HomeMy WebLinkAboutEngineer's OWTS Design PlanJuly 15, 2025
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311• Fax (970) 704-0313
S OPRIS E NGINEERING • LLC civil consultants
Keith Brand
147 Grange Lane
Carbondale CO. 81623
Keith Brand<keith@terralinkstructures.com>
RE: OWTS Design Report-Brand Residence-Lot 2, Reception #1007096, 55 S Cedar Street, (Pending Change
TBD), Carbondale Garfield County, CO Parcel Number: 239328411005, (Pending Change TBD)
SE Job No. 33149.01, Garfield County OWTS Permit No.________________
Dear Keith:
This letter report presents the applicable findings in regard to the design and construction of a new Onsite
Wastewater Treatment System (OWTS) at the above referenced site, in support of your building permit application
to Garfield County. The proposed improvements include a 1200 S.F. 3-bedroom single family residence on Lot 2.
Findings
The subject site is suitable for residential development and construction per the provisions of the Garfield County
land use code. The proposed residential site improvements and the engineered OWTS have been designed in
accordance with Garfield County and the State of Colorado Regulations, based on the site assessment findings that
included relevant review of the amended plat reception # 1007096, physical, legal, regulatory, existing and proposed
conditions. The proposed site specific OWTS improvements are presented on the attached OWTS design plan, C2 -
OWTS Plan, dated 07-15-2025, by Sopris Engineering, for obtaining a Garfield County OWTS construction permit.
Existing Site Conditions
The existing abandoned buildings on Lot 2 parcel will be demolished and the site will be prepared for new
construction. The subject property is located at 55 S Cedar Street near the intersection of Sopris Road and Cedar
Street. The proposed improvements on Lot 2 consist of a new residential dwelling, drive/parking and OWTS. The
site is designed with respect to the existing topography, development envelopes, vegetation, and other existing and
proposed site features as shown on the Improvement Survey Plat prepared by Sopris Engineering dated July 15,
2025 and the Final Plat, Cooperton Twnste. Block:11 Lot: B recorded under Reception #1007096, dated May 20,
2025, that delineates Lot 2.
The proposed development site is situated on a flat to gently sloped (approximately 0%-9%) lightly wooded and
grassy area. Topography within the development is generally flat from the western envelope boundary draining into
a small ditch on the eastern edge of the property. A new well was installed near the southeastern corner of the site.
The site is included in the Basalt Water Conservancy District per civil action no. 5593, augmented per BWCD
contract number: 3.3.5.811, March 14, 2023 for two single family homes.
Proposed OWTS System
The system is designed for demand dosing of advanced secondary treated effluent to a shallow pressure dosed
chamber dispersal field bed, pursuant to Regulation 43 design criteria. The design includes a gravity service pipe
from the proposed dwelling to a 1,500-gal concrete 3-compartment septic/Advantex treatment tank. The treatment
tank will be equipped with Advantex treatment and operational components to provide advanced secondary
treatment of the septic tank effluent. The treatment will include an AX-20 filter pod that has the capacity to treat
effluent from an equivalent 3-bedroom residence or up to 600 gpd for residencial wastewater strength . The
Keith Brand,
OWTS Permit Application Report
SE Job No. 33149.01
July 15, 2025
Page 2
treatment system will include all controls and Vericomm telemetry. The system will be maintained and service on a
bi-annual basis by a qualified service provider. The upgraded OWTS is designed and intended to be operated in
compliance with the County adopted OWTS Regulation 43 for higher level treatment systems..
Design Calculations OWTS
The OWTS is designed as a level 3N treatment System with installation of an OWTS advanced treatment unit to
allow for a smaller system footprint and efficiently serve the proposed usage.
Minimum population based on 3 bedrooms = 6 persons. Gallons per day per person =75 gal/person/day,
Design flow (qd) gal/day = (# of people) x (avg. flow) gal/person/day.
Design Flow qd = 6*75=450 gpd
Septic Tank Design
The septic tank capacity required for a minimum 48 hour retention time is calculated as follows:
V = 450 gal/day * 2 = 900 gallons.
