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Journal of Membrane Science 186 (2001) 41–52

Role of backpulsing in fouling minimization incrossflow filtration with ceramic membranes

Rishi Sondhi, Ramesh Bhave∗

US Filter, Ceramic Membrane Products, 1750 Filter Drive, DeLand, FL 32724, USA

Received 20 July 2000; received in revised form 13 November 2000; accepted 16 November 2000

Abstract

Effect of backpulsing on crossflow filtration of different process streams was studied. Laboratory scale experiments were

conducted with synthetic electroplating wastewater containing Cr(OH)3 suspension. Porous ceramic membranes of variouspore sizes (0.05–5.0m) were evaluated. Filtration experiments with and without backpulsing show that backpulsing is

effective in minimizing membrane fouling. Up to five-fold increase in steady-state permeate flux and 100% flux recovery

were observed. Theoretical aspects are reviewed to develop a better understanding of the critical parameters associated with

high-pressure backpulsing.

Pilot and commercial scale operating results on several industrial applications, such as yeast filtration, process slurry

filtration and oily wastewater filtration are presented. Data analysis shows the critical importance of backpulsing in reducing

long-term membrane fouling while allowing the realization of high product recovery. Optimization of process parameters

with backpulsing typically results in higher flux and reduces the total capital cost required to achieve the desired production

rate. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Microfiltration; Ultrafiltration; Ceramic membranes; Backpulsing; Fouling

1. Introduction

Transmembrane pressure pulsing or backpulsing

(BP) is an effective technique for reducing fouling

phenomenon in membranes, improving the overall

filtration rate and extending the cleaning interval

(the time between two consecutive membrane clean-

ings). Backpulsing is an in-situ method for cleaning

the membrane by periodically reversing the trans-

membrane pressure. When transmembrane pressure

is reversed, permeate liquid is forced back through

the membrane to the feed side. This flow reversal

∗ Corresponding author. Tel.: +1-904-822-8000;

fax: +1-904-822-8010.

E-mail address: [email protected] (R. Bhave).

dislodges deposited foulants, which are then carriedout of the membrane module by the tangential flow of

retentate or are redeposited on the membrane surface

[1]. It should be noted, that backpulsing is most effec-

tive in removing deposits on the membrane surface.

Should severe pore plugging occur, backpulsing will

most likely be ineffective in preventing precipitous

flux decline. This type of irreversible fouling may

only be corrected by chemical cleaning.

There are several parameters associated with back-

pulsing. Backpulse duration is defined as the amount

of time the filtration system operates under negative

transmembrane pressure. Pulse amplitude is defined

as the absolute value of maximum transmembranepressure during backpulsing. Backpulse interval is the

duration of time in between two consecutive pulses.

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 6 6 3 - 3



42 R. Sondhi, R. Bhave/ Journal of Membrane Science 186 (2001) 41–52

Nomenclature

H m membrane thickness

J permeate flux

K Kozeny–Carman constant

L membrane pore length

Rm membrane resistancer membrane pore radius

S i internal surface area per unit

volume of membrane

Greek letters

µ permeate viscosity

P transmembrane pressure (TMP)

ε membrane porosity

Backpulsing should be distinguished from the more

familiar technique of backflushing or backwashing.

The fundamental difference between a backpulse andbackwash is the speed and force utilized to dislodge

accumulated matter on the membrane surface. In

backflushing, flow reversal through the membrane oc-

curs for 5–30 s once every 30 min to several hours. In

backpulsing, flow reversal occurs every few minutes

and reverse high-pressure pulses (up to 10 bar) are

applied for very short periods of time (typically <1s).

In addition, backpulsing is a dynamic process and

introduces transient effects not found in conventional

backflushing.

A variety of backpulse devices can be used to

produce reverse flow to periodically remove accumu-

lated foulants on the membrane elements. The choiceis dependent on the size and number of modules in

the filtration loop, along with cost considerations.

Typically for each backpulse, about 0.5 l of permeate

volume is required per square meter of filtration area.

The simplest backpulse device is a pump and bladder

assembly. The bladder assembly holds the permeate

volume and includes a membrane barrier to prevent

direct contact of pressurizing air with permeate. The

pump (gear or diaphragm) is connected to the air

intake and generates the required air pressure for an

effective backpulse. A variation of this approach uses

a tank and air compressor. The tank is filled with

permeate and pressurized to 80–100 psi. The tank issized to deliver adequate volume based on the total

filtration area. The frequency is set with a timer. The

disadvantage of this type of device is the relatively

slower speed of permeate discharge, inconsistent

discharge volume and potential leaks. These factors

contribute to the lower efficiency of such devices

compared to the backpulse device used in this work.

