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

Systematic investigation and modeling prediction of virus inactivation by ozone in wastewater: Decoupling the matrix effects

Water disinfection is undoubtedly regarded as a critical step in ensuring the water safety for human consumption, and ozone is widely used as a highly effective disinfectant for the control of pathogenic microorganisms in water. Although the diminished ozone efficiencies in complex water matrices have been widely reported, the specific extent to which individual components of matrix act on the virus inactivation by ozone remains unclear, and effective methodologies to predict the comprehensive effects of various factors are needed. In this study, the decoupled impact of the intricate water matrix on the ozone inactivation of viruses was systematically investigated and assessed from a simulative perspective. The concept of "equivalent ozone depletion rate constant" (k') was introduced to quantify the influence of different species, and a kinetic model was developed based on the k' values for simulating the ozone inactivation processes in complex matrix. The mechanisms through which diverse species influenced the ozone inactivation effectiveness were identified: 1) competition effects (k' = 105∼107 M-1s-1), including organic matters and reductive ions (SO32-, NO2-, and I-), which were the most influential species inhibiting the virus inactivation; 2) shielding effects (k' = 103∼104 M-1s-1), including Ca2+, Mg2+, and kaolin; 3) insignificant effects (k' = 0∼1 M-1s-1), including Cl-, SO42-, NO3-, NH4+, and Br-; 4) promotion effects (k' = ∼-103 M-1s-1), including CO32- and HCO3-. Prediction of ozone disinfection efficiency and evaluation of species contribution under complex aquatic matrices were successfully realized utilizing the model. The systematic understanding and methodologies developed in this research provide a reliable framework for predicting ozone inactivation efficiency under complex matrix, and a potential tool for accurate disinfectant dosage determination and interfering factors control in actual wastewater treatment processes.

Deciphering the Roles of Extracellular Polymeric Substances (EPS) in Shaping Disinfection Kinetics through Permanent Removal via Genetic Disruption

Extracellular polymeric substances (EPS) ubiquitously encapsulate microbes and play crucial roles in various environmental processes. However, understanding their complex interactions with dynamic bacterial behaviors, especially during the disinfection process, remains very limited. In this work, we investigated the impact of EPS on bacterial disinfection kinetics by developing a permanent EPS removal strategy. We genetically disrupted the synthesis of exopolysaccharides, the structural components of EPS, in Pseudomonas aeruginosa, a well-known EPS-producing opportunistic pathogen found in diverse environments, creating an EPS-deficient strain. This method ensured a lasting absence of EPS while maintaining bacterial integrity and viability, allowing for real-time in situ investigations of the roles of EPS in disinfection. Our findings indicate that removing EPS from bacteria substantially lowered their susceptibility threshold to disinfectants such as ozone, chloramine B, and free chlorine. This removal also substantially accelerated disinfection kinetics, shortened the resistance time, and increased disinfection efficiency, thereby enhancing the overall bactericidal effect. The absence of EPS was found to enhance bacterial motility and increase bacterial cell vulnerability to disinfectants, resulting in greater membrane damage and intensified reactive oxygen species (ROS) production upon exposure to disinfectants. These insights highlight the central role of EPS in bacterial defenses and offer promising implications for developing more effective disinfection strategies.

Advanced oxidation of recalcitrant chromophores in full-scale MBR effluent for non-potable reuse of leachate co-treated municipal wastewater

Wastewater non-potable reuse involves further processing of secondary effluent to a quality level acceptable for reuse and is a promising solution to combating water scarcity. Recalcitrant chromophores in landfill leachate challenge the water quality for non-potable reuse when leachate is co-treated with municipal wastewater. In this study, we first use multivariate statistical analysis to reveal that leachate is an important source (with a Pearson's coefficient of 0.82) of recalcitrant chromophores in the full-scale membrane bioreactor (MBR) effluent. We then evaluate the removal efficacies of chromophores by chlorination, breakpoint chlorination, and the chlorination-UV/chlorine advanced oxidation treatment. Conventional chlorination and breakpoint chlorination only partially remove chromophores, leaving a colour level exceeding the standards for non-potable reuse (>20 Hazen units). We demonstrate that pre-chlorination (with an initial chlorine dosing of 20 mg/L as Cl2) followed by UV radiation (with a UV fluence of 500 mJ/cm2) effectively degraded recalcitrant chromophores (>90%). By quantifying the electron donating capacity (EDC) and radical scavenging capacity (RSC) of the reclaimed water, we demonstrate that pre-chlorination reduces EDC and RSC by up to 64%, increases UV transmittance by 32%, and increases radical yields from UV photolysis of chlorine by 1.7-2.2 times. The findings advance fundamental understanding of the alteration of dissolved coloured substances by (photo)chlorination treatment and provide implications for applying advanced oxidation processes in treating wastewater effluents towards sustainable non-potable reuse.

