The aerobic granular sludge (AGS) system is highly-sensitive to the mixed carbon sources ratios. Despite the combination of sodium acetate/glucose is widely applied in AGS systems, a knowledge gap in the nitrous oxide (N2O) emission mechanisms and production pathways was yet filled. In this study, the impacts of sodium acetate/glucose (S/G) ratios of 3, 1 and 1/3 on N2O emissions, production pathways and sink capability were systematically unveiled in AGS systems by biochemical batch tests and microbial community analysis. The results revealed that during the sole denitrification, higher N2O reduction rates (i.e., 16.7 and 12.3 mg/g VSS/h in sole and multiple nitrogen oxide schemes, respectively) and greater N2O-sink capability (i.e., 3.77) were invariably obtained with higher sodium acetate ratios (i.e., S/G = 3), probably due to the enrichment of denitrifiers harboring N2O reduction genes and key enzymes. Nevertheless, higher sodium acetate ratios relatively induced a higher N2O emissions (i.e., 4.02 %) during the sole nitrification, driven by the higher contribution of nitrifier denitrification (34.1 %) and increasing Nitrosomonas (i.e., ammonia-oxidizing bacteria) abundance. Conversely, the elevated glucose ratio (i.e., S/G = 1/3) enhanced nitrite-oxidizing bacteria (NOB) abundance, thereby suppressing nitrifier denitrification pathway-derived N2O emissions. The counterbalancing effects of elevated sodium acetate ratios on mitigating N2O production during nitrification and higher glucose proportions promoting N2O reduction in denitrification collectively resulted in slight difference in N2O emissions factors (0.43-0.5 %, p > 0.05) throughout cyclic operations. The gained insights can be as a reference by wastewater treatment plants (WWTPs) to reduce the N2O footprint in AGS systems.

Given the ubiquitous presence of humic substances (HS) in swimming pool waters (SPW) and its potential impact on haloacetic acids (HAA) removal, this study made a systematic investigation on their potential interaction during nanofiltration (NF) process. The water flux decreased by 26.6% and 11.1% for NF90 and NF270, respectively, after 24 h exposure to 30 mg/L HS, whereas the rejections of NaCl increased by 8.9% and 14.6%, HAA by 6.9% and 20.5%, and boron by 2.6% and 7.1%. Fulvic acid features the smallest particle size and the strongest hydrophobicity of all HS fractions tested, therefore showing the highest potential of depositing on the surfaces and/or into the pore channels of membranes with loose structures, and serving as the main component of HS responsible for the variations of membrane performance. The decreased hydrophilicity and pore size of membrane surface were jointly responsible for the reduced water flux. The enhanced size exclusion effect either competed against the weakened charge repulsion effect, which happened for NF270, or cooperated with the strengthened charge repulsion effect, which happened for NF90, leading to overall increased rejections of charged HAA and NaCl. For neutral compound boron, the reduced pore size was likely the main factor leading to its increased rejection. By exploring underlying interaction mechanism among trace contaminants, HS, and membranes, this study provides novel theoretical insights for the assessment of NF process.

Aniline is a prevalent contaminant in wastewater treatment plants (WWTPs) affected by industrial discharges. However, its impact on nitrous oxide (N2O) emissions during biological nitrogen removal (BNR) is not well understood. In this study, two sequencing batch reactors (SBRs) were operated under identical conditions, with the only difference being continuous addition of 20 mg/L aniline to the experimental reactor (SBR-A). SBR-A exhibited an N2O emission factor of 1.13 ± 0.10%, 43% lower than that of the control reactor (SBR-C), and N2O emission during aerobic nitrification was clearly delayed. Batch tests were performed to elucidate the underlying mechanisms. Despite similar nitrifier abundances, SBR-A exhibited a 30% lower maximum ammonia oxidation rate than SBR-C, indicating metabolic suppression rather than biomass loss. During nitrification, oxygen competition and aniline toxicity further constrained AOB activity, with negligible N2O emission detected prior to the complete degradation of aniline. Across a dissolved oxygen range of 0.3-2.0 mg/L, N2O production during nitrification decreased by 37-57%, consistent with inhibition of the AOB denitrification pathway. Conversely, the maximum heterotrophic N2O reduction rate in SBR-A increased 2.28-fold, which was higher than the enhancement for NO2- reduction. This resulted in lower N2O accumulation, indicating preferential stimulation of nitrogen removal. Microbial community analysis further demonstrated significant enrichment of denitrification in SBR-A, particularly Thauera, whose relative abundance was 1.45 times that in SBR-C. Overall, sustained aniline loading reduced nitrification-derived N2O production and strengthened its reduction, providing new insight into how industrial co-contaminants affect N2O emissions in conventional BNR systems.

