Investigation of pre-coagulation and powder activate carbon adsorption on ultrafiltration membrane fouling

Designing sustainable membrane-based water treatment via fouling control through membrane interface engineering and process developments

Membrane-based water treatment processes have been established as a powerful approach for clean water production. However, despite the significant advances made in terms of rejection and flux, provision of sustainable and energy-efficient water production is restricted by the inevitable issue of membrane fouling, known to be the major contributor to the elevated operating costs due to frequent chemical cleaning, increased transmembrane resistance, and deterioration of permeate flux. This review provides an overview of fouling control strategies in different membrane processes, such as microfiltration, ultrafiltration, membrane bioreactors, and desalination via reverse osmosis and forward osmosis. Insights into the recent advancements are discussed and efforts made in terms of membrane development, modules arrangement, process optimization, feed pretreatment, and fouling monitoring are highlighted to evaluate their overall impact in energy- and cost-effective water treatment. Major findings in four key aspects are presented, including membrane surface modification, modules design, process integration, and fouling monitoring. Among the above mentioned anti-fouling strategies, a large part of research has been focused on membrane surface modifications using a number of anti-fouling materials whereas much less research has been devoted to membrane module advancements and in-situ fouling monitoring and control. At the end, a critical analysis is provided for each anti-fouling strategy and a rationale framework is provided for design of efficient membranes and process for water treatment.

Dynamic Membranes for Enhancing Resources Recovery from Municipal Wastewater

This paper studied the feasibility of using dynamic membranes (DMs) to treat municipal wastewater (MWW). Effluent from the primary settler of a full-scale wastewater treatment plant was treated using a flat 1 µm pore size open monofilament polyamide woven mesh as supporting material. Two supporting material layers were required to self-form a DM in the short-term (17 days of operation). Different strategies (increasing the filtration flux, increasing the concentration of operating solids and coagulant dosing) were used to enhance the required forming time and pollutant capture efficiency. Higher permeate flux and increased solids were shown to be ineffective while coagulant dosing showed improvements in both the required DM forming time and permeate quality. When coagulant was dosed (10 mg L⁻¹) a DM forming time of 7 days and a permeate quality of total suspended solids, chemical oxygen demand, total nitrogen, total phosphorous and turbidity of 24 mg L⁻¹, 58 mg L⁻¹, 38.1 mg L⁻¹, 1.2 mg L⁻¹ and 22 NTU, respectively, was achieved. Preliminary energy and economic balances determined that energy recoveries from 0.032 to 0.121 kWh per m³ of treated water at a cost between €0.002 to €0.003 per m³ of treated water can be obtained from the particulate material recovered in the DM.

Exploring the influence mechanism of ozonation on protein fouling of ultrafiltration membranes as a result of the interfacial interaction of foulants at the membrane surface

Ozonation was widely used before ultrafiltration processes, but its effect mechanism on protein fouling is still controversial. Ozonation of bovine serum albumin (BSA) solutions was performed in the present work. The interfacial forces of BSA at the membrane surface were measured before and after ozonation. The adsorption behaviour of BSA onto the membrane surface and the fouling layer structures under different ozone dosages were also investigated. These results were combined with the membrane fouling behaviour to identify the effect of ozonation on protein fouling. The results showed that ozonation could weaken the interaction forces between the membrane and BSA effectively, but this did not have any effect on membrane fouling. In contrast, in terms of membrane fouling behaviour after pre-ozonation, the contribution of the changes in the covalent disulfide bonds between BSA molecules outweighs those of the non-covalent bonds. The number of disulfide bonds gradually increased as the O₃:DOC ratio increased from 0 to 0.3, and began to decline when the O₃:DOC ratio was further increased to 0.45 and 0.6. This could have altered the deposition rate of foulants onto the membrane surface and the structure of the fouling layers, and may have caused the membrane fouling first to be enhanced and then to decline with increasing ozone dosages.

Effect of Pre-Oxidation on Coagulation/Ceramic Membrane Treatment of Yangtze River Water

The membrane separation process is being widely used in water treatment. It is very important to control membrane fouling in the process of water treatment. This study was conducted to evaluate the efficiency of a pre-oxidation-coagulation flat ceramic membrane filtration process using different oxidant types and dosages in water treatment and membrane fouling control. The results showed that under suitable concentration conditions, the effect on membrane fouling control of a NaClO pre-oxidation combined with a coagulation/ceramic membrane system was better than that of an O₃ system. The oxidation process changed the structure of pollutants, reduced the pollution load and enhanced the coagulation process in a pre-oxidation-coagulation system as well. The influence of the oxidant on the filtration system was related to its oxidizability and other characteristics. NaClO and O₃ performed more efficiently than KMnO₄. NaClO was more conducive to the removal of DOC, and O₃ was more conducive to the removal of UV₂₅₄.

