Effect of PH on Fouling Attachments and Power Consumption In

Effect of pH on Fouling Attachments and Power Consumption inUltrafiltration of Latex Solution

Amira Abdelrasoul,* Huu Doan and Ali LohiDepartment of Chemical Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3

The goal of the present study was to remediate ultrafiltration fouling through the altering of the latex paint solution pH, in order to reduce the totalmass fouling and the specific power consumption. Polycarbonate flatmembranewith a pore size of 0.05mmwas used under a constant feed flow rateand crossflowmode in ultrafiltration of a latex paint solution. It was observed that the ionic strengthof the latex solution had a significant effect on themembrane hydrophilicy and the particle surface charge, which in turn influenced the particleparticle and particlemembrane attachment. At thetransmembrane pressure of 15psi, a feed flow rate of 1 LPM, and a feed concentration of 1.3 kg/m3, increasing solution pH from 7 to 12 resulted in aconsiderable 50% reduction in membrane fouling. This reduction of fouling led to a 53.9% decrease in the specific power consumption, while thepermeate flux increased by 97.4%.On the other hand, themass of fouling noticeably decreased by 35.8%, the permeate flux increased by 64.5%andthe specific power consumption dramatically increased by 127.5%, when the feed flow rate was increased from 1 to 4 LPM at the solution pH of 7.Alternatively, when the feed flow rate and the solution pH were increased simultaneously, the mass of fouling was reduced by 56.8%, with animprovement in the specific power consumption augmented only by 54.1%, while the permeate flux increased by 136.84%.

Keywords: membrane fouling, ultrafiltration, attachments, power consumption, pH, repulsion

INTRODUCTION

In the manufacturing of paint products, cleaning of reactors andmixing basins generate a large quantity of wastewater. Painteffluents typically have high levels of biological oxygendemand (BOD of >580mg/L), chemical oxygen demand (COD>5500mg/L), suspended solids and turbidity.[1] Therefore, thewastewater needs be treated before being discharged. One of thebenefits of treating the waterbased paint effluent with UF overother available processes is the potential to reclaim the solid fromthe paint effluent for recycling purposes, and the possible reuse ofthe permeate as processed for cleaning water.[13] However,membrane fouling is one of the major factors that hinders a morewidespread application of UF.

Membrane fouling leads to a significant increase in hydraulicresistance,manifested as the decline in permeate flux or increase inthe transmembrane pressure (TMP), when the process is operatedunder constantTMP or constantflux conditions, respectively.[4,5]

In a system where the permeate flux is maintained by increasingTMP, the energy required to achieve filtration escalates and causeshigher power consumption. Notably, in cases with a longer periodof filtration, membrane fouling is not completely reversible bybackwashing.[6] In fact, fouling leads to a higher operational cost,higher energy demand, reduced membrane lifetime and increasedcleaning/washing frequency. In a seawater desalination plant thecost of the membrane unit is about 2025% of the total capitalcost.[7] Various researches on the subject indicate that theimplementation of different fouling remediation techniques resultsin a significant reduction in the power consumption in filtrationprocess.[79] For a plant capable of producing 100m3/day water,the energy consumption is more than 10 kWh/m3; however, thiscan be reduced to 3.5 kWh/m3 if fouling is being adequatelycontrolled.[10] The energy consumption of a crossflow system isabout 5 kWh/m3 permeate. For deadend filtration the energyconsumption is only 0.10.2 kWh/m3 permeates.[11] One of theresearch studies shows that the total energy consumed for 40minfiltration was from 50.08 to 62.54 kJ/L at 3 bar while the energy

consumed in membrane cleaning ranged from 87.08 to 107 kJ for50100min at different operational pressures.[12] Therefore,innovative approaches to fouling remediation are necessary forthe reduction of power consumption and for future membranedevelopment and its application.

The question underlying membrane fouling and its practicalimplementation is the origin of the attachment of foulants on themembrane surface. Two major forces contributing to this foulantattachment are dispersion interaction force and polar interactionforce.[13] Foulants that remain attached to the membrane surfacesare most likely caused by the balance of the van der Waalsattraction force and the electrostatic repulsion force betweenparticles and the membrane surface due to the surface charge. VanOss postulated the concepts of apolar and polar interactions inorder to classify and predict interactions in an aqueous medium,based on the Derjaguin, Landau, Verwey and Overbeek (DLVO)theory.[13] The apolar interaction is mainly a dispersion interac-tion. On the other hand, the polar interaction is the interactionbetween Lewis acidbase pairs in the system, including the twointeracting entities and the surrounding water molecules. Theseinteractions are useful in explaining the benefit of hydrophilisingthe membrane surface to reduce the irreversible attachment ofparticles and other foulingmaterials on themembrane surface. Thestrength of chemical attachment depends on the dispersioninteraction (apolar), as well as, predominantly on the polarinteraction. The latter can be either attractive or repulsive,depending on the hydrophilicity of the two interactingsurfaces.[13,14]

*Author to whom correspondence may be addressed.Email address: [email protected]. J. Chem. Eng. 9999:113, 2014 2014 Canadian Society for Chemical EngineeringDOI 10.1002/cjce.21974Published online in Wiley Online Library(wileyonlinelibrary.com).

