HomeMy WebLinkAboutOWTS Design
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
ONSITE WASTEWATER TREATMENT SYSTEM DESIGN
PROPOSED RESIDENCE
LOT 4, ORR-SMITH SUBDIVISION
TBD COUNTY ROAD 312
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-688
DECEMBER 14, 2021
PREPARED FOR:
DON REBOUL
P. O. BOX 994
NEW CASTLE, COLORADO 81647
(donreboulconstruction@gmail.com)
Kumar & Associates, Inc. ® Project No. 21-7-688
TABLE OF CONTENTS
INTRODUCTION ...................................................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
SUBSURFACE CONDITIONS ................................................................................................. - 1 -
OWTS ANALYSIS .................................................................................................................... - 2 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
SOIL TREATMENT AREA................................................................................................... - 3 -
OWTS COMPONENTS ......................................................................................................... - 4 -
OWTS OPERATION AND MAINTENANCE.......................................................................... - 7 -
OWTS HOUSEHOLD OPERATION .................................................................................... - 7 -
OWTS MAINTENANCE ....................................................................................................... - 8 -
OWTS CONSTRUCTION OBSERVATION ............................................................................ - 8 -
LIMITATIONS ........................................................................................................................... - 8 -
FIGURES
FIGURE 1 – OWTS SITE PLAN
FIGURE 2 – USDA SOIL GRADATION TEST RESULTS
FIGURE 3 – SOIL TREATMENT AREA CALCULATIONS
FIGURE 4 – SOIL TREATMENT AREA PLAN VIEW
FIGURE 5 – SOIL TREATMENT ARE CROSS SECTION
ATTACHMENTS
VALLEY PRECAST 1,500 GALLON, TWO-CHAMBER SEPTIC TANK DETAIL
VALLEY PRECAST 1,000 GALLON, TWO-CHAMBER SEPTIC TANK DETAIL
VALLEY PRECAST 8-HOLE DISTRIBUTION BOX
Kumar & Associates, Inc. ® Project No. 21-7-688
INTRODUCTION
This report provides the results of an onsite wastewater treatment system (OWTS) design for a
proposed residence located on Lot 4, Orr-Smith Subdivision, TBD County Road 312, Garfield
County, Colorado. A vicinity map of the site is included on Figure 1. The purpose of the report
was to provide design details for the OWTS in accordance with the 2018 Garfield County
Environmental Health Department Onsite Wastewater Treatment System Regulation #43. The
design services are supplemental to our agreement for professional services to Don Reboul dated
August 23, 2021.
PROPOSED CONSTRUCTION
The proposed residence will be single-story, five-bedroom structure with an attached garage
located as shown on Figure 1. The ground floor will be slab-on-grade at a finish elevation at to
slightly above the existing ground surface. The OWTS will be designed for 7 bedrooms to allow
the possibility of a future two-bedroom accessory dwelling unit (ADU). The location of the
ADU is not currently planned. The soil treatment area (STA) will be located to the northwest of
the proposed residence, see Figure 1. Water service will be provided by an onsite well to the
west of the residence located as shown on Figure 1. The route of the water line is not yet
determined, but the route is unlikely to impact the OWTS.
If proposed construction is different than that described, we should be contacted to re-evaluate
our design recommendations.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The driveway was under
construction and an existing well was located near the west side of the lot, see Figure 1. The
ground surface was gently to moderately sloping down to the north. Garfield Creek crosses the
northern portion of the lot. The lot is crossed by several irrigation ditches. The subject site was
previously an irrigated field and vegetation on the edge of the field near the creek consists of oak
brush.
SUBSURFACE CONDITIONS
The field exploration for the OWTS was conducted on October 18, 2021. Two profile pits
(Profile Pits 1 and 2) were excavated with a mini-excavator at the approximate locations shown
on Figure 1 to evaluate the subsurface conditions at proposed STA for the OWTS. The pits were
logged by a representative of Kumar & Associates. Logs of the profile pits are provided below.
