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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S. Ognier et al. /Desalination 146 (2002) 141-147 145
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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S. Ognier et al. /Desalination 146 (2002) 141-147 147
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.
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