Handbook of Civil Engineering Calculations

tivated Sludge (WAS). As outlined previously in Part 5 "Compute the Food to Microor-
ganism Ratio and the Volumetric Loading," the most commonly used parameters for con-
trolling the activated sludge process are the F:M ratio and the mean cell residence time
(0C). The Mixed Liquor Volatile Suspended Solids (MLVSS) concentration may also be
used as a control parameter. Return Activated Sludge (RAS) is important in maintaining
the MLVSS concentration and the Waste Activated Sludge (WAS) is important in con-
trolling the mean cell residence time (0C).
The excess waste activated sludge produced each day (see step 3 "Compute the Quan-
tity of Sludge Wasted") is wasted from the system to maintain a given F:M or mean cell
residence time. Generally, sludge is wasted from the return sludge line because it is more
concentrated than the mixed liquor in the aeration tank, hence smaller waste sludge
pumps are required. The waste sludge is generally discharged to sludge thickening and di-
gestion facilities. The alternative method of sludge wasting is to withdraw mixed liquor
directly from the aeration tank where the concentration of solids is uniform. Both meth-
ods of calculating the waste sludge flow rate are illustrated below.
Use Figs. 1 and 2 when performing mass balances for the determination of RAS and
WAS.

X= Mixed Liquor Suspended Solids (MLSS)—see Part 1 "Compute the Reactor

Volume."

Qr = Return activated sludge pumping rate (Mgd)

Xr = Concentration of sludge in the return line (mg/L). When lacking site specific op-

erational data, a value commonly assumed is 8000 mg/L.

Q 6 = Effluent flow rate (Mgd)

Xe = Concentration of solids in effluent (mg/L). When lacking site specific opera-

tional data, this value is commonly assumed to be zero.

Qw = Wasted Activated Sludge (WAS) pumping rate from the reactor (Mgd)

Qw> = Waste Activated Sludge (WAS) pumping rate from the return line (Mgd)

Other variables are as defined previously.
The actual amount of liquid that must be pumped to achieve process control depends
on the method used and the location from which the wasting is to be accomplished. Also
note that because the solids capture of the sludge processing facilities (i.e., thickeners, di-

Control Volume

Return Line

FIGURE 1. Settling tank mass balance.