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Kumar & Associates, Inc. ® Project No. 21-7-688
Log of Profile Pit 1
Depth USDA Classification
0’-1½’ TOPSOIL; organic loam, firm, slightly moist, dark brown.
1½’-8’ SILTY CLAY LOAM; strong blocky structure, very stiff, slightly moist, tan to
brown.
• Disturbed bulk sample obtained from 4 to 5 feet.
• Bottom of pit @ 8 feet.
• No redoximorphic features observed at time of pit excavation.
Log of Profile Pit 2
Depth USDA Classification
0’-1½’ TOPSOIL; organic loam, firm, moist, dark brown.
1½’-8’ SILTY CLAY LOAM; strong blocky structure, very stiff, slightly moist, tan to
brown.
• Bottom of pit @ 8 feet.
• No free water or redoximorphic features observed at time of pit excavation.
The soils encountered in profile pits below, about 1½ feet of topsoil, consisted of Silty Clay
Loam to the pit depths of 8 feet. Hydrometer and gradation analyses were performed on a
disturbed bulk sample of the soils from Profile Pit 1 from 4 to 5 feet depth with the results
provided on Figure 2. The tested sample (minus No. 10 size sieve fraction) classifies as Silty
Clay Loam per the USDA system.
Based on the profile pit tactile evaluations and laboratory testing, the design soils have been
classified as Soil Type 3 per State regulations, which equates to a long-term acceptance rate
(LTAR) of 0.35 gallons per square foot per day. No free water was encountered in the pits nor
indications of seasonal high groundwater observed at the time of the pit excavations.
OWTS ANALYSIS
Based on the site conditions and profile pit evaluations, the evaluated area should be suitable for
a conventional infiltration soil treatment system consisting of chambers in a trench configuration.
The base of the soil treatment trenches will be in Silty Clay Loam soils (Soil Type 3) for which
we will use an LTAR of 0.35 gallons per square foot per day to size the STA. The system will
be designed for the five bedrooms of the proposed residence with and an average daily flow of
75 gallons per person for double occupancy in the first three bedrooms and single occupancy in
the fourth and fifth bedrooms, and two additional bedrooms with double occupancy for the future
ADU. In total, the OWTS will be designed for seven bedrooms and 12 persons.
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Kumar & Associates, Inc. ® Project No. 21-7-688
The effluent will gravity flow from the residence to the septic tank for primary treatment and
then gravity flow to the STA for dispersal and final treatment. The STA will consist of six
trenches with Infiltrator Quick 4 Plus Standard chambers. Equal distribution will be provided to
the chambers and trenches by a distribution box.
We recommend using separate septic tanks for the residence and the future ADU to limit
distances for the transportation of waste solids in the sewer lines. We should review our design
when location and plans for the ADU have been developed.
A 10 feet minimum setback from the STA is required to the property lines. A 100 feet minimum
setback is required from the STA to the well. The setbacks are shown on Figure 1. A 25 feet
setback is required from the proposed STA to any potable water supply lines. A 5 feet setback is
required from the sewer and effluent lines to the water supply line. The location of the water
supply line is not currently designed. If the water supply line is within 25 feet of the proposed
STA or 5 feet of the sewer or effluent lines, we should be notified for additional
recommendations.
DESIGN RECOMMENDATIONS
The design recommendations presented below are based on the proposed construction, the site
and subsurface conditions encountered, and our experience in the area. A layout of the proposed
OWTS components is provided on Figure 1. The location of the future ADU and its separate
septic tank are not shown on Figure 1. If conditions encountered during construction are
different than those that are described in this report, contact us to re-evaluate our design
recommendations.
SOIL TREATMENT AREA
• The STA will consist of six trenches with 25 Infiltrator Quick 4 Plus Standard chambers
placed in the native Sandy Loam soils (150 chambers total). Equal distribution will be
provided to the trenches by a distribution box.
