the effect of membrane module configuration on extraction efficiency in an extractive membrane...

E L S E V I E R Journal of Membrane Science 128 (1997) 231-242

j m t r n ~ o f MEMBRANE

SCIENCE

The effect of membrane module configuration on extraction efficiency in an extractive membrane bioreactor

L . F . S t r a c h a n , A . G . L i v i n g s t o n *

Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine~ Prince Consort Road, London SW7 2BY, UK

Received 6 March 1996; received in revised form 4 November 1996; accepted 5 November 1996

Abstract

The extractive membrane bioreactor (EMB) is a new technology for the treatment of wastewaters containing poorly water- soluble organic compounds. It consists of a bioreactor linked to membrane modules which contain bundles of silicone rubber tubes. Wastewater is pumped through the tube lumens and the biomedium is circulated around the shell side. This paper describes two simple mathematical models developed to compare EMB flow configurations and the results of experiments performed to test the models. We show that tube side Reynolds number has a significant effect on the mass-transfer coefficient of the test pollutant (monochlorobenzene) through the membrane. We also show that, on scale up, for a constant wastewater membrane residence time, an EMB consisting of three membrane modules with tube side recycle flows ~40 times the wastewaster flow will have a higher extraction efficiency than an equivalent EMB with plug flow inside the module.

Keywords: Extractive membrane bioreactor; Waste detoxification; Mass transfer

1. Introduction

Current pressure to lower emissions of toxic wastes into the environment is leading many chemical man- ufacturers to reconsider the way in which wastewaters are treated. Particular problems are encountered with poorly water-soluble, volatile organic compounds, which can be difficult to treat by conventional methods [ 1]. Mixed bacterial cultures, such as those found in activated sludge, are often unable to degrade such compounds. However, bacterial strains have been isolated which are capable of full degradation of many of these otherwise recalcitrant molecules [2-4]. The

*Corresponding author.

0376-7388/97/$17.00 ~(~ 1997 Elsevier Science B.V. All rights reserved. P l l S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 3 2 4 - 9

treatment of individual wastewater streams containing these molecules is one way in which such bacteria could be applied directly to the compounds which they are able to degrade. However, biological treatment of such streams can be adversely affected by the extremes of pH and ionic strength commonly found in such 'point source' wastewaters.

The extractive membrane bioreactor, or EMB [5], overcomes this problem. The wastewater stream and a biomedium are kept apart by means of a silicone rubber membrane (Fig. 1). Poorly water-soluble organic compounds can pass rapidly through the non-porous silicone rubber membrane, while ionic species in the wastewater stream cannot. Having passed through the membrane, the organic compounds

232 L.F. Strachan, A.G. Livingston./Journal of Membrane Science 128 (1997) 231-242

tube ~ shell side side~

inorganic species I remain in wastewater ]

organic molecules diffuse across mei into biofilm

WASTEWATER silicone rubber membrane

\ BIOMEDIUM

biofilm

Fig. 1. Principle of the extractive membrane bioreactor.

can be degraded by microorganisms which have been specifically chosen for their ability to carry out degradation of the particular organics present in the wastewater.

The EMB process has been used extensively at laboratory scale [6,7] to treat a number of simulated and industrially produced wastewaters. Laboratory scale EMBs employed to date have consisted of a single shell-and-tube membrane module with the wastewater flowing in a single pass through the inside of the tubes in plug flow, at Reynolds number (Re) <12. Recent studies [8] have shown that, for hydrophobic compounds which have high silicone rubber- aqueous phase partition coefficients, the major resistance to mass transfer lies in the fluid films, and the flow rate on the tube (wastewater) side of the membrane can have a significant effect on mass transfer and hence on the operational efficiency of the EMB. Under plug flow conditions, low wastewater Reynolds numbers mean low mass-transfer coefficients, and thus large modules with high membrane areas. An alternative is to have a number of membrane modules connected in series with respect to wastewater flow, each module having an internal recycle of wastewater on the tube side (Fig. 2). If the recycle rate is suffi- ciently high, each module then operates essentially as a continuously stirred tank reactor (CSTR), with no significant axial concentration gradients in the tubes. The aim of this research was to compare pollutant removal in each of the two alternative membrane

module configurations, using both mathematical models and experimental data.

