Variation of effluent organic matter (EfOM) during anaerobic/anoxic/oxic (A2O) wastewater treatment processes

Destabilization mechanisms of Semi-aerobic aged refuse biofilters under harsh treatment conditions: Evidence from fluorescence and microbial characteristics

Semi-aerobic aged refuse biofilters (SAARB) are commonly-used biotechnologies for treating landfill leachate. In actual operation, SAARB often faces harsh conditions characterized by high concentrations of chemical oxygen demand (COD) and Cl-, as well as a low carbon-to-nitrogen ratio (C/N), which can disrupt the microbial community within SAARB, leading to operational instability. Maintaining the stable operation of SAARB is crucial for the efficient treatment of landfill leachate. However, the destabilization mechanism of SAARB under harsh conditions remains unclear. To address this, the study simulated the operation of SAARB under three harsh conditions, namely, high COD loading (H-COD), high chloride ion (Cl-) concentration environment (H-Cl-), and low C/N ratio environment (L-C/N). The aim is to reveal the destabilization mechanism of SAARB under harsh conditions by analyzing the fluorescence characteristics of effluent DOM and the microbial community in aged refuse. The results indicate that three harsh conditions have different effects on SAARB. H-COD leads to the accumulation of proteins; H-Cl- impedes the reduction of nitrite nitrogen; L-C/N inhibits the degradation of humic substances. These outcomes are attributed to the specific effects of different factors on the microbial communities in different zones of SAARB. H-COD and L-C/N mainly affect the degradation of organic matter in aerobic zone, while H-Cl- primarily impedes the denitrification process in the anaerobic zone. The abnormal enrichment of Corynebacterium, Castellaniella, and Sporosarcina can indicate the instability of SAARB under three harsh conditions, respectively. To maintain the steady operation of SAARB, targeted acclimation of the microbial community in SAARB should be carried out to cope with potentially harsh operating conditions. Besides, timely mitigation of loads should be implemented when instability characteristics emerge, and carbon sources and electron donors should be provided to restore treatment performance effectively.

Molecular-level insights into dissolved organic matter and its variations of the full-scale processes in a typical petrochemical wastewater treatment plant

Petrochemical wastewater (PCWW) treatment poses challenges due to its unique and complex dissolved organic matter (DOM) composition, originating from various industrial processes. Despite the addition of advanced treatment units in PCWW treatment plants to meet discharge standards, the mechanisms of molecular-level sights into DOM reactivity of the upgraded full-scale processes including multiple biological treatments and advanced treatment remain unclear. Herein, we employ water quality indexes, spectra, molecular weight (MW) distribution, and Fourier transform ion cyclotron resonance mass spectrometry to systematically characterize DOM in a typical PCWW treatment plant including influent, micro-oxygen hydrolysis acidification (MOHA), anaerobic/oxic (AO), and micro-flocculation sand filtration-catalytic ozonation (MFSF-CO). Influent DOM is dominated by tryptophan-like and soluble microbial products with MW fractions 〈 1 kDa and 〉 100 kDa, and CHO with lignin and aliphatic/protein structures. MOHA effectively degrades macromolecular CHO (10.86 %) and CHON (5.24 %) compounds via deamination and nitrogen reduction, while AO removes CHOS compounds with MW < 10 kDa by desulfurization, revealing distinct DOM conversion mechanisms. MFSF-CO transforms unsaturated components to less aromatic and more saturated DOM through oxygen addition reactions and shows high CHOS and CHONS reactivity via desulfurization and deamination reactions, respectively. Moreover, the correlation among multiple parameters suggests UV254 combined with AImod as a simple monitoring indicator of DOM to access the chemical composition. The study provides molecular-level insights into DOM for the contribution to the improvement and optimization of the upgraded processes in PCWW.

Exploring transformation of dissolved organic matters and dissolved organic nitrogen in full-scale anammox wastewater treatment: Temperature and microbial roles

Variation of dissolved organic matters (DOM) in mainstream anammox process has received limited attention. This study systematically characterized DOM and dissolved organic nitrogen (DON) in a full-scale mainstream anammox wastewater treatment plant (WWTP) using spectroscopy and Fourier transform-ion cyclotron resonance mass spectrometry. Roles of bacterial community structures related with temperatures on DOM and DON transformations were analyzed. Results indicated that the WWTP removed highly bioavailable, S-containing DOM while producing more unsaturated, aromatic, and N-containing DOM. Higher relative abundances of Proteobacteria and Chloroflexi at low temperature resulted in greater removal rates of proteins, SMP-like and humic acid-like substances. At high temperature, higher relative abundance of Actinobacteriota increased lignin production. Principal component analysis revealed that temperature significantly impacted DOM characteristics compared to DON. These findings are crucial for understanding DOM and DON transformation during mainstream anammox WWTP.

Phycospheric bacteria limits the effect of nitrogen and phosphorus imbalance on diatom bloom

Human activities have caused an imbalance in the input nitrogen and phosphorus (N/P) in the biosphere. The imbalance of N/P is one of the characteristics of water eutrophication, which is the fundamental factor responsible for the blooms. The effects of the N/P imbalance on diatom and phycospheric bacteria in blooms are poorly understood. In this study, the N/P molar ratio in real water (14:1) and the predicted N/P molar ratio in future water (65:1) were simulated to analyze the response of Cyclotella sp. and phycospheric bacteria to the N/P imbalance. The results showed that the N/P imbalance inhibited the growth of Cyclotella sp., but prolonged diatom bloom duration. The resistance of Cyclotella sp. to the N/P imbalance is related to phycospheric bacteria, and there are dynamic regulatory mechanisms within the phycospheric bacteria community to resist the N/P imbalance: (1) the increase of HNA bacterial density, the decrease of LNA bacterial density, (2) the increase of phycospheric bacterial diversity and eutrophic bacteria abundance, and the change of denitrifying bacteria abundance, (3) the activity of nitrogen and phosphorus metabolism of HNA bacteria enhanced, while that of LNA bacteria decreased. And the gene hosts of nitrogen and phosphorus metabolism were most enriched in Proteobacteria, indicating that Proteobacteria played an important role in maintaining the stability of phycospheric bacteria and was the dominant phylum resistant to the N/P imbalance. This study clarified that the algal-bacteria system was resistant to the N/P imbalance and implied that the N/P imbalance had little effect on the occurrence of diatom bloom events due to the presence of phycospheric bacteria.

