This study was conducted to assess the merits and limitations of various high-pressure membranes, tight nanofiltration (NF) membranes in particular, for the removal of trace organic compounds (TrOCs). The performance of a low-pressure reverse osmosis (LPRO) membrane (ESPA1), a tight NF membrane (NF90) and two loose NF membranes (HL and NF270) was compared for the rejection of 23 different pharmaceuticals (PhACs). Efforts were also devoted to understand the effect of adsorption on the rejection performance of each membrane. Difference in hydrogen bond formation potential (HFP) was taken into consideration. Results showed that NF90 performed similarly to ESPA1 with mean rejection higher than 95%. NF270 outperformed HL in terms of both water permeability and PhAC rejection higher than 90%. Electrostatic effects were more significant in PhAC rejection by loose NF membranes than tight NF and LPRO membranes. The adverse effect of adsorption on rejection by HL and ESPA1 was more substantial than NF270 and NF90, which could not be simply explained by the difference in membrane surface hydrophobicity, selective layer thickness or pore size. The HL membrane had a lower rejection of PhACs of higher hydrophobicity (log D>0) and higher HFP (>0.02). Nevertheless, the effects of PhAC hydrophobicity and HFP on rejection by ESPA1 could not be discerned. Poor rejection of certain PhACs could generally be explained by aspects of steric hindrance, electrostatic interactions and adsorption. High-pressure membranes like NF90 and NF270 have a high promise in TrOC removal from contaminated water.

•A new method was developed for TFN membrane preparation.•The incorporated zeolite nanoparticles had little agglomeration.•Water permeability was greatly improved with slight sacrifice of MgSO4 rejection.•The TFN membrane performed well in rejecting pharmaceuticals.A novel approach to fabricating thin-film nanocomposite (TFN) nanofiltration membranes was reported in this study. It involved the preparation of a polysulfone support in situ embedded with zeolite nanoparticles followed by interfacial polymerization to form the polyamide layer. Compared with the TFN membranes prepared by the conventional method (TFN-C), the new TFN membrane (TFN-I) had higher loading and more uniform dispersion of nanoparticles in the polyamide layer. The nanoparticles incorporation resulted in an increase of surface roughness but no change of surface hydrophilicity. The TFN-I membrane doubled the water permeability compared with the control membrane (TFC). The TFN-I membrane had a similar rejection of MgSO4 (>93% at 150 psi) and negatively charged pharmaceuticals (PhACs) with TFC, but a reduced rejection of NaCl and a slightly lower rejection of neutral and positively charged PhACs of small molecular weights. The TFN-I membrane performed much better than the TFN-C membrane. The demonstrated performance of TFN-I could be due to the internal pores of zeolite nanoparticles, the increased membrane surface roughness and, though undesirable, the microporous defects between the nanoparticles and the polyamide matrix. The newly developed approach is highly promising for the fabrication of TFN nanofiltration and reverse osmosis membranes of improved performance by incorporating various nanoparticles.

Dissolved organic matter (DOM) plays a substantial role in wastewater treatment systems. Fluorescence is an important property of DOM and its use is promising for DOM characterization, but has rarely been extended to probing the basic physicochemical properties such as hydrophobicity and molecular weight. This study explores the possible linkages between the fluorescence properties and hydrophobicity/molecular weight of DOM, through case studies from three wastewater treatment plants (two membrane bioreactors and one oxidation ditch). The fluorescence properties of different hydrophobic/hydrophilic and molecular-weight fractions of DOM were obtained using excitation–emission matrix (EEM) spectroscopy and size-exclusion chromatography with fluorescence detection. The EEM spectra were interpreted using techniques of fluorescence regional integration, parallel factor analysis, fluorescence spectroscopic indices, and a novel energetic mapping based on fluorophore energy levels. It was found that for all the three plants, the hydrophobic fractions of DOM had a higher fluorescence intensity per UV absorbance (indicating a higher quantum yield) as well as a larger Stokes shifts than the hydrophilic fraction. The lower-molecular-weight fractions generally exhibited a higher fluorescence intensity per total organic carbon (indicating a higher fluorophore density), with the fluorescence distribution at slightly smaller excitation and emission wavelengths. These phenomena were explained via analysis of the fluorophore energy state during the excitation/emission process. The scale of the π-conjugated system in DOM molecules may serve as an intermediate factor in the correlations between the hydrophobicity/molecular weight and the fluorescence properties. These correlations may assist in developing fluorescent probes for the DOM characteristics during the process monitoring of wastewater treatment plants.

