HomeMy WebLinkAboutOWTS Design ReportJune 18, 2021
Haines, Steven G & Bailey A
18 Pine Street,
Carbondale Co 81623
"Steven Haines" coffgbdsteve a7gmail.com>
RE: Design Report, On-site Wastewater Treatment System (OWTS), 41 S. Pine Drive, Carbondale,
Garfield County, CO, Copperton Townsite Block: 13 Lot: 5 Amended Lots 5-8
SE Job No. 31132, Parcel No: 239328313016, New EHD Permit #
Dear Steven:
This letter report presents the applicable findings in regard to the design and construction of a replacement
Onsite Wastewater Treatment System (OWTS) at the above referenced Site, in support of a new OWTS
permit application to Garfield County. A new OWTS construction permit is required for the proposed usage
on the subject property.
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 a new OWTS in accordance with County OWTS Regulations and State of
Colorado Regulation 43. 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 layout, we have
prepared a civil OWTS design plan with construction details for permit approval. The design drawing is
based on a previous Improvement Location Certificate by Survco Inc. Site observation sketches and GIA
database mapping. The design and construction recommendations with specifications are delineated on the civil
design plans, C 1-OWTS Plan dated 06-18-2021, by Sopris Engineering.
OWTS Conditions and Desien Criteria
Existing OWTS Components:
The current OWTS system serves the existing residential dwelling that currently has four bedrooms. The OWTS
components consist of gravity service pipes, a 1,500 -gallon Norwesco plastic septic tank, that discharges ST effluent
to a 741 S.F. chamber bed field consisting of 8 chamber rows approximately 31' long by 24' . The effluent is gravity
discharged to a buried plastic distribution box with (4) 4" effluent distribution pipes each connected to a manifold pipe
that connect to 1 of the 4 pairs of chamber rows. The existing bed is compromised by exhibiting reduced infiltrative
capacity performance and needs to be replaced with and upgraded system. The existing chamber is deep with the top
of chambers over 5 feet below the existing surface grades. The bed will be abandoned in place or removed as needed
to install the new proposed shallow OWTS Geomat beds in and around the general vicinity. The absorption field was
installed in 1998, under County building permit no. 6961.
Proposed OWTS Replacement STU Technology:
The systems is designed based 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
502 Main Street ■ Suite A3 ■ Carbondale, CO 81623 ■ (970) 704-0311 • Fax (970) 704-
0313
SopRis ENGINEERING • LLC civd consultants
QWTS Design and Construction
41 S. Pine
SE Jab No. 31132.01
June 18, 2021
Page 2
with area calculations based on the LTAR loading rate for the in-situ soils utilizing Geomat material installed over a
6 -inch layer of ASTM -33 filter sand in shallow pressure dosed beds per the estimated design flow capacities.
Proposed 4WTS System Replacement
Proposed site improvements and expected usage result in a design flow of 525 gpd for the existing four bedroom
residence. The system will be constructed to provide for demand dosing of septic tank effluent to an engineered
enhanced media soil treatment area (STA) pursuant to Regulation 43 design criteria. The QWTS will maintain the
gravity service pipe from the existing dwelling to the existing 1,500 -gal concrete 2 -compartment plastic septic tank-
The
ankThe replacement system will include the installation of a new single compartment dosing basin/tank in series with the
primary tank that provides for a minimum 70 gallon dosing volume and minimum 150 gallon storage volume.. Due to
the current material supply challenges, the dosing basin/tank alternative may be a 300 -540-gallon Norwesco, Roth or
Infiltrator single compartment plastic septic tank or a 500 gallon concrete tank. A 30" ribbed PUC Pump Basin may
also be used. The new dosing basin/tank will be equipped with an Grenco System bio tube simplex 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 a common manifold pipe that will connect to the custom perforated
distribution laterals at the head of the Geomat treatment/absorption beds.
Design Calculations OWTS
The existing residential structure has 4 bedrooms. The total peak residential population is 7 persons.
From County adopted State Regulation 4, Design Flow Criteria. Minimum population is based on 2 persons per
bedroom for the fust 3 bedrooms = 6 persons and 1 person per bedroom for each additional bedroom. Therefore.
design population is 3 bedrooms x 2 persons + 1 bedroom x 1 person = 7 persons total.
Gallons per day per person =75 gal/person/day, No peaking factor
Design capacity = Max. Design flow (Qd)gallonslday = (# of people) x (avg. flow) gal/person/day.
Design flow Qd = 7 + 75 = 525 gpd
Septic Tank Design. 2 -Bay detention time = 525 x 2 = 1,050 gallons required.
Continue use of the existing 1,500 gallon plastic two-compartment primary septic tank and install in series a
minimum 300 gallon dosing pump tank with a bio -tube simplex high head pump dosing assembly or minimum 150
gallon pump basin with a bio -tube simplex high head pump dosing assembly.
Sub Surface Conditions and Testing
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on dune 10, 2021. The
soils in the proposed field envelope area were sampled and characterized by application of the USDA visual/tactile
soil texture method analysis from samples in a 7 foot deep profile pits and 5 foot deep excavations exposing the tank,
effluent pipe and buried distribution box. The shallow soils in these excavations were sampled, observed and tested at
2, 3 and 6 foot depths.
