Formation mechanisms of emerging organic contaminants during on-line membrane cleaning with NaOCl in MBR

Influence of biodegradable plastics on the generation of disinfection byproducts in the chlorination process

Biodegradable plastics (BPs) have seen a continuous increase in annual production and application due to their environmentally sustainable characteristics. However, research on the formation of disinfection byproducts (DBPs) from biodegradable microplastics (BMPs) during chlorination is limited, and the effects of aqueous solution chemistry on this process have yet to be explored. Therefore, two biodegradable microplastics, polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT), were investigated in this study to examine the changes in their physicochemical properties before and after chlorination, and the formation of DBPs under different environmental conditions. The results showed that PLA was more chlorine-responsive, and generated more DBPs. The pH converted some of the intermediates into more stable DBPs by affecting the concentration of HClO and base-catalyzed reactions, whereas ionic strength slightly reduced DBP concentration by ion adsorption and promoting the aggregation of BMPs. Finally, since PLA has a slightly greater volume of mesopores and micropores compared to PBAT, it may more effectively adsorb DBP precursors beyond natural organic matter (NOM), such as some anthropogenic pollutants, thus potentially decreasing the formation of chlorinated DBPs in surface water. This research explored the potentiality for DBP formation by BMPs under different water quality conditions during the disinfection process, which is useful for assessing the environmental hazards of BMPs.

Emerging algal organic matter from simulated on-line chemical cleaning of ultrafiltration membranes treating algae-containing water

Massive reproduction of algae due to the eutrophication of water body poses a new challenge to the water ecosystem. Despite ultrafiltration (UF) acting as an effective method to treat algae-containing waters, on-line chemical cleaning is frequently utilized to sustain the permeability of UF membranes. However, little attention is currently paid on the side-effects of practical on-line chemical cleaning on aqueous environments. Therefore, this work evaluated the generation of algae organic matter triggered by diverse membrane cleaning reagents (i.e., HCl, NaOH, NaClO, SDS and CA), and their subsequent fate in terms of biodegradation and membrane retention. The results indicated that NaOH, HCl and NaClO caused serious damage and lysis of algal cells, leading to the significant release of dissolved organic matter (DOM), while CA and SDS induced negligible DOM release. The occurrence of DOM release was able to cause extra biofouling, thus deteriorating the UF permeability. Furthermore, DOM was characterized in terms of three molecular weight ranges, i.e., high molecular weight (HMW, > 3400 Da), medium molecular weight (MMW, 150-3400 Da), and low molecular weight (LMW, <150 Da). Protein-related substances in the range of HMW and MMW were primarily produced under HCl and NaOH exposures. In contrast, NaClO led to an obvious release of humic-like materials with MMW. During the next round of UF operation, roughly 17 % to 31 % of these released DOM could be removed by via the joint actions of suspended algae biodegradation and fouling layer retention. Nevertheless, roughly 69 % to 83 % of these produced DOM eventually entered into the UF permeate, resulting in the deterioration of permeate quality. Consequently, the detailed mechanisms of DOM generation and subsequent removal by UF were proposed, which re-examined the origins of emerging contaminants in aqueous environment and shed new light on the strategies to ameliorate current practice of on-line membrane chemical cleaning.

Bacteria and Virus Inactivation: Relative Efficacy and Mechanisms of Peroxyacids and Chlor(am)ine

