Exploring the relative changes in dissolved organic matter for assessing the water quality of full-scale drinking water treatment plants using a fluorescence ratio approach

A novel chitosan and polyferric sulfate composite coagulant for biogas slurry pretreatment by simultaneous flocculation and floatation: Performance and underlying mechanisms

Biogas slurry from anaerobic digestion is rich in nutrients but has not been fully utilized due to a high content of suspended solids (SS) causing clogging during agricultural irrigation. This study aimed to evaluate the performance of a novel chitosan and polyferric sulfate (CTS-PFS) composite coagulant for simultaneous flocculation and floatation to enhance SS removal while preserving nutrients in biogas slurry. Orthogonal method was used for experimental design to determine the optimal synthesis and operational conditions of CTS-PFS. Results show that CTS-PFS outperformed individual CTS and PFS coagulant in terms of SS removal and nutrient (nitrogen, phosphorus, and potassium) preservation. Compared to individual CTS and PFS coagulation, the combination of CTS and PFS at the mass ratio of 1:6 showed significantly higher performance by 41.5 % increase in SS removal and 5.2 % reduction in nutrient loss. The improved performance of CTS-PFS was attributed to its formation of polynuclear hydroxyl complexes with ferric oxide groups (e.g. Fe-OH, Fe-O-Fe, Fe-OH-Fe and COO-Fe) to strengthen charge neutralization and adsorption bridging. Data from this study further confirm that CTS-PFS enhanced the removal of small suspended particles and dissolved organic matter in the molecular weight range of 0.4-2.0 kDa and preserved ammonia and potassium better in biogas slurry. Bubbles were generated as hydrogen ions from coagulant hydrolysis interacted with bicarbonate and carbonate in biogas slurry for removing the produced flocs by floatation. Floc flotation was more effective in CTS-PFS coagulation due to the significant production of uniform bubbles, evidenced by the reduction in the viscosity of biogas slurry.

Tracking the changes of dissolved organic matter throughout the city water supply system with optical indices

Dissolved organic matter (DOM) is important in determining the drinking water treatment and the supplied water quality. However, a comprehensive DOM study for the whole water supply system is lacking and the potential effects of secondary water supply are largely unknown. This was studied using dissolved organic carbon (DOC), absorption spectroscopy, and fluorescence excitation-emission matrices-parallel factor analysis (EEM-PARAFAC). Four fluorescent components were identified, including humic-like C1-C2, tryptophan-like C3, and tyrosine-like C4. In the drinking water treatment plants, the advanced treatment using ozone and biological activated carbon (O3-BAC) was more effective in removing DOC than the conventional process, with the removals of C1 and C3 improved by 17.7%-25.1% and 19.2%-27.0%. The absorption coefficient and C1-C4 correlated significantly with DOC in water treatments, suggesting that absorption and fluorescence could effectively track the changes in bulk DOM. DOM generally remained stable in each drinking water distribution system, suggesting the importance of the treated water quality in determining that of the corresponding network. The optical indices changed notably between distribution networks of different treatment plants, which enabled the identification of changing water sources. A comparison of DOM in the direct and secondary water supplies suggested limited impacts of secondary water supply, although the changes in organic carbon and absorption indices were detected in some locations. These results have implications for better understanding the changes of DOM in the whole water supply system to help ensure the supplied water quality.

The treatment of petrochemical wastewater via ozone-persulfate coupled catalytic oxidation: mechanism of removal of soluble organic matter

Petrochemical wastewater contains a variety of organic pollutants. Advanced oxidation processes (AOPs) are used for deep petrochemical wastewater treatment with distinct advantages, including the complete mineralization of organic substances, minimal residual byproducts, and compatibility with biological treatment systems. This work evaluates the effectiveness of three methods, namely, ozone, persulfate, and O3-PMS (ozone-persulfate) processes, which were compared to remove soluble organic matter. The O3-PMS process offered significant advantages in terms of organic matter removal efficiency. This process involves ozone dissolution in an aqueous persulfate solution, producing a more significant amount of hydroxyl radicals in comparison to single AOPs. The production of hydroxyl radicals and the synergistic effect of hydroxyl radicals and persulfate radicals were investigated. In the O3-PMS process, transition metal ions were added to understand the mechanism of the O3-PMS coupled catalytic oxidation system. The results showed that when the ozone concentration was in the range of 5 ~ 25 mg/L, the dosage of persulfate was in the range of 0.01 ~ 0.05 mol/L, the dosage of metal compounds was in the range of 0:0 ~ 2:1, and the reaction time was in the range of 0 ~ 2 h; the optimum chemical oxygen demand (CODCr) and total organic content (TOC) removal effect was achieved under the coupled system with an ozone concentration of 10 mg/L, a persulfate dosage of 0.02 mol/L, a 1:2 dosage ratio of between Fe2+ and Cu2+ compounds, and a reaction time of 2 h. Under optimal reaction conditions, the rates of CODCr and TOC removal reached 70% and 79.3%, respectively. Furthermore, the removal kinetics of the O3-PMS coupled catalytic oxidation system was analyzed to optimize the removal conditions of COD and TOC, and the mechanism regulating the degradation of dissolved organic matter was explored by three-dimensional fluorescence and GC-MS technology. Thus, O3-PMS coupled catalytic oxidation is an effective process for the deep treatment of wastewater. The careful selection of transition metal ions serves as a theoretical foundation for the subsequent preparation of catalysts for the ozone persulfate oxidation system, and this study provides a suitable reference for removing organic matter from petrochemical wastewater.