•Rejection ratios for 24 pharmaceuticals by two FO membranes were ranked.•The solution–diffusion model could predict the rejection ratio well.•The permeability coefficient obtained by the diffusion cell method was more appropriate for model prediction.•Permeability coefficient is primarily determined by molecular weight.Two commercial forward osmosis (FO) membranes (HTI-ES and HTI-NW) were employed to study the rejection performance of 24 pharmaceuticals (PhACs) using NaCl as the draw solute. The PhAC permeability coefficient (B value) was determined for each PhAC by using both the reverse osmosis (RO) mode method and the diffusion cell method. The B values were used to predict the rejection ratios in the FO mode. The rejection ratio increased with the increase of draw solute (NaCl) concentration for each PhAC. Under a NaCl concentration of 1 mol/L, all PhACs were highly rejected by >90%, except for a few including nalidixic acid, gemfibrozil, carbamazepine and sulfamethoxazole, which were rejected by 80–90% when HTI-ES membrane was used. The HTI-NW membrane could reject PhACs better than the HTI-ES membrane; however, the PhACs followed almost an identical sequence in terms of the rejection ratios. Results showed that the B values for several charged PhACs of relatively low molecular weight obtained by the diffusion cell method could be substantially larger than that determined by the RO mode method. In comparison with the experimental data, the B values obtained by the diffusion cell method were more appropriate to be used to predict the rejection ratios of the PhACs by the solution–diffusion model during FO operation. The underestimation of the B values by using the RO mode method might be primarily due to the ion exchange mechanism caused by reverse draw solute permeation during FO operation. Compared with the hydrophobicity and the charge properties, the molecular weight of PhAC was a more important factor in determining its B value. Very low B value is expected if the molecular weight is higher than 300 Da. Exceptions, however, were found including clofibric acid, gemfibrozil and sulfadiazine. The solute-membrane affinity should also be taken into consideration when trying to link the B values with physicochemical properties of the PhACs.

Swimming has become a popular exercising and recreational activity in China but little is known about the disinfection by-products (DBPs) concentration levels in the pools. This study was conducted as a survey of the DBPs in China swimming pools, and to establish the correlations between the DBP concentrations and the pool water quality parameters. A total of 14 public indoor and outdoor pools in Beijing were included in the survey. Results showed that the median concentrations for total trihalomethanes (TTHM), nine haloacetic acids (HAA9), chloral hydrate (CH), four haloacetonitriles (HAN4), 1,1-dichloropropanone, 1,1,1-trichloropropanone and trichloronitromethane were 33.8, 109.1, 30.1, 3.2, 0.3, 0.6 μg·L−1 and below detection limit, respectively. The TTHM and HAA9 levels were in the same magnitude of that in many regions of the world. The levels of CH and nitrogenous DBPs were greatly higher than and were comparable to that in typical drinking water, respectively. Disinfection by chlorine dioxide or trichloroisocyanuric acid could substantially lower the DBP levels. The outdoor pools had higher TTHM and HAA9 levels, but lower trihaloacetic acids (THAA) levels than the indoor pools. The TTHM and HAA9 concentrations could be moderately correlated with the free chlorine and total chlorine residuals but not with the total organic carbon (TOC) contents. When the DBP concentration levels from other survey studies were also included for statistical analysis, a good correlation could be established between the TTHM levels and the TOC concentration. The influence of chlorine residual on DBP levels could also be significant.