The soils below nil to 4 inches of topsoil consist of medium dense sandy silt loam texture soil with slightly blocky to
granular structure to a depth of 7 feet. The soil profile contains less than 20% scattered gravel or rock content to the
b -feet depth below the surface grades. The soils appear to be consistent across the site from observations in the
QWTS Design and Construction
41 S. Pine
SE Job No. 31132.01
June 18, 2021
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excavations. No free water was encountered in the pit excavations. Seasonal high Groundwater levels are expected to
be below 10 feet from the existing surface grades on the relatively flat terrain.
The native soils sampled from 2-3 feet below the surface, in the vincinity of the proposed field, are characterized as a
soil type 2 consisting of silt loam texture with granular structure. This soil has an effective loading rate for
conventional soil treatment of 0.6 Gal/S.F./day for a level 1 conventional treatment system The equivalent percolation
rate is assumed to be 20-30 minutes per inch. In order to provide optimal treatment in the limited available area, a
pressure dosed STA utilizing enhanced manufactured media will be installed to provide for an enhanced treatment
level TL -1 system.
Additional observations and soil evaluation will be performed during construction prior to placement of the enhanced
media materials, by Sopris Engineering in the area of the proposed treatment/absorption field and tank to inspect the
soils and installation criteria. The soils will be further observed to determine the presence of any unsuitable soil or
other conditions in order to assist the contractor in installing the design material in 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.
GeoMatTM 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 6 -inches of washed filter sand (ASTM -
C33, "Concrete sand") below 4 rows of 39 -inch wide GeoMat model 3900 manufactured treatment/distribution media,
in accordance with Regulation 42 sections (43.10(C), 43.13.E.Ld, Table 10-2 and Table 10-3 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, the treatment level -1 for GeoMat requires a minimum 6" depth of filter sand media with
the bottom of the sand layer defined as the infiltrative surface being at least 4 -feet above a limiting layer (43.8, Table
7-2.4); Maximum hydraulic loading rate for TU effluent is the long term acceptance rate of the receiving soil Type 2
is 0.6gal.IS.F.lday) (Table 10-1 &10-1A.).
A(sf) = Qd x L.F.1 x L.F.2 .: A = Area;
LTAR LTAR = 0.6 gaUS.F.lday for silt loam
Qd = flow (gal/day) for QWTS = 525 gpd
L.F.1=1.0 pressure dosed bed application
L.F.2=0.7 Enhanced Manufactured Media
OWTS: A(sf) = 525 gpd x 1.0 x 0.7 = 612.5 S.F.
0.6 Gal/S.F./day
The treatment field will consist of two 25.5 feet long by 12 feet wide pressure dosed Geomat sand filter beds each with
of 306+ square feet of absorption treatment area with (4) custom perforated pressure dose distribution laterals per bed
to provide a total 612.5 S.F. of treatment field area. Excavate two beds to a minimum 18" depth below finish grade.
QWTS Design and Construction
41 S. Pine
SE Job No. 31132.01
June 18, 2021
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Scarify surface, remove any random cobbles and backfill with 6" 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 geo mat beds with suitable
excavated materials to finish top of bed grade slightly mounded approximately 4 inches above the surrounding surface
grades.
Effluent Distribution System
The 612.5 S.F. treatment field will be installed with a total of 8 pressure dose distribution laterals that will be equally
pressured dosed ,by the high head pump assembly in the dosing basin, to a common 1.5" manifold pipe connected to
each lateral running the width of the two beds . The dosing pump assembly will be installed in the pump basin between
the septic tank and the field to provide for equal pressure distribution of effluent across each treatment/absorption bed.
The septic tank effluent will be dosed through each individual 1.5" custom perforated pressure distribution lateral
running the length of each bed centered on each 39" GeoMat material row. The target dosing volume per cycle is 70
gallons that provides over 3 times the total pipe volume anticipating 3-4 dosing cycles per day.
A ball valve will he installed on the terminal end of each distribution lateral centered below and housed in 612.5a 10"
valve box with lid set 2" above surface grades. The minimum slope of the transport pipe from the dosing basin to the
manifold shall be installed at a minimum 2.5% upward slope to allow for drain back to the basin. A piezometer
monitoring well pipe riser with threaded cap will be installed at two corners of the field and be extended down to the
excavated surface depth at the bottom of the placed sand.
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-QWTS construction meeting and calls are essential and required prior to installation of the QWTS.
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. Photo documentation will be included with the As -Built Record
drawing submittal package to finalize the OWTS permit. Therefore the Engineer should be contacted as needed and
notified of the construction progress and scheduling to allow time to schedule specific inspection times.
QWTS Design and Construction
41 S. Pine
SE Job No. 31132.01
June 18, 2021
Page 5
OWTS Operation and Maintenance
All components of the engineered QWTS shall be inspected on a regular basis and be properly maintained. The
responsibility for repair and maintenance of the system will remain with the property Owner. The owner shall retain
the services of qualified personnel to inspect the QWTS and to perform all maintenance and repairs necessary to
ensure that the system is in good operating condition and is in compliance with the manufacturer's performance
requirements. The operating components of the QWTS shall be inspected by the engineer within 30 days of being
placed into operation and thereafter every six months by the owner or qualified service provider. In general, for a
properly utilized system, septic tanks should be pumped every 3 - 5 years. The effluent filter should be cleaned every
six months and at the time of pumping. Absorption fields should be maintained with suitable vegetative cover and
kept free of root invasive plants. Positive surface drainage away from the absorption field should be maintained.
If you have any question or need any additional information, please call.
Sincerely,
SOPRIS ENGINEERING, LLC
28377 ;
E�wr 78, X027
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Paul E. Rutledge Yancy Nichol, PE
Design Engineer Principal