Which floc properties to better run a membrane filtration operation?

摘要

Treatment of waters and wastewaters by microfiltration requires the addition of chemical coagulants to enhance the removal of NOM substances. Enlarging particle size by coagulation also results in an increase of steady permeate flux as both back-transport in cross flow filtration and cake permeability will be higher for larger particles. A much favorable improvement is expected from intra-floc porosity which is expected from the fractal nature of such aggregates. The control and ultimately the characterization of floc properties are thus of considerable interest for a microfiltration operation to be run in the best economical conditions as for instance with low cake resistance. Indeed, a crucial pace forward to “tailor” floc properties to suit with a low specific resistance is not at once their initial structure but their final arrangement in the cake. rnFor this purpose we have developed specific experiments to investigate the structural and mechanical features of cakes. We are now able to release valuable information on the impact of floc properties on both specific cake resistance and compressibility as characterized by the dependence of cake solid fraction on cake pressure drop. In this work, we shall present some results for dead-end filtration of silica particles (Ludox HS 40, 14 nm diameter spherical particles) coagulated by CaCl_2, Al 13(7+ polycation) and FeCl_3. in a RLCA regime. Analysis indicates that the specific cake resistance and the compressibility which are strongly influenced by the nature of coagulant, primarily result from the resistance of interparticle bonds within flocs rather than fractal properties.