FILTRATION OF BACTERIA AND YEAST BY ULTRASOUND-ENHANCED SEDIMENTATION

Continuous flow filtration of suspensions of eukaryotic cells by ultra sonic standing wave enhanced sedimentation has recently been reported. The filtration efficiency for Escherichia coli in such a filter has b een characterized at frequencies of 1 and 3 MHz in the present work an d compared with results for Saccharomyces cerevisiae. The yeast can be filtered at greater than 99% efficiency at a flow rate of 5 ml min(-1 ) at either frequency. The filtration efficiency of the smaller E. col i at 3 MHz is in excess of 80% at concentrations in the region of 10(1 0) ml(-1) but decreased at lower concentrations. However, E. coli in a mixed suspension with yeast were, because of inter-particle interacti ons, removed with the filtrate at an efficiency ranging from 80 to 50% over the eight orders of bacterial concentrations tested (down to 10( 3) ml(-1)) at 3 MHz. Quantitative considerations show that poor filtra tion of pure suspensions of the smaller cells at the lower frequency a rises because, at reasonable flow rates, the residence time is not suf ficient for the cells to reach the pressure nodal cell concentration r egions. The filtration efficiencies of both cell types are comparable at 3 MHz. It is suggested that the more comparable efficiencies arise because concentration regions are narrower at the high frequency and S tokes drag by the filter bulk flow inhibits sedimentation of the conce ntrated cells.