Membrane Fouling In Water Treatment Research Articles

Heterogeneous electro-Fenton catalytic FeOCl@NCNT/ceramic membrane filtration for in-situ membrane fouling control: Performance and mechanism

Homogeneous electro-Fenton (EF) technique is effective to mitigate membrane fouling through oxidizing organic foulants by hydroxyl radicals (HO·), but suffers from acid pH working environment (2.5 ∼ 3.5), low iron recyclability, and iron sludge accumulation. Herein, we developed a heterogeneous EF catalytic membrane to overcome these problems. A FeOCl@NCNT-functionalized ceramic membrane (FeOCl@NCNT/CM) was prepared to control the membrane fouling caused by humic acid (HA). At near neutral pH (∼ 6.0), the membrane enabled the in-situ generation of HO·. It also imparted an effective regeneration of Fe(II) through cathodic reduction and the electron transfer to Fe(III) from pyrrolic-N. The continuously generated HO· could oxidize the HA on/in the membrane, thus effectively mitigating membrane fouling. Electrostatic repulsion also contributed to fouling control by reducing the adhesion of negatively charged HA on the membrane. In continuous electrification mode, the FeOCl@NCNT/CM showed a superior antifouling ability. The water flux of the membrane at an applied voltage of −2.2 V was 1.79 times as high as that without voltage. Both reversible and irreversible membrane fouling were greatly reduced, and the transition of membrane fouling mechanism from pore blockage to cake filtration was significantly delayed. In intermittent electrification mode, the membrane exhibited an excellent self-cleaning performance. The fouled membrane showed considerable water flux recovery ratios (> 95%) after the EF cleaning at −2.2 V. The excellent antifouling and self-cleaning properties, low energy consumption, good stability and reusability of the FeOCl@NCNT/CM make it promising for the in-situ control of membrane fouling in water treatment.

Pre-deposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons

In this study, ordered mesoporous carbons (OMCs), i.e., CMK-3 and CMK-8 were pre-deposited on ultrafiltration membrane surface to alleviate membrane fouling by organic matter in the water. CMK-3 shows a typical 2D hexagonal symmetrical structure with long straight parallel channels, while CMK-8 exhibits a 3D porous structure with cubic channels. The pre-deposited CMK-3/CMK-8 layers exhibited significantly superior performance for both flux enhancement and fouling mitigation, whereas powdered activated carbon (PAC) showed limited influence. The fouling mechanism was apparently altered by the OMCs pre-deposition layers, and pore blocking always dominated throughout the filtration process without the formation of cake filtration. In comparison with PAC, CMK-3 and CMK-8 were more effective for the rejection of tryptophan-like protein substances, which were the major foulants causing membrane fouling. The effects of pollutant fraction and deposited amount (i.e., 6, 12, 25 and 50 g/m2) were systematically explored, and the results indicated that this technology was more efficient for alleviating the fouling induced by humic acid and bovine serum albumin, whereas ineffective for sodium alginate fouling, and the optimal deposited amount varied with the pollutant fraction. CMK-3 exhibited significantly superior performance than CMK-8, likely due to its long straight parallel channels with lower internal resistance for pollutants adsorption. Both adsorption and size exclusion of the pre-deposited layers contributed to fouling mitigation. These results demonstrate that pre-depositing with OMCs as alternatives to conventional PAC shows promising potential for mitigating membrane fouling in water treatment.

The Combination of Coagulation and Adsorption for Controlling Ultra-Filtration Membrane Fouling in Water Treatment

Ultra-filtration technology has been increasingly used in drinking water treatment due to improvements in membrane performance and lowering of costs. However, membrane fouling is the main limitation in the application of ultra-filtration technology. In this study, we investigated the impact of four different pre-treatments: Coagulation, adsorption, coagulation followed by adsorption (C-A), and simultaneous coagulation and adsorption (C+A), on membrane fouling and natural organic matter removal efficiency. The results showed that adsorption process required a large amount of adsorbent and formed a dense cake layer on the membrane surface leading to severe membrane fouling. Compared to adsorption alone, the coagulation and C-A processes decreased the transmembrane pressure by 4.9 kPa. It was due to less accumulation of particles on the membrane surface. As for water quality, the C-A ultra-filtration process achieved the highest removal efficiencies of natural organic matter and disinfection by-product precursors. Therefore, the addition of adsorbent after coagulation is a potentially important approach for alleviating ultra-filtration membrane fouling and enhancing treatment performance.