VOLUME 9999, 2014 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 1

The ionic strength of the feed solution significantly alters thesurface properties and interactions of the colloidal foulants, andtherefore, affects the fouling potential.[15,16] For hydrophilicmembranes, membrane fouling decreased when the ionic strengthof the feed solution was amplified by increasing the solution pH.This was attributed to the augmentation of the electrostaticrepulsion forces between the foulants and the membrane surface,due to the increased negative charge on the membrane surface.[1724] The influence of the ionic strength on fouling can be directlyobserved by performing filtration experiments. The experimentalobservation, however, may not be sufficient for a thoroughunderstanding of the fouling potential of latex at differentsolution pH. The relationship between the solution ionic strengthand the particletoparticle and particletomembrane attachmentis necessary in elucidating the underlying factors affectingmembrane fouling. In our previous work a mathematical modelfor ultrafiltration was developed.[25] The model can predict themass of the fouling retained by the membrane and the increase inthe transmembrane pressure. However, the developed modelexpressions are dependent on the particletoparticle and particletomembrane attachment probabilities. Therefore, the influence ofthe ionic strength, via varying the solution pH, on the attachmentprobability, and hence, on membrane fouling was investigated inthe present study. The influence of the operational conditions onthe total mass of fouling, and the specific power consumption wasalso investigated.

ATTACHMENT MATHEMATICAL MODEL

In our previous work, a mathematical model was developed topredict the amount of fouling based on analyses of particleattachments. The mathematical model was combined with theHermia model to further develop models for transmembranepressure prediction. Fouling was considered primarily due to theattachments among foulant entities (coagulation attachment) andbetween foulants and membrane surfaces (depositional attach-ment). The coagulation attachment is the particletoparticleattachment (app) and the depositional attachment is the particletomembrane attachment (apm). Notably, the two attachmentprobabilities are dependent on the properties of the foulants andmembranes, as well as the solution chemistry and operatingconditions. Based on the mathematical model expressions,[25] themass of particles contributing to pore blocking can be calculatedfrom Equation (1). The mass of particles accountable to the cakelayer can be calculated from Equation (2). These equations arefunctions of themembranes physical properties, particle projectedarea, feed concentration, volumetric permeate flux and attachmentfouling probabilities. The summation of mp and mc represents thetotal mass of fouling retained by themembrane (mt). Equations (1)and (2) are solved for app and apm using mp and mc valuesmeasured experimentally, as described in Experimental Methodsection. Furthermore, the Response Surface Methodology (RSM)was used to design experiments that provided sufficient data andallowed to establish the necessary correlations for the prediction ofthe attachment fouling probabilities at a given operationalcondition. These correlations could be then used in combinationwith the mathematical model in order to make a complete set ofpredictive models, as illustrated in our previous work.

Equation (3) represents the increase in the transmembranepressure due to membrane fouling.[25] This equation is a functionof themembranes physical properties, particle projected area, feedconcentration, volumetric permeate flux, cake resistance andattachment fouling probabilities. The equation indicates that the

coagulation and depositional attachments have an effect on theincrease transmembrane pressure. Consequently, that increase intransmembrane pressure will result in a significant increase in thepower consumption through the filtration process

mp apmes4appsL 1 exp4appsLCinVs apmes

ss1 expssCinVs

NmsxsapmB

pDmLmBapm ln 1 exp pDmLmBapm sxsapmBNm CinVs

1

mc appCinVs apm1 es app

4app1exp4appsLCinVs

sL 1exp4

appssCinVsss

2

P0 1 11 apmes=4app1 exp4appsLCinVs

11 apmes1 expssCinVs

1f1 Nm=sxsrapmBLmf pDmLmBapmln1exppDmLmBapmsxsapmB=NmCinVsgg

2

RcRm

CinVs apmes4appsL

1 exp4appsLCinVs

apmesss

1 expssCinVs NmsxsapmB

pDmLmBapm

ln 1 exp pDmLmBapm sxsapmBNm CinVs

3

where mp (kg/m2) is the total mass of particles attached to

membrane pores per unit membrane surface area, mc (kg/m2) is

the total mass of particles in the cake layer per unit membranesurface area, P0 is the increase in transmembrane pressure throughthe filtration time, es is the cake porosity, sL (m

2/kg) is theprojected area of a unit mass of the large particles (particlediameter pore diameter) onmembrane surface, ss (m2/kg) is theprojected area of a unit mass of the small particles on themembrane surface (pore diameter/6

consumption. The specific power consumption per unit volume offiltrate is defined as:

Specific power consumption kWh=m3 1:916 106TMPAVGQVs4

where TMPAVG is the timeaveraged transmembrane pressurethroughout the filtration duration. TMPAVG (psigmin) can becalculated based on the area under the curve, as shown in Figure 1.Q (LPM) is the feed flow rate and Vs (m3) is the cumulativepermeate volume.

MATERIALS AND METHODS

Experimental Method

A schematic diagram of the experimental setup is shown inFigure 2.[25] Further details about the experimental setup,membranefiltration unit, latex paint used and the particulars of the procedure,were published in the previous study.[25] Samples of the feed and thepermeate streams were collected during each experimental run. Ameasured mass of each sample was then transferred to a metal dishthat was placed in an oven at 1058C for the duration of a week. Themass of each dish was periodically measured, approximately every3 days, until there was no discernible change. From the measuredmasses of the solution and the solid in each metal dish, the solidcontent of the paint solution was then determined. The particle sizedistribution of the latex paint solution was determined using aparticle size analyser (Model 2000; Malvern Mastersizer, Worces-tershire, UK) which had a range of 0.022000mm. The particle sizedistribution of the latex solution had a range of 0.03800mm. Theconcentration of samples did not affect the particle size distributionobtained during this experiment.