Install new 1,500 gallon 3-compartment concrete Septic/Treatment Tank with Orenco Systems Advantex AX-20
treatment components. The first compartment chamber has a 1000 gallon capacity. The recirculation and dosing
compartments each have a 500 gallon capacity for a total tank Volume of 2000 gallons.
Sub Surface Soil Conditions
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on August 21, 2024.
The soils in the proposed soil treatment areas (STA) and adjacent building envelopes (STA) were sampled and
characterized by application of the USDA visual/tactile soil texture method analysis from samples in 5 profile pits
on Lot B. No evidence of seasonal high ground water, redox conditions or any other limiting condition was observed
in the 3 applicable profile pits located on Lot 2. Groundwater levels are expected to be below 10 feet from the
existing surface grades on the flat terrain.
Profile Pit-1
The soils below approximately 8 inches of rocky gravelly topsoil consists of sandy silt loam texture soil with slightly
blocky to granular shape of moderate structural grade to a depth of 5 feet containing less than 5% scattered gravel or
rock content. Soils below 5' become increasingly rocky with gravel and cobbles transitioning to an R type soil. No
free water was encountered at the 7' extent of the pit excavation.
Profile Pit-4
The soils below approximately 8 inches of topsoil consists of sandy silt loam texture soil with fine granular shape
containing less than 1% rock content to a depth of 6-feet. Soils below 6 feet become increasingly rocky with gravel
and cobbles transitioning to an R type soil to the profile pit depth of 7-feet. No free water was encountered in the pit
excavation.
Profile Pit-5
The soils below approximately 8 inches of topsoil consists sandy silt loam texture soil with fine granular shape
containing less than 1% rock content to a depth of 4-feet. Soils below 4-feet consists of gravels, cobbles and small
boulders a silt loam soil matrix, containing over 30% rock content, to the profile pit depth of 5-feet. No free water
was encountered in the pit excavation.
Keith Brand,
OWTS Permit Application Report
SE Job No. 33149.01
July 15, 2025
Page 3
The native soils sampled in the pits are generally characterized as a soil type 2 consisting of dark to light brown
slightly sandy silt loam texture with blocky to fine granular shape with moderate structural grade. The sandy silt
loam soil tactile test ribbons were 3/4-inch to 1 -inch long. Soil type 2 has an effective loading rate for conventional
soil treatment of 0.60 Gal/S.F./day for a level 1 conventional treatment system. For treatment level 3N the loading
rate is 1.1 gal/S.F./day. The equivalent percolation rate of these soils is assumed to be 20-30 minutes per inch.
Additional observations and soil evaluation will be performed during construction prior to the shallow placement of
the bed chambers in the area of the proposed dispersal field to inspect the soils and provide installation guidance for
installations of the distribution materials over the in-situ native soils. If any adjustments to design or installation
recommendations are necessary, notice will be made to County EHD and applicable revisions to design/construction
plans and report will be submitted. All conditions and installations will be included in the As-Constructed
documentation.
Treated Effluent Soil Dispersal/Absorption Area System Design
The treated effluent soil dispersal/absorption bed is designed based on utilizing the effective Long Term Acceptance
Rate (LTAR) for the native soils receiving treatment level 3N quality effluent and the full pressure dose distribution
to a gravelless chamber bed. The new treatment system and dispersal field will consist of controlled pressure dosing
of treated septic tank effluent, via a highhead dosing pump,to the chamber rows with custom perforated distribution
laterals that provides for equal pressure distribution across the native soil absorptive surface.
Control Systems.
Receiving Wastewater Treatment Level 3N;
LTAR; (Table 10-1 Soil type 2 loading rate for a sandy silt loam is (1.1 gal./S.F./day);
Design Flow; Qd = flow (gal/day) = 450 gpd
Loading factors; (Table 10-2, Pressure application bed, L.F.1 = 1.0 )
Treated Effluent Soil Dispersal/Absorption Area ; A = Area S.F.;
A(sf) = Qd x L.F.1 : A(sf) = 450 gpd x 1.0 = 409 S.F.
