Identification of key water parameters and microbiological compositions triggering intensive N2O emissions during landfill leachate treatment process

Towards a high-rate operation of contact stabilization process: A microscopic view of carbon capture properties

Carbon capture performance is a key factor determining the chemical energy recovery potential of the high-rate contact stabilization (HiCS) process. However, the mechanisms of organic carbon capture are complex, involving surface adsorption, extracellular adsorption, and intracellular storage. A unique characteristic of the HiCS process is its low sludge residence time (SRT). Unfortunately, the influence of SRT on carbon capture has not been thoroughly studied, especially in terms of the underlying mechanisms. In this study, the microscopic changes in carbon capture performance during the transition from a conventional contact stabilized (CS) system to a high-rate mode of operation were demonstrated using intracellular carbon sources, extracellular polymeric substances (EPS), signaling molecules, and microbial community assays. The results showed that the extracellular carbon adsorption and intracellular carbon storage performance increased, and the microbial community structure changed significantly with converting the CS system to the high-rate operation mode. The enhancement of extracellular carbon adsorption performance mainly relied on the growth of EPS, which was accomplished by the strong growth of the relative abundance of the dominant bacterial group Cloacibacterium within the HiCS system, offsetting the negative effect produced by the decline of acyl-homoserine lactones. 98 mgCOD/gSS, 343 mgCOD/gSS, and 500 mgCOD/gSS of polyhydroxyalkanoates (PHAs) per sludge unit were obtained at SRT-24d, 8d, and 2d, respectively, suggesting that the HiCS system is more advantageous for rapid PHAs production.

Dual isotope labelling combined with multi-omics analysis revealing the N2O source evolution in aerobic biological systems driven by salinity gradient

Salinity is considered a major factor influencing nitrous oxide (N2O) emissions from biochemical treatment of high-salinity wastewater, but its mechanism has not been thoroughly investigated. In this study, we investigated the effects of salinity on N2O emissions under aerobic conditions. As salinity rose from 0.66 % to 3.66 %, N2O emission flux first increased and then decreased, while the emission factor (EF) consistently increased, likely due to significant inhibition of nitrification at 3.66 % salinity. Nitrogen‑oxygen dual isotope labeling experiments demonstrated that the dominant N2O production pathway shifted with salinity: from nitrifier nitrification (NN, 36.07 %-40.97 %) at low salinity (0.66 %, 1.66 %), to nitrification-coupled denitrification (NCD, 51.67 %) at 2.66 %, and to nitrifier denitrification (ND, up to 80.81 %) at the salinity of 3.66 %. From the changes in bacterial relative abundances and expressions of 4 key functional genes (amoA, hao, nor, and nosZ) revealed by metatranscriptomic sequencing, Nitrosomonas, unclassified Rhodospirillales, and Nitrospira were identified as key contributors to NN, NCD, and ND pathways, respectively, as salinity increased. We also found that the differential expressed genes and metabolites involved in energy metabolism, oxidative phosphorylation, and metabolism of amino acids, pyrimidines, and nucleotides may affect N-cycling bacteria, thereby influencing nitrogen conversion and salinity tolerance as well. This study sheds light on nitrification process in response to salinity stress and offers insights for mitigating greenhouse gas emissions from high-salinity wastewater treatment.

Towards a high-rate operation of contact stabilization process: Challenges of flocculation and floc stability

The high-rate contact stabilization (HiCS) process, a variant of high-rate activated sludge, has gained attention for its superior energy recovery and enhanced biosorption capabilities. The need for efficient energy recovery in HiCS necessitates a high settling efficiency to minimize resource loss due to endogenous sludge consumption. However, the low sludge retention time (SRT) required for HiCS can significantly affect sludge floc stability and flocculation performance, warranting a deeper analysis of the factors influencing these characteristics. This study investigates the impact of SRT reduction on sludge performance, focusing on energy potential, viscoelasticity, and critical pressure. The analysis was conducted using rheological tests, contact angle measurements, zeta potential analysis, Fourier transform infrared spectroscopy, XDLVO theory, and the PARAFAC model. Results indicate that due to the contribution of hydrophobicity, the HiCS system maintained the large flocs morphology of the sludge even when the SRT was maintained for 2d. However, a combination of aerobic microbial activity, high concentrations of loosely bound extracellular polymeric substances, and the presence of the filamentous bacterium Thiothrix contributed to reduced flocculation performance.

