The treatment procedure for a liquid e.g. water-based effluent containing a fermentable organic charge uses an e.g. bacterial biomass (1) fed into an input flow (2) of the effluent, and extracting from the biomass a water-based output flow, poor or rich in the biomass. The procedure uses two aerobic or anaerobic digestion stages: one (A) in which the input flow (2) is mixed with a flow (4) of recycled liquid rich in the biomass to produce a liquid flow (51) poor in organic charge, and a residual gas flow (61) separated from it, and a final stage (Z) in which a liquid circulation loop (7) is established, the latter being enriched with biomass in a separation stage (8). The separation stage, using permeation or centrifugal decantation produces a flow (3) of water poor in or exempt of biomass, and a flow of recycled liquid (4) rich in biomass. A residual gas, e.g. methane, is removed in intermediate stages (B, C).
