riorate if solids loading is increased beyond the characteristic value for the suspension.
Without extensive experimental work covering all seasons and operating variables, high-
er rates should not be used for design. The recommended solids loading rates vary de-
pending upon the type of activated sludge process selected. However, solids loading rates
ranging from 0.8 to 1.2 Ib/ft
2
-h (3.9 to 5.86 kg/m
2
-h) for average flow, and 2.0 Ib/ft
2
-h
(9.77 kg/m
2
-h) for peak flow are accepted design values.
For a Qp of 10.0 Mgd and a design surface loading rate of 1,000 gal/day/ft^2 at peak
flow, the surface area (A) of a settling tank may be calculated:

0 1Ox^10

(^6) gal/day
1000 gal/day/ft
2

/.
-i£SQf-
iM
»<»''">

3. Find the settling tank surface area using solids loading criteria
   The total solids load on a clarifier consists of contributions from both the influent and the
   return activated sludge (RAS). Assume the following (see "Design of A Complete-Mix
   Activated Sludge Reactor"):

Qr = RAS flow rate = 1.25(0 =. 1.25(4.0 Mgd) - 5.0 Mgd (18,925 m^3 /d)

X = MLSS in aeration tank = 4,500 mg/L

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 (Ib/d) = (Qp + gr)(A)(8.34 lb-L/mg-Mgal)

Using values given above;

Max Solids (Ib/d) = (10 Mgd + 5 Mgd)(4,500 mg/L)(8.34 Ib-L/mg-Mgal)

= 562,950 Ib/d

= 23,456 Ib/h (10,649 kg/h) of Suspended Solids

Therefore, using a solids loading rate of 2.0 Ib/ft^2 -h (9.77 kg/m^2 -h) at peak flow, the sur-
face area of a settling tank may be calculated:

A-^-".™<'«».

In this case, the solids loading dominates and dictates the required settling tank area.

4. Select the number of settling tanks
   Generally, more than one settling thank would be constructed for operational flexibility.
   Two tanks will be sufficient for this example. Therefore, the surface area of each tank will
   be 11,728 ft
   2
   /2 = 5,864 ft
   2
   (544.8 m
   2
   ). The diameter of each settling tank is then 86.41 ft.
   Use 87 ft (26.5 m). The total area of the two settling tanks is 11,889 ft
   2
   (110.4.5 m
   2
   ). For
   an average flow rate of 4.0 Mgd (15,140 m
   3
   /d) and a RAS flow rate of 5.0 Mgd (18,925
   m
   3
   ), the total solids entering the settling tanks at average daily flow is: