Environmental Engineering Reference
In-Depth Information
5.3.3.5 Grit Removal Calculations
Wastewater systems typically average 1 to 15 ft
3
of grit per million
gallons of flow (sanitary systems, 1 to 4 ft
3
/MG; combined wastewater
systems, 4 to 15 ft
3
/MG), with higher flow occurring during storm events.
Generally, grit is disposed of in sanitary landfills. Because of this prac-
tice, for planning purposes, operators must keep accurate records of grit
removal. Most often, the data are reported as cubic feet of grit removed
per million gallons of flow:
Grit Volume (ft )
Flow
3
3
(5.5)
Grit Removed (ft /MG)
=
(MG)
Over a given period, the average grit removal rate at a plant (at least
a seasonal average) can be determined and used for planning purposes.
Typically, grit removal is calculated as cubic yards, because excavation
is normally expressed in terms of cubic yards:
TotalGrit (ft )
27 ft /y
3
3
Grit Removed(yd )
=
(5.6)
3
d
3
Example 5.5
Problem:
A treatment plant removes 10 ft
3
of grit in one day. How many
cubic feet of grit were removed per million gallons if the plant flow was
9 MGD?
Solution:
Grit Volume (ft )
Flow (MG)
3
10
ft
9MG
3
3
Grit Removed
=
=
=
1.1ft MG
Example 5.6
Problem:
The total daily grit removed for a plant is 250 gal. If the plant
flow is 12.2 MGD, how many cubic feet of grit are removed per million
gallons of flow?
Solution:
First, convert gallon grit removed to cubic feet:
250 gal
7.48 gal/ft
3
=
33 ft
3
Next, complete the calculation of cubic feet per million gallons:
Grit Volume (ft )
Flow (MG)
3
ft
12.2 MG
33
3
3
Grit Removed
=
=
=
2.7ft MG