Peroxyacids (POAs) are a promising alternative to chlorine for reducing the formation of disinfection byproducts. However, their capacity for microbial inactivation and mechanisms of action require further investigation. We evaluated the efficacy of three POAs (performic acid (PFA), peracetic acid (PAA), and perpropionic acid (PPA)) and chlor(am)ine for inactivation of four representative microorganisms (Escherichia coli (Gram-negative bacteria), Staphylococcus epidermidis (Gram-positive bacteria), MS2 bacteriophage (nonenveloped virus), and Φ6 (enveloped virus)) and for reaction rates with biomolecules (amino acids and nucleotides). Bacterial inactivation efficacy (in anaerobic membrane bioreactor (AnMBR) effluent) followed the order of PFA > chlorine > PAA ≈ PPA. Fluorescence microscopic analysis indicated that free chlorine induced surface damage and cell lysis rapidly, whereas POAs led to intracellular oxidative stress through penetrating the intact cell membrane. However, POAs (50 μM) were less effective than chlorine at inactivating viruses, achieving only ∼1-log PFU removal for MS2 and Φ6 after 30 min of reaction in phosphate buffer without genome damage. Results suggest that POAs' unique interaction with bacteria and ineffective viral inactivation could be attributed to their selectivity toward cysteine and methionine through oxygen-transfer reactions and limited reactivity for other biomolecules. These mechanistic insights could inform the application of POAs in water and wastewater treatment.

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.

Microbial Responses to Various Types of Chemical Regents during On-Line Cleaning of UF Membranes

Ultrafiltration is widely used to treat various environmental waters, and on-line membrane cleaning with various chemical reagents is frequently employed to sustain the filtration flux. However, the residue of cleaning agents in the ultrafiltration system is unavoidable, which may affect microbiological properties and biofilm formation during the next-round filtration. By investigating the changes in microbial characteristics, and their biofouling behaviors after exposure to HCl, NaOH, NaClO, citric acid (CA), and sodium dodecyl sulfonate (SDS), this study fills a knowledge gap in microbial responses to various types of chemical cleaning agents in an ultrafiltration system. The result shows that HCl, NaOH, and NaClO affect the bacterial properties and subsequent attachment on the membrane surface, while CA and SDS have no obvious influence on microorganisms. Specifically, HCl, NaOH, and NaClO reduce the hydrophobicity and mean size of suspended microorganisms, increase the extracellular polymeric substances (EPS) release, and trigger intracellular reactive oxygen species (ROS) generation, resulting in the death of a large quantity of microorganisms. Due to the self-protecting strategy, plenty of living cells aggregate on the membrane surface and form a cake layer with a stratified structure, causing more severe membrane biofouling.

Identification of the key biochemical component contributing to disinfection byproducts in chlorinating algogenic organic matter

Disinfection byproducts (DBPs) remains an ongoing issue because of their widespread occurrence and toxicity. Various organic substances in Algogenic organic matter (AOM) can produce DBPs in the chlorination process. To provide specific suggestions for the targeted removal of DBP precursors in AOM, the main biochemical components in AOM were qualitatively and quantitatively analyzed. An accurate model for predicting the DBP formation potentials (DBPFPs) of AOM was herein developed based on the dissolved organic carbon of the five main biochemical components in AOM and the DBPFPs of their corresponding surrogates. The contributions of each biochemical component to the three DBP species were evaluated, and the key components were identified. The results showed that lipids, proteins, carbohydrates, humic acid-like substances, and fulvic acid-like substances were the main biochemical components in AOM. Thereof, proteins (71.2 ± 2.1%) and carbohydrates (53.1 ± 2.1%) were the major contributor to the carbon content in intracellular organic matter and extracellular organic matter, respectively. The contribution results of biochemical components to the formation of DBPs showed that proteins were the key contributor to DBPs, suggesting that the targeted removal of proteins before the chlorination process would effectively reduce DBPs from AOM.