Brownian diffusion, inter-particle interactions, shear-induced diffusion and hydrodynamic lift forces all contribute to restricting the formation of concentration polarization (CP) during cross-flow membrane filtration of particles. In this study, a unified CP model was developed for the quantification of the CP formation of colloidal particles. The basic methods adopted in previous models such as mass balance equation accounting for particle diffusion and convection, disintegration of the solid pressure and osmotic pressure and derivation of particle drift velocity are kept. In the new model, the hydrodynamic lift force is taken into account in the force balance equations and shear-induced diffusion is included in the particle transport process. The simulation results show that the inter-particle interactions and the hydrodynamic lift force are predominant for relatively small and large particles, respectively. More specifically, while the inter-particle interactions are important for relatively small (e.g. 2a = 10 nm) particles, the hydrodynamic lift force dominates for relatively large particles (e.g. 2a = 1 μm). Neither inter-particle interactions nor hydrodynamic lift forces play a significant role in CP formation of sub-micro particles (e.g. 2a = 100 nm), for which the solid fraction buildup within the CP formation is most severe. For filtration of relatively large particles under a fixed hydrodynamic condition, it appears that there is a critical flux, over which the particle deposition rate will increase dramatically. Similarly, at a fixed filtration flux, there exists a critical shear stress induced by the cross-flow, over which the membrane fouling rate will decrease sharply. Use of a higher shear intensity can effectively alleviate membrane fouling caused by particles of all size ranges.Graphical abstractMass balance is utilized for concentration polarization (CP) quantification. Both long-range inter-particle interactions and hydrodynamic lift forces reduce the particle motion rate, which impedes the CP formation extent.Highlights► A unified CP model was developed for cross-flow filtration of particles. ► Drift velocities of the particles determine the particle convection rate. ► Hydrodynamic lift forces and inter-particle interactions determine drift velocity. ► Critical filtration flux exists during filtration of relatively large particles.

•Chloroacetic acids were used to evaluate biodegradation performance of BAC filters.•Disinfectant-enhanced backwash substantially increased removal efficiency of attached biomass.•BAC filter biodegradation performance was recovered in one day after backwash.•Chloramines-enhanced backwash was suggested to sustain the operation of BAC filters.Disinfectant-enhanced backwash is frequently required to control the over-growth of biomass in biologically active carbon (BAC) filters for drinking water treatment. This study was conducted to investigate the impact of different backwashing strategies on the biodegradation and adsorption performance of BAC filters and attached biomass concentration in the filters. The biodegradation performance was evaluated using the three chloroacetic acids (CAAs) as indicator chemicals. Results showed that both free chlorine- and chloramines-enhanced backwashes could significantly increase the removal efficiency of attached biomass, but they also impaired the CAA degradation in BAC filters. The deterioration of CAA degradation could not be correlated with the removed attached biomass. Use of CAAs was a feasible approach to evaluate the biodegradation performance of BAC filters either during operation or after backwash. Chloramines-enhanced backwash is suggested to be employed to sustain the operation of BAC filters when excessive biomass growth takes place, due to its higher efficiency in removing attached biomass and lower adverse impact on BAC adsorption properties compared with free chlorine-enhanced backwash. However, the more pronounced adverse impact on organic matter degradation and the inconvenience of using chloraminated water must be considered.Download full-size image

•Real rejection ratios for TOrCs by Desal HL were determined.•The NF membrane rejected most TOrCs effectively except for a few.•Steric effect was the primary mechanism for NF rejection of TOrCs.•DSPM&DE model generally over-predicted the rejection ratios.This study aimed to investigate the rejection ratios for 40 trace organic compounds (TOrCs) by the Desal HL nanofiltration (NF) membrane and to assess the applicability of the DSPM&DE model in predicting the rejection performance. Steady state rejection ratios were considered. A higher water flux normally led to a higher rejection ratio. At a filtration flux of 6.59 × 10− 6 m/s, the NF membrane could effectively reject most of the TOrCs (> 80%), except for a few that included acyclovir, caffeine, carbamazepine, chloramphenicol, metronidazole, nandrolone, oxytetracycline, sodium nifurstyrenate and trenbolone. Most poorly rejected TOrCs had a molecular weight lower than 275 Da. Steric hindrance effect was the primary mechanism that contributed to the rejection of TOrCs by the NF membrane. The rejection ratio for methylparaben decreased when the water flux increased. The DSPM&DE model was successful in predicting the rejection ratios for acyclovir, caffeine and ranitidine, which all have relatively low molecular weight and are hydrophilic. The model generally over-predicted the rejection ratios for the remaining TOrCs. The over-prediction could not be explained by high hydrophobicity only. Lack of consideration of the TOrC-membrane interactions on the partitioning of TOrCs to the membrane material was probably the primary reason for over-prediction by the model.All the studied 40 TOrCs other than acyclovir, caffeine, carbamazepine, chloramphenicol, metronidazole, nandrolone, oxytetracycline, sodium nifurstyrenate and trenbolone were effectively rejected by the Desal HL membrane. The DSPM&DE model over-predicted the rejection ratios for most TOrCs, except for acyclovir, caffeine and ranitidine which have low molecular weight and hydrophobicity.Download full-size image

