Journal of Membrane Science 376 (2011) 196–206Contents lists available at ScienceDirectJournal of MembraneSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m e m s ciFouling and cleaning of RO membranes fouled by mixtures of organic foulants simulating wastewater effluentWui Seng Ang 1,Alberto Tiraferri,Kai Loon Chen 2,Menachem Elimelech ∗Department of Chemical and Environmental Engineering,P.O.Box 208286,Yale University,New Haven,CT 06520-8286,USAa r t i c l e i n f o Article history:Received 6December 2010Received in revised form 7April 2011Accepted 9April 2011Available online 20 April 2011Keywords:Reverse osmosis FoulingWastewater effluent CleaningOrganic foulantsWastewater treatment Effluent organic matter Wastewater reclamation Membranesa b s t r a c tThe fouling and subsequent cleaning of RO membranes fouled by a mixture of organic foulants sim-ulating wastewater effluent has been systematically investigated.The organic foulants investigated included alginate,bovine serum albumin (BSA),Suwannee River natural organic matter,and octanoic acid,representing,respectively,polysaccharides,proteins,humic substances,and fatty acids,which are ubiquitous in effluent organic matter.After establishing the fouling behavior and mechanisms with a mixture of organic foulants in the presence and absence of calcium ions,our study focused on the clean-ing mechanisms of RO membranes fouled by the mixture of organic foulants.The chemical cleaning agents used included an alkaline solution (NaOH),a metal chelating agent (EDTA),an anionic surfactant (SDS),and a concentrated salt solution (NaCl).Specifically,we examined the impact of cleaning agent type,cleaning solution pH,cleaning time,and fouling layer composition on membrane cleaning effi-ciency.Foulant–foulant adhesion forces measured under conditions simulating chemical cleaning of a membrane fouled by a mixture of the investigated organic foulants provided insights into the chemical cleaning mechanisms.It was shown that while alkaline solution (NaOH)alone is not effective in dis-rupting the complexes formed by the organic foulants with calcium,a higher solution pH can lead to effective cleaning if sufficient hydrodynamic shear (provided by crossflow)prevails.Surfactant (SDS),a strong chelating agent (EDTA),and salt solution (NaCl)were effective in cleaning RO membranes fouled by a mixture of foulants,especially if applied at high pH and for longer cleaning times.The observed cleaning efficiencies with the various cleaning agents were consistent with the related measurements of foulant–foulant intermolecular forces.Furthermore,we have shown that an optimal cleaning agent con-centration can be derived from a plot presenting the percent reduction in the foulant–foulant adhesion force versus cleaning agent concentration.© 2011 Elsevier B.V. All rights reserved.1.IntroductionAs demand for potable water increases worldwide,the paradigm for selecting water sources to meet this demand is transitioning from conventional sources,such as reservoirs and lakes,to less con-ventional sources,such as treated secondary wastewater effluent.In order to produce water of superior quality,the use of mem-branes in desalination and wastewater reclamation has become more widespread.Membrane fouling is a major impediment to the use of membrane technology for such applications,because fouling is inevitable.Despite research efforts to develop better anti-fouling membranes [1]and improved fouling-control strategies [2,3],membrane fouling still occurs over time.Thus,a long-term∗Corresponding author.Tel.:+12034322789;fax:+12034324387.E-mail address:menachem.elimelech@ (M.Elimelech).1Current address:Public Utility Board of Singapore,Singapore.2Current address:Department of Geography and Environmental Engineering,Johns Hopkins University,Baltimore,MD 21218,United States.solution would be to remove the foulant deposited on the mem-brane via chemical cleaning.To select the appropriate cleaning agents and adopt an effective chemical cleaning protocol for fouled membranes in wastewater reclamation,the implications of wastewater effluent characteris-tics on membrane fouling have to be well-understood.Wastewater effluent contains dissolved organic matter,commonly known as effluent organic matter (EfOM),which comprises polysaccha-rides,proteins,aminosugars,nucleic acids,humic and fulvic acids,organic acids,and cell components [2–4].Organic fouling of the RO membranes by the EfOM can be extensive since EfOM is gener-ally small enough to pass through the pores