HomeMy WebLinkAboutEngineer's OWTS Design PlanAugust 27, 2025
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
S OPRIS E NGINEERING • LLC civil consultants
Gregg Mackey
Po Box 398,
Basalt, CO, 81621
"gmack pers" gmackey3@gmail.com
Cc: Jorden Voskuil <jordenvoskuil@rocketmail.com>
RE: Design Report–Upgrade/Repair Onsite Wastewater Treatment System (OWTS), Mackey
Commercial Shop Facility, 0050 C.R. 110, Garfield County, Colorado
SE Job No. 18197.02 Parcel Number: 2393-074-00-012
Current Garfield County OWTS Permit: SEPT-11-18-5581
Dear Gregg:
This report presents the applicable findings in regard to the design and construction of an upgrade/replacement
Onsite Wastewater Treatment System (OWTS) at the above-referenced Site. A previous county septic permit #:
SEPT-11-18-5581 was issued 11/29/2018.
The OWTS design is based on our evaluation of the site conditions for use in obtaining a County OWTS
construction permit. We have evaluated the existing and proposed site conditions with respect to the operation and
installation of site-specific system design in accordance with County OWTS Regulations and State of Colorado
Regulation 43 in support of your building permit application. The proposed system meets all required setback
requirements and operational demands for the increased usage of the two repair shops. Based on the physical site
features, soils, and existing building layout, we have prepared a civil OWTS design plan with construction details
for permit approval that was prepared with GIS, Survey Plat information and aerial mapping. The design criteria and
system sizing information is summarized below. The design and construction recommendations with specifications
are delineated on the civil design plans, C1-OWTS Upgrade Plan dated 08-27-2025, by Sopris Engineering.
Summary Design Criteria
The upgraded OWTS is designed with a maximum treatment capacity of 160 gallons per day based on the estimated
maximum daily wastewater flow per population usage of up to 8 employees, 1 small washing machine and 5 person
transients in accordance with Table 6-2 values, Section 43.6, A.4. The OWTS will use the previously installed 1000
gallon 2-compartment concrete septic tank with PVC risers and lids installed at the surface that will be maintained.
A gravity sewer pipe with a cleanout was installed from the facility structure to the tank. The tank was installed with
an inlet sanitary tee and an outlet effluent filter sanitary tee. However, upon inspection the effluent filter was missing
and observation of the existing outlet pipe appeared to have snapped off. The outlet pipe will be cleaned and a new
Orenco 4" effluent filter assembly will be installed.
The existing STA consisted of a single 30’ long chamber trench providing 90 S.F. of absorptive area. The proposed
upgrade soil treatment area (STA) will be expanded to consist of a single, two row bed (40’ x 6’) with leaching
chambers that will provide 270 S.F. of total absorption area. The septic tank effluent will be distributed by gravity
via the existing 4” effluent pipe connected to a new distribution box. The 4"-sewer pipe to be recovered, severed and
installed into inlet of new plastic or concrete distribution box with risers and lid set to surface grade. The D-box will
be installed level on compacted ABC. Individual distribution pipes will connect for D-Box outlet ports to the head
end cap of each chamber row. Observation ports with caps will be installed on each end chamber unit. The design is
in compliance with the current County regulations with design calculations based on the soil type, texture and
structure with an appropriate long term acceptance rate (LTAR).
Gregg Mackey, Upgrade/ repair OWTS
050 C.R. 110
SE Job No. 18197.02
August 27, 2025
Sub Surface Conditions and observation
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering on
October 26, 2018. The soils were tested to determine the hydraulic loading rate and to confirm the appropriate
absorptive surface suitable for a conventional STU in native soils. The soils below nil to 6 inches of mixed surface
fill & topsoil consist of medium dense sandy silt loam with moderate granular to slightly blocky structure with some
cobbles and gravels.
Shallow and deep excavations were observed on site adjacent to the proposed building location. The native soils
sampled from 2-3 feet below the surface are characterized as a soil type 2 consisting of silt loam texture with
moderate granular structure. The percentage of cobbles and gravels below 24 inches was less than 15% in the silt
loam to the 7 foot depth explored. This soil has an effective loading rate for conventional soil treatment of 0.60
gal/S.F./day for a level 1 conventional treatment system. The equivalent percolation rate is assumed to be
approximately 16-25 minutes per inch. No free water was encountered in excavations on site. No excessive boulders
or cobbles were encountered during excavation. Observations of the soils, in the excavation during construction,
appeared to be consistent with the previous subsurface soil investigation.
Design Flow
The proposed upgraded onsite wastewater treatment system (OWTS) design flow is calculated as follows for low
usage commercial shop facility that serves two mechanical repair shops each with a single restroom (toilet and
lavatory) & 1 small washer/dryer in 1 shop.