Using machine learning to identify environmental factors that collectively determine microbial community structure of activated sludge

Activated sludge (AS) microbial communities are influenced by various environmental variables. However, a comprehensive analysis of how these variables jointly and nonlinearly shape the AS microbial community remains challenging. In this study, we employed advanced machine learning techniques to elucidate the collective effects of environmental variables on the structure and function of AS microbial communities. Applying Dirichlet multinomial mixtures analysis to 311 global AS samples, we identified four distinct microbial community types (AS-types), each characterized by unique microbial compositions and metabolic profiles. We used 14 classical linear and nonlinear machine learning methods to select a baseline model. The extremely randomized trees demonstrated optimal performance in learning the relationship between environmental factors and AS types (with an accuracy of 71.43%). Feature selection identified critical environmental factors and their importance rankings, including latitude (Lat), longitude (Long), precipitation during sampling (Precip), solids retention time (SRT), effluent total nitrogen (Effluent TN), average temperature during sampling month (Avg Temp), mixed liquor temperature (Mixed Temp), influent biochemical oxygen demand (Influent BOD), and annual precipitation (Annual Precip). Significantly, Lat, Long, Precip, Avg Temp, and Annual Precip, influenced metabolic variations among AS types. These findings emphasize the pivotal role of environmental variables in shaping microbial community structures and enhancing metabolic pathways within activated sludge. Our study encourages the application of machine learning techniques to design artificial activated sludge microbial communities for specific environmental purposes.

Characteristics and key driving factors of nitrous oxide emissions from a full-scale landfill leachate treatment system

The characteristics of N2O emission from a full-scale landfill leachate treatment system were investigated by in-situ monitoring over 1.4 years and driving factors responsible for these emissions were identified by statistical analysis of multidimensional environmental variables. The results showed that the maximum N2O emission flux of 2.21 × 107 mg N·h-1 occurred in the nitrification tanks, where 98.5 % of the total N2O was released, with only 1.5 % of the total N2O emitted from the denitrification tanks. Limited oxygen in nitrification tank was responsible for N2O hotspot. The N2O emissions from the parallel lines A and B (both comprising the primary biochemical system) accounted for 52.6 % and 46.6 %, respectively, while the secondary biochemical system contributed only 0.8 % to the total emissions. Higher nitrite concentration in line A and lower nitrogen loading in the secondary biochemical system caused these discrepancies. We found that during the steady state of leachate treatment, intensive N2O emissions of 253.4-1270.5 kg N·d-1 were measured. The corresponding N2O emission factor (EF) ranged from 8.86 to 49.6 %, much higher than those of municipal wastewater treatment. But N2O EF was inconceivably as low as 0.42 % averagely after system maintenance. Influent with low salinity was the key reason, followed by the high MLSS and varying microbial community after maintenance. The dominant genus shifted from Lentimicrobium and Thauera to Norank-F-Anaerolineaceae and Unclassified-F-Rhodocyclaceae. This study underscores the significance of landfill leachate treatment in urban nitrogen management and provides valuable insights into the characteristics and driving factors of N2O emissions from such systems. The findings offer important references for greenhouse gas emission inventories and strategies for N2O control in full-scale wastewater treatment plants.

Low-voltage stimulated denitrification performance of high-salinity wastewater using halotolerant microorganisms

Nitrate is a common contaminant in high-salinity wastewater, which has adverse effects on both the environment and human health. However, conventional biological treatment exhibits poor denitrification performance due to the high-salinity shock. In this study, an innovative approach using an electrostimulating microbial reactor (EMR) was explored to address this challenge. With a low-voltage input of 1.2 V, the EMR reached nitrate removal kinetic parameter (kNO3-N) of 0.0166-0.0808 h-1 under high-salinities (1.5 %-6.5 %), which was higher than that of the microbial reactor (MR) (0.0125-0.0478 h-1). The mechanisms analysis revealed that low-voltage significantly enhanced microbial salt-in strategy and promoted the secretion of extracellular polymeric substances. Halotolerant denitrification microorganisms (Pseudomonas and Nitratireductor) were also enriched in EMR. Moreover, the EMR achieved a NO3-N removal efficiency of 73.64 % in treating high-salinity wastewater (salinity 4.69 %) over 18-cycles, whereas the MR only reached 54.67 %. In summary, this study offers an innovative solution for denitrification of high-salinity wastewater.