Mathematical models have been developed in which an infinite number of CSTRs in series is used to appro- ximate a plug flow system [9]. As the number of CSTRs is increased, the approximation improves. Operation of a membrane module as a CSTR with an internal recycle of the wastewater will, however, increase the tube side flow rate without increasing the overall flow rate of wastewater through the EMB. At higher tube side flow rates, the fluid film resistance will be much reduced and so operation of an EMB with modules in a CSTRs-in-series configuration can improve the mass transfer of an organic pollutant across the membrane. It is therefore proposed that a CSTRs-in-series configuration will not simply app- roach the plug flow system as the number of CSTRs is increased, but will, in fact, have superior performance.

This paper describes two simple mathematical models of EMB performance, one based on a plug flow configuration and one on a CSTRs-in-series configuration. The effect of the tube side wastewater flow rate on the predicted removal efficiency was investigated using these models. Experiments were performed using an EMB with three membrane modules operating either with or without internal recycles of the wastewater stream, The organic pollutant used in the experimental work was monochlorobenzene (MCB). MCB is toxic [10] and is included in the North Sea Treaty reference list of priority pollutants,

L.E Strachan, A.G. Livingston./Journal of Membrane Science 128 (1997) 231-242 233

2 3

$ wastewater in ~ •

(a)

i n W a S t e w a t e T ~ l ~ , ~ weir______~ wastewater box out

I : wastewater [~ out membrane [ ~ " - 1 : 1

modules

(b)

Fig. 2. Three module EMB showing plug flow (a) and CSTRs-in-series (b) modes of operation (biomedium streams not shown).

making techniques for the treatment of MCB-containing wastewaters important. It is hydrophobic and has a strong affinity for the silicone rubber phase; the partition coefficient between silicone rubber and water for MCB is 70, compared, for example, to a value for phenol of 0.5 [8]. Thus, mass transfer of MCB is controlled by fluid film resistances, making it an appropriate test compound for this study. MCB degradation on the shell side of the membrane was performed by Pseudomonas strain JS150 [11].

2. M a t h e m a t i c a l m o d e l

2.1. Development o f the mathematical model

Two simple mathematical models were developed to evaluate plug flow versus CSTRs-in-series configurations in an EMB, as shown in Figs. 2 and 3. The EMB consisted of three membrane modules, in which the wastewater streams could be linked such that the three modules operated either as a single pass plug flow system (Fig. 2a) or as three separate modules with internal recycle streams (Fig. 2b). Pseudomonas strain JS 150 readily utilises MCB as a sole carbon and energy source [11], and it was assumed that in the

presence of a biofilm on the shell side of the membrane, any pollutant crossing the membrane was rapidly degraded. This assumption has been verified experimentally by measuring the flux of MCB across a short length of membrane while simultaneously recording the biofilm thickness [12]. Shell side MCB concentration was therefore assumed to be near zero. Pseudomonas strain JSI50 converts between 60 and 80% of the MCB carbon into carbon dioxide rather than biomass [13], and thus biofilm growth is slow. The thickness of the biofilm was therefore assumed to be approximately constant and to have little or no effect on the system under investigation.

2.2. Plug f l o w model

For a plug flow system, a mass balance over a small length of membrane tube, dx, gives:

- F d S = k(S - C) • 7rdm. d_~ ( 1 )

where F is the wastewater flow rate, S is the pollutant concentration inside the membrane tube, d is the tube internal diameter, C is the pollutant concentration outside the membrane tube, m is the number of silicone rubber tubes in the module and k is the overall mass-transfer coefficient. Assuming C=O, (pollutant


wastewater in

exha .., box gas - [ ~ n u t r i e n t [ [

biomedium ~ I I _ . . f e e d . . . . . / .

o.t / I . r . . . . . . .

airlift ~ . o membrane "~ bioreactor ° ° modules

O o

0 - o 0

o o l 0

o

° ~ ,

air in

w e l r _ _ _ _ ~ ~

1

* i l l l l ~ [ ]

I I"

biomedium recycle

• wastewater

~. out

3"' 1]

2 3

i

Fig. 3. Diagram of apparatus (modules shown in CSTRs-in-series configuration).

being rapidly degraded outside the membrane tube), and since A = 7rdLm :

Sout : e_kA/F (2) Sin

where Sin is the inlet concentration of pollutant in the wastewater and Sou t is the outlet concentration of pollutant in wastewater.