Refractory fluorescent dissolved organic matter in conventional and membrane-based drinking water treatment processes

Fluorescent dissolved organic matter (fDOM) has been generally considered a refractory DOM component for drinking water treatment. However, this judgement is made without clear understandings on the removal behaviors of individual fDOM fractions. Therefore, the removals of high, medium and low molecular weight (MW), as well as hydrophobic fDOM fractions in a natural surface water were determined in this study for selected bench- and full-scale water treatment processes. The results showed that low MW (<1000 Da) and hydrophobic fractions of protein-like fDOM were more refractory than other fractions and even released during coagulation and ozonation processes. The corresponding removal efficiencies ranged -25.7%-68.6%. Besides, similar-sized, tyrosine- and tryptophan-like fDOM (F-Tyr and F-Trp) fractions exhibited distinct removal behaviors. Coagulation and powdered activated carbon (PAC) adsorption were ineffective in removing both types of fractions. Ozonation and ion exchange (IX) more effectively removed F-Trp, while F-Tyr fractions were more prone to nanofiltration (NF). Moreover, the integration of coagulation and IX pretreatment moderately enhanced F-Trp removal, but not F-Tyr removal by NF. However, the release of protein-like substances during ozonation, coagulation, and activated carbon-sand filtration adversely affected fDOM removal in a full-scale treatment plant. These findings highlighted the persistency of protein-like fDOM fractions in drinking water treatment processes.

Fluorescence excitation-emission matrix as a novel indicator of assimilable organic carbon in wastewater: Implication from a coal chemical wastewater study

Assimilable organic carbon (AOC) is recognized as an important parameter to evaluate the biostability of water. Studies have been carried out to investigate the easier and faster AOC detection methods in recent years. In our study, the relationship between AOC and excitation-emission matrix (EEM) was investigated through analysis of wastewater from a coal chemical industrial corporation, including biochemical effluent, ultrafiltration effluent, and reverse osmosis concentrate. Considering the influence of water sample properties on AOC distribution, these water samples were fractionated according to their hydrophilicity and acid/base properties. Neutrals and hydrophobic acids were major components of total organic carbon and AOC concentration of these fractions was measured. EEM spectra of water samples were divided into five regions according to fluorescence peaks. Distribution of fluorescence region integration (FRI) of water samples was also calculated, as well as other fluorescence parameters. Statistical analysis showed that the concentration of AOC presented high positive correlation with the FRI in region H2, with R² = 0.696. Monte Carlo simulation also proved that the proportion of significant R² (p < 0.05) was high at 89.1%, suggesting that the model was reliable at least at the qualitative level. In that case, FRI in Region H2 could be an indication for AOC concentration in water samples. Our findings focus on fundamental insights into establishing relationship between spectroscopy method and AOC in wastewater and provide an easier way of accessing AOC in coal chemical industrial wastewater. Further investigation could be oriented to the dynamic analysis of AOC transformation and tracing.

Current status of hypochlorite technology on the wastewater treatment and sludge disposal: Performance, principals and prospects

As cost-effective and high-efficient oxidants, the hypochlorite chemicals have been widely utilized for bleaching and disinfection. However, its potential applications in wastewater treatment and sludge disposal were less concerned. This paper mainly summarized the state-of-the-art applications of hypochlorite technology in wastewater and sludge treatment based on the main influencing factors and potential mechanisms of hypochlorite treatment. The results indicated that the hypochlorite approaches were not only effective in pollutants removal and membrane fouling mitigation for wastewater treatment, but also contributed to sludge dewatering and resource recovery for sludge disposal. The ClO^- and large generated free active radicals (i.e., reactive chlorine species and reactive oxygen species), which possessed strong oxidative ability, were the primary contributors to the pollutants decomposition, and colloids/microbes flocs disintegration during the hypochlorite treatment process. The performance of hypochlorite treatment was highly associated with various factors (i.e., pH, temperature, hypochlorite types and dosage). In combination with the reasonable activators (i.e., Fe²⁺ and ultraviolet), auxiliary agents, and innovative processes (i.e., hydrothermal and electro-oxidation), the operational performance of hypochlorite technology could be further enhanced. Finally, the feasibility and benefits of hypochlorite application for wastewater and sludge treatment were analyzed, and the existing challenges and future research efforts that need to be made have also prospected. The review can hopefully provide a theoretical basis and technical guidance to extend the application of hypochlorite technology for wastewater treatment and sludge disposal on large scale.

