MIL-HDBK-1138
wood is attached across the end of the stick to form a "foot,"
and the stick is pushed down through the scum to the liquid
layer. When the stick is moved up, the foot meets resistance on
the bottom of the scum layer, and the contractor marks the stick
at the top of the layer to measure the total thickness. As a
general guideline, if the scum layer is within 3 inches (8 cm)
of the bottom of the inlet baffle, the tank should be pumped.
The solids layer is measured by wrapping cloth around
the bottom of the stick and lowering it to the bottom of the
tank. Insert the stick either through a hole in the scum layer
or through the baffle or tee, if possible, to avoid getting scum
on the cloth. The solids depth can be estimated by the length of
solids sticking to the cloth. If the solids depth is equal to
one-third or more of the liquid depth, the tank should be pumped.
Removal of Settleable Solids. Clean the tank
3.2.2
whenever the bottom of the floating scum layer is within 8 inches
(20 cm) of the bottom of the outlet device. Table 1 shows the
estimated tank pumping frequencies, based on tank size and
household size.
Table 1
Estimated Septic Tank Pumping Frequencies in Years(1)
Household Size
Tank Size
(number of people)
(gallons)
1
2
3
4
5
6
500
5.8
2.6
1.5
1.0
0.7
0.4
750
9.1
4.2
2.6
1.8
1.3
1.0
900
11.0
5.2
3.3
2.3
1.7
1.3
1,000
12.4
5.9
3.7
2.6
2.0
1.5
1,250
15.6
7.5
4.8
3.4
2.6
2.0
1,500
18.9
9.1
5.9
4.2
3.3
2.6
1,750
22.1
10.7
6.9
5.0
3.9
3.1
2,000
25.4
12.4
8.0
5.9
4.5
3.7
2,250
28.6
14.0
9.1
6.7
5.2
4.2
2,500
31.9
15.6
10.2
7.5
5.9
4.8
(1) These figures assume no garbage disposal is in use.
Source: Pennsylvania State University Cooperative Extension
Service, as reprinted in Pipeline: Maintaining Your Septic
23
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