CIVIL ENGINEERING FORMULAS

366 CHAPTER THIRTEEN

It is recommended that aeration equipment be designed with a factor of
safety of at least 2.
The food to microorganism ratio is defined as

F:MSO Xa (13.17)

where F:M is the food to microorganism ratio in d–1.
F:M is simply a ratio of the “food” or BOD 5 of the incoming waste, to the
concentration of “microorganisms” in the aeration tank or MLVSS.
Typical values for F:M reported in literature vary from 0.05 d–1to 1.0 d–1
depending on the type of treatment process used.
The volumetric (organic) loading (VL) is defined as

(13.18)

VLis a measure of the pounds of BOD 5 applied daily per thousand cubic feet
of aeration tank volume.
To determine the waste activated sludge (WAS) and return activated sludge
(RAS), Figs. 13.3 and 13.4 are used, along with the following variables:

VLSOQ/VrSO/

Q,S 0

X,S,Vr

Control volume

Aeration tank

Qr,xr,S

Return

line

Settling

tank

Qe,S,Xe

Qw,X

Q + Qr

X

FIGURE 13.3 Aeration tank mass balance. (Hicks—Handbook of Civil

Engineering Calculations, McGraw-Hill.)

Settling

tank

Q,So

X,S,Vr

Aeration tank

Qr,Xr,S

Qw,Xr

Return line

Q + Qr Qe,S,Xe

X

Control volume

FIGURE 13.4 Settling tank mass balance. (Hicks—Handbook of Civil Engineering

Calculations, McGraw-Hill.)