The backpulse valve assembly containing a fixed

volume reservoir (such as that used in the laboratory

scale unit) is more effective due to the ability to deliver

consistent permeate volume at high pressure almost in-

stantaneously. Backpulse devices are available in sizes

that provide from 100 ml to 5.7 l of reverse flow. A sin-

gle assembly can be used to backpulse several modules

at one time, whether connected in series or parallel.

Backpulsing is of special significance in ceramic

membrane filtration because unlike polymeric mem-

branes, ceramic membranes are able to withstand

the high pressures associated with backpulsing.

Ceramic membranes have been used in several indus-

trial applications for more than 15 years. They are

well suited for slurry filtration, oil–water emulsionseparations, surface water filtration, aqueous cleaner

recovery, as well as food, dairy and beverage (e.g.

sugar juice clarification, milk protein concentration,

fruit juice clarification, etc.) applications [2]. Some

interesting results have been reported on the use of

ceramic membranes in crossflow filtration of proteins

[3], yeast [4] and other suspensions [5–8]. Several

research groups [9–12] have investigated the use

of backpulsing with ceramic membranes. However,

these studies were limited to bench-scale testing and

theoretical analysis of the backpulse technique. This

paper endeavors to combine the theoretical aspect of

backpulsing with laboratory and industrial data. Inaddition to bench-scale studies, we report the oper-

ating results from pilot testing and commercial scale

installations, which illustrate the importance of back-

pulse in reducing long-term membrane fouling while

allowing the realization of high product recovery.

2. Theory

During filtration, particles accumulate on the mem-

brane surface, forming a cake or gel layer. At the same

time, some particles may adsorb on or block the sur-

face pores. It is believed that the backpulsing processrestores the flux by dislodging the particles blocking

the membrane pores and those particles forming a



R. Sondhi, R. Bhave / Journal of Membrane Science 186 (2001) 41–52 43

Fig. 1. Schematic representation of membrane cleaning during

backpulsing.

cake on the membrane surface (Fig. 1). It is assumed

that the cake layer is instantly lifted and swept into the

retentate flow [12]. Thus, only particle fouling on the

membrane surface (external fouling) is considered and

internal pore plugging effects are assumed to be mini-mal [12,13]. If the above assumption is valid, then for

complete membrane cleaning, the pore-blocking and

cake-forming particles should be pushed back into

the crossflow and swept away into the retentate flow.

The apparent distance traveled by the solvent

molecule during pulsing should be greater than the

apparent pore length. Then the membrane cleaning

time, t c (the time required to dislodge the particles

from the membrane surface) can be defined as the

ratio of distance traveled by the solvent to the solvent

velocity. Considering a limiting case, when the dis-

tance traveled by the solvent is assumed to be equal to

the pore length L, it can be approximated by using the

d’Arcy’s permeability model, assuming a tortuosity

factor of 2.5 [14,15]

L = 15(εRmr

2) (1)

where µ is the membrane porosity, Rm the mem-

brane resistance and r the pore radius. The particle

velocity is governed by hindered transport in the

narrow pore. However, when the particle size approx-

imates the pore size, the average particle velocity is

equal to the average solvent velocity. This is true for

ceramic membranes as the individual ceramic parti-

cles are dense. The porosity is created by interstitialspace between the solid particles. Assuming the pores

to be interstices between close-packed spheres, the

flow through the membrane can be described by the

Kozeny–Carman equation [16]

J =ε3 P

KH mµS 2i (1 − ε)2

(2)

where J is the permeate flux, P the applied trans-

membrane pressure (TMP), µ the permeate viscosity, H m the membrane thickness, K the Kozeny–Carman

constant and S i the internal surface area per unit vol-

ume of membrane. For spherical particles and assum-

ing K = 5 [17], the above equation can be written as

J =ε3r2 P

45H mµ(1 − ε)2 (3)

The cleaning time (t c) can now be written as

t c =εRmr

2/5

ε3r2 P/45H mµ(1 − ε)2

= 9µRmH m(1 − ε)2

Pε2 (4)

From the above equation, it can be seen that the

cleaning time is proportional to permeate viscosity,

membrane resistance, membrane thickness and inver-

sely proportional to TMP.