Analysing N-nitrosamine occurrence and sources in karst reservoirs, Southwest China

N-nitrosamines in reservoir water have drawn significant attention because of their carcinogenic properties. Karst reservoirs containing dissolved organic matter (DOM) are important drinking water sources and are susceptible to contamination because of the fast flow of various contaminants. However, it remains unclear whether N-nitrosamines and their precursor, DOM, spread in karst reservoirs. Therefore, this study quantitatively investigated the occurrence and sources of N-nitrosamines based on DOM properties in three typical karst reservoirs and their corresponding tap water. The results showed that N-nitrosamines were widely spread, with detection frequencies > 85%. Similar dominant compounds, including N-nitrosodimethylamine, N-nitrosomethylethylamine, N-nitrosopyrrolidine, and N-nitrosodibutylamine, were observed in reservoirs and tap water, with average concentrations of 4.7-8.9 and 2.8-6.7 ng/L, respectively. The average carcinogenic risks caused by these N-nitrosamines were higher than the risk level of 10-6. Three-dimensional fluorescence excitation-emission matrix modeling revealed that DOM was composed of humus-like component 1 (C1) and protein-like component 2 (C2). Fluorescence indicators showed that DOM in reservoir water was mainly affected by exogenous pollution and algal growth, whereas in tap water, DOM was mainly affected by microbial growth with strong autopoietic properties. In the reservoir water, N-nitrosodiethylamine and N-nitrosopiperidine were significantly correlated with C2 and biological indicators, indicating their endogenously generated sources. Based on the principal component analysis and multiple linear regression methods, five sources of N-nitrosamines were identified: agricultural pollution, microbial sources, humus sources, degradation processes, and other factors, accounting for 46.8%, 36.1%, 7.82%, 8.26%, and 0.96%, respectively. For tap water, two sources, biological reaction processes, and water distribution systems, were identified, accounting for 75.7% and 24.3%, respectively. Overall, this study presents quantitative information on N-nitrosamines' sources based on DOM properties in typical karst reservoirs and tap water, providing a basis for the safety of drinking water for consumers.

Seasonal variations in dissolved organic matter concentration and composition in an outdoor system for bank filtration simulation

Dissolved organic matter (DOM) in surface waters can vary markedly in character depending on seasonal variations such as rainfall intensity, UV radiations and temperature. Changes in DOM as well as temperature and rainfall intensity over the year can affect the biochemical processes occurring in bank filtration (BF). Identification and characterization of DOM in the surface water could help to optimize the water treatment and provide stable and safe drinking water. This study investigated year-long variations of DOM concentrations and compositions in a surface water of a circulated outdoor pond (research facility) connected to a BF passage. DOM was dominated by humic substances and a changing pattern of DOM in surface water was observed throughout the year. A significant increase of DOM (∼ 38%) in surface water was noted in August compared to November. The fluorescent DOM showed that DOM in summer was enriched with the degradable fraction whilst non-degradable fraction was dominated in winter. A constant (1.7 ± 0.1 mg/L) effluent DOM was recirculated in the system throughout the year. DOM removal through BF varied between 4% to 39% and was achieved within a few meters after infiltration and significantly correlated with influent DOM concentration (R2 = 0.82, p < 0.05). However, no significant (p > 0.05) change in the removal of DOM was observed in two subsurface layers (upper and lower). This study highlights the presence of a constant non-degradable DOM in the bank filtrate, which was not affected by temperature, redox conditions and UV radiations.

Fluorescence spectroscopy in the detection and management of disinfection by-product precursors in drinking water treatment processes: A review

Monitoring and prevention of the formation of disinfection by-products (DBPs) is paramount in drinking water treatment plants (DWTP) to ensure human health safety. This review provides an overview of how fluorescence techniques are developed to predict DBP formation and to evaluate the reduction of fluorescence components and DBPs following individual DWTP processes. Evidence has shown that common DBPs, nitrogenous DBPs and specific emerging DBPs exhibit positive linear relationships with terrestrial, anthropogenic, tryptophan-like, and eutrophic humic-like fluorescence. Due to the interrelationships of both regulated and emerging DBP types with fluorescence components, the limitations arise when attempting to predict emerging DBPs solely through linear relationships. Monitoring the reduction of DBP precursors after each treatment process can be achieved by studying the relationship between fluorescence components and DBPs. During the coagulation process, highest reduction rates are observed for terrestrial humic-like fluorescence. Advanced treatments such as granular, powdered, silver-impregnated activated carbon, magnetic ion exchange resins, and reverse osmosis, have revealed a significant reduction of fluorescent DBP precursors, ranging from 53% to 100%. During chlorination, the reduction rate follows the order: terrestrial humic-like > microbial humic-like > protein/tryptophan-like fluorescence. This review provides insights into the reduction of fluorescence signatures following individual DWTP processes, which offers information regarding DBP formation. These insights could assist in optimizing the treatment process to more effectively manage DBP formation. For the identification of emerging DBP generation, the utilization of advanced models is imperative to precisely predict emerging DBPs and to more accurately trace DBP precursors within DWTPs.

Characteristics of optical properties of DOM and nutrients in rainwater of different ecological areas of a large reservoir in China

Numerous studies have focused on the spectral characteristics and seasonal variations of dissolved organic matter (DOM) in rainwater. However, the relationship between the optical indices of DOM and nutrients in rainwater from different ecological areas of large reservoirs is poorly understood. A one-year monitoring study was conducted between March 2019 and February 2020 in the Danjiangkou Reservoir in Henan Province, China, to compare the composition, spectral characteristic parameters, and relationship between the optical indices of DOM and nutrients in rainwater under different ecological environments. The study showed that the average value of a300 in all samples was 5.29 ± 2.16 m-1 and showed a seasonal trend of higher in spring and winter and lower in summer and autumn as well as a regional difference of agricultural area > urban area > reservoir area. A three-dimensional fluorescence with parallel factor analysis (EEM-PARAFAC) revealed four components of the rainwater: C1 and C2 as UV humic-like substances, and C3 and C4 as protein-like substances. The protein-like components of rainwater from agricultural areas had a high fluorescence intensity, whereas the UV humic-like components of rainfall from urban and reservoir areas had a high fluorescence intensity. Analysis of the fluorescence indices showed that rainwater DOM humification was low and had a strong endogenous character in the Danjiangkou Reservoir. The redundancy analysis revealed that NO3--N, DTN, and SO42- mainly influenced the DOM optical indices of rainwater in urban areas, EC, DTN, and DOC had the highest interpretation of the DOM optical indices of rainwater in agricultural areas, and SO42-, DOC, and DTN had the highest interpretation of the DOM optical indices of rainwater in the reservoir. Overall, understanding the characteristics of rainfall DOM fluorescence and the relationships with nutrients in different ecological regions provides important information for comprehending biogeochemical processes in reservoirs.

