Environmental Engineering Reference
In-Depth Information
Zoogleal slime
Oxygen
Media
Sloughings
Organic
matter
Wa stewater holding tank
figure 7.6 Rotating biological contactor (RBC) cross-section and treat-
ment system.
7.4 roTaTing biologiCal ConTaCTors
The rotating biological contactor (RBC) is a biological treatment
system (see Figure 7.6) that is a variation of the attached growth idea
provided by the trickling filter. Because it relies on microorganisms that
grow on the surface of a medium, the basic biological process of this
fixed-film
biological treatment device is similar to that of the trickling
filter. An RBC consists of a series of closely spaced (mounted side by
side), circular, plastic (synthetic) disks that are typically about 3.5 m in
diameter and attached to a rotating horizontal shaft (see Figure 7.6).
Approximately 40% of each disk is submersed in a tank contain-
ing the wastewater to be treated. As the RBC rotates, the attached bio-
mass film (zoogleal slime) that grows on the surface of the disk moves
into and out of the wastewater. While submerged in the wastewater,
the microorganisms absorb organics; when they are rotated out of the
wastewater they are supplied with the oxygen necessary for aerobic
decomposition. As the zoogleal slime reenters the wastewater, excess
solids and waste products are stripped off the media as
sloughings
.
These sloughings are transported with the wastewater flow to a settling
tank for removal.
Modular RBC units are placed in series (see Figure 7.7), simply
because a single contactor is not sufficient to achieve the desired level
of treatment; the resulting treatment achieved exceeds conventional
secondary treatment. Each individual contactor is called a
stage
and
the group is known as a
train
. Most RBC systems consist of two or
more trains with three or more stages in each. The key advantage in
using RBCs instead of trickling filters is that RBCs are easier to operate
under varying load conditions, as it is easier to keep the solid medium
wet at all times. Moreover, the level of nitrification that can be achieved
by an RBC system is significant, especially when multiple stages are
employed.