Multi-endpoint assays reveal more severe toxicity induced by chloraminated effluent organic matter than chloraminated natural organic matter

Disinfection by chloramination produces toxic byproducts and the difference in toxicity of reclaimed and drinking water treated by chloramination remains unclear. This study investigated cytotoxic effects at the same concentrations of dissolved organic matter and showed that chloraminated effluent organic matter (EfOM) induced 1.7 times higher cytotoxicity than chloraminated natural organic matter (NOM) applied to simulate drinking water. Chloraminated EfOM induced more reactive nitrogen species than chloraminated NOM, and chloraminated EfOM and NOM induced similar and higher levels of reactive oxygen species than the negative control, respectively. Consequently, intracellular macromolecule damage indicated by DNA/RNA damage marker 8‑hydroxy-(deoxy)guanosine and the intracellular protein carbonyl concentration induced by chloraminated EfOM was higher and slightly more than that induced by chloraminated NOM, respectively. These data were consistent with the effects on cell physiological processes. Cell cycle arrest mainly occurred in G2 phase by chloraminated EfOM and NOM. Early apoptotic cells, which could return to normal, increased upon exposure to high concentrations of chloraminated EfOM and NOM. Moreover, necrotic cells were significantly increased from 0.5% to 2.5% when the concentration increased from 20- to 60-fold chloraminated EfOM, but were not obviously changed by chloraminated NOM. These results indicated that the comprehensive intracellular changes induced by toxic substances in chloraminated EfOM were more irreversible and induced more cell death than chloraminated NOM.

Effects of alternative disinfection methods on the characteristics of effluent organic matter and the formation of disinfection byproducts

The characteristics of effluent organic matter (EfOM) and the type of disinfection methods are closely related to the formation of disinfection byproducts (DBPs) in reclaimed water. In this study, five disinfection methods, i.e., chlorination, ultraviolet (UV) followed by chlorination (UV + Cl), UV/chlorine (UV/Cl), chloramination, and chlorine dioxide (ClO2), were applied to investigate the changes in the properties of EfOM, the formation of DBPs, and the relationship between EfOM properties and DBP formation during the disinfection of four secondary biological effluents. The results showed that EfOM with medium molecular weight (MW) (0.5-6 kDa) was the dominant fraction for all WWTPs. From a fluorescence perspective, the EfOM of the AAO process was rich in humic matter, while the EfOM of the oxidation ditch (OD) process was rich in protein matter. Disinfectants tended to transfer EfOM with high molecular weight (MW) (>6 kDa) to those with low MW (<0.5 kDa). Chlorination, UV + Cl and UV/Cl were more reactive to humic matter, while chloramination and chlorine dioxide were more reactive to protein matter. The formation of known DBPs was mainly dependent on humic matter, while protein matter was more likely to generate unknown DBPs. N-DBPs only accounted for 5.7%-17.7% of the total DBPs, but contributed more than 70% of the calculated toxicity, among which bromochloroacetonitrile (BCAN), dichloroacetonitrile (DCAN), and monobromoacetamide (MBAcAm) were the most important contributors to the calculated cytotoxicity. Monobromoacetic acid (MBAA) and MBAcAm were the primary drivers of the calculated genotoxicity. Overall, UV + Cl was the suggested optimal disinfection method for WWTPs.

Chemodiversity transformation of organic matters in a full scale MBR-NF wastewater reclamation plant