Carbon dots/TiO2 enhanced visible light-assisted photocatalytic of leachate: Simultaneous effects and Mechanism insights

The persistence and potential fouling risks associated with humic substances and bacteria present in leachate have gained increasing attention. Therefore, developing efficient and environmentally compatible technologies for their removal is essential. This study presented the hydrothermal synthesis of a photocatalyst by coupling carbon dots (CDs) and bulk TiO2 (P25). The incorporation of CDs increased the photocatalytic performance by enhancing visible light absorption and facilitating the separation of electrons/holes. Compared to P25, the CDs/P25 exhibited optimal photocatalytic activity for humic acid (HA), fulvic acid (FA), and leachate, with 1.64, 1.02, and 1.12 times higher activity, respectively. Remarkedly, the CDs/P25 accelerated the conversion of large HA molecules into small molecules at a faster rate and higher amount than the bulk P25, due to the increase of hydroxyl radicals, monoclinic oxygen radicals, and superoxide radicals. Additionally, the CDs/P25 demonstrated better bacterial-deactivation ability than the P25, with dead bacteria percentages of 83.3% and 34.6%, respectively. This study provides a promising strategy for efficiently applying CDs/P25 photocatalysis to leachate treatment.

Fluorescence-based indicators predict the performance of conventional drinking water treatment processes: Evaluation based on the changes in the compositions of dissolved organic matter

This study investigated the treatability of dissolved organic matter (DOM) by the selected lab-scale drinking water treatment processes using fluorescence excitation-emission matrix (EEM) analysis. The fluorescence ratio Peak 3/Peak 2 was established from well-defined fluorescence peak intensity of humic-like components (Ex/Em: 225 nm/425 nm) and protein-like components (Ex/Em: 230 nm/345 nm). Peak 3/Peak 2 predicted the aromatic characteristics of DOM and their origins in the different natural surface water feeding the different drinking water treatment plants. The drinking water treatment processes confirmed the treatability of DOM using Peak 3/Peak 2 and was well-confirmed by specific UV₂₆₀ absorbance relative to dissolved organic carbon (DOC) (SUVA) and fluorescence-based indices. Peak 3/Peak 2 was demonstrated to have a strong correlation with SUVA and DOC removal for the water after treatment by coagulation, adsorption, and chlorination. Compared to the humification index and fluorescence index, Peak 3/Peak 2 is better for indicating the DOM composition in terms of treatability. These findings can broaden the use of fluorescence spectroscopy in water treatment applications, by developing the fluorescence ratio to evaluate the performance of drinking water treatment plants.

FexO/FeNC modified activated carbon packing media for biological slow filtration to enhance the removal of dissolved organic matter in reused water

The biological slow filtration reactor (BSFR) process has been found to be moderately effective for the removal of refractory dissolved organic matter (DOM) in the treatment of reused water. In this study, bench scale experiments were conducted using a mixture of landscape water and concentrated landfill leachate as feed water, to compare a novel iron oxide (Fe_xO)/FeNC modified activated carbon (Fe_xO@AC) packed BSFR, with a conventional activated carbon packed BSFR (AC-BSFR), operated in parallel. The results showed that the Fe_xO@AC packed BSFR had a refractory DOM removal rate of 90%, operated at a hydraulic retention time (HRT) of 10 h at room temperature for 30 weeks, while under the same conditions the removal by the AC-BSFR was only 70%. As a consequence, the treatment by the Fe_xO@AC packed BSFR substantially reduced the formation potential of trihalomethanes, and to a less extent, haloacetic acids. The modification of Fe_xO/FeNC media raised the conductivity and the oxygen reduction reaction (ORR) efficiency of the AC media to accelerate the anaerobic digestion by consuming the electrons that are generated by anaerobic digestion itself, which lead to the marked improvement in refractory DOM removal.

Transformation of dissolved organic matter at a full-scale petrochemical wastewater treatment plant

Transformation of dissolved organic matter (DOM) in petrochemical wastewater (PCW) treatment has rarely been studied. In this work, low- and high-salinity PCW were collected from a treatment plant and the transformations of DOM at molecular level along the treatment processes of both PCW were comparatively investigated. By using Orbitrap MS, the polar DOM constituents were categorized into five molecular classes namely saturated compounds, aliphatics, highly unsaturated and phenolic compounds (Huph), polyphenols and condensed polycyclic aromatics (Cpla). Aliphatics (58.62%) with low molecular weight (150-250 Da) and O/C (0-0.2) were dominant in raw low-salinity PCW; while Huph (65.03%) with O/C at 0.2-0.8 were rich in raw high-salinity PCW. After full-scale treatment, differentiated DOM constituents in both raw PCWs were transformed into aliphatics and Huph with O/C at 0.3-0.5. Anoxic/Oxic treatment of low-salinity system (L-A/O) removed a high fraction of aliphatics (53.05%); while Huph with low O/C (0.1-0.3) (65.68%) in the effluent of L-A/O were further mineralized by ozonation of low-salinity system (L-ozonation). In comparison, anoxic/oxic treatment of high-salinity system (H-A/O) mainly removed unsaturated Huph (34.10%) and aliphatics (30.86%). This resulted in a decrease of dissolved organic carbon as indicated via Spearman correlation. Different from L-ozonation, ozonation of high-salinity system (H-ozonation) degraded aliphatics (26.09%) and Huph (41.85%) with a relatively high O/C (0.2-1.2). After L-A/O and L-ozonation treatments, remaining saturated compounds that were originated from raw low-salinity PCW, were removed by subsequent biological aerated filter. Comparatively, after H-A/O and H-ozonation treatments, residual Huph and aliphatics which were mainly bio-derivates and ozonated intermediates, were further removed by air flotation filter. Hence, DOM transformation of different PCWs along similar treatments varied significantly. This study provides in-depth insights on DOM transformation along a full-scale PCW treatment process.

