Current through Register Vol. 30, No. 38, September 20, 2024
A. A 4.05 General
Permit allows for the use of a gravelless trench with less than 3000 gallons
per day design flow receiving wastewater treated to a level equal to or better
than that specified in R18-9-E302(B).
1.
Definition. For purposes of this Section, a "gravelless trench" means a
disposal technology characterized by installation of a proprietary pipe and
geocomposite or other substitute media into native soil instead of the
distribution pipe and aggregate fill used in a trench allowed in
R18-9-E302.
2. A permittee may use
a gravelless trench if suitable gravel or volcanic rock aggregate is
unavailable, excessively expensive, or if adverse site conditions make movement
of gravel difficult, damaging, or time consuming.
B. Performance. An applicant shall design a
gravelless trench so that treated wastewater released to the native soil meets
the following criteria:
1. TSS of 75
milligrams per liter, 30-day arithmetic mean;
2. BOD5 of 150
milligrams per liter, 30-day arithmetic mean;
3. Total nitrogen (as nitrogen) of 53
milligrams per liter, 5-month arithmetic mean; and
4. Total coliform level of 100,000,000
(Log10 8) colony forming units per 100 milliliters, 95th
percentile.
C. Notice of
Intent to Discharge. In addition to the Notice of Intent to Discharge
requirements specified in
R18-9-A301(B)
and
R18-9-A309(B),
an applicant shall submit the following:
1.
The soil absorption area that would be required if a conventional disposal
trench filled with aggregate was used at the site,
2. The configuration and size of the proposed
gravelless disposal works, and
3.
The manufacturer's installation instructions and warranty of performance for
absorbing wastewater into the native soil.
D. Design requirements. In addition to the
applicable requirements in
R18-9-A312,
an applicant shall:
1. Ensure that the top of
the gravelless disposal pipe or similar disposal mechanism is at least 6 inches
below the surface of the native soil and 12 to 36 inches below finished grade
if approved fill is placed on top of the installation;
2. Calculate the infiltration surface as
follows:
a. For 8-inch diameter pipe, 2 square
feet of absorption area is allowed per linear foot;
b. For 10-inch diameter pipe, 3 square feet
of absorption area is allowed per linear foot;
c. For bundles of two pipes of the same
diameter, the absorption area is calculated as 1.67 times the absorption area
of one pipe; and
d. For bundles of
three pipes of the same diameter, the absorption area is calculated as 2.00
times the absorption area of one pipe;
3. Use a pressure distribution system meeting
the requirements of R18-9-E304 in medium sand, coarse sand, and coarser soils;
and
4. Construct the drainfield of
material that will not decay, deteriorate, or leach chemicals or by products if
exposed to sewage or the subsurface soil environment.
E. Installation requirements. In addition to
the applicable requirements in
R18-9-A313(A),
an applicant shall:
1. Install the gravelless
pipe material according to manufacturer's instructions if the instructions are
consistent with this Chapter,
2.
Ensure that the installed disposal system can withstand the physical
disturbance of backfilling and the load of any soil cover above natural grade
placed over the installation, and
3. Shape any backfill and soil cover in the
area of installation to prevent settlement and ponding of rainfall for the life
of the disposal works.
F.
Operation and maintenance requirements. In addition to the applicable
requirements in
R18-9-A313(B),
the permittee shall inspect the finished grade in the vicinity of the
gravelless disposal works for maintenance of proper drainage and protection
from damaging loads.