Membrane bioreactor (MBR) and nanofiltration (NF) process has been attractive in wastewater reclamation, and was set as the target process in this study. Dissolved organic matter (DOM) and trace organic contaminants (TrOCs), closely associated with water safety, are noteworthy pollutants. Though the general DOM characteristics and TrOCs removal in MBR-NF reclamation process have been reported in lab-/pilot-scale experiment, the molecular characteristics of DOM revealed by high resolution mass spectrometry, and the correlation between DOM and TrOCs have been rarely studied in full-scale MBR-NF wastewater reclamation plant. In this work, biological and NF processes contributed significantly to the removal of DOM and TrOCs, while MBR filtration contributed slightly. Spectroscopic analyses revealed that DOM with higher aromaticity and lower molecular weight were more recalcitrant along the treatment. Aromatic protein-like substances were preferentially removed comparing to humic-like substances. Fourier transform ion cyclotron resonance mass spectrometry was applied to investigate DOM transformation at molecular level. DOM molecules with higher H/C and lower O/C, especially the aliphatics and peptides, were readily biodegraded into higher‑oxygenate, highly unsaturated, and aromatic compounds. The generated species mainly included condensed aromatics, polyphenols, and highly unsaturated compounds. Filtration in MBR tended to reject higher oxygenated molecules. NF effectively removed most of the DOM molecules, especially higher oxygenated molecules with low H, N and S. The residual TrOCs in the NF effluent, including sulfamethoxazole, ofloxacin, and bisphenol A, still displayed above medium environmental risk. Significant correlations were found among organic compounds, spectral indices, and peptides molecules. Positive correlation between most of the TrOCs and several DOM parameters implied that they were synchronously removed in biological and membrane filtration processes. SUVA and FI might be potential indexes in monitoring the performance of MBR-NF process in both DOM and TrOC removal. These findings would expand the understanding of DOM and TrOCs behavior in wastewater reclamation process and simplify an in-depth system monitoring.

Characteristics in dissolved organic matter and disinfection by-product formation during advanced treatment processes of municipal secondary effluent with Orbitrap mass spectrometry

Dissolved organic matter (DOM) is reported to be a precursor to disinfection by-products (DBPs), which have adverse effects on human health. Therefore, it is crucial to effectively remove DOM before water disinfection. Characteristics of DOM and DBPs formation during advanced treatment processes including coagulation, adsorption, ultraviolet (UV) irradiation, and ozone (O3) oxidation in municipal secondary effluent were investigated in this research. DOM was characterized by Fourier transform infrared spectroscopy (FTIR), excitation-emission matrix fluorescence spectroscopy (EEM), and Orbitrap mass spectrometry (Orbitrap MS). Moreover, DBPs formation potential under different advanced treatment processes was also discussed. FTIR results indicated that various functional groups existing in DOM may react with the disinfectant to form toxic DBPs. EEM analysis indicated that DOM in all water samples was dominated by soluble microbial product-like (SMPs) and humic acid-like (HA) substances. The municipal secondary effluent was abundant with DOM and rich in carbon, hydrogen, oxygen, and nitrogen atoms, contained a certain dosage of phosphorus and sulfur atoms, and the highest proportion is lignin. Most of the precursors (CHO features) had positive double bond equivalent subtracted oxygen per carbon [(DBE-O)/C] and negative carbon oxidation state (Cos) in all four different advanced treatment processes. DBPs formation potential (DBPFP) of coagulation, adsorption, UV irradiation, and O3 oxidation advanced treatment processes were 487 μg L-1, 586 μg L-1, 597 μg L-1, and 308 μg L-1, respectively. And the DBPs precursors removal efficiency of coagulation, adsorption, UV irradiation, and O3 oxidation advanced treatment processes were 50.8%, 40.8%, 39.8%, and 69.0%, respectively. This study provides in-depth insights into the changes of DOM in municipal secondary effluent at the molecular level and the removal efficiency of DBPs precursors during coagulation, adsorption, UV irradiation, and O3 oxidation advanced treatment processes.

Removal of disinfection byproducts through integrated adsorption and reductive degradation in a membrane-less electrochemical system

Proper control/removal of disinfection byproducts (DBPs) is important to drinking water safety and human health. In this study, a membrane-less electrochemical system was developed and investigated to remove DPBs through integrated adsorption and reduction by granular activated carbon (GAC)-based cathode. Representative DPBs including trihalomethanes and haloacetonitriles at drinking water concentrations were used for removal experiments. The proposed system achieved >70% removal of most DBPs in a batch mode. The comparison with control tests under either open circuit or hydrolysis demonstrated the advantages of electrochemical treatment, which not only realized higher DPBs removal but also extended GAC cathode lifetime. Such advantages were further demonstrated with continuous treatment. High dechlorination and debromination efficiencies were obtained in both batch (82.2 and 94.3%) and continuous (79.3 and 87.6%) reactors. DBPs removal was mainly contributed by the electrochemical reduction and adsorption by the GAC-based cathode, while anode showed little oxidizing effect on DBPs and halide ions. Dehalogenated products of chloroform and dichloroacetonitrile were identified with toxicity reduction. The energy consumption of the continuously operated system was estimated to be 0.28 to 0.16 kWh m-3. The proposed system has potential applications for wastewater reuse or further purification of drinking water.

