Current through Register Vol. 39, No. 6, September 16, 2024
The following are modifications to septic tank systems or
sites which may be utilized singly or in combination to overcome selected soil
and site limitations. Except as required in this Rule, the provisions for
design and installation of Rule .1955 and .1970 of this Section shall
apply:
(1) SHALLOW SYSTEMS: Sites
classified UNSUITABLE as to soil depth or soil wetness may be reclassified as
PROVISIONALLY SUITABLE with respect to soil depth or soil wetness conditions by
utilizing shallow placement of nitrification trenches in the naturally
occurring soil. Shallow trenches may be used where at least 24 inches of
naturally occurring soil are present above saprolite, rock, or soil wetness
conditions and all other factors are PROVISIONALLY SUITABLE or SUITABLE.
Shallow trenches shall be designed and constructed to meet the vertical
separation requirements in Rule .1955(m) or .1970 of this Section. The
long-term acceptance rate shall be based on the most hydraulically limiting
naturally occurring soil horizon within 24 inches of the ground surface or to a
depth of one foot below the trench bottom, whichever is deeper. Soil cover
above the original grade shall be placed at a uniform depth over the entire
nitrification field and shall extend laterally five feet beyond the
nitrification trench. The type and placement of soil cover shall be approved by
the local health department.
(2)
DRAINAGE AND RESTRICTIVE HORIZONS: Sites classified UNSUITABLE as to soil
wetness conditions or restrictive horizons may be reclassified PROVISIONALLY
SUITABLE as to soil wetness conditions or restrictive horizons when:
(a) Soils are Soil Groups I or II with
SUITABLE structure, and clay mineralogy;
(b) Restrictive horizons, if present, are
less than three inches thick or less than 12 inches from the soil
surface;
(c) Modifications can be
made to meet the requirements in Rule .1955(m) of this Section for the
separation between the water table and the bottom of the nitrification trench
at all times and when provisions are made for maintenance of the drainage
systems;
(d) Easements are recorded
and have adequate width for egress and ingress for maintenance of drainage
systems serving two or more lots; and
(e) Maintenance of the drainage system is
made a condition of any permit issued for the use or operation of a sanitary
sewage system.
Drainage may be used in other types of soil when the
requirements of Rule .1942, .1970 or .1948(d) in this Section are met.
(3) MODIFIED TRENCHES:
Modified nitrification trenches or lines, including large diameter pipe
(greater than four inches I.D.), and specially designed porous block systems
may be permitted by the local health department as follows:
(a) GRAVELLESS TRENCHES: Gravelless
nitrification trench systems may be substituted for conventional trench systems
on any site found to be SUITABLE or PROVISIONALLY SUITABLE in accordance with
Rules .1940 to .1948 of this Section to eliminate the need for gravel, minimize
site disturbance, or for other site planning considerations. Gravelless
nitrification trench systems shall not be used, however, where wastes contain
high amounts of grease and oil, such as restaurants. Large diameter pipe
systems and porous block systems may be permitted by the local health
department as follows:
(i) Large diameter pipe
systems shall consist of eight-inch or 10-inch (inside diameter), corrugated,
polythylene tubing encased in a nylon, polyester, or nylon/polyester blend
filter wrap installed in a nitrification trench, 12 or more inches wide and
backfilled with soil classified as soil group I, II, or III. Nitrification area
requirement shall be determined in accordance with Rules .1955(b) and .1955(c),
or in Rule .1956(6)(b), Table III(a) of this Section, when applicable, with
eight-inch tubing considered equivalent to a two-foot-wide conventional trench
and 10-inch tubing considered equivalent to a two and one-half-foot-wide
conventional trench. The long-term acceptance rate shall not exceed 0.8 gallons
per day per square foot. Tubing and fittings shall comply with the requirements
of ASTM F-667, "Standard Specification for Large Diameter Corrugated
Polyethylene Pipe and Fittings, "which is hereby incorporated by reference
including any subsequent amendments and editions. Copies of the standards may
be inspected at the Division of Environmental Health Central Office, located at
2728 Capital Blvd., Raleigh, NC, and copies may be downloaded from the Internet
at
http://www.astm.org, or obtained
from ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West
Conshohocken, PA 19438-2959, at a cost of thirty dollars ($30.00). The
corrugated tubing shall have two rows of holes, each hole between three-eighths
and one-half-inch in diameter, located 120 degrees apart along the bottom half