of pretreatment (MF or UF)membranes [4].In particular,recent findings suggest that while biofouling can prevail on the tail-element of the membrane module,fouling of the lead-element exposed to reclaimed water is dominated by EfOM adsorption [5].In addition,higher potential of fouling was observed for the higher molecular weight hydropho-bic/aromatic fraction of the EfOM [6,7].The presence of Ca 2+in the feed source for the RO membranes has been reported to form complexes with the constituents of EfOM,such as polysaccharidesW.S.Ang et al./Journal of Membrane Science376 (2011) 196–206197[8]and natural organic matter[9],and to significantly enhance membrane fouling.While our previous studies have addressed the fouling of RO membranes by individual organic foulant types,such as polysaccharides[10],proteins[11],and fatty acids[12],only recently have investigations reported on the effects of a combina-tion or mixture of foulants on the fouling of RO membranes[13,14].A variety of chemical cleaning agents are commonly used to clean RO membranes fouled by organic matter[15].Alkaline solutions remove organic foulants on membranes by hydroly-sis and solubilization of the fouling layer.Alkaline solutions also increase the solution pH,and,therefore,increase the negative charges and solubility of the organic foulant.Metal chelating agents remove divalent cations from the complexed organic molecules and weaken the structural integrity of the fouling layer matrix[16]. Surfactants solubilize macromolecules by forming micelles around them[17],thereby facilitating removal of the foulants from the membrane surface.In our earlier study on salt cleaning of organic matter-fouled RO membranes[18],we demonstrated that NaCl and other common inert salts can be used as an effective alternative for the cleaning of RO membranes fouled by gel-forming hydrophilic organic foulants.In the presence of a salt solution,the fouling layer swells and becomes more porous.As a result,this would facil-itate the diffusion of Na+into the fouling layer and breakup of Ca2+–alginate bonds by ion exchange.Understanding the fouling layer characteristics and the interaction of chemical agents with foulants is therefore critical for the effective cleaning of organic matter-fouled RO membranes.Atomic force microscopy(AFM)has been applied in mem-brane fouling/cleaning research to quantify intermolecular forces [10,19–21].Our research has shown that foulant–foulant inter-actions could be determined by performing force measurements using a carboxylate-modified latex colloid probe in an AFMfluid cell[10,20].The technique has been used to quantify the foul-ing behavior of a nanofiltration membrane fouled by humic acid and the cleaning efficiencies of EDTA and SDS[20],and has been extended to quantify RO membrane fouling by organic foulant in the form of alginate[10],BSA[11],and octanoic acid[12].In this study,the AFM has also been employed as an alternative tool to indicate the optimal concentration of cleaning agent for cleaning fouled membranes.The original protocol[11,12]for using the AFM has been modified to investigate the intermolecular adhesion force between different foulants.The objective of this study is to explore the mechanisms govern-ing the fouling of RO membranes by mixtures of organic foulants simulating wastewater effluent,and the ensuing chemical cleaning of the fouled membranes by cleaning agents.To make this study rel-evant to wastewater reclamation,we systematically investigate the fouling of RO membranes by each individual organic foulant type (polysaccharides,proteins,humic acids,or fatty acids)and mix-tures containing several types of organic foulants in the absence and presence of calcium ions.Cleaning experiments are performed with the fouled membranes using NaOH,EDTA,SDS,and NaCl as model alkaline solution,metal chelating agent,surfactant,and salt cleaning solution,respectively.The intermolecular adhesion forces between the different foulants and estimated aggregate sizes in foulant mixtures were used to explain the fouling mechanism of RO membranes and the cleaning behavior of a cleaning agent on the fouled membranes.2.Materials and methodsanic foulants Louis,MO),Suwannee River natural organic matter(SRNOM) (International Humic Substances Society,St.Paul,MN),bovine serum albumin(BSA)(Sigma–Aldrich,St.Louis,MO),and octanoic acid(OA)(Sigma–Aldrich,St.Louis,MO),respectively.According to the manufacturer,the molecular weight of the sodium alginate ranges from12to80kDa.Other characteristics of