•Forward osmosis rejects haloacetic acids (HAAs) well.•The active-layer facing feed water (AL-FW) orientation is preferred.•The solution-diffusion model predicts HAA rejection better for AL-FW.•The solution-diffusion model accurately predicts reverse draw solution permeation.The rejection of haloacetic acids (HAAs) by forward osmosis (FO) and the coupled reverse draw solute permeation were experimentally determined and mathematically modeled by using the solution-diffusion model for both the AL-FW (active layer facing the feed water) and the AL-DS (active layer facing the draw solution) orientations. The rejection ratio for each HAA increased with the increase of draw solute concentration for the AL-FW orientation. In contrast, the HAA rejection ratio could reach its maximum under a medium osmotic pressure difference for the AL-DS orientation. The rejection ratios for all HAAs were higher than 94.6% for the AL-FW orientation and ranged from 73.8% to 89.1% for the AL-DS orientation under a draw solute concentration of 1 mol/L NaCl. The reverse draw solute flux for the AL-FW orientation was lower than that for the AL-DS orientation. The model-predicted HAA rejection results matched well with the experimental rejection ratios for the AL-FW orientation. However, the model over-estimated the rejection ratios for the AL-DS mode, likely due to the adoption of inaccurate mass transfer coefficient for internal concentration polarization. Regarding the reverse draw solute permeation, a general agreement between the model prediction and experimental data was observed for both orientations.The behaviors of haloacetic acid (HAA) rejection by forward osmosis are obtained by experiments and predicted by the solution-diffusion model combined with concentration polarization for both the AL-FW (active layer facing the feed water) and the AL-DS (active layer facing the draw solution) orientations.Download full-size image

•Alumina and LTL zeolite nanoparticles suited the in situ embedment method.•The nanoparticle embedded membranes had higher hydrophilicity and filterability.•Embedded nanoparticles exhibited anti-adhesion ability but no bacteriocidal function.•Anti-adhesion ability was responsible for the anti-biofouling performance.Nanoparticle embedded polysulfone ultrafiltration (UF) membranes were prepared by using the in situ embedment method, and the anti-biofouling properties of the prepared membranes were evaluated by conducting bacteria adhesion test, bacterium inactivation test and biofilm formation test separately. Among the several aluminum and/or silicon oxide nanoparticles tested, alumina (Al2O3) and Linda type L (LTL) zeolite nanoparticles were successfully embedded which could be evenly dispersed on membrane surface with high coverage ratio (38% and 49%, respectively) and were resistant to hydraulic shear detachment. The water contact angles for the nanoparticle embedded membranes (UF-Al2O3 and UF-LTL) and the control membrane (UF-C) were 57°, 40° and 66°, respectively. Owing to the higher surface hydrophilicity, both UF-Al2O3 and UF-LTL demonstrated a higher filterability than UF-C. Biofouling was inhibited on both UF-Al2O3 and UF-LTL, indicated by the lower Pseudomonas aeruginosa biofilm formation rate. Further investigation showed that both UF-Al2O3 and UF-LTL exhibited a high anti-adhesion efficiency to both Escherichia coli and P. aeruginosa, but no bacteriocidal effect on E. coli. The anti-biofouling ability of UF-Al2O3 and UF-LTL mainly benefited from the anti-adhesion ability attributed to the embedded nanoparticles. The improved anti-adhesion ability could not be simply explained by the enhanced hydrophilicity.Download high-res image (245KB)Download full-size image