Spectral change of dissolved organic matter after extracted by solid-phase extraction and its feasibility in predicting the acute toxicity of polar organic pollutants in textile wastewater

Spectroscopic parameters can be used as proxies to effectively trace the occurrence of organic trace contaminants, but their suitability for predicting the toxicity of discharged industrial wastewater with similar spectra is still unknown. In this study, the organic contaminants in treated textile wastewater were subdivided and extracted by four commonly-used solid-phase extraction (SPE) cartridges, and the resulting spectral change and toxicity of textile effluent were analyzed and compared. After SPE, the spectra of the percolates from the four cartridges showed obvious differences with respect to the substances causing the spectral changes and being more readily adsorbed by the WAX cartridges. Non-target screening results showed source differences in organic micropollutants, which were one of the main contributors leading to their spectral properties and spectral variations after SPE in the effluents. Two fluorescence parameters (C1 and humic-like) identified by the excitation emission matrix-parallel factor analysis (EEM-PARAFAC) were closely correlated to the toxicity endpoints for Scenedesmus obliquus (inhibition ratios of cell growth and Chlorophyll-a synthesis), which can be applied to quantitatively predict the change of toxicity effect caused by polar organic pollutants. The results would provide novel insights into the spectral feature analysis and toxicity prediction of the residual DOM in industrial wastewater.

Enhanced removal of trace pesticides and alleviation of membrane fouling using hydrophobic-modified inorganic-organic hybrid flocculants in the flocculation-sedimentation-ultrafiltration process for surface water treatment

Pesticide concentrations in surface water occasionally exceed regulated values due to seasonal events (rainy season in high intensity agricultural areas) or intermittent discharges (leakage, spillage, or other emergency events). The need to remove pesticide compounds in these situations poses a challenge for drinking water treatment plants (DWTPs). In this work, the performance of dosing hydrophobic-modified inorganic-organic hybrid flocculants (HOC-M; lower acute toxicity than corresponding metal salt coagulants; acceptable economic costs when M=Al or Fe; prepared in large-scale quantities), for the removal of four different pesticides (each initial concentration: 0.25 μg/L) from Yangtze River water, and in mitigating membrane fouling, by an integrated flocculation-sedimentation-ultrafiltration (FSUF) process, was evaluated over a period of 40 days; the FSUF is well-established in many DWTPs. The mechanisms underlying the treatment were unveiled by employing a combination of instrumental characterizations, chemical computations, material flow analyses, and statistical analyses. Efficient pesticide removal (80.3%∼94.3%) and membrane fouling reduction (26.6%∼37.3% and 28.3%∼57.6% for reversible and irreversible membrane resistance, respectively) in the FSUF process were achieved by dosing HOC-M, whereas conventional inorganic coagulants were substantially inferior for pesticide removal (< 50%) and displayed more severe fouling development. Hydrophobic association between the pesticides and the hydrophobic organic chain of HOC-M played a predominant role in the improvement in pesticide removal; coexisting particulate/colloid inorganic minerals and natural organic matter with HOC-M adsorbed on the surface, acting as floc building materials, provided sites for the indirect combination of pesticides into flocs. The observed fouling alleviation from dosing HOC-M was ascribed to both the pre-removal of fouling-causing materials in the flocculation-sedimentation prior to UF, and a stable hydrophilization modification effect of residual HOC-M in the UF unit. The latter effect resulted from a hydrophobic association between the PVDF substrate of the membranes and the hydrophobic organic chains of the HOC-M, causing the hydrophilic ends of the HOC-M to be exposed away from the membrane surface, thereby inhibiting foulant accumulation. This work has not only demonstrated the superior performance of dosing HOC-M in the FSUF process for trace pesticide removal in DWTPs, but also clarified the underlying mechanisms.

A review of long-term change in surface water natural organic matter concentration in the northern hemisphere and the implications for drinking water treatment

Reduced atmospheric acid deposition has given rise to recovery from acidification - defined as increasing pH, acid neutralization capacity (ANC), or alkalinity in surface waters. Strong evidence of recovery has been reported across North America and Europe, driving chemical responses. The primary chemical responses identified in this review were increasing concentration and changing character of natural organic matter (NOM) towards predominantly hydrophobic nature. The concentration of NOM also influenced trace metal cycling as many browning surface waters also reported increases in Fe and Al. Further, climate change and other factors (e.g., changing land use) act in concert with reductions in atmospheric deposition to contribute to widespread browning and will have a more pronounced effect as deposition stabilizes. The observed water quality trends have presented challenges for drinking water treatment (e.g., increased chemical dosing, poor filter operations, formation of disinfection by-products) and many facilities may be under designed as a result. This comprehensive review has identified key research areas to be addressed, including 1) a need for comprehensive monitoring programs (e.g., larger timescales; consistency in measurements) to assess climate change impacts on recovery responses and NOM dynamics, and 2) a better understanding of drinking water treatment vulnerabilities and the transition towards robust treatment technologies and solutions that can adapt to climate change and other drivers of changing water quality.

Molecular-level characterization of natural organic matter in the reactive electrochemical ceramic membrane system for drinking water treatment using FT-ICR MS

Applications of electrochemical advanced oxidation processes are rising in drinking water treatment for effective mitigation of refractory organic compounds. This study explored the fate of natural organic matter (NOM) (lake water and standard NOM (SRNOM solution)) at molecular-level in the reactive electrochemical membrane (REM) system utilizing Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Fluorescence spectroscopy showed above 90 % removal of the humic-like component in both lake water and SRNOM solution in 10 min of REM operation compared to 70-80 % removal of the fulvic-like component after 30 min. REM-based treatment effectively eliminated (>70 %) the disinfection byproduct precursors. The lake water, sharing ~70 % of similar compounds with SRNOM, displayed a different propensity toward electrochemical oxidation, and its finished water was characterized with relatively lower double-bond equivalent (DBE), nominal oxidation state of carbon (NOSC), and aromaticity compared to that of SRNOM. The chloride ions in the water matrix of lake water impacted the electrochemical oxidation and generated significantly different transformation products than SRNOM solution. The heteroatoms (N and S) containing compounds (CHON and CHOS) were preferentially degraded in lake water; however, CHOS compounds were removed fewer in SRNOM. The electrosorption and electrochemical oxidation on the REM surface were the significant contributors for NOM removal. The newly formed compounds were mostly retained on the REM surface and fewer were released in finished water. This study is believed to help understand the fate of NOM in real source drinking water during electrochemical treatment.