Temperature-Driven Activated Sludge Bacterial Community Assembly and Carbon Transformation Potential: A Case Study of Industrial Plants in the Yangtze River Delta

Temperature plays a critical role in the efficiency and stability of industrial wastewater treatment plants (WWTPs). This study focuses on the effects of temperature on activated sludge (AS) communities within the A2O process of 19 industrial WWTPs in the Yangtze River Delta, a key industrial region in China. The investigation aims to understand how temperature influences AS community composition, functional assembly, and carbon transformation processes, including CO2 emission potential. Our findings reveal that increased operating temperatures lead to a decrease in alpha diversity, simplifying community structure and increasing modularity. Dominant species become more prevalent, with significant decreases in the relative abundance of Chloroflexi and Actinobacteria, and increases in Bacteroidetes and Firmicutes. Moreover, higher temperatures enhance the overall carbon conversion potential of AS, particularly boosting CO2 absorption in anaerobic conditions as the potential for CO2 emission during glycolysis and TCA cycles grows and diminishes, respectively. The study highlights that temperature is a major factor affecting microbial community characteristics and CO2 fluxes, with more pronounced effects observed in anaerobic sludge. This study provides valuable insights for maintaining stable A2O system operations, understanding carbon footprints, and improving COD removal efficiency in industrial WWTPs.

The continuous increased stability of sediment dissolved organic matter implies ecosystem degradation of lakes in the cold and arid regions

The characteristics of lake dissolved organic matter (DOM) pool and lake ecosystem interact, and studying the responses between sediment DOM characteristics and lake ecosystem changes may shed light on the inherent connection between ecosystem evolution and carbon biogeochemical cycles. Lakes in cold and arid regions are sensitive to changes and accumulate large amounts of carbon as DOM, which may provide a window into more explicit relationships between ecosystem evolution and changes in sediment DOM characteristics in time dimension. However, considerable blind spots exist in the responses between the sediment DOM and ecosystem evolution on time scale and the underlying mechanisms. In this study, multiple approaches were combined to investigate the relationship between the variation trend of sediment DOM characteristics and the evolution of fragile lake ecosystems across three different lake ecosystems in cold and arid regions of China. A strong positive relationship between sediment DOM stabilities, especially humification, and ecosystem degradation was found, consistent for the three lakes. Ultra-high-resolution mass spectrometry and structural equation modeling revealed that the changes of ecosystems affected sediment DOM stability through direct pathways (0.24), such as the contents of terrestrial DOM in lake DOM pool, and indirect pathways, including algae-mediated (0.43) and salinity-mediated pathways (0.22), which all increased the contents of refractory DOM in the lake DOM pool and sediments. Based on the fact that DOM stability changes could act on the ecosystem in turn, a possible positive feedback mechanism between ecosystem degradation and increased DOM stability was further inferred. These results suggested that the continuous increased stability of sediment DOM in may implies ecosystem degradation of lakes in the cold and arid regions. This study provides a new perspective for recognizing ecosystem evolution through sediment DOM and improves the understanding of the interaction of lake ecosystem evolution and the biogeochemical cycle of DOM.

The formation and control of disinfection by-products by two-step chlorination for sewage effluent: Role of organic chloramine decomposition among molecular weight fractions

With the increasing discharge of wastewater effluent to natural waters, there is an urgent need to achieve both pathogenic microorganism inactivation and the mitigation of disinfection by-products (DBPs) during disinfection. Studies have shown that two-step chlorination, which injected chlorine disinfectant by splitting into two portions, was more effective in inactivating Escherichia coli than one-step chlorination under same total chlorine consumption and contact time. In this study, we observed a substantial reduction in the formation of five classes of CX3R-type DBPs, especially highly toxic haloacetonitriles (HANs), during two-step chlorination of secondary effluent when the mass ratio of chlorine-to-nitrogen exceeded 2. The shift of different chlorine species (free chlorine, monochloramine and organic chloramine) verified the decomposition of organic chloramines into monochloramine during second chlorination stage. Notably, the organic chloramines generated from the low molecular weight (< 1 kDa) fraction of dissolved organic nitrogen in effluent organic matter tended to decompose during the second step chlorination leading to the mitigation of HAN formation. Furthermore, the microbiological analysis showed that two-step chlorinated effluent had a slightly lower ecological impact on surface water compared to one-step chlorination. This work provided more information about the two-step chlorination for secondary effluent, especially in terms of organic chloramine transformation and HAN control.

Chemical characteristics of dissolved organic matter in effluent from sludge alkaline fermentation liquid-fed sequencing batch reactors

Sludge alkaline fermentation liquid (SAFL) is a promising alternative to acetate for improving biological nitrogen removal (BNR) from wastewater. SAFL inevitably contains some refractory compounds, while the characteristics of dissolved organic matter (DOM) in effluent from SAFL-fed BNR process remain unclear. In this study, the molecular weight distribution, fluorescent composition and molecular profiles of DOM in effluent from SAFL and acetate-fed sequencing batch reactors (S-SBRs and A-SBRs, respectively) at different hydraulic retention time (12 h and 24 h) was comparatively investigated. Two carbon sources resulted in similar effluent TN, but a larger amount of DOM, which was bio-refractory or microorganisms-derived, was found in effluent of S-SBRs. Compared to acetate, SAFL increased the proportion of large molecular weight organics and humic-like substances in effluent DOM by 74.87%-101.3% and 37.52%-48.35%, respectively, suggesting their bio-refractory nature. Molecular profiles analysis revealed that effluent DOM of S-SBRs exhibited a more diverse composition and a higher proportion of lignin-like molecules. Microorganisms-derived molecules were found to be the dominant fraction (71.51%-72.70%) in effluent DOM (<800 Da) of S-SBRs. Additionally, a prolonged hydraulic retention time enriched Bacteroidota, Haliangium and unclassified_f_Comamonadaceae, which benefited the degradation of DOM in S-SBRs. The results help to develop strategies on reducing effluent DOM in SAFL-fed BNR process.

