HomeMy WebLinkAboutEngineer's OWTS Design PlanMarch 10, 2025
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
S OPRIS E NGINEERING • LLC civil consultants
Eric Rudd
Rudd Limited Liability Company
0132 Park Avenue
Basalt, Colorado 81621
eric@ruddconstruction.com
RE: OWTS Design Report, On-site Wastewater Treatment System (OWTS), Rudd Limited Liability
Company, Buildings I & K, 62 County Road 113, Carbondale, Garfield County, CO
SE Job No. 34003.01, Parcel No: 239318100358. New Garfield County Permit # ____________
Dear Eric:
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 current building
permit application for Building I. The new OWTS is designed to serve the required design capacities for
current proposed Building I and future proposed Building K usage on the subject property.
The OWTS design is based on our evaluation of the existing and proposed site conditions with respect to
the operation and installation of a new OWTS in accordance with County OWTS Regulation 43 for use in
obtaining a Garfield County OWTS construction permit. The new system meets all required setback
requirements and operational demands for the proposed usage. Based on the physical site features, soils,
and proposed building layouts, we have prepared a civil OWTS design plan with construction details for
permit approval. The design drawing is based on a recent Topographic Survey, by Stars to Stones
Surveying, dated 11-14-2022. The design and construction recommendations with specifications are
delineated on the civil design plans, C1.0-OWTS Plan dated 03-10-2025, by Sopris Engineering.
OWTS Conditions and Design Criteria
The proposed OWTS System will serve the current and future proposed usage for buildings I and K,
respectively, based on a total design flow of 1160 gpd. The OWTS will include gravity sewer service pipe
from the currently proposed building I to a new 1250 gallon 2-comparment concrete septic tank that will
provide for sewage detention of building I waste. The second tank compartment will be equipped with a
high head effluent duplex pump assembly used for dosing effluent from the proposed building I and future
Building K combined STA. The future proposed building K will have a new 2500-gallon 2-compartment
concrete septic tank equipped with a high head effluent pump assembly and 2” discharge transport pipe.
The 2” effluent discharge pipe will transport the building K septic tank effluent to the second chamber of
the building I, 1250-gallon septic/dosing tank to dose the combined effluent to the proposed STA sized to
treat the total current and future design flow of both buildings.
The system will include the installation of an Orenco System bio tube effluent pump assembly for pressure
distribution of septic tank effluent to a new Geotextile Sand filter (GSF) bed system consisting of Geomat
enhanced manufactured media, filter sand and custom perforated distribution laterals. The effluent
discharge piping will be installed to connect to an automatic distribution valve that will have individual
distribution pipes connecting to a manifold pipe at the head of each pair of rows set in the Geomat
treatment beds each containing two pairs of rows with custom perforated distribution laterals. Each bed
row will have appropriate inspection ports and appurtenances.
OWTS Design and Construction
Rudd, 62 113 County Road, Carbondale, Garfield County
SE Job No. 32247.01
March 10, 2025
Page 2
The system is designed based on the current design criteria approved, on February 19, 2020, by CDPHE
for the use of a pressure dosed geotextile sand filter (GSF) type system. Specifically, the preferred
treatment technology is Geomatrix System, LLC “Geomat” system approved by CDPHE for use as a
proprietary distribution product for use in Colorado OWTS’s utilizing applicable multiplier loading rates
for enhanced manufactured media. The design is in compliance with area calculations based on the LTAR
loading rate for the in-situ soils utilizing Geomat material installed over a 24-inch layer of ASTM-33
secondary filter sand in shallow pressure dosed beds sized per the estimated total design flow.
Design Calculations OWTS
The current proposed commercial warehouse, Building I will have a single ½ bath restroom and may have
up to a maximum of 8 workers. The future proposed Building K commercial structure is planned to have
25 shop suites with a single ½ bath restroom per suite with up to 50 workers.
Design Flow Criteria: Minimum population for Building I is based on 8 persons for a conservative
estimate of warehouse shift employees and/or transients and for Building K a peak average 2 persons per
workshop = 50 persons. Design flows are based on 20 gal/day for 8-hour shift employees for light
industrial workers (factory or plants, no showers).
Max. Design flow (Qd) gallons/day = (# of people) x (avg. flow) gal/person/day.
Design capacity = (25 x 2) + 8 = 58 persons @ 20 gal/person/day = 1160 gpd.
Septic Tank Design: Each septic tank will provide for 2-Day detention time capacity for the respective
design flows from each building
Building I: Design flow Qd = 8* 20 = 160 gpd x 2hr = 320 gal detention capacity
Building K: Design flow Qd = 50 * 20 = 1000 gpd x 2hr = 2000 gal detention capacity
.
Building I: Install a 1250 gallon two compartment concrete septic tank with a high head pump assembly in
the second compartment to dose combined buildings I and K effluent to the soil treatment Area.
Building K: Install a 2500 gallon two compartment concrete septic tank with a high head pump assembly
in the second compartment to transfer building K septic tank effluent to the second Building I tank
compartment.
Sub Surface Conditions and Testing
Based on site observations, review of previous permit documentation and geotechnical evaluations the
soils below nil to 4" of topsoil consist of sandy silt loam texture soils. A site assessment, OWTS inspection
and subsurface soil investigations were performed by SE on November 1, 2022. Two test profile pits were
excavated to sample and characterize the in-situ soil conditions where the STA will be located. The soils
encountered in the pits to a depth of 6’ consist of silt loam and scattered gravels with slightly granular to
massive structure of weak grade. The evaluation indicated the presence of wetted soils at 5' deep in pit one
and 6’ in pit two. This soil type 2A has an effective loading rate for conventional soil treatment of 0.5
gal/S.F./day for a level 1 conventional treatment system. For a unlined sand filter with secondary treatment
sand the TL-3 LTAR is 0.8. No free water is expected to be encountered within the field excavations.