Microplastics (MPs) and short-chain per- and polyfluoroalkyl substances (PFAS) frequently co-occur in contaminated waters, yet their interactions during electrochemical treatment remain poorly understood. Here, we demonstrate that in a heterogeneous electro-Fenton system, short-chain PFAS—specifically perfluorobutanoic acid (PFBA) and perfluorobutanesulfonic acid (PFBS)—severely inhibit polyethylene terephthalate (PET)-MPs degradation, reducing efficiency by up to ~70% at 200 μM, with significant inhibition persisting even at environmentally relevant concentrations, e.g., 0.01 μM. We identify a distinct interfacial shielding mechanism: electrochemical oxidation generates hydrophilic hydroxyl groups on PET-MPs, creating high-affinity binding sites that selectively anchor PFAS transformation products (rather than parent compounds) via hydrogen bonding. This forms a persistent, fluorine-rich passivating layer that physically blocks reactive oxygen species from accessing the polymer surface. Molecular dynamics simulations confirm markedly enhanced binding affinities of these derivatives to oxidized PET surfaces compared to pristine polymers (−6.48 vs −1.35 kcal/mol for 4H-perfluorobutanoic acid, the dominant derivative). Notably, this inhibitory effect extends across multiple short-chain PFAS congeners and MP polymer types, suggesting that transformation product-driven surface passivation may be a general phenomenon in PFAS-MPs co-contaminated systems. Our findings advance the understanding of co-contaminant interference in advanced oxidation processes and underscore the necessity of accounting for transformation product-driven interfacial interactions and passivation when designing electrochemical treatment technologies for real-world water matrices.

The coexistence of residual antibiotics in wastewater raises concerns about the functional stability of sulfur-driven autotrophic denitrification (SDAD) mediated by sulfur-oxidizing bacteria (SOB) and the fate of antibiotics during treatment. This study investigated the functional tolerance of SOB to three representative antibiotics-ciprofloxacin (CIP), oxytetracycline (OTC), and sulfamonomethoxine (SMM)-under 360-min batch exposure and elucidated their removal behaviors during SOB-mediated sulfide (S2-) oxidation and NO3-/NO2- reduction. Batch experiments revealed distinct fates among the three antibiotics. OTC was removed to a limited extent via adsorption, and SMM remained largely stable. In contrast, CIP exhibited selective transformation closely associated with active S2- oxidation, rather than NO3-/NO2- reduction, with removal efficiency increasing from 11.6 ± 1.7% to 38.1 ± 0.3% as the initial S2- concentration rose from 0 to 12 mg-S/L. Notably, none of the three antibiotics significantly inhibited SOB-mediated S2- oxidation or NO3-/NO2- reduction under 360-min batch exposure, indicating appreciable short-term functional tolerance of the enriched SOB community. Five CIP transformation products were tentatively identified, with predicted lower ecotoxicity than the parent compound. A proof-of-concept sequential-substrate experiment further demonstrated that reintroducing S2- after denitrification substantially enhanced CIP removal, supporting the validity of a phased sulfur-based treatment strategy. To gain mechanistic insight, AlphaFold 3-based molecular docking analyses were performed, which suggested that S2- enhanced the association between CIP and sulfide-quinone oxidoreductase, with calculated binding energy decreasing from -5.2 to -7.0 kcal/mol in the presence of S2-, whereas no such change was observed for OTC or SMM. These findings advance the understanding of antibiotic removal in SDAD systems and provide a basis for mechanism-guided process optimization in the treatment of antibiotic-containing low C/N wastewaters.

Mixotrophic denitrification (MDN) has emerged as a promising nitrogen removal technology for wastewater treatment plants, due to its low carbon source costs and minimal sulfate discharge. While denitrification is recognized as effective in reducing N2O emissions, the specific mitigation potential of MDN remains unclear. This study investigated the N2O mitigation capacity and electron distribution of MDN, revealing its significant potential and strong resistance to shock load for N2O mitigation. The maximum N2O-N reduction rate observed was 519.7 mg N/(gVSS x h), significantly surpassing the maximum NO2--N reduction rate (101.3 mg N/(gVSS x h)), underscoring the effectiveness of MDN in mitigating N2O emissions. Despite electron competition from nitrogen oxides inhibiting the N2O-N reduction rate, MDN maintained a higher N2O-N consumption rate than production. Notably, MDN exhibited stable N2O mitigation performance via electron distribution toward N2O reductase. It shows no statistically significant shift change under a 50% reduction in organic carbon or sulfur supply. Additionally, optimal performance, with maximum nitrogen removal efficiency and superior N2O reduction potential, were achieved at a COD/N ratio of 2.5 and an S/N ratio of 4.6. Economically, this strategy reduced dosing costs of chemical input costs by 26% compared to heterotrophic denitrification. Overall, this study provides fundamental insights into electron flow during MDN, demonstrating its significant potential for N2O mitigation under tested conditions and offering a theoretical basis for future development of efficient N2O mitigation strategies.