From the Regulation 43 usage Table 6-2 :
Max. Design flow (Qd) = # of people x (avg.per capita flow)
Qd=daily worker office: 8 persons @ 15 gpd = 10 x 15 = 120 gpd
Qd=transient Vistor: 4 persons @ 5 gpd = 4 x 5 = 20 gpd
Qd=single small washing machine: 1 @ 20 gpd
Total Design flow Qd = 120+20+20 =160 gpd
Septic Tank Design
Continue use of the existing 1000 gallon, two compartment concrete septic tank with a new effluent filter to be
installed on the existing 4" outlet pipe end that extends into the tank and accessible below the riser lid in in the
second chamber. Extend new filter handle to within 6" of lid. Tank was installed with sealed PVC risers and plastic
lids set at the finish grade surface to be maintained.
Sub Surface Conditions and Testing
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering on
October 25, 2018. The soils in the area of the proposed absorption field were sampled and characterized by
application of the USDA soil texture method analysis. The soils were tested to determine the hydraulic loading rate
and to confirm the appropriate absorptive surface suitable for conventional absorption systems in native soils. A
shallow and deep excavated pit was observed in the East central portion of the site adjacent to the proposed field
location. The soils below 4 to 9 inches of mixed surface fill & topsoil consist of medium dense sandy silt loam with
moderate granular to slightly blocky structure with some cobbles and gravels. The percentage of cobbles and gravesl
below 24 inches was less than 20% in the sandy silt loam to the 7 foot depth explored. The soil appears to be
consistent across the site.
The native soils sampled from 2-3 feet below the surface are characterized as a soil type 2 consisting of silt loam
texture with moderate granular structure. This soil has an effective loading rate for conventional soil treatment of
Gregg Mackey, Upgrade/ repair OWTS
050 C.R. 110
SE Job No. 18197.02
August 27, 2025
0.60 gal/S.F./day for a level 1 conventional treatment system. The equivalent percolation rate is assumed to be
approximately 16-25 minutes per inch. Increased quantities of cobbles and gravel are encountered at shallow depths
within 2-feet of the existing surface grade. No free water was encountered in the excavations on site. Groundwater
levels are expected to be well below 10 feet from the existing surface grades. The soils are suitable for a
conventional shallow absorption field consisting of gravelless infiltration chamber units.
Upgraded Soil Treatment Area Design
The upgrade treatment/absorption field is designed based on utilizing the effective Long Term Acceptance Rate
(LTAR) for the native soils and the application of appropriate loading factors for a soil treatment unit system
utilizing gravity distribution to a gravelless chamber bed. The new treatment system and absorption field will consist
of gravity distribution of septic tank effluent via a distribution box to individual chamber rows with treatment across
the scarified native soil absorptive surface of the expanded bed excavation.
Long Term Acceptance Rate (LTAR)
Wastewater Treatment Level 1; loading rate for silt loam is (0.60 gal./S.F./day) for soil type 2.;
Loading factors; (Table 10-2, Gravity application bed = 1.2 ) and
(Table 10-3, gravelless chambers = 0.7)
A(sf) = Qd x LF 1 X LF 2.: A = Area;
LTAR LTAR = 0.60 Gal/S.F./day for silt loam
Qd = flow (gal/day) = 160 gpd
L.F.1=1.2 Gravity Distribution bed
L.F.2=0.7 Leaching Chambers
A(sf) = 160 gpd x 1.2 x 0.7 = 224 S.F.
0.60 Gal/S.F./day
Number of Arc-36 (5' long ) chambers:
224 S.F = 14.9 chambers, Install 16 ADS Arc-36 Biodiffuser chambers
15 S.F./Chamber Total Absorption area provided is 240 S.F.
Install 20 Biodiffuser chambers in two rows 50' long by 3' wide plus two end caps as delineated on the plan or
equivalent Quick 4 chambers. The septic tank effluent will be gravity distributed to the head of each chamber row
via a 4" distribution pipe from the concrete distribution box.
Construction and Inspections
Prior to construction of the permitted system, the engineer should be contacted by the contractor and owner well in
advance to provide adequate time to discuss the system components with the contractor, answer questions, resolve
any conflict issues and schedule inspection site visits based on construction progress. A pre-OWTS construction
meeting is essential and required prior to installation of the OWTS. All septic system components shall be staked in
the field for approval by the Engineer prior to excavation.
During initial construction the engineer will evaluate the soils in the absorption treatment field excavations to
confirm soil conditions and make adjustments as needed. All septic system components and trench installations are
to be approved by the Engineer prior to backfilling. All installation will be inspected, measured, reported and
delineated in the Certification letter and Record drawing. The contractor needs to photo document all OWTS
construction phases and supply photos to the Engineer.