| (v) Color coding of distribution system. All new distribution
piping, fittings, valve box covers, and sprinkler tops shall be permanently
colored purple to identify the system as a reclaimed water system
according to Chapter 210 of this title (relating to Use of Reclaimed
Water).
(3) Mound drainfields. A mound drainfield is an absorptive
drainfield constructed above the native soil surface. The mound consists
of a distribution area installed within fill material placed on the
native soil surface. The required area of the fill material is a function
of the texture of the native soil surface, the depth of the native
soil, basal area sizing considerations, and sideslope requirements.
A description of mound construction, as well as construction requirements
not addressed in this section can be found in the North Carolina State
University Sea Grant College Publication UNC-SG-82-04 (1982).
(A) A mound drainfield shall only be installed at a
site where there is at least one foot of native soil; however, approval
for installation on sites with less than one foot of native soil may
be granted by the permitting authority on a case-by-case basis.
(B) Mounds and mound distribution systems must be constructed
with the longest dimension parallel to the contour of the site.
(C) Soil classification, loading rates (R(a)), and
wastewater usage rates (Q) shall all be obtained from this chapter.
(D) The depth of soil material (with less than 30%
gravel) between the bottom of the media and a restrictive horizon
must be at least 1.5 feet to the restrictive horizon or two feet to
groundwater. The soil material includes both the fill and the native
soil.
(E) The distribution area is defined as the interface
area between the media containing the distribution piping and the
fill material or the native soil, if applicable. The distribution
length is the dimension parallel with the contour and equivalent to
the length of the distribution media which must also run parallel
with the contour. The distribution lines within the distribution media
must extend to 12 inches of the end of the distribution media. The
distribution width is defined as the distribution area divided by
the distribution length.
(i) The formula A(d) = Q/R(a) shall be used for calculating
the minimum required distribution area of the mound where:
Attached Graphic
(ii) The area credited toward the minimum required
distribution area can be determined in either of the following ways.
(I) If the distribution area consists of a continuous
six-inch layer of media over the fill, the credited area is the bottom
interface area between the media and soil beneath the media.
(II) If the distribution area consists of rows of media
and distribution piping, the credited area can be calculated using
the formulas listed in paragraph (1)(C)(i)(I) or (II) of this subsection
depending on the depth of the media.
(iii) For sites with greater than 2% slopes and solid
bedrock, saturated zones, or class IV horizons within two feet of
the native soil surface, the length to width ratio of the distribution
area must be at least 7:1. For sites with greater than 2% slopes and
no solid bedrock, saturated zones, or class IV horizons within two
feet of the native soil surface, the length to width ratio of the
distribution area must be at least 4:1. No length to width ratio is
required on a site with 2% slope or less.
(iv) Effluent must be pressure dosed into the distribution
piping to ensure equal distribution and to control application rates.
(v) If a continuous layer of media is used, the dosing
lines must not be spaced more than three feet apart. If rows of media
are used, the rows may be as close as three feet apart, measured edge
to edge.
(vi) The dosing holes must not be greater than three
feet apart.
(F) The basal area is defined as the interface area
between the native soil surface and the fill material. The formula
A(b) = Q/R(a) must be used for calculating the minimum required basal
area of the mound where: A(b) = minimum required basal absorptive
area in square feet; Q = design wastewater usage rate in gallons per
day; R(a) = application rate of the native soil surface in gallons
per square foot per day.
(i) On sites with greater than 2% slope, the area credited
toward the required minimum basal area is computed by multiplying
the length of the distribution system by the distance from the upslope
edge of the distribution system to the downslope toe of the mound.
(ii) On sites with 2% slopes or less, the area credited
toward the minimum required basal area sizing includes all areas below
the distribution system as well as the side slope area on all side
slope areas greater than six inches deep.
(G) Mounds shall only be installed on sites with less
than 10% slope.
(H) The toe of the mound is considered the edge of
the soil absorption system.
(I) The side slopes must be no steeper than three to
one.
(J) There must be at least six inches of backfill over
the distribution media and the mound shall be crowned to shed water.
(4) Soil substitution drainfields. Soil substitution
drainfields may be constructed in Class Ia soils, highly permeable
fractured rock, highly permeable fissured rock, or Class II and III
soils with greater than 30% gravel.
(A) A soil substitution drainfield must not be used
in Class IV soils or Class IV soils with greater than 30% gravel.
Class III or IV soil shall not be used as the substituted soil in
a soil substitution drainfield. There must be at least two feet of
substituted soil between the bottom of the media and groundwater.
(B) A soil substitution drainfield is constructed similar
to a standard absorptive drainfield except that a minimum two foot
thick Class Ib or Class II soil buffer shall be placed below and on
all sides of the drainfield excavation. The soil buffer must extend
at least to the top of the media. The two-foot buffer area along the
sides of the excavation is not credited as bottom area in calculating
absorptive area. However, the interface between the media and the
substituted soil is credited as absorptive area.
(C) Soil substitution drainfields must be designed
to address soil compaction to prevent unlevel disposal. It is recommended
that low-pressure dosing be used for effluent distribution. The edge
of the substituted soil is considered the edge of the soil absorption
drainfield in determining the appropriate separation distances as
listed in §285.91(10) of this title.
(D) Class Ia soils do not provide adequate treatment
of wastewater through soil contact. A soil substitution drainfield
may be constructed in Class Ia soils in order to provide adequate
soil for treatment. Absorptive area sizing must be based on the textural
class of the substituted soil and must follow the formulas in subsection
(b)(1)(A)(vii)(I) of this section.
(E) Highly permeable fractured and fissured rock, which
contains soil in the fractures and fissures, does not provide adequate
treatment of wastewater through soil contact. A soil substitution
drainfield can be constructed in this permeable fractured and fissured
rock in order to provide adequate soil for treatment. Absorptive area
sizing must be based on the most restrictive textural class between
either the native soil residing in the fractures or fissures or the
substituted soil. The sizing must follow the formulas in subsection
(b)(1)(A)(vii)(I) of this section.
(F) Class II and III soils with greater than 30% gravel
do not provide adequate treatment of wastewater through soil contact.
A soil substitution drainfield can be constructed in Class II or III
soils with greater than 30% gravel in order to provide adequate soil
for treatment. Absorptive area sizing must be based on the most restrictive
textural class between either the non-gravel portion of the native
soil or the substituted soil. The sizing must follow the formulas
in subsection (b)(1)(A)(vii)(I) of this section.
(5) Drainfields following secondary treatment and disinfection.
Subsurface drainfields following secondary treatment and disinfection
may be constructed in Class Ia soils, fractured rock, fissured rock,
or other conditions where insufficient soil depth will allow septic
tank effluent to reach fractured rock or fissured rock, as long as
the following conditions are met.
(A) Drainfield sizing.
(i) If the unsuitable feature is Class Ia soil, the
disposal area sizing shall be based on the application rate for Class
Ib soil. Some form of pressure distribution shall be used for effluent
disposal.
Cont'd... |