Molecular-level insights into the transformation and degradation pathways of dissolved organic matter during full-scale swine wastewater treatment

The rapid development of swine farming has resulted in the generation of a large amount of swine wastewater (SW), and dissolved organic matter (DOM) has a crucial role in determining the efficiency and safety of SW treatment. In this study, the transformation and influential mechanisms of DOM on the quality of SW effluent during full-scale SW treatment in actual engineering were systematically investigated using multispectral analysis and the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) technique. The results showed that S-containing, reduced, saturated, and less aromatic molecules were preferentially removed in the C-AF, while C-S preferentially removed reduced, unsaturated, and aromatic molecules, as well as molecules with large molecular weights. And in the two-stage A/O, the degradation of organic matter and DOM transformation occurred mainly in the A/O-1, with the A/O-2 acting as a supplement to further enhance the humification of DOM. Furthermore, the AOP preferentially removed lignin-like and highly unsaturated compounds, replacing them with a new generation of substances such as proteins and tannins with low aromaticity and unsaturation. More deeply, oxygen addition reactions dominate in both A/O and AOP. Specifically, the most common types of reactions in the A/O were the corresponding potential precursor-product pairs based on methyl to carboxylic acid (-H2 + O2) and alcohol to carboxylic acid (-H2 + O), while tri-hydroxylation (+O3) and di-hydroxylation (+H2O2) reactions were predominant in the AOP. Finally, the study's findings might suggest improving the actual engineering by prioritizing the AOP before the A/O-2 and using the C-S for safeguard treatment of the A/O-2 effluent. It is reliable that this kind of adjustment guarantees safe drainage indications and raises each process unit's efficiency in purifying.

Investigation on molecular characteristics of organic compounds during a full-scale landfill leachate treatment process based on non-targeted analysis

In this study, a new methodology for evaluating full-scale landfill leachate treatment processes by non-targeted analysis using comprehensive two-dimensional gas chromatography quadrupole time-of-flight mass spectrometry (GC × GC-QTOF-MS) was proposed. The method revealed the chemical complexity of organic compounds in landfill leachate samples at the molecular level and evaluated the removal efficiency of the anaerobic-anoxic-oxic (A2O) - membrane bioreactor (MBR) - nanofiltration (NF) treatment process in conjunction with multi-level classification of organic compounds. Results showed that the results of non-targeted analysis combined with multi-level classification of organic compounds had a significant correlation with the conventional water quality parameters and can be used to evaluate the treatment process. A total of 2508 organic compounds were detected in 6 samples. 17 emerging contaminants (ECs) with known potentially hazards were detected, including Diisobutyl Phthalate (DIBP), which is toxic to male reproduction and development, and 4-Tert-Butylphenol, which causes endocrine disruption in animals. The removal rate of organic compounds by this full-scale landfill leachate treatment processes reached 79.14%. The anaerobic tank played a crucial role with 64.98% contribution. For compounds, the removal rate of heterocyclics was as high as 94.67%, and the removal rate of aliphatics was poor, only 63.49%. This treatment process had almost perfect removal effect on the steroids in alicyclics and phenols in aromatics, but poor treatment effect on saturated alkanes in aliphatics and naphthenes in alicyclics. This study provides a methodology for accurate assessment of the molecular level of treatment processes, new insights for process optimization in waste treatment plants, and data support for the detection of emerging contaminants. The environmental hazards of landfill leachate can be further evaluated in the future in conjunction with ecotoxicity assessment studies.

Molecular insights into the dissolved organic matter of leather wastewater in leather industrial park wastewater treatment plant

Leather wastewater (LW) effluent is characterized by complex organic matter, high salinity, and poor biodegradability. To meet the discharge standards, LW effluent is often mixed with municipal wastewater (MW) before being treated at a leather industrial park wastewater treatment plant (LIPWWTP). However, whether this method efficiently removes the dissolved organic matter (DOM) from LW effluent (LWDOM) remains debatable. In this study, the transformation of DOM during full-scale treatment was revealed using spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry. LWDOM exhibited higher aromaticity and lower molecular weight than DOM in MW (MWDOM). The DOM properties in mixed wastewater (MixW) were similar to those in LWDOM and MWDOM. The MixW was treated using a flocculation/primary sedimentation tank (FL₁/PST), anoxic/oxic (A/O) process, secondary sedimentation tank (SST), flocculation/sedimentation tank, denitrification filter (FL₂/ST-DNF), and an ozonation contact reactor (O₃). The FL₁/PST unit preferentially removed the peptide-like compounds. The A/O-SST units had the highest removal efficiencies for dissolved organic carbon (DOC) (61.34 %) and soluble chemical oxygen demand (SCOD) (52.2 %). The FL₂/ST-DNF treatment removed the lignin-like compounds. The final treatment showed poor DOM mineralization efficiency. The correlation between water quality indices, spectral indices, and molecular-level parameters indicated that lignin-like compounds were strongly correlated with spectral indices and CHOS compounds considerably contributed to the SCOD and DOC. Although the effluent SCOD met the discharge standard, some refractory DOM from LW remained in the effluent. This study illustrates the composition and transformation of DOM and provides theoretical guidance for improving the current treatment processes.

Improving nutrients removal of Anaerobic-Anoxic-Oxic process via inhibiting partial anaerobic mixture with nitrite in side-stream tanks: role of nitric oxide

A side-stream tank which was in parallel with the anoxic tank was used to improve the performance of the Anaerobic-Anoxic-Oxic process. The partial mixtures from the anaerobic tank were injected into the side-stream tank with the initial nitrite nitrogen (NO₂^--N) concentrations of 10 mg/L and 20 mg/L. When the initial NO₂^--N concentration in the tank was 20 mg/L, total nitrogen and total phosphorus removal efficiencies of the A²/O process increased from 72% and 48% to 90% and 89%, respectively. 2.23 mg/L of nitric oxide (NO) were observed in the side-stream tank. The abundance of Nitrosomonas sp. and Nitrospira sp. were varied from 0.98% and 6.13% to 2.04% and 1.13%, respectively. The abundances of Pseudomonas sp. and Acinetobacter sp. were increased from 0.81% and 0.74% to 6.69% and 5.48%, respectively. NO plays an important role for improving the nutrients removal of the A²/O process in the side-stream nitrite-enhanced strategy.

Unraveling the evolution of dissolved organic matter in full-scale A/A/O wastewater treatment process using size exclusion chromatography with PARAFAC and nonnegative matrix factorization analysis

Changes in spectral features and molecular weight (MW) of dissolved organic matter (DOM) along the A/A/O processes in eight full-scale wastewater treatment plants (WWTPs) were characterized using size exclusion chromatography with a diode array detector, a fluorescence detector and an organic carbon detector in tandem (SEC-DAD-FLD-OCD) as well as bulk water quality parameters. The parallel factor (PARAFAC) and the nonnegative matrix factorization (NMF) analyses have been effectively applied to the postprocessing of SEC-FLD fingerprints and SEC-OCD chromatograms, respectively. Individual SEC-FLD-PARAFAC or SEC-OCD-NMF components may span a broad range of MW, indicating that these SEC fractions within the same component were cognate and varied coherently across the dataset samples. The SEC-FLD-PARAFAC modeling and SEC-OCD-NMF analysis have clearly and concisely presented that the dramatic decreases of dissolved organic carbon, UV absorbance at 254 nm and protein-like fluorescence at Ex280/Em350 nm in the anaerobic process were primarily associated with the degradation of the large MW proteinaceous and polysaccharide-like biopolymers. It has also revealed that fluorescence of humic acid-like fractions increased significantly during the anaerobic process, but fluorescence of fulvic acid-like and humic substances' building blocks decreased slightly. Laboratory experiments further confirmed the presence of the humification process in anaerobic processes, and the formation of humic acid-like fluorophores may be associated with carbohydrate metabolism. The combination of SEC-FLD-PARAFAC and SEC-OCD-NMF helped to establish the links between changes in bulk water quality parameters and the evolution of SEC MW fractions, which provides a more in-depth insight into wastewater DOM treatability and enables the optimization of wastewater treatment processes.