Correlation Analysis of the Carboxyl and Carbonyl Groups of Natural Organic Matter and the Formation Potential of Trihalomethanes and Chloral Hydrate

Natural organic matter (NOM) has always been considered the main precursor of disinfection by-products (DBPs) during the chlorine disinfection of drinking water. This research focuses on investigating the correlation between the functional group (carboxyl and carbonyl groups) content of NOM and the formation of trichloromethane (TCM) and chloral hydrate (CH). The quantitative determination of carboxyl groups, carbonyl groups, TCM, and CH were conducted during the drinking water treatment processes with different coagulant dosages and with/without pre-oxidation by KMnO4 or NaClO. The most appropriate coagulant for the removal of conventional components was polyaluminum chloride (PAC), and the dosage was 110 mg/L. Up to 43.7% and 14.5% of the carboxyl and carbonyl groups, respectively, were removed through the coagulation and sedimentation processes, which can be enhanced by increasing PAC dosage. The filtration process further increased the removal rates of these two functional groups to 59.8% and 33.5%, respectively. The formation potential of the TCM and CH decreased as the PAC dosage increased. Pre-oxidation by KMnO4 (0.8–1.0 mg/L) effectively controlled the formation of DBPs while increasing the carboxyl and carbonyl group content. Pre-oxidation by NaClO decreased the formation of TCM rather than CH, and a suitable amount (0.5–1.0 mg/L) decreased the carboxyl and carbonyl groups. It was found that there was a good linear correlation between carboxyl groups and TCM and CH. The linear fit R2 values of the carboxyl groups to TCM and CH were 0.6644 and 0.7957, respectively. The linear fit R2 values of the carbonyl groups to TCM and CH were 0.5373 and 0.7595, respectively.

Using EEM-PARAFAC to identify and trace the pollution sources of surface water with receptor models in Taihu Lake Basin, China

The identification and apportionment of the multiple pollution sources are essential and crucial for improving the effectiveness of surface water resources management. In this study, the surface water samples were collected from Taihu Lake Basin, and the optimal water quality parameters for the receptor models were selected firstly with multivariate statistical analyses. In order to identify the potential pollution sources in surface water, dissolved organic matter (DOM) was analyzed with the excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC). Through the Pearson correlation analysis of water quality parameters and DOM components, the pollution sources were further verified, i.e., agricultural activities, domestic sewage, phytoplankton growth/terrestrial input and industrial sources. In addition, principal component analysis (PCA) combined with the absolute principal component score-multiple linear regression (APCS-MLR) and positive matrix factorization (PMF) models were employed to quantify pollution sources. Compared with PCA-APCS-MLR model, PMF model resulted in higher performance on evaluation statistics and lower proportion of unexplained variability, thus showed more realistic and robust representation. The results of PMF showed that agricultural activities (42.08%) and domestic sewage (21.16%) were identified as the dominant pollution sources of surface water in the study area. This study highlights the effectiveness of EEM-PARAFAC in identifying the pollution sources, and the applicability of PMF in apportioning the contributions of each potential pollution source in surface water.

Contribution of filtration and photocatalysis to DOM removal and fouling mechanism during in-situ UV-LED photocatalytic ceramic membrane process

The use of ceramic membranes and ultraviolet light-emitting diodes (UV-LEDs) has advanced the application of photocatalytic membrane for water treatment. We systematically evaluated the contribution of filtration and photocatalysis to dissolved organic matter (DOM) removal and fouling mechanism during in-situ UV-LED photocatalytic ceramic membrane filtration. The results showed that physical rejection primarily led to removal of 4-15 kDa molecules and photocatalysis further increased the removal of 1-4 kDa molecules, causing small sized microbial humic-like or protein-like materials in the permeate. In-situ UV-LED photocatalysis had an excellent effect on membrane fouling mitigation regardless of DOM sources. The dominant fouling mechanism changed from partial blockage to gel layer formation with increasing Ca²⁺ concentration but did not change with UV treatment. Correlation analysis revealed that the removal of 1-4 kDa molecules contributed to the mitigation of both reversible and irreversible fouling resistance, and the small molecules were the major cause of irreversible fouling resistance. Removal of 1-4 kDa terrestrial humic acid-like contributed to the pore blockage mechanism for synthetic water. Removal of 4-15 kDa protein-like materials was closely correlated to the pore blockage mechanism for real water. Trihalomethanes (THMs) and haloacetic acids (HAAs) formation potential (FP) were both significantly reduced after photocatalytic ceramic membrane process, but precursors of nitrogenous disinfection by-products (N-DBPs) with high toxicity were not removed by filtration or by photocatalysis, which deserves attention. Membrane rejection made higher contribution to better DBPFP control than photocatalysis. This study provides novel insights into the impact of UV-LED on DOM removal, DBPFP control and fouling mitigation, promoting the development of photocatalytic ceramic membrane filtration.

Response of electron transfer capacity of humic substances to soil microenvironment

The humic substances (HS) - mediated electron transfer process is of great significance to the reduction and degradation of pollutants and the improvement of soil quality. Different soil conditions lead to different characteristics of HS, resulting in differences in the electron transfer capacity (ETC) of HS. It is unclear how the environmental conditions in soil affect the ETC by affecting on HS. In this study, the response relationship of soil microenvironment, HS and ETC has been studied. The results show that the ETC follows the descending order of: Langshan > Nanchang > Anqing > Beijing > Guilin. There were significant differences in ETC in soil HS in different regions. There were significant differences in electron-donating capacity (EDC) in soil HS in different regions and depths. EDC in soil was higher than electron-accepting capacity (EAC), and on average, are 22.4 times higher than the EAC. The HS components of soils in different regions are different. The most significant differences were in tyrosine-like substances and soluble microbial by-products (SMPs). The five components of the soil HS from Langshan were the most different from those in other regions. There were differences in SMPs and humic-like substances in soils of different depths in Anqing and Guilin. ETC can be affected by the composition of HS components in different regions. The composition of HS at different soil depths in the same regions had little effect on ETC. SMPs can promote ETC and EDC, and tyrosine-like substance can promote EDC. Moisture content, pH and TOC are the main factors affecting the composition of HS components. This results can provide a research basis for the sustainable and safe utilization of agricultural soil.