2.3. CSTRs-in-series model

For a single CSTR (or a membrane module in which the wastewater is well-mixed), a mass balance on pollutant over the system gives:

F(Sin - Sout) ~--- kASout (3)

For n modules, of equal membrane area, in series, assuming that F and k are the same for each module, with a total membrane area for all the modules of A:

Sin ~ + 1 (4)

If k is assumed to be the same in the CSTRs-in- series modules and the plug flow module, it can easily

be seen that as n---~e~, the CSTRs-in-series model (Eq. (4)) approaches the plug flow model (Eq. (2)), since In(1 + x) ~ x for small values of x; and therefore, from Eq. (4):

In = - n l n ~-ff + 1 (5)

-nkA -kA "~ nF as n ~ oc F (Eq.(2))

2.4. Effect of f low rate on mass-transfer coefficient

The overall mass-transfer coefficient, k, can be divided into a tube side liquid film resistance and a membrane resistance, and is given by:

I 1 [Rinln(Rout/Rin)] = k-T + L D--~mK ] (6)

where kT is the tube side liquid film mass-transfer coefficient, Rin is the inside radius of the membrane tube, Rout is the outside radius of the membrane tube, Dsm is the diffusion coefficient of the substrate in the membrane and K is the partition coefficient of the

L.F. Strachan, A.G. Livingston./Journal of Membrane Science 128 (1997) 231-242 235

substrate between the membrane and the aqueous phases.

Eq. (6) has no term for mass transfer on the outside of the membrane tube (i.e. through the biofilm), since MCB is assumed to be consumed instantaneously after crossing the membrane and, as mentioned previously, the concentration of substrate is assumed to be neg- ligible at the membrane/biofilm interface.

The flow rate inside the membrane tubes has an effect on the mass-transfer coefficient via the liquid film coefficient (kx). Under laminar conditions, such as those used in the following experiments, the relationship between Sherwood number (Sh) and Rey- nolds number (Re) is often described by a modification of the L~v~que correlation [14,15]:

I

Sh = 1.62(Re)3(Sc)3 (7)

Expanding the terms, we obtain:

1 1

kv = 1.62(Re)-~ (8) t s_,) t a )

In a plug flow configuration, the effect of an increase in wastewater flow rate (F) will be to increase Sout/Sin. This happens because wastewater residence time in the membrane tubes is reduced. However, the increase in Sout/Sin is moderated to some extent by a corresponding increase in the overall mass-transfer coefficient due to the higher tube side velocity.

In a CSTRs-in-series configuration, the recycle flow rate (Q) can be more than one order of magnitude greater than the wastewater flow rate (F). Under these

conditions, the increase in wastewater flow rate increases Sout/ain due to the decrease in wastewater residence time in the membrane tubes; however, there is no significant improvement in mass-transfer coefficient, since in this case, tube side Reynolds number remains approximately constant. Values used in cal- culating model results are given in Table 1.

3. Materials and methods

3.1. Microorganisms

Pseudomonas strain JS150, obtained from J.C. Spain and S. Nishino and described by Haigler et al. [11], was used for MCB degradation. The original organism was isolated from a mixture of sewage samples collected at the Tyndall Air Force Base and Panama City, both in Florida, USA. The JS150 strain was derived by Haigler et al. (ibid.) from a strain originally isolated for its ability to degrade p-dichlor- obenzene and was reported to degrade monochlorobenzene in small scale fixed bed cultures. It was obtained in freeze-dried form and was reconstituted using brain-heart broth (Merck, UK) containing 1% yeast extract (Sigma, UK), and then grown in 250 ml shake flasks containing 100 ml minimal nutrient medium and 100 mg 1-1 MCB before transfer to the EMB. The minimal nutrient medium contained the following compounds:MgSO4.7H20(224 mg 1 1),ZnSOa.7H20 (10mg 1 - l ) Na2MoOa-2H20 (5 mg l-l) , K H 2 P O 4

(680 mg 1-1), Na2HPO4.7H20 (1340 mg 1--I), Ca(H2 PO4)2 (59 mg 1 1), FeSO4 (0.44 mg 1 i), and (NH4)2 SO4 (2500 mg l-F).

