NOVELTY - Effluent treatment of water containing (non)pathogenic micro organisms, using a membrane bioreactor for biological purification by a biomass to reduce micro-organism concentration by forming a treatment loop (B) of bioreactor mixed liquor, comprises: solid/liquid separation with a cut-off threshold to keep the microorganism and biomass in the liquid and solid phases, respectively; disinfection and/or sterilization of the liquid phase, where the mixed liquor flow is determined to nullify the concentration; and return of the treated liquid phase to the bioreactor.    USE - The process is useful for effluent treatment e.g. of surface water, sea waters, urban waste effluents and industrial waste effluents.    ADVANTAGE - The process is effective and industrially applicable.    DETAILED DESCRIPTION - Effluent treatment of water containing (non)pathogenic micro organisms, using a membrane bioreactor for biological purification by a biomass to reduce micro-organism concentration in the bioreactor by forming a treatment loop (B) of bioreactor mixed liquor, comprises: solid/liquid separation with a cut-off threshold to keep the microorganism and the biomass to remain in the liquid phase and solid phase, respectively; disinfecting and/or sterilizing treatment of the liquid phase, where the flow of mixed liquor subjected to the disinfecting treatment is determined to nullify the concentration phenomenon of intrinsic microorganism in the bioreactor; and return of the treated liquid phase to the bioreactor. An INDEPENDENT CLAIM is included for an installation of effluent treatment i.e. to carry out the process, comprising a solid/liquid separation device (12), a disinfection and/or sterilization device (D) of the liquid phase and a restitution device (17) of the treated liquid phase in bioreactor.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic diagram of the effluent treatment installation (Drawing includes non-English language text):    Biological reactor; (1, R)    Filtration cell; (2)    Recirculation pipe; (10)    Solid/liquid separation device; (12)    Restitution device; (17)    Biomass treatment loop; (B)    Disinfection and/or sterilization device. (D)