Abstract
Flux decline due to concentration polarisation and
membrane fouling is a serious problem in membrane
filtration. In this thesis the effect of an external DC
electric or ultrasonic field separately on the flux in
cross-flow membrane filtration of wastewater samples was
studied.
Significant enhancement of the limiting flux compared
with the flux with no electric field was achieved in the
filtration of model wastewater. The most important
parameters for the limiting flux enhancement were the
electrophoretic mobility and the applied electric field
strength. The electric field especially prevented the
cake formation on the membrane surface. Its effect on
other kinds of fouling was minor. The critical electric
field strength was determined both theoretically and
experimentally.
Electrofiltration was also studied in the industrial
wastewater applications. The average electrophoretic
mobility of the charged particles and colloids in the
samples studied was usually only slightly negative. The
best flux improvement in electrofiltration was achieved
when filtering a sample with the highest electrophoretic
mobility. In that case the limiting flux could be
increased many-fold. Gas was produced on the electrodes
in the filtration of the wastewater samples. The flux
enhancement decreased significantly when the membrane
worked as an electrode and gas was produced on the
membrane. The problem did not exist when a non-conductive
ceramic membrane was used and an electric field was
applied across the membrane. However, the high
conductivity caused high energy consumption, which is a
problem in electrofiltration of industrial wastewater.
Ultrasound irradiation also provided enhancement in
cross-flow membrane filtration. It increased the flux
primarily by breaking the cake layer at the membrane
surface. Liquid jets produced by cavitation served as a
basis for ultrasonic membrane cleaning. There are several
factors, which affect the cavitation and thus influence
the effectiveness of ultrasound in membrane fouling
prevention. In this thesis important factors were studied
from the literature and from experimental investigations.
The experimental part was focused on the suitable
ultrasound propagation direction and the effect of the
transmembrane pressure, which previously have got little
attention in the research of ultrasound assisted membrane
filtration. Also some aspects of the effects of
ultrasound frequency, particle size and cross-flow were
studied experimentally. According to these studies a low
frequency ultrasound irradiation during a short pause in
filtration from the permeate side of the tight membrane,
an ultrafiltration membrane, is efficient and, at the
same time, a gentle method in membrane cleaning. For open
membranes the ultrasound propagation direction should be
different or the irradiation from the feed side should be
combined with other cleaning techniques like
backflushing.
Electrofiltration is not a universal method for the
filtration of industrial wastewater. It is a competitive
method, when the average electrophoretic mobility in the
sample is high and the conductivity is low. Ultrasound
assisted filtration is less dependent on the feed
properties and could be more useful in the cleaning of
membranes in industry. However, there are some factors,
especially the development of transducer technology for
membrane filtration applications and the control of
membrane erosion caused by cavitation, which need further
development.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 11 Nov 2005 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-6663-7 |
Electronic ISBNs | 951-38-6664-5 |
Publication status | Published - 2005 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- membrane filtration
- fouling
- electrofiltration
- electric field
- ultrasound
- flux
- transmembrane pressure
- filters
- wastewaters
- wastewater purification