Table 1

Value used in mathematical modelling

Variable Value

# 0.8×10 3Pas

t) 1 0 0 0 kg m - 3

D~w 8.75×10-1Om2s i a

d 3×10 3m

L 6 m (plug flow) or 2 m (CSTRs-in-series) D~m 1×10 l ° m 2 s Ib

K 70 Rin 1.5×10 3 m

Rout 1.9×10 3 m

a Calculated using the Wilke-Chang estimation method [14]. b From Brookes and Livingston [8].

3.2. Assays

MCB concentration in the wastewater, exit gas and biomedium was analysed using a Perkin-Elmer gas chromatograph with a flame ionisation detector and a megabore column, 25 m long and 0.23 mm i.d., with BP1 (SGE, Australia) as the stationary phase. 1 ~1 samples were injected directly into the column. The temperature program ran from 40 to 120°C. Peak areas were compared with those for solutions of known MCB concentration. The uncertainty in this assay (quoted as the standard deviation of three separate determinations at the 100 mg 1 - I level) was 4.2%. The

236 L.E Strachan, A.G. Livingston./Journal of Membrane Science 128 (1997) 231-242

detection limit was 0.2 mg 1-1 MCB. The bioreactor exit gas was also analysed in this way with a sample gas volume of 1 ml.

Chloride release during MCB degradation was fol- lowed using the colorimetric chloride ion assay as described by Iwasaki et al. [ 16]. The uncertainty in this assay (quoted as the standard deviation of five separate determinations at the 20 mg 1 ~ level) was 4%.

The extraction efficiency is defined as:

Sin -- Sout Extraction efficiency -- - - × 100% (9)

Sin

3.3. Chloride balance

The expected chloride release was calculated from the amount of MCB which crossed the silicone rubber membrane, i.e.:

35.5 Expected chloride (mg h -1 ) : (Sin - Sout) × F × 112.---5

where 35.5/112.5 is the proportion of chloride in MCB. The measured chloride release was calculated directly from measurements of chloride concentration in the biomedium, as follows:

Measured chloride (mg h - l ) : ([Cl-]t~ - [C1-]tl × V T

where [C1-]t I and [C1-]t 2 are the biomedium chloride 1 concentrations (mg 1 ) at times t~ and tz, respectively,

Tis the time elapsed between tl and t 2 (h) and Vis the volume of the bioreactor plus the volume of the biomedium lost in the biomedium overflow in time t2-q (1).

3.4. Apparatus

The EMB is shown in Fig. 3. The airlift bioreactor was made of glass, wiph an internal diameter of 120 mm and a total volume of 2.5 1. Air entered through a sintered glass disc at the base of the reactor and flowed up through the draft tube, creating an airlift effect before exiting at the top. Nutrients were pro- vided continuously to the biomedium at a rate of 0.069 1 h-a and the excess biomedium left the reactor at the same rate. A pH controller held the pH in the biomedium at pH 7.0:k0.05. A temperature controller, with a thermistor and a heating element, kept the temperature in the biomedium constant at 30°C.

The membrane module shells were constructed from QVF glass sections (Coming, UK). The membrane itself consisted of a bundle of twenty silicone rubber tubes (2 m long, 3 mm i.d., 0.4 mm wall thickness) held together at top and bottom in an ABS plastic collar. Wastewater was pumped through the insides of the tubes and the biomedium was recirculated around the shell side of the module.

Recirculation of wastewater within each membrane module was accomplished by a perspex weir box. For each module, wastewater was circulated out of the associated compartment of the weir box, up through the membrane module and back into the same compartment of the box. Weirs between successive sections allowed the recycle flow rates to be manipulated independent of the overall wastewater flow rate.

Tubing throughout the system was Teflon in order to minimise the losses of MCB through the tubing walls. Connections between sections of Teflon were made using flexible Viton tubing.

3.5. Membrane resistance

The overall mass-transfer coefficient for MCB through the silicone rubber membrane is assumed to be dominated by the wastewater liquid film term. This assumption can be validated for this experimental system by considering the membrane resistance, given in Eq. (6) as:

Membrane resistance = Rinln(Rout/Rin) (10) DsmK

Using Rin and Rout values for this experimental system, and taking values of K and Dsm for MCB from Brookes and Livingston [8] of 70 and 1.8× 10 -~° m 2 s -~, respectively, the membrane resistance is equal to 21 705 m -~ s.