SRNOM,includ-ing molecular weight and mass fraction of hydrophobic NOM,can be found elsewhere[22,23].According to the manufacturer,the molecular weight of the BSA is about66kDa.BSA is reported to have an isoelectric point at pH4.7[24].Octanoic acid(Sigma–Aldrich,St. Louis,MO)was selected to model fatty acids in EfOM because of its presence in food and solubility in water(saturation concentration of4.7mM at20◦C)[12].Sodium alginate,BSA,and SRNOM were received in powder form,and stock solutions(2g/L)were prepared by dissolving each of the foulants in deionized(DI)water.DI water was supplied from a Milli-Q ultrapure water purification system(Millipore,Billerica, MA).Mixing of the stock solutions was performed for over24h to ensure complete dissolution of the foulants,followed byfil-tration with a0.45-mfilter(Durapore,Millipore,Billerica,MA). Thefiltered stock solutions were stored in sterilized glass bottles at4◦C.Octanoic acid was received in solution(≥98%concentra-tion)and was stored at room temperature.To achieve the intended octanoic acid concentration during fouling,octanoic acid was dis-solved separately for at least8h prior to fouling so that,at the initiation of fouling,octanoic acid could be introduced as a solu-tion.A few hours before the initiation of fouling,the ionic strength of the stock solution was adjusted to the same concentration as that of the feed solution(10mM)and the stock solution pH was elevated,as needed,from ambient pH of3.9–9.0by adding small amounts of1M NaOH.2.2.Chemical cleaning agentsThe chemical cleaning agents used were:NaOH(pH11.0)as an alkaline solution,certified grade disodium ethylenediaminete-traacetate(Na2–EDTA)as a metal chelating agent,certified grade sodium dodecyl sulfate(SDS)as an anionic surfactant,and NaCl as a salt cleaning solution.The agents were purchased from Fisher Sci-entific(Pittsburgh,PA)and used with no further purification.The stock chemical solutions were prepared fresh by dissolving each chemical in deionized(DI)water.The pH of the EDTA,SDS,and NaCl cleaning solutions was adjusted with1.0M NaOH as necessary.2.3.RO membraneThe relatively well-characterized thin-film composite LFC-1 membrane(Hydranautics,Oceanside,CA)was used as a model RO membrane.The average hydraulic resistance was determined to be 9.16(±0.11)×1013m−1corresponding to a hydraulic permeabil-ity of10.9(±0.13)×10−11m s−1Pa−1.The observed salt rejection was98.7–99.3%,determined with a10mM(584mg/L)NaCl feed solution at an applied pressure of300psi(2068.5kPa)and a cross-flow velocity of8.6cm/s.Membrane samples were received as dry large sheets,and were cut and stored in DI water at4◦C.The membrane has been reported to be negatively charged at solu-tion chemistries typical to wastewater effluents,with an isoelectric point at about pH4.6[25].The membrane has been reported to be coated with a neutral polyalcohol layer rich in–COH functional groups,which renders the surface less charged than the surfaces of other polyamide RO membranes without a coating layer[25,26].2.4.Crossflow test unit198W.S.Ang et al./Journal of Membrane Science376 (2011) 196–206unit consists of a membrane cell,pump,feed reservoir,temper-ature control system,and data acquisition system.The membrane cell consisted of a rectangular plate-and-frame unit,which con-tained aflat membrane sheet placed in a rectangular channel with dimensions measuring7.7cm long,2.6cm wide,and0.3cm high. Both permeate and retentate were recirculated back to the feed reservoir.Permeateflux was registered continuously by a digital flow meter(Optiflow1000,Humonics,CA),interfaced with a com-puter.Afloating disc rotameter(King Instrument,Fresno,CA)was used to monitor the retentateflow rate.The crossflow velocity and operating pressure were adjusted using a bypass valve(Swagelok, Solon,OH)in conjunction with a back-pressure regulator(U.S.Para Plate,Auburn,CA).Temperature was controlled by a recirculating chiller/heater(Model633,Polysciences)with a stainless steel coil submerged in the feed water reservoir.2.5.Fouling and cleaning experimentsThe membrane wasfirst compacted with DI water until the permeateflux became constant,followed by the initial baseline performance for1h.The membrane was then stabilized and equi-librated with a foulant-free electrolyte solution for2h.Theflux at which the baseline run was performed was predetermined so that the initialflux would drop to a specifiedflux of2.3×10−5m s−1(or 83L m−2h−1)after adding the electrolyte solution.The chemistry of the foulant-free electrolyte solution