• The STA was sized based on an LTAR of 0.35 gallons per square feet per day.
• Each chamber was allowed 12 square feet of area. A 30% reduction of the STA size was
applied for use of Infiltrator chambers in a trench configuration. The total STA design
size is 1800 square feet. Soil treatment area calculations are shown on Figure 3.
• A minimum of four feet of undisturbed soil between the trenches is required.
• A minimum of 12 inches and a maximum of 24 inches of cover soil should be placed
over the top of the chambers.
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Kumar & Associates, Inc. ® Project No. 21-7-688
• The trenches should be oriented along ground contours to minimize soil cut and cover.
• The base and sidewalls of the trench excavations should be scarified prior to the chamber
placement.
• Backfill should be graded to deflect surface water away from the STA and should be
sloped at 3 horizontal to 1 vertical or flatter. A swale to divert surface water run-off may
be needed along the up-slope side of the STA.
• Disturbed soil should be re-vegetated as soon as possible with a native grass mix. No
trees, shrubs or other plants with deep roots should be planted on or near the STA as this
may damage the system piping.
• Four-inch diameter inspection ports should be installed vertically into the knockouts
provided in the Infiltrator end caps. Install ports at each end of each trench. The
inspection port piping should be screwed into the top to the chambers and should not
extend down to the ground surface inside the chambers. The inspection ports should
extend at least 8 inches above the finished ground surface or be protected in a valve box
at finished grade.
• A plan view of the STA is presented on Figure 4 and a cross section of the STA is
presented on Figure 5.
• The STA must maintain a minimum 25 feet setback from the water supply line.
• The STA must maintain a minimum 10 feet setback from the lot property line.
• The STA must maintain a minimum 100 feet setback from the well.
• The STA must maintain a minimum 50 feet setback from Garfield Creek.
• The STA must maintain a minimum 50 feet setback from irrigation ditches.
OWTS COMPONENTS
Recommended OWTS components provided below are based on our design details and our
experience with the specific component manufacturers. Equivalent components may be feasible
but need to be approved by us prior to construction.
Septic Tank
• A Valley Precast 1,500 gallon, two-chamber septic tank should be be used for primary
treatment of wastewater from the residence. A detail of the tank is provided as an
attachment to the report.
• A Valley Precast 1,000 gallon, two-chamber septic tank should be used for primary
treatment of wastewater from the ADU. A detail of the tank is provided as an attachment
to this report.
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Kumar & Associates, Inc. ® Project No. 21-7-688
• An effluent filter should be installed in the tank outlet tee.
• The tanks must be a minimum of five feet from any structure and from the STA. The
tank must be a minimum of 10 feet from any potable water supply lines.
• The tanks must be set level. The excavation bottom must be free of large rocks or other
objects that could damage the tank during placement. A road base or gravel bedding
material may be needed to prevent tank damage during placement and act as a leveling
course.
• The tank lids must extend to final surface grade and made to be easily located.
• Install the tank with 2 feet minimum cover soil for frost protection. Maximum tank soil
cover is 4 feet.
Sewer Pipe
• The sewer line to the septic tank should not be less than the diameter of the building
drains and not less than 4 inches in diameter.
• The sewer pipe should have a rating of SDR35 or stronger. Schedule 40 PVC pipe is
required beneath all driveway surfaces.
• The sewer pipe should be sloped between 2% to 4% to help limit disturbance of solids in
the tank and potential sewage bypass of the first chamber of the tank. If a steeper slope is
needed, this can be accomplished with vertical step-downs in the sewer line.
• A minimum 36 inches of cover soil should be provided over the sewer pipe. Paved areas,
patios or other areas without vegetative cover may be more susceptible to frost and we
recommend 48 inches of soil cover over the sewer pipe in these areas, and the pipe be
insulated on top and sides with 2-inch-thick blue foam insulation board. If adequate soil
cover is not possible, we should be contacted for re-evaluation prior to installation.