Evaluation of ferrate as a coagulant aid/oxidant pretreatment for mitigating submerged ultrafiltration membrane fouling in drinking water treatment

Although pre-coagulation can mitigate ultrafiltration (UF) membrane fouling in the treatment of surface waters for drinking water supply, biological activities (‘biofouling’) can induce a continuous increase in membrane fouling. To meet this challenge, potassium ferrate, K2FeO4, a combined oxidant and coagulant, was evaluated as a pre-treatment chemical for controlling submerged UF membrane fouling in water treatment. Ferrate use as an alternative to- (phase 1: ∼23days), and in combination with- (phase 2: ∼30days), conventional FeCl3, have been studied using parallel continuous bench-scale submerged membrane systems, using FeCl3 as the reference. The poorer performance of ferrate (alone) as a pre-treatment compared to FeCl3 (phase 1) was the result of a lower coagulation efficiency, which outweighed the beneficial impact of the ferrate on bacterial inactivation. The net reduction in pre-treatment performance led to an increase in the concentration of residual, active bacteria in the membrane tank, and bacteria associated large molecular weight (MW) organic substances, such as extracellular polymeric substances (EPS) or biopolymers, which were the principal cause of the higher rate of membrane fouling observed. In contrast, ferrate performed best as a coagulant aid/oxidant (FeCl3/K2FeO4) (phase 2), with the rate of membrane fouling (increase in transmembrane pressure) 4.5 times lower than conventional FeCl3 pre-treatment. This pre-treatment arrangement resulted in less bacteria (and EPS) and suspended solids in the membrane tank, and less accumulation of materials in the cake layer and within the membrane pores. The results indicated clearly the potential benefit of applying ferrate as a coagulant aid/oxidant with a coagulant, in UF pre-treatment, with the control of bacteria and EPS a key factor in reducing membrane fouling.

Effect of pre-ozonation on mitigation of ceramic UF membrane fouling caused by algal extracellular organic matters

Soluble extracellular organic matters (EOM) resulting from algal blooms in water sources can cause severe membrane fouling in water treatment. The effect of pre-ozonation on mitigation of ceramic UF membrane fouling caused by EOM released from Microcystis aeruginosa and the associated fouling mitigation mechanism were investigated through the characterization of hydraulic performance, dissolved organic carbon (DOC), fluorescence excitation–emission matrix (EEM) spectra, molecular weight (MW) distribution, hydrophilicity and model fit of five combined fouling models. Pre-ozonation achieved a remarkable effect of hydraulically reversible fouling mitigation as a result of the selective oxidation of the very high MW hydrophobic biopolymers (⩾20kDa) to lower MW and more hydrophilic compounds. However, pre-ozonation had a very limited effect on mitigation of hydraulically irreversible fouling dominated by the high MW (1–10kDa) hydrophilic organics. Modeling results indicated that the EOM-related membrane fouling mitigation by pre-ozonation was more likely ascribed to the alleviation of cake layer and standard pore blocking with standard pore blocking playing a more important role.

Granular iron oxide adsorbents to control natural organic matter and membrane fouling in ultrafiltration water treatment