The total mass of fouling (mt) is the difference in weight of themembrane before and after filtration. For measuring the mass ofparticles contributing to the cake layer (mc), the cakewas removed(scratched off) under the microscope, so as to keep the membranespores blocked with the particles contributing to the pore blocking.The scratched off solid portion was then weighted to determine theamount of the cake layer. The blocked pores of themembranewerechecked using SEM. The mass of particles contributing to poreblocking (mp), is the difference between the weight of themembrane before filtration (clean membrane) and the weightafter the scratching off the cake layer. Themass of particles (mt,mc

andmp) wasmeasured using an electronic balance (Mettler ToledoModel AB 54S Fact, Switzerland, 0.1mg).

The solution pH was adjusted using a pH transmitter(Mettlertoledo pH Transmitters 2100e, MettlerToledo GmbH,Urdorf, Germany,

of fouling and the specific power consumption over the filtrationprocess. Each point on the curves in thefigures represents the resultof an individual experimental run. As shown in Figure 3a, at a feedflow rate of 4.0 L/min and a feed solid concentration of 1.3 kg/m3,when the pressure was raised from 15 to 35 psig, the particletoparticle (app) and the particletomembrane (apm) attachmentprobabilities increased from 0.4 to 0.75 and from 0.5 to 0.8,respectively. However, further increase in pressure did not affectapp and apm significantly. When the transmembrane pressure wasraised from 15 to 35 psig, the volumetric permeate flux increasedfrom 0.0125 to 0.0178m3/m2. Higher permeate flow through themembrane brought more particles to the membranes surface. As aconsequence, the chances for the particletoparticle and particletomembrane collision and attachment increased, resulting in

higher attachment probabilities. Notably, increasing the pressurefrom 35 to 45 psig resulted in a insignificant change in thevolumetric permeateflux (Vs) from0.0178 to 0.0179m

3/m2. This isdue to the fact that the transmembrane pressure higher than thecritical pressure (35 psig) has an insignificant effect on thepermeate flux or the total mass of fouling retained by themembrane. Thus, the fouling attachments probabilities valueslevelled off to a relatively constant value. Moreover, Figure 3ashows that the depositional attachment probability apm was higherthan the coagulation attachment probabilityapp. This indicates thatthe transmembrane pressure might have a more significant impacton the pore blocking and the membranetoparticle attachmentthan the cake layer buildup. Higher fouling attachment probabili-ties resulted in an increase in the total mass of fouling from 0.0104

Figure 2. Schematic diagram of experimental setup.[25]

Table 2. Process parameters levels

Factor Variable

Range

UnitLow (1) High (1)

A Transmembrane pressure 15 35 psigB Feed flow rate 1 7 LPMC Feed concentration 0.78 1.82 kg/m3

Table 1. Properties of GE membranes

Membrane material Pore sizea (mm) Pore densityb (pore/cm2) Nominal weight (mg/cm2) Nominal thicknessc (mm) Typical flow rated (mL/min/cm2)

Polycarbonate 0.05 6108 0.7 6 0.4aTolerance 0%, 20%.bTolerance 15%.cTolerance 10%.dInitial flow rates measured using prefiltered water at 10 psi.

4 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 9999, 2014

to 0.0145 kg/m2 as shown in Figure 3b. The specific powerconsumption also increased from 10.17 to 21.02 kWh/m3.

Figure 4a depicts the effect of the feed flow rate on the foulingattachment probability at the initial transmembrane pressure of15 psig and the feed solid concentration of 1.3 kg/m3. Under theseconditions, the particletoparticle attachment probability (app)was significantly reduced from 0.86 at 1 LPM (crossflow velocityof 10.4 cm/s) to 0.19 at 6 LPM (crossflow velocity of 62.5 cm/s)while the particletomembrane attachment probability (apm)decreased from 0.82 to 0.49. The probable cause for this substantialreduction in app with the feed flow rate is that at a higher flow rate,a higher shear rate is applied on the particles at the cake layer.Consequently, a higher number of particles were washed awayfrom the cake layer. In particular, as the feed flow rate wasincreased to 3.0 LPM, therewas a significant decrease in app, whicharguably marks the crucial flow rate for membrane fouling in thissystem. On the other hand, at a higher flow rate, the cumulative

permeate flux (Vs) increased from 0.0076 to 0.014m3/m2. As a

result, more particles were brought into contact with themembrane, leading to an increase in collisions and attachmentsbetween particles and the membrane surface. Consequently, apmdecreased at a lower rate as comparedwith that of app. As shown inFigure 4a, app and apm approach the highest values at flow rate of1.0 LPM. It was also observed that as the fouling attachmentprobabilities decreased, the mass of particles retained by themembrane dropped from 0.0162 kg/m2 at a feed low rate of 1 LPMto 0.00838 kg/m2 at 6 LPM, as illustrated in Figure 4b. Due to thereduction of the total mass of fouling by increasing the feed flowrate from 1 to 6 LPM, the transmembrane pressure over theduration of the filtration process only increased by 1 psig,compared to an increase of 5 psig at 1 LPM. Although the totalmass of fouling was decreased, with the increase in the flow ratefrom1 to 6 LPM, the power consumption per unit volume increasedfrom 4.47 to 13.1 kWh/m3 as illustrated in Figure 4b. This aligned

