(a) The
following procedure shall be used for disposal fields consisting of a disposal
bed or disposal trenches, which are at equal elevations.
1. Step One: Determine the length, number and
spacing of distribution laterals based upon the required size of the disposal
field, determined as prescribed in
7:9A-10.2, and the requirements for
spacing of disposal trenches or the requirements for spacing of distribution
laterals within disposal beds as prescribed in
7:9A-10.3(d). The
number of distribution laterals will also depend upon whether a central or end
manifold arrangement is used.
2.
Step Two: Select the hole diameter and spacing. The hole diameter shall be a
minimum of one-quarter inch but no larger than one-half inch. The minimum
allowed hole spacing shall be 30 inches. The maximum allowed hole spacing shall
be 60 inches, except in the case of systems installed in soils or fill material
with a permeability faster than six inches per hour or a percolation rate
faster than 15 minutes per inch, in which case the maximum allowed hole spacing
shall be 36 inches.
3. Step Three:
Based upon the hole diameter and the hole spacing selected and the length of
the laterals, determine the required diameter of laterals using Figure 14 of
Appendix A. If the disposal field configuration is such that it is beyond the
applicable limits of Figure 14, other methods of hydraulically evaluating
adequate lateral diameter may be used subject to prior approval by the
administrative authority.
4. Step
Four: Pressure distribution systems shall be designed so that a minimum
pressure head of 2.5 feet shall be maintained at the distal end of the
laterals. Based upon the hole diameter and the design pressure head at the
distal end of the laterals, determine the hole discharge rate from the table
below. Determine the lateral discharge rate by multiplying the hole discharge
rate by the number of holes per lateral.
| Pressure |
Discharge Rate (gallons
per minute) |
| Head (ft) |
based on Hole Diameter
(inches) |
| 1/4 |
5/16 |
3/8 |
7/16 |
1/2 |
| 2.5 |
1.18 |
1.85 |
2.66 |
3.63 |
4.73 |
| 3.0 |
1.28 |
1.99 |
2.87 |
3.91 |
5.10 |
| 3.5 |
1.40 |
2.19 |
3.15 |
4.29 |
5.60 |
| 4.0 |
1.47 |
2.30 |
3.31 |
4.51 |
5.89 |
| 4.5 |
1.59 |
2.48 |
3.57 |
4.86 |
6.35 |
| 5.0 |
1.65 |
2.57 |
3.71 |
5.04 |
6.59 |
5.
Step Five: Based upon the number of laterals and the lateral spacing, determine
the manifold length. Based upon the manifold length, the lateral discharge rate
and the number of laterals, using Figure 15 of Appendix A, determine the
required manifold diameter. If the disposal field configuration is such that it
is beyond the applicable limits of Figure 15, other methods of hydraulically
evaluating proper manifold diameter may be used subject to approval by the
administrative authority.
6. Step
Six: Determine the necessary system discharge rate by multiplying the lateral
discharge rate by the number of laterals; and
7. Step Seven: For pump systems, select the
proper pump as follows:
i. Using Figure 16 of
Appendix A, determine the friction head based upon the system discharge rate
and the diameter and length of the delivery pipe. If the system discharge rate
is such that it is beyond the applicable limits of Figure 16, then other
methods of determining friction head in the delivery pipe may be used subject
to approval by the administrative authority.
ii. Calculate the total operating head, H[t],
using the following formula:
H[t], ft = H[f] + H[e] + H[p]
where:
H[f] is the friction head, in feet, determined in (a)7i
above;
H[e] is the elevation head, in feet, calculated by
subtracting the dosing tank low water elevation from the elevation of the
invert of the distribution laterals; and
H[p] is the design pressure head to be maintained at the
distal end of the laterals, in feet.
iii. Choose a pump which is rated by the
manufacturer to deliver a flow rate equal to or greater than the system
discharge rate calculated in Step Six when working against a total dynamic head
equal to the total operating head calculated in (a)7ii above.
8. Alternate Step Seven: For
systems using syphons, determine the syphon elevation as follows:
i. Determine the friction head in the
delivery pipe as in (a)7i above.
ii. Calculate the velocity head using the
following formula:
H[v], ft = (D/A)[LESS THAN]2[GREATER THAN]/2g
where:
D = System Discharge Rate, ft[LESS THAN]3[GREATER
THAN]/sec.
= (System Discharge Rate, gpm) (1 ft[LESS THAN]3[GREATER
THAN]/7.48 gal) (1 min/60 sec)
A = pipe area, ft[LESS THAN]2[GREATER THAN]
= [(internal pipe diameter, in/2) (1 ft/12 in)][LESS
THAN]2[GREATER THAN] (3.14)
g = 32.2 ft/sec[LESS THAN]2[GREATER THAN]
iii. Calculate the total operating head,
H[t], by the following equation:
H[t], ft = H[f] + H[v] + H[p]
where:
H[f] is the friction head, in feet, determined from Figure 16
of Appendix A.
H[v] is the velocity head, in feet, determined in (a)8ii
above.
H[p] is the design pressure head to be maintained at the
supply end of the laterals, in feet.
iv. Choose a siphon rated to discharge at a
flow rate equal to or greater than the system discharge rate. Install the
siphon at an elevation such that the siphon invert is higher than the invert of
the distribution laterals by a distance equal to the total operating head
calculated in (a)7iii above.
(b) If a trench system is proposed where the
elevation of the infiltrative surface will not be the same in all trenches, the
design engineer must demonstrate by means of appropriate calculations to the
satisfaction of the administrative authority, that all portions of all trenches
will receive equal hydraulic loading in conformance with the requirements of
7:9A-10.2. One way of accomplishing
this would be to divide the disposal field into sections consisting of
individual trenches or groups of trenches which are at the same elevation and
which are dosed individually in conformance with the requirements of this
section.