3. Experimental

3.1. Bench-scale system

For laboratory scale studies, Membralox® (USFilter, DeLand, FL) ceramic membranes of various

pore sizes (0.05–5.0m) were used in the filtration

experiments. These tubular membranes were 250 mm

in length with internal diameter of 7 mm. Filtration ex-

periments with synthetic wastewater were performed

using a bench-top filter unit (US Filter 1T1-70) as

shown in Fig. 2. The unit was equipped with a 12 l

conical bottom feed tank, a 50 psi two-stage centrifu-

gal pump, which feeds a 250 mm membrane housing

connected with a backpulse unit. The unit incorpo-

rated a tubular heat exchanger, a paddlewheel flow

meter with analog display, six valves (V1–V6), and

three pressure gauges (PI, PO, and PP). In filtrationexperiments, the feed solution was pumped to the

filter unit and during forward filtration the pressure




Fig. 2. Ceramic membrane filtration system.

gradient was maintained by adjusting the flow rate

(using valves V1 and V2) of the feed solution.

The backpulse unit was attached on the permeate

side of the membrane housing. Fig. 3 is the schematic

of the particular backpulse device studied in this

work. During reverse filtration, a timer initiated puls-

ing. A solenoid valve, pressurized with nitrogen gas,

activated the backpulse unit by directing a nitrogen

pulse to the backpulse valve. The backpulse deviceassembly incorporates a known volume permeate

reservoir and a cylindrical piston. When the backpulse

valve is actuated, the piston moves rapidly, displac-

ing permeate and causing a flow reversal through the

membrane for a very short duration. The flow rever-

sal removes adsorbed particles and accumulated cake

from the membrane deposited during filtration. This

very short reverse filtration was followed by a longer

forward filtration period during which permeate was

collected. Permeate side pressure was measured by a

high accuracy sensor (PX 800-100GV, Omega Engi-neering, Stamford, CT) used in conjunction with data

acquisition software (Strawberry Tree, Sunnyvale,




Fig. 3. Schematic representation of N2 actuated backpulsing apparatus.

CA) and an IBM computer. Backpulse duration could

be varied between 0.5 and 30 s and the backpulse

interval could be varied between 30 s and 1 h.

3.2. Operating procedures

Synthetic wastewater was prepared for laboratory

experiments by mixing chromium sulfate (Cr2(SO4)3·

12H2O), which is the principal component of elec-

troplating wastewater, with de-ionized water and 1N

NaOH and stirred overnight to produce a homogenoussuspension. The following reaction takes place in

water to produce chromium hydroxide particles:

Cr2(SO4)3 · 12H2O + 6NaOH

→ 2Cr(OH)2 + 3Na2SO4 + 12H2O

Typical chromium sulfate concentration in the simu-

lated electroplating wastewater was about 50 mg/l and

the suspension had a pH of about 7. For the suspension

at pH 7, the diameter of the aggregates, measured by

a laser zeta-potential and particle size analyzer (Coul-

ter, Delsa 440 SX), ranged from 1.5 to 10 m, with

an average of 5m.Permeate flux through the membrane, reported as

l/m2 h (LMH), was measured by collecting the perme-

ate in a graduated cylinder and timing the collection

period. Flux was calculated as the amount of permeate

collected divided by the product of membrane filtra-

tion area and time taken to collect a known volume.

The transmembrane pressure (TMP) was calculated as

TMP = 12(P I + P O)− P P

where PI is the inlet pressure, PO the outlet pressure,

and PP the permeate side pressure.

The total suspended solids (TSS) in the feed (F ) and

permeate (P) were measured and the rejection coeffi-cient was calculated by R = 1−(P/F). All laboratory

experiments were carried out at a crossflow velocity of

5.7 m/s, unless specified otherwise, and at a tempera-

ture of 40◦C. The Reynolds number was calculated to

be well above 4000, implying turbulent flow. Permeate

was open to atmosphere (except when backpulse was

applied) such that the typical transmembrane pres-

sure (TMP) was 140 kPa (20 psi). Both permeate and

retentate were recirculated to the feed tank to maintain

the concentration of the feed (total recycle mode).

Most experiments were repeated two to four times

for each set of conditions and the results discussed

in this paper report the average values. Each experi-ment began with filtering clean tap water through the

clean membrane until permeate flux was stabilized.