Enhancing ultrafiltration performance by gravity-driven up-flow slow biofilter pre-treatment to remove natural organic matters and biopolymer foulants

Membrane fouling by influent biopolymers, and the formation of surface biofilms, are major obstacles to the practical application of membrane technologies. Identifying reliable and sustainable pre-treatment methods for membrane filtration remains a considerable challenge and is the subject of continuing research study worldwide. Herein, the performance of a bench-scale gravity-driven up-flow slow biofilter (GUSB) as the pre-treatment for ultrafiltration to reduce membrane fouling is presented. Dissolved organic carbon (DOC) was shown efficiently removed by the GUSB (around 80%) when treating a natural water influent. More significantly, biopolymers, with molecular weight (MW) between 20 kDa and 100 kDa, were effectively removed (62.8% reduction) and this led to a lower rate of transmembrane pressure (TMP) development by the UF membrane. Microbial diversity analysis further unraveled the function of GUSB in shaping microbes to degrade biopolymers, contributing to lower accumulation and different distribution pattern of SMP on the membrane surface. Moreover, the biofilm formed on the membrane surface after the pre-treatment of GUSB was observed to have a relative porous structure compared to the control system, which can also affect the fouling development. Long-term operation of GUSB further revealed its robust performance in reducing both natural organic matters and UF fouling propensity. This study overall has demonstrated the potential advantages of applying a GUSB to enhance UF process performance by reducing biofouling and improving effluent quality.

Hybrid UF/NF process treating secondary effluent of wastewater treatment plants for potable water reuse: Adsorption vs. coagulation for removal improvements and membrane fouling alleviation

Coagulation and adsorption are gradually adopted as pre-treatments to produce reclaimed potable water. However, previous researches on membrane fouling mechanisms were currently insufficient to minimize dual membrane fouling. This study aimed at investigating the effects of pre-coagulation and pre-adsorption on the removal performance and membrane fouling alleviation of dual membrane UF/NF process in treating secondary effluent from a wastewater treatment plant. The results indicated that both types of pretreatments conferred positive effects on organic membrane fouling removal of the UF process whereas diverse effects on NF process. Pre-coagulation could enhance the removal of nitrogen and phosphorus to contribute towards producing microbiologically-stable water. On the other hand, introduction of Al³⁺ reduced the removal efficiency of UF/NF systems on heavy metals. From the perspective of UF membrane fouling, two pretreatments employed could increase the flux of UF, but simultaneously aggravating irreversible membrane fouling. Hermia and Tansel models revealed an unstable cake filtration was caused by pre-coagulation and pre-adsorption. Both the models consistently demonstrated the rapid formation of cake filtration onto UF membrane surface. Interestingly, the powdered activated carbon (PAC) adsorption could significantly reduce cake layer fouling onto the surface of NF membrane, while pre-coagulation aggravated the NF fouling. These results are essential to developing robust, cost-effective and energy-efficient strategies based on membranes to produce reclaimed potable water.

Abatement of the membrane biofouling: Performance of an in-situ integrated bioelectrochemical-ultrafiltration system

The practical applications of membrane-based water treatment techniques are constrained by the problem of membrane fouling. Various studies have revealed that interactions between extracellular polymeric substances (EPS) and the membrane surface determine the extent of irreversible fouling. Herein, we describe a novel bioelectrochemical system (BES) integrated with an ultrafiltration (UF) membrane in order to provide an enhanced antifouling property. It was found that the integrated BES membrane system had a superior performance compared to a conventional (control) UF system, as manifested by a much lower development of transmembrane pressure. The BES significantly reduced microbial viability in the membrane tank and the imposed electrode potential contributed to the degradation of biopolymers, which favored the alleviation of membrane fouling. Notably, the electron transfer between the acclimated microorganisms and the conductive membrane in the BES integrated system exhibited an increasing trend with the operation time, indicating a gradual increase in microbial electrical activity. Correspondingly, the accumulation of extracellular polymeric substances (EPS) on the membrane surface of the BES integrated system showed a substantial decrease compared to the control system, which could be attributed to a series of synergistic effects induced by the BES integration. The differences in the microbial diversity between the control and the BES integrated system revealed the microbial selectivity of the poised potential. Specifically, microbial strains with relatively high EPS production, like the genus of Zoogloea and Methyloversatilis, were reduced significantly in the BES integrated system, while the expression of the electroactive bacteria was promoted, which facilitated extracellular electron transfer (EET) and therefore the bioelectrochemical reactions. Overall, this study has presented a feasible and promising new approach for membrane fouling mitigation during the process of water treatment.

