• Kyllönen, Hanna

Abstract

Flux decline due to concentration polarisation andmembrane fouling is a serious problem in membranefiltration. In this thesis the effect of an external DCelectric or ultrasonic field separately on the flux incross-flow membrane filtration of wastewater samples wasstudied.Significant enhancement of the limiting flux comparedwith the flux with no electric field was achieved in thefiltration of model wastewater. The most importantparameters for the limiting flux enhancement were theelectrophoretic mobility and the applied electric fieldstrength. The electric field especially prevented thecake formation on the membrane surface. Its effect onother kinds of fouling was minor. The critical electricfield strength was determined both theoretically andexperimentally.Electrofiltration was also studied in the industrialwastewater applications. The average electrophoreticmobility of the charged particles and colloids in thesamples studied was usually only slightly negative. Thebest flux improvement in electrofiltration was achievedwhen filtering a sample with the highest electrophoreticmobility. In that case the limiting flux could beincreased many-fold. Gas was produced on the electrodesin the filtration of the wastewater samples. The fluxenhancement decreased significantly when the membraneworked as an electrode and gas was produced on themembrane. The problem did not exist when a non-conductiveceramic membrane was used and an electric field wasapplied across the membrane. However, the highconductivity caused high energy consumption, which is aproblem in electrofiltration of industrial wastewater.Ultrasound irradiation also provided enhancement incross-flow membrane filtration. It increased the fluxprimarily by breaking the cake layer at the membranesurface. Liquid jets produced by cavitation served as abasis for ultrasonic membrane cleaning. There are severalfactors, which affect the cavitation and thus influencethe effectiveness of ultrasound in membrane foulingprevention. In this thesis important factors were studiedfrom the literature and from experimental investigations.The experimental part was focused on the suitableultrasound propagation direction and the effect of thetransmembrane pressure, which previously have got littleattention in the research of ultrasound assisted membranefiltration. Also some aspects of the effects ofultrasound frequency, particle size and cross-flow werestudied experimentally. According to these studies a lowfrequency ultrasound irradiation during a short pause infiltration from the permeate side of the tight membrane,an ultrafiltration membrane, is efficient and, at thesame time, a gentle method in membrane cleaning. For openmembranes the ultrasound propagation direction should bedifferent or the irradiation from the feed side should becombined with other cleaning techniques likebackflushing.Electrofiltration is not a universal method for thefiltration of industrial wastewater. It is a competitivemethod, when the average electrophoretic mobility in thesample is high and the conductivity is low. Ultrasoundassisted filtration is less dependent on the feedproperties and could be more useful in the cleaning ofmembranes in industry. However, there are some factors,especially the development of transducer technology formembrane filtration applications and the control ofmembrane erosion caused by cavitation, which need furtherdevelopment.

Topics

• impedance spectroscopy
• surface
• mobility
• laser emission spectroscopy
• strength
• ultrasonic