Handbook of Civil Engineering Calculations

FIGURE 2. Aeration tank mass balance.

gesters, etc.) is not 100 percent and some solids are returned, the actual wasting rate will
be higher than the theoretically determined value.
(a) Waste Activated Sludge (WAS) pumping rate from the return line. If the mean cell
residence time is used for process control and the wasting is from the sludge return line
(Fig. 1), the wasting rate is computed using the following:

0

V

'

X

C (QW'*r +QeXe)

Assuming that the concentration of solids in the effluent from the settling tank (Xe) is

low, then the above equation reduces to:

_ vrx _ vrx

°

c

~ Qw,xr

Qw> =

~e^xr

Using values defined previously:

*< - TSffST -""H*-*—-*"" -*>

To determine the WAS pumping rate using this method, the solids concentration in

both the aeration tank and the return line must be known.

If the food to microorganism ratio (F :M) method of control is used, the WAS

pumping rate from the return line is determined using the following:

P*& = 2^(8.34 Ib/Mgal/mg/L)

Therefore:

C" = (8000 (^9) m^(d8.34) = °' 059 M^ = (^59) '°° (^0) ^(223' 3 rf/«
In this case, the concentration of solids in the sludge return line must be known. Note
that regardless of the method used for calculation, if wasting occurs from the return line,
the WAS pumping rate is approximately the same.
Return Line
Control Volume