Start-up of pilot-scale ANAMMOX reactor for low-carbon nitrogen removal from anaerobic digestion effluent of kitchen waste

The pilot-scale simultaneous denitrification and methanation (SDM)-partial nitrification (PN)-anaerobic ammonia oxidation (Anammox) system was designed to treat anaerobic digestion effluent of kitchen waste (ADE-KW). The SDM-PN was first started to avoid the inhibition of high-concentration pollutants. Subsequently, Anammox was coupled to realize autotrophic nitrogen removal. Shortcut nitrification-denitrification achieved by the SDM-PN. The NO2--N accumulation (92 %) and NH4+-N conversion (60 %) were achieved by PN, and the removal of TN and COD from the SDM-PN was 70 % and 73 %, respectively. After coupling Anammox, the TN (95 %) was removed with a TN removal rate of 0.51 kg·m-3·d-1. Microbiological analyses showed a shift from dominance by Methanothermobacter to co-dominance by Methanothermobacter, Thermomonas, and Flavobacterium in SDM during the SDM-PN. While after coupling Anammox, Candidatus kuenenia was enriched in the Anammox zone, the SDM zone shifted back to being dominated by Methanothermobacter. Overall, this study provides new ideas for the treatment of ADE-KW.

Micron zero-valent iron chitosan hydrogel balls boosts nitrate removal in constructed wetlands for secondary effluent treatment

Reducing nitrate in the secondary effluent from municipal wastewater treatment plants can prevent eutrophication, which can be achieved by constructed wetlands. Zero-valent iron has been used as electron donors for nitrate removal in constructed wetlands to deal with the low carbon-to-nitrogen ratio (C/N) problem, but the effects are often limited by passivation. In this study, micron zero-valent iron chitosan hydrogel balls were prepared as part of the substrate. The total nitrogen removal efficiency maintained at 85 %-96 % in 70 days. The chelating ability of chitosan could reduce the formation of iron oxides on the surface of iron particles and microbial cells, thus eliminating the passivation. Denitrification microorganisms were enriched and the expressions of denitrification genes were increased. The study provides new understandings of further improving the nitrate removal efficiency of constructed wetlands under low C/N and efficient use of iron materials.

Overlooked role in bacterial assembly of different-sized granules in same sequencing batch reactor: Insights into bacterial niche of nutrient removal

The unique ecosystem within different-sized granules affects microbial assembly, which is crucial for wastewater treatment performance. This study operated an aerobic granular sludge system to evaluate its performance in treating synthetic municipal wastewater. Subsequently, the microbial community within different-sized granules was characterized to investigate bacterial assembly, and elucidated their biological potential for nutrient removal. The nutrient removal efficiencies were as follows: 93.8 ± 2.8 % chemical oxygen demand, 65.4 ± 4.0 % total nitrogen, and 93.8 ± 6.8 % total phosphorus. The analysis of microbial assembly unveiled remarkable diversity among different-sized sludges, the genus relative abundance displayed 61.4 % positive and 33.0 % negative correlation with sludge size. The excellent potential for organic degradation, denitrification, and polyphosphate accumulation occurred in sludge sizes of > 0.75 mm, 0.20-0.50 mm, and < 0.20 mm, respectively. Functional annotation further confirmed the nutrient removal potential within different-sized sludges. This study provides valuable insights into the bacterial niche of different-sized sludges, which can enhance their practical application.

Organic substitution stimulates ammonia oxidation-driven N2O emissions by distinctively enriching keystone species of ammonia-oxidizing archaea and bacteria in tropical arable soils