Table 1

Summary of typical parameter values for laboratory filtration

experiments

Membrane characteristics

Ceramic membrane tube i.d 7 mm

Tube length 25 cm

Nominal pore size 0.05, 0.2, 0.8m

Porosity 50%Feed conditions of sulfate suspension

Solid concentration 50 mg/l

pH 7

Temperature 40◦C

Typical process parameters

Forward filtration pressure 140 kPa (20 psi)

Backpulse amplitude 170 kPa (25 psi)

Crossflow velocity 5.7 m/s

Backpulse duration 0.5 s

Backpulse interval 30 s

At this point the feed was changed to the synthetic

wastewater. Membranes were reused, and underwent amulti-step cleaning procedure involving 1% (wt./wt.)

HNO3 followed with 2% (wt./wt.) NaOH solutions.

The system was thoroughly rinsed with clean water

prior to the start of a new experiment. The membranes

were stored in dilute NaOCl to prevent bacterial

growth. Typical values of key operating parameters

are given in Table 1.

The objective of pilot testing is to obtain data suit-

able for scale-up. It is, therefore, necessary to operate

the unit for a sufficiently long period to obtain in-

formation on long-term fouling characteristics. Many

industrial applications require continuous filtration at

constant filtrate flow. The transmembrane pressureincreases as the run progresses to maintain constant

flow. For batch processes the filtrate flow is initially

high and decreases with concentration factor. The

industrial examples discussed later in the paper (see

Section 4.3) show filtration performance under both

variable flux and constant flux conditions representa-

tive of batch and continuous processing, respectively.

The backpulse time and duration are set from the

start of the run. The filtration cycle is continued until

the filtrate flow drops substantially below the starting

value, indicating irreversible fouling. At this time,

chemical cleaning is performed to remove foulants

on the membrane. The cleaning interval between twosuccessive filtration cycles is an important design

parameter. The primary purpose for pilot testing is

to establish reproducibility of process flux, cleaning

intervals and to validate the cleaning protocol.

4. Results and discussion

4.1. Laboratory filtration results

Fig. 4a–c show permeate flux versus time (with

and without backpulsing) for synthetic electroplating

wastewater filtration using 0.05, 0.2 and 0.8 m mem-

branes, respectively. It was observed that not only

was there 100% flux recovery with backpulsing, but

the steady-state flux was also two to five times greater

than average flux without backpulse. It was observed

that filtrate flux could be increased by a factor of 5

for the 0.8m microfiltration membrane. The 0.2 and

0.5m ceramic membranes also showed substantial

increase in permeate flux (up to 2.5 times) with back-

pulsing. Thus, the larger the pore diameter, the greateris the effectiveness of backpulse. These results also

show that backpulsing is effective in reducing the

fouling effect and in maintaining higher steady-state

flux. The operating conditions are given in Table 1.

In a recent study, Sondhi et al. [18] illustrated the

effect of process parameters, crossflow velocity and

TMP, on permeate flux. They reported results for

wastewater containing 50 mg/l suspended solids using

a 0.8m membrane at crossflow velocity in the range

of 3–6.5m/s and at a fixed feed pressure of 138 kPa

(20 psi). All of the operating velocities corresponded

to Reynolds number well above 4000, indicating tur-

bulent region. It was seen that, the higher the crossflowvelocity, the higher the observed permeate flux. In

general, this can be attributed to increased shear (due

to high crossflow velocity) reducing the fouling layer

on the membrane surface [3,7]. For filtration experi-

ments with the 0.8m membrane, backpulsing was

applied with a fixed pulse interval of 30 s and pulse

duration of 0.5 s. Steady-state flux reached a value

of 11250 l/m2 h (LMH) at a crossflow velocity of

6.5 m/s. Flux with backpulsing was about four times

greater than the steady-state flux without backpulsing.

This may be due to the combined effect of high cross-

flow (scouring action) and backpulsing, that helps to

remove the cake layer deposited on the membrane.Transmembrane pressure can have a significant ef-

fect on permeate flux. Sondhi et al. [18] also discussed




Fig. 4. ((a)–(c)) Filtration results with synthetic electroplating water

with and without backpulsing for (a) 0.05, (b) 0.2 and (c) 0.8m

membrane.

the effect of TMP on flux during wastewater filtra-

tion with a 0.8m membrane. TMP was in the range

of 100–175 kPa and crossflow velocity was at 5.0 m/s.

