HomeMy WebLinkAboutOWTS Installation Observations 03.08.2021March 8, 2021
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311
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S OPRIS E NGINEERING • LLC civil consultants
Craig Wilcox
0188 County Rd 226
Rifle Co 81650
RE: As-Constructed–Report–Onsite Wastewater Treatment System (OWTS), Wilcox Residence, 0188 C.R. 211,
A Parcel in Sections 9 & 11 T5S R92w 6th PM, M/B, Garfield County, Colorado
SE Job No. 30211.01 Parcel No: 2727-091-00-018, Previous Garfield County ISDS Permits: 4419
Current Garfield County OWTS Permit: SEPT-6617, Issued 12/02/2020
Dear Craig:
Pursuant to County requirements, this letter provides documentation that the Onsite Wastewater Treatment System
(OWTS) upgraded components recently installed at the above referenced site is in compliance with the permitted
design. Sopris Engineering inspected the excavations, soils, materials and the installation of the system components
during construction, prior to final backfill and after all installations were completed. The replacement absorption
field and tank installations were designed and constructed to serve the existing 4-bedroom residential property.
Sopris Engineering performed site visits to inspect and document the as-constructed conditions of the OWTS.
Inspections were coordinated with the property owner and the Contractor, Kurtis Geenman, KEG enterprises, that
installed the upgraded system components. The as-constructed conditions and installation of the upgraded and new
OWTS components is in compliance with Garfield County Regulations; the recommendations and specifications
delineated on the civil C1 OWTS plan drawing dated 11-030-2020 and the OWTS design Report dated 11-30-2020
by Sopris Engineering. The design criteria and system sizing information is summarized below. The As-
Constructed OWTS components were completed as delineated on the design construction document plan previously
submitted for permit.
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. The replacement
OWTS utilizes the existing 1,000 gallon single compartment concrete septic tank and the new 1,000 gallon 2-
compartment reinforced plastic septic tank installed in series adjacent to the existing tank within the lawn area east
of the dwelling structure. New pipe connections from the existing tank to the new septic/dosing tank were installed
with the required the sanitary tees. An existing concrete riser on the outlet access of the septic tank was replaced
and the assembly reset and sealed water tight. The new 1,000 gallon 2-compartment septic/dosing tank was installed
with sealed PVC risers with fiber lids at the surface, inlet sanitary tee, an Orenco biotube high head effluent pump
assembly in the second compartment. The existing gravity sewer pipe was maintained from the residential
structures to the inlet of the existing tank.
A 1.5-inch schedule-40 effluent transport pipe from the discharge assembly in the 2nd compartment of the new
septic/dosing tank was installed to an automatic distribution valve (ADV) adjacent to the new replacement STU
absorption field with individual 1.5-inch schedule-40 distribution pipes installed to the head of each new chamber
trench.
The new replacement soil treatment unit (STU) consists of leaching chamber trenches that provide 840 S.F. of total
absorption area installed at shallow depths not exceeding 36” below existing surface grades. The septic tank
effluent is pressure discharged through the ADV for alternative dosing of 4 full pressure dosed trenches. The
Wilcox Residence
OWTS, 0188 C.R. 226
SE Job No. 30211.01
March 8, 2021
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effluent is equally distributed along the length of each trench via a custom perforated 1.5-inch schedule-40
distribution lateral installed on the absorptive surface from the head to the terminal end cap of each trench.
Observation ports with caps are installed on each end chamber unit. Each terminal end of the distribution lateral
was capped with a 1.5-inch ball valve centered under the inspection port. 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). All installations were performed during dry weather. The designed system
meets all required setbacks and was installed within the general location indicated on the plan. The existing well
serving the residence is located over 100 feet Northwest of the proposed field area. Our design is outlined below
and delineated on the attached C-1 As-Constructed OWTS plan.
Design Flow
The design flow is calculated as follows for the total current and design 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 4-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 bedroom = 1 persons for a total of 7 persons at 75 gal/person/day.
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 = 1050 gallons. Used the existing 1,000 gallon, single compartment concrete tank and a new
1,000 gallon 2-compartment reinforced plastic septic tank installed in series for a total tank capacity of 2,000
gallons. The new septic/dosing tank is equipped with a biotube effluent pump vault.
Sub Surface Conditions and Observation
A subsurface soil investigation and site assessment was performed by Sopris Engineering on November 19, 2020.
