Abstract: A single unit fluidized-bed reactor having a reaction space for purification of waste water with biomass attached to carrier particles, comprising a liquid distribution device above the bottom of the reaction space particularly suited for introduction of waste water in wide reactors to achieve uniform fluidization therein, said device consisting of a number of substantially horizontal pipes, each having in the under-surface a number of regularly distributed openings for downward introduction of the liquid, and on top of the reaction space united therewith a multifunctional separation compartment for complete separation of the gas-liquid-solids mixture leaving the reaction space and for complete return of attached biomass into the reaction space, the separation compartment being construction so as to handle treatments involving large or small gas fractions in reactors of varying width.