Rotating Biological Contactor

A rotating biological contactor (or RBC) is a type of fixed media filter which removes both organic matter and ammonia from water.  It can be added to a packaged plant for more efficient ammonia removal, replacing the aerator in both location and function.  Although RBC's are less prevalent than trickling filters or oxidation ditches, they produce a high quality effluent and wastewater operators should be familiar with them.  

The RBC consists of a series of rotating discs.  These discs are coated with a biological slime like the slime on rocks in a healthy stream.  This slime is rotated through the air and and then through the wastewater so that it picks up oxygen in the air and breaks down B.O.D. in the wastewater.  Since the discs rotate through the air, there is no need to pump air into the wastewater.  And since the slime stays on the discs, there is no need to recycle sludge.  

There are only two RBC's operating in the southwest Virginia area, one in Conaway and the other in Haysi.  The water/wastewater students at Mountain Empire Community College were recently given the opportunity to tour the site at Haysi.  This treatment system will be described below.  It is typical of an RBC system and is very similar to a trickling filter system.  


The influent from the service area is collected in a 50-foot wet well.  Pretreatment begins when the wastewater is passed through a basket and is pumped to the grit chamber.  Then the influent flows through the grinder pump and into the primary clarifier.  In the primary clarifier, solids are removed and pumped to the digester.  The rest of the influent is now ready for the RBC.  


The RBC's discs are supported on a single shaft which is slowly rotated through the wastewater by an air driven motor.  The RBC is covered by a removable fiberglass housing which has access portals at each end.  

The discs are covered with a thick coating of slime.  This slime is the microorganisms, both aerobic and anaerobic, which treat the wastewater.  RBC's act much like a trickling filter in that the contactors perform well at removing B.O.D. and converting ammonia ( NH4OH) to nitrates (NO3).  The efficiency of an RBC is about 85%.

Clarifier and Digester

After being treated in an RBC, the influent is sent to the secondary clarifier.  Excess slime which has built up on the RBC and has sloughed off into the wastewater is carried to the secondary clarifier as well.  This sloughed off slime and other sludge settles to the bottom of the clarifier and is pumped to the sludge digester.

The supernate collected from the top of the digester is re-circulated through the plant as in the trickling filter system.  

The Rest of the Process

The supernate from the top of the secondary clarifier is treated with chlorine to destroy most of the remaining coliforms and bacteria.  Then the water is dechlorinated using sulphur dioxide which removes the excess chlorine from the water.  This step protects the environment from the harmful effects of chlorine.  

The final treatment step is to raise the oxygen level in the water.  This is achieved by running the effluent over step aerators before releasing the water into the south fork of the Big Sandy River.