• The sewer pipe should be bedded in compacted ¾ inch road base or native soils provided
that the native soils contain no angular rocks or rocks larger than 2½ inches in diameter
to help prevent settlement of the pipe. Sags in the pipe could cause standing effluent to
freeze and damage the piping.
• Install cleanout pipes within 5 feet of the building foundations, where the sewer pipes
bend 90 degrees or more and every 100 feet of sewer pipe.
• All 90-degree bends should be installed using a 90-degree long-sweep or by using two
45-degree elbows.
• The sewer line location shown on Figure 1 is considered conceptual. We assume that
there will be only one sewer line exiting the residence. It is the responsibility of the
owner/contractors to locate all sewer line exit locations and connections to the septic
tank. We should be notified if there is more than one sewer line exiting the residence.
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Kumar & Associates, Inc. ® Project No. 21-7-688
Effluent Transport Piping
• The effluent transport pipe should be 4-inch diameter SDR35 piping sloped at a 2%
minimum to flow from the septic tank to the distribution box and from the distribution
box to each trench. Piping should extend at least 6 inches into the top knockout provided
in the Infiltrator end caps and be screwed in place. Splash plates should be installed
beneath the inlet piping to help limit scouring of the infiltrative soil surface in the
chambers.
• The effluent transport pipe should be bedded in compacted ¾ inch road base or native
soils provided that the native soils contain no angular rocks or rocks larger than 2½
inches in diameter to help prevent settlement of the pipe. Sags could cause standing
effluent to freeze and damage to the piping.
• A minimum 18 inches of cover soil should be provided over the effluent transport pipe.
Paved areas, patios or other areas without vegetative cover may be more susceptible to
frost. We recommend 48 inches of soil cover over the effluent pipe and insulation with at
least 2 inches of rigid foam insulation in these areas. If adequate cover soil is not
possible, we should re-evaluate the condition prior to installation.
• Schedule 40 PVC pipe is required underneath driveway surfaces and should be provided
with at least 48 inches of soil cover and be insulated on top and sides with 2-inch-thick
foam insulation board.
• All 90-degree bends should be installed using a 90 degree long-sweep or by using two
45-degree elbows.
• The effluent line must maintain a minimum 10 feet setback to a piped or lined irrigation
ditch or intermittent irrigation lateral.
• The effluent line must maintain a minimum 50 feet setback to an unlined irrigation ditch.
• Where the effluent line encroaches on any of the above setbacks, the effluent line must be
encased with larger diameter Schedule 40 PVC pipe in a watertight manner throughout
the encroachment.
Distribution Box
• The distribution box will be used to equally divide effluent flow between the trenches.
We recommend the Valley Precast 8-Hole Distribution Box since it is easily accessible
from ground surface for maintenance. A copy of the distribution box detail is provided as
an attachment to this report.
• The distribution box outlet pipes should be equipped with flow equalizers for ease of
initial setup and adjustment over time.
• The distribution box should be a minimum of 18 inches below grade with risers and an
access lid at or above grade.
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Kumar & Associates, Inc. ® Project No. 21-7-688
• The distribution box must be accessible from ground surface for inspection and
maintenance per State guidelines.
• The effluent line from the septic tank should be directed downward into the distribution
box with a 90-degree elbow inside the box.
• The distribution box must be installed level on a compacted road base subgrade to ensure
equal effluent distribution. If settlement of the distribution box occurs, unequal effluent
distribution may take place which could cause hydraulic overload of one of the trenches
and potential failure. The distribution box should be monitored periodically, and the flow
equalizers should be adjusted as necessary for levelling.
• The ground surface above and around the distribution box should be graded to drain away
surface water to prevent standing water near the distribution box.
