(4500 mg/L)(4^0 Mgd - 0.104 Mgd) =
*r 8000 mg/L-4500 mg/L^5 v 'The ratio of RAS pumping rate to influent flow rate, or recirculation ratio (a), may now
be calculated:
Qr 5.0 Mgd = f
a Q 4.0 MgdRecirculation 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.
It should be noted that if the control volume were placed around the aeration tank in
Fig. 1 and a mass balance performed, or the control volume placed around the settling
tank in Fig. 2 and a mass balance performed, that a slightly higher RAS pumping rate
would result. However, the difference between these RAS pumping rates and the ones
calculated above is negligible.
DESIGN OFA CIRCULAR SETTLING TANK
Domestic wastewater with an average daily flow of 4.0 Mgd (15,140 m
3
/d) exits the aera-
tion tank of a standard activated sludge treatment process. Design a circular settling tank
to separate the sludge from the effluent. The settling tank will work in conjunction with
the aeration tank. Assume a peaking factor of 2.5.
Calculation Procedure:- Determine the peak flow
Conventional examples of circular settling tank design utilize settling tests to size the
tanks. However, it is more common that settling facilities must be designed without the
benefit settling tests. When this situation develops, published values of surface loading
and solids loading rates are generally used. Because of the large amount of solids that
may be lost in the effluent if design criteria are exceeded, surface loading rates should be
based on peak flow conditions. Using a peaking factor of 2.5, the daily peak flow (Qp) is:
Qp = 2.5 x 4.0 Mgd = 10.0 Mgd (37,850 m^3 /d^- Find the settling tank surface area using surface loading criteria
The recommended surface loading rates (settling tank effluent flow divided by settling
tank area) vary depending upon the type of activated sludge process used. However, sur-
face loading rates ranging from 200 to 800 gal/day/ft^2 (8.09 to 32.4 L/m^2 -d) for average
flow, and a maximum of 1,000 gal/day/ft^2 (40.7 L/m^2 -d) for peak flow are accepted de-
sign values.
The recommended solids loading rate on an activated sludge settling tank also varies
depending upon the type of activated sludge process used and may be computed by divid-
ing the total solids applied by the surface area of the tank. The preferred units are Ib/ft^2 -h
(kg/m^2 -h). In effect, the solids loading rate represents a characteristic value for the sus-
pension under consideration. In a settling tank of fixed area, the effluent quality will dete-