Ognier et al., 2002, DS

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DESALINATION

ELSEVIER Desalination 146 (2002) 141-147www.elsevier.com/locate/desal

Characterisation and modelling of fouling

in membrane bioreactors

S. Ognier*, C. Wisniewski, A. Grasmick

Laborutoire Genie des Procedes de Montpellier 2, CC 024, Place Eugene Bataillon 34095 Montpellier Cedex 05 France

Tel. +33 (4) 67 14 48 54; Fax +33 (4) 67 14 48 54; emails: [email protected]

[email protected] [email protected]

Received 7 February 2002; accepted 6 March 2002

Abstract

A membrane bioreactor used for denitrification of a synthetic substrate was studied in term of membrane fouling.

For standard pH and temperature conditions, subcritical conditions were defined to ensure the process stability. The

stepwise method was used to determine the critical flux for the deposition of colloidal particles. Under standard

physicochemical conditions, only a low and constant fouling resistance was observed if the permeate flux was

maintained below the critical flux. The influence of physicochemical variations was then investigated by varying pHand temperature in the biological reactor. It was observed that, when the pH value was higher than a critical one, the

membrane was rapidly fouled. This maximum admissible pH value decreased when the temperature increased. On

analysing the reversible nature of fouling and the variations of ionic concentrations with the pH, the role of carbonate

calcium precipitation was pointed out. By using classical filtration models, it was shown that the fouling mechanism

could be the deposition of CaCO, particles formed in the bulk suspension by bulk crystallisation.

Keywords: Membrane bioreactor; Membrane fouling; Subcritical regime; Precipitation

1. Introduction deposits from building up on the membrane

Critical flux is an interesting notion to define

optimal hydrodynamic conditions; subcritical

conditions can be defined to avoid macroscopic

*Corresponding author.

surface [ 11. However, the membrane permeability

can decrease during the operation due to the

interactions between soluble compounds and

membrane material, which do not depend on

hydrodynamic conditions. Therefore, the stability

Presented ut the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France,

July 7-l 2, 2002.

001 l-91 64/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved

PII: SO0 II-9 164(02)00508-8

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142 S. Ognier et al. /Desalination 146 (2002) 141-147

of the system depends not only on hydrodynamic

conditions but also on biological and physico-

chemical suspension properties, and it is of

utmost importance to define subcritical hydro-

dynamic conditions as well as physico-chemicaland biological conditions to obtain a stable filtration

regime. In these conditions, long filtration periods

without having to use chemical cleaning pro-

cedures can be obtained. Fan et al. [2] showed

that a membrane bioreactor (MBR) system for

the treatment of raw municipal wastewater can

be run continually over 70 d with a stable trans-

membrane pressure.

However, the biological and physico-chemical

properties of the suspension are not always stable

due to influent composition or temperature changes.As these variations are weak, they are not always

taken into account when defining the operating

conditions. Therefore, subcritical conditions are

generally defined experimentally in fixed biological

and physicochemical conditions. Based on these

considerations, the objective of this work was (i)

to ensure that stable filtration conditions could

be obtained in an MBR under constant hydro-

dynamic, biological and physico-chemical con-

ditions and (ii) to study the influence of weakvariations of two physicochemical parameters

(pH and temperature) on process stability. The

fouling phenomena were analysed by using classical

filtration models.

2. Experimental

2.1. Membrane bioreactor

Experiments were conducted on a pilot MBR,

which consisted of a 20-l bioreactor tank and a

ceramic ultrafiltration membrane module

(Membralox@) having a 0.24-m* surface area and

a mean pore size of 0.05 pm and a resistance of

5x10” m-l. The recirculated pump integrated to

the system ensured the perfect mixing of the

reactor and made the retentate circulate with a

I .6 m/s tangential velocity in the membrane

module, corresponding to a wall shear stress of

13 Pa. A cooling system kept the whole system

at a constant temperature of 25?1 “C. A constant

permeation flux was maintained by using a

suction pump (Watson-Marlow 505 RS).

2.2. Denitrifcation process

The system worked in denitrification of a

synthetic substrate. The biodenitrification was

realised by an activated sludge, mixed culture,

taken from the aeration tank from the municipal

wastewater treatment plant in Montpellier

(France) and acclimated to the synthetic substrate

used in the experiments. The synthetic substrate

was prepared by diluting potassium nitrate and

ethanol in tap water so that the concentrationswere 200 mg,,,/l and 1000 mg,n/l, respectively.

In these conditions, the ratio COD/N was equal

to 5. (NH,),HPO, was also added so that COD/P

= 150.