Water quality and periphyton functional response to input of dissolved manure-derived hydrochars (DHCs)

Dissolved organic matter (DOM) plays a critical role in the global carbon cycle and provides food and energy for aquatic organisms. Recently, hydrochar, as a solid carbonaceous substance derived from hydrothermal carbonization, has been increasingly used as a soil amendment. Upon entering the soil, dissolved components (DHCs) were released from hydrochar as exogenous DOM, finally entering the aquatic ecosystems by runoff, which participates in environmental geochemical processes. However, relevant reports revealing the response of the aquatic ecosystem to the input of DHCs remain insufficiently elucidated. For the first time, the fundamental features of DHCs and their influence on water quality and aquatic biological function were investigated in this study. DHCs at 260 °C (DHC260) had lower yields, a greater [C/N], worse biodegradability, and larger humic acid relative amounts than did DHCs at 180 °C (DHC180). The DHC structural alterations in periphyton-incubated aquatic ecosystems suggested that protein substances were more easily degraded or assimilated by periphyton, especially for DHC180, with rates of decrease of 34.5-63.5%. The increased chemical oxygen demand (COD) degradation in the DHC260 treatments was most likely due to humic acid substances with higher COD equivalents. Furthermore, DHC260 caused phosphorus to accumulate in periphyton, reducing aquatic phosphorus concentration. Notably, the abundances of Flavobacteria and Cyanobacteria associated with water blooms increased 12.7-25.5- and 1.3-8.3-fold, respectively; consequently, the promotional impact of DHCs on algal blooms should be considered. This result extends the nonnegligible role of DHCs in aquatic ecosystems and underlines the need to regulate the hydrochar application process.

Unraveling dissolved organic matter in drinking water through integrated ozonation/ceramic membrane and biological activated carbon process using FT-ICR MS

The performance of an integrated process comprising coagulation, ozonation, and catalytic ceramic membrane filtration (CMF) followed by treatment with biological active carbon (BAC) was evaluated in a pilot-scale (96 m³/d) experiment to understand the biostability and quality of the finished water. The fate of dissolved organic matter (DOM) at the molecular level was explored using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Biostable finished water with an assimilable organic carbon (AOC) concentration of 30.2-45.4 µg/L was obtained by the integrated process, and the high hydraulic retention time (HRT) (≥ 45 min) of the BAC filter was necessary to provide biostable finished water. The coagulation/O₃/CMF unit efficiently transformed nitrogen-containing polyaromatic hydrocarbons (PAH) with aromaticity and large molecular weight (Mw) (500-1000 Da) into CHO-type highly unsaturated phenolic compounds (HuPh) with less aromaticity and medium Mw (300-500 Da), which were effectively removed by subsequent BAC filtering. The main reaction was oxygen addition, followed by deamination and dealkylation of the coagulation/O₃/CMF unit and decarboxylation of the BAC filter. Principal component analysis revealed that N-containing and large-Mw PAH are potential AOC precursors, and the chemical characteristics of CHO-type and medium-Mw HuPh make them AOC candidates (correlation coefficients > 0.96). This study provides insights into the management of drinking water biostability and its suitability for the practical application of the integrated coagulation/O₃/CMF-BAC process in drinking water treatment plants.

Use of activated carbon to reduce ammonia emissions and accelerate humification in composting digestate from food waste

Management of digestate from food waste (DFW) is becoming the bottleneck of the food waste anaerobic digestion. Composting is a feasible method to dispose the DFW and convert it to organic fertilizer; however, high ammonia (NH₃) emissions and long composting time are key concerns in this process. In this study, the mechanism of activated carbon (AC) on the loss of NH₃ and humification during DFW composting was investigated. The use of AC could promote humification, shorten 50% of the DFW composting period, and decrease the NH₃ emissions by 34%. Results of the microbial analysis indicated that the AC could promote the growth of key microbes (i.e., Wallemia genus for fungi; and Fastidiosipila genus for bacteria). The Cladosporium and Fastidiosipila genera developed in the fractions closely and loosely attached to the AC, respectively, leading to faster degradation of lignocellulose matter. In addition, AC could enrich the Ammoniibacillus genus, reducing nitrogen loss.

Exploring the fate of dissolved organic matter at the molecular level in the reactive electrochemical ceramic membrane system using fluorescence spectroscopy and FT-ICR MS

This research evaluated the performance of reactive electrochemical ceramic membrane (REM) in treating secondary effluent and investigated the fate of dissolved organic matter (DOM) at the molecular level. The role of adsorption, electrosorption, and oxidation in DOM removal was comprehensively elucidated based on fluorescence spectroscopy and high-resolution mass spectrometry (FT-ICR MS). Among the fluorescence components (C1-C3) in secondary effluent, microbial humic-like C2 showed fewer adsorption on the REM surface without applying an electrical potential. The electrosorption helped an enhanced uptake of all DOM components and transformed them onto the electrode surface. The fluorescence components and all three fractions (hydrophilic, transphilic, and hydrophobic) were rapidly degraded, and finished water with stable DOM was obtained. The leading degradation phenomena were the change of the unsaturated compounds to the aliphatic and transformation of large-sized molecules to medium and small-sized ones. Above 70% of the compounds in the secondary effluent acted as precursors, which were mineralized/degraded and transformed products were found on the REM surface and in the finished water. The compounds containing sulfur (CHOS) were easily and preferably degraded/mineralized, followed by the compounds containing nitrogen (CHON) and CHO. The oxidation of DOM led to the extensive formation of organo-chlorinated compounds, which contributed above 80% in products. Overall, the combination of fluorescence spectroscopy and FT-ICR MS provided unique behavior of DOM in the secondary effluent toward electro-oxidation in the REM system. These findings could help explore the potential of REM for different water matrices to project the possible composition of DOM in the finished water.

Removal of disinfection by-product precursors in drinking water treatment processes: Is fluorescence parallel factor analysis a promising indicator?

