HomeMy WebLinkAboutOWTS Design Report 03.31.2022March 31, 2022
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313
S OPRIS E NGINEERING • LLC civil consultants
Jeffrey K & Worthen, Angela F, Huber
524 County Rd 259
Silt, CO
"Jeff Huber" <jklineman44@yahoo.com>
RE: Design Report–Onsite Wastewater Treatment System (OWTS), Huber Residence, 524 County Rd 259,
AKA Rolling Meadows Lot: Tr G, Section 34 T5S R92W 6th PM, Garfield County, Colorado
SE Job No. 31260.01 Parcel No: 212734200159, Previous Garfield County ISDS Permit: 1892, 9-9-91
Dear Jeff:
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 your OWTS permit application
to Garfield County. The OWTS design is based on recent site assessment observations by Sopris Engineering of
existing physical site features, conditions geotechnical soils evaluation with respect to the proposed residential
usage and applicable regulatory design criteria in accordance with State and County OWTS Regulations. The
design recommendations, specifications and construction details are delineated on the attached OWTS
permit/construction plan that was prepared with GIS, Survey Plat information and aerial mapping. The design
criteria and system sizing information is summarized below.
Summary Existing System Conditions
The waste flow from the existing residence has been served by an OWTS permitted at the time of building permit
in 1992, under County ISDS permit No. 1892 to serve a 3-bedroom residence. The system included the installation
of a two-compartment Copeland 125 0-gallon concrete septic tank with effluent pipe routed to a 4-way distribution
box. The existing 30 year old infiltrator chamber absorption trench system is currently functional however it is
assumed to have partially clogged soils, exhibiting slower infiltrative capacity and reduced recovery. The field is in
need of replacement. The previously installed septic tank is functional and sound with normal liquid levels.
However some root growth and degradation affecting the integrity of the concrete access was observed.
Replacement of the risers and lids is recommended. Based on the existing observed system conditions, the owners
have decided to replace the existing ISDS with a new regulatory compliant OWTS to insure effective sewage
management and as a value added upgrade to the property.
Summary Design Criteria
The replacement OWTS is designed for a treatment capacity of 525 gallons per day based on the design wastewater
flow to serve a 4-bedroom residence in accordance with Table 6-2 values, Section 43.6, A.4. This design provides
for an upgrade to the existing 1250 gallon septic tank, installation of a dosing tank with dosing siphon and
replacement soil treatment unit (STU). A new 500 gallon 1-compartment reinforced plastic dosing tank will be
installed in series with the existing tank within the landscape lawn embankment. A dosing siphon will be installed
in the tank to automatically dose septic tank effluent to a distribution box. The existing backfill over the 1,250
gallon septic tank, outlet pipe, raw gravity sewer inlet pipe, from primary dwelling will be excavated to the tank lid
and below the pipe inverts as necessary to inspect, repair and install new risers and pipe connection as needed. The
existing concrete risers will be removed. New 24” ribbed PVC risers with new plastic or fiber lids will be installed.
The risers will be set in new set on bolt down bases, secured and sealed to the top of the tank lid. The risers will
extend to 3” above finish surface grades. A new outlet effluent filter will be installed on the existing septic tank
along with appropriate pipe connection to the new dosing tank in series. A gravity schedule-40 or SDR-35 effluent
pipe from the new dosing tank/siphon discharge assembly will be installed to the new replacement STU absorption
field with appropriate stepped fittings and cleanouts as needed to a new distribution box at the head of the STU
chamber trench field.
Huber Residence
OWTS 524 CR 259
SE Job No. 301260.01
March 31, 2022
Page2
The new replacement soil treatment unit (STU) will consist of leaching chamber trenches that provide 662 S.F. of
total absorption area. The septic tank effluent will be gravity discharged through a new 4 -inch schedule-40 or SDR-
35, installed to the STU via a distribution box. The effluent will be equally distributed from the distribution box via
2” distribution pipes connected to the head end cap of each of 6 chamber trenches. 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). The designed system meets all required setbacks and will be installed within the general location
indicated on the plan. The existing well serving the residence, (Permit No. 39336-F) is located over 200 feet
South Southeast of the proposed field area. Our design is outlined below and delineated on the attached C-1
OWTS plan.
