Effect of Long-Term Sodium Hypochlorite Cleaning on Silicon Carbide Ultrafiltration Membranes Prepared via Low-Pressure Chemical Vapor Deposition.

Sodium hypochlorite (NaClO) is widely used for the chemical cleaning of fouled ultrafiltration (UF) membranes. Various studies performed on polymeric membranes demonstrate that long-term (>100 h) exposure to NaClO deteriorates the physicochemical properties of the membranes, leading to reduced performance and service life. However, the effect of NaClO cleaning on ceramic membranes, particularly the number of cleaning cycles they can undergo to alleviate irreversible fouling, remains poorly understood. Silicon carbide (SiC) membranes have garnered widespread attention for water and wastewater treatment, but their chemical stability in NaClO has not been studied. Low-pressure chemical vapor deposition (LP-CVD) provides a simple and economical route to prepare/modify ceramic membranes. As such, LP-CVD facilitates the preparation of SiC membranes: (a) in a single step; and (b) at much lower temperatures (700-900 °C) in comparison with sol-gel methods (ca. 2000 °C). In this work, SiC ultrafiltration (UF) membranes were prepared via LP-CVD at two different deposition temperatures and pressures. Subsequently, their chemical stability in NaClO was investigated over 200 h of aging. Afterward, the properties and performance of as-prepared SiC UF membranes were evaluated before and after aging to determine the optimal deposition conditions. Our results indicate that the SiC UF membrane prepared via LP-CVD at 860 °C and 100 mTorr exhibited excellent resistance to NaClO aging, while the membrane prepared at 750 °C and 600 mTorr significantly deteriorated. These findings not only highlight a novel preparation route for SiC membranes in a single step via LP-CVD, but also provide new insights about the careful selection of LP-CVD conditions for SiC membranes to ensure their long-term performance and robustness under harsh chemical cleaning conditions.

Modeling the antifouling properties of atomic layer deposition surface-modified ceramic nanofiltration membranes.

This work investigates the enhancement of antifouling properties of ceramic nanofiltration membranes by surface modification via atomic layer deposition (ALD) of TiO2. Feed solutions containing bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) were used as model foulants. The classic fouling mechanism models and the modified fouling indices (MFI) were deduced from the flux decline profiles. Surface roughness values of the ALD coated and uncoated membranes were 63 and 71 nm, respectively, while the contact angles were 34.2 and 59.5°, respectively. Thus, coating increased the water affinity of the membrane surfaces and consequently improved the anti-fouling properties. The MFI values and the classic fouling mechanism correlation coefficients for cake filtration for the ALD coated and the uncoated membrane upon SA fouling were 42,963 (R2 = 0.82) and 143,365 sL-2 (R2 = 0.98), respectively, whereas the correlation coefficients for the combined foulants (SA + BSA + HA) were 267,185 (R2 = 0.99) and 9569 sL-2 (R2 = 0.37), respectively. The study showed that ALD can effectively enhance the antifouling properties of ceramic membranes.

Oil-in-water emulsion separation: Fouling of alumina membranes with and without a silicon carbide deposition in constant flux filtration mode.

Ceramic membranes have drawn increasing attention in oily wastewater treatment as an alternative to their traditional polymeric counterparts, yet persistent membrane fouling is still one of the largest challenges. Particularly, little is known about ceramic membrane fouling by oil-in-water (O/W) emulsions in constant flux filtration modes. In this study, the effects of emulsion chemistry (surfactant concentration, pH, salinity and Ca2+) and operation parameters (permeate flux and filtration time) were comparatively evaluated for alumina and silicon carbide (SiC) deposited ceramic membranes, with different physicochemical surface properties. The original membranes were made of 100% alumina, while the same membranes were also deposited with a SiC layer to change the surface charge and hydrophilicity. The SiC-deposited membrane showed a lower reversible and irreversible fouling when permeate flux was below 110 L m-2 h-1. In addition, it exhibited a higher permeance recovery after physical and chemical cleaning, as compared to the alumina membranes. Increasing sodium dodecyl sulfate (SDS) concentration in the feed decreased the fouling of both membranes, but to a higher extent in the alumina membranes. The fouling of both membranes could be reduced with increasing the pH of the emulsion due to the enhanced electrostatic repulsion between oil droplets and membrane surface. Because of the screening of surface charge in a high salinity solution (100 mM NaCl), only a small difference in irreversible fouling was observed for alumina and SiC-deposited membranes under these conditions. The presence of Ca2+ in the emulsion led to high irreversible fouling of both membranes, because of the compression of diffusion double layer and the interactions between Ca2+ and SDS. The low fouling tendency and/or high cleaning efficiency of the SiC-deposited membranes indicated their potential for oily wastewater treatment.

Characterization of natural organic matter in South African drinking water treatment plants: Towards integrating ceramic membrane filtration.

