NOVELTY - A biological reactor vessel (24) comprises a tank having a treatment volume for receiving and delivering a liquid to be treated; an assembly of media substrate provided in the treatment volume to serve as a biological support to carry a biomass; and a drive system for agitating the biological support while the assembly of media substrate is located at least partially inside the treatment volume. The drive system causes the assembly of media substrate to oscillate with a reciprocating motion while the media substrate remains at least partially within the treatment volume.    USE - The biological reactor vessel is used for effecting biological oxygen demand removal through an anaerobic process.    ADVANTAGE - The biological reactor vessel prevents excessive bacterial growth on the bacterial support assemblies and maximizes the prospect that healthy bacteria are in contact with the bacterial support assemblies, thus enhancing the efficiency of the biological reaction.    DETAILED DESCRIPTION - A biological reactor vessel (24) comprises a tank having an inlet (320B), an outlet (336B), and a treatment volume for receiving and delivering a liquid to be treated; an assembly of media substrate provided in the treatment volume within the tank to serve as a biological support to carry a biomass; and a drive system for agitating the biological support while the assembly of media substrate is located at least partially inside the treatment volume. The drive system causes the assembly of media substrate to oscillate with a reciprocating motion while the media substrate remains at least partially within the treatment volume. Guide rails (361) are provided which guide the motion of the assembly of media substrate as the assembly of media substrate is agitated by the drive system. The rails guide the media substrate in a rectilinear motion. The reactor vessel is subdivided into interconnected chambers, with biological supports respectively contained within each chamber. A fluid flow conduit is provided between adjacent chambers for allowing stages of treatment of reaction liquor to be contained. The reactor vessel is sealed to provide a separation between an internal atmosphere of the reactor vessel and an atmosphere exterior of the reactor vessel. The separation allows gas to evolve within the reactor vessel to escape from the reactor. A feed unit is provided for effecting chemical or nutrient injection into the treatment volume to support a biological or chemical process. The drive system produces a smooth velocity curve in the driven assembly of media substrate.    DESCRIPTION OF DRAWING(S) - The drawing is a pictorial exploded assembly of a two-compartment biological reactor with a cover for the reactor.    Biological reactor vessel (24)    Hydraulic dampers (41)    Inlet (320B)    Outlet (336B)    Guide rails (361)