The reaction of denitrification can be written

as follows:

SCH,CH,OH + 12 NO;+ 10 HCO,- + 6 N,

+ 9 H,O + 2 CO,

Hydroxide and hydrogenocarbonate ions are

metabolically produced by the reaction of

denitrification. In theory, the increase of alkalinity

is equal to 3.6 mg CaCO3/mg N-NO,--N denitrified.

The bioreactor pH value could increase to 8.5 or

more when no acidic solution was added to the

substrate (consequently, chlorydic acid was added

to the substrate to maintain the pH value between

7.5 and 8).

By keeping the biological parameters (hydraulic

retention time, sludge retention time, organic

loading) constant, the biomass concentration wasstable, equal to a constant value of 1.5 g&l. Table 1

presents the main biological characteristics of the

system.

2.3. Fouling characterisation

TMP evolution was monitored in the MBR

by recording data of pressure transducers P,, P,

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S. Ognier et al. /Desalination 146 (2002) 141-147 143

Table I

Biological conditions

Volumetric loading rate, gcoD/lld 3

Hydraulic retention time, h 8

Sludge retention time, d 3.5

Specific denitrification rate, gNlgv&d 0.4

Yield, g&gco, 0.2

Biomass concentration, g&l 1.5

and P,. P, and P, are the pressures of the retentate

measured at the input and the output of the

membrane module and P, is the pressure on the

permeate side. During the filtration operation, P,

and P, are constant and P, decreases due to mem-

brane fouling. TMP was calculated by the relation:

To characterise the nature of the fouling,

several cleaning methods were tested: intermittent

filtration, forward flush with water, back flush

with water and slight acid cleaning. Except for

the intermittent filtration, the membrane resistance

to a water permeation was determined after eachcleaning method. TMP was measured when

filtering pure water at lo,20 and 30 l.m-*.h-‘.The

tangential velocity was the one used during the

filtration operation. Details of the different methods

in chronological order of their applications are

as follows:

l Intermittent filtration. The suction pump was

switched off for approximately 10 min, then

the filtration operation was reinitiated. During

the intermittent filtration, the recirculationpump continued to make the retentate circulate.

l Forward flush. The filtration ws stopped

when the reactor was filled up with pure water.

Then, the water was recirculated for 10 min

without filtering.

l Back flush. Pure water was filtered in the

opposite direction of the normal filtration

operation with a TMP of 0.5 bar.

l Acidic cleaning. Last, the membrane was

chemically cleaned with a slightly diluted

solution of nitric acid (HNO,) at room temp-

erature.

l Complete chemical cleaning. To restore theinitial permeability of the membrane, a

complete chemical cleaning was done. An

alkaline solution (diluted hydroxide sodium

solution) and then an acid solution (diluted

nitric acid solution) were filtered at 60°C.

3. Results and discussion

3. I. Definition of operating conditions ensuring

process stability

The operating conditions of the MBR were

defined under “standard” conditions of pH and

temperature, that is to say, a pH value between

7.5 and 8 and a temperature equal to 25kl”C. The

objective of this preliminary study was to define

hydro-dynamic conditions where no deposition

of colloidal particles on the membrane occurred.

Therefore, the increase of membrane resistance

is controlled and a stable regime should be

obtained. In theory, such conditions are possiblewhen the permeate flux is inferior to the critical

flux value.

To determinate the critical flux value in the MBR,

the stepwise method was used. The permeate flux

was stepwise increased with a step length of

30 min. Below the critical flux value, the TMP

stabilised rapidly after each flux increase, and the

stabilised value of TMP increased linearly with

the flux imposed. Above the critical flux value,

this linear relationship did not apply any more due

to a deposition phenomenon. Fig. 1 shows the TMPmeasured for each flux value. The subcritical regime

corresponds to the first part of the curve where the

resistance stays constant (2x10’* m-l) for permeate

flux values below 38 l.m-*.h-‘. This resistance

differed from the clean membrane one due to an

instantaneous fouling phenomenon taking place

at the very beginning of the filtration. Above

38 l.m-2.h-‘, the fouling resistance increased

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I

1 5 1 7 1 9 2 1 2 3 2 5 2 7 2 9 3 1 3 3 3 5

Temperature (“C)

Fig. 3. Influence of temperature on critical pH value.

cleaning methods. Two experiments were con-

ducted: (1) Case A was initiated during an intensive

fouling phase, (2) Case B was done at the end of

the filtration run presented in Fig. 2, when the

process was stabilised again (t = 575 h). The results

are presented in Table 2.

As shown by these results, filtration resting is

totally ineffective in removing the fouling

resistance in both cases. This result signifies that

the fouling mechanism is not the formation of a

reversible deposit on the membrane surface.

