NOVELTY - Wastewater treatment involves feeding wastewater at a flow rate Qinf to a bioreactor system (5); leading the wastewater in the form of a mixed liquor containing activated sludge through the bioreactor system; and performing wastewater treatment in accordance with a first mode and a second mode. When switching from the first to the second mode and vice versa, mixed liquor suspended solids concentration of bioreactor effluent (MLSSbio) is adjusted so that in the first mode MLSSbio is larger than in the second mode.    USE - For wastewater treatment.    ADVANTAGE - The bioreactor volume is more effectively used for the biological conversion processes so that, on average, the treatment capacity of the bioreactor system is increased per unit reactor volume.    DETAILED DESCRIPTION - Wastewater treatment involves feeding wastewater at a flow rate Qinf to a bioreactor system; and leading the wastewater in the form of a mixed liquor containing activated sludge through the bioreactor system. The mixed liquor leaves the bioreactor system as a bioreactor effluent which has a mixed liquor suspended solids content MLSSbio. The wastewater treatment is performed in accordance with a first mode, for a flow rate Qinf which is within a first range, and in accordance with a second mode, for a flow rate Qinf which is within a higher, second range and switching between both modes depending on the magnitude of Qinf. Both in the first and in the second mode, at least a portion of the bioreactor effluent is supplied to a membrane filtration system (25) where the portion of the bioreactor effluent is filtrated to produce a purified waste water stream at a flow rate Qeff,ft and a mixed liquor retentate which has a mixed liquor suspended solids content MLSSft. Both in the first and in the second mode, at least a portion of the mixed liquor retentate is recycled at a flow rate Qr,ft to the bioreactor system. At least in the second mode, a further portion of the bioreactor effluent is supplied to a settling tank system (7) which comprises at least one settling tank where the further portion of the bioreactor effluent is clarified to produce a further purified waste water stream and a further mixed liquor containing settled activated sludge, at least a portion of the further mixed liquor being recycled at a flow rate Qr,st,bio to the bioreactor system. When switching from the first to the second mode and vice versa, MLSSbio is adjusted so that in the first mode MLSSbio is larger than in the second mode. An INDEPENDENT CLAIM is also included for a wastewater treatment installation.    DESCRIPTION OF DRAWING(S) - The figure shows schematically a hybrid wastewater treatment system.    Mechanical treatment system (3)    Bioreactor system (5)    Settling tank system (7)    Discharge point (9)    Membrane filtration system (25)