Steroid hormone micropollutant removal from water with activated carbon fiber-ultrafiltration composite membranes

Short activated carbon fibers (ACF) with high surface area were fabricated via carbonization in N₂ and activation in CO₂ at high temperatures, with cellulose fibers as the raw materials. The obtained ACF were subsequently deposited into the support layer of a polyethersulfone (PES) ultrafiltration membrane by a facile filtration process to obtain the sandwich structured ACF-PES composite membrane. The hormone (17β-estradiol, E2) adsorption kinetics and isotherm of ACF in static conditions, as well as E2 removal by filtration with the ACF-PES composite membrane were investigated. In static conditions, ACF rapidly and efficiently adsorbs E2 evidenced by a high removal of >97 %. The fitting of second order kinetics and linear (Henry) adsorption isotherm models indicated the availability of easily accessible adsorption sites. Besides, such efficient E2 adsorption was contributed by many interactions between E2 and ACF, namely hydrophobic interactions, hydrogen bonding and π-π stacking. The incorporation of ACF in a PES membrane resulted in a minor loss of filtration flux compared with the control membrane, but significantly improved E2 removal through adsorption pathway. With only 1.0 mg ACF incorporated (loading 2.0 g/m²), the composite membrane could reject 76 % of E2 from a 100 ng/L solution at a flux of 450 L/m²∙h, demonstrating that ACF-PES can overcome the permeability-selectivity trade-off of traditional UF membranes.

Membrane fouling control by Ca2+ during coagulation-ultrafiltration process for algal-rich water treatment

Seasonal algal bloom, a water supply issue worldwide, can be efficiently solved by membrane technology. However, membranes typically suffer from serious fouling, which hinders the wide application of this technology. In this study, the feasibility of adding Ca²⁺ to control membrane fouling in coagulation-membrane treatment of algal-rich water was investigated. According to the results obtained, the normalized membrane flux decreased by a lower extent upon increasing the concentration of Ca²⁺ from 0 to 10 mmol/L. Simultaneously, the floc particle size increased significantly with the concentration of Ca²⁺, which leads to a lower hydraulic resistance. The coagulation performance is also enhanced with the concentration of Ca²⁺, inducing a slight osmotic pressure-induced resistance. The formation of Ca²⁺ coagulation flocs resulted in a looser, thin, and permeable cake layer on the membrane surface. This cake layer rejected organic pollutants and could be easily removed by physical and chemical cleaning treatments, as revealed by scanning electron microscopy images. The hydraulic irreversible membrane resistance was significantly reduced upon addition of Ca²⁺. All these findings suggest that the addition of Ca²⁺ may provide a simple-operation, cost-effective, and environmentally friendly technology for controlling membrane fouling during coagulation-membrane process for algal-rich water treatment.

High-Flux, Antifouling Hydrophilized Ultrafiltration Membranes with Tunable Charge Density Combining Sulfonated Poly(ether sulfone) and Aminated Graphene Oxide Nanohybrid

In this work, a new protocol was developed for creating charge-tuned, hydrophilic hybrid ultrafiltration (UF) membranes with high flux, rejection rate, and fouling resistance. The membranes were fabricated using a combination of sulfonated poly(ether sulfone) (SPES) and aminated graphene (GO-SiO₂-NH₂) nanohybrid via the non-solvent-induced phase separation (NIPS) method. The GO-SiO₂-NH₂ nanohybrid was first synthesized using GO nanosheets and 3-aminopropyl triethoxysilane (APTES) through the covalent condensation reaction at 80 °C and was thoroughly characterized. Then, 2-8 wt% of the nanohybrid was incorporated into the matrix of SPES for the fabrication of the hybrid membranes. The resulting membranes were characterized using an electrokinetic analyzer, a contact angle goniometer, and Raman, field emission scanning electron microscopy-energy-dispersive X-ray spectrometry (FESEM-EDX), and atomic force microscopy experiments. The porosity, charge density, and surface morphology were altered, and the hybrid membranes became more hydrophilic after the incorporation of the nanohybrid. The pure water flux of the hybrid membranes systematically increased with the loading amount of the nanohybrid. The pure water flux of the hybrid membrane containing 6 wt% GO-SiO₂-NH₂ nanohybrid at a 2 bar feed pressure was 537 L m^-2 h^-1, about 3-fold that of pristine membrane (186 L m^-2 h^-1). The fouling resistance of the hybrid membranes was evaluated and confirmed using several representative foulants, including bovine serum albumin, humic acid, sodium alginate, and a synthetic solution of natural organic matter (NOM). The fabricated membranes were capable of removing more than 97% of NOM, without a compromise of their rejection rate.