This work investigated the removal efficiency of disinfection by-product (DBP) precursors by different drinking water treatment processes and evaluated the feasibility of using fluorescence components removal as an indicator. A four-component (including tryptophan-like, protein-bound, tyrosine-like, and humic-like components) parallel factor analysis model was developed basing on 288 fluorescence excitation-emission matrices. Among all treatment processes, coagulation-sedimentation process showed the best performance, with mean removal ratios of 30% in total fluorescence intensity and 31% in total formation potential (FP) of DBPs, respectively. It preferentially removed humic-like component C4 (43%). Advanced treatment processes were less effective in comparison. Ozone and biological activated carbon (BAC) combined process reduced 20% of total fluorescence intensity, while ultrafiltration process reduced < 3%. Ozonation and BAC filtration preferentially removed free amino acids (i.e., C1 and C3) and protein-bound (i.e., C2) components, with mean removal ratios of 12% and 17%, respectively. Significant correlations (p < 0.01, double-tailed) were observed between four fluorescence components removal and FPs reduction of three trihalomethanes, dichloroacetonitrile (DCAN), and 1,1-dichloropropanone (1,1-DCP). Specifically, the correlation coefficients for three trihalomethanes and 1,1-DCP followed the order of C4 > C1 > C2 > C3, while the order for DCAN was C2 > C4 > C1 > C3.

Source identification and characteristics of dissolved organic matter and disinfection by-product formation potential using EEM-PARAFAC in the Manas River, China

Dissolved organic matter (DOM) is ubiquitous in natural water and reacts with disinfectants to form disinfection by-products (DBPs). The analysis of DOM in raw water is helpful in evaluating the formation potential of DBPs. However, there is relatively little research on the DOM identification of raw water in northern China. In this study, the sources and characteristics of DOM were investigated in the samples collected from the Manas River. Dissolved organic carbon (DOC), UV₂₅₄, specific ultraviolet absorbance, and fluorescence indices (fluorescence index, humification index, and biological index) were measured to characterize the DOM, and trihalomethanes (THMs) were quantified following formation potential tests with free chlorine. The maximum amount of total trihalomethane formation potential (THMsFP) was 225.57 μg L⁻¹. The DOM of the Manas River consisted of microorganisms and soil resources. The excitation–emission matrix combined with parallel factor analysis (EEM-PARAFAC) identified microbial humus (C1, 54%) and tryptophan-like protein (C2, 46%). PARAFAC components were evaluated as the precursor surrogate parameters of THMsFP. Additionally, the linear THMsFP correlation was stronger with C1 + C2 (r = 0.529, p < 0.01) than with C1 (r = 0.485, p < 0.01). Thus, C1 + C2 is an accurate THMsFP precursor surrogate parameter for the Manas River, and the use of fluorescence spectroscopy may be a robust alternative for predicting DOC removal.

Humic-like and protein-like components were identified by PARAFAC. THMs FP was significantly correlated with components C1 and C1 + C2. The source, types and humification degree of DOM affect the formation of DBPs.

EEM-PARAFAC characterization of dissolved organic matter and its relationship with disinfection by-products formation potential in drinking water sources of northeastern China

Dissolved organic matter (DOM) is the precursor of disinfection by-products (DBPs) which is widely found in the aquatic environment. The analysis of DOM in raw water is helpful to evaluate the formation potentials of DBPs. However, there is relatively little research on the DOM identification of raw water in northern China. In this study, the variation in DOM in M reservoir water in one year by fluorescence excitation-emission matrix-parallel factor analysis (EEM-PARAFAC) was investigated to evaluate the DBP formation potential (DBPFP). The results suggested that five components, namely, two humic-like substances (C2, C3), two fulvic-like substances (C1, C4) and one protein-like substance (C5), were identified in the DOM of M reservoir water. The content of DOM in autumn and winter was higher than that in spring and summer. The source of DOM in the water body of M reservoir was mainly from terrestrial source, but less from aquatic source. The source, types and humification degree of DOM affect the formation of DBPs. The formation potential of DBPs had the following order: trihalomethanes (THMs) > dichloroacetic acid (TCAA) > trichloroacetic acid (DCAA) > chloral hydrate (CH). The formation potentials of THM and TCAA were strongly correlated with C2 (r_THM = 0.805, r_TCAA = 0.857). The formation potential of CH has a good correlation with C1 (r = 0.722). The formation of DCAA has a good correlation with C4 (r = 0.787). DOM and DBPFP were negatively correlated with the biological index (BIX) and fluorescence index (FI) of the raw water, and positively correlated with the humification index (HIX).

Spatial variation of dissolved organic nitrogen in Wuhan surface waters: Correlation with the occurrence of disinfection byproducts during the COVID-19 pandemic

Intensified sanitization practices during the recent coronavirus disease-2019 (COVID-19) led to the release of chlorine-based disinfectants in surface water, potentially triggering the formation of disinfection byproducts (DBPs) in the presence of dissolved organic nitrogen (DON). Thus, a comprehensive investigation of DON's spatial distribution and its association with DBP occurrence in the surface water is urgently needed. In this study, a total of 51 water samples were collected from two rivers and four lakes in May 2020 in Wuhan to explore the regional variation of nitrogen (N) species, DON's compositional characteristics, and the three classes of DBP occurrence. In lakes, 53.0% to 86.3% of N existed as DON, with its concentration varying between 0.3-4.0 mg N/L. In contrast, NO₃^--N was the dominant N species in rivers. Spectral analysis revealed that DON in the lakes contained higher humic and fulvic materials with higher A₂₅₄, A₂₅₃/A_203, SUVA₂₅₄, and P_III+IV/P_I+II+V ratios, while rivers had higher levels of hydrophilic compounds. Trihalomethanes (THMs) were the most prevalent DBPs in the surface waters, followed by N-nitrosamines and haloacetonitriles (HANs). The levels of N-nitrosamines (23.1-97.4 ng/L) increased significantly after the outbreak of the COVID-19 pandemic. Excessive DON in the surface waters was responsible for the formation of N-nitrosamines. This study confirmed that the presence of DON in surface water could result in DBP formation, especially N-nitrosamines, when disinfectants were discharged into surface water during the COVID-19 pandemic.