Microbiome recognition of virulence-factor-governed interfacial mechanisms in antibiotic resistance and pathogenicity removal by functionalized microbubbles

The frequent occurrence of epidemics around the world gives rise to increasing concerns of the pollution of pathogens and antibiotic resistant bacteria in water. This study investigated the impacts of virulence factors (VFs) on the removal of antibiotic resistant and pathogenic bacteria from municipal wastewater by ozone-free or ozone-encapsulated Fe(III)-coagulant-modified colloidal microbubbles (O3_free-CCMBs or O3-CCMBs). The highly interface-dependent process was initiated with cell-capture on the microbubble surface where the as-collected cells could be further inactivated with the bubble-released ozone and oxidative species if O3-CCMBs were used. The microbiome sequencing analyses denote that the O3_free-CCMB performance of antibiotic resistant and pathogenic bacteria removal was dependent on the virulence phenotypes related to cell-surface properties or structures. The adhesion-related VFs facilitated the effective attachment between cells and the coagulant-modified bubble-surface, which further enhanced cell inactivation by bubble-released ozone. On the contrary, the motility-related VFs might help cells to escape from the bubble capture by locomotion; however, this could be overcome by O3-CCMB-induced oxidative demolition of the movement structures. Besides, the microbubble performance was also impacted with the cell-membrane structure related to antibiotic resistance (i.e., efflux pumps) and the dissolved organic matter through promoting the surface-capture and decreasing the oxidation efficacy. The ozone-encapsulated microbubbles with surface functionalization are robust and promising tools in hampering antibiotic resistance and pathogenicity dissemination from wastewater to surface water environment; and awareness should be raised for the influence of virulence signatures on its performance.

Emerging disinfection by-products formation of various molecular weight organic matter fractions in raw water contaminated with treated wastewater

Combining dissolved organic matter (DOM) in raw water (RW) with DOM in treated wastewater (TWW) can react with chlorine and pose emerging disinfection by-products (DBPs). This study evaluated DOM based on the molecular weight (MW) size fractionation, trihalomethane, iodinated-trihalomethane, haloacetonitrile, and trichloronitromethane formation potential (THMFP, I-THMFP, HANFP, and TCNMFP) of the RW from the U-Tapao Canal, Songkhla, Thailand and the RW mixed with TWW (RW + TWW) samples. The RW and RW + TWW were treated by coagulation with poly aluminum chloride. The DOM of RW and RW + TWW and their treated water was distributed most in the MW below 1 kDa. The MWs of 3-10 kDa and 1-3 kDa were the active DOM involved in the specific THMFP for the RW + TWW. The MW of < 1 kDa in the RW + TWW resulted in a slightly high specific I-THMFP and HANFP. The MW of 1 - 3 kDa in the coagulated samples had a high specific I-THMFP. The MW of > 10 kDa in the coagulated RW + TWW was a precursor for a particular HANFP. Monitoring systems for measuring the level of TWW mixed with RW and an effective process to enhance the efficiency of traditional water treatment must be set up to produce a consumer-safe water supply.

Molecular probing of dissolved organic matter and its transformation in a woolen textile wastewater treatment station

Woolen textile industry produces enormous wastewater (WTIW) with high pollution loads, and needs to be treated by wastewater treatment stations (WWTS) before centralized treatment. However, WTIW effluent still contains many biorefractory and toxic substances; thus, comprehensive understandings of dissolved organic matter (DOM) of WTIW and its transformation are essential. In this study, total quantity indices, size exclusion chromatography, spectral methods, and Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) were used for comprehensively characterizing DOM and its transformation during full-scale treatments, including influent, regulation pool (RP), flotation pool (FP), up-flow anaerobic sludge bed (UA), anaerobic/oxic (AO) and effluent. DOM in influent featured a large molecular weight (5-17 kDa), toxicity (0.201 HgCl₂ mg/L), and a protein content of 338 mg C/L. FP largely removed 5-17 kDa DOM with the formation of 0.45-5 kDa DOM. UA and AO removed 698 and 2042 chemicals, respectively, which were primarily saturated components (H/C > 1.5); however, both UA and AO contributed to the formation of 741 and 1378 stable chemicals, respectively. Good correlations were found among water quality indices and spectral/molecular indices. Our study reveals the molecular composition and transformation of WTIW DOM during treatments and encourages the optimization of the employed processes in WWTS.