Pre-coagulation coupled with sponge-membrane filtration for organic matter removal and membrane fouling control during drinking water treatment

A new hybrid system was developed in this study for the treatment of drinking water consisting of pre-coagulation using polyaluminium chloride (PACl) and membrane filtration (MF) with sponge cubes acting as biomass carriers (P-SMF). When compared to a conventional MF (CMF) and a MF after coagulation by utilizing PACl (P-MF), better removal of nutrients, UV₂₅₄ and dissolved organic carbon (DOC) (>65%) was obtained from the P-SMF. The accumulation of biopolymers (including polysaccharides and proteins), humic substances, hydrophilic organics, and other small molecular weight (MW) organic matter in the CMF led to the most severe membrane fouling coupled with the highest pore blocking and cake resistance. Pre-coagulation was ineffective in eliminating small MW and hydrophilic organic matter. Conversely, the larger MW organics (i.e. biopolymers and humic substances), small MW organics and hydrophilic organic compounds could be removed in significantly larger quantities in the P-SMF by PACl coagulation. This was achieved via adsorption and the biodegradation by attached biomass on these sponges and by the suspended sludge. Further analyses of the microbial community indicated that the combined addition of PACl and sponges generated a high enrichment of Zoolgloea, Amaricoccus and Reyranella leading to the reduction of biopolymers, and Flexibacter and Sphingobium were linked to the degradation of humic substances. Moreover, some members of Alphaproteobacteria in the P-SMF may be responsible for the removal of low MW organics. These results suggest that the pre-coagulation process coupled with adding sponge in the MF system is a promising technology for mitigating membrane fouling.

Prevention of UF membrane fouling in drinking water treatment by addition of H2O2 during membrane backwashing

Although conventional coagulation pre-treatment can mitigate the fouling of ultrafiltration (UF) membrane when treating raw waters, it is insufficient to restrict the development of irreversible fouling and reversible fouling to a low level. In this paper we demonstrate that the intermittent addition of H₂O₂ into the membrane tank during backwash events (after coagulation pre-treatment) successfully prevented the development of any significant membrane fouling. Laboratory-scale tests were undertaken using two membrane systems operated in parallel over 60 days, one serving as a reference coagulation-ultrafiltration (CUF) process, and the other receiving the H₂O₂ (CUF-H₂O₂), with a decreasing dose in three successive phases: 10, 5 and 2 mg/L. The results showed that the addition of H₂O₂ (via a separate dosing tube) during a 1 min backwash process (at 30 min intervals) reduced the growth of bacteria in the membrane tank, and the associated concentrations of soluble microbial products (SMP, including protein and polysaccharide). This resulted in a much reduced cake layer, which contained significantly less high MW organic matter (>50%), such as EPS, thereby improving the interaction between particles in the cake layer and/or particles and the membrane surface. There was also less organic matter, of all MW fractions, adsorbed in the membrane pores of the CUF-H₂O₂ system. The addition of H₂O₂ in the membrane tank appeared to alter the nature of the organic matter with a conversion of hydrophobic to hydrophilic fractions, which induced less organics adsorption within the hydrophobic PVDF membrane pores, and a reduced bonding ability for particles. There was no physico-chemical evidence of any deterioration of the membrane from exposure to H₂O₂, which indicates the feasibility of applying this novel method of fouling control for full-scale UF based water treatment processes.

