Desalination 199 (2006) 487–489

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy.

Fouling effects and critical flux in relationwith module design and aeration conditions for

a side stream outside/in filtration system

Maricarmen Espinosa Bouchot, Benjamin Espinasse, Corinne Cabassud*INSA, LIPE, 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France

email: corinne.cabassud@insa-toulouse.fr

Received 20 October 2005; accepted 1 March 2006

1. Introduction

Submerged membranes are now widelyused for domestic wastewater treatment. Inthese systems gas bubbling allows to preventor to remove biofilm and particle deposits.Because of the high energy consumption andloss of gas flow efficiency in these systems,new membrane bioreactors are under develop-ment, which are based on outside/in dead-endfiltration in modules disposed externally to thebioreactor. In this paper, we will consider anew concept of MBR, in which the membranesare disposed externally to the bioreactor andoperated in dead-end filtration mode with arecycling of the concentrate in the aerationtank, which generates a very small liquidvelocity in the module (in the range of somecm/s). This system could be called an “externalloop dead-end MBR”. Its main advantages are(i) to separate the two aeration functions: forbiomass activity (using fine bubbles inside thebioreactor) and for fouling control (using bigbubbles in a small volume close to the membrane)

(ii) to facilitate the maintenance of the installa-tions. The development of this new process raisesthe problem of designing and optimizing itsparameters: geometry of the fibre bundle, air flowrate and liquid velocity. The aim of this work wasto study the influence of these different parame-ters on critical fluxes and pressures in order to beable to describe more accurately the process.

2. Material and methods

Experiments were performed with semi-industrial scale modules (from Polymem) onpilot plants. Clay suspensions were used as“model” suspensions. To explain the data weused the definition of the critical flux [1] basedon determination of the limit for irreversibilityusing the flux step method with increasing anddecreasing steps. Variation of irreversible resis-tance was also used.

3. Results and discussion

The first tests were performed with differentbundle geometry (in U-shape and with freefibres) and different fibre properties. These*Corresponding author.

doi:10.1016/j.desal.2006.03.1970011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

488 M.E. Bouchot et al. / Desalination 199 (2006) 487–489

modules were operated in dead-end filtrationfor a 10 g/L suspension at different permeatefluxes and for different air superficial veloci-ties. Critical fluxes and transmembrane pres-sure were determined for the different moduleconfigurations.

In Fig. 1, one can note that the module withU fibres is much less efficient than the modulewith free fibres. This phenomenon was relatedto the less important mixing effects and thereduction of fibre motion in the module with theU fibres. On the basis of these experiments andof computations to predict pressure uniformityinside the fibres during backwashes, some opti-mal membrane properties (inner diameter,length) were determined for a range of perme-ability. This allowed to define optimal bundleand fibre properties. An experimental modulebased on these results was made by Polymem.Other experiments were performed with thismodule in order to study the influence of airvelocity and liquid velocity on critical flux andcritical transmembrane pressure. To begin with,those two parameters were set so as to obtain aconstant mixing Reynolds number in order todiscuss the pertinence of this parameter to

describe fouling phenomena for a two-phaseflow outside the fibres.

In Fig. 2, the apparition of the irreversibilityappears at 1.5 ´ 10–6 m/s for the system with thehigher liquid velocity and at 2 ´ 10–6 m/s whenthe air flow rate is the higher.

Then for a same mixing Reynolds numberone can notice that the apparition of the criticalflux and the variation of resistances are differ-ent. The mixing Reynolds number is shown tobe an irrelevant parameter to describe foulingphenomena outside fibres.

The effect of the air is then (in certain con-ditions) more effective that the effect of thewater at the same superficial velocity in thesystem. Video observations of the gas flowfor both operating conditions showed thatlower cake resistances were obtained whenthe gas velocity is higher (at a constant Rem)because the gas is in shape of big slugs thatare able to make the fibre move whereas whenthe air velocity is smaller the bubbles aresmaller. Results obtained in term of criticalflux and critical transmembrane pressure fordifferent liquid and gas velocities are intro-duced in Fig. 3.

Fig. 1. Variation of the irreversible resistance over themembrane resistance with the permeation flux for twogeometry at the same air superficial velocity.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 6 10-6 1.2 10-5

Module "free fibers"Module with "U" fibers

Ri/R

m

J (m.s-1)

Fig. 2. Variation of the irreversible resistances during afiltration with 1 g/L of bentonite and the same mixingReynolds number for the optimised module.

0

1

2

3

4

5

0 1 10-6 2 10-6 3 10-6 4 10-6

Qc 200L/h Air: 100L/h

Qc 250L/h Air: 50L/h

Ri/R

m

J (m.s-1)

M.E. Bouchot et al. / Desalination 199 (2006) 487–489 489

Critical fluxes are increasing when the airsuperficial velocity increases until a limit valueof about 0.08–0.1 m/s. However critical fluxesare decreasing when the liquid velocity isincreasing, which can be explained by the sensi-tivity of air flow patterns to the liquid velocity.

4. Conclusions

This study allowed to design a module forside-stream filtration, that could be used forapplication to waste water treatment. The influ-ence of air and liquid superficial velocities oncritical flux and the observation of air flow out-side the fibres allowed to conclude on the rela-tive influence of air and liquid flows on foulingcontrol and to define some ranges for theseoperating parameters.

Acknowledgements

This work was supported by the ESF (Euro-pean Social Fund) and Polymem.

Reference

[1] B. Espinasse, P. Bacchin and P. Aimar, On anexperimental method to measure critical flux inultrafiltration, Desalination, 146 (2002) 91–96.

Fig. 3. Critical TMP and flux as a function of airsuperficial velocity and liquid velocity.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Ug (m.s-1)

PTM

crit

ique

(bar

)

02468101214161820

Flux

crit

ique

(L-1

.h-2

.m)

PTMc - 0.012PTMc - 0.025PTMc - 0.037Fc - 0.012Fc - 0.025Fc - 0.037