Basic Wastewater Treatment Terms: What Is a Rotating Biological Contactor (RBC)?
The rotating biological disk (rotating biological contactor) is a biological treatment structure composed of a water tank and a rotating disk body partly immersed in the wastewater. The microbial film growing on the disk surface repeatedly contacts the wastewater in the tank and the oxygen in the air, purifying the wastewater.
After primary treatment in a sedimentation tank, the wastewater contacts the biofilm; the microorganisms on the biofilm take up the organic pollutants in the wastewater as nutrients, purifying the water. In a pneumatic rotating biological contactor, the dissolved oxygen needed for microbial metabolism is supplied through an aeration pipe located below the rotating disk. The disk surface is covered with an air hood; the compressed air released from the aeration pipe drives the air hood to rotate the disk. When the disk leaves the wastewater, a thin layer of water forms on its surface, which also absorbs dissolved oxygen from the air.
The rotating biological contactor process is an artificial intensification of sewage irrigation and land treatment. In this treatment method, bacteria and microbial flora, and microfauna such as protozoa, grow and reproduce on the biofilm carrier of the rotating biological contactor, forming a membranous biological sludge — the biofilm. After primary treatment in a sedimentation tank, the wastewater contacts the biofilm; the microorganisms on the biofilm take up the organic pollutants in the wastewater as nutrients, purifying the water. In a pneumatic rotating biological contactor, the dissolved oxygen needed for microbial metabolism is supplied through an aeration pipe located below the rotating disk. The disk surface is covered with an air hood; the compressed air released from the aeration pipe drives the air hood to rotate the disk. When the disk leaves the wastewater, a thin layer of water forms on its surface, which also absorbs dissolved oxygen from the air.
As an aerobic biological reactor for wastewater treatment, the rotating biological contactor may be said to have appeared with the popularization of plastics. The reactor consists of a water tank and a set of disks: dozens or nearly a hundred plastic or FRP disks are strung on a shaft and laid flat on a strip-shaped tank with a semi-circular cross-section. The disk diameter generally does not exceed 4 m, the tank diameter is about a few centimeters larger, and a motor and reduction gear rotate the disk shaft at about 1.5–3 rpm depending on the disk diameter, with a peripheral linear velocity of the disk of about 15 m/min. Wastewater flows from one end of the tank to the other; the shaft is above the water surface, and about 40% of the disk surface is immersed in water and about 60% exposed to air. As the shaft rotates, the disk surface alternately contacts the wastewater and the air. The disk surface is covered by a membranous substance formed by microbial growth — the biofilm. The biofilm alternately contacts the wastewater and air fully, continuously acquiring pollutants and oxygen and purifying the wastewater. A shear stress is generated between the membrane and the disk surface as a result of rotation; as the membrane thickness increases, this stress increases until, at a certain point, the membrane falls off the disk surface and flows away with the water. Rotating biological contactors are generally used when water volumes are small. Compared with a biological filter, the rotating biological contactor method gives the wastewater and biofilm a longer contact time, and is somewhat controllable. The water tank is often divided into sections and the disks into groups, which both prevents short-circuiting and helps improve the load rate and effluent quality, since the load rate decreases step by step. If the rotating biological contactor produces an odor, it can be covered.
The integrated wastewater treatment device is a high-efficiency wastewater treatment device with the rotating biological treatment unit — the rotating biological contactor — at its core. The whole device is divided into the following treatment units:
1. Primary sedimentation tank: The wastewater is lifted by a lift pump from the equalization tank into the SW device, first entering the primary sedimentation tank, which uses an inclined-plate sedimentation tank; under gravity, using the shallow-sedimentation principle, most suspended solids and inorganic particles in the wastewater settle out, while also removing some organic matter. To facilitate the occasional removal of an inclined plate to clean off sludge that is difficult to slide off, the device uses movable inclined plates. The bottom of the primary sedimentation tank is separated from the anoxic zone to avoid disturbing the mixed liquor of the anoxic tank and affecting the sedimentation effect of the primary tank; the sludge of the primary tank is regularly removed by a vacuum truck.
2. Anoxic tank: The anoxic tank is located in the interlayer between the rotating biological contactor shell and the external tank. In this space, the inflow from the primary sedimentation tank, the returned nitrifying liquor lifted by a hydraulic rotor, and the returned sludge from the secondary sedimentation tank are mixed and fully blended by a submersible mixer, completing the denitrification process. Under the action of denitrifying bacteria, nitrate nitrogen finally forms nitrogen gas, escapes from the water, and ultimately achieves the purpose of nitrogen removal.
3. Rotating biological treatment unit — rotating biological contactor: The denitrified effluent from the interlayer anoxic tank flows by gravity to the rotating biological treatment unit, which is the core part of the device. It adopts a unique composite biochemical technology that can efficiently degrade pollutants under low-energy conditions. The entire rotating biological treatment unit consists of three biological reactors, each composed of a biological rotor and a biochemical tank; each biological rotor internally consists of multi-stage biological impellers, and each biological impeller is provided with a large number of spiral biological blades. Driven by the transmission device, the three biological rotors rotate synchronously; air (oxygen) enters through the gas-water holes on the end face of the biological rotor and mixes with the wastewater; through the three-phase contact and mass transfer of oxygen, wastewater and microorganisms, the degradation of carbon-containing organics and the nitrification of nitrogen-containing organics are achieved. At the same time, the rotating biological blades are scoured by the wastewater, aging biofilm falls off, and new biofilm forms, achieving the continuous renewal of the biological system. The nitrified wastewater is lifted by the hydraulic rotor to the intermediate distribution tank, which, controlled by a weir gate, distributes the flow to the sedimentation tank and the anoxic tank.
4. Secondary sedimentation tank: The secondary sedimentation tank uses an inclined-plate sedimentation tank; under gravity, using the shallow-sedimentation principle, the large amount of aged, fallen-off biofilm in the effluent from the rotating biological treatment unit is precipitated, and the clarified treated effluent enters the next unit. Part of the settled sludge is returned by a return-sludge pump to the anoxic tank, and the rest is periodically removed externally by a vacuum truck as excess sludge.