Removal of organics by combined process of coagulation-chlorination-ultrafiltration: optimization of overall operation parameters

To gain the run parameters of the combined process of coagulation/in situ chlorination/ultrafiltration (UF) so that the system can remove as much organic contaminants as possible without serious membrane fouling, the impacts of operation conditions in coagulation and pre-chlorination unit were investigated in a pilot-scale test. The characteristics of organics in UF influent were examined by excitation emission matrix spectroscopy to find out fouling behavior of different natural organic matter compositions to UF membrane. Thereafter, the operation parameters of different processing units of the hybrid device were optimized by response surface methodology (RSM). The results showed that the tests with the agitation speed of 40 r min^-1 had the lowest membrane fouling rate and the highest COD_Mn removal, in addition, inappropriate dosage of sodium hypochlorite in membrane influent might exert negative impacts on membrane by lowering UV₂₅₄ rejection, especially during the high algae laden period. The predominant factors of membrane fouling were the existence of tryptophan protein-like substances and the soluble microbial products. Optimum values of the mechanical rotation speed in coagulation unit, chemical dosage in pre-chlorination unit, and membrane flux in UF unit of the integrative process were 41.79 r min^-1, 1.40 mg L^-1, and 82.26 LMH, respectively.

The effects of hydrogen peroxide pre-oxidation on ultrafiltration membrane biofouling alleviation in drinking water treatment

Pre-oxidation is widely used to reduce ultrafiltration membrane fouling. However, the variation in the composition of microbial communities and extracellular polymeric substances (EPSs) accompanying pre-oxidation in drinking water treatment has received little attention. In this study, hydrogen peroxide (H₂O₂) was used in a coagulation-ultrafiltration process with Al₂(SO₄)₃·18H₂O. A long-term reactor experiment (60d) showed that pre-oxidation alleviated membrane fouling, mainly due to its inhibition of microbial growth, as observed by flow cytometry measurements of the membrane tank water. Further analysis of the formed cake layer demonstrated that the corresponding levels of EPS released from the microbes were lower with than without H₂O₂ treatment. In comparison to polysaccharides, proteins dominated the EPS. 2D-electrophoresis showed little difference (p>0.05, Student's t-test) in the composition of proteins in the cake layer between the treatments with and without H₂O₂. The molecular weights of proteins ranged from approximately 30-50kDa and the majority of isoelectric points ranged from 6 to 8. High-throughput sequencing showed that the predominant bacteria were Proteobacteria, Bacteroidetes, and Verrucomicrobia in both cake layers. However, the relative abundance of Planctomycetes was higher in the cake layer with H₂O₂ pre-oxidation, which was likely probably due to the strong oxidative resistance of its cell wall. Overall, our findings clarify the fundamental molecular mechanism in H₂O₂ pre-oxidation for ultrafiltration membrane bio-fouling alleviation in drinking water treatment.

Insights and Model for Understanding Natural Organic Matter Adsorption onto Mixed Adsorbents

Adsorption-based processes are commonly used to remove natural organic matter (NOM) from drinking water sources and thereby mitigate its impacts on other water treatment processes and the quality of the finished water. These processes are complicated by the fact that NOM comprises multiple fractions that can exhibit disparate adsorption behaviors. Prior modeling of NOM adsorption has invariably focused on systems with a single adsorbent, but results presented here demonstrate surprising and counterintuitive behavior in systems containing two or more adsorbents. Specifically, if the sequence of affinities of the different NOM fractions are the same for two adsorbents, then overall adsorption changes monotonically as one adsorbent is gradually replaced by the other. However, if the sequence differs for the two adsorbents, overall adsorption can increase even when the nominally stronger adsorbent is gradually replaced by the weaker one. This work demonstrates and explains such behavior for a particular mixture of adsorbents and introduces a mathematical model that illustrates how other mixtures might behave.

Ultrafiltration membrane fouling induced by humic acid with typical inorganic salts

Severe ultrafiltration (UF) membrane fouling is always induced by humic acid (HA). However, little attention has been paid to the influence of inorganic salts, and even the studies related have been limited to only a single kind of salt. In addition, the concentration of the inorganic salts reported in previous studies is much high. Herein, the effect of HA on UF membrane performance was investigated in the presence of typical inorganic salts, with concentrations similar to those in natural waters or actually used in most current water plants. The results showed that membrane performance was influenced little by monovalent inorganic salts (NaCl and KCl), while divalent inorganic salts (CaCl₂ and MgCl₂) could exacerbate the membrane fouling. For trivalent inorganic salts (AlCl₃·6H₂O and FeCl₃·6H₂O), floc adsorption was the dominant HA removing mechanism, and AlCl₃·6H₂O behaved better than FeCl₃·6H₂O. Relating to the floc properties, severe membrane fouling occurred with low dosage, while it was mitigated with high dosage. Compared with the trivalent inorganic salts, more severe membrane fouling was caused by divalent inorganic salts. Additionally, little synergistic or inhibitory effect occurred with mixtures of divalent inorganic salts and trivalent inorganic salts. Furthermore, analysis with the classical fouling models showed that cake filtration was the main fouling mechanism with/without inorganic salts. Based on the findings, we believe these different HA behaviors exhibited during coagulation process with inorganic salts will have a large potential application in UF membrane fouling alleviation in water treatment.

