Electrically or ultrasonically enhanced membrane filtration of wastewater:Dissertation

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