HomeMy WebLinkAboutEngineer's OWTS Design PlanJune 1 3,2025
Moises and Kenia Perez
1534 East 12th Street
Rifle, Colorado 81650
Keniacardenas @ live.com
RE: OWTS Design Report, On-site Wastewater Treatment System (OWTS), Moises & Kenia Rebecca
Perez,Perez Residence, Rapids on the Colorado Lot 15, New Castle,
SE Job No. 35055, Parcel No: 218104307015. New Garfield County
Garfield County, CO
Permit #
Dear Kenia:
This letter report presents the applicable findings regarding 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. A new OWTS construction permit is required for the proposed urug" on
the subject property.
The OWTS design is based on our evaluation of the site conditions for use in obtaining a Garfield 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 proposedbuilding layout,
we have prepared a civil OWTS design plan with construction details for permit approval. The design
drawing is based on a recent site plan, by Studio M Engineers, dated 05-12-2025.1he design and
construction recommendations with specifications are delineated on the civil design plans, Cl.0-OWTS
PIan dated 06-13-2025, by Sopris Engineering.
OWTS Conditions and Desien Criteria
Proposed OWTS System
Proposed site improvements and expected usage result in a design flow of 450 gpd. The system will be
constructed to provide for demand dosing of septic tank effluent to an engineered soil treatment area
(STA), pursuant to Regulation43 design criteria. The OWTS will include a gravity service pipe from the
new dwelling to a new 1500-gallon concrete septic tank equipped with a high head effluent pump
assembly.
The system is designed based on the current design criteria for the use of a pressure dosed sand filter bed
system using leaching chambers as spray shields. The STA area calculations are based on the design flow,
and the LTAR loading rate for the in-situ soils utilizing applicable multiplier loading rates.
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 pressure dosed sand filter system. The effluent discharge
piping will be installed to connect to a pressurized manifold connecting to the head of each chamber row's
custom perforated distribution laterals. Each distribution lateral will have appropriate inspection port
cleanouts and appurtenances.
502 Main Street a Su ite A3 a Carbondale, C0 B1 623 a (970) 704-031 1 a Fax (970) 704-031 3
$oPRts tnetltunllc o llc civil consultants
OWTS Design and Construction
Perez, Rapids on the Colorado Lot 15, New Castle, Garfield County
SE Job No. 35055.01
June 13,2025
Page2
Desien Calculations OWTS
The proposed residential facility design includes a 3-bedroom single family residence.
The design flow is based on a total residential population of 6 persons.
From County adopted State Regulation 43, Design Flow Criteria: Minimum population is based on 2
persons per bedroom for the first 3 bedrooms = 6 persons and 1 person per bedroom for each additional
bedroom.
Design capacity =3 x2 = 6 persons @ 75 gal/person/day,
Max. Design flow (Qd) gallons/day - (# of people) x (avg. flow) gallperson/day.
Design flow Qd = 6 * 75 = 450 gpd
Septic Tank Design: 2-Day detention time = 450 x 2 = 900 gallons required.
Use a 1500 gallon, two compartment concrete septic tank with a high head pump assembly in the second
compartment.
Sub Surface Conditions and Testins:
A subsoil study for foundation design was performed by Kumar & Associates on January 14,2024 with
findings presented in their report dated February 24,2025 under project no.25-7-LI1. The soils
encountered in two boring holes were sampled and characteizedby laboratory testing. The soil conditions
were evaluated and sampled to determine the presence of any Iimiting soil or hydrological conditions,
assess appropriate hydraulic loading rate and to assess appropriate treatment level design alternatives. The
soils below about 1 foot of topsoil consist of medium dense silty sand to a depth of about 54 inches below
the surface, overlying dense, silty sandy gravel and cobbles to the maximum drilled depth
of 7% feet.. The soil appears to be consistent in both profile excavations and across the sloped terrain in the
vicinity of the proposed field. No evidence of free water was encountered or observed in the borings
excavations. Groundwater levels are expected to be below 8 feet from the existing surface grades.
The soils 2 to 3 feet below the surface are charactenzed as a soil type 2 consisting of a silty sand texture of
moderate structure with rock content less than 357o. Soil type 2 has an effective loading rate for
conventional soil treatment of 0.6 Gal/S.F./day, respectively for a level I conventional treatment system.
The equivalent percolation rate is assumed to be 25-50 minutes per inch. The soils below 4.5 feet consist
of an Rl silty sand soil with rock content greater than35Vo, that presents a limiting layer for a conventional
STA. Therefore a treatment level-3 full pressure dosed sand filter system is recommended.
Additional observations and soil evaluations from two profile pits will be performed by Sopris Engineering
in the area of the proposed treatment/absorption field and tank during intial stages of construction prior to
placement of the distribution and sand filter media materials. Sopris Engineering will verify the soil
conditions, OWTS components and provide installation guidance to the contractor. If any adjustments to
design criteria or component modification are necessary, notice will be made to County EHD and
applicable revisions to design/construction plans and report will be submitted. All findings, conditions and
installation documentation will be included in the As-Constructed plan and report.