of the pipe (each 60 degrees from the bottom center line) and staggered so that
one hole is present in the valley of each corrugation. The tubing shall be
marked with a visible top location indicator, 120 degrees away from each row of
holes. Filter wrap shall be spun, bonded, or spunlaced nylon, polyester, or
nylon/polyester blend nylon filter wrap meeting the minimum requirements in
Table III(a):
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Table III(a): Minimum Filter Wrap Requirements for
Large Diameter Pipe Systems
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PROPERTY
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VALUE
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Unit Weight
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1.0 ounce per square yard
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Sheet Grab Tensile Strength
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Machine Direction: 23 pounds
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Trapezoid Tear Strength
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Machine Direction: 6.2 pounds
Cross Direction: 5.1 pounds
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Mullen Burst Strength
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40 pounds per square inch or
276 kilopascals
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Frazier Air Permeability
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500 cubic feet per minute per square foot at
pressure differential of 0.5 inches of water
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Corrugated tubing shall be covered with filter wrap at the
factory and each joint shall be immediately encased in a black polyethylene
sleeve which shall continue to encase the large diameter pipe and wrap until
just prior to installation in the trench to prevent physical damage and
ultraviolet radiation deterioration of the filter wrap. Large diameter pipe
systems shall be installed in accordance with this Rule and the manufacturer's
guidelines. The trench bottom and pipe shall be level (with a maximum fall of
one inch in 100 feet). Rocks and large soil clumps shall be removed from
backfill material prior to being used. Clayey soils (soil group IV) shall not
be used for backfill. The near end of the large diameter pipe shall have an
eight-inch by four-inch offset adapter (small end opening at top) suitable for
receiving the pipe from the septic tank or distribution device and making a
mechanical joint in the nitrification trench.
(ii) A Prefabricated, Permeable Block Panel
System (PPBPS), utilizing both horizontal and vertical air chambers and
constructed to promote downline and horizontal distribution of effluent, may be
used under the following conditions:
(A) the
soil and site criteria of this Section shall be met;
(B) in calculating the required linear
footage for a PPBPS's nitrification field, the linear footage for the
nitrification line as determined in Rule .1955 (b) and (c), or in Rule .1956
(6)(b), Table III(a) of this Section when applicable, shall be multiplied by
0.5 for a 16 inch PPBPS;
(C)
installation of the PPBPS shall be in accordance with Rule .1955 except:
(I) the PPBPS trench shall be located not
less than eight feet on centers;
(II) the installation shall be in accordance
with the manufacturer's specifications; and
(III) the sidewalls of nitrification trenches
placed in Group IV soils shall be raked to open pores which were damaged or
sealed during excavation;
(D) where design sewage flow is more than 480
gallons per day, the system shall be pressure-dosed; and
(E) the long-term acceptance rate shall not
exceed 0.8 gallons per day per square foot.
(b) Other types of nitrification trenches or
lines may be approved by the local health department on a site-specific basis
in accordance with Rule .1969 of this Section.
(4) INTERCEPTOR DRAINS: Sites classified as
UNSUITABLE as to soil wetness conditions because of the presence of lateral
water movement may be reclassified PROVISIONALLY SUITABLE as to soil wetness
conditions when such water is intercepted and diverted to prevent saturation of
the soil absorption system.
(5)
STEEP SLOPES: Stable slopes greater than 30 percent may be reclassified as
PROVISIONALLY SUITABLE when:
(a) The soil
characteristics can be classified as SUITABLE or PROVISIONALLY SUITABLE to a
depth of at least one foot below the bottom of the nitrification trench at the
upslope side of the trench;
(b)
Surface water runoff is diverted around the nitrification field if necessary to
prevent scouring or erosion of the soil over the field; and
(c) The finished grade over the nitrification
field site is returned to the original topography and seeded to establish a
permanent vegetative cover, unless otherwise specified by the local health
department.