4. Results

A chloride balance is shown in Fig. 4. This enables us to compare the amount of chloride produced through degradation of MCB with the expected production based on the quantity of MCB extracted across the membrane. Variations in inlet MCB concentration, due to the volatility of the pollutant and its consequent

E @

o H

= @

"o o H

t . . .

@

L.F Strachan, A.G. Livingston./Journal of Membrane Science 128 (1997) 231-242

140

120

100

80

60

40

20

b

,,O

- - o , - - e x p e c t e d

-- m e a s u r e d

I i I i I

0 l0 20 30 40 50 60

237

Time (days)

Fig. 4. Chloride balance.

loss from the feed solution, made the balance difficult to close, since the system was not at steady state. However, a reasonable level of agreement is obtained between expected chloride evolution (calculated from the amount of MCB extracted through the membrane from the wastewater) and measured chloride evolution. This verifies that biodegradation (as opposed to air stripping, adsorption to apparatus, etc.) was the dominant MCB removal mechanism.

Fig. 5 shows the relationship between the tube side Reynolds number and extraction efficiency for an EMB, containing three membrane modules in series. It can be seen that the extraction, obtained experimentally, was superior to that predicted by the model in all cases, with the exception of the CSTRs-in-series results at low Reynolds numbers. At low tube side Reynolds numbers, CSTR-in-series performance is worse than the performance of the plug flow set-up (Re=8).

Fig. 6 shows the relationship between the mass- transfer coefficient (k) and Reynolds number, as predicted by the L6v~que correlation, and as obtained experimentally, from data for single membrane modules. The L~v~que correlation underestimates the improvement in mass transfer brought about by an increased Reynolds number. It can also be seen that, taking a typical experimental mass-transfer coefficient for the experiment of 4x 10 - 6 m s ~, the membrane resistance of 21 705 m i s makes up only 8% of the total resistance to mass transfer.

Fig. 7 shows the predicted effect of wastewater flow rate on both plug flow and CSTRs-in-series configurations. It can be seen that, as expected, in both cases an increase in flow rate results in a predicted reduction in extraction efficiency. A plug flow configuration will always be superior to a CSTRs-in-series configuration when Q=0. However, the model predicts that if a recycle stream is introduced, with Q>10 F, the extrac-


100

"-" 95

o~

o ,m

¢~ 90

85

Ii

i M e a n E x p e r i m e n t a l Resu l t s (P lug F l o w )

M e a n E x p e r i m e n t a l Resu l t s ( C S T R s )

0 I O0 200 300 400 500 600

Re (tube side)

Fig. 5. Relationship between tube side Reynolds number and extraction efficiency for an EMB containing three membrane modules in series.

tion efficiency of the CSTRs-in-series configuration is superior at all except very low wastewater flow rates.

5. Discussion

The effect of tube side Reynolds number on extraction efficiency is predicted to be such that, in a CSTRs- in-series configuration, an increased Re results in an increased extraction efficiency, due to the effect on the tube side liquid film coefficient. This prediction is supported by the low membrane resistance calculated for this experimental system. Fig. 5 depicts this relationship as predicted by the CSTRs-in-series model, and also shows the experimental data obtained for both plug flow and CSTRs-in-series operation, using three membrane modules. It can be seen that the use of the L6v~que correlation in the mathematical model tends to underestimate the effect of Re on extraction efficiency. It can also be seen that, as predicted, the

extraction efficiency under plug flow conditions is superior to the results for CSTRs-in-series at Re<500. This result is borne out by Fig. 7, which compares the extraction efficiency of CSTRs-in-series versus plug flow configurations. At zero recycle flow rate, the performance of the CSTRs-in-series configuration is inferior to that of the plug flow configuration; however, as the recycle ratio is increased, the CSTRs-in-series configuration outperforms the plug flow configuration.