Navigating the emerging pollutant crisis appears increasingly daunting, with the interaction between micro- and nanoplastics (M/NPs) and antimicrobial resistance (AMR) in complex microbial consortia remaining poorly understood. Here, mixed-culture microcosms are subjected to polymer- and size-resolved plastic exposures, and resistome and mobilome dynamics are quantified using phenotyping and multi-omics. M/NP exposure increases AMR gene abundance and reshapes resistance profiles in a polymer-dependent manner, dominated by efflux and target alteration. Particle miniaturization amplifies resistome diversity and gene mobility, and nanoplastics show the highest horizontal gene transfer activity and strongest co-localization of AMR genes with mobile genetic elements, forming dense cross-phylum transfer networks. Mechanistically, nanoplastics elevate ROS and membrane damage, activate the SOS response, and upregulate conjugation, competence, and transposase functions. Increased ATP generation and efflux activity sustain stress tolerance and energy-intensive DNA exchange, turning nanoplastics into hotspots of transferable resistance with implications for microbial evolution and ecological resilience.

Abstract The extensive use of antibiotics and their persistent residues have posed significant ecological and public health concerns. Electro-Fenton (EF) process, as an advanced oxidation process, has emerged as a promising approach for antibiotic degradation in wastewater. First, the EF process is briefly compared with other wastewater treatment technologies, followed by an overview of its reaction mechanisms and major process configurations. Particular emphasis is placed on the generation and identification of reactive oxygen species (ROS), especially the emerging role of singlet oxygen, and the limitations of current ROS identification strategies. Recent developments in catalyst design, including metallic materials, carbon-based composite metallic materials, and nonmetallic doped carbon materials, are critically reviewed. The effects of key operational parameters and water matrix components, including pH, current density, electrode spacing, coexisting ions, dissolved organic matter, and reactor configuration, are discussed. Special attention is also given to the dual role of chloride ions. Finally, current challenges, knowledge gaps, and future research directions related to process optimization, reactor engineering, scale-up implementation, sustainability assessment, and intelligent design strategies are highlighted to facilitate the practical application of EF technology for sustainable water treatment.

The environmental dissemination of antibiotic resistance genes (ARGs) poses a significant threat to both ecosystem and human health, yet their efficient removal continues to present considerable challenges. Hydroxyl radicals (•OH), as highly reactive oxidants, are capable of effectively degrading extracellular ARGs (eARGs). However, conventional Fenton reaction systems typically demand substantial chemical inputs and high energy consumption. Here, we report a biologically driven pathway that couples microbial ammonia oxidation with •OH generation to enable sustainable eARGs degradation. Alcaligenes sp. CHO6, a heterotrophic ammonia-oxidizing bacterium, was isolated from aquaculture sediment and shown to couple aerobic ammonia oxidation with Fe(III) reduction, leading to •OH production and subsequent eARGs degradation. During ammonia oxidation, strain CHO6 accumulated hydroxylamine (NH2OH), which acted as a key reductant driving Fe(III)/Fe(II) cycling and iron-mediated •OH formation. Genome sequencing revealed a dnfT1RT2ABCD cluster homologous to the direct ammonia oxidation (Dirammox) pathway. Heterologous expression of dnfABC in Escherichia coli confirmed its functional role in NH2OH accumulation and •OH generation. In vitro, the •OH produced by strain CHO6 degraded >90% of several eARGs (bla, tetA, gmR) within 3 h. This study unveils a previously unrecognized mechanism linking microbial nitrogen metabolism to reactive oxygen species (ROS) formation and offers a proof-of-concept for a biologically driven approach for mitigating eARGs in wastewater and natural systems.