Table 3. Experimental parameters and results based on the experimental design for the total mass of fouling and the specific power consumption

Exp no.Initial TMP

(psig)TMPAVG

a

(psig)Final TMP(psig)

Independent variables

mt(kg/m2)

Power consumed(kWh/m3)

Feed flow rate Q(LPM)

Feed concentration Cf(kg/m3)

1 15 (1) 15 17 1 (1) 0.78 (1) 0.0088 2.812 35 (1) 35 40 1 (1) 0.78 (1) 0.013 5.743 15 (1) 15 16 7 (1) 0.78 (1) 0.0048 14.744 35 (1) 35 39 7 (1) 0.78 (1) 0.012 28.465 15 (1) 16.47 20 1 (1) 1.82 (1) 0.018 4.826 35 (1) 35.27 42 1 (1) 1.82 (1) 0.019 8.667 15 (1) 16.27 21 7 (1) 1.82 (1) 0.0155 25.988 35 (1) 35 43 7 (1) 1.82 (1) 0.023 37.979 15 (1) 15 18 4 (0) 1.3 (0) 0.01 10.1710 35 (1) 35 41 4 (0) 1.3 (0) 0.015 16.6111 25 (0) 25 32 1 (1) 1.3 (0) 0.0195 5.3412 25 (0) 25 27 7 (1) 1.3 (0) 0.0082 19.6413 25 (0) 25 27 4 (0) 0.78 (1) 0.0076 12.0114 25 (0) 26.8 33 4 (0) 1.82 (1) 0.024 17.2315 25 (0) 25.5 29 4 (0) 1.3 (0) 0.012 13.9816 25 (0) 25 29 4 (0) 1.3 (0) 0.0117 13.7717 25 (0) 25 29 4 (0) 1.3 (0) 0.0118 13.6118 25 (0) 25 29 4 (0) 1.3 (0) 0.012 13.8419 25 (0) 25 29 4 (0) 1.3 (0) 0.0121 13.6620 25 (0) 25 29 4 (0) 1.3 (0) 0.0119 13.51aTMPAVG (psig) TMPAVG (psigmin)/filtration time (min), TMPAVG (psigmin) calculated as mentioned in Specific Power Consumption section.

a b

Figure 3. Effect of initial transmembrane pressure at [Q4 LPM], [Cf1.3 kg/m3], [pH 7] on (a) fouling attachment probabilities (app, apm) and thevolumetric permeate flux (Vs). (b) Total mass of fouling and specific power consumption.


with the specific power consumption as directly proportional to thefeed flow rate in Equation (4). Specifically, this indicates that theincreasing of the flow rate six times results in a three timesincreasing of the power consumption. This, in turn, implies that theremediation of the fouling is greatly required to reduce the totalmass of fouling in lower flow rates, increase the volumetricpermeate flux, and, at the same time, reduce the powerconsumption per unit volume of filtrate.

Figure 5a shows the effects of the feed solid concentration on thefouling attachment probability at a feed flow rate of 1 L/min and15psig. Increasing the feed concentration from 0.78 to 1.82kg/m3

resulted in a significant increase in the coagulation attachmentprobability, app, and a substantial decrease in the depositionalattachment probability, apm. The particletoparticle attachment, app,increased from 0.3 to 0.9, while the particletomembrane attach-ment,apm, decreased from0.95 to 0.6.At ahigher feed concentration,the chance of particletoparticle collision and attachment would behigher. As a consequence, the coagulation attachment probabilityand cake layer buildup were enhanced. On the other hand,increasing of the feed concentration lead to a decrease in thevolumetric permeate flux (Vs) from 0.011 to 0.007m

3/m2. A reducedpermeate flow resulted in a lower chance of particletomembranecollision and attachment. The depositional attachment probability,apm, thus decreased from0.95 to 0.6. Figure 5b indicates that the totalmass of fouling increased from 0.0088 to 0.0188kg/m2 when theconcentration was increased from 0.78 to 1.82kg/m3, as expected.Consequently, the increase in transmembrane pressure over theduration of filtration was elevated from 2 to 5psig to overcome thehigher resistance to the permeate flow; hence, the specific powerconsumption was also almost doubled from 2.8 to 4.8kWh/m3.

