During operation of membrane systems, biofilms develop from the onset and participate in the sepn. process as a secondary membrane. Biofilms mainly consist of bacteria, the extracellular polymer substances - which are excreted by the cells and which immobilize these cells and entrap particles on the membrane surfaces. The biofilm has an hydraulic resistance. If this leads to an intolerable loss of performance, the effect is called \"biofouling\" and cleaning is needed. The assumed action of cleaners is to remove the biofouling layer. However, in many cases removal of the biomass is not actually obsd., although performance improves after cleaning. An explanation is that the cleaner improves the permeation properties of the biofilm instead of removing it. It is shown here that the application of com. cleaning agents substantially affects the permeability of the matrixes used to model biofouling. Improved performance due to redispersion of the matrix is small compared to the improved performance due to increased matrix permeability. Formaldehyde tends to decrease biofilm permeability. Thus, the hydraulic characteristics of biofilms on membranes are very important for the sepn. process and can be improved or made worse by adding chems. for cleaning and disinfection.