CIVIL ENGINEERING FORMULAS

368 CHAPTER THIRTEEN

The ratio of RAS pumping rate to influent flow rate, or recirculation ratio (),
may now be calculated:

(13.24)

Recirculation ratio can vary from 0.25 to 1.50 depending upon the type of
activated sludge process used. Common design practice is to size the RAS
pumps so that they are capable of providing a recirculation ratio ranging from
0.50 to 1.50.

DESIGN OF A CIRCULAR SETTLING TANK

Circular settling tanks are widely used in domestic wastewater treatment. Sur-
face loading rates for settling tanks should be based on the peak flow condition
anticipated, using a peaking factor to adjust for unanticipated overloads. Or,

Qp(average daily load, Mgd) (peaking factor) (13.25)

A peaking factor of 2.0 to 4.0 might be chosen by a designer based on the prob-
able load on the system.
The surface area of the settling tank is

AQp/(design surface loading rate, gal/day/sq ft) (13.26)

The toal solids load on a clarifier consists of loads from the influent and the
Return Activated Sludge (RAS). Then:

QrRAS flow rate 1.25 (Q) (13.27)

XMLSS in aeration tank

Therefore, the maximum solids loading occurs at peak flow and maximum
RAS flow rate. The maximum solids entering the clarifier is calculated using:

Max solids (lb/d) (QpQr)(X)(8.34) lbL/mgMgal) (13.28)

The surface area needed, based on an assumed solids loading rate (such as
2.0 (lb/sqfth) at peak flow) is

A(maximum solids)/(assumed solids loading rate) (13.29)

Since two required areas have been computed, designers choose the larger
of the two areas to provide safety in the design. For backup purposes two tanks
are normally chosen. The actual solids loading can be compared with the
assumed loading using

A(solids processed, lb/h)/(total settling tank surface area, sq ft) (13.30)

α

Qr

Q