Wastewater Calculations - Mathematics and Statistics for the Environment - page 672

3/2

QCAgHCH = 2 = (24.13)

d w

where
Q = Width of the channel.
C d = Discharge coefficient.
A = Cross-sectional area of the channel.
H = Depth of flow in the channel.
C w = Equal to Cg 2 .

d

The horizontal velocity ( v h ) is related to the discharge rate and channel velocity by

1/3

Q
A

Q
wH

Q
C

1/2

v == = CH = C

(24.14) h

w

24.3 PRIMARY TREATMENT
Primary treatment (primary sedimentation or clarification) should remove both settleable organic
and floatable solids. Poor solids removal during this step of treatment may cause organic overload-
ing of the biological treatment processes following primary treatment. Normally, each primary
clarification unit can be expected to remove 90 to 95% of settleable solids, 40 to 60% of total sus-
pended solids, and 25 to 35% of BOD.

24.3.1 p roCess C ontrol C alCulations
As with many other wastewater treatment plant unit processes, several process control calculations
may be helpful in evaluating the performance of the primary treatment process. Process control
calculations are used in the sedimentation process to determine

• Surface loading rate (surface settling rate)
• Weir overlow rate (weir loading rate)
• BOD and suspended solids removed (lb/day)
• Percent removal
• Hydraulic detention time
• Biosolids pumping
• Percent total solids (%TS)

In the following sections, we take a closer look at a few of these process control calculations and
provide example problems.

Note: The calculations presented in the following sections allow us to determine values for each
function performed. Again, keep in mind that an optimally operated primary clarifier should
have values within an expected range. Recall that the expected ranges of percent removal for
a primary clarifier are

• Settleable solids, 90-95%
• Suspended solids, 40-60%
• Biochemical oxygen demand (BOD), 25-35%

The expected range of hydraulic detention time for a primary clarifier is 1 to 3 hr. The expected
range of surface loading/settling rate for a primary clarifier is 600 to 1200 gpd/ft 2 (ballpark esti-
mate). The expected range of weir overflow rate for a primary clarifier is 10,000 to 20,000 gpd/ft.