Rotating Biological Contactors | Activated Sludge Process | Wastewater Treatment Plant

Rotating Biological Contactors

A previously common fixed film reactor that seems to be losing favor of late is the rotating biological contactor, or rotating biological contactors (RBCs) unit. These units are constructed of bundles of plastic sheets attached to a shaft. The shaft rotates over a rounded bottom tank with a portion of the plastic submerged at all times. The concept behind a RBC unit is to provide a constantly wet surface for a biological slime to grow on while minimizing the amount of air that needs to be supplied to the wastewater to maintain aerobic conditions. By rotating large bundles of plastic sheets into and out of the wastewater on a regular, set time basis, wastewater slime can accumulate on the surface of the plastic disks. When a portion of the disk is submerged, the slime absorbs nutrients from the wastewater for use in biological growth. When that area of the disk is out of the water, the slime absorbs oxygen from the surrounding atmosphere and assimilates the nutrients into new plant growth that is ready to receive new nutrients when the disk re-enters the wastewater.

RBCs typically have about 40 percent of the depth immersed in the wastewater at any time, but can be greater than 50 percent submerged if good aeration of the wastewater can be assured. The addition of the needed aeration capacity to maintain submergence greater than about 40 percent is typically so high for operating costs that the lower submergence and greater number of disks is more cost-effective over time. If the dissolved oxygen in the wastewater falls below a certain level and a portion of the disc is always submerged, the biological growth on that portion of the disc can become anaerobic, resulting in odours and potential operational problems with sludge management. Therefore, if submergence is deep, aeration is added to enhance the aeration of the biological film and to assist with rotation of the bundles. Air cups are provided on the outside perimeter of the bundles by some manufacturers to assist with the bundle rotation, thereby saving energy costs associated with turning the massive bundles.