Formation and quantification of soluble microbial products and N2O production by ammonia-oxidizing bacteria (AOB)-enriched activated sludge

Mitigating greenhouse gas emissions from municipal wastewater treatment in China

Municipal wastewater treatment plays an indispensable role in enhancing water quality by eliminating contaminants. While the process is vital, its environmental footprint, especially in terms of greenhouse gas (GHG) emissions, remains underexplored. Here we offer a comprehensive assessment of GHG emissions from wastewater treatment plants (WWTPs) across China. Our analyses reveal an estimated 1.54 (0.92-2.65) × 104 Gg release of GHGs (CO2-eq) in 2020, with a dominant contribution from N2O emissions and electricity consumption. We can foresee a 60-65% reduction potential in GHG emissions with promising advancements in wastewater treatment, such as cutting-edge biological techniques, intelligent wastewater strategies, and a shift towards renewable energy sources.

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.

Modeling nitrous oxide emissions in membrane bioreactors: Advancements, challenges and perspectives

Membrane bioreactors (MBRs) have become a well-established wastewater treatment technology owing to their extraordinary efficiency and low space advantage over conventional activated sludge processes. Although the extended activated sludge models can predict the general trend of nitrous oxide (N₂O) emissions in MBRs, the simulation results usually deviate from the actual values. This review critically evaluates the recent advances in the modeling of N₂O emissions in MBRs, and proposes future directions for the development and improvement of models that better match the MBR characteristics. The quantitative impact of MBR characteristics on N₂O emissions is identified as a key knowledge gap demanding urgent attention. Accurately clarification of the N₂O emission pathways governed by MBR characteristics is essential to improve the reliability and practicability of existing models. This article lays a momentous foundation for the optimization of N₂O models in MBRs, and proposes new demands for the next-generation model. The contents will assist academics and engineers in developing N₂O production models for accurate prediction.

Characteristics of the extracellular products of pure oxygen aerated activated sludge in batch mode

The effects of pure oxygen aeration on compositional characteristics of soluble microbial products (SMP) and extracellular polymeric substances (EPS) of the activated sludge acclimated in a sequential batch reactor (SBR) were explored in batch mode. The structure of the extracellular products would change with different aeration methods or aeration rates. The proportion of SMP of most oxygen aerated sludge was less than 10%, while that in air aerated sludge was as high as 30-40%. The proportion of TB-EPS decreased from 56.95% to 30.63%, and the proportion of LB-EPS increased obviously with the increase of oxygen aeration rate. The contents of the protein (PN) and the polysaccharide (PS) of extracellular products with oxygen aeration were significantly different, and the PN was much higher than PS (p < 0.05). The zeta potential of each component in activated sludge was negative, gradually decreasing with the progress of biological treatment. The fluorescence peaks in SMP, LB-EPS and TB-EPS with pure oxygen aeration were attributed to tryptophan PN-like and humic acid-like fractions. The results showed that the proportion of the components in the extracellular products could be regulated by adjusting the aeration rate and aeration mode, so as to optimize the treatment process of activated sludge.

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.

Characteristics of extracellular polymeric substances and soluble microbial products of activated sludge in a pulse aerated reactor

This study aimed to investigate the stratification characteristics of extracellular polymeric substances (EPS) and the properties of soluble microbial products (SMP) of the activated sludge with pulse aeration. The activated sludge was acclimated with aeration on/off time of 5 min/10 min for 60 days. The results showed that both polysaccharides (PS) and proteins (PN) increased in the loosely bound EPS (LB-EPS) and the tightly bound EPS (TB-EPS) with the increase of operational time. Both the PN/PS ratio and the total LB-EPS increased in the later period of the pulse aerated acclimation process. There was an obvious positive correlation between sludge volume index (SVI) and LB-EPS (R ² = 0.871), mainly due to the PS in LB-EPS which was also significantly correlated with SVI (R ² = 0.954). A downward trend of SMP concentrations occurred at the end of acclimation which was opposite to the upward change of EPS contents. Two obvious fluorescence peaks were detected respectively in EPS and SMP by 3D-EEM fluorescence spectroscopy. Peak A was detected in both LB-EPS and TB-EPS, which was associated with tryptophan protein-like substances. Peak B representing humus carbon and carboxylic acids was mainly detected in SMP. The release of humus-like components in SMP from activated sludge was mainly in accordance with the dissolution and hydrolysis of PN in TB-EPS.

