Removal of organic foulants from capillary ultrafiltration membranes by use of ultrasound.
Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006
Fouling is a serious problem in membrane filtration, caused by pore plugging and adsorption of rejected macromolecules or other solutes in the membrane system. This requires periodic cleaning of membranes, which can add considerably to the overall cost of plant operation owing to lost productivity related to down-time, the cost of the chemicals used in cleaning, higher pressures and associated pumping costs to maintain membrane productivity, as well as reduced lifetime of the membranes. Ultrasound has recently been suggested as a promising approach to combating fouling in membranes. In principle it can be used on-line and may even eliminate the use of chemical cleaning or alternative measures completely, which could lead to major advances in the development and implementation of membrane technology. The objective of this investigation was therefore to assess the feasibility of using ultrasound to mitigate fouling in capillary ultrafiltration systems applied to water containing natural organic matter. Experimental work was conducted with a small laboratory-scale capillary membrane module. Ultrasound was introduced into the system by means of an ultrasonic probe operating at a fixed frequency of approximately 30 kHz, generating a maximum acoustic power density of 130 W/cm2 with a nominal power output of 50 W (IKA Labortechnik Staufen, United Kingdom, U50). Five systems were investigated, viz. aqueous solution of Congo Red dye, ultrapure water, coloured ground water from the George region, water from the Steenbras dam, as well as an aqueous solution of dextran. In most cases, ultrasonication resulted in an increase in the permeate flux. This increase could partly be attributed to an increase in the temperature and thus a decrease in the viscosity of the fluid and partly to enhanced mass and energy transfer due to sonication. Based on experiments done with the Congo Red dye and ultrapure water, no damage as a result of ultrasonication could be discerned in the membrane filter, except when there was direct contact between the ultrasonic probe and the membrane materials. Permeate quality analyses confirmed that sonication does not damage the membrane material – no degradation of permeate quality was found specifically during sonication intervals. In conclusion, ultrasound indeed appeared to be an effective approach to remove foulants associated with natural organic matter from membranes. However, an issue not addressed by this study, but apparent from the literature, is that the effect of ultrasound is strictly local and this has major implications for the scaleup of such ultrasound systems.