Evaluation of Efficiently Removing Secondary Effluent Organic Matters (EfOM) by Al-Based Coagulant for Wastewater Recycling: A Case Study with an Industrial-Scale Food-Processing Wastewater Treatment Plant

The reuse of wastewater has been identified as an important initiative for the sustainable development of the environment; thus, the removal of secondary effluent organic matter (EfOM) to ensure the safety of reused wastewater is the key step and a subject of extensive research. In this study, Al₂(SO₄)₃ and anionic polyacrylamide were selected as coagulant and flocculant, respectively, for the treatment of secondary effluent from a food-processing industry wastewater treatment plant to meet the standard regulatory specifications for water reuse. In this process, the removal efficiencies of chemical oxygen demand (COD), components with UV₂₅₄, and specific ultraviolet absorbance (SUVA) were 44.61%, 25.13%, and 9.13%, respectively, with an associated reduction in chroma and turbidity. The fluorescence intensities (Fmax) of two humic-like components were reduced during coagulation, and microbial humic-like components of EfOM had a better removal efficiency because of a higher Log Km value of 4.12. Fourier transform infrared spectroscopy showed that Al₂(SO₄)₃ could remove the protein fraction of the soluble microbial products (SMP) of EfOM by forming a loose SMP protein complex with enhanced hydrophobicity. Furthermore, flocculation reduced the aromaticity of secondary effluent. The cost of the proposed secondary effluent treatment was 0.034 CNY t^-1 %COD^-1. These results demonstrate that the process is efficient and economically viable for EfOM removal to realize food-processing wastewater reuse.

Changes of nitrogen and phosphorus removal pattern caused by alternating aerobic/anoxia from the perspective of microbial characteristics

The purpose of this study was to compare the impact of different numbers of alternating aerobic/anoxic (A/O) cycles on pollutant removal. Three sequential batch reactors (SBRs) with varying numbers of alternating A/O cycles were established. Under the tertiary anoxic operating conditions, the removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N), total nitrogen (TN), and total phosphorus (TP) were 88.73%, 89.56%, 72.15%, and 77.61%, respectively. Besides, alternating A/O affected the dominant microbial community relative abundance (Proteobacteria and Bacteroidetes) and increased microbial richness and diversity. It also increased the relative abundance of aerobic denitrifying, heterotrophic nitrifying, and denitrifying phosphorus removal bacteria to change N and P removal patterns. Furthermore, the abundance of carbohydrate metabolism and amino acid metabolism was improved by alternating A/O to improve organic matter and TN removal.

Greenhouse gas emission during swine manure aerobic composting: Insight from the dissolved organic matter associated microbial community succession

Greenhouse gas emissions during aerobic composting is unavoidable, but good practices can minimize emission. Therefore, to explore the key factors influencing the release of greenhouse gas emissions during composting, the inaction of organic matter conversion, greenhouse gas emissions and bacterial community structure during co-composting with different ratio (pig manure and corn straw) over a 6-week period was studied. The excitation-emission matrix fluorescence spectroscopy with the parallel factor was used to identify that dissolved organic matter associated microbial community succession mainly influenced greenhouse gas emissions. Protein-like fractions of dissolved organic matter were more likely to decompose and promote CH₄ and CO₂ emissions, while the humic-like fractions of dissolved organic matter positively affected N₂O emissions. The largest of greenhouse gas emissions was appeared in MR2 with 12.7 kg CO₂-eq, and the MR3 and MR4 reduced greenhouse gas emissions by 26.8 % and 11.4 %, respectively.

Temperature effects on microbial dissolved organic matter metabolisms: Linking size fractions, fluorescent compositions, and functional groups

This study elucidated the compositional and structural variations of size fractions of microbially-induced dissolved organic matter (DOM) caused by short-term temperature changes (5 to 35 °C), taking riverine DOM as an example. A simple and efficient method combining fractionation-[parallel factor analysis and two-dimensional Fourier-transform infrared correlation spectroscopy (PARAFAC-2D FTIR COS)]-correlation was introduced to link fluorescent DOM components and their structures in terms of surface functional groups. Results indicated that the higher temperature stimulated the decomposition of aromatics (sizes decreased from 10 kDa-0.22 μm to <10 kDa) and the transformation of proteins to humics (with sizes <0.22 μm); while both the higher and lower temperatures inhibited the utilization of larger-sized DOM (>0.22 μm, especially the non-fluorescence part) and synthesis of larger-sized microbial-derived proteins and humics (>0.22 μm), which may result in more smaller-sized (<10 kDa) and refractory aromatics transported from rivers to oceans in the warming future. However, the structure-determined DOM behaviors could be less affected by temperature since the fluorescent proteins and humics revealed similar functional group compositions, such as carboxyl, hydroxyl, carbonyl/aldehyde, carboxylic anhydride, and carboxamide groups. These findings have strong implications for DOM biogeochemistry in future temperature-shock scenarios. The proposed method will support in-depth analyses of structure-regulated processes from a mechanistic perspective.

