All raw waters contain microorganisms: bacteria, algae, fungi, viruses and higher organisms. The typical size of bacteria is 1 to 3 µm. Microorganisms can be regarded as colloidal matter and removed by the pretreatment as discussed in separate information on Collodial Fouling Prevention. The difference from dead particles however, is their ability to reproduce and form a biofilm under favorable living conditions. Microorganisms entering an RO system find a large membrane surface where the dissolved organic nutrients of the water are concentrated (due to concentration polarization): an ideal environment for the formation of a biofilm. Biological fouling of the membranes may seriously affect the performance of the RO system. The symptoms are an increase of the differential pressure from feed to concentrate, finally leading to telescoping and mechanical damage of the membrane elements and a membrane flux decline. Sometimes biofouling develops even on the permeate side, thus contaminating the product water. A biofilm is difficult to remove, because it protects its microorganisms against the action of shear forces and disinfection chemicals. In addition, incompletely removed biofilms lead to a rapid regrowth.