Steady-state flux increased with transmembrane pres-

sure. However, the rate of increase decreased at higher

transmembrane pressure of 172 kPa and the flux ap-

peared to level out. This phenomenon could be at-

tributed to the fact that at higher operating pressure,

the effect of fouling is more important as the cake

layer thickness on the membrane increases. Sondhi

et al. [18] reported that when backpulsing was applied,

the steady-state flux reached a maximum of 12,300

LMH at a TMP of 170 kPa, which was about 2.6 times

greater than the nonpulsed steady-state flux at the same

conditions. It should be noted that the high flux val-

ues reported in this study are not indicative of the flux

values achieved in typical wastewater treatment appli-

cations. These values are high due to the dilute nature

of synthetic samples. However, dilute feed systems are

ideally suited for fundamental analysis and modelingof backpulsing.

4.2. Laboratory backpulse results

Fig. 5 shows typical permeate pressure variation

during backpulsing. This particular run was done with

Fig. 5. Typical permeate side pressure profiles during a backpulse

period for a 0.2m membrane showing two distinct phases: flow

reversal (permeate is forced to the feed side) and refilling (permeate

refilling of the membrane module).




a 0.2m membrane, pulse duration of 0.5 s, and pulse

interval of 30 s. It can be seen from the pressure versus

time profile that the backpulse process consists of two

distinct phases. In the first phase, pressure is applied

on the piston which forces permeate back through the

membrane. This represents the steep rise in the perme-

ate side pressure as shown in the curve. It is believed

that reverse flow and hence, membrane cleaning takes

place during this time. In the second phase, the piston

remains at the extended position for some time to al-

low the cylinder to refill with permeate as the piston

returns to the original position before the next pulse

(refilling time). The backpulse cycle is followed by

a longer forward filtration cycle. Levesley and Hoare

[10] observed similar permeate side pressure trends

during ceramic membrane filtration of yeast suspen-

sion with backpulsing.

Figs. 6 and 7 show the comparison between the

theoretical (solid lines) and experimental (symbols)

membrane cleaning time as a function of backpulseamplitude (maximum transmembrane pressure during

reverse flow phase of backpulse cycle) and membrane

pore size (diameter), respectively. The experimen-

tal cleaning time was calculated from the permeate

side pressure variation data during the backpulse

period. It is assumed that the membrane cleaning

takes place during the initial phases of backpulse

(flow reversal). Experimental data shown in Fig. 6

were obtained for synthetic wastewater filtration with

Fig. 6. Predicted and experimental cleaning time (t c) vs. pulse

amplitude for a 0.2m membrane.

Fig. 7. Predicted and experimental cleaning time for different pore

size membranes.

a 0.2m membrane (BP duration = 0.5s and BP

interval =

30 s) at different backpulse amplitudes.Experimental data shown in Fig. 7 were obtained

for synthetic wastewater filtration with different pore

diameter membranes (BP duration = 0.5s and BP

interval = 30 s).

The theoretical cleaning time (Eq. (4)) was calcu-

lated by estimating membrane resistance ( Rm) from

clean water filtration data and assuming that the resis-

tance to flow is controlled by the top and the interme-

diate layer of the membrane having an approximate

thickness ( H m) of 50m [2,19]. Although membrane

pore size does not directly influence cleaning time, it

affects membrane resistance ( Rm). The larger pore di-

ameter membrane has a lower resistance to flow. Theapplied transmembrane pressure (P) was estimated

by the pressure sensor on permeate side. The mem-

brane porosity (ε) was assumed to be 50% [2,19].

Typically, the porosities of membrane layer and sup-

port range from about 35 to 50% depending on the

pore size and ceramic material properties.

Membrane cleaning time decreases with an increase

in backpulse amplitude and membrane pore diam-

eter. Increasing the pulse amplitude and membrane

pore diameter probably results in better and quicker

cleaning and hence, less time is required. Note that

the zirconia ultrafiltration membrane (0.05 m) has a

different cleaning time due to its different membranestructure compared to the alumina microfiltration

membranes (0.2, 0.8 and 5.0 m). It can be seen from




Figs. 6 and 7 that, there is a close agreement between

the theoretical and experimental cleaning times for

different backpulse amplitudes and membrane pore

sizes. It can be inferred that the model is applicable

to both ultrafiltration and microfiltration processes.