The comparison of predicted and experimental mass-transfer coefficients is shown in Fig. 6. It is immediately clear that the L6v~que correlation under-predicts the effect of Reynolds number on mass-transfer coefficient. This also explains why the experimentally observed extraction efficiency is greater than that predicted by the mathematical models; the problem with the models is their basis on the inaccurate (in this case) L6v~que correlation. The L~v~que correlation was originally developed for heat transfer and was subsequently adapted


1.0e-5

"7

8.0e-6

6.0e-6

4.0e-6

i 2.0e-6

0.0e+0

o Leveque Prediction • Mean Experimental Results (Plug Flow)

Mean Experimental Results (CSTRs) •

1

0 100 200 300 400 500 600

Re (tube side)

Fig. 6. Relationship between tube side Reynolds number and overall mass transfer coefficient for a single membrane module.

for mass-transfer applications. It has a number of limitations. The correlation holds only for laminar flow (a condition which is, in fact, satisfied in this research) and when:

Q - - < 400 (11) DswL

i.e. in the tube entrance region, with fully developed flow but without a fully developed concentration profile. This condition is not satisfied in the EMB, since at Re=50, Q/DswL = 7400, while at Re=600, Q/DswL = 89500. However, one would expect lower, not higher, mass-transfer coefficients in the fully developed region [15]; hence, this does not explain the poor agreement. The mass-transfer improvement observed may be due to factors intrinsic to the experimental system employed, such as the use of a diaphragm pump for the wastewater recycling stream which produces pulsation in the tube side wastewater flow rate. The pulsing nature of the flow makes it

unlikely that there was anything like a laminar profile established in the tubes, and it may aid mass transfer through the production of eddies in the flow. However, the diaphragm pump was only used in the CSTRs-in- series configuration, and hence this does not explain the significant difference between the predicted and actual extraction efficiency under plug flow conditions, for which a peristaltic pump with a smoother flow pattern was used.

Both models assume that the presence of biofilm and its thickness have no effect on the extraction process, and previous experiments with Pseudomonas JS150 showed little biofilm growth due to the con- version of the bulk of the MCB carbon into carbon dioxide [13]. In the present work, however, as shown in Fig. 6, an increase in mass transfer of ca. three times is observed when Re is increased, from 2.5× 10 to 7.9×10 6 m s 1; this would result in a three-fold increase in pollutant flux, and therefore a thicker biofilm which exerts a greater resistance to mass

2 4 0 L.F. Strachan, A.G. Livingston./Journal of Membrane Science 128 (1997) 231-242

100

80

4o

20

0

0.0e+0

o

v

Plug Flow Model (Q = 0)

CSTRs-in-series Model (Q = 0)

CSTRs-in-series Model (Q = 10F)

CSTRs-in-series Model (Q = 20F)

1.0e-6 2.0e-6 3.0e-6 4.0e-6 5.0e-6 6.0e-6

Flow Rate (F) m3/s

Fig. 7. Predicted relationship between flow rate and extraction efficiency for an EMB containing three membrane modules in series. Plug flow and CSTRs-in-series models both shown, with the CSTR recycle ratio (Q) at 0, 10 or 20. Total membrane surface area=0.9 m 2.

transfer might be expected. Work performed in this group, using 1,2-dichloroethane as the model pollutant and a Xanthobacter strain as the degradatory organism, suggests that the presence of a very thin biofilm (<50 ~tm) enhances the flux of the organic compound across the membrane, while a thicker biofilm results in a reduction in flux [17]. However, with Pseudomonas JS 150 and MCB, the development of biofilms has no apparent effect on flux. Further recent work in our group [12] has showed that even with tube Reynolds numbers of 8000, there is no appreciable fall in MCB flux over time with the Pseudomonas biofilms. This is assumed to be due to faster reaction kinetics of the Pseudomonas, coupled with a looser, less dense biofilm than is observed with Xanthobacter. In any case, assuming a shell side MCB concentration of zero will have broadly the same effects on either the plug flow or CSTRs-in-series models, and so even if it is not

entirely accurate it does not affect the comparison between the two configurations made in this paper.

During operation in plug flow mode, tube side Reynolds number is directly dependent on the flow rate of wastewater through the EMB. However, if the EMB is instead operated in the CSTRs-in-series configuration, the presence of a recycle stream allows tube side flow rate, and hence Reynolds number, to be varied independent of overall wastewater flow rate. The predicted relationship between wastewater flow rate and extraction efficiency is shown in Fig. 7. In all cases, as wastewater flow rate increases, efficiency decreases, since mass-transfer coefficient is modelled as being proportional to Re 1/3. As tube side recycle ratio is increased in the CSTRs-in-series model, the result is that this configuration becomes superior to plug flow.