Powdered activated carbon doping improves the mechanical and adsorption properties of cementitious microfiltration membrane

Cementitious membrane (CM) is a promising microfiltration membrane with low cost for raw materials and low energy consumption of non-sintering fabrication process. A novel carbon-cementitious microfiltration membrane (CCM) was fabricated with powdered activated carbon (PAC) as an additive based on CM, to solve the low mechanical strength of CM during multiple practical uses. While maintaining adequate pure water flux and porosity, the mechanical strength of the membrane was greatly improved to ensure the stability of the membrane in the filtration process. The bending strength of the CCM was 2-3 times higher than that of CM. 10 wt% CCM has the smallest critical pore size and optimal permeability, which was chosen to be the optimal PAC doping ratio. The X-ray diffraction and FT-IR results indicated that the addition of PAC did not change the mineral composition of cement hydration products, and the appropriate amount of PAC acted as a nucleation site and accelerated hydration. The effect of size effect on bending strength was more obvious with the decrease of membrane thickness. In the membrane adsorption experiments of benzophenone-4, nitrobenzene and p-chloronitrobenzene, the CCM exhibited prominent adsorption properties than CM. These results broaden the application scope of microfiltration membranes in water treatment process.

Characterizing membrane fouling formation during ultrafiltration of high-salinity organic wastewater

High-salinity organic wastewater usually consists of diverse highly concentrated ions such as Na⁺, Ca²⁺ and Al³⁺ etc., which may significantly influence the fouling propensity when membrane technique is employed for contaminants removal. The current work investigated the effects of high salinity especially high-concentration Na⁺, Ca²⁺ and Al³⁺ on UF fouling characteristics, where 2 M Na⁺ and 0.5-1.0 M Ca²⁺ or Al³⁺ were applied according to the general composition of high-salinity wastewater. The results demonstrated that the presence of high-concentration Na⁺ alone benefited the ultrafiltration of bovine serum albumin (BSA) solution, but posed adverse effects on the ultrafiltration of humic acid (HA) solution. Further addition of Ca²⁺ or Al³⁺ on the basis of Na⁺ was found to aggravate the development of BSA fouling. Such differentiated behaviors were further elucidated by the comprehensive fouling characterizations in terms of foulant properties, specific resistances, filtration modelling and fouling layer observations. Correlation analysis suggested that irreversible fouling had strong relationship with Al³⁺ addition, while reversible fouling seemed to be primarily influenced by foulant size. Meanwhile, membrane rejection in the presence of various salts remarkably decreased, which was negatively correlated with zeta potential. Consequently, this study should shed light on the membrane fouling formation for treating high-salinity organic wastewater using membrane techniques.

Tracing dissolved organic matter in inflowing rivers of Nansi Lake as a storage reservoir: Implications for water-quality control

Quantitative characterization of dissolved organic matter (DOM) in various aquatic ecosystems has become of increasing importance as its transformation plays a key role in inland water carbon, yet few studies have quantified water DOM inputs to storage lakes for water quality control and safety assurance. This study assessed the quantity and quality of DOM in 21 inflow rivers of Nansi Lake as the important storage lake of large-scale water transfer projects by using excitation-emission matrix spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) and ultraviolet-visible (UV-Vis) spectroscopy. The results showed that DOM contents varied significantly with an average value of 5.8 mg L^-1 in different inflow rivers, and three fluorescence substances (including UVC humic-like, UVA humic-like and tyrosine-like components) were identified by EEM-PARAFAC. The distribution of the DOM components was distinctively different among sampling sites, and UVA humic-like component mainly dominated in Nansi Lake. Meanwhile, DOM components with higher aromaticity and molecular weight were found in the west side of lake. Fluorescence spectral indexes manifested that the source of DOM was mainly from allochthonous or terrestrial input. Moreover, significant correlations between water quality and DOM characteristics were observed in Nansi Lake. These findings would be beneficial to understand the biogeochemical role and impact of DOM in inflowing rivers in the water-quality monitoring and control of storage lakes.