OWTS Design and Consffuction
Perez, Rapids on the Colorado Lot 15, New Castle, Garfield County
SE Job No. 35055.01
Jane 13,2025
Page 3
Sand Filter Treatment Unit/Absorption Svstem Desien
The treatment/absorption field is designed based on utilizing the effective Long Term Acceptance Rate
(LTAR) for an intermittent sand filter treatment system utilizing pressure dosed distribution laterals and
gravelless chamber units as spray shields. The new treatment system will consist of a fully pressure dosed
sand filter Treatment bed System with a Z4-inch layer of filter sand (ASTM-33, "washed ASTM-33
Concrete sand").
Long Term Acceptance Rate (LTAR)
Considering the application of regulation 43 design criteria of unlined sand filters receiving septic tank
effluent, pressure dosed sand filter results in the applied effluent to be treated to Wastewater Treatment
Level 3 quality prior to infiltration into native soils:
The design basal area for a treatment level 3 STA is sized per the maximum LTAR of ( l.l gal./S.F./day)
for type 2 receiving soils or the maximum LTAR of 0.8 gallS.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.lday The STA is designed utilizing a
loading factor of l: (Table 10-2, pressure dosed application bed = 1.0 )
A(s0 =Odx L.F.A = Area;
LTAR = 0.8 Gal/S.F./day for secondary sand, ASTM-33, "
Concrete sand"
Qd = flow (gallday) = 450 gpd
L.F.1=1.0 pressure dosed bed
LTAR
A(s0 =450gpd x 1.0 = 562.5 S.F.
0.8 Gal/S.F./day
Construct a single 47'by 12' sand filter bed with a minimum Z|-inchdepth of ASTM-33, concrete sand
placed over shallow native soils. The maximum excavation depth of the bed to be 48". Install 3 rows of
custom perforated 1.5 inch distribution laterals on top of sand layer and cover with l1 Quick-4 Standard
Chambers per row for use as pressure dose spray shields, as delineated on the plan. Backfill entire bed with
native soil l0- to 12 inches above the top of chambers.
Effluent Distribution Svstem
The septic tank effluent will be pressure discharged to a common manifold pipe, installed at the head of
the lateral rows, via a 1.5" transport pipe from the high head pump discharge assembly in a biotube filter
vault installed in the second compartment of the 1,500 gallon tank The minimum slope of the transport
pipe from the tank's dosing compartment to the manifold shall be installed at a minimum2To slopeio allow
for drain back to the tank.
The distribution system will be installed for effluent to be equally distributed across the absorptive surface
in each row via individual custom perforated pressure distribution laterals connected to the manifold pipe.
OWTS Design and Construction
Perez, Rapids on the Colorado Lot 15, New Castle, Garfield County
SE Job No. 35055.01
Iune 13,2025
Page 4
The distribution laterals will be installed on the infiltrative surface, under the chambers, along the length of
the bed. The 5/32-inch orifices will be drilled on 2'centers oriented upward along the length of tfr"
distribution lateral except for the first and last orifice that will be oriented downward with an attached
orifice sheild to allow for drainage of the lateral. A ball valve will be installed on the distal end of each
distribution lateral centered below the 4" inspection port, housed in a 6" valve box with lid set 2" above
surface grades.
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 discusi the system components
with the contractor, answer questions, resolve any conflict issues and schedule inspectiron 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 th"
absorption treatment field to confirm design assumptions made and existing conditions. The engineer will
provide construction recommendations and make adjustments to the field liyout and sizing if nJeded 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 instaliation insiection 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 pu"kag" to finalize ttr. OW1S
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.
The county environmental health department needs to be contacted at least 48 hours prior to final backfill
of components to provide advanced notice of construction schedule in order to perform a final inspection
of the in place components. The Garfield county environmental health contact number is 970-945-6614 x
8106 (main office).
OWTS Operation and Maintenance
All components of the engineered OWTS shall be inspected on a regular basis and be properly maintained
The responsibility for repair and maintenance of the system will remain with the propertfowner. The
owner shall retain the services of qualified personnel to inspect the OWTS and toperform all maintenance
perez" Rapids on rhe colorad" r", ,1H*T,i:?t:.?ffiffi8:Jfi;tt*t,T:;t1:'rill
Page 5
and repairs necessary to ensure that the system is in good operating condition and is in compliance with the
manufacturer's perforrnance requirements. The operating cornponents of the OWTS shall bi inspected by
the engineer within 30 days of being placed into operation and thereaffer evsry six months by thl owner or
qualified service provider. In general, for a properly utilized system, septic tanlcs should be pumped every
3-5 years- The effluent filter should be cleaned every six rnonths and at the time of pumping. Absorption
fields should be maintained with suitable vegetative cever and kept free of root invasive ptints. 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 ENCINEERING, LLC
1837?
13,20?5
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Paul E- Rutledge
Design Engineer
AL
Yancy PE
Principal
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