Effects of domestic sewage from different sources on greenhouse gas emission and related microorganisms in straw-returning paddy fields

Reusing domestic sewage for crop irrigation is a promising practice, particularly in developing countries, since it is a substitute for chemical fertilizer and reduces water contamination. More attention was paid to the effect of sewage irrigation on crop yield and soil nutrients, but little attention was paid to greenhouse gas (GHG) emission from straw-returning paddy fields. In this study, a soil column monitoring experiment was conducted to assess the effects of untreated domestic sewage (dominated with ammonia) and treated domestic sewage (dominated with nitrate) irrigation on methane (CH₄), nitrous oxide (N₂O) emission, and related soil microorganisms in straw-returning paddy fields. Results showed that straw-returning dramatically promoted CH₄ emission but had little effect on N₂O emission. Both untreated and treated domestic sewage irrigation decreased CH₄ emission of straw-returning paddy whether nitrogen fertilizer applied or not. The mitigating effect of treated sewage irrigation on CH₄ emission was greater than untreated sewage irrigation. CH₄ emission had a significant correlation with the abundance of soil methanogens and methanogens/methanotrophs. N₂O emission increased with untreated or treated domestic sewage irrigation, although the total N input, including the N carried by sewage water, was the same for all treatments. No significant correlation between N₂O and denitrification functional genes was found in this study. Treated domestic sewage irrigation reduced the global warming potential (GWP) by 66.7%, but untreated domestic sewage had no evident influence on the GWP. Results indicated that treated domestic sewage irrigation could significantly inhibit CH₄ emission and the GWP by decreasing the ratio of methanogens to methanotrophs, and is promising in mitigating GWP from straw-returned paddy fields.

Exploring the linkage between free nitrous acid accumulation and nitrous oxide emissions in a novel static/oxic/anoxic process

In this work, four batch tests were conducted to comprehensively explore the effects of free nitrous acid (FNA) accumulation on nitrous oxide (N₂O) emissions in a novel energy-saving and N₂O-reducing static/oxic/anoxic (SOA) process. With the accumulation of FNA, the N₂O emission factor increased from 1.51% to 4.32%, and the N₂O emission ratio contributed by ammonia-oxidizing bacteria (AOB) increased from 74.0% to 78.6%, accordingly. Mechanism studies show that produced FNA and weakened aerobic metabolism bring synergy to competition between reductases. Aeration conditions and FNA cytotoxicity exert a greater impact on nitrite-oxidizing bacteria than on AOB, thus enhancing the potential for nitrite accumulation. Considering the removal of nitrogen and phosphorus and the reduction of N₂O emissions in the SOA process, it is feasible to keep the average dissolved oxygen above 2.0 mg/L under the premise of nitrite accumulation.

Reducing Cost and Environmental Impact of Wastewater Treatment with Denitrifying Methanotrophs, Anammox, and Mainstream Anaerobic Treatment