A year-long cyclic pattern of dissolved organic matter in the tap water of a metropolitan city revealed by fluorescence spectroscopy

Delivering drinking water with stable quality in metropolitan cities is a big challenge. This study investigated the year-long dynamics of dissolved organic matter (DOM) in the tap water and source water of a metropolitan city in southern China using fluorescence spectroscopy. The DOM detected in the tap water, and source water of Shenzhen city was season and location-dependent. A year-long cyclic trend of DOM was found with predominate protein-like fluorescence in the dry season compared to the humic-like enriched DOM in the wet season. A general DOM pattern was estimated by measuring the shift in dominant fluorescence regions on the excitation-emission matrix (EEM). The difference in fluorescent DOM (FDOM) composition (in terms of the ratio of protein-like to humic-like fluorescence) was above 200% between wet and dry seasons. The taps associated with reservoirs receiving water from the eastern tributary of Dongjiang River showed significant changes in protein-like contents than the taps with source water originating from the western part of the river. This study highlights the importance of optimizing drinking water treatment plants' operational conditions after considering seasonal changes and source water characteristics.

Spectroscopic fingerprinting of dissolved organic matter in a constructed wetland-reservoir ecosystem for source water improvement-a case study in Yanlong project, eastern China

The coupling between constructed wetlands and reservoir (CWs-R) afforded a novel ecosystem to improve the water quality and increase the emergency storage capacity of micro-polluted river drinking water source. In this study, spectroscopic characteristics of DOM in YL CWs-R ecosystem were first systematic studied based on a three-year field monitoring to investigate the chemical composition, sources and track the involved biogeochemical processes in the ecosystem. Three humic-like components (C1, C2, and C4, em >380 nm) and one protein-like component (C3, em < 380 nm) were identified by PARAFAC model. Significant spatiotemporal variations in concentration and composition of FDOM were observed in YL CWs-R ecosystem. The improved water transparency (SD) and, the increased hydraulic retention time (HRT) along YL CWs-R ecosystem enhance photochemical processes, leading to significant decreases in the intensities of humic-like components in effluent (P < 0.05) with lower degrees of aromaticity, molecular weights, and humification (decrease in HIX and increases in S_R and BIX). In contrast, no significant spatial difference was observed for protein-like component (P > 0.05), which implies that the biodegradation and production of protein-like component may balance each other in the CWs-R ecosystem. The ecological pond unit plays a major role in the removal and transformation of DOM, especially in summer, while wetland purification unit contributes little to DOM reduction. In addition, the decay of aquatic macrophytes in wetland purification unit and the risk of algal bloom in the ecological pond unit might become important autochthonous sources of DOM, especially in summer and autumn. These findings are critical for further understanding the transformation processes of DOM in large-scale CWs-R ecosystems, and could provide important implications to improve sustainable safety of drinking water sources.

Novel insights into impacts of the COVID-19 pandemic on aquatic environment of Beijing-Hangzhou Grand Canal in southern Jiangsu region

In 2020, a sudden COVID-19 pandemic unprecedentedly weakened anthropogenic activities and as results minified the pollution discharge to aquatic environment. In this study, the impacts of the COVID-19 pandemic on aquatic environment of the southern Jiangsu (SJ) segment of Beijing-Hangzhou Grand Canal (SJ-BHGC) were explored. Fluorescent component similarity and high-performance size exclusion chromatography analyses indicated that the textile printing and dyeing wastewater might be one of the main pollution sources in SJ-BHGC. The water quality parameters and intensities of fluorescent components (WT-C1(20) and WT-C2(20)) decreased to low level due to the collective shutdown of all industries in SJ region during the Spring Festival holiday and the outbreak of the domestic COVID-19 pandemic in China (January 24th to late February, 2020). Then, they presented a gradual upward trend after the domestic epidemic was under control. In mid-March, the outbreak of the international COVID-19 pandemic hit the garment export trade of China and consequently inhibited the production activities of textile printing and dyeing industry (TPDI) in SJ region. After peaking on March 26th, the intensities of WT-C1(20) and WT-C2(20) decreased again with changed intensity ratio until April 12th. During the study period (135 days), correlation analysis revealed that WT-C1 and WT-C2 possessed homology and their fluorescence intensities were highly positively correlated with conductivity and COD_Mn. With fluorescence fingerprint (FF) technique, this study not only excavated the characteristics and pollution causes of water body in SJ-BHGC, but also provided novel insights into impacts of the COVID-19 pandemic on production activities of TPDI and aquatic environment of SJ-BHGC. The results of this study indicated that FF technique was an effective tool for precise supervision of water environment.

Front-face fluorescence excitation-emission matrix (FF-EEM) for direct analysis of flocculated suspension without sample preparation in coagulation-ultrafiltration for wastewater reclamation

Fluorescence spectroscopy has been suggested as a promising online monitoring technique in water and wastewater treatment processes due to its high sensitivity and selectivity. However, a pre-filtration is still indispensable in fluorescence measurement for removing ubiquitous particles and flocs in real samples to eliminate the strong light scattering that could attenuate fluorescence detection significantly. This study proposed a front-face fluorescence spectroscopy, which could characterize the liquid sample with suspended solids directly without pre-filtration. Front-face excitation-emission matrix (FF-EEM) coupled with parallel factor (PARAFAC) analysis was used for analyzing fluorescence components and to probe coagulation of secondary effluent and fouling in the subsequent ultrafiltration (UF), and conventional right-angle fluorescence EEM (RA-EEM) was also compared. The results showed that FF-EEM was less susceptible to turbidity (induced by standard particles) in the secondary effluent compared to RA-EEM. FF-EEM could successfully measure dissolved fluorophores in coagulated suspension without pre-filtration, while conventional RA-EEM was undermined significantly due to the existing flocs. FF-EEM coupled with PARAFAC could accurately probe dissolved organic matter and fouling in coagulation- UF wastewater reclamation processes. Therefore, it was demonstrated that this front-face fluorescence without any sample preparation step might be highly promising in real-time online fluorescence monitoring in multi water and wastewater treatment processes.