However, in the first case, half of the foulingresistance can be eliminated by the forward flush.

As the cleaning methods of filtration resting and

forward flush differ only in the use of water (the

hydrodynamic conditions are identical), the

cleaning efficiency of the forward flush points

out once again the crucial role of physicochemical

conditions. The forward flush effectiveness could

be due to the use of water with a neutral pH value.

The importance of the pH value in fouling removal

had been already noticed during the experiments.

This result was confirmed by the resistance values

obtained after acid cleaning: in both cases, acid

cleaning proved to be very efficient in removing

the fouling resistance that remained after the

forward flush.

The irreversible fouling can be of organic (bio-film, metabolites, etc.) or inorganic (precipitated

salts) nature. Alkaline cleaners are generally

considered as the most effective against biofilms

and organic foulants whereas acidic cleaning is

required to ensure the removal of inorganic preci-

pitants [4]. Therefore, the effectiveness of the

acidic cleaning indicates that the fouling could

be mainly due to precipitation phenomena. The

continuous fouling increase observed is not in

disagreement with this hypothesis. Actually, if

precipitation can be instantaneous, the continuous

feed of hard tap water and the biological reaction

can induce continuously a salt precipitation as long

as the suspension pH is beyond the critical value

for precipitation.

To determine the nature of the precipitants,

the influence of pH on the suspension composition

was analysed. When the pH was increased, only

hydroxide ions (OH-) and carbonate ions (CO:-)

concentrations were increased. Therefore, the pre-

cipitation was supposed to depend on the concen-trations of hydroxide and/or carbonate ions. As the

substrate was prepared with hard tap water (Ca2+=

120 mg/l and Mg*+ = 8 mg/l), the reaction quotient

was compared with the solubility product for the

main hydroxide and carbonate precipitates involving

calcium and magnesium ions. Table 3 presents

the solubility product at 25°C the reaction quotient

calculated at pH 9 with the calcium and magnesium

concentrations in the tap water used for the substrate.

These calculations indicate that two inorganic

crystals, CaCO, and Mg(OH),, can precipitate

Table 2

Membrane resistance values obtained after the different cleaning methods tested (m-l)

Before intermittent After intermittent After forward flush After back flush After acidic

filtration filtration cleaning

Case A 20x10” 20x10” 10x10’* 8.6x10’* 1.9x10’*

Case B 9x1012 9x1012 6.5. lOI 6.5~10’~ 1.8x1o’2

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considered. Considering that better fittings are

obtained with models describing an external

fouling mechanism, the fouling would be located

at the pore entrance or on the membrane surface

rather than in the whole membrane matrix. Thisresult allows one to think that the fouling could

be caused by the deposition of CaCO, particles

formed in the bulk suspension (bulk crystallisation)

on the membrane surface. Actually, a pore con-

striction mechanism should have been obtained

with heterogeneous crystallisation. However, it

is of course difficult to base conclusions on the

only use of the models and further research would

be necessary to confirm this hypothesis.

4. Conclusions

An MBR for denitrification was studied in

terms of process stability. Unusual membrane

fouling in an MBR system was investigated. The

following conclusions could be drawn:

l In an MBR for denitrification, the great alkalinity

of the suspension can cause the precipitation

of calcium carbonate for pH values between

8 and 9.

l

The role of precipitation can be pointed out as acause of system instability, even if the system

works in subcritical conditions.

l The fouling mechanism could be the deposition

of CaCO, particles formed in the bulk suspension

by bulk crystallisation. Further research would

be necessary to confirm this hypothesis.

References

[I] J.A. Howell, Subcritical flux operation of micro-

filtration, J. Membr. Sci., 107 (1995) 165-171.

[2] C. Wrsniewski, F. Persin, T. Cherif, R. Sandeaux, A.

Grasmick and C. Gavach, Denitrification of drinking

water by the association of an electrodialysis and a

membrane bioreactor: feasability and application,

Desalination, 139 (2001) 199-205.

[3] X.-J. Fan, V. Urbain, Y. Qian and J. Manem, Ultra-

filtration of activated sludge with ceramic membranes

in a cross-flow membrane bioreactor process, Water

Sci. Technol., 41(10-l 1) (2000) 243-250.

[4] R. Liikanen, J. Yli-Kuivila, R. Laukkanen, Efficiency

of various chemical cleanings for nanofiltration

membrane fouled by conventionally-treated surface

water, J. Membrane Sci., 195 (2002) 265-276.

[5] A. Ould-Dris, M.Y. Jaffrin, D. Si-Hassen and Y.

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127 (2000) 47-58.

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Ognier et al., 2002, DS