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.

Effects of High Salinity on Alginate Fouling during Ultrafiltration of High-Salinity Organic Synthetic Wastewater

Ultrafiltration is widely employed in treating high-salinity organic wastewater for the purpose of retaining particulates, microbes and macromolecules etc. In general, high-salinity wastewater contains diverse types of saline ions at fairly high concentration, which may significantly change foulant properties and subsequent fouling propensity during ultrafiltration. This study filled a knowledge gap by investigating polysaccharide fouling formation affected by various high saline environments, where 2 mol/L Na⁺ and 0.5–1.0 mol/L Ca²⁺/Al³⁺ were employed and the synergistic influences of Na⁺-Ca²⁺ and Na⁺-Al³⁺ were further unveiled. The results demonstrated that the synergistic influence of Na⁺-Ca²⁺ strikingly enlarged the alginate size due to the bridging effects of Ca²⁺ via binding with carboxyl groups in alginate chains. As compared with pure alginate, the involvement of Na⁺ aggravated alginate fouling formation, while the subsequent addition of Ca²⁺ or Al³⁺ on the basis of Na⁺ mitigated fouling development. The coexistence of Na⁺-Ca²⁺ led to alginate fouling formed mostly in a loose and reversible pattern, accompanied by significant cracks appearing on the cake layer. In contrast, the fouling layer formed by alginate-Na⁺-Al³⁺ seemed to be much denser, leading to severer irreversible fouling formation. Notably, the membrane rejection under various high salinity conditions was seriously weakened. Consequently, the current study offered in-depth insights into the development of polysaccharide-associated fouling during ultrafiltration of high-salinity organic wastewater.

Evaluating an on-line cleaning agent for mitigating organic fouling in a reverse osmosis membrane

Cleaning-in-place (CIP) is a representative fouling management process from which the filtration performances of fouled membranes can be recovered. However, CIP can cause significant inefficiency in water production because frequent system restabilization is necessary for cleaning processes. This study applied a newly developed on-line cleaning agent (OCA, a feed water additive for fouling mitigation), to reduce the number of CIP by enhancing water productivity. Reverse osmosis filtration was performed to evaluate the effect of on-line cleaning on the mitigation of organic fouling originating from humic acid (HA) and bovine serum albumin. OCA increased the permeate flux in proportion to OCA concentration. In particular, OCA effectively reduced the fouling layer thickness by 22% when fouling was influenced by HA-Ca²⁺ complexation, increasing water production by 5%. It also had a minor influence on bovine serum albumin fouling, producing a 1.4% increase in permeate flux. Furthermore, the pore blockage-cake filtration model was used to evaluate OCA cleaning performance through the reduction in fouling layer resistance and the growth parameter. The results demonstrated the advantages of OCA utilization for mitigating cake layer development. These findings imply that OCA can be an effective cleaning additive, especially in seawater and groundwater treatment processes with a high proportion of HA and calcium ions.

Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH

Sodium hypochlorite (NaClO) is a commonly applied cleaning agent for ultrafiltration membranes in water and wastewater treatment. Long-term exposure to NaClO might change the properties and performance of polymeric membranes, and ultimately shorten membrane lifespan. Active species in NaClO solution vary with solution pH, and the aging effects can change depending on the membrane material. In this study, the aging of polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes by NaClO at pH 3-11 was investigated by examining variations in chemical composition, surface charge, surface morphology, mechanical strength, permeability, and retention ability. Polyvinyl pyrrolidone (PVP), which was blended in both membranes, was oxidized and dislodged due to NaClO aging at all investigated pH values, but the oxidation products and dislodgement ratio of PVP varied with solution pH. For the PVDF membrane, NaClO aging at pH 3-11 caused a moderate increase in permeability and decreased retention due to the oxidation and release of PVP. The tensile strength decreased only at pH 11 because of the defluorination of PVDF molecules. For the PES membrane, NaClO aging at all investigated pH resulted in chain scission of PES molecules, which was favored at pH 7 and 9, potentially due to the formation of free radicals. Therefore, a decrease in tensile strength and retention ability, as well as an increase in permeability, occurred in the PES membrane for NaClO aging at pH 3-11. Overall, the results can provide a basis for selecting chemical cleaning conditions for PVDF and PES membranes.