Comparison of the effects of aluminum and iron(III) salts on ultrafiltration membrane biofouling in drinking water treatment

Coagulation plays an important role in alleviating membrane fouling, and a noticeable problem is the development of microorganisms after long-time operation, which gradually secrete extracellular polymeric substances (EPS). To date, few studies have paid attention to the behavior of microorganisms in drinking water treatment with ultrafiltration (UF) membranes. Herein, the membrane biofouling was investigated with different aluminum and iron salts. We found that Al₂(SO₄)₃·18H₂O performed better in reducing membrane fouling due to the slower growth rate of microorganisms. In comparison to Al₂(SO₄)₃·18H₂O, more EPS were induced with Fe₂(SO₄)₃·xH₂O, both in the membrane tank and the sludge on the cake layer. We also found that bacteria were the major microorganisms, of which the concentration was much higher than those of fungi and archaea. Further analyses showed that Proteobacteria was dominant in bacterial communities, which caused severe membrane fouling by forming a biofilm, especially for Fe₂(SO₄)₃·xH₂O. Additionally, the abundances of Bacteroidetes and Verrucomicrobia were relatively higher in the presence of Al₂(SO₄)₃·18H₂O, resulting in less severe biofouling by effectively degrading the protein and polysaccharide in EPS. As a result, in terms of microorganism behaviors, Al-based salts should be given preference as coagulants during actual operations.

Reducing ultrafiltration membrane fouling during potable water reuse using pre-ozonation

Wastewater reclamation has increasingly become popular to secure potable water supply. Low-pressure membrane processes such as microfiltration (MF) and ultrafiltration (UF) play imperative roles as a barrier of macromolecules for such purpose, but are often limited by membrane fouling. Effluent organic matter (EfOM), including biopolymers and particulates, in secondary wastewater effluents have been known to be major foulants in low-pressure membrane processes. Hence, the primary aim of this study was to investigate the effects of pre-ozonation as a pre-treatment for UF on the membrane fouling caused by EfOM in secondary wastewater effluents for hydrophilic regenerated cellulose (RC) and hydrophobic polyethersulfone (PES) UF membranes. It was found that greater fouling reduction was achieved by pre-ozonation for the hydrophilic RC membrane than the hydrophobic PES membrane at increasing ozone doses. In addition, the physicochemical property changes of EfOM, including biopolymer fractions, by pre-ozonation were systemically investigated. The classical pore blocking model and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theories were employed to scrutinize the fouling alleviation mechanism by pre-ozonation. As a result, the overarching mechanisms of fouling reduction were attributed to the following key reasons: (1) Ozone degraded macromolecules such as biopolymers like proteins and polysaccharides into smaller fractions, thereby increasing free energy of cohesion of EfOM and rendering them more hydrophilic and stable; (2) pre-ozonation augmented the interfacial free energy of adhesion between foulants and the RC/PES membranes, leading to the increase of repulsions and/or the decrease of attractions; and (3) pre-ozonation prolonged the transition from pore blocking to cake filtration that was a dominant fouling mechanism, thereby reducing fouling.

Fluorescent natural organic matter responsible for ultrafiltration membrane fouling: Fate, contributions and fouling mechanisms

Membrane fouling has been a main obstacle to the success of ultrafiltration (UF) technology. Recently, fluorescent natural organic matter (FNOM), including humic-like substances (HS) and protein-like substances, has been recognized as substances responsible for membrane fouling. In this study, the matrix of FNOM in natural river water was substantially modified by combined coagulation and powdered activated carbon adsorption to enhance the diversity of the FNOM matrix. Fluorescence excitation emission matrix spectroscopy was employed to characterize FNOM components during the UF process. The correlations between FNOM components of the feedwater and membrane fouling were evaluated for the initial period and long-term operation. Reliable correlations of the maximum fluorescence intensity of HS with initial membrane fouling indicated that HS were major foulants in the initial period. Furthermore, the protein-like component exhibited significant correlation with the concentration effect fouling (R² = 0.6131) and with irreversible fouling (R² = 0.8711). We found that the fouling mechanism changed from pore obstruction to a protein concentration polarization layer followed by protein cake layer filtration. Total fouling of the UF membrane over long-term operation was alleviated with powdered activated carbon (PAC) adsorption; however, the mitigation of irreversible fouling was dependent on whether PAC adsorbed protein-like substances.