Formation of microorganism-derived dissolved organic nitrogen in intermittent aeration constructed wetland and its stimulating effect on phytoplankton production: Implications for nitrogen mitigation

To control eutrophication in aquatic ecosystems, enhancing nitrogen removal in the constructed wetland (CW) by upgrading conventional CW to aeration CW is commonplace. However, regulatory efforts have only focused on reducing dissolved inorganic nitrogen (DIN) discharge and disregarding dissolved organic nitrogen (DON). Here, we used experimental mesocosms to investigate the effect of aeration mode on the characteristics of effluent DON in CW. The results showed that intermittent aeration is prone to introduce large amounts of DON and bioavailable DON (ABDON) in the effluents, although it greatly decreases effluent total nitrogen (TN). Analysis of DON fluorescent components and molecular characteristics indicated that suddenly shifting the environment from anoxic condition to aerobic condition in the intermittent aeration CW (IACW) would stimulate microorganisms to release tryptophan and simple aromatic proteins-like substances, which does not occur in the limited continuous aeration CW (CACW). Consequently, the abundance of DON resembling lipids, proteins/amino sugars, and carbohydrates-like molecules in IACW were about 2.1 times higher than that in CACW. Bioassay results showed that Selenastrum capricornutum and Microcystis aeruginosa incubated with effluent from IACW both generate larger phytoplankton biomass than that with CACW effluent, even though IACW effluent contains less TN than its counterpart. Moreover, Microcystis aeruginosa can simultaneously utilize DON and DIN, while Selenastrum capricornutum seem to utilize the DON only when DIN was not available. This result implies that increasing DON discharge may also influence phytoplankton composition and stimulate harmful phytoplankton species. Overall, this study indicates that upgrading CW solely depending on DIN removal level cannot ensure a mitigation of nitrogen-related eutrophication, and more efforts should be paid to curb DON discharge.

Modeling and optimizing of an actual municipal sewage plant: A comparison of diverse multi-objective optimization methods

The activated sludge process of an actual municipal sewage treatment plant was systematically modeled, calibrated, and verified in this study. Identified multi-objective optimization (MOO) methods were employed to optimize the process parameters of the validated model, and the optimal MOO algorithm was obtained by comparing Pareto solution sets. The optimization model consisted of three key evaluation indicators (objective functions), which are the average effluent quality (AEQ), overall cost index (OCI), and total volume (TV) of the biochemical tank, along with 12 more process parameters (decision variables). Three optimization algorithms, i.e., adaptive non-dominated sorting genetic algorithm III (ANSGA-III), non-dominated sorting genetic algorithm II (NSGA-II), and particle swarm algorithm (PSO), were adopted using MATLAB. The comparison of these algorithms demonstrated that the ANSGA-III algorithm had better Pareto solution sets under the triple objective optimization, and the effluent quality of COD, TN, NH₄⁺-N, and TP after optimization decreased by 2.22, 0.47, 0.13, and 0.02 mg/L, respectively. Additionally, the simulated AEQ was reduced by 13% compared to the original effluent, and the OCI and TV decreased from 21,023 kWh d^-1 and 17,065 m³ to 20,226 kWh d^-1 and 16,530 m³, respectively. The reported ANSGA-III algorithm and the proposed multi-objective method have a promising ability for energy conservation, emission reduction, and upgrading of municipal sewage treatment plants.

Variation of the toxicity caused by key contaminants in industrial wastewater along the treatment train of Fenton-activated sludge-advanced oxidation processes

Industrial wastewater contains a mixture of refractory and hazardous pollutants that have comprehensive toxic effects. We investigated the treatment of a long-chain industrial wastewater treatment train containing Fenton, biological anoxic/oxic (AO), and heterogeneous ozone catalytic oxidation (HOCO) processes, and evaluated their detoxification effect based on the analysis of the genic toxicity of some key contaminants. The results showed that although the effluent met the discharge standard in terms of traditional quality parameters, the long-chain treatment process could not effectively detoxify the industrial wastewater. The analysis results of summer samples showed that the Fenton process increased the total toxicity and genotoxicity of the organics, concerned metals, and non-volatile pollutants, whereas the A/O process increased the toxicity of the organics and non-volatile pollutants, and the HOCO process led to higher toxicity caused by metals and non-volatile pollutants. The outputs of the winter samples indicated that the Fenton process reduced the total toxicity and genotoxicity caused by non-volatile pollutants but increased that of the organics and concerned metals. The effect of the A/O process on the effluent toxicity in winter was the same as that in summer, whereas the HOCO process increased the total toxicity and genotoxicity of the metals in winter samples. Correlation analysis showed that various toxicity stresses were significantly correlated with the variation of these key pollutants in wastewater. Our results could provide a reference for the optimization of industrial wastewater treatment plants (IWTPs) by selecting more suitable treatment procedures to reduce the toxicity of different contaminants.

Impact of environmental factors and tributary contributions on tidal dissolved organic matter dynamics

Riverine dissolved organic matter (DOM) transport was a key step in the carbon biogeochemical cycle while we had limited understanding of its contribution to the estuary DOM dynamics. This study focused on the river downstream-to-tidal estuary DOM variation and the control of environmental factors on it. The contributions of three tributaries with varing urbanization degrees to the tidal DOM dynamics were evaluated. Though more aromatics were introduced to the urban tributary, the A₂₅₀/A₃₆₅ values and fluorescent index values indicated the DOM molecular size was uniformly reduced due to the enhanced microbial degradation during transport. The tidal DOM showed less varied spectroscopic indexes than the tributary DOM, but tidal cycles strongly impacted the fluorescent DOM quantified by the fluorescence regional integration (FRI). Salinity range can differentiate the fluorescent DOM variation patterns in river tributaries (e.g., <2.5, positive correlations; >2.5, negative correlations) and tidal cycles (>10, negative correlations). For tidal DOM, the high salinity decreased more humic-related components, resulting in increased proportions of protein-related components in high tides. The dissolved oxygen and nitrogen contents were negatively correlated with salinity, suggesting the microbial contributions and anthropogenic inputs in tributaries increased the tidal DOM quantity. The less urbanized tributaries contributed more to the low-tide DOM compositions/properties while the dynamic contribution of the urban tributary impacted more the tidal DOM dynamics. Our results highlighted the uneven declines of FRI values of different components by freshwater-saltwater mixing in estuaries and suggested the different functioning of urban, agro-urban, and suburban tributaries contributed to tidal DOM dynamics.

Trace metal complexation with dissolved organic matter stresses microbial metabolisms and triggers community shifts: The intercorrelations

The response of microorganisms to heavy metal-dissolved organic matter (Me-DOM) complexation is critical for the microbial-mediated coupled biogeochemical cycling of metals and DOM. This study investigated the impact of typical metals [As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn (at an environmentally-relevant concentration of 200 ppb)], model DOM substrates [humic acids (HA) and bovine serum albumin (BSA)], and their complexation on riverine microbial DOM metabolisms. DOM biodegradability decreased after the metal complexation (especially Co, Cr, and Mn for HA and Ni for BSA). While microbial transformation of humics and proteins was observed, components with lower aromaticity and hydrophobicity were accumulated during the cultivation. The substrate difference and metal speciation changed community compositions and resulted in distinctive community member networks, which accounted for the varied metabolic DOM patterns. The correlations indicated that rather than metal uptakes, Me-DOM complexation and community shifts controlled microbial DOM metabolisms. Microbial BSA metabolisms were less correlated to the key genera identified by network analysis or community diversity. Instead, they were sensitive to metal speciation, which may be attributed to the complicated utilization and production of proteins and their essential roles in detoxification. The constructed correlations among metals (Me-DOM complexes), DOM metabolisms, and community shifts provide strong implications for the biogeochemical function of Me-DOM complexes and highlight the effect of metal speciation on microbial protein metabolisms even at trace concentrations.