The operating conditions are given in Table 1. A

minimum pulse amplitude of about 170 kPa (25 psi)

was necessary for effective membrane cleaning and

to obtain an increase in flux. It was observed that for

pulse amplitude <170kPa, the force acting on the

piston was too small to initiate backpulsing. Rodgers

and Sparks [20] also reported that for short pulses

with low amplitude, significant flow reversal was

not observed during ultrafiltration of totally retained

solutes using transmembrane pressure pulsing.

Figs. 8 and 9 show the membrane cleaning time as

a function of pulse duration and interval, respectively.

The experimental data for Fig. 8 were obtained with

a 0.2m membrane at feed concentration of 50 and

80 mg/l, and pulse duration ranging from 0.5 to 3 s.The experimental data for Fig. 9 were obtained with

a 0.2m membrane, feed concentration of 50 and

80 mg/l, and pulse intervals ranging from 30 to 160 s.

No definite trends were observed in this study and it

can be said that the feed concentration did not show

any significant effect on the membrane cleaning time

as predicted by Eq. (4) (solid lines). Small increases

Fig. 8. Membrane cleaning time versus pulse duration for a 0.2 m

membrane, feed concentration of 50 and 80 mg/l. See Table 1 for

operating conditions.

Fig. 9. Membrane cleaning time versus pulse interval for a 0.2 m

membrane, feed concentration of 50 and 80 mg/l. See Table 1 for

operating conditions.

in cleaning time were observed when the pulse

interval was increased. This can be expected as longer

pulse intervals allow greater time for the accumu-

lation of foulants on the surface thus, increasing

the potential to foul the membrane. Further exper-

imentation is necessary to verify these preliminary

findings. In particular, it will be useful to study the

effect of higher concentration on membrane cleaning

time (t c).

4.3. Industrial scale

4.3.1. Effect of backpulse on the filtration of dilute

yeast suspensions

The separation of cells from fermentation broth

is a very important unit operation in the recovery of

fermentation products. Membrane processes are gen-

erally preferred over centrifugation or rotary drum

filtration since they minimize product losses. Typi-

cally, filtration rates are strongly dependent on broth

concentration and TMP. Bhave [5] has discussed the

effect of concentration factor and TMP on flux for

fermentation broth filtration.

Matsumoto et al. [9] studied the effect of back-

pulse on the filtration of dilute yeast suspensions(prepared from commercial packed wet baker’s yeast

to simulate fermentation broth) using the 0.2m




Membralox® ceramic membrane. The filtration

surface area was 0.2 m2. The concentration of yeast

suspensions ranged from 8 to 30 g/l (dry basis). In

the absence of backpulsing, the membranes fouled

rapidly and flux dropped from 1100 LMH to about

40 LMH within the first hour of filtration. Backpulse

frequency was varied to evaluate its effect on main-

taining flux. With a backpulse interval of 1 min an

average flux of about 900 LMH could be sustained

compared to about 600 LMH at backpulse interval

of 5 min. The significant impact of backpulse on

flux is clearly evident. Typically, it is seen that a

higher pulse frequency significantly improves flux.

However, this would also depend on the net flux.

At low flux values, high cleaning frequency cannot

be used because the amount of permeate generated

may not be adequate to maintain the effectiveness of

backpulsing.

4.3.2. Effect of backpulsing on the filtration of

process slurries

The filtration of process slurries containing high

concentrations of solids, which may be abrasive in na-

ture, is a prominent industrial application for ceramic

membranes. These process slurries often contain a

variety of metal (e.g. iron, copper, tungsten, alu-

minum) compounds and organic constituents (e.g. oils,

pigments, wetting agents). The effect of backpulsing

on the filtration rate of metallic slurry was demon-

strated on the 50 nm Membralox® ceramic ultrafil-

tration membrane (P37-30, 3 mm channel diameter).

Two filtration skids were operated in parallel each

containing four modules with a total filtration area of 106m2.

An important advantage of using tubular multichan-

nel membranes was that recoveries greater than 97%

could be achieved. This corresponds to a volumetric

concentration factor (VCF) of about 40. Tests were

performed to investigate the effect of concentration

factor on flux, with and without backpulse. Approx-

imately 200,000 gal of slurry was processed, batch-

wise, per day. The initial suspended solids ranged

between 0.5 and 1% (by volume). At VCF of 40, the

average flux without backpulse was about 305 LMH.

