On-Site Wastewater Treatment and Disposal Systems - Forced ...

Step 3: Select lateral diameter. For l/4-in. hole diameter, 30-in.
hole spacing, and 40-ft length, Figure 7-28 indicates either a
l-l/4-in. or l-l/2-in. diameter lateral could be used. The l-
l/2-in. diameter is selected for this example.
Step 4: Calculate lateral. discharge rate. By maintaining higher
pressures in the lateral, small variations in elevation along
the length of the lateral and between laterals do not signifi-
cantly affect the rates of discharge from each hole. This
reduces construction costs, but increases pump size. For this
example, a 2-ft head is to be maintained in the lateral. For a
l/4-in. hole at 2 ft of head, Table 7-13 shows the hole dis-
charge rate to be 1.04 gpm.
Number of holes/lateral = 40-ft 'a~~a~p~,':~,"h
. -
= 16
Lateral discharge rate = (16 holes/lateral) x (1.04 gpm/hole)
= 16.6 gpm/lateral
Step 5: Select manifold size. There are to be five laterals spaced 9
ft apart. A manifold length of 36 ft is therefore required.
For five laterals and 16.6 gpm/lateral, Figure 7-29 indicates
that a 3-in. diameter manifold is required.
Step 6: Determine minimum dose volume (Figure 7-30).
With: lateral diameter = l-l/Z in.
lateral length = 40 ft
number of laterals = 5
Then: pipe volume = 3.7 gal
Minimum dose volume = approx. 200 gal
The final dose volume may be larger than this minimum depending
on the desired number of doses per day (see Table 7-4).
See Figure 7-31 for completed network design.
Step 7: Determine minimum discharge rate.
Minimum discharge rate = (5 laterals) x (16.6 gpm/lateral)
= 83 gpm
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