Cooperation of heterotrophic bacteria enables stronger resilience of halophilic assimilation biosystem than nitrification system under long-term stagnation

Long-term stagnation of biosystems (with no or very little wastewater) owing to seasonal downtime or failure maintenance brings great challenges to the performance recovery after system restart. In particular, the reduction of microbial activity and change of dissolved organic matter (DOM) affect the effluent quality and subsequent treatment procedures. Monitoring the dynamics and resilience of biosystems after long-term stagnation is important to formulate targeted countermeasures for system stability. However, the influence of long-term stagnation on autotrophic nitrification (AN) and heterotrophic assimilation (HA) biosystems has not been systematically explored. Here, we used halophilic AN and HA systems to study the stability and resilience of two nitrogen removal consortia after long-term stagnation. The results showed that 97.5 % and 93 % of ammonium and 47.0 % and 90.1 % of total nitrogen were removed using the halophilic AN and HA systems, respectively, in the stable period. After four weeks of stagnation, the HA system showed stronger resilience than AN system, in terms of faster recovery of treatment performance, and less fluctuations in sludge settleability and extracellular polymeric substances. In addition, after the stagnation period, the DOM of AN system was rich in low-molecular refractory humic acid, whereas that of HA system was rich in high-molecular proteins. The stagnation period led to the replacement of the dominant heterotrophic functional microorganisms, Paracoccus and Halomonas, with Muricauda and Marinobacterium in the HA system. The microbial network results revealed that the cooperation of heterotrophic bacteria enables stronger resilience of the HA system from prolonged stagnation than the AN system. In addition, the nitrogen removal efficiency, protein to polysaccharide ratio of EPS and fluorescence intensity of DOM were significantly correlated with the microbial community composition. These results suggest that AN system has greater risks in terms of treatment performance and sludge stability than the system after long-term stagnation.

Chemical oxygen demand and nitrogen removal from real membrane-manufacturing wastewater by a pilot-scale internal circulation reactor integrated with partial nitritation-anammox

A pilot-scale system integrating internal circulation and partial nitritation-anammox successfully treated real high-strength membrane-manufacturing wastewater in this study. With this pilot-scale system, a high chemical oxygen demand (COD) removal efficiency of 85 % and a nitrogen removal of 90 % are achieved at an organic loading rate of 6.0 kg COD/m3/d. The nitrogenous organic matters in the internal circulation zone are degraded into ammonia nitrogen. In the partial nitrification zone, nitrite accumulation is achieved, providing a suitable NH4+-N/NO2--N ratio for anammox reaction. Partial nitritation is achieved by maintaining an operational temperature at 30-35 °C, free ammonia concentration at 5-7 mg/L and dissolved oxygen at 0.4-0.7 mg/L with a reflux ratio of 150 %. The COD to nitrogen ratio in the internal circulation effluent is maintained below 3.0 to inhibit nitrite oxidizing bacteria. This study demonstrates that a pilot-scale system can efficiently remove organic matters and nitrogen from wastewater of membrane-manufacturing industry.

Immobilized BiOCl0.75I0.25/g-C3N4 nanocomposites for photocatalytic degradation of bisphenol A in the presence of effluent organic matter

The BiOCl_0.75I_0.25/g-C₃N₄ nanosheet (BCI-CN) was successfully immobilized on polyolefin polyester fiber (PPF) through the hydrothermal method. The novel immobilized BiOCl_0.75I_0.25/g-C₃N₄ nanocomposites (BCI-CN-PPF) were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy EDS, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) to confirm that BCI-CN was successfully immobilized on PPF with abundant oxygen vacancies reserved. Under simulated solar light irradiation, 100 % of bisphenol A (BPA) with an initial concentration of 10 mg·L^-1 was degraded by BCI-CN-PPF (0.2 g·L^-1 of BCI-CN immobilized) after 60 min. A similar photocatalytic efficiency of BPA was obtained in the presence of effluent organic matter (EfOM). The photocatalytic degradation of BPA was not affected by EfOM <5 mg-C/L. In comparison, the photocatalytic performance was considerably inhibited by EfOM with a concentration of 10 mg-C/L. Furthermore, photogenerated holes and superoxide radicals predominated in the photocatalytic degradation processes of BPA. The total organic carbon (TOC) removal efficiencies of BPA and EfOM were 75.2 % and 50 % in the BCI-CN-PPF catalytic system. The BPA removal efficiency of 94.9 % was still achieved in the eighth cycle of repeated use. This study provides a promising immobilized nanocomposite with high photocatalytic activity and excellent recyclability and reusability for practical application in wastewater treatment.

Rapid detection and speciation of illicit drugs via a thin-film microextraction approach for wastewater-based epidemiology study

High detection frequency of illicit drugs in water samples urges the development of rapid detection method for wastewater-based epidemiology (WBE) study. Here, we first developed a fast, convenient, and cost-effective method by combining thin-film microextraction (TFME) with gas chromatography-mass spectrometry (GC-MS) for sensing illicit drugs in wastewater sample. A divinylbenzene particle-loaded membrane was prepared by dip coating on a copper mesh. The sampling conditions of three illicit drugs were optimized and the performance of the proposed method was evaluated. The limit of detection was 5.5 2.0, and 1.1 ng L^-1 for methamphetamine (MAMP), ketamine (KET), and methaqualone (MEQA), respectively, with acceptable precision (< 6.1 % for membrane to membrane reproducibility) and recovery from influent water (95 % - 111 %). Then, the proposed method was applied to study the occurrence and distribution of the target compounds in a wastewater treatment plant. The presence of methamphetamine, ketamine, and methaqualone was confirmed and their concentrations in the influent sample were 57 ± 8, 40 ± 4, and 75 ± 2 ng L^-1, respectively. The speciation of the target compounds in different ponds was also investigated. Results showed that the content of organic matter and the pH of the sample significantly affected the binding state of the compounds. This work provides an efficient and accurate analytical protocol for WBE investigation of illicit drugs.