Mainstream partial nitrification/anammox (PN/A) provides a low-carbon process pathway for municipal nitrogen removal. However, one-stage PN/A deployment remains constrained by unstable ammonia-oxidizing bacteria (AOB)-mediated nitritation, adaptive resurgence of nitrite-oxidizing bacteria (NOB), and inadequate nitrite availability for anammox under variable influent conditions. This review evaluates complete ammonia-oxidizing (comammox) bacteria as an ecological strategy to enhance mainstream PN/A stability by reconfiguring nitrifier competition and nitrite partitioning. Comammox Nitrospira exhibit high ammonia affinity, persistence under low dissolved oxygen, and preferential retention under long solids retention times. These traits enable sustained ammonia oxidation under conditions that often compromise canonical AOB. Although comammox possess nitrite-oxidizing capacity encoded by nitrite oxidoreductase, reactor observations indicate that ammonia oxidation by comammox exceed their subsequent nitrite oxidation. This kinetic imbalance enables transient nitrite accumulation and facilitates anammox nitrite uptake when biomass spatial organization promotes nitrite transfer to anammox before Nitrospira reoxidation. Conditions that favour comammox also diminish the competitive advantage of many canonical NOB, reframing NOB control from transient chemical inhibition toward ecological constraint through comammox niche occupation. The resulting comammox-anammox interaction is governed by nitrite partitioning among ammonia oxidation, nitrite oxidation and anammox uptake. Endogenous partial denitrification further recycles residual nitrate to nitrite when nitrate accumulation limits nitrogen removal. Current evidence supports comammox integration as a promising, boundary-defined strategy for stabilizing anammox-dominated one-stage mainstream PN/A. Future efforts should focus on validating nitrogen transformation pathways, improving the retention of functional biomass, demonstrating long-term reactor-scale performance, and developing predictive modelling that accounts for uncertainty.

Membrane fouling is a persistent challenge that hinders the widespread application of membrane technology for water treatment. This study introduced an innovative layer-by-layer-assembly approach (LBL), i.e., chitosan (Cs), beta-cyclodextrin (beta-CD), glycidyl trimethyl ammonium chloride (GTAC), to modify the semi-aromatic polyamide nanofiltration membrane to enhance the separation performance and fouling resistance. The results show that the LBL surface coating, i.e., amino group in Cs and/or hydroxyl group in beta-CD reacted with the unhydrolyzed acyl chloride group in TMC and/or epoxy group in GTAC, partially neutralized the negatively charged membrane surface with enhanced hydrophilicity. The modified membrane (named as PTCBG) was on par with or even better than the leading commercial NF270 membrane in terms of permeability and selectivity (A value: 13.1-13.7 vs. 14.2-14.3 L & sdot;m- 2 & sdot;h- 1 & sdot;bar- 1; A/B value: 9.2-46.9 vs. 3.1-8.8 bar-1, for rejection tests of typical divalent salts including CaCl2, MgCl2 and MgSO4). The flux decline rate (FDR) decreased significantly from 89% for virgin membrane to 29% for PTCBG membrane using dodecyl trimethyl ammonium bromide (DTAB) as a model foulant. The corresponding flux recovery rate (FRR) was 102%. It indicates the enhanced reversible antiorganic fouling property, which is attributed to the weakened hydrophobic and electrostatic attraction. The antibacterial rate of PTCBG relative to virgin membrane was 75% and 68% for Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The FDR of PTCBG membrane exposed to E. coli and S. aureus was 20% and 51%, much lower than 35% and 65% for virgin one. It consistently reflected the enhanced biofouling resistance for PTCBG membrane, as a result of combined effects of weak adsorption and strong sterilization on bacteria, attributed to quaternary ammonium introduced by GTAC. This study provides an innovative and straightforward strategy to fabricate polyamide membranes with good permeability/selectivity and excellent fouling resistance.

The pervasive co-occurrence of microplastics (MPs) and per- and polyfluoroalkyl substances (PFAS) poses a significant challenge for remediation technologies. Electrochemical advanced oxidation processes like electro-Fenton (EF) are promising for MPs degradation, yet their efficacy in complex, co-contaminant systems remains poorly understood. This work revealed a previously overlooked mechanism by which perfluorooctanoic acid (PFOA) and its derivatives severely inhibited the electrochemical degradation of polyethylene terephthalate (PET)-MPs in a pyrite-modified heterogeneous EF system. The inhibition evolved through two distinct phases: an initial phase (0-5 h) dominated by reactive oxygen species (ROS) and electron competition, followed by a subsequent phase (5-10 h) governed by interfacial shielding. This shielding arose from the formation of a persistent fluorine-rich layer on the MPs' surface, facilitated mainly by hydrogen bonding between the oxidized MPs' surface and PFOA-derived intermediates (i.e., short-chain fluorotelomer carboxylic acids), which blocked further ROS attack. Such inhibition led to a drastic reduction in the degradation efficiency of PET-MPs by > 50% at 50 mg/L PFOA and was still effective under more environmentally relevant conditions (1-10 μg/L PFOA). These findings underscore that in heterogeneous co-contaminant systems, interfacial interactions could induce a more profound and persistent inhibitory effect than homogeneous competition alone, providing critical insights for designing effective remediation strategies for complex environmental matrices.