In water resource recovery facilities, sidestream biological nitrogen removal via anaerobic ammonium oxidation (anammox) is more energy and cost efficient than conventional nitrification-denitrification. However, under mainstream conditions, nitrite oxidizing bacteria (NOB) out-select anammox bacteria for nitrite produced by ammonium oxidizing bacteria (AOB). Therefore, nitrite production is the bottleneck in mainstream anammox nitrogen removal. Nitrate-dependent denitrifying anaerobic methane oxidizing archaea (n-damo) oxidize methane and reduce nitrate to nitrite. The nitrite supply challenge in mainstream anammox implementation could be solved with a microbial community of AOB, NOB, n-damo, and anammox with methane from anaerobic sludge digestion or a mainstream anaerobic membrane bioreactor (AnMBR). The cost and environmental impact of traditional nitrification/dentrification relative to AOB/anammox and AOB/anammox/n-damo systems, with and without an AnMBR, were compared with a stoichiometric model. AnMBR implementation reduced costs and emission rates at moderate to high nutrient loading by lowering aeration and sludge handling demands while increasing methane available for cogeneration. AnMBR/AOB/anammox systems reduced cost and GHG emission by up to $0.303/d/m³ and 1.72 kg equiv. CO₂/d/m³, respectively, while AnMBR/AOB/anammox/n-damo systems saw a similar reduction of at least $0.300/d/m³ and 1.65 kg equiv. CO₂/d/m³ in addition to alleviating the necessity to stop nitrification at nitrate, allowing easier aeration control.

Heterotrophic denitrifiers growing on soluble microbial products contribute to nitrous oxide production in anammox biofilm: Model evaluation

In this work, a model framework was constructed to assess and predict nitrous oxide (N₂O) production, substrate and microbe interactions in an anammox biofilm bioreactor. The anammox kinetics were extended by including kinetics of autotrophic soluble microbial products (SMP) formation, which consisted of utilization-associated products (UAP) and biomass-associated products (BAP). Heterotrophic bacteria growing on UAP, BAP and decay released substance (SS) were modelled to perform four-step sequential reductions from nitrate to dinitrogen gas. N₂O was modelled as an intermidiate of heterotrophic denitrification via three pathways with UAP, BAP and SS as the electron donors. The developed model framework was evaluated using long-term operational data from an anammox biofilm reactor and satisfactorily reproduced effluent nitrogen and SMP as well as N₂O emission factors under different operational conditions. The modeling results revealed that N₂O was mainly produced with UAP as the electron donor while BAP and SS play minor roles. Heterotrophic denitrifiers growing on UAP would significantly contribute to N₂O emission from anammox biofilm reactor even though heterotrophs only account for a relatively small fraction of active biomass in the anammox biofilm. Comprehensive simulations were conducted to investigate the effects of N loading rate and biofilm thickness, which indicated that maintaining a low N loading rate and a thick biofilm thickness were essential for high total nitrogen removal efficiency and low N₂O emission.

A review on nitrous oxide (N2O) emissions during biological nutrient removal from municipal wastewater and sludge reject water

Nitrous oxide (N₂O) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as nitritation/denitritation and completely autotrophic N-removal. The microbial pathways leading to the N₂O emission include hydroxylamine oxidation and nitrifier denitrification, both activated by ammonia oxidizing bacteria, and heterotrophic denitrification. In this work, a critical review of the existing literature on N₂O emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the N₂O emissions either per se or combined are identified: low dissolved oxygen, high nitrite accumulation, low chemical oxygen demand to nitrogen ratio, slow growth of denitrifying bacteria, uncontrolled pH and temperature. However, there is no common pattern in reporting the N₂O generation amongst the cited studies, a fact that complicates its evaluation. When simulating N₂O emissions, all microbial pathways along with the potential contribution of abiotic N₂O production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of such models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous N₂O dynamics. Thus, the choice of the N-removal process, the optimal selection of operational parameters and the establishment of validated dynamic models combining multiple N₂O pathways are essential for studying the emissions mitigation.