Fine iron oxide particles (IOPs) are effective in removing natural organic matter (NOM) that causes membrane fouling in water treatment, but the separation of used IOPs is problematic. This study focused on the fabrication and use of granular iron oxide adsorbents, in combination with ultrafiltration (UF) membranes while investigating the NOM removal efficiency and fouling control. Sulfonated styrene-divinylbenzene copolymer beads were coated with two types of iron oxides (ferrihydrite and magnetite) and their performances were compared to that of fine IOPs. A significant amount of iron oxide coating (52–63 mg of Fe per g bead) was achieved by means of electrostatic binding and hydrolysis of iron ions. Iron oxide coated polymer (IOCP) beads were able to remove some amounts (∼20%) of dissolved organic carbon (DOC) comparable to that achieved by IOPs within a short period of time (<15 min). Regenerated IOCPs exhibited the same sorption capacity as the fresh ones. The integrated IOCP/UF system operation with a 15-min empty bed contact time and 10-h cyclic regeneration maintained the 20% DOC removal with no sign of significant membrane fouling. In contrast, a sharp transmembrane pressure buildup occurred in the UF system when no iron oxide pretreatment was applied, regardless of the types of membranes tested. Iron oxide adsorbed the NOM fraction with molecular weights of >1000 kDa which is believed to be responsible for severe UF fouling.

Use of surface enhanced Raman spectroscopy for studying fouling on nanofiltration membrane

Membrane fouling is a key constraint for the widespread application of membrane-based technologies in water and wastewater treatment because it reduces the production volume of water, thereby increasing operating and maintenance costs. Characterization of membrane materials and foulant layers is crucial for an accurate understanding of fouling processes. This study used surface-enhanced Raman spectroscopy (SERS) and normal Raman spectroscopy as a novel technique to examine fouling caused by naturally occurring organics on a polymeric nanofiltration (NF) membrane, which is used for the treatment of drinking water. The results from this study suggest that normal Raman spectroscopy is useful for characterization of polyamide NF membranes used in drinking water treatment, but is not sensitive enough to resolve the foulant materials present on the membrane surface. Scanning electron microscopy (SEM) analysis of virgin and used membrane suggests significant membrane fouling. The observed SERS peak of fouled membrane at wavenumber 1543cm−1 suggests the presence of protein and those at 1444, 1305, and 1239cm−1 indicate the possible presence of carbohydrate on the membrane surface. These results demonstrate that SERS has the potential for identification of functional groups of organics involved in NF membrane fouling in water treatment that could lead to improved strategies for membrane fouling reduction. The use of SERS for possible foulant identification offers a rapid, cost effective and portable tool for on-site analysis of membrane surfaces.

Novel approach for the analysis of bench-scale, low pressure membrane fouling in water treatment

The concept of the unified membrane fouling index (UMFI) was introduced recently for assessing the fouling of low pressure membranes (LPM) used in water treatment. The major advantages of the UMFI over most other fouling indices are the universality (independent of filtration scale or mode) and specificity (based on the actual membrane of interest for full-scale application). This study applied the concept of the UMFI to the assessment of fouling results obtained in a bench-scale fouling study. This study was conducted with combinations of four commercially available LPMs and four natural waters (including secondary wastewater effluent) under systematically varied hydrodynamic conditions. It was found that, with the adoption of the UMFI, the fouling potential of different membrane and water combinations can be presented in a unified, concise, and quantitative way, enabling quantitative and integrated assessment of the fouling data and their relationships with different potential fouling factors. Expansion of the UMFI concept to other studies is anticipated to assist in the integration of the vast amount of bench-scale fouling results obtained with different LPMs; this integration is crucial to the mechanistic understanding of LPM fouling.

Membrane fouling in water treatment: effect of solution composition on resistance and compressibility of gel-forming filter cakes

Formation of highly impermeable assemblages on membranes in water treatment can result in significant problems in maintaining permeate flux and necessitate frequent cleaning of the membrane. In this study, we describe the hydraulic properties of montmorillonite clay particles and examine the link between microscopic particle–particle interactions and the bulk properties of the clay assemblage. In particular, we show that changes in the ionic composition of feed waters can have a dramatic impact on the fouling properties of montmorillonite particles with highly impermeable, voluminous filter cakes formed at low ionic concentrations and dramatically more porous assemblages formed at higher ionic strengths.

Towards a better characterisation and understanding of membrane fouling in water treatment

This paper presents two successful case studies where LC-OCD (Liquid Chromatography-Organic Carbon Detection) tool was used to characterise fouling material in membrane processes. It has been demonstrated that this technique combined with other analytical methods is a powerful tool to identify species responsible for fouling, understand membrane fouling mechanisms and therefore provide solutions to prevent or/and minimise fouling.