Efficient control of membrane fouling in MF by removal of biopolymers: Comparison of various pretreatments

In recent studies on membrane fouling in microfiltration (MF) and ultrafiltration (UF) for drinking water production, hydrophilic macromolecular organics referred to as biopolymers have been shown to be major players in the fouling. In this study, various pretreatments were compared to maximize removal of biopolymers and to control membrane fouling efficiently. Multiple water samples were collected from different drinking water sources and were used in this study. Coagulation using polyaluminum chloride (PACl) was carried out under conditions of different dosages and different pHs and was also carried out in combination with the use of powdered activated carbon (PAC) or magnetic ion exchange (MIEX^®) resin. The efficiency of removal of biopolymers was highest by the combination of MIEX^® and coagulation regardless of the type of sample. Efficient removal of biopolymers achieved by the combination of MIEX^® and coagulation led to efficient control of membrane fouling in MF, which was confirmed by bench-scale filtration tests conducted under a constant flux of 62.5 LMH using commercially available hollow-fiber membranes. Enhanced coagulation with increased coagulant dosage or acidic coagulation (pH = 6) also exhibited good removal of biopolymers in some cases and led to control of fouling. In contrast, the combination of PAC and coagulation sometimes caused more rapid evolution of fouling by forming cake layers on the membrane surface. Results of bench-scale tests showed that the concentration of biopolymers in the feed water correlated well with the degree of physically irreversible fouling, which was dominant in this study. The strong correlation was shown with multiple water samples treated by various pretreatments, demonstrating that biopolymer concentration in feed water is a good index for fouling studies.

The impact of anionic polyacrylamide (APAM) on ultrafiltration efficiency in flocculation-ultrafiltration process

With the purpose of improving the ultrafiltration (UF) efficiency, anionic polyacrylamide (APAM) has been used as a coagulant aid in the flocculation-UF process. In this study, the impact of APAM on UF efficiency has been investigated with regard to membrane fouling, membrane cleaning and effluent quality. The results indicated that the optimal dosage of APAM had positive impacts on membrane fouling control, membrane cleaning and effluent quality. According to the flux decline curve, scanning electron microscopy and contact angle characterization, the optimal dosage of APAM was determined to be 0.1 mg/L coupled with 2 mg/L (as Al³⁺) poly-aluminium chloride. Under this optimal condition, membrane fouling can be mitigated because of the formation of a porous and hydrophilic fouling layer. APAM in the fouling layer can improve the chemical cleaning efficiency of 0.5% NaOH due to the disintegration of the fouling layer when APAM is dissolved under strong alkaline conditions. Furthermore, with the addition of APAM in the flocculation-UF process, more active adsorption sites can be formed in the flocs as well as the membrane fouling layer, thus more antipyrine molecules in the raw water can be adsorbed and removed in the flocculation-UF process.

Impacts of epichlorohydrin-dimethylamine on coagulation performance and membrane fouling in coagulation/ultrafiltration combined process with different Al-based coagulants

Two kinds of aluminum-based coagulants and epichlorohydrin-dimethylamine (DAM-ECH) were used in the treatment of humic acid-kaolin simulated water by coagulation-ultrafiltration (C-UF) hybrid process. Coagulation performance, floc characteristics, including floc size, compact degree, and strength were investigated in this study. Ultrafiltration experiments were conducted by a dead-end batch unit to implement the resistance analyses to explore the membrane fouling mechanisms. Results showed that DAM-ECH aid significantly increased the UV254 and DOC removal efficiencies and contributed to the formation of larger and stronger flocs with a looser structure. Aluminum chloride (Al) gave rise to better coagulation performance with DAM-ECH compared with poly aluminum chloride (PACl). The consequences of ultrafiltration experiments showed that DAM-ECH aid could reduce the membrane fouling mainly by decreasing the cake layer resistance. The flux reductions for PACl, Al/DAM-ECH (dosing both Al and DAM-ECH) and PACl/DAM-ECH (dosing both PACl and DAM-ECH) were 62%, 56% and 44%, respectively. Results of this study would be beneficial for the application of PACl/DAM-ECH and Al/DAM-ECH composite coagulants in water treatment processes.

