of sewage treatment. N₂O is a greenhouse gas with a global warming potential approximately 190-270 times that of CO₂ and 4-21 times that of CH₄, which was produced and released into the environmental in denitrifying P removal systems under conditions of a low C/N ratio, high dissolved oxygen, and low activity of denitrifying phosphorus accumulating organisms (DPAOs). This paper reviews the emission characteristics and influencing factors of N₂O during denitrifying P removal processes and proposes appropriate strategies for controlling the emission of N₂O. This work serves as a basis for the development of new sewage treatment processes and the reduction of greenhouse gas emissions in future wastewater treatment plants.

Recent advances in nitrous oxide production and mitigation in wastewater treatment

Nitrous oxide (N₂O) emitted from wastewater treatment plants has caused widespread concern. Over the past decade, people have made tremendous efforts to discover the microorganisms responsible for N₂O production, elucidate metabolic pathways, establish production models and formulate mitigation strategies. The ultimate goal of all these efforts is to shed new light on how N₂O is produced and how to reduce it, and one of the best ways is to find key opportunities by integrating the information obtained. This review article critically evaluates the knowledge gained in the field within a decade, especially in N₂O production microbiology, biochemistry, models and mitigation strategies, with a focus on denitrification. Previous research has greatly deepened the understanding of the N₂O generation mechanism, but further efforts are still needed due to the lack of standardized methodology for establishing N₂O mitigation strategies in full-scale systems. One of the challenges seems to be to convert the denitrification process from a net N₂O source into an effective sink, which is recommended as a key opportunity to reduce N₂O production in this review.

Position paper - progress towards standards in integrated (aerobic) MBR modelling

Membrane bioreactor (MBR) models are useful tools for both design and management. The system complexity is high due to the involved number of processes which can be clustered in biological and physical ones. Literature studies are present and need to be harmonized in order to gain insights from the different studies and allow system optimization by applying a control. This position paper aims at defining the current state of the art of the main integrated MBR models reported in the literature. On the basis of a modelling review, a standardized terminology is proposed to facilitate the further development and comparison of integrated membrane fouling models for aerobic MBRs.

Strategies for mitigating nitrous oxide production and decreasing the carbon footprint of a full-scale combined nitrogen and phosphorus removal activated sludge system

Nitrous oxide (N₂O) emitted from biological nutrient removal activated sludge systems contributes significantly to the total carbon footprint of modern wastewater treatment plants. In the present study, N₂O production and emissions were experimentally determined in a large-scale plant (220,000 PE) employing combined nitrogen (N) and phosphorus (P) removal. As a modelling tool, the Activated Sludge Model 2d (ASM2d) was extended with modules describing multiple N₂O production pathways and N₂O liquid-gas transfers. The new model was calibrated and validated using the results of laboratory experiments and full-scale measurements. Different operational strategies were evaluated following the proposed model-based procedure. Heterotrophic denitrification was found to be the predominant pathway of N₂O production under both anoxic and aerobic conditions. This behaviour could primarily be attributed to the predominant abundance of heterotrophic denitrifiers over nitrifiers. Simulations revealed that the optimal solution for minimizing liquid N₂O production is to set the dissolved oxygen concentration in the aerobic zone from 1 to 2 mg O₂/L and to enhance the mixed liquor recirculation rate (MLR) (>500% of the influent flowrate) while not compromising effluent standards. Regarding the actual conditions, the potential reduction in the carbon footprint was estimated to be 10% by applying the proposed operational strategy. The results suggest that considerable improvements can be achieved without substantial upgrades and increased costs.