Partial organic substitution (POS) is pivotal in enhancing soil productivity and changing nitrous oxide (N₂O) emissions by profoundly altering soil nitrogen (N) cycling, where ammonia oxidation is a fundamental core process. However, the regulatory mechanisms of N₂O production by ammonia oxidizers at the microbial community level under POS regimes remain unclear. This study explored soil ammonia oxidation and related N₂O production, further building an understanding of the correlations between ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) activity and community structure in tropical arable soils under four-year field management regimes (CK, without fertilizer N; N, with only inorganic N; M1N1, with 1/2 organic N + 1/2 inorganic N; M1N2, with 1/3 organic N + 2/3 inorganic N). AOA contributed more to potential ammonia oxidation (PAO) than AOB across all treatments. In comparison with CK, N treatment had no obvious effects on PAO and lowered related N₂O emissions by decreasing soil pH and downregulating the abundance of AOA- and AOB-amoA. POS regimes significantly enhanced PAO and N₂O emissions relative to N treatment by promoting the abundances and contributions of AOA and AOB. The stimulated AOA-dominated N₂O production under M1N1 was correlated with promoted development of Nitrososphaera. By contrast, the increased AOB-dominated N₂O production under M1N2 was linked to the enhanced development of Nitrosospira multiformis. Our study suggests organic substitutions with different proportions of inorganic and organic N distinctively regulate the development of specific species of ammonia oxidizers to increase associated N₂O emissions. Accordingly, appropriate options should be adopted to reduce environmental risks under POS regimes in tropical croplands.

Generation, toxicity, and reduction of chlorinated byproducts: Overcome bottlenecks of electrochemical advanced oxidation technology to treat high chloride wastewater

Electrochemical advanced oxidation process (EAOP) is recommended for high-strength refractory organics wastewater treatment, but the accompanying chlorinated byproduct generation becomes a bottleneck that limits the application of this technology to actual wastewater. In this study, we applied EAOP (0.4-40 mA cm^-2) to treat ultrafiltration effluent of an actual landfill leachate, and quantitatively assessed the toxicities of the dominant chlorinated byproducts in EAOP-treated effluent. Considering both toxic effect and dose, it followed the order: active chlorine > chlorate > perchlorate > organochlorines. The toxic active chlorine could spontaneously decompose by settling. And secondary bioreactor originally serving for denitrification could be used to reduce perchlorate and chlorate. The effects of residual active chlorine and extra carbon addition on simultaneous denitrification, perchlorate, and chlorate reduction were investigated. It seemed that 20 mg of active chlorine was an acceptable level to bioactivity, and sufficient electron donors favored the removal of chlorate and perchlorate. Pseudomonas was identified as an active chlorine tolerant chlorate-reducing bacteria. And Thauera was responsible for perchlorate reduction under the conditions of sufficient carbon source supply. Our results confirmed that the perchlorate and chlorate concentrations in the effluent below their health advisory levels were achievable, solving the issue of toxic chlorinated byproduct generation during EAOP. This study provided a solution to realistic application of EAOP to treat high chloride wastewater.

Insights into N2O turnovers under polyethylene terephthalate microplastics stress in mainstream biological nitrogen removal process

The ubiquitous microplastics in wastewater have raised growing concerns due to their unintended effects on microbial activities. However, whether and how microplastics affect nitrous oxide (N₂O) (a potent greenhouse gas) turnovers in mainstream biological nitrogen removal (BNR) process remain unclear. This work therefore aimed to fill such knowledge gap by conducting both long-term and batch tests. After over 100 days of feeding with wastewater containing polyethylene terephthalate (PET) microplastics (0-500 μg/L), the long-term results showed that both production and reduction of N₂O during denitrification were reduced, as well as the N₂O production during nitrification. Accordingly, 60% reduction in N₂O accumulation and 70% reduction in N₂O production were observed in the denitrification and nitrification batch tests, respectively. Nevertheless, the long-term N₂O emission factors under PET microplastics stress were comparable to that in the control reactor, mainly because PET microplastics led to more nitrite accumulation in anoxic period. With the aid of online N₂O sensors and site-preference analysis, it was demonstrated that the heterotrophic bacteria pathway and ammonia oxidizing bacteria denitrification pathway for N₂O production were negatively affected by PET microplastics, whereas a clear increase in the contribution of hydroxylamine pathway (+ 22.9%) was observed. Further investigation revealed that PET microplastics even at environmental level (i.e. 10 μg/L) significantly reshaped the BNR sludge characteristics (e.g. much larger particle size) and microbial communities (e.g. Thauera, Rhodobacte and Nitrospira) as well as the nitrogen metabolism pathways, which were chiefly responsible for the changes of N₂O turnovers and N₂O production pathways under the PET microplastics stress.