This work presents the first comprehensive investigation of natural organic matter (NOM) fraction removal using ceramic membranes in South Africa. The rate of removal of bulk NOM (measured as UV254 and DOC % removal), the biodegradable dissolved organic carbon (BDOC) fraction, polarity-based fractions, and fluorescent dissolved organic carbon (FDOM) fractions was investigated from water abstracted from drinking water treatment plants (WTPs) in South Africa. Further, mechanisms of ceramic membrane fouling by waters of South Africa were studied. Ceramic membranes removed more than 80% DOC from samples from coastal WTPs, whereas for inland plants, the removal was between 60% and 75% of DOC. FDOM was removed to at least 80% regardless of the site of the plant. The BDOC removal by the ceramic membranes was above 85%. The hydrophobic fraction was the most amenable to removal by ceramic membranes regardless of the site of sample abstraction (above 60% for all sites). The freshness index (ß:α) correlated strongly to UV254 removal (R2 = 0.96), thus UV254 removal can serve as a proxy for the susceptibility to removal of such class of NOM by ceramic membranes. This investigation demonstrated that ceramic membranes could be a valuable technology if integrated into the existing WTPs. PRACTITIONER POINTS: The removal of bulk parameters by ceramic membrane was greater than unit conventional processes used in all the sampled water treatment plants. The hydrophobic polarity-based fraction of NOM was the most amenable to removal by ceramic membranes regardless of the site of the WTP. Polarity-based fractions, aromaticity, and initial DOC had a combined influence on the removal of organic matter by ceramic membranes as explained by principal component three.

State-of-the-Art Ceramic Membranes for Oily Wastewater Treatment: Modification and Application.

Membrane filtration is considered to be one of the most promising methods for oily wastewater treatment. Because of their hydrophilic surface, ceramic membranes show less fouling compared with their polymeric counterparts. Membrane fouling, however, is an inevitable phenomenon in the filtration process, leading to higher energy consumption and a shorter lifetime of the membrane. It is therefore important to improve the fouling resistance of the ceramic membranes in oily wastewater treatment. In this review, we first focus on the various methods used for ceramic membrane modification, aiming for application in oily wastewater. Then, the performance of the modified ceramic membranes is discussed and compared. We found that, besides the traditional sol-gel and dip-coating methods, atomic layer deposition is promising for ceramic membrane modification in terms of the control of layer thickness, and pore size tuning. Enhanced surface hydrophilicity and surface charge are two of the most used strategies to improve the performance of ceramic membranes for oily wastewater treatment. Nano-sized metal oxides such as TiO2, ZrO2 and Fe2O3 and graphene oxide are considered to be the potential candidates for ceramic membrane modification for flux enhancement and fouling alleviation. The passive antifouling ceramic membranes, e.g., photocatalytic and electrified ceramic membranes, have shown some potential in fouling control, oil rejection and flux enhancement, but have their limitations.

Simultaneous removal of ammonium ions and sulfamethoxazole by ozone regenerated high silica zeolites.

Continuous development of industry and civilization has led to changes in composition, texture and toxicity of waste water due to the wide range of pollutants being present. Considering that the conventional wastewater treatment methods are insufficient for removing micropollutants and nutrients to a high level, other, alternative, treatment methods should be used to polish wastewater treatment plant effluents. In this study we developed an alternative, polishing concept for removal of ammonium and micropollutants that could potentially be incorporated in existing wastewater treatment plants. We demonstrated a method to use high silica MOR zeolite granules as an adsorbent for simultaneous removal of the micropollutant sulfamethoxazole (SMX) and ammonium (NH4+) ions from aqueous solutions. At an initial NH4+ concentration of 10 mg/L the high silica zeolite mordenite (MOR) granules removed 0.42 mg/g of NH4+, similar to the removal obtained by commonly used natural zeolite Zeolita (0.44 mg/g). However, at higher NH4+ concentrations the Zeolita performed better. In addition, the Langmuir isotherm model showed a higher maximum adsorption capacity of Zeolita (qmax, 4.08 mg/g), which was about two times higher than that of MOR (2.11). The adsorption capacity of MOR towards SMX, at both low (2 µg/L) and high (50 mg/L) initial concentrations, was high and even increased in the presence of NH4+ ions. The used adsorbent could be regenerated with ozone and reused in consecutive adsorption-regeneration cycles with marginal decrease in the total adsorption capacity.

Fundamental fouling mechanisms of dissolved organic matter fractions and their implications on the surface modifications of ceramic nanofiltration membranes: insights from a laboratory scale application.

This work reports on the fundamental factors influencing inter-foulant and foulant-membrane interactions during simulated dissolved organic matter removal using ceramic nanofiltration. Fouling tests were performed using sodium alginate (SAL), humic acid (HA) and bovine serum albumin (BSA) as model foulants. Fouling potentials of each foulant and their mixtures were investigated using feed solutions containing fixed concentrations of K+, Na+, Mg2+ and Ca2+ with a total ionic strength of 10 mM. The impact of modification by atomic layer deposition on fouling mitigation was also assessed. The flux decline in the first 100 min for single foulants was 4.16 × 10-2, 2.69 × 10-2 and 1.60 × 10-2 Lm-2 for SAL, HA and BSA, respectively. These results demonstrated that for the single foulants, deposition on the membrane surface in the early stages of filtration was primarily governed by membrane-foulant interactions. Interestingly, cake filtration was the least fouling mechanism in feed solutions composed of BSA and SAL (R2 = 0.519, 0.374 for BSA + SAL and BSA + SAL + HA, respectively) and the most favorable fouling mechanism of feed solution which included HA and SAL (R2 = 0.972). The water contact angle dropped from 58o to 35° after coating, thus improving its anti-fouling properties.

