A rotating biological contactor is formed about a polygonal shaft. The contactor includes a hub formed of a series of hub rings disposed side by side along the length of the shaft with each hub ring formed of hub segments which are joined end to end. In one embodiment the hub segments are identical and there is one hub segment for each corner of the polygonal shaft. In a second embodiment there is one hub segment for each two corners. The hub segments also include a mounting portion which has welded to it alternating flat and formed thin walled sectors of contactor media. The sectors are joined to each by welding and successive layers of the sectors are angularly offset from each other so that some of the sectors span the joints between adjacent hub segments of a hub ringRotating biological contactors are used in secondary biological wastewater treatment processes. The contactors provide surfaces for the growth of a biomass which has the ability to absorb, adsorb, coagulate and oxidize undesirable organic constituents of the wastewater and to change them into unobjectionable forms of matter. The contactors are typically rotated partially submerged in wastewater in a treatment tank so that the surfaces are alternately exposed to the wastewater and to oxygen in the overlying atmosphere. A film of wastewater is carried into the air and trickles down the surfaces of the contactor while absorbing oxygen from the air. Organisms in the biomass remove dissolved oxygen and organic materials from the film of wastewater and unused dissolved oxygen in the wastewater film is mixed with the contents of the mixed liquor in the tank.
Initially the rotating biological contactors were simple flat discs spaced along a support shaft. Originally formed of sheet metal, the flat discs have more recently been formed of a foam plastic material. Discs offer a limited surface area in relation to the volume which they occupy, and other approaches have been employed to increase the surface area in relation to the volume of the envelope of the contactor. One approach has been to build up the contactor from sheets of thermoformed plastic which are joined together along the length of the axis of the shaft and define a series of passageways through which the wastewater flows. The passageways define large surface areas which are contacted by the wastewater and support the growth of the biomass. Another approach is to wind a formed sheet of thermoplastic in a spiral about a central shaft. The formed sheet can be provided with a series of cups or other protuberances which hold the layers of the convolute apart and also increase the surface area. A rotating biological contactor in accordance with the invention is formed of a series of hub segments joined together end to end to form a ring adapted to surround a polygonal shaft.