This study compared the simulation results of the side-stream membrane-aerated biofilm reactor (MABR) in terms of total nitrogen (TN) removal and N2O production obtained by the conventional comammox-exclusive biological nitrogen removal (BNR) model and the novel BNR model with comammox-related model structures/parameters. Even though the conventional comammox-exclusive MABR obtained >85 % TN removal over a wide range of substrate conditions and achieved up to ∼92.0 % TN removal, it suffered from 0.90 %-4.80 % N2O production. Comparatively, despite the significantly lower N2O production (0.01 %-0.06 %), due to the undesired full nitrification of comammox bacteria, the novel comammox-inclusive MABR failed to provide adequate nitrite for anammox bacteria under exceeding substrate conditions and only obtained the maximum ∼88.0 % TN removal through comammox bacteria-based partial nitritation/anammox. Both MABRs should be operated at a moderate hydraulic retention time (e.g., 4.0 d) with a sufficient biofilm thickness (e.g., ≥300 μm) to attain efficient TN removal and reduced N2O production.

The kinetics of polyamide membrane degradation by free chlorine and halide ions (Br- and Cl-) were innovatively evaluated based on physicochemical properties and filtration performance, using water/solute permeability coefficient in addition to bromide incorporation as important indicators. The reaction rate constants for the reduced water and H3BO3 permeability coefficient were 1-2 orders of magnitude higher at 0-1 h than 1-10 h. N-bromination and bromination-promoted hydrolysis are dominant degradation mechanisms at 0-1 h (reflected by the breakage of hydrogen bond, the increased Ca binding content, and the increased charge density), and ring-bromination further occurs at 1-10 h (reflected by the disappearance or weakening of aromatic amide band and the nearly constant hydrogen bond). The more reactive but less abundant brominating agents (Br2O, BrOCl, BrCl, and Br2) played significant roles in membrane degradation, contradicting the conventional belief that HOBr is the only reactive species. BrCl at pH 4.0 and BrOCl and Br2O at pH 7.0 made significantly higher contributions to membrane degradation than HOBr (>76 % vs. <13 %). The increased contribution of BrCl and Br2 with the increased [Cl-] and [Br-]ex (the excess bromide, defined as [Br-]o - [HOCl]o when [Br-]o > [HOCl]o), respectively, was responsible for the greater reduction of water permeability coefficient. The innovative and simple approach developed in this study provides important insights to evaluate and predict membrane degradation.

This study aims to formulate the degradation mechanism of polyamide membrane by chlorine, and to assess the role of Ca2+ or Mg2+ involved in chlorination. By adjusting chlorination pH, two competing degradation mechanisms, namely chlorination-promoted hydrogen bond cleavage and chlorination-promoted hydrolysis, were first time proposed. Hydrogen bond cleavage promoted severe compaction (reduced pore radius), while hydrolysis led to a loose but non-compactable structure (increased pore radius), causing opposite trends in membrane filtration performance at different pHs. The pore radius and water flux were reduced by 33% and 69% at chlorination pH 4.0, however, water flux was increased by 45% at chlorination pH 10.0. Therefore, intermolecular rather than intramolecular bonds regulate the rotational freedom and then affect compactness of polyamide layers under pressure. Ca2+ or Mg2+ further amplified these effects of chlorine, i.e., water flux was further reduced by 7%-10% at pH 4.0 and further increased by 23%-48% at pH 7.0-10.0. The coordination between carbonyl oxygen and Ca2+ or Mg2+, evidenced by simulated molecular electrostatic potential and binding energies, initiated excessive hydrogen bond breakage between C--O and N-H. Consequently, it prompted N-chlorination, as non-hydrogen-bonded N-H has a higher chlorination priority than hydrogen- bonded N-H. In addition, Ca2+ or Mg2+ accelerated chlorination-promoted hydrolysis.

