Handbook of Civil Engineering Calculations

ment be designed with a safety factor of at least 2. Therefore, in sizing aeration equipment

a value of (2)(6816 Ib O 2 AI) - 13,632 Ib O 2 /d (6188.9 kg O 2 /d) is used.

5. Compute the food to microorganism ratio (F:M)
   and the volumetric loading (VJ
   In order to maintain control over the activated sludge process, two commonly used pa-
   rameters are (1) the food to microorganism ratio (F :M) and, (2) the mean cell residence
   time (0C). The mean cell residence time was assumed in Part 1 "Compute Reactor Vol-
   ume" to be 8 days.
   The food to microorganism ratio is defined as:

?:M = S 0 OXa

where F:M is the food to microorganism ratio in d~l.
F:M is simply a ratio of the "food" or BOD 5 of the incoming waste, to the concentra-
tion of "microorganisms" in the aeration tank or MLVSS. Therefore, using values defined
previously:

F:M = (0.208 d)(3600 mg/L) = °' 321 d '

Typical values for F:M reported in literature vary from 0.05 d~l to 1.0 d~l depending
on the type of treatment process used.
A low value of F:M can result in the growth of filamentous organisms and is the most
common operational problem in the activated sludge process. A proliferation of filamen-
tous organisms in the mixed liquor results in a poorly settling sludge, commonly referred
to as "bulking sludge."
One method of controlling the growth of filamentous organisms is through the use of a
separate compartment as the initial contact zone of a biological reactor where primary ef-
fluent and return activated sludge are combined. This concept provides a high F:M at con-
trolled oxygen levels which provides selective growth of floe forming organisms at the
initial stage of the biological process. An F:M ratio of at least 2.27 d~l in this compart-
ment is suggested in the literature. However, initial F:M ratios ranging from 20-25 d~l
have also been reported.
The volumetric (organic) loading (V 1 ) is defined as:

VL = S 0 QW, = S 0 /O

V 1 is a measure of the pounds OfBOD 5 applied daily per thousand cubic feet of aera-

tion tank volume. Using values defined previously:

VL = (240 mg/L)/(0.208 d) = 1154 mg/L-d = 72 Ib/10^3 ft^3 -d (1.15 kg/Mm^3 -d)

Volumetric loading can vary from 20 to more than 200 Ib/10^3 ft^3 -d (0.32 to 3.2

kg/Mm^3 •</), and may be used as an alternate (although crude) method of sizing aeration

tanks.

6. Compute the waste activated sludge (WAS) and return activated
   sludge (RAS) requirements
   Control of the activated sludge process is important to maintain high levels of treatment
   performance under a wide range of operating conditions. The principle factors used in
   process control are (1) maintaining dissolved-oxygen levels in the aeration tanks, (2) reg-
   ulating the amount of Return Activated Sludge (RAS), and (3) controlling the Waste Ac-