Transformation of dissolved organic matter during biological wastewater treatment and relationships with the formation of nitrogenous disinfection byproducts

Nitrogenous disinfection byproducts (N-DBPs) can be produced from dissolved organic matter (DOM) during the disinfection of secondary wastewater effluent. This study examined the transformation of DOM and the abatement of N-DBP precursors during different types of biological wastewater treatment (e.g., anaerobic/anoxic/oxic activated sludge processes and membrane bioreactor) using high-performance size exclusion chromatography (HPSEC) with dissolved organic carbon, UV, and fluorescence detectors. DOM with molecule weight (MW) larger than 3 kDa and protein-like substances smaller than 0.3 kDa was effectively bio-transformed, whereas DOM fractions with MW in the range of 0.3-3 kDa were the most bio-refractory. Complete nitrification was beneficial to the removal of small amino sugar-like and protein-like molecules (< 0.3 kDa). Haloacetonitrile (HAN) precursors were recalcitrant to biological treatment with a median removal of 17%. Halonitromethane (HNM) and N-nitrosamine (NA) precursors tended to be effectively removed in complete nitrification conditions. The abundance of low-molecular-size protein-like substances (< 0.3 kDa) was significantly correlated with the formation potential of HNM, NA, and total N-nitrosamine (TONO) in post-chloramination (r = 0.81, 0.62, and 0.68, respectively, p < 0.01). This study improved the understanding of DOM transformation and the removal of N-DBPs precursors in wastewater treatment and pointed out the benefit of provision of complete nitrification in removing low-molecular-size protein-like substances and NA and HNM precursors.

Microbial metabolism changes molecular compositions of riverine dissolved organic matter as regulated by temperature

This study investigated the control of dissolved organic matter (DOM) molecular compositions by microbial community shifts under temperature regulation (range from 5 to 35 °C), using riverine DOM and in situ microorganisms as examples. The functioning of different microbial metabolisms, including the utilization and generation processes, was comprehensively analyzed. Though the overall quantity of DOM was less temperature-affected, more molecules were identified at moderate temperatures (e.g., 15 and 25 °C) and their accumulated mass peak intensities increased with the temperature. The results were ascribed to 1) the microbial production of macromolecular (m/z > 600) CHO, CHON, and CHONS species was stimulated at higher temperatures; 2) the microorganisms consumed more DOM molecules at both higher and lower temperatures; and 3) the simultaneously decreased utilization and increased generation of recalcitrant CHO and CHON molecules with m/z < 600 at higher temperatures. The strong correlations among the temperature, community structures, and DOM chemodiversity suggested that temperature promoted the community evenness to increase the DOM generation. In addition, the higher temperature decreased the abundance of microorganisms that utilized more recalcitrant molecules and produced fewer new molecules (e.g., Proteobacteria, Acinetobacter, and Erythrobacter) while increased others that functioned the opposite (e.g., Verrucomicrobia, Bacteroidetes, and Flavobacterium) to increase the DOM production. The constructed temperature-community-DOM chemistry relationship deepened the molecular-level understanding of DOM variations and provided implications for the warming future.

Microbial community response of the full-scale MBR system for mixed leachates treatment

In practice, mature landfill leachate and incineration (young) leachate are mixed to improve the biodegradability and enhance biological treatment performance. However, the ratio of mature-to-young leachates greatly influences MBR treatment efficiency and microbial community structure. This study investigated the treatment efficiency and microbial community structure of full-scale MBR systems operated under two mix ratios, mature leachate: young leachate = 7:3 (v/v, denoted as LL) and 3:7 (v/v, denoted as IL). LL group showed lower Cl- and COD concentrations but a higher aromatic organic content comparing to IL group, and the COD and UV254 removals for LL were significantly lower than those for IL by MBR treatment. Microbial community structures were similar in both groups at phylum level, with dominant phyla being Proteobacteria (23.8%-32.3%), Bacteroidetes (15.25%-20.7%), Chloroflexi (10.5%-23.1%), and Patescibacteria (9.9%-13.2%). However, the richness and diversity of LL group were lower, and differences were observed at lower taxonomy levels. Results indicated that salinity mainly changed the structure of microbial community, resulting in greater abundance of salt-tolerant microbials, while refractory organics affected microbial community structure, and also led to decreased diversity and metabolic activity. Therefore, in mixed leachates biological treatment, a higher young leachate ratio is recommended for better organics removal performance. PRACTITIONER POINTS: The trade-off between refractory organics and salinity in mixed leachate treatment should be paid attention. Refractory organics reduced alpha and functional diversities of microorganisms. Mixed leachate with a higher young leachate ratio reached a better organic removal.

Molecular insight into the variation of dissolved organic phosphorus in a wastewater treatment plant

To date, eutrophication becomes a great concern of vulnerable aquatic systems. Dissolved organic phosphorus (DOP) discharged from wastewater treatment plant (WWTP) holds a large source of phosphorus in receiving water. However, due to the complexity of DOP, their variation and fate in WWTP remain unknown at the molecular level, and are always overlooked. Here, the variation of DOP in a WWTP was uncovered via Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Results show that 95% of DOP in the influent could be removed by the secondary biological treatment processes. The removed DOP species were mainly lipids with the molecular characteristics of low oxygen content, low unsaturation and low aromaticity. Meanwhile, during biological treatments, some new DOP species, especially lignin/carboxylic rich alicyclic molecules (CRAM) that possessed high oxygen content, high unsaturation and high aromaticity, were produced and released into the secondary effluent. In the subsequent tertiary treatment, coagulation by aluminum salt tended to remove high molecular weight and high oxygen content DOP species in the secondary effluent, which was complementary to the biological treatment. However, the sand filter usually retained microorganisms, which would result in the generation of new DOP species in this process. During the final ultraviolet disinfection process, DOP was effectively mineralized to phosphate, especially the species with high molecular weight and highly unsaturated aromatic DOP species (e.g., lignin/CRAM and tannin), which had higher UV absorbance. The revealed variation of DOP in WWTP is beneficial to optimize the treatment processes to enhance the removal of DOP.