and operating conditions adjusted in this stage were similar to those used for the ensuing fouling runs.As octanoic acid takes time to dissolve completely,the mixture of organic foulant solution has to be prepared8h before the fouling run.The feed foulant solution was prepared separately in another container.The chemistry of the feed foulant solution was adjusted to be identical to that of the foulant-free electrolyte solution so that the overall ionic strength and solution chemistry would not change when the feed foulant solution was added to initiate fouling. Fouling runs were carried out for17h.At the end of the fouling run, the solution in the feed reservoir was disposed off and chemical cleaning solution was added to the feed reservoir to clean the fouled membrane.At the end of the cleaning stage,the chemical cleaning solution in the reservoir was discarded,and both the reservoir and membrane cell were rinsed with DI water toflush out the residual chemical cleaning solution.Finally,the cleaned RO membrane was subjected to the second baseline performance with DI water to re-determine the pure waterflux.The crossflow velocity throughout the experiment,except dur-ing cleaning,was maintained at8.6cm/s.The operating conditions (i.e.,initialflux,crossflow velocity,and temperature)at this stage were identical to those applied during the initial baseline perfor-mance,so as to determine the cleaning efficiency by comparing the pure waterfluxes determined before fouling and after clean-ing.Throughout all the fouling/cleaning stages,the feed water in the reservoir,which was located on top of a magnetic stirrer,was mixed rigorously to ensure complete mixing of the feed water and cleaning solution.To confirm the reproducibility of determined cleaning effi-ciency,selected fouling/cleaning runs were duplicated.Results showed that fouling rate and cleaning efficiency obtained from the duplicate runs were within less than a5%difference.To investigate the change in the permeate quality during the fouling stage,permeate samples taken before and at the start and end of fouling were analyzed for salt(NaCl)rejection using an ICP-AES(ICP Optima3000,Perkin Elmer,Waltham,MA).Permeate and feed samplings obtained before the fouling run were collected at the end were collected during thefinal40min of the fouling run.2.6.AFM adhesion force measurementsAtomic force microscopy(AFM)was used to measure the inter-facial force between the foulant in the bulk solution and the foulant in the fouling layer on the membrane.The force measurements were performed with a colloid probe,modified from a commercial-ized SiN AFM probe(Veeco Metrology Group,Santa Barbara,CA).A carboxylate modified latex(CML)particle(Interfacial Dynam-ics Corp.,Portland,OR)was used as a surrogate for the organic foulants,because organic foulants(alginate and SRNOM)carry pre-dominantly carboxylic functional groups.To make a colloid probe, a CML particle with a diameter of4.0m was attached using Nor-land Optical adhesive(Norland Products,Inc.,Cranbury,NJ)to a tipless SiN cantilever.The colloid probe was cured under UV light for20min.The AFM adhesion force measurements were performed in a fluid cell using a closed inlet/outlet loop.The solution chemistries of the test solutions injected into thefluid cell were identical to those used in the bench-scale fouling/cleaning experiments.Once all the air bubbles had beenflushed out of thefluid cell,the injection would stop and the outlet was closed.The membrane was equilibrated with the test solution for30–45min before force measurements were performed.The force measurements were conducted at three tofive different locations,and at least10measurements were taken at each location.Because the focus of this study was on the foulant–foulant interaction(adhesion),only the raw data obtained from the retracting force curves were processed and converted to obtain the force versus surface-to-surface separation curves.The force curves presented were the averages of all the representative force curves obtained at the different locations.The protocol for AFM analysis has been modified slightly to investigate the interaction between different foulant types. The AFM colloidal probe is soaked in organic foulant solution (2000mg/L alginate,BSA,or SRNOM,or>98%octanoic acid)for at least24h(at4◦C for alginate,BSA,and SRNOM solutions to prevent organic degradation,and at room temperature for octanoic acid). The membrane is fouled with200mg/L organic foulant(alginate, BSA,SRNOM,or octanoic acid)using the crossflow unit for about 17h.After transferring the colloidal probe to the AFMfluid cell and the membrane to the AFM disc puck,an electrolyte solution con-taining0.5mM CaCl2and8.5mM NaCl(adjusted to pH6.5±0.2) (identical solution chemistry as during fouling)is injected into the fluid cell.The volume of electrolyte solution added is just enough tofill up thefluid cell so as to minimize the possibility offlush-ing away the foulants on the membrane and probe surfaces.AFM force measurements are taken after20min of equilibration time.To investigate the effect of cleaning agent on the intermolecular adhe-sion force,the cleaning agent was added to the electrolyte solution at the same concentration as that used in the cleaning experiments.2.7.Light scatteringDynamic light scattering experiments were performed on foulant solution to determine the effective hydrodynamic diam-eters of the foulant aggregates in foulant mixtures using a multi-detector light scattering unit(ALV-5000,Langen,Germany). New glass vials(Supelco,Bellefonte,PA)for containing foulant solu-tions under various solution chemistries were cleaned prior to use by soaking overnight in a cleaning solution(Extran MA01,Merck KGaA,Darmstadt,Germany),rinsing with DI water,and drying inW.S.Ang et al./Journal of Membrane Science376 (2011) 196–206199Fig.1.Influence of individual foulant type on fouling of LFC-1membranes:(a)in the absence of Ca2+and(b)in the presence of0.5mM Ca2+.The total ionic strength of the feed solution wasfixed at10mM by adjusting with NaCl and the feed solution pH was adjusted to6.0±0.2,as necessary,by adding NaOH.Fouling conditions: foulant concentration of25mg/L,initial permeateflux of23m/s(or83L m−2h−1), crossflow velocity of8.6cm/s,and temperature of21.0±0.5◦C.of1M NaOH.The vial containing the foulant solution was vortexed (Mini Vortexer,Fisher Scientific)to homogenize the solution.The vial was then allowed to sit for30min before starting the light scattering experiment.All light scattering measurements were conducted by employ-ing the detector positioned at a scattering angle of90◦from the incident laser beam.The detector signal was fed into the correla-tor,which accumulated each autocorrelation function for15s.The intensity-weighted hydrodynamic radius of the colloidal aggre-gates was determined with second-order cumulant analysis(ALV software)[27].The reported size is the average of thefirst20mea-surements.3.Results and discussion3.1.Membrane fouling3.1.1.Fouling with individual foulantsFig.1presents the normalizedflux profiles for LFC-1mem-branes fouled by each individual foulant(alginate,BSA,SRNOM, or octanoic acid)in the absence(Fig.1a)and presence(Fig.1b)of Ca2+,respectively.In the absence of Ca2+,theflux decline profiles of membranes fouled by the various foulants are insignificant.The 2+Fig.2.Influence of a mixture of(a)2foulants or(b)more than2foulants on fouling of LFC-1membranes in the presence of0.5mM Ca2+.The total ionic strength of the feed solution wasfixed at10mM by adjusting with NaCl and the feed solution pH was adjusted to6.0±0.2,as necessary,by adding NaOH.Fouling conditions were identical to those in Fig.1.RO membranes by BSA,SRNOM,or octanoic acid is minimal.How-ever,we have observed that the presence of Ca2+can affect fouling behavior when the foulant concentrations are higher(300mg/L BSA;2mM or288mg/L octanoic acid)[11,12].3.1.2.Fouling with mixture of foulantsTo investigate the implications for wastewater reclamation, the effect of Ca2+on fouling of RO membranes by all possible combinations of two or more foulant types is investigated.The con-centration of each foulant type was maintained at25mg/L.Fig.2a shows the normalizedflux profiles of membranes fouled by a mix-ture of two foulants in the presence of Ca2+.The effect of Ca2+is most significant for feed solutions containing alginate as one of the two foulant types.This mechanism will be further investigated with the aid of DLS and AFM paring theflux profiles of mem-branes fouled by alginate as a co-foulant,theflux-decline profile of membrane fouled by alginate and octanoic acid is the least sig-nificant due to the formation of octanoic acid–calcium complexes, which increase the hydrophilicity of the fouling layer[12].Fig.2b shows the normalizedflux profiles of membranes fouled by a mixture of three foulant types and all foulant types in the presence of Ca2+.In the presence of Ca2+,for membranes fouled by mixtures containing alginate,the effect of Ca2+onflux profiles is most significant,especially for the