Statistical Analysis

Central Composite FaceCentered CCF Response Surface Method(RSM) was selected as the experimental design method asillustrated in details in Experimental Method section. The totalmass of fouling was measured for each experimental run, and thespecific power consumption was calculated, as shown in Table 3using Equation (4). The total mass of fouling and the powerconsumed per unit volume of permeate varied in the ranges of0.00480.024 kg/m2 and 2.8137.97 kWh/m3, respectively.The resultswere analysed usingmultiregression so as to identify

the effects of the process parameters on the total mass of foulingand the specific power consumption in ultrafiltration of a latexsolution. Moreover, the process parameters were examined inorder to identify the optimum conditions that would allow for theminimum mass of fouling and power consumption.Based on the ANOVA analysis, linear models best fitted the

experimental data for the total mass of fouling and the specificpower consumption. The ANOVA result is presented in Table 4.The Pvalue is the probability value used to set the level ofconfidence for the Ftest and to ascertain the significance of each ofthe coefficients. The Pvalue should be less than or equal to 0.05 tobe significant.[27,28] According to regression coefficients andprobability values (Pvalue), as shown in Table 4, the linear termswere all functionally significant for the totalmass of fouling and thespecific power consumption. The models for the total mass offouling and the specific power consumption are given inEquations (5) and (6), respectively. In addition, the goodness offit was determined by evaluating the coefficients of determination(R2). R2 was 0.97 for the linear models

a b

Figure 4. Effect of feed flow rate at [TMP15psig], [Cf1.3 kg/m3] and [pH 7] on (a) fouling attachment probabilities (app, apm) and the volumetricpermeate flux (Vs). (b) Total mass of fouling and power consumption.

a b

Figure 5. Effect of feed concentration at [Q1 LPM], [TMP15psig] and [pH 7] on (a) fouling attachment probabilities (app, apm) and the volumetricpermeate flux (Vs). (b) Total mass of fouling and power consumption.


Total mass of fouling mt kg=m2 0:00499 0:000299A 0:0003267B 0:00929C 5

Specific Power Consumption kWh=m3 16:2412 0:3978A 3:3427B 5:777C 6

where the total mass of fouling (mt) and the specific powerconsumption are the function of average transmembrane pressureTMPAVG (A) (psig), feed flow rate (B) (LPM) and feed concentra-tion (C) (kg/m3). The linear models were compatible with theresults in Effect of Operating Conditions on Fouling Attachmentsand Power Consumption section. As illustrated in Equation (5), inorder to minimise the total mass of fouling, the transmembranepressure and feed concentration should be decreasedwhile the feedflow rate should be increased. On the other hand, to minimise thespecific power consumption, the transmembrane pressure, feedflow rate and feed concentration should be decreased. Furthermorethe average transmembrane pressure TMPAVG (psig) can beestimated so as to predict the specific power consumption fromEquation (6) using the initial transmembrane pressure, variation oftransmembrane pressure (P0) with time estimated from Equa-tion (3) and the filtration time.

Fouling Remediation by Changing the Solution pH

Following previous results, the fouling remediation can beconsidered critical in reducing membrane fouling, increasing thepermeate flux and reducing the power consumption per unitvolume of filtrate at given operating conditions. The effects of thesolution pH over a range from 3 to 12 on particleparticleattachment and particlemembrane were investigated sincethose attachment potentials strongly affect the volumetricpermeates flux, the total mass of fouling and the specific powerconsumption.

Effect of pH on ionic strength of latex solution and membranesurface hydrophilicity

The ionic strength is directly proportional to the solutionconductivity. Therefore, the effect of the solution pH on the ionicstrength of the solution is evaluated based on the change in thesolution conductivity with pH. Figure 6a illustrates the effect of pHon the conductivity of the latex solution. At a solid concentration of1.3 kg/m3, the pH of the latex solution was 7. The solutionconductivity increased from 0.094 to 38.8mS/cm when thesolution pH was increased from 7 to 14. On the other hand,decreasing pH from 7 to 2 using HCl and H2SO4 only resulted in aslight increase in the conductivity to 2.57 and 5.5mS/cm,respectively. Figure 6b depicts the zeta potential values of thelatex particles and membrane surface. Zeta potential of latexparticles at pH 7 is approximately 26.61mV. For the pH values>7, the zeta potential becomes more negative as the ionic strengthincreases. The zeta potential of latex particles achieved42.66mVat pH 12.

The zeta potential of the membrane surface is negative at all pHvalues investigated. As the pH is increased from 3 to 12, the zetapotential of the membrane surface becomes increasingly negativefrom 18.11 to 46.77mV, respectively. This indicates that thehydrophilicity of the membrane surface increased with the riseof pH value.

Effect of pH change on membrane fouling and power consumption

As previously shown in Effect of Operating Conditions on FoulingAttachments and Power Consumption section and StatisticalAnalysis section, at the initial transmembrane pressure of15 psig, the feed flow rate of 4 LPM and the feed solid concentrationof 1.3 kg/m3, the power consumption per unit volume of filtratewas 10.17 kWh/m3; and the volumetric permeate flux and the totalmass of fouling were 0.0125 and 0.0104, respectively. Under thesame operating conditions, increasing the solution pH from 7 to 12resulted in a significant reduction in the particletoparticle

Table 4. Regression coefficients and probability values of statistical analysis

Coded factor Actual factorCoefficient for mt

(kg/m2) PvalueCoefficient for specific

power consumption (kWh/m3) Pvalue

A TMPAVG (psig) 0.000299 0.0044 0.39780 0.0005B Feed flow rate (LPM) 0.000326667 0.0005 3.34267

attachment from 0.4 to 0.19, as shown in Figure 7a. This indicatesthat the ionic strength of the latex solution increased with pH,which in turn caused an increased negative charge on the latexparticles due to the OH group added to the solution. Raisingthe pH from 7 to 12 resulted in an increase of the zeta potential ofthe latex particles from 26.61 to 42.66mV, respectively. As aconsequence, the electrostatic repulsive forces between the latexparticles were increased, while the particletoparticle attachmentdecreased significantly. Also, the depositional attachment de-creased from 0.55 to 0.26. This could be attributed to theenhancement of the hydrophilicy of the membrane and theincrease in the negative charge on the membrane surface withincreases in the solution pH. As the pH is increased from7 to 12, the

