Environmental Engineering Reference
In-Depth Information
In the following sections we take a closer look at a few of these process
control calculations and example problems.
Note:
The calculations presented in the following sections allow us to
determine values for each function performed. Keep in mind that an
optimally operated primary clarifier should have values in an expected
range.
6.3.1 Percent removal
The expected ranges of percent removal for a primary clarifier are:
Settleable solids
90-95%
Suspended solids
40-60%
BOD
5
25-35%
6.3.2 detention Time
The primary purpose of primary settling is to remove settleable
solids. This is accomplished by slowing the flow down to approximately
1 fpm. The flow at this velocity will stay in the primary tank from 1.5
to 2.5 hr. The length of time the water stays in the tank is called the
hydraulic detention time
.
6.3.3 surface loading rate
The surface loading rate is the number of gallons of wastewater
passing over 1 ft
2
of tank per day. This can be used to compare actual
conditions with design. Plant designs generally use a surface loading
rate of 300 to 1200 gpd/ft
2
. Other terms used synonymously with surface
loading rate are
surface overflow rate
and
surface settling rate
.
Flow (gpd)
2
=
Surface SettlingRate(gpd/ft )
(6.1)
2
S
ettlingTankArea (ft )
Example 6.1
Problem:
A settling tank is 120 ft in diameter, and the flow to the unit is
4.5 MGD. What is the surface loading rate in gpd/ft
2
?
Solution:
4.5MGD
×
1,000,000 gal/
MGD
398gpd/ft
2
Surface LoadingRate
=
=
0.785
×
120ft120 ft
×
