THE PERMEABILITY OF BIOFOULING LAYERS ON MEMBRANES

During operation of membrane systems, biofilms develop from the onset and participate in the separation process as a secondary membrane. Bio films mainly consist of bacteria, the extracellular polymer substances - which are excreted by the cells and which immobilize these cells an d entrap particles on the membrane surfaces. The biofilm has an hydrau lic resistance. If this leads to an intolerable loss of performance, t he effect is called ''biofouling'' and cleaning is needed. The assumed action of cleaners is to remove the biofouling layer. However, in man y cases removal of the biomass is not actually observed, although perf ormance improves after cleaning. An explanation is that the cleaner im proves the permeation properties of the biofilm instead of removing it . It is shown here that the application of commercial cleaning agents substantially affects the permeability of the matrices used to model b iofouling. Improved performance due to redispersion of the matrix is s mall compared to the improved performance due to increased matrix perm eability. Formaldehyde tends to decrease biofilm permeability. Thus, t he hydraulic characteristics of biofilms on membranes are very importa nt for the separation process and can be improved or made worse by add ing chemicals for cleaning and disinfection.