Wastewater Calculations - Mathematics and Statistics for the Environment - page 680

■ EXAMPLE 24.29
Problem: A trickling filter receives a primary effluent flow of 5 MGD. If the recirculated flow is 4.6
MGD, what is the recirculation ratio?

Solution:

Recirculated flow (MGD) 4.6 MGD

5MGD

Recirculationratio = = = .92 0

Primary effluent flow (MGD)

24.5.1.5 Trickling Filter Design
In trickling filter design, the parameters used are hydraulic loading and BOD:

QR
A

o

Hydraulic loading = (2 4.21)

where
Q o = Average wastewater flow rate (MGD).
R = Recirculated flow = Q o × circulation ratio.
A = Filter area (acres).

8340 (BOD )(

Q

)

s

BODloading = (24.22)

V

where
BOD s = Settled BOD 5 from primary treatment (mg/L).
Q o = Average wastewater flow rate (MGD).
V = Filter volume (ft 3 ).
8340 = Conversion of units.

24.6 ROTATING BIOLOGICAL CONTACTORS
The rotating biological contactor (RBC) is a variation of the attached growth idea provided by the
trickling filter. Still relying on microorganisms that grow on the surface of a medium, the RBC is
instead a fixed-film biological treatment device (see Figures 24.5 and 24.6). The basic biological
process, however, is similar to that occurring in trickling filters. An RBC consists of a series of
circular plastic disks mounted side by side and closely spaced; they are typically about 11.5 ft in
diameter. Attached to a rotating horizontal shaft, approximately 40% of each disk is submersed in
a tank that contains the wastewater to be treated. As the RBC rotates, the attached biomass film
(zoogleal slime) that grows on the surface of the disks 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 needed oxygen 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.

24.6.1 rbC p roCess C ontrol C alCulations
Several process control calculations may be useful in the operation of an RBC. These include sol-
uble BOD, total media area, organic loading rate, and hydraulic loading. Settling tank calculations
and biosolids pumping calculations may be helpful for evaluation and control of the settling tank
following the RBC.