OWTS OPERATION AND MAINTENANCE
The OWTS will require periodic inspection and maintenance to function properly. A properly
designed, installed and maintained system can greatly increase its lifespan. The level of
maintenance will vary depending on the complexity of the system and water use habits of the
residents. We recommend that an OWTS Operation and Maintenance (O&M) Manual be
developed. Depending on the complexity of the system, a contract with an OWTS maintenance
provider may be prudent. Below are some basic recommendations for the OWTS O&M.
OWTS HOUSEHOLD OPERATION
• Use of high efficiency water fixtures is recommended to decrease the hydraulic load on
the OWTS system.
• Fix plumbing leaks immediately as this may cause a hydraulic overload of the soil
absorption system.
• Do not irrigate the area on top of or directly upgradient of the STA as this may cause a
hydraulic overload.
• Do not dispose of household waste down drains as this may clog or damage OWTS
components. Examples of household waste includes: dental floss, cotton swabs, coffee
grounds, paper towels, feminine products and many other kitchen and bath items.
• Use of kitchen garbage disposals is not recommended. If a garbage disposal is utilized,
kitchen wastewater should be screened thoroughly. Many kitchen solids are not
decomposed in the septic tank and may cause increased tank pumping frequency.
• Do not dispose of household chemicals, greases, oils, paints, hot tub water or water
softener backwash in household drains. A separate drywell, if feasible, may be necessary
for hot tub water or water softener backwash disposal.
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Kumar & Associates, Inc. ® Project No. 21-7-688
• Limit the use of bleach as this may harm useful bacteria in the septic tank and soil
absorption system.
• Liquid dishwasher and clothes washer detergent is recommended for households served
by an OWTS. Clay substances used as fillers in powder detergents may result in
clogging of the soils absorption system.
• The effluent in septic tanks can freeze during extended periods of non-use in cold
weather. We recommend that a tank heater be installed in this system to help prevent
freezing.
OWTS MAINTENANCE
• Inspect the septic tanks, distribution box and STA at least annually for problems or signs
of failure.
• The filter should be cleaned annually by spray washing solids into the first chamber of
the septic tank.
• The septic tanks should be pumped and cleaned every 3 to 5 years depending on use.
Longer pumping intervals may increase the amount of solids that reach the STA, which
may shorten its life span.
• Pumping of the septic tanks should take place when the level of the sludge and scum
layers combined take up about 25 to 33% of the capacity of the first chamber of the tank.
OWTS CONSTRUCTION OBSERVATION
The Garfield County Environmental Health Department requires that the designer of the system
provide a record drawing and certification of the OWTS construction (As-Built inspection). We
should be provided with at least 48-hour notice prior to the installer needing the inspections.
Prior to issuance of our certification letter, we require observation of all system components
prior to backfill. The number of site visits required for the inspection will depend on the
installer’s construction schedule.
LIMITATIONS
We have conducted this design in accordance with generally accepted engineering principles and
practices in this area at this time. We make no warranty either expressed or implied. The
recommendations provided in this report are based on the site conditions, profile pit evaluations
and soil texture analysis, the proposed construction and our experience in the area. Variations in
the subsurface conditions may not become evident until excavation is performed. If conditions
Kumar & Associates
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Infiltrator Quick 4 Plus
Standard Chamber
Installed Level
Scarify ground surface
prior to chamber
placement.
Excavate 36"
Wide Trench
Approx. Existing
Ground Surface
12" min. 24"
max. Cover Soil
Graded to
Deflect Surface
Water
Native Soil
4' Min.
Undisturbed
Soil
Between
Trenches
Kumar & Associates
Notes
1.Chambers should be installed level on a scarified ground surface. Concrete
sand may be used as a leveling course if necessary.
2.Care should be taken by the contractor to avoid compaction of the native soils in
the trench areas. The base and side walls of trench excavations should be
scarified prior to chamber installation.
3.Changes to this design should not be made without consultation and approval
by Kumar & Associates.