The role of shear conditions on floc characteristics and membrane fouling in coagulation/ultrafiltration hybrid process – the effect of flocculation duration and slow shear force

The impact of shear conditions during coagulation on the ultrafiltration permeate flux in a coagulation–ultrafiltration (C–UF) process was investigated.

Phenols, flame retardants and phthalates in water and wastewater - a global problem

Organic pollutants in water and wastewater have been causing serious environmental problems. The arbitrary discharge of wastewater by industries, and handling, use, and disposal constitute a means by which phenols, flame retardants (FRs), phthalates (PAEs) and other toxic organic pollutants enter the ecosystem. Moreover, these organic pollutants are not completely removed during treatment processes and might be degraded into highly toxic derivatives, which has led to their occurrence in the environment. Phenols, FRs and PAEs are thus highly toxic, carcinogenic and mutagenic, and are capable of disrupting the endocrine system. Therefore, investigation to understand the sources, pathways, behavior, toxicity and exposure to phenols, FRs and PAEs in the environment is necessary. Formation of different by-products makes it difficult to compare the efficacy of the treatment processes, most especially when other organic matters are present. Hence, high levels of phenols, FRs and PAEs removal could be attained with in-line combined treatment processes.

Powder Activated Carbon Pretreatment of a Microfiltration Membrane for the Treatment of Surface Water

This study focused on the effect of powder activated carbon (PAC) adsorption on microfiltration (MF) membrane performance. The results showed that PAC pretreatment offered high organic matter removal rates for both dissolved organic carbon (DOC) and ultraviolet absorbance at 254 nm (UV₂₅₄) during 10–200 mg/L PAC dosage. The removal efficiencies of organic matter by MF membrane filtration decreased with the increase of organic matter removal rate by PAC adsorption. PAC mainly removed organic matter of about 3 kDa molecular weight (MW). MF membrane maintained more than 5 kDa MW organic matter on the membrane after PAC adsorption. The results of membrane filtration indicated that PAC pretreatment slightly promoted membrane flux, regardless of PAC dosage. It seems that the organic matter fouling membrane was concentrated in more than 3 kDa MW. PAC removed markedly less than 3 kDa MW organic matter and had less effect on more than 3 kDa organic matter. Thus, PAC cannot reduce membrane fouling.

Contribution of Fe3O4 nanoparticles to the fouling of ultrafiltration with coagulation pre-treatment

A coagulation (FeCl3)-ultrafiltration process was used to treat two different raw waters with/without the presence of Fe3O4 nanoparticle contaminants. The existence of Fe3O4 nanoparticles in the raw water was found to increase both irreversible and reversible membrane fouling. The trans-membrane pressure (TMP) increase was similar in the early stages of the membrane runs for both raw waters, while it increased rapidly after about 15 days in the raw water with Fe3O4 nanoparticles, suggesting the involvement of biological effects. Enhanced microbial activity with the presence of Fe3O4 nanoparticles was evident from the measured concentrations of extracellular polymeric substances (EPS) and deoxyribonucleic acid (DNA), and fluorescence intensities. It is speculated that Fe3O4 nanoparticles accumulated in the cake layer and increased bacterial growth. Associated with the bacterial growth is the production of EPS which enhances the bonding with, and between, the coagulant flocs; EPS together with smaller sizes of the nano-scale primary particles of the Fe3O4-CUF cake layer, led to the formation of a lower porosity, more resilient cake layer and membrane pore blockage.

Membrane fouling controlled by coagulation/adsorption during direct sewage membrane filtration (DSMF) for organic matter concentration

Unlike the role of the membrane in a membrane bioreactor, which is designed to replace a sediment tank, direct sewage membrane filtration (DSMF), with the goal of concentrating organic matters, is proposed as a pretreatment process in a novel sewage treatment concept. The concept of membrane-based pretreatment is proposed to divide raw sewage into a concentrated part retaining most organics and a filtered part with less pollutant remaining, so that energy recovery and water reuse, respectively, could be realized by post-treatment. A pilot-scale experiment was carried out to verify the feasibility of coagulant/adsorbent addition for membrane fouling control, which has been the main issue during this DSMF process. The results showed that continuous coagulant addition successfully slowed down the increase in filtration resistance, with the resistance maintained below 1.0×10(13) m(-1) in the first 70 hr before a jump occurred. Furthermore, the adsorbent addition contributed to retarding the occurrence of the filtration resistance jump, achieving simultaneous fouling control and chemical oxygen demand (COD) concentration improvement. The final concentrated COD amounted to 7500 mg/L after 6 days of operation.