Biological-based control strategies for MBR membrane biofouling: a review

Membrane bioreactor (MBR) technology has been paid extensive attention for wastewater treatment because of its advantages of high effluent quality and minimized occupation space and sludge production. However, the membrane fouling is always an inevitable problem, which causes high operation and maintenance costs and prevents the wide use of MBR technology. The membrane biofouling is the most complicated and has relatively slow progress among all types of fouling. In recent years, many membrane biofouling control methods have been developed. Different from the physical or chemical methods, the biological-based strategies are not only more effective for membrane biofouling control, but also milder and more environment-friendly and, therefore, have been increasingly employed. This paper mainly focuses on the mechanism, unique advantages and development of biological-based control strategies for MBR membrane biofouling such as quorum quenching, uncoupling, flocculants and so on. The paper summarizes the up-to-date development of membrane biofouling control strategies, emphasizes the advantages and promising potential of biological-based ones, and points out the direction for future studies.

Variations of disinfection byproduct precursors through conventional drinking water treatment processes and a real-time monitoring method

In this investigation, raw water (RW), settled water (SW), and filtered water (FW) collected from a drinking water treatment plant were fractionated into 24 natural organic matter (NOM) fractions with varying molecular weights and hydrophobicity. The yields of disinfection byproducts (DBPs) obtained during the chlorination of the NOM fractions were explored. Results revealed that the 0-1 kDa, 5-10 kDa, and hydrophobic DBP precursors dominated RW. Hydrophobic fractions cannot be effectively removed, which contributed to the high DBP precursors remaining in the FW. The optional optical parameters, including UVA (UV₃₄₀, UV₃₆₀, and UV₃₈₀), UVB (UV₂₈₀, UV₃₀₀, and UV₃₁₀), and UVC (UV₂₅₄, UV₂₆₀, and UV₂₇₂), were analyzed to determine the DBP yields during chlorination of different NOM fractions. Results revealed that UVC could be applied to indicate the regulated DBP yields of the humified precursors. Contrary to the generally accepted view, for biologically derived precursors, their regulated DBPs and dichloroacetonitrile correlated better with UVA (e.g. UV₃₄₀). Moreover, PARAFAC analysis was applied to decompose an array of 24 EEM spectra. Good linear correlations were found between the PARAFAC components and most DBP yields. Furthermore, four fluorescence parameters were proposed via a modified fluorescence picking method, which can serve as excellent surrogates of PARAFAC components. These fluorescence parameters were found to be effective in indicating most DBP yields. Finally, the fluorescence intensity at excitation wavelength/emission wavelength = 310/416 nm was found to be a promising built-in parameter for the real-time monitoring of DBP precursors, regardless of the humification degree of the precursors.

Formation of disinfection by-products during sodium hypochlorite cleaning of fouled membranes from membrane bioreactors

