Primary Treatment (Sedimentation) - Wastewater Operators - page 253

 Example 6.2

Problem: A circular clarifier has a diameter of 50 ft. If the primary efflu-
ent flow is 2,150,000 gpd, what is the surface overflow rate in gpd/ft 2 ?

Solution:

Area = 0.78550ft50ft × ×

Flow (gpd)
Area (ft )

2,150,000
0.785

1096 gpd/ft 2 Surface Overflow Rate = = =

2

× 50 ft × 50 ft

6.3.4 Weir overflow rate

Weir overflow rate (weir loading rate) is the amount of water leaving
the settling tank per linear foot of weir. The result of this calculation
can be compared with design. Normally, weir overflow rates of 10,000 to
20,000 gpd/ft are used in the design of a settling tank.

Flow (gpd)
Weir Len

Weir Overflow Rate (gpd) = (6.2)

gth (ft)

 Example 6.3

Problem: The circular settling tank is 90 ft in diameter and has a weir
along its circumference. The effluent flow rate is 2.55 MGD. What is the
weir overflow rate in gallons per day per foot?

Solution:

2.55 MGD1,000,000 gal/M × G

Weir Overflow Rate = = 9023 gpd/ft

3.14 × 90 ft

6.3.5 sludge Pumping

Determination of sludge pumping (the quantity of solids and vola-
tile solids removed from the sedimentation tank) provides the accurate
information required for process control of the sedimentation process.

Solids (lb/day) = PumpRate × PumpTime8.34lb × /gal × %Solids

(6.3)

Volatile Matter (lb/day) = Pump Rate × Pump Time × 8.34 lb/gal
× % Solids × % Volatile Matter

(6.4)