zeta potential of the membrane surface became increasinglynegative from 36.67 to 46.77mV, respectively. Therefore, theelectrostatic attraction force between latex particles and themembrane surface decreased. Consequently, the particletomembrane attachment was significantly reduced. The noticeablereduction of the fouling attachments caused a considerabledecrease in the total mass of fouling from 0.0104 to 0.007 kg/m2,as can be seen in Figure 7b. This indicates that an increase in theionic strength via an increase in pH resulted in some enhancementof the antifouling property of the feed and the membrane. Thedecrease in the total mass of fouling caused a reduction in theresistance to the flow. Therefore, the increase in the transmem-brane pressure over the duration of the filtration process was

a b

Figure 7. Effect of pH value at [TMP15psig], [Cf1.3 kg/m3] and [Q4 LPM] (using 1NNaOH and 0.1N H2SO4) on (a) fouling attachment probabilities(app, apm) and the volumetric permeate flux (Vs). (b) Total mass of fouling and power consumption (kWh/m

3).

Figure 8. SEM images of membrane after ultrafiltration [Q4 LPM], [Cf1.3 kg/m3], [TMP15psig] (a) cake height5.05mm at pH 7, (b) cakeheight3.6mm at pH 10 and (c) cake height1.85mm at pH 12.


reduced, and hence, the specific power consumption decreasedfrom 10.17 to 6.9 kWh/m3. In addition, decreasing of bothattachments resulted in increasing permeate flux from 0.0125 to0.018m3/m2, as shown in Figure 7a.

On the other hand, in acidic conditions the hydrophilicmembrane became less negatively charged (more hydrophobic).This can be attributed to the more solubility of H functionalgroup. As the pH decreased from 7 to 3, the zeta potential of themembrane surface became significantly less negative, decreasingfrom 36.67 to 18.11mV, respectively. This in turn results in a

stronger electrostatic attractive force between the latex particlesand the membrane surface. As a consequence, more particletomembrane attachment would occur. As illustrated in Figure 7a,when pH was decreased from 7 to 3, a considerable increase in thedepositional attachment from 0.55 to 0.82 was observed.Moreover, the lowering of the pH reduced the negative chargeon the particles, thus causing a stronger electrostatic attractionforce between particles and encouraging particle aggregation. Asthe pH decreased from 7 to 3, the zeta potential of the latex particlesbecame significantly less negative, decreasing from 26.61 to

a b

Figure 9. Effect of pH value at [TMP15psig], [Cf1.3 kg/m3] and [Q1 LPM] (using 1NNaOH and 0.1N H2SO4) on (a) fouling attachment probabilities(app, apm) and the volumetric permeate flux (Vs). (b) Total mass of fouling and power consumption (kWh/m

3).

Figure 10. SEM images of membrane after ultrafiltration. (a) Cake height 13.8mm at [TMP15psig], [Cf1.3 kg/m3], [Q1 LPM] and pH 7. (b) Cakeheight 6.1mmat [TMP15psig], [Cf1.3 kg/m3], [Q1 LPM] and pH 10. (c) Cake height 2.3mmat [TMP15psig], [Cf1.3 kg/m3], [Q1 LPM] and pH12. (d) Cake height 2.4mm at [TMP15psig], [Cf1.3 kg/m3], [Q6 LPM] and pH 7.


11.2mV, respectively. A stronger electrostatic attraction andparticle aggregation resulted in an increased coagulation attach-ment from 0.4 to 0.59. Moreover, SEM images obtained after theultrafiltration provided further evidence in support of the resultsacquired previously. Particularly, at pH 7 the cake thickness was5.05mm, as shown in Figure 8a. The cake height decreased to3.6mm at pH 10, as can be seen in Figure 8b. The cake thicknessfurther diminished to 1.85mm at pH 12, as illustrated in Figure 8c.This was due to the significant reduction in the particletoparticleattachment, which was the predominant factor responsible for thecake formation.

As presented in Effect of Operating Conditions on FoulingAttachments and Power Consumption section and StatisticalAnalysis section, the minimum specific power consumptionoccurred at the lowest flow rate of 1 LPM. On the other hand, atthe lowest flow rate the total mass of fouling and the cake layerthickness were highest. Therefore, the effect of pH on themembrane fouling and the specific power consumption at a lowfeed flow rate of 1 LPM was further investigated. It was observedthat at a feed flow rate of 1 LPM, the transmembrane pressure of15 psig and the feed solid concentration of 1.3 kg/m3, when thesolution pH was increased from 7 to 12, the total mass of foulingdecreased from 0.0162 to 0.0081 kg/m2, as can be seen inFigure 9b. This contributed to the considerable reduction in bothattachment probabilities app and apm, from 0.86 to 0.40 and from0.82 to 0.37, respectively, as shown in Figure 9a. Therefore, themembrane fouling was lessened, leading to a decrease in thetransmembrane pressure elevation over the duration of theexperiment from 6 to 1 psig. Consequently, the specific powerconsumption decreased from 4.62 to 2.06 kWh/m3, as presented inFigure 9b. Moreover, at the solution pH of 12, a volumetric

permeate flux of 0.015m3/m2 was obtained, which is higher thanthe permeate flux at a higher feed flow rate of 6 LPM with thesolution pH of 7 and the same other operating conditions.Furthermore, the cake height at Q 1 LPM, TMP 15 psig,