membrane fouled by a mixture of alginate,BSA,and SRNOM(without octanoic acid).In compar-ing the latter with theflux profile of the membrane fouled by all200W.S.Ang et al./Journal of Membrane Science376 (2011) 196–206Fig.3.Sodium ion(Na+)rejection of RO membranes measured before,and at the start and end of the fouling runs,at an adjusted feed solution pH of6.0.The membranes were fouled by combined foulant types,composed of25mg/L each of alginate,BSA,SRNOM,and octanoic acid.Permeate and feed samples obtained before the fouling run were collected30min before the onset of fouling.Samples taken at the start of the fouling run were initiated afterfirst discarding20mL of permeate(duration of8min).Permeate and feed samples taken at the end were collected during thefinal40min of the fouling run.Error bars indicate one standard deviation.Fouling conditions were identical to those in Fig.1.contained alginate and octanoic acid in the presence of Ca2+.The inhibitory effect of octanoic acid onflux-decline profiles can also be observed by comparing theflux profile of combined foulant types of alginate,SRNOM,and octanoic acid in Fig.2b with the profile of alginate and SRNOM in Fig.2a.3.1.3.Impact of fouling on salt rejectionFig.3presents Na+rejection of the RO membranes fouled by combined foulant types of alginate,BSA,SRNOM,and octanoic acid in the presence of Ca2+,at the start and end of the fouling runs.The trend of observed Na+rejection is similar both in the absence and presence of Ca2+.At the onset of fouling,the Na+rejection instan-taneously increases.This phenomenon is consistent with previous observations,which attributed the decrease in Na+permeability to the fouling layer acting as an additional selective barrier[12]. Toward the end of the fouling runs,the fouling layer becomes thicker and denser,resulting in even higher Na+rejection.It can be inferred that the presence of Ca2+resulted in a more compact fouling layer,which improves the ability of the fouling layer to fur-ther act as a selective barrier against the transport of Na+across the membrane.3.2.Fouling mechanisms3.2.1.Role of foulant–foulant interaction and foulant sizeRecent studies have demonstrated that the long-term organic fouling of RO membranes and the consequent behavior of water flux are dominated by the feed water chemistry and strong foulant–foulant interactions[4,20,21,28].Quantifying these inter-actions provides a basis for the understanding of the fouling mechanisms and for the rational selection of a suitable cleaning strategy.As discussed in Section3.1.2,fouling behavior becomes significant when alginate is one of the co-foulants.When alginate is absent from the feed solution,regardless of the other foulant types present,fouling is relatively insignificant.This behavior can be explained by evaluating the interaction forces among the differ-ent foulants.To investigate the effect of interactions of alginate with other foulant types,DLS analysis is performed on a solution contain-ing2foulant types(200mg/L alginate plus200mg/L of another foulant type)in the presence of Ca2+.Fig.4a shows that the alginate molecules in the solution have an effective hydrodynamic diame-ter of84nm,which is larger than the effective diameter of51nm of alginate molecules in a solution in which the foulant concentration is halved.The results imply that aggregation of alginate molecules is concentration dependent.The larger effective diameter of aggre-gates formed in400mg/L alginate solution as opposed to those formed in200mg/L alginate solution also implies a more exten-sive gel network at a higher concentration.The effective diameters of the foulant molecules in mixtures of alginate and BSA,alginate and SRNOM,and alginate and octanoic acid are,respectively,48, 63,and73nm.The effective diameter is an indirect indication of the foulant size due to aggregation between the foulants.Because of the varying interactions between alginate and another foulant type in the presence of Ca2+,the aggregate size differs for foulant aggregates of different foulant combinations.Fig.4b shows the intermolecular forces between foulant adsorbed on a colloidal probe and a membrane fouled by algi-nate as determined by AFM.For a membrane fouled by alginate and octanoic acid,the dominant foulant interactions are between alginate and alginate molecules(1.03mN/m)and between octanoic acid and octanoic acid molecules(0.90mN/m).For a membrane fouled by alginate and SRNOM,the dominant foulant interaction is between alginate and alginate molecules(1.03mN/m).For a membrane fouled by alginate and BSA,the dominant foulantinter-Fig.4.