LTAR 1.1 Gal/S.F./day
Number of Quick-4 Plus LP Infiltrator chambers 4'x3'x8"chambers:
409 S.F. = 34 chambers; Use 36 chambers
12 S.F./Chamber
Install 36 Quick-4 Plus LP Infiltrator chambers in 4 rows 36' long by 3' wide with 9 chamber units per row
plus two end caps as delineated on the plan. The treated effluent will be pressure distributed to a common
manifold at the proximal head of the chamber rows and for discharge to each perforated distribution
lateral pipe in each chamber row.
The treated effluent soil dispersal/absorption bed is designed based on utilizing the effective Long Term
Acceptance Rate (LTAR) for the native soils receiving treatment level 3N quality effluent and the full
pressure dose distribution to a gravelless chamber bed. The new treatment system and dispersal field will
consist of controlled pressure dosing of treated septic tank effluent, via a highhead dosing pump,to the
chamber rows with custom perforated distribution laterals that provides for equal pressure distribution
across the native soil absorptive surface.
Keith Brand,
OWTS Permit Application Report
SE Job No. 33149.01
July 15, 2025
Page 4
System Controls
An Orenco systems control panel will be installed by an electrician in coordination with the qualified treatment
system installer. The control panel will control all functions and monitoring of the installed treatment and
distribution components. The controls will record all operational pump cycles, run times, operational conditions and
is capable of making automatic adjustments as necessary. Operational floats and alarms for the treatment system
components in the re-circulation septic tank vault will be recorded and reset by the control panel. A Vericomm
Telemetry system will be installed via a com connection in the control panel to allow offsite monitoring, data
collection and remote operational setting control. The system is set up with a web-based technology for use by
authorized operators and/or the contracted maintenance providers.
The control panel will be configured for 240 volt electric service for to the recirculation pump and controls. The
power supply for the pumps and controls shall consist of (1) 20 amp circuits for the dosing and circulation pump and
(1) 10-amp circuit for controls. Provide line of site disconnect at the control box to the pump and control
components at the tank. The electrician or qualified installer shall install electrical wires in conduit from the service
panel and or junction box to the Orenco control panel. Direct bury electrical wire may be used and installed in the
gravity sewer trench from the control panel to the respective junction box mounted on the riser over re-circulation
tank compartment.
OWTS Operation and Maintenance
All components of the engineered OWTS should be inspected on a regular basis and be properly maintained. The
responsibility for repair and maintenance of the system will remain with the Lot Owner. The owner will retain the
services of a qualified service provider to inspect the OWTS and to perform all required maintenance and repairs to
ensure that the system is in good operating condition in compliance with the manufacturer’s performance
requirements. The operating components of the OWTS system will be inspected within 30 days of being placed into
operation and thereafter inspected and serviced every six months in accordance with Regulation 43 Applicability,
Section 43.4 and Operation and Maintenance Section 43.14. The treatment system supplied by Valley Precast will
be purchased by the owner with a minimum 3-year maintenance contract.
A clearly visible, permanently attached label or plate giving instructions for obtaining service must be placed at a
conspicuous location for a higher level treatment system with applicable components under a service contract. The
owner is responsible for maintaining a contract with a qualified service provider to ensure compliance with the
provisions of the Gunnison County OWTS Regulations, any permit conditions and the manufacturer’s system
maintenance protocols.
The OWTS should require minimal maintenance. Several factors influencing the need for maintenance include:
actual wastewater flow versus design flow, the volume of kitchen/domestic waste (excluding human waste and toilet
paper), excessive household chemicals and other toxic liquids. The tank, dispersal field and other applicable
treatment system components should be visually inspected bi-annually for debris, wear, damage, leaks, or other
potential problems. In general, for a properly utilized system, septic tanks should be pumped and inspected every 2 -
4 years. The effluent filters should be cleaned every six months and at the time of pumping. Infiltration fields
should be maintained with suitable cover and kept free of root invasive plants. Positive surface drainage away from
the infiltration field should be maintained.
Construction and Inspections
Prior to construction of the permitted system, the engineer should be contacted by the contractor and construction
project manager well in advance to provide adequate time to discuss the system components with the contractor,