Description of Sewage / Effluent Treat Process RBC Process
The
RBC process involves allowing the wastewater to come in contact with a biological
medium in order to remove pollutants in the wastewater before discharge of the
treated wastewater to the environment, usually a body of water (river, lake or
ocean). A rotating biological contactor is a type of secondary treatment
process. It consists of a series of closely spaced, parallel discs mounted on a
rotating shaft which is supported just above the surface of the waste water.
Microorganisms grow on the surface of the discs where biological degradation of
the wastewater pollutants takes place.
The
rotating packs of disks (known as the media) are contained in a tank or trough
and rotate at between 2 and 5 revolutions per minute. Commonly used plastics
for the media are polyethylene, PVC and expanded polystyrene. The shaft is
aligned with the flow of wastewater so that the discs rotate at right angles to
the flow with several packs usually combined to make up a treatment train.
About 40% of the disc area is immersed in the wastewater.
Biological
growth is attached to the surface of the disc and forms a slime layer. The
discs contact the wastewater with the atmospheric air for oxidation as it
rotates. The rotation helps to slough off excess solids. The disc system can be
staged in series to obtain nearly any detention time or degree of removal required.
Since the systems are staged, the culture of the later stages can be acclimated
to the slowly degraded materials.
The
discs consist of plastic sheets ranging from 2 to 4 m in diameter and are up to
10 mm thick. Several modules may be arranged in parallel and/or in series to
meet the flow and treatment requirements. The discs are submerged in waste
water to about 40% of their diameter. Approximately 95% of the surface area is
thus alternately submerged in waste water and then exposed to the atmosphere above
the liquid. Carbonaceous substrate is removed in the initial stage of RBC.
Carbon conversion may be completed in the first stage of a series of modules,
with nitrification being completed after the 5th stage. Most design of RBC
systems will include a minimum of 4 or 5 modules in series to obtain
nitrification of waste water.
Biofilms,
which are biological growths that become attached to the discs, assimilate the
organic materials in the wastewater. Aeration is provided by the rotating
action, which exposes the media to the air after contacting them with the
wastewater, facilitating the degradation of the pollutants being removed. The
degree of wastewater treatment is related to the amount of media surface area
and the quality and volume of the inflowing wastewater.
The
system is typically covered but not enclosed; air can circulate through the
white vents visible in the above photographs. Intermittent odours will also be produced
be from irregular activities such as the bi-quarterly de-sludge, where
displaced air from tankers being filled will constitute the most likely source.