OWTS Design and Construction
Rudd, 62 113 County Road, Carbondale, Garfield County
SE Job No. 32247.01
March 10, 2025
Page 3
Additional observations and soil evaluation will be performed during construction prior to installation of
field components, by Sopris Engineering in the area of the proposed STA and tank to confirm the previous
soil evaluation and design criteria. If soil conditions encountered require adjustments to design criteria or
the design plan a notice will be made to County EHD and applicable revisions to design/construction plans
and report will be submitted. The results of the additional soil evaluation and soil characterization will be
included in the As-Constructed documentation.
GeoMat™ Treatment Unit/ Absorption System Design
The treatment/absorption fields are designed based on utilizing the effective Long Term Acceptance Rate
(LTAR) for a geotextile sand filter (GSF) treatment system utilizing pressure dosed distribution laterals.
The new treatment systems will consist of pressure dosed GeoMat sand filter treatment beds with 24-
inches of filter sand (ASTM-C33, "Concrete sand") below rows of 39-inch wide GeoMat model 3900
manufactured treatment/distribution media, in accordance with Regulation 42 sections (43.10(C),
43.13.E.1.d, Table 10-2 and Table 10-2 as approved by CDPHE for this technology.
Long Term Acceptance Rate (LTAR)
Considering the allowable application of the OWTS regulation 43 and CDPHE approved proprietary
Distribution Product for Geomatrix Systems, GeoMat product:
Receiving septic tank effluent a treatment level-3 system for an unlined sand filter requires a minimum 24"
depth of filter sand media with the infiltrative surface being at least 2.5-feet above a limiting layer (43.8,
Table 7-2.4); A pressure dosed sand filter results in the applied effluent to be treated to Wastewater
Treatment Level 3 quality prior to infiltration into native soil:
The design basal area for a treatment level 3 STA is sized per the maximum LTAR of (0.9 gal./S.F./day)
for type 2A receiving soils or the maximum LTAR of 0.8 gal/S.F./day for the minimum 24" depth of
secondary sand treatment media whichever results in the larger area;
The maximum Loading rate for this design is therefore 0.8 gal./sq.ft./day The STA is designed utilizing a
loading factor of 1: (Table 10-2, pressure dosed application bed = 1.0 )
A(sf) = Qd x L.F.1.: A = Area;
LTAR LTAR = 0.80 gal/S.F./day for secondary sand
Qd = flow (gal/day) for OWTS = 1160 gpd
OWTS: A(sf) = 1160 gpd = 1450 S.F.
0.80 Gal/S.F./day
Install two 12' by 61' Geomat beds with 4 rows of full pressure dose distribution laterals. The 2 beds will
provide 1464 S.F. of treatment/absorption area. Excavate the beds to a minimum 32” depth below average
existing surface grade. Scarify surface, remove any cobbles and place 24-inches of ASTM-C33, "Concrete
sand". Install the specified length of GeoMat rows with the specified custom perforated distribution
laterals and manufacturer supplied orifice shields below the specified orifices on top of the GeoMat
material. The distribution laterals to be installed per manufacturer’s recommendations below the top
geofabric cover. Backfill the completed geomat beds with suitable excavated materials to finish top of bed
grade mounded approximately 6 inches above the surrounding surface grades.
OWTS Design and Construction
Rudd, 62 113 County Road, Carbondale, Garfield County
SE Job No. 32247.01
March 10, 2025
Page 4
Effluent Distribution System
Filtered effluent will discharge to the Geomat field laterals via a high head pump biotube discharge
assembly to an automatic distribution valve between the head of each treatment area bed.
The distribution system will be installed to allow for effluent to be alternately and equally distributed
across the absorption surface in each pair of bed rows via individual custom perforated full pressure
distribution laterals connected to the manifold pipe at the head of each pair of geo mat rows installed on
the enhanced manufactured media surface along the entire length of the individual beds. The 5/32 inch
orifices will be drilled at 2’ centers oriented downward along the length of the distribution lateral. A ball
valve will be installed on the terminal end of each distribution lateral centered below and housed in a 8"
valve box with lid set 2" above surface grades.
The minimum slope of the transport pipe from the tank's dosing compartment to the ADV shall be installed
at a minimum 2.5% upward slope to allow for drain back to the tank. The individual distribution pipes
from the ADV shall be installed at a minimum 1% downward slope and will connect to the distribution
lateral manifold at the head of each paired Geomat row.
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, answer questions, resolve any conflict issues and schedule inspection site visits based
on construction progress. A pre-OWTS construction meeting and calls are essential and required prior to
installation of the OWTS.
The engineer, prior to excavation shall inspect the staked location of the proposed treatment/absorption
bed and tank. 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 location of the
absorption treatment field to confirm design assumptions made and existing conditions. The engineer will
provide construction recommendations and make adjustments to the field layout and sizing if needed with
discussion of construction procedures.
As necessary a revised Construction document drawing will be submitted to the County as needed if
changes are required. All installations shall be inspected, surveyed, documented and reported/delineated in
the Certification letter and As-Constructed drawing.
County regulations require that the design engineer of record perform site inspections of the permitted
system during construction and provide “As-Built” documentation of the installed system to the county
after construction is complete. Photo documentation during construction and site system inspections is
required to be performed by the contractor and engineer. System component installation inspection and
review of photo documentation is required of the exposed system components prior to backfilling.