Page 440 - 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:M S X (13.17)
O a
–1
where F:M is the food to microorganism ratio in d .
F:M is simply a ratio of the “food” or BOD of the incoming waste, to the
5
concentration of “microorganisms” in the aeration tank or MLVSS.
Typical values for F:M reported in literature vary from 0.05 d –1 to 1.0 d –1
depending on the type of treatment process used.
The volumetric (organic) loading (V ) is defined as
L
V L S O Q/V r S O / (13.18)
V is a measure of the pounds of BOD applied daily per thousand cubic feet
L 5
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:

Q , X Control volume
w
Q, S 0 Q + Q r Q e , S, X e
X
Aeration tank Settling
X, S, V r Return tank
Q , x , S line
r
r
FIGURE 13.3 Aeration tank mass balance. (Hicks—Handbook of Civil
Engineering Calculations, McGraw-Hill.)

Control volume

Q, S o Q + Q r Q , S, X e
e
X
Settling
Aeration tank
tank
X, S, V r
r
Q r , X , S Return line
, X
Q w r
FIGURE 13.4 Settling tank mass balance. (Hicks—Handbook of Civil Engineering
Calculations, McGraw-Hill.)