With backpulsing, the average flux increased to about

365 LMH representing a 20% increase (Fig. 10).This is particularly significant given the concentrated

nature of slurry at VCF of 40.

Fig. 10. Effect of backpulsing on oily wastewater.

4.3.3. Effect of backpulsing on the filtration of oily

wastewater

The treatment of oily wastewater to remove oils

and suspended solids with ceramic membranes is well

established. The wastewater typically contains emul-

sified oils that are difficult to separate with conven-

tional treatment technologies such as coalescers and

oil skimmers. Furthermore, many polymeric mem-

branes are unsuitable due to their limited stability in

aggressive chemical environments such as highly con-

taminated oily wastewater (e.g. lube oils, petroleum

fractions). In the petrochemical industry, the treat-

ment of produced water with ceramic membranes was

demonstrated in a number of land-based and offshore

locations [21]. In this application, the produced wateris pretreated with coagulants to improve filtration

rates. Permeate is free of oils, grease and suspended

solids and is suitable for reuse, discharge or deep-well

injection.

Flux stability using backpulsing is dependent on

pretreatment, membrane selection and operating flux.

If the operating flux is set too high, the cleaning inter-

val is considerably shorter compared to that realized

at lower operating flux. Chen et al. [21] studied the

produced water filtration characteristics with 0.2 and

0.5m Membralox® ceramic membranes. In the ab-

sence of backpulsing and feed pretreatment, the runs

lasted only a few hours. When backpulse was initi-ated, with pulse interval in the range of 1–2 min, the

0.2m membrane fouled in <24 h at high operating




flux value of 1600 LMH. However, with 0.5m

membrane and backpulsing (at 2 min interval), run

duration increased to about 48 h with the same oper-

ating flux of 1600 LMH. The cleaning interval can be

greatly increased if operating flux is decreased to a

sustainable range. This may be accomplished by con-

trolling the rate of increase of membrane resistance

(membrane fouling) over time.

Long-term flux stability was demonstrated on a

commercial scale installation for the treatment of

produced water containing 10–100 ppm oils and sus-

pended solids using 0.8m Membralox® membranes

[22]. The 16 module filtration system (∼115m2 fil-

tration area) handles a continuous flow rate of about

850m3 per day. It was observed that the operation

was extremely stable and an average flux of 300

LMH was maintained over several months. It should

be noted, however, that a typical cleaning interval for

most commercial microfiltration and ultrafiltration

applications ranges between less than a week up to4 weeks.

5. Conclusions

The role of backpulsing is clearly demonstrated

with experimental data on the bench-scale, pilot

scale and large-scale filtration systems using ceramic

membranes. The paper also describes the theoretical

aspects governing the process of backpulsing and pro-

vides analysis of the critical parameters influencing

the effectiveness of backpulse. The results on the fil-

tration of synthetic electroplating wastewater on labo-ratory scale modules show that the filtrate flux can be

increased by a factor of 5 for the 0.8 m membrane.

The 0.2 and 0.5m ceramic membranes also show

substantial increases in permeate flux up to 2.5 times

with backpulsing. Thus, the larger the pore diameter,

the greater is the effectiveness of backpulsing. Per-

meate flux also increased with increasing crossflow

velocity and transmembrane pressure, in the presence

of backpulsing. The dependence of membrane clean-

ing time on backpulse parameters (amplitude, fre-

quency, and duration) and membrane pore size is also

discussed.

Several industrial applications are discussed thatfurther support the beneficial effects of backpulse in

reducing long-term membrane fouling and increasing

the cleaning interval. Filtration experiments for dilute

yeast suspensions with the 0.2m membrane show

that high flux of about 900 LMH can be sustained at

backpulse interval of 1 min. In the absence of back-

pulse the flux decreases rapidly in the first 15 min of

filtration. For process slurry filtration, it was shown

that backpulse increased flux by 20–25% at concen-

tration factor of 40 (∼97.5% recovery). The industrial

scale filtration of oily produced water showed that the

flux stability and cleaning interval are strongly depen-

dent on not only backpulse but also on the membrane

pore diameter and the absolute value of flux. At low

flux values of about 300 LMH, it was shown that clean-

ing interval could be extended to several months using

a 0.8m membrane. On the other hand, if high flux

values are desired, the membrane cleaning frequency

could drastically increase.

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