(6)
SAPROLITE SYSTEM: Sites classified UNSUITABLE as to soil depth, with saprolite
present, may be reclassified PROVISIONALLY SUITABLE as to soil depth when:
(a) An investigation of the site using pits
at locations specified by the local health department is conducted. The
following physical properties and characteristics shall be present in the two
feet of saprolite below the proposed trench bottom:
(i) the saprolite texture is sand, loamy
sand, sandy loam, loam, or silt loam;
(ii) clay mineralogy is suitable;
(iii) greater than two-thirds of the material
has a moist consistence that is loose, very friable, friable, or
firm;
(iv) the saprolite wet
consistence is nonsticky or slightly sticky and nonplastic or slightly
plastic;
(v) the saprolite is in an
undisturbed, naturally occurring state; and
(vi) the saprolite has no open and continuous
joints, quartz veins, or fractures that are relic of parent rock to a depth of
two feet below the proposed trench bottom.
(b) Table III(b) is used in determining the
long-term acceptance rate. The long-term acceptance rate shall be based on the
most hydraulically limiting, naturally occurring saprolite to a depth of two
feet below trench bottom.
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Table III(b): Long-Term Acceptance Rates for
Saprolite Systems
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SAPROLITE GROUP
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SAPROLITE TEXTURAL CLASS
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LONG-TERM ACCEPTANCE RATE (gallons per day per
square foot)
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I
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Sands
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Sand
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0.8 - 0.6
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Loamy Sand
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0.7 - 0.5
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II
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Loams
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Sandy Loam
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0.6 - 0.4
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Loam
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0.4 - 0.2
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Silt Loam
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0.3 - 0.1
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If a low pressure pipe system is used, the long-term
acceptance rate in Table III(b) shall be reduced by one-half and the system
shall be designed in accordance with Rule 1957(a) of this Section, except that
Rule .1957(a)(2)(B) and Rule .1957(a)(3) shall not apply. Other design criteria
may also be used in conjunction with an advanced pretreatment system pursuant
to Rule .1970. Saprolite textural classifications shall be determined from
disturbed materials and determined by Rule .1941(a)(1) of this Section.
Low-pressure distribution shall be used when the total length of nitrification
lines exceeds 750 feet in a single system.
(c) The design daily flow does not exceed
1000 gallons.
(d) The nitrification
field is constructed using nitrification trenches with a maximum width of three
feet and a maximum depth of three feet on the downslope side of the
nitrification trench. The bottom of a nitrification trench shall be a minimum
of two feet above rock or saprolite that does not meet the requirements of
Subparagraph (6)(a) of this Rule, or the requirements of Subparagraph (h)(2) of
Rule .1970 in conjunction with an advanced pretreatment system. However, where
SUITABLE or PROVISIONALLY SUITABLE soil underlies the trench bottom, this
separation distance may be reduced by subtracting the actual soil depth beneath
the trench bottom from 24 inches to establish the minimum separation distance
from the trench bottom to rock.
(e)
The bottom of any nitrification trench is a minimum of two feet above any
wetness condition.
(f) Surface and
subsurface interceptor drains are required on sites with more slowly permeable
horizons above the usable saprolite to intercept laterally flowing waters or
perched waters.
Exceptions to the provisions of Rule .1950(a) found in Rule
.1950 and .1951 of this Section shall not apply to systems installed pursuant
to this Item [Rule .1956(6)]. Other saprolite systems may be approved on a
site-specific basis in accordance with Rule .1948(d) or .1970 of this
Section.