Design Flow
The design flow is calculated as follows for the total current and anticipated future waste flows of the existing
single family dwelling. The replacement OWTS is designed for a treatment capacity of 525 gallons per day based
on the design wastewater flow to serve a 5-bedroom capacity residence in accordance with Table 6-2 values,
Section 43.6, A.4. Regulation 43:
Minimum population based on 2 person per bedroom for the first 3 bedrooms = 6 persons and 1 person per
bedroom for each additional 1 bedroom = 1 persons for a total of 7 persons. 75 gal/person/day, No PF
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qd = 7*75=525 gpd
Septic Tank Design
The septic tank capacity required for a minimum 48 hour retention time is calculated as follows:
V = 525 gal/day * 2 = 1,05 0 gallons. Use the existing 1,250 gallon, two- compartment concrete tank and a new
500 gallon 1-compartment reinforced plastic dosing tank installed in series. All tanks to be installed and or
retrofitted with sealed ribbed PVC risers and fiber glass lids set to the finish grade surface.
Sub Surface Conditions and Observation
A site specific subsurface soil evaluation was performed by Sopris Engineering, March 23, 2022, in the vicinity of
the proposed OWTS field. Two profile pits were excavated, by backhoe, to a depth of 7-8 feet in the vicinity of
the proposed soil treatment area (STA). The soils encountered in open pit excavations were sampled and
characterized by application of the USDA soil texture method test. The soils were tested to determine the
presence of any limiting soil or hydrological conditions, assess appropriate hydraulic loading rate, range of
absorptive surface levels and to assess appropriate treatment level design alternatives for installation of
treatment/absorption systems the in-situ native soils. The soils below 12 inches of topsoil consist of medium dense
silt loam from 1.0 to 7 feet below the surface.
The native soils from 2 to 3 feet below the surface are characterized as a soil type 2 consisting of sandy silt loam
texture of granular with rock content less than 35%. The tactile soil test ribbons were approximately 1 inch long.
the soils below 4 feet contained increased fine sand and were denser with slightly granular to massive weak
structure to depths of 7 -8 feet. The type 2 soil has an effective loading rate for conventional soil treatment of 0.5 to
0.6 Gal/S.F./day for a level 1 conventional treatment system. The equivalent percolation rate is 26-40 minutes per
inch. No free water or other limiting conditions were encountered or observed in the profile pit excavations on site.
Groundwater levels are expected to be below 12 feet from the existing surface grades.
Huber Residence
OWTS 524 CR 259
SE Job No. 301260.01
March 31, 2022
Page3
Treatment-Soil Treatment Unit/ Absorption System Design
The 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 dosed
effluent via a dosing siphon (Fluid Dynamic 200 Series, Model 207 ) to a distribution box to equally disperse
approximately 95 gallons per cycle to the six gravelless chamber trenches. The new gravity dosed volumetric
distribution of septic tank effluent to the individual chamber trenches will insure treatment across the native soil
absorptive surface and adequate recovery time between dosing cycles. Based on design flow the system will be
dosed 5 times per day. Realistically the system will be dosed 2-3 times per day.
The shallow STU trenches shall be installed with a maximum depth to the absorptive surface of 36 inches from
finish surface grades and backfilled with approximately 3-inches of final grading topsoil mounded over the trench
rows placed to allow positive drainage away across the surface. Chambers shall be installed level in each individual
trench overlying the native soil. Each trench will be constructed level along the contours with successive trenches
installed following natural contour grades accordingly. A minimum separation distance of 4 feet will be
maintained between the individual trench sidewalls.
The engineer, prior to installation shall inspect the trench excavation to confirm soil conditions and installation
procedures. All septic system components and trench installations are to be approved by the Engineer prior to
backfilling.
Field Sizing:
Long Term Acceptance Rate (LTAR)
Considering the application of the state regulation 43:
Receiving Wastewater Treatment Level 1;
Soil type 2A loading rate for silt loam with granular to weak massive structure at depth is (0.5 gal./S.F./day).
Loading factors; (Table 10-2, Dosed Siphon trenches = 0.9) and
(Table 10-3, gravelless chambers = 0.7 )
A(sf) = Qd x L.F.1 x L.F.2 .: A = Area;
LTAR LTAR = 0.5 Gal/S.F./day for silt loam
Qd = flow (gal/day) = 525 gpd
L.F.1=0.9Dosed Siphon Distribution
L.F.2=0.7 Chambers
A(sf) = 525 gpd x 0.9 x 0.7 = 661.5 S.F.
0.5 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
661.5 S.F = 56 chambers; Use 56 chambers
12 S.F./Chamber
Number of Equivalent Arc chambers 5'x3'x12"chambers:
661.5 S.F. = 44.4 chambers; Use 42 chambers
15 S.F./Chamber
STU Layout
Use 56 Quick-4 Infiltrator chambers in 4 trenches 34' long by 3' wide with 9 chambers units and 2 trenches 38'
long by 3' wide with 10 chambers units per trench plus two end caps as delineated on the plan.