The present invention concerns a method for biological treatment of carbon, nitrogen and optionally phosphorus in water, in a reactor system (1) comprising a sequencing batch reactor (SBR) (2) and a moving bed biofilm reactor (MBBR) (3). The method comprises a step (10) of filling said SBR reactor (2) with water to be treated (5), a step (20) of anoxic/aerobic biological treatment in said reactor system (1) and a step (30) of discharging treated water (35) from said SBR reactor (2). The anoxic/aerobic biological treatment step (20) comprises: a biological treatment (210) under largely anoxic conditions in the SBR reactor (2), producing a first effluent (215), a biological treatment (220) under aerobic conditions in the MBBR reactor (3), producing a second effluent (225), and a continuous recirculation of the first and second effluents. The present invention also concerns a corresponding facility.