Periodic chemical cleaning with sodium hypochlorite (NaClO) is essential to restore the membrane permeability in a membrane bioreactor (MBR). However, the chlorination of membrane foulants results in the formation of disinfection by-products (DBPs), which will cause the deterioration of the MBR effluent and increase the antibiotic resistance in bacteria in the MBR tank. In this study, the formation of 14 DBPs during chemical cleaning offouled MBR membrane modules was investigated. Together with the effects of biofilm extracellular polymeric substances (EPS), influences of reaction time, NaClO dosage, initial pH, and cleaning temperature on the DBP formation were investigated. Haloacetic acids (HAAs) and trichloromethane (TCM), composed over 90% of the DBPs, were increasingly accumulated as the NaClO cleaning time extended. By increasing the chlorine dosage, temperature, and pH, the yield of TCM and dichloroacetic acid (DCAA) was increased by up to a factor of 1-14, whereas the yields of haloacetonitriles (HANs) and haloketones (HKs) were decreased. Either decreasing in the chlorine dosage and cleaning temperature or adjusting the pH of cleaning reagents toward acidic or alkaline could effectively reduce the toxic risks caused by DBPs. After the EPS extraction pretreatment, the formation of DBPs was accelerated in the first 12 h due to the damage of biofilm structure. Confocal laser scanning microscopy (CLSM) images showed that EPS, particularly polysaccharides, were highly resistant to chlorine and might be able to protect the cells exposed to chlorination.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available in the online version of this article at 10.1007/s11783-021-1389-3 and is accessible for authorized users.

Relationship between fluorescence excitation-emission matrix properties and the relative degree of DOM hydrophobicity in wastewater treatment effluents

Excitation-emission matrix (EEM) fluorescence spectroscopy is a powerful tool for the characterization of dissolved organic matter (DOM) in wastewater systems. It is of particular value if its utility could be extended by connecting the spectral features to hydrophobicity, one of the fundamental physicochemical properties of DOM. In this study, we employed a DAX-8 resin column to fractionate the hydrophobic/philic components of DOM and determine the relative degree of hydrophobicity by adjusting the critical retention factor (k'_cr, the ratio of treated water sample volume to column volume). A higher k'_cr would result in a higher hydrophobicity of the column effluent. At different k'_cr values (5, 10, 25, 50, 100, and 200), the EEM characteristics of the obtained DOM components were inspected, including overall properties (average fluorescence per total organic carbon and UV absorbance), regional properties (fluorescence regional integration (FRI) and its secondary parameters), and energy-related properties (energy level of the excited states, Stokes shift for relaxation of the excited states, and fluorescence lifetime). In case studies of a wastewater membrane bioreactor and an oxidation ditch, plenty of the EEM properties varied significantly with logk'_cr (r > 0.9, p < 0.05). The average fluorescence per UV absorbance (reflecting quantum yield), fluorescence proportion at Stokes shift ≥ 1.1 μm^-1, and some secondary FRI parameters presented the best linear fitting with logk'_cr, suggesting a smooth variation of the π-conjugated structures with the relative degree of DOM hydrophobicity. This may help to further understand the relationship between EEM fingerprints and DOM hydrophobicity.

Optimization of In Situ Backwashing Frequency for Stable Operation of Anaerobic Ceramic Membrane Bioreactor

The cost-effective and stable operation of an anaerobic ceramic membrane bioreactor (AnCMBR) depends on operational strategies to minimize membrane fouling. A novel strategy for backwashing, filtration and relaxation was optimized for stable operation of a side stream tubular AnCMBR treating domestic wastewater at the ambient temperature. Two in situ backwashing schemes (once a day at 60 s/day, and twice a day at 60 s × 2/day) maintaining 55 min filtration and 5 min relaxation as a constant were compared. A flux level over 70% of the initial membrane flux was stabilized by in situ permeate backwashing irrespective of its frequency. The in situ backwashing by permeate once a day was better for energy saving, stable membrane filtration and less permeate consumption. Ex situ chemical cleaning after 60 days’ operation was carried out using pure water, sodium hypochlorite (NaOCl), and citric acid as the order. The dominant cake layer was effectively reduced by in situ backwashing, and the major organic foulants were fulvic acid-like substances and humic acid-like substances. Proteobacteria, Firmucutes, Epsilonbacteria and Bacteroides were the major microbes attached to the ceramic membrane fouling layer which were effectively removed by NaOCl.