Abstract: A biological rotor for biomass treatment in a sewage treatment plant is erected from sector shaped packs removably attached between spider-like support assemblies each of which comprises a ring and radial channel-shaped arms. The packs include support members. One of the support members has its ends located in the channels of the arms and the other support members are clamped to the ring. A separator plate can be built up from sections formed in each pack.

Abstract: To desludge the final settlement tank of a waste water treatment plant which includes a bio zone equipped with slowly rotating rotors on which biomass deposited from the waste water is digested by microorganisms as the biomass is periodically exposed to air, the level of liquid in the final settlement tank is raised above that in the bio zone so that gravity induced desludging can occur. One of the rotating rotors can be equipped with water lifting means to generate the required hydraulic head for desludging.Skimming off of scum from the surface of the water in the final settlement tank can also be effected using the hydraulic head between the tank and the bio zone.