Cf 1.3 kg/m3 and pH 7 was 13.8mm as shown in Figure 10a.Increasing pH value to 10 resulted in a significant reduction in thecake thickness to 6.1mm. At pH 12 the cake thickness was furtherreduced to 2.3mm, which is very close to the cake thickness of2.4mm at Q 6 LPM, TMP 15 psig, Cf 1.3 kg/m3 and pH 7.However, at the lower feed flow rate of 1 LPM, TMP 15 psig,Cf 1.3 kg/m3 and pH 12, the specific power consumption wasonly 2.06 kWh/m3 (Figure 9b), compared with 13.1 kWh/m3 atQ 6 LPM, TMP 15 psig, Cf 1.3 kg/m3 and pH 7 (Figure 4b).Table 5 summarises the difference between the effects achieved

by either increasing the feed flow rate or by raising the pH valuewith respect to the fouling attachment, the totalmass of fouling, thevolumetric permeate flux, and the specific power consumption attransmembrane pressure of 15 psig and 1.3 kg/m3. A sole increaseof the feed flow rate reduced membrane fouling, but wasaccompanied by high power consumption. On the other hand,an increased feed flow rate accompanied by an augmented pHvalue had a noticeable improvement in the specific powerconsumption. On the overall, a reduced flow rate in combinationwith an increased pH had the best effects on both the membranefouling and the specific power consumption.The effects of ionic strength on the membrane fouling and the

power consumption were also studied at a higher TMP of 25 psig,Q 4 LPM and Cf 1.3 kg/m3. The solution pHwas variedwithin arange from 3 to 12. Figure 11 shows similar trends for the foulingattachments, the volumetric permeate flux, the total mass offouling, and the specific power consumption as those at TMP 15

Table 5. Comparison of total mass of fouling, volumetric permeate flux, cake height and the power consumption at different pH. [TMP15psig],[Cf1.3 kg/m3] and 25minQ (LPM) pH app apm mt (kg/m

2) Vs (m3/m2) Specific power consumption (kWh/m3) Cake height (mm)

1 7 0.86 0.82 0.0162 0.0076 4.47 13.81 10 0.56 0.55 0.011 0.012 2.65 6.11 12 0.4 0.37 0.0081 0.015 2.06 2.34 7 0.4 0.55 0.0104 0.0125 10.17 5.054 10 0.26 0.34 0.008 0.016 7.87 3.64 12 0.19 0.26 0.007 0.018 6.89 1.85

a b

Figure 11. Effect of pH value at [TMP25psig], [Cf1.3 kg/m3] and [Q4 LPM] (using 1N NaOH& 0.1N H2SO4) on (a) fouling attachment probabilities(app, apm) and the volumetric permeate flux (Vs). (b) Total mass of fouling and power consumption (kWh/m

3).


psig shown Figure 7. However, ultrafiltration at TMP of 25 psigresulted in higher values for the particletoparticle and particletomembrane attachments. For instance, at pH 10, the particletoparticle attachment was 0.40 and 0.26, at TMP of 25 and 15 psig,respectively. At a higher TMP, the permeate flux was higher. Moreparticles were thus brought to the membrane surface; hence, morechances for the particletoparticle and particletomembranecollision and attachment occurred. Consequently, the particletoparticle and particletomembrane attachment increased. At TMP 25 psig and Q 4 LPM, increases in pH from 7 to 12 resulted in asignificant reduction in the total mass of fouling from 0.012 to0.0077 kg/m2. Consequently, the specific power consumption wasreduced from 14.0 to 10.1 kWh/m3, as shown in Figure 11b.Figure 12a and b presents SEM images of the cake height afterultrafiltration at the solution pH of 7 and 10, respectively. The cakethickness decreased from 6.65 to 5.30mmwhen the pH was raisedfrom 7 to 10. SEM images in Figure 12 and Figure 8 provide aconfirmation of the effect of the transmembrane pressure on thecake height.

Effect of divalent and monovalent ions

The effect of themonovalent ion on fouling attachments, totalmassof fouling, and specific power consumption was also investigatedusing sulphuric acid and hydrochloric acid. As shown in Table 6at pH of 3 and 5, lower fouling attachment probabilities, lower totalmass of fouling, higher volumetric permeate flux and lower powerconsumption were achieved with sulphuric acid. This can beattributed to the more negative charge sulphate ion, SO4

2, ofsulphuric acid, which makes the membrane less hydrophobic incomparison to the chlorine ion, Cl, of hypochloric acid. It isrelevant to note that the zeta potential of the membrane surfaceat pH 3 was 18.11 and 16.31mV with sulphuric acid and

hydrochloric acid, respectively. When pH was increased to 5, thezeta potential of the membrane surface increased to 24.78mVusing sulphuric acid, in comparison to 20.55mV using hydro-chloric acid. This again reconfirms the advantage of sulphuric acidover hydrochloric acid in foulingmediation. Figure 13 presents theSEM images of the cake thickness at pH 3 using sulphuric acid andhypochloric acid. The cake thickness increased to 6.35mmfor 0.1NH2SO4 to 7.65mm for 0.1N HCl.