(7)
SAND LINED TRENCH SYSTEM: Sites classified UNSUITABLE as to soil wetness, soil
morphology, restrictive horizon or soil depth where a horizon with higher
permeability underlies less permeable horizons, may be reclassified
PROVISIONALLY SUITABLE as to soil wetness, soil morphology, restrictive horizon
or soil depth (soil depth to parent material, not rock) when:
(a) An investigation of the site using pits
or auger borings at locations specified by the local health department is
conducted. The following physical properties and characteristics shall be
present:
(i) if the receiving permeable
horizon is deeper than five feet below the natural grade, the effluent is to
receive pretreatment to TS-I or TS-II level prior to pressure dispersal in the
sand lined trenches. If the receiving permeable horizon is encountered at
depths of five feet or less below the natural grade, pretreatment to TS-I or
TS-II level and pressure dispersal is not required;
(ii) the texture of the receiving permeable
horizon is sand, loamy sand, sandy loam, loam, or silt loam;
(iii) the structure of the receiving horizon
is classified as SUITABLE or PROVISIONALLY SUITABLE;
(iv) the moist consistence of the receiving
permeable horizon is loose, very friable, friable, or firm;
(v) if the receiving permeable horizon has
zones of heavier textured materials, these zones are discontinuous with an
average thickness not exceeding 1/3 of the required thickness of the receiving
permeable horizon;
(vi) if the
texture of the receiving permeable horizon is sandy loam or loam, and the
system design flow is greater than 600 gallons per day, the saturated hydraulic
conductivity of the permeable horizon shall be field-determined; and
(vii) if the texture of the receiving
permeable horizon is silt loam, the saturated hydraulic conductivity of the
permeable horizon shall be field-determined.
(b) The minimum thickness required of the
receiving permeable horizon is dependent upon the texture of the receiving
horizon as follows:
(i) sand or loamy sand -
1 foot thick;
(ii) sandy loam or
loam - 2 feet thick; or
(iii) silt
loam - 3 feet thick.
(c)
Table III(c) is used in determining the long-term acceptance rate (LTAR) for
all sand-lined trench systems. The long-term acceptance rate shall be:
(i) the rate set forth in Table III(c), based
on the most hydraulically limiting, naturally occurring soils overlying the
permeable receiving layer, or
(ii)
when the saturated hydraulic conductivity of the underlying horizons is
required to be determined pursuant to Subitem (7)(a)(vi) or Subitem (7)(a)(vii)
of this Rule, either the rate set forth in Table III(c), based on the most
hydraulically limiting, naturally occurring soils overlying the permeable
receiving layer, or 10 percent of the saturated hydraulic conductivity of the
underlying permeable horizon (or 20 percent with TS-I or TS-II pretreatment),
whichever is less.
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Table III(c): Long-Term Acceptance Rates for Sand
Lined Trench Systems
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SOIL GROUP
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TEXTURAL CLASS OF MOST HYDRAULICALLY LIMITING
OVERLYING SOIL HORIZON
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DISTRIBUTION OR PRETREATMENT CONDITION
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LONG-TERM
ACCEPTANCE RATE (LTAR) (gallons per day per square
foot, on trench bottom area basis)*
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I
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Sands (Sand, Loamy Sand)
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Gravity Distribution
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0.7 - 0.9
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Pressure Dispersal
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0.8 - 1.2
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Pressure Dispersal and TS-I or TS-II
pretreatment
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0.9 - 1.4
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II
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Coarse Loams (Sandy Loam, Loam)
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Gravity Distribution
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0.5 - 0.7
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Pressure Dispersal
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0.6 - 0.8
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Pressure Dispersal and TS-I or TS-II
pretreatment
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0.7 - 1.0
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III
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Fine Loams (Sandy Clay Loam, Silt Loam, Clay Loam,
Silty Clay Loam, Silt)
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Gravity Distribution
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0.2 - 0.4
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Pressure Dispersal
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0.3 - 0.6
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Pressure Dispersal and TS-I or TS-II
pretreatment
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0.4 - 0.8
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IV
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Clays (Clay, Sandy Clay, Silty Clay)
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Gravity Distribution
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0.1 - 0.2
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Pressure Dispersal
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0.15 - 0.3
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Pressure Dispersal and TS-I or TS-II
pretreatment
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0.2 - 0.4
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*The LTAR for all sand-lined trench systems shall not
exceed the loading rates set forth in this table or 10 percent of the saturated
hydraulic conductivity of the underlying permeable horizon (or 20 percent with
TS-I or TS-II pretreatment) when required to be determined pursuant to Subitem
(7)(a)(vi) or Subitem (7)(a)(vii) of this Rule, whichever is less. There shall
be no reduction in trench length compared to a conventional gravel trench when
accepted or innovative nitrification trenches are used.