(a)Effective diameter of foulant aggregates in solutions of various foulant combinations that contain alginate as co-foulant.The foulant solution consists of200mg/L alginate plus200mg/L of another foulant type in an electrolyte solution of0.5mM CaCl2and8.5mM NaCl(same solution chemistry as that used in fouling experiments).TheW.S.Ang et al./Journal of Membrane Science376 (2011) 196–206201Fig.5.Proposed structure of fouling layer on membrane surface under different combinations of foulants.actions are between alginate and alginate molecules(1.03mN/m) and between alginate and BSA molecules(0.73–0.79mN/m).We observe that when alginate is present in the feed,regardless of the co-foulant,the interaction of alginate molecules among them-selves is most dominant,with the possibility of alginate molecules interacting with other molecules,especially BSA molecules.Comparing the effective diameters of the foulant aggregates in various2-foulant mixtures(Fig.4a)with the intermolecular adhe-sion force between different foulants(Fig.4b)reveals that there is an inverse correlation between the foulant aggregate size and the intermolecular adhesion force(foulant aggregate size generally decreases as intermolecular adhesion force increases).It is hypoth-esized that the interaction among the foulant types within the aggregates would affect the conformation,and hence,the size of the aggregates in the foulant solution.For example,the relatively stronger intermolecular adhesion force between alginate and BSA molecules in the feed solution in the presence of Ca2+results in a more‘compact’or‘tighter’conformation of the foulant aggregates as compared to the foulant aggregates formed from a solution of alginate and SRNOM.The deposition of the smaller and more‘com-pact’alginate–BSA aggregates results in a tighter fouling layer and a lowerfinalflux(Fig.5)[29].We note that the SA–SA aggregate does not follow the trend of a decrease in aggregate size with increasing adhesion force because alginate molecules tend to form extended gel networks in the presence of calcium ions[29],as opposed to the other combinations of foulants.3.2.2.Proposed structure of fouling layerThe fouling experiments reveal that membrane fouling in the presence of Ca2+is controlled by alginate.From the AFM force mea-surement analysis and the DLS experiments,the proposed structure of the fouling layer when severe fouling occurs under various solu-tion chemistries is schematically shown in Fig.5.The top drawing shows the likely conformation of the cross-linked alginate fouling layer when the feed contains alginate in the presence of Ca2+.In this case,the fouling layer has the typical structure resulting from the formation of an‘egg-box’shaped gel network on the mem-brane surface[29].The middle drawing shows the proposed fouling layer formed by a feed solution containing a mixture of alginate, BSA,SRNOM,and octanoic acid(each foulant has the same concen-tration).The DLS experiments show that the aggregates of foulant mixtures containing alginate as a co-foulant have smaller effective feed solution in which alginate is the sole foulant.When the algi-nate concentration is increased while maintaining the same total foulant concentration,the fouling layer becomes more porous due to the increase in the highly ordered alginate–calcium complexes on the membrane surface.The state of the fouling layer would affect the transfer of a cleaning agent to the fouling layer,and hence,the cleaning efficiency of the cleaning agent as delineated in the next sections.3.3.Cleaning of fouled membranes3.3.1.Type of cleaning agentFig.6presents the cleaning efficiencies of various cleaning agents on membranes fouled by combined foulant types compris-ing alginate,BSA,SRNOM,and octanoic acid in the presence of 0.5mM Ca2+.Cleaning was performed for15min without an oper-ating pressure(i.e.,no permeate)and at a crossflow velocityfive times higher than that during fouling.Cleaning the fouled mem-brane with DI water resulted in19%cleaning efficiency,which implies that the fouling layer on the membrane surface was largely irreversible.Conventional cleaning agents,such as NaOH(pH11),Fig.6.Cleaning efficiencies of various cleaning agents on membranes fouled by combined foulant types comprising alginate,BSA,SRNOM,and octanoic acid,with the concentration of each foulant type at25mg/L,in the presence of0.5mM Ca2+.Cleaning conditions:time,15min;temperature,21±0.5◦C;and no applied。