The soils from two profile pits, in the vicinity of the proposed STU field envelope area were sampled and
characterized by application of the USDA visual/tactile soil texture method analysis. The soils below 12 inches of
topsoil consist of medium dense clay loam soil with less than 2% scattered gravel or rock content to7 feet below the
surface grades. The soils within the excavated trenches were observed during construction and appeared to be
consistent with the soil conditions previously observed in the excavated profile pits.
The native soils sampled from 2-3 feet below the surface are characterized as a soil type3 consisting of clay loam
texture with slightly blocky granular structure. This soil has an effective loading rate for conventional soil treatment
of 0.35 Gal/S.F./day for a level 1 STU system. The equivalent percolation rate is assumed to be 40-60 minutes per
inch. No free water was encountered in excavations on site. Seasonal high Groundwater levels are expected to be
below 10 feet from the existing surface grades. The soils are suitable for a soil treatment area (STA).
Treatment-Soil Treatment Unit/ Absorption System Design
The treatment/absorption field was 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
pressure distribution to full pressure dosed gravelless chamber trenches. The new treatment system and absorption
field consist of pressure dosed distribution of septic tank effluent from a biotube high head pump assembly vault to
an automatic disturbing valve with individual distribution pipes routed to each chamber trench installed with custom
Wilcox Residence
OWTS, 0188 C.R. 226
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March 8, 2021
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perforated distribution laterals. Each of the four trenches are alternately dosed with equal distribution across the
native soil absorptive surface.
The shallow STU trenches were installed with approximately 12-inches of final topsoil backfill over the trenches
that will be further graded out in the spring and revegetated to maintain a minimum 6” mound arcos the field that
will provide for positive drainage away from the surface. A minimum separation distance of 4 feet was maintained
between the individual trench sidewalls. All septic system components and trench installations were approved by
the Engineer prior to backfilling.
Field Sizing:
Based onLong Term Acceptance Rate (LTAR) for Receiving Wastewater Treatment Level 1;
Soil type 3 loading rate for clay loam with blocky to granular structure is (0.35 gal./S.F./day) utilizing applicable
loading factors for full pressure dosed gravelless chamber trenches.
A(sf) = Qd x L.F.1 x L.F.2 .: A = Area;
LTAR LTAR = 0.35 Gal/S.F./day for clay loam
Qd = flow (gal/day) = 525 gpd
L.F.1=0.8 (Table 10-2, Pressure dosed trenches distribution)
L.F.2=0.7 (Table 10-3, gravelless chambers Category 3)
A(sf) = 525 gpd x 0.8 x 0.7 = 840 S.F. absorption treatment area required
0.35 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
840 S.F = 70 chambers; Use 72 chambers
12 S.F./Chamber
STU Layout
A total of 72 Quick-4 Infiltrator chambers in four trenches 74' long by 3' wide with 18 chamber units per trench
plus two end caps were installed as delineated on the plan. The septic tank effluent is pressure distributed to the
head of each chamber trench via individual distribution pipes from the installed automatic distribution valve housed
in a 30” fiberglass riser installed between the tanks and the center of the field.
Effluent Distribution System
The OWTS field is fully pressure dosed along the entire length of each trench. An Orenco Systems Bio Tube pump
vault system was installed with an effluent screen in the second compartment of the septic/dosing tank. The simplex
pump assembly provides demand dosing of septic tank effluent through a single 1.5-inch diameter PVC transport
pipe from the pump assembly connected to a four outlet ADU (Model 6404). The ADU was installed with the inlet
pipe at a high point relative to the discharge assembly outlet and the individual distribution pipe connections at the
head of each trench allowing for drain back of the pipes to the pump vault and chambers respectively.
The effluent is alternately equally pressure dosed through each 1.5-inch custom perforated distribution lateral
running the length of the trench installed on the absorptive surface centered below the chambers. The distribution
laterals have 5/32-inch orifices drilled every 2.0 feet oriented upward to spray against the interior chamber units.
The first and last orifice was drilled and oriented downward with an orifice shield to allow drainage of the line.
Individual 1.5-inch ball valves were installed at the end of each perforated distribution lateral centered below
vertical inspection ports at the terminal end of each trench. A 4-inch inspection port was installed from the end caps
at the head of each trench. All inspection ports extend a minimum 3-inches above the finish grade with a cap.