The installation comprises three reaction vessels (100, 200, 300) in series, each containing an aqueous phase and a specific bacterial population. The vessels serve in succession to dissolve the liquid feedstock for treatment by enzymatic hydrolysis, then to biodegrade the resulting effluent by biological digestion, and finally to reduce the residual bacterial components by biological action. The vessels are mutually isolated by tangential filters (108, 208, 308) and by centrifugal separators (212, 312) for filtering the biomass from the reaction products. The result is a final liquid effluent (EFF/N) that is essentially non-organic, with the filtered biomass downstream constituting an ultimate organic waste (DU) of reduced volume.