IWA Poster Template

1
Characteristics of particle deposit and
resistance build-up on ultrafiltration membranes
of different MWCOs application of a PDA online
monitoring method
Introduction
In many cases, removal of suspended particles
from water is the main task in drinking water
treatment or tertiary treatment of secondary
effluent for wastewater reuse. Therefore, a
matter of concern for UF membrane application is
the behaviour of higher quantity of suspended
particles on flux decline. UF membranes often
have a broad MWCO range from several thousand to
several hundred thousand Da, but few literatures
have dealt with the effect of MWCO on particle
deposit and resistance build-up on the membrane
surface during filtration, though this is a
factor to be considered in MWCO selection. In
order to evaluate the weight of particles
deposited on the membrane surface and its
variation during a filtration run, an online
optical technique, namely particle diffusion
analyzer (PDA) was introduced in the study.
Experimental System
Fig. 1 shows the schematic diagram of the
experimental system used in this study. The feed
solution was boosted to the membrane cell which
was constructed in transparent plastics housing a
piece of flat sheet membrane. The membrane was a
round sheet of 80mm diameter. Polyethersulfone
(PES) membranes of three MWCOs were applied for
this study 100k, 50k and 10k Da. In the
experiments, the feed solution was pressurised
and controlled to a constant value of 0.2MPa in
the process of filtration. The membrane cell
could be operated in either crossflow or dead end
mode. An electric magnetic stirrer was placed in
the membrane cell so that the feed solution could
be fully mixed and a shear force on the membrane
surface could be provided. A steady concentrate
flow was delivered to a particle diffusion
analyser (PDA2000, Rank Brothers Co. Ltd.) where
the variation of suspended particles in the
rejected flow could be monitored online. The
permeate quantity was online monitored by an
electronic balance on which the permeate tank was
mounted. Both the PDA and electronic balance were
connected with a personal computer for data
processing.
Figure 1 The schematic diagram of the
experimental system
Results Discussion
Variation of membrane deposit weight
Analysis of membrane resistance
The PDA online monitoring results are shown in
Fig. 2 for PES membranes of MWCO10k, 50k and
100k. With kaolinite suspensions of 40NTU, the
average CI value should be about 1.9 according to
the preliminary experiment results shown in Fig.
2. However, as membrane filtration started, quick
decrease of the CI value was noticed from Fig. 7.
In the case of MWCO10k (curve a), the initial CI
value became about 1.0 and it decreased
continuously as filtration time elapsed. In the
cases of membranes with larger MWCOs (50k and
100k), the initial CI values did not depart from
1.9 so much but they decreased quickly with time.
After about 2000 sec filtration, the three CI
curves almost coincided with each other and
reached a final value about 0.6. Based on the
PDA monitoring results, the membrane deposit
weight was calculated. Fig. 3 shows the variation
of the deposit weight per permeate volume with
filtration time. Generally speaking, the membrane
deposit weight resulted from the filtration of
unit volume permeate decreased with increasing
MWCO of the membranes. However, the pattern of
variation with time differed with different
MWCOs. In the case of MWCO10k, the initial
deposit weight was above 200g/m3, and it kept
increasing till the end of filtration.
Contrarily, the initial deposit weights for
larger MWCO membranes were much lower (near
100g/m3 in the case of MWCO50k and about 50g/m3
in the case of MWCO100g/m3) and they tended to
decrease as filtration time elapsed.
The decline of permeate flux is in any sense
resulted from an increase of membrane resistance
and in this study because kaolinite clay
particles were the only material to deposit on
the membrane surface or penetrate into membrane
pores, an analysis of the relationship between
membrane resistance and the deposit weight on the
membrane can provide useful information about the
behavior of particles on membranes of different
MWCOs. An estimation of the total resistance can
be performed by using Darcys law. The
calculation results are shown in Fig. 4. The
initial resistance Rm0, i.e. the calculated Rm at
t0 was evaluated as 1.201013, 1.431012, and
5.751011 m-1 for the PES membranes of MWCO10k,
50k and 100k, respectively. As filtration time
elapsed, Rm increased in each case, but the
increasing speed was greater for larger MWCO
membranes. The membrane resistance Rm can be
considered as the initial membrane resistance
Rm0, i.e. the hydraulic resistance of clean
membrane itself plus an increment ?Rm during the
filtration process. As a result, Fig. 5 was
obtained to show the variation of
?Rm/q(RmRm0)/q with time for PES membranes of
MWCO10k, 50k and 100k, where q is the deposit
weight per unit membrane surface. It can be seen
that a PES membrane of smaller MWCO not only
tends to have more suspended matter deposited
(Fig. 3) but is also easy to build up higher
hydraulic resistance from unit weight deposit.
Fig.2 CI curves recorded by PDA Fig.3
Deposit weight on membranes
Fig. 4 Membrane resistance Fig. 5
deposited kaolinite clay to the


with filtration time
build-up of hydraulic
resistance
Conclusions
The method of PDA online monitoring and mass
balance analysis used in this study provided an
indirect way to monitor particle concentration
change in the concentrate flow and then to
evaluate the deposit weight, which may assist the
investigation of membrane fouling by suspended
particles.
Acknowledgement This study is supported by the
National Natural Science Foundation of China
(Grant No. 50138020)