Quantification of nitrous oxide (N2O) emissions and soluble microbial product (SMP) production by a modified AOB-NOB-N2O-SMP model

A modified AOB-NOB-N₂O-SMP model able to quantify nitrous oxide (N₂O) emissions and soluble microbial product (SMP) production during wastewater treatment is proposed. The modified AOB-NOB-N₂O-SMP model takes into account: (1) two-step nitrification by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), (2) N₂O production by AOB denitrification under oxygen-limited conditions and (3) SMP production by microbial growth and endogenous respiration. Validity of the modified model is demonstrated by comparing the simulation results with experimental data from lab-scale sequencing batch reactors (SBRs). To reliably implement the modified model, a model calibration that adjusts model parameters to fit the model outputs to the experimental data is conducted. The results of this study showed that the modeling accuracy of the modified AOB-NOB-N₂O-SMP model increases by 19.7% (NH₄), 51.0% (NO₂), 57.8% (N₂O) and 16.7% (SMP) compared to the conventional model which does not consider the two-step nitrification and SMP production by microbial endogenous respiration.

Effects of soluble microbial products (SMP) on the performance of an anammox attached film expanded bed (AAFEB) reactor: Synergistic interaction and toxic shock

The accumulation of soluble microbial production (SMP) in an anammox attached film expanded bed (AAFEB) and its effect on the reactor performance were investigated in this study. During the long-term experiment, an extended HRT resulted in the accumulation of SMP and the change of treatment performance. When the SMP increased from 10.5±1.5mgL^-1 to 31.7±6.4mgL^-1 with the increase of influent TN concentration from 313mgL^-1 to 2500mgL^-1, the TN removal efficiency was stable. However, when the influent TN concentration was 3500mgL^-1, the SMP concentration increased higher than 100mgL^-1, the reactor soon became inhibited. Bath tests indicated that both the specific anammox activity (SAA) and the substrate tolerance ability decreased during the stable operation phases, whereas the specific denitrification activity (SDA) was significantly enhanced. In addition, N₂O emissions in the anammox-denitrifier symbiotic system were greater than in the conventional nitrogen removal process.

Assessment of Heterotrophic Growth Supported by Soluble Microbial Products in Anammox Biofilm using Multidimensional Modeling

Anaerobic ammonium oxidation (anammox) is known to autotrophically convert ammonium to dinitrogen gas with nitrite as the electron acceptor, but little is known about their released microbial products and how these are relative to heterotrophic growth in anammox system. In this work, we applied a mathematical model to assess the heterotrophic growth supported by three key microbial products produced by bacteria in anammox biofilm (utilization associated products (UAP), biomass associated products (BAP), and decay released substrate). Both One-dimensional and two-dimensional numerical biofilm models were developed to describe the development of anammox biofilm as a function of the multiple bacteria-substrate interactions. Model simulations show that UAP of anammox is the main organic carbon source for heterotrophs. Heterotrophs are mainly dominant at the surface of the anammox biofilm with small fraction inside the biofilm. 1-D model is sufficient to describe the main substrate concentrations/fluxes within the anammox biofilm, while the 2-D model can give a more detailed biomass distribution. The heterotrophic growth on UAP is mainly present at the outside of anammox biofilm, their growth on BAP (HetB) are present throughout the biofilm, while the growth on decay released substrate (HetD) is mainly located in the inner layers of the biofilm.

Performance of aerobic granular sludge in a sequencing batch bioreactor for slaughterhouse wastewater treatment

Lab-scale experiment was conducted to investigate the formation and characteristics of aerobic granular sludge for biological nutrient removal of slaughterhouse wastewater. Experimental results showed that removal performances of chemical oxygen demand (COD), ammonia and phosphate were enhanced with sludge granulation, and their removal efficiencies reached 95.1%, 99.3% and 83.5%, respectively. The aerobic granular sludge was matured after 90days cultivation, and protein-like substances were the main components. Simultaneously, the mass ratio of proteins and polysaccharides (PN/PS) was enhanced to 2.5 from 1.7. The granules with particle sizes of 0.6-1.2 and 1.2-1.8mm, accounting for 69.6%, were benefit for the growth of ammonia oxidizing bacteria (AOB) and nitrate oxidizing bacteria (NOB), and corresponding specific oxygen demand rates (SOUR) of AOB and NOB were 31.4 and 23.3mgO2/gMLSSh, respectively.