High-silica zeolites for adsorption of organic micro-pollutants in water treatment: A review.

High-silica zeolites have been found to be effective adsorbents for the removal of organic micro-pollutants (OMPs) from impaired water, including various pharmaceuticals, personal care products, industrial chemicals, etc. In this review, the properties and fundamentals of high-silica zeolites are summarised. Recent research on mechanisms and efficiencies of OMP adsorption by high-silica zeolites are reviewed to assess the potential opportunities and challenges for the application of high-silica zeolites for OMP adsorption in water treatment. It is concluded that the adsorption capacities are well-related to surface hydrophobicity/hydrophilicity and structural features, e.g. micropore volume and pore size of high-silica zeolites, as well as the properties of OMPs. By using high-silica zeolites, the undesired competitive adsorption of background organic matter (BOM) in natural water could potentially be prevented. In addition, oxidative regeneration could be applied on-site to restore the adsorption capacity of zeolites for OMPs and prevent the toxic residues from re-entering the environment.

Effect of PAC dosage in a pilot-scale PAC-MBR treating micro-polluted surface water.

To address the water scarcity issue and advance the traditional drinking water treatment technique, a powdered activated carbon-amended membrane bioreactor (PAC-MBR) is proposed for micro-polluted surface water treatment. A pilot-scale study was carried out by initially dosing different amounts of PAC into the MBR. Comparative results showed that 2g/L performed the best among 0, 1, 2 and 3g/L PAC-MBR regarding organic matter and ammonia removal as well as membrane flux sustainability. 1g/L PAC-MBR exhibited a marginal improvement in pollutant removal compared to the non-PAC system. The accumulation of organic matter in the bulk mixture of 3g/L PAC-MBR led to poorer organic removal and severer membrane fouling. Molecular weight distribution of the bulk liquid in 2g/L PAC-MBR revealed the synergistic effects of PAC adsorption/biodegradation and membrane rejection on organic matter removal. Additionally, a lower amount of soluble extracellular polymer substances in the bulk can be secured in 21 days operation.

Tight ceramic UF membrane as RO pre-treatment: the role of electrostatic interactions on phosphate rejection.

Phosphate limitation has been reported as an effective approach to inhibit biofouling in reverse osmosis (RO) systems for water purification. The rejection of dissolved phosphate by negatively charged TiO2 tight ultrafiltration (UF) membranes (1 kDa and 3 kDa) was observed. These membranes can potentially be adopted as an effective process for RO pre-treatment in order to constrain biofouling by phosphate limitation. This paper focuses on electrostatic interactions during tight UF filtration. Despite the larger pore size, the 3 kDa ceramic membrane exhibited greater phosphate rejection than the 1 kDa membrane, because the 3 kDa membrane has a greater negative surface charge and thus greater electrostatic repulsion against phosphate. The increase of pH from 6 to 8.5 led to a substantial increase in phosphate rejection by both membranes due to increased electrostatic repulsion. At pH 8.5, the maximum phosphate rejections achieved by the 1 kDa and 3 kDa membrane were 75% and 86%, respectively. A Debye ratio (ratio of the Debye length to the pore radius) is introduced in order to evaluate double layer overlapping in tight UF membranes. Threshold Debye ratios were determined as 2 and 1 for the 1 kDa and 3 kDa membranes, respectively. A Debye ratio below the threshold Debye ratio leads to dramatically decreased phosphate rejection by tight UF membranes. The phosphate rejection by the tight UF, in combination with chemical phosphate removal by coagulation, might accomplish phosphate-limited conditions for biological growth and thus prevent biofouling in the RO systems.

Influence of solute-membrane affinity on rejection of uncharged organic solutes by nanofiltration membranes.

A simple, analytical method for predicting transport of uncharged organic solutes through nanofiltration (NF) and reverse osmosis (RO) membranes is presented in this paper. The method requires characterization of key solute and membrane parameters-namely, solute size, membrane pore size, and solute-membrane affinity. All three parameters can be experimentally determined from relatively simple permeation tests and contact angle analyses. The parameters are fed into an analytical model of solute transport, which accounts for hindered convection and diffusion of solutes in the membrane pores, as well as the combined effects of steric exclusion and solute-membrane affinity on solute partitioning from the feed solution into the membrane pores. Overall model predictions for organic solute rejection agreed well with experimental data for three different solutes and two different polymeric NF membranes. Further, the model demonstrates the dramatic influence of solute-membrane affinity on organic rejection by NF and RO membranes. Solute transport predictions made assuming only steric exclusion significantly overestimated rejections for solutes with strong affinity for membrane polymers and similarly underestimated rejections for solutes that were strongly repelled by membrane polymers.