The discovery of comammox bacteria has revolutionized our understanding of nitrification, challenging the conventional paradigm that this process is mediated by two distinct microbial groups. Although comammox bacteria, particularly Candidatus Nitrospira nitrosa, are prevalent with significant activities in wastewater treatment systems, their physiological and biochemical properties, particularly growth substrates-based kinetics, are yet to be fully disclosed. To this end, we first attempted to enrich Ca. N. nitrosa in a laboratory-scale continuous stirred tank reactor fed with mainstream-level ammonium. Following the 40 d operation, a suite of microbial analyses jointly confirmed the unprecedentedly rapid specific enrichment and absolute dominance of Ca. N. nitrosa with the entire gene repertoire needed for completing the full nitrification process in the sludge. Dedicated batch tests then revealed and validated the affinity constants of Ca. N. nitrosa for total ammonium, nitrite and oxygen (0.393 ± 0.058 mg-N/L, 0.365 ± 0.001 mg-N/L and 0.006 ± 0.001 mg-O2/L, respectively), demonstrating high affinities for substrates that could underpin its widely reported competitive presence and functionality in wastewater treatment systems. This work provides the first kinetic characterization of Ca. N. nitrosa, complementing the currently limited understanding of the comammox process while enabling a better evaluation of its important role in the nitrogen cycle across diverse environments.

This study attempted to compare the enrichment of complete ammonium oxidation (comammox) bacteria, which are affiliated with Nitrospira and not able to generate nitrous oxide (N2O, a potent greenhouse gas) through biological pathways, in two commonly-utilized configurations of floccular sludge reactors, i.e., sequencing batch reactor (SBR) and continuous stirred tank reactor (CSTR), under the ammonium condition of mainstream wastewater (i.e., 40.0 g-N/m3). The results in terms of nitrification performance and microbial analyses during 216-d operation showed that compared with SBR offering a fluctuating but generally higher in-situ ammonium concentration (i.e., 1.0–6.0 g-N/m3) which was favorable for the growth of ammonium-oxidizing bacteria (AOB, belonging to Nitrosomonas in this study), CSTR managed to lower the in-situ ammonium level to < 2.0 g-N/m3, thus creating a competitive advantage for comammox bacteria with a highly oligotrophic lifestyle. Such an argument was further supported by dedicated batch tests which revealed that Nitrospira-dominant sludge had a lower maximum ammonium oxidation rate and lower apparent ammonium and oxygen affinity constants than Nitrosomonas-dominant sludge (i.e., 33.5 ± 2.1 mg-N/h/g-MLVSS vs. 139.9 ± 26.7 mg-N/h/g-MLVSS, 1.1 ± 0.1 g-N/m3 vs. 17.6 ± 4.6 g-N/m3, and 0.017 ± 0.002 g-O2/m3 vs. 0.037 ± 0.013 g-O2/m3, respectively), proving the nature of comammox bacteria as a K-strategist. Overall, this study not only provided useful insights into the effective enrichment of comammox bacteria in floccular sludge but also further revealed the interactions between comammox bacteria and AOB, thereby contributing to the future development of comammox-inclusive biological nitrogen removal technologies for sustainable wastewater treatment.

This study provides novel insights into the interactions among humic acid (HA), calcium ion (Ca2+), and nanofiltration membranes, revealing their critical impact on rejecting nine antibiotics and six estrogens, common environmental pollutants. In the presence of HA, the normalized water flux of NF270 and NF90 was decreased to 92% and 89%, respectively, and was further reduced to 71%-83% and 74%-79% with the addition of Ca2+, resulting from the reduced hydrophilicity by HA-Ca-induced deposition. HA reduced antibiotic rejection by 27% for NF270 and 3% for NF90, attributed to weakened charge repulsion and size exclusion (reflected by decreased membrane surface charge and boron rejection). Adding Ca2+ further decreased antibiotic rejection by up to 48% for NF270 and 6% for NF90, as charge repulsion and size exclusion were further diminished. HA also decreased estrogen rejection by 8%-16% for NF270 and 5%-14% for NF90, due to weakened size exclusion. After adding Ca2+, estrogen rejection showed inconsistent trends due to competing effects of size exclusion and hydrophobic interaction. Therefore, size exclusion and charge repulsion are the main rejection mechanisms for the negatively charged antibiotics, however, size exclusion and hydrophobic interaction are for the neutral and hydrophobic estrogens. The reduced membrane permeability and selectivity highlight the importance of effectively managing organic matter and divalent cations in feedwater to ensure both treatment efficiency and long-term membrane stability.