CONCLUSION

According to the results obtained in the present study, it wasobserved that higher fouling probabilities caused an increase in the

Figure 12. SEM images of membrane after ultrafiltration [Q4 LPM], [Cf1.3 kg/m3], [TMP25psig] (a) Cake height 6.65mm at pH 7, (b) cake height5.30mm at pH 10.

Figure 13. SEM images ofmembrane after ultrafiltration [Q4 LPM], [Cf1.3 kg/m3], [TMP15psig], pH 3 (a) Cake height6.35mmusing 0.1NH2SO4.(b) Cake height7.65mm using 0.1N HCl.

Table 6. Comparison of the effect of the monovalent acid (HCl) and thedivalent acid (H2SO4) at pH 3 and 5 at transmembrane pressure 15psig,4 LPM and 1.3 kg/m3

Using0.1N H2SO4

Using0.1N HCl

pH 3app 0.59 0.62apm 0.82 0.97mt (kg/m

2) 0.0135 0.0145Vs (m

3/m2) 0.0098 0.0088Power consumption (kWh/m3) 14.14 15.94

pH 5app 0.5 0.54apm 0.71 0.8mt (kg/m

2) 0.0123 0.013Vs (m

3/m2) 0.0115 0.01Power consumption (kWh/m3) 11.7 13.86


total mass of fouling retained by the membrane and the cakethickness, which in turn resulted in an increased resistance to thepermeate flux. Higher permeate volume could be achieved byincreasing the transmembrane pressure, thus causing a greaterenergy consumption per unit volume of filtrate. For example, at afeed flow rate of 4.0 L/min and feed concentration of 1.3 kg/m3,increase in the transmembrane pressure from 15 to 45 psig leads tohigher coagulation and depositional attachments. The total mass offouling also increased accordingly from 0.0104 to 0.0145 kg/m2.Furthermore, the specific power consumption increased from10.17 to 21.02 kWh/m3. In addition, increasing the feed concen-tration from 0.78 to 1.82 kg/m3 caused a substantial increase in thetotal mass of fouling that is, from 0.0088 to 0.0188 kg/m2. Notably,in this case the specific power consumption was almost doubledfrom 2.8 to 4.8 kWh/m3. It was also observed that both attachmentprobabilities considerably decreased when the feed flow rate wasincreased from 1 to 6 LPM; resulting lower totalmass of fouling andcake height as well. On the other hand, the specific powerconsumption increased significantly from 4.47 to 13.1 kWh/m3.

It was also observed that the lowest specific power consumptionoccurred at the minimum flow rate with the minimum transmem-brane pressure and the lowest feed concentration. In addition,lower membrane fouling, higher volumetric permeate flux andlower specific power consumption were obtained with increasingthe solution pH. It was noted that at given transmembranepressure, feed solid concentration and pH, increasing the feed flowrate resulted in a reduced total mass of fouling. However, thespecific power consumption increased. On the other hand, increasein the pH from 7 to 12 lead to decrease in the total mass of foulingand the specific power consumption. A combination of anincreased feed flow rate and a higher solution pH was thus shownto enhance both the permeate flux and the specific powerconsumption with less membrane fouling.

NOMENCLATURE

B mass transfer coefficient (m2)Cin the mean mass concentration of particles at the inlet of

a membrane pores (kg/m3)Cf concentration of foulants in the feedwater (kg/m

3)Dm membrane pore diameter (m)Lm length of membrane pores (m)mp total mass of particles attached to membrane pores per

unit area (kg/m2)mc total mass of particles in the cake layer per unit area

(kg/m2)mt total mass of particles retained per unit membrane

surface area (kg/m2)Nm number density of membrane pores per a unit

membrane surface areaP0 the increase in transmembrane pressure due to

membrane fouling through the filtration timePvalue probability in statistical significance testing (ANOVA

test)Q feed flow rate (LPM)R2 the percent of the variation of the response explained

by the modelRm membrane resistance (m

1)Rc Cake resistance (m/kg)TMP initial transmembrane pressureTMPAVG timeaveraged transmembrane pressure throughout

the filtration duration (psigmin)TMPAVG average transmembrane pressure throughout the

filtration duration (psig)Vs cumulative volume of the permeate normalized to

membrane surface area (m3/m2)Vs cumulative permeate volume (m3)

Greek Symbols

s projected area of a unit mass of the particles on membranesurface (m2/kg)

sL projected area of a unit mass of the large particles (Particlediameter pore diameter) on membrane surface (m2/kg)

sS projected area of a unit mass of the small particles onmembrane surface (m2/kg) (pore diameter/6< particlesize< pore diameter/2)

sxS projected area of a unit mass of the very small particles onmembrane surface (m2/kg) (particle size< pore diameter/6)

es cake porosityapm the attachment probabilities between a particle and the

membraneapp the attachment probabilities between two particles

ACKNOWLEDGEMENTS

The authors are grateful for the financial support from NaturalScience and Engineering Research Council of Canada (NSERC).The assistance and facilities provided by the Department ofChemical Engineering, Ryerson University, are also highlyappreciated.

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Manuscript received September 24, 2013; revised manuscriptreceived October 28, 2013; accepted for publication October 28,2013.


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