The impact of inorganic ions on the solar photolysis of chlorinated dissolved organic matter from different sources: Spectral characteristics, disinfection byproducts, and biotoxicities

Dissolved organic matter (DOM) from wastewater treatment plant (WWTP) effluent is chlorinated and then discharged into natural waters, where it is subject to solar irradiation. However, the impacts of inorganic ions in natural waters on the photochemical transformations of the chlorinated DOM (DOM-Cl) have not been studied comprehensively. In this study, variations in the spectral characteristics, disinfection byproducts (DBPs), and biotoxicities of DOM-Cl under solar irradiation at different pH values and in the presence of NO3- and HCO3- were revealed. Three sources of DOM, including DOM from a WWTP effluent, natural organic matter from the Suwannee River, and DOM from plant leaf leachate, were investigated. Solar irradiation resulted in the oxidation of the highly reactive aromatic structures and then reduced the amounts of chromophoric and fluorescent DOM, especially under alkaline conditions. Moreover, alkaline conditions significantly promoted the detected DBPs degradation and the biotoxicities attenuation, while NO3- and HCO3- generally impeded them (or did not work). Dehalogenation of the unknown halogenated DBPs and photolysis of the nonhalogenated organics were the main mechanisms for the DOM-Cl biotoxicity reductions. Hence, improving the ecological safety of WWTP effluents could be achieved through solar irradiation by removing the DBPs formed.

UV/chlorine and chlorination of effluent organic matter fractions: Tracing nitrogenous DBPs using FT-ICR mass spectrometry

UV/chlorine process is a promising advanced treatment to eliminate pathogen and remove refractory micropollutants for reclamation of municipal secondary effluent. However, effluent organic matter (EfOM) featuring high organic nitrogen content serves as a potential precursor for nitrogenous disinfection byproducts (N-DBPs) of health concern. The molecular-level alteration of a hydrophobic (HPO) EfOM fraction and a transphilic (TPI) EfOM fraction isolated from the same municipal effluent and the formation of N-DBPs in the UV/chlorine were tracked by ultrahigh-resolution mass spectrometry. Compared with chlorination, UV/chlorine induced a significantly greater modification on the molecular composition of EfOM and resulted in formation of unique formulae and chlorinated molecules with higher degree of oxidation, lower aromaticity, and less carbon number due to the involvement of reactive radical species. For both EfOM fractions, UV/chlorine formed more diverse DBPs with higher intensity and Cl-incorporation than chlorination. The TPI fraction of EfOM characterized by higher O/C and N/C ratios generated more N-DBPs with higher intensity clustered in the high O/C region than the HPO fraction of EfOM by both UV/chlorine and chlorination. Totally, 207 and 117 nitrogen-containing chlorinated formulae were recorded after UV/chlorine treatment of TPI and HPO, respectively. Precursor tracking found a greater number of DBPs were originated from raw EfOM through electrophilic substitution pathway rather than chlorine addition. Toxicity bioassays demonstrated that DBPs can trigger oxidative stress-induced DNA damage, while HPO fraction of EfOM dominated the induction of cytotoxicity. However, no correlation could be established between the diversity/abundance of N-DBPs and the level of DNA damage. A total of 22 DBPs with a significant rank correlation with DNA damage were identified, while C₈H₆O₅NCl was found as the N-DBP with the strongest correlation. The potential toxic chlorine-containing formula with the most abundant intensity was assigned to C₅HO₃Cl₃. This study suggests that the character and transformation of EfOM and associated toxicity is critical to evaluate the UV/chlorine process toward practical application.

Microbial Transformation of Dissolved Organic Sulfur during the Oxic Process in 47 Full-Scale Municipal Wastewater Treatment Plants

Dissolved organic sulfur (DOS) is a significant part of effluent organic matter of wastewater treatment plants (WWTPs) and poses a potential ecological risk for receiving waters. However, the oxic process is a critical unit of biological wastewater treatment for microorganisms performing organic matter removal, wherein DOS transformation and its mechanism are poorly understood. This study investigated the transformation of DOS during the oxic process in 47 full-scale municipal WWTPs across China from molecular and microbial aspects. Surprisingly, evident differences in DOS variations (ΔDOS) separated sampled WWTPs into two groups: 28 WWTPs with decreased DOS concentrations in effluents (ΔDOS < 0) and 19 WWTPs with increased DOS (ΔDOS > 0). These two groups also presented differences in DOS molecular characteristics: higher nitrogen/carbon (N/C) ratios (0.030) and more peptide-like DOS (8.2%) occurred in WWTPs with ΔDOS > 0, implying that peptide-like DOS generated from microbes contributed to increased DOS in effluents. Specific microbe-DOS correlations (Spearman correlation, p < 0.05) indicated that increased effluent DOS might be explained by peptide-like DOS preferentially being produced during copiotrophic bacterial growth and accumulating due to less active cofactor metabolisms. Considering the potential environmental issues accompanying DOS discharge from WWTPs with ΔDOS > 0, our study highlights the importance of focusing on the transformation and control of DOS in the oxic process.