Advanced treatment of secondary effluent organic matters (EfOM) from an industrial park wastewater treatment plant by Fenton oxidation combining with biological aerated filter

This study investigated the advanced treatment of secondary effluent organic matters (EfOM) from an industrial park wastewater treatment plant (IPWTP) by Fenton oxidation process and its combination with biological aerated filter (BAF). The constituents of EfOM were characterized by using fluorescence excitation-emission matrix, and the results showed that the major components included aromatic proteins, soluble microbial products, humic and fulvic acid-like substances, and compounds associated with fluorescent region of Ex 250-300 nm/Em 600-700 nm. The EfOM was strongly resistant to biodegradation (biochemical oxygen demand (BOD₅):chemical oxygen demand (COD) ratio at 0.11), resulting in less than 15% dissolved organic carbon (DOC) removal efficiency by the BAF reactor. The advanced treatment of EfOM by Fenton oxidation process led to maximum ~50% mineralization efficiency of EfOM under the optimal conditions of 2.0 mM Fe^II, 10 mM H₂O₂, pH 3.0 and 3.0 h of the reaction time. Particularly, Fenton oxidation treatment effectively improved the biodegradability of EfOM in the IPWTP secondary effluents, e.g., increasing the BOD₅:COD ratio from 0.11 to 0.42. A synergistic combination of Fenton oxidation process with the BAF reactor offered desirable mineralization efficiencies of EfOM (>70%) at lower dosages of Fenton's reagents. The present results suggest that Fenton oxidation process combining with the BAF reactor can be a promising strategy for the advanced treatment of EfOM in IPWTP secondary effluents. This study provides guidance for the characterization and advanced treatment of EfOM in IPWTP secondary effluents for practical purpose.

Microbial Roles in Dissolved Organic Matter Transformation in Full-Scale Wastewater Treatment Processes Revealed by Reactomics and Comparative Genomics

Understanding the degradation of dissolved organic matter (DOM) is vital for optimizing DOM control. However, the microbe-mediated DOM transformation during wastewater treatment remains poorly characterized. Here, microbes and DOM along full-scale biotreatment processes were simultaneously characterized using comparative genomics and high-resolution mass spectrometry-based reactomics. Biotreatments significantly increased DOM's aromaticity and unsaturation due to the overproduced lignin and polyphenol analogs. DOM was diversified by over five times in richness, with thousands of nitrogenous and sulfur-containing compounds generated through microbe-mediated oxidoreduction, functional group transfer, and C-N and C-S bond formation. Network analysis demonstrated microbial division of labor in DOM transformation. However, their roles were determined by their functional traits rather than taxa. Specifically, network and module hubs exhibited rapid growth potentials and broad substrate affinities but were deficient in xenobiotics-metabolism-associated genes. They were mainly correlated to liable DOM consumption and its transformation to recalcitrant compounds. In contrast, connectors and peripherals were potential degraders of recalcitrant DOM but slow in growth. They showed specialized associations with fewer DOM molecules and probably fed on metabolites of hub microbes. Thus, developing technologies (e.g., carriers) to selectively enrich peripheral degraders and consequently decouple the liable and recalcitrant DOM transformation processes may advance DOM removal.

Conventional bioretention column with Fe-hydrochar for stormwater treatment: Nitrogen removal, nitrogen behaviour and microbial community analysis

An FeCl3-modified rice husk hydrochar ('Fe-hydrochar') was used as the filler in a conventional bioretention column to remove nitrogen from synthetic stormwater. When the ammonia nitrogen (NH4-N) and nitrate nitrogen (NO3-N) concentrations of the influent were both 20 mg/L, the average removal rates of NH4-N and total nitrogen (TN) were approximately 97% and 50%, respectively. Nitrogen was mainly removed by microbial nitrification and denitrification, with 25% of NH4-N being adsorbed by the Fe-hydrochar. The remaining NH4-N was converted into NO3-N by nitrification in the upper layer, and NO3-N was mainly converted to nitrogen gas (N2) by denitrification in the lower layer. The organic matter released by the Fe-hydrochar was degraded and used as the carbon source for denitrification. The dominant bacteria were Pseudomonas, Rhizobium, and Flavobacterium at the genus level. Pseudomonas and Rhizobium were responsible for heterotrophic nitrification-aerobic denitrification, while Flavobacterium was related to the degradation of organic matter.

Dynamics of dissolved organic matter and dissolved organic nitrogen during anaerobic/anoxic/oxic treatment processes

With Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and fluorescence spectroscopy, this study investigated the transformation of dissolved organic matter (DOM) and nitrogen (DON) during the widely-applied anaerobic/anoxic/oxic (A²O) processes to provide molecular insights into the removal, generation, and reduction of DOM/DON species in different biological treatment units. Results indicated that the anaerobic process decomposed the macromolecules of influent DOM/DON and decreased their mass. The anoxic process denitrified DON and generated DOM, as indicated by the decreased molecule number of CHON and CHONS and the increased CHO and CHOS species, as well as the increased overall DOM intensities. DOM mineralization and ammonia nitrogen-DON conversion occurred in the oxic process. Aromaticity and unsaturation degree increased slightly after the A²O processes, which was correlated with the relative abundance of Proteobacteria (positively) and Bacteroidetes (negatively). The results have strong implications to the understanding of DOM/DON dynamics in wastewater treatment plants.

DOM chemodiversity pierced performance of each tandem unit along a full-scale "MBR+NF" process for mature landfill leachate treatment

Mature landfill leachate contains a substantial fraction of recalcitrant dissolved organic matters (DOM) that is a challenging for conventional wastewater treatment that is typically focused on the removal of biodegradable organic matter. "Biological treatment + membrane treatment" has been widely employed to treat complex leachate. However, the performance of each unit based on both conventional bulk indicators and molecular information has not been well understood. Therefore, the fate of DOM chemodiversity along the full-scale treatment process across ten sampling points over three different seasons were analyzed to determine the efficiency of every unit process with the assistance of ultra-performance liquid chromatography coupled with hybrid quadrupole Orbitrap mass spectrometry. Results showed that the process performance, visualized through the molecular signals, were relatively stable in the temporal dimension. The process removed 83.2%-92.2% of DOM molecules in terms of richness, where lignin/carboxyl-rich alicyclic compounds (CRAM)-likes with relatively high saturation was preferentially removed, while newly generated bio-derived N-containing compounds (N/C_wa 0.15-0.17) became resistant. The relationship between conventional bulk physicochemical indicators and molecular indexes suggested that soluble chemical oxygen demand (sCOD) and dissolved organic carbon (DOC) were contributed by the refractory DOM with high weighted average double bond equivalents (DBE_wa), which was distributed in the region of O/C 0.2-0.5 and H/C 1.2-1.8. This refractory DOM required ultrafiltration and nanofiltration for removal. DOM molecules were positively correlated with five-day biochemical oxygen demand (BOD₅) and revealed that approximately 96.9%-98.4% of the DOM could be removed or transformed in the primary anoxic zone. In addition, the bio-derived aliphatic/proteins, lipids and lignin/CRAM-likes (O/C > 0.2) with condensed aromatization were the sources of dissolved organic nitrogen (DON) and still remained in the final effluent. The present study suggests that the design and operation of the combination process with biological and membrane treatment could be specifically optimized based on the DOM molecular characteristics of the wastewater.