Rotating Biological Contactor
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.
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.