If a low pressure pipe system is used, the system shall be
designed in accordance with Rule .1957(a) of this Section, except that Rule
.1957 (a)(2)(B) and Rule .1957(a)(3) shall not apply and trenches shall be a
maximum of three feet in width. Textural classifications of the overlying
material shall be determined from disturbed materials and determined by Rule
.1941(a)(1) of this Section. Pressure distribution shall be used when the total
length of nitrification lines exceed 600 linear feet in a single system and
pressure dispersal such as LPP or drip irrigation shall be used when the total
length of nitrification lines exceeds 1200 linear feet in a single
system.
(d) A
Certified Operator or a Public Management Entity with a Certified Operator is
required for all sand lined trench systems, if required by Article 3 of G.S.
90A. A Public Management Entity with a Certified Operator, if required by
Article 3 of G.S. 90A, shall be required for sand lined trench systems when
drainage is utilized to lower the water table on a site.
(e) The sand lined trench system is
classified as a type V system in accordance with Rule .1961 of this Section,
except that the required inspection frequency shall be at least once per year
by the operator and greater frequency for advanced pretreatment and pressure
dispersal systems as required by Rule .1961, Rule .1969 or Rule
.1970.
(f) The design daily flow
does not exceed 1000 gallons.
(g)
The nitrification field is constructed using nitrification trenches with a
maximum width of three feet. The bottom of the gravel portion of the sand lined
trench shall be no deeper than 24 inches below finished grade.
(h) The sand lined trenches are constructed
to extend into the permeable horizon. If the sand lined trench bottoms are
deeper than five feet below the natural grade, the effluent shall receive
pretreatment to TS-I or TS-II level prior to dispersal in the sand lined
trench.
(i) Filter media used in
the sand lined portion of the trench is sand or loamy sand in texture. If
required by the local health department in the Construction Authorization, the
installer shall provide written laboratory verification of the media textural
classification and quality prior to the sand lined trench being installed. When
laboratory analysis is required, the material shall be determined to be a
clean, uncoated fine, medium, or coarse sand with at least 90 percent in sizes
ranging from 0.1 to 1.0 millimeters, with no more than one percent smaller than
0.002 millimeters.
(j) Drainage is
required when the sand lined trench is used and soil wetness conditions are
present that are not related to lateral water movement. Drainage shall extend
into the permeable layer. Drainage shall be maintained on the site to provide
for 18 inches of separation between the water table and the bottom of the
gravel portion of the trench. This separation distance may be reduced to 12
inches if pressure dispersal is utilized, nine inches if advanced pretreatment
meeting TS-I or TS-II is utilized and six inches if both pressure dispersal and
TS-I or TS-II pretreatment are utilized.
(k) The drainage plan is prepared by a person
or persons who are licensed or registered to consult, investigate, evaluate,
plan or design wastewater systems, soil and rock characteristics, ground water
hydrology, or drainage systems if required in G.S. 89C, 89E, 89F, or 90A
Article 4. The drainage shall have an outlet accessed by gravity or by a
designed pump drainage system. The outlet location and elevation must be shown
with relative water level elevations and drainfield site elevations labeled on
the drainage plan.
(l) Plans and
specifications for a drainage system serving two or more lots are prepared in
accordance with Rule .1938(c) of this Section.
(m) All required drainage components are
considered to be a part of the wastewater system and subject to ownership and
easement requirements in Sub-item (2)(d) of this Rule and Paragraphs (c) and
(j) of Rule .1938.
(n) Side ditches
or surface swales in a U shape around the system are used to facilitate surface
water removal. Swales shall be at least 18 inches deep and located at least 25
feet from the outer edge of the nitrification trenches.
(o) The drainfield area is crowned at a
minimum grade of one percent as measured from the centerline of the drainfield
to the top of the bank of the side ditches or surface swales.
(p) No depressions are allowed over the
drainfield area, including no linear depressions shall be allowed over the
trenches.
Exceptions to the provisions of Rule .1950(a) found in Rule
.1950 and .1951 of this Section shall not apply to systems installed pursuant
to this Item [Rule .1956(7)]. Other sand lined trench systems may be approved
by the local health department on a site-specific basis in accordance with Rule
.1948(d) of this Section.