It is predicted that tube side flow rate will be critical in determining the efficiency of the EMB when treat-


ing a wastewater containing compounds, such as MCB, which have a high partition coefficient in silicone rubber. This prediction is borne out by the results of the experimental work and explained by a comparison of the value of the membrane resistance with the overall resistance to mass transfer. Membrane resistance in this system can be seen to account for ~8% of the total resistance. It should be noted that this model holds only when the mass transfer through the liquid film on the tube side of the membrane is the limiting step in the overall extraction and degradation. Compounds with lower diffusivities in silicone rubber than chlorobenzene may well be limited instead by the diffusion through the membrane, in which case alternative strategies for improving extraction efficiency must be proposed, such as the use of thin, supported membranes. In many cases, both the liquid film mass- transfer coefficient and the membrane thickness will be important for the efficiency of the system.

An increase in plug flow configuration efficiency could be brought about by doubling the length and halving the number of the membrane tubes in the module (thus keeping the total membrane area the same). Such a change would double the wastewater flow rate through each tube, and thus double the tube side Reynolds number. The L6v~que correlation predicts that such a change would have no effect on extraction efficiency, since the improvement brought about by increased Reynolds number would be balanced by a reduction in efficiency due to the effect of the term d/L, since both terms are raised in the correlation to the same power. Data presented here and ongoing work in this laboratory, however, suggest that Re should, in fact, be raised to a power of >1/3, and d~ L, to a power of <1/3. If this relationship were true, an increase in the tube length would improve the performance of a plug flow configuration.

The models assume that the only difference between the plug flow system and the CSTRs-in-series system is the flow rate inside the tubes. A result of this increase in flow rate, however, is that the pressure inside the tubes is higher in the latter mode. Low tube pressure in plug flow mode can result in partial collapse of the membrane tubes, reducing the effective membrane surface area. High tube pressure ensures that the tubes are fully dilated, although work within this group suggests that over ~1.5 bar internal pressure, the 3 mm tubes are at risk of bursting.

It should be noted that operation in a CSTRs-in- series configuration will require significantly more process plant (pumps, tanks, tubing). In the experimental configurations used for this research, three additional pumps, one per membrane module, were required for CSTRs-in-series operation, over the three pumps needed for plug flow operation. This will increase the capital and operating cost of the EMB, a consideration which must be offset against any increase in pollutant removal efficiency.

In summary, this work has shown us that, with sufficient recycle flow, the CSTRs-in-series configuration has a superior extraction efficiency to the plug flow configuration of the EMB. The mathematical model for CSTRs-in-series operation predicted that an increase in Reynolds number would result in an increase in mass-transfer coefficient and, subsequently, in extraction efficiency; this was borne out by the experimental results, although the model was somewhat pessimistic in its predictions. Further work is necessary to examine the effect of wastewater flow rate on each mode of operation and to improve the predictive capability of the model.

6. List of symbols and abbreviations

A C

CSTR d Osw

D~m

EMB F k K kT

L 1H

MCB Rin

Rout Re

total membrane area of module (m 2) pollutant concentration outside membrane tube (kg m 3) continuous stirred tank reactor tube internal diameter (m) diffusion coefficient of pollutant in water (m 2 S --l )

diffusion coefficient of pollutant in membrane (m 2 s i extractive membrane bioreactor wastewater flow rate (m 3 s i) overall mass-transfer coefficient (ms 1) partition coefficient (-) tube side liquid film mass-transfer coefficient (ms t) tube length (m) number of membrane tubes in module monochlorobenzene inside radius of membrane tube (m) outside radius of membrane tube (m) Reynolds number (-)


S

Sin

Sout Sc Sh u

#

P

pollutant concentration inside membrane tube (kg m 3) inlet pollutant concentration (kg m -3) outlet pollutant concentration (kg m 3) Schmidt number (-) Sherwood number (-) velocity (m s -1) viscosity (kg m - t s -1) density (kg m -3)

Acknowledgements

The authors wish to acknowledge the Biotechnol- ogy and Biological Sciences Research Council for their financial support, S. Nishino and J.C. Spain for providing Pseudomonas sp. JS150, and S.J. Puttick and E.S. Ortiz for useful discussions while writing this paper.

References

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[2] D.T. Gibson (Ed.), Microbial Degradation of Organic Compounds, Marcel Dekker, New York, 1984.

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the effect of membrane module configuration on extraction efficiency in an extractive membrane...

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