HomeMy WebLinkAbout19. OWTS Report 12.09.2020December 9, 2020
Jack Albright
RFLANDCO LLC
402 Park Avenue, Unit A
Basalt Co 81621
"Jack Albright" <jack@albright-associates.com>
RE: Design Report, On -site Wastewater Treatment System (OWTS), 17352 Highway 82 Professional
Office, 17352 82 Hwy, Carbondale, CO , Section: 32 Township: 7 Range: 87 A Tr In Lot 18
SE Job No. 30201, Parcel No: 2391-321-00-043.
Dear Jack:
This letter report presents the applicable findings in regard to the design and construction of an Onsite
Wastewater Treatment System (OWTS) at the above referenced Site, in support of your building permit
application to Garfield County. Research of available information and County Records did not reveal any
permit, construction or other documentation regarding an existing ISDS that currently serves the existing
dwelling. It is assumed that any existing components discovered during construction will be appropriately
abandoned in place or removed as needed. 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 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 Garfield 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 building permit approval. The
design drawing is based on a recent Improvement Location Certificat (ILC), by Tuttle Survey Services,
dated 7-8-20. The design and construction recommendations with specifications are delineated on the civil
design plans, C2-OWTS Plan dated 12-09-2020, by Sopris Engineering.
OWTS Design Criteria and Sizing
Based on our findings the design and installation of an engineered OWTS with enhanced treatment is feasible in
accordance with applicable County and the State of Colorado Regulation 43. The existing residential structure has 4
bedrooms and detached garage structure that can be considered for use as a 1-bedroom ADU. In order to ensure
adequate OWTS capacity for existing usage and potential future usage the new replacement system is designed for
for the maximum design flow under anticipated re -development scenarios. The residential facility design assumes a
4-bedroom single family residence plus a 1-bedroom detached ADU. The system is designed based on the current
County design criteria and the design criteria approved for the use of a pressure dosed sand filter (SF) system. The
design is in compliance with the current County Regulation No. 43 with area calculations based on the LTAR
loading rate for ASTM-33 filter sand per a minimum equivalent 4-bedroom residential design plus a 1 bedroom
ADU residential use flow capacity.
Design recommendations include the installation of a three -compartment 1,500 gallon concrete septic tank that will
be equipped with an Orenco Systems simplex high head pump in a Biotube vault installed in the third 500-gallon
volume chamber with associated piping and controls. The total tank volume is 2,000 gallons.
502 Main Street • Suite A3 ❑ Carbondale, CO 81623 ❑ (970) 704-0311 ❑ Fax (970) 704-0313
SOPRIS ENGINEERING • LLC
civil consultants
OWTS Design and Construction
RFLANDCO LLC
SE Job No. 13201.01
December 9, 2020
Page 2
We recommend the use of a full pressure dosed Sand Filter (SF) system to provide good secondary treatment
performance in the relatively rocky soils. The SF field will consist of a single bed installed to provide for equal
pressure distribution of effluent across a 840 S.F. treatment/absorption SF field with 24-inches of filter sand below
gravelless chamber spray shield units. The effluent will be dosed across the three distribution rows via an
individual custom perforated pressure distribution pipe running the length of the 70 feet by 12 feet sand filter bed.
Design Calculations OWTS
Design Flow Criteria: Minimum population based on 2 person per bedroom for the first 3 bedrooms = 6 persons and
1 person per bedroom for each additional bedrooms = 4 bedroom residence for a total of 7 persons. 75
gal/person/day,
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qdl Main house Residential: 7 persons @ 75 gpd = 7 x 75 = 525 gpd
Design flow Qd2 ADU a Residential: 2 persons @ 75 gpd = 2 x 75 = 150 gpd
Design flow Qd = Qdl + Qd2 = 525 + 150 = 675 gpd
Septic Tank Design: 2-Day detention time = 675 x 2 = 1350 gallons required.
Use a 1,500 gallon, three -compartment septic tank with a bio-tube dosing assembly in the 500-gallon third chamber.
The total tank volume is 2,000 gallons.
Sub Surface Conditions and Testing
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering on
November 27, 2020. The soils in the area of the proposed absorption field 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, appropriate hydraulic loading rate and to assess appropriate treatment level design
for the in -situ native soils. Currently onsite construction activities, associated with the remodel of the existing
structure, include foundation construction that provided the opportunity to observe, test and evaluate the onsite soils
in the open excavations. The soils below nil to 3 inches of topsoil consist of medium dense gravelly sandy loam to
sandy silt loam with cobbles material to 0.3 to 5 feet below the surface. The soils become increasingly rocky with
depth and appear to be consistent across the site.
The native soils sampled from 1-2 feet below the surface are characterized as a soil type 2 soil consisting of sandy
gravelly silt loam texture with moderate 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. Soils below 2 feet are increasingly
rocky and are characterized as Type R1 soils with rock content greater than 35% and less than 65% with 50% of the
rock content size less than 3/4 inch. The rocky soils at depth with 24 inches of filter sand replacement media have
an effective loading rate of of 0.8 Gal/S.F./day for a level-3 treatment system. Due to limited area and to provide
optimal treatment an intermittent sand filter is recommended to provide for a treatment level TL3 sand filter system.
No free water was encountered in excavations observed on site. Normal Groundwater levels are expected to be
below 10 feet from the existing surface grades. The soils are suitable for the installation of a sand filter treatment
bed system consisting of a minimum 24" depth of secondary sand Media that will be fully pressure dosed meeting
the requirements under sections (43.10.C,D,E,F & H and 43.11.C.). Additional soil evaluation will be performed
during construction to verify the soils in the entire field excavation.
OWTS Design and Construction
RFLANDCO LLC
SE Job No. 13201.01
December 9, 2020
Page 3
Intermittent Sand Filter Treatment Unit/ Absorption System Design
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 24-inch layer of filter sand (ASTM-33, "washed ASTM-33 Concrete sand").
Long Term Acceptance Rate (LTAR)
Considering the application of the new state regulation 43:
Receiving septic tank effluent, the treatment level for intermittent sand filters is considered TL3 for the minimum
24" depth of treatment media with the upper infiltrative surface being at least 4-feet above a limiting layer;
Maximum hydraulic loading rate for TL 1 effluent applied to "Secondary Sand Media" in an unlined sand filter is
0.8 gal./sq.ft./day, or the long term acceptance rate of the receiving soil Type 2 R-1 for TL3 (1.1 gal./S.F./day) The
maximum Loading rate is therefore 0.8 gal./sq.ft./day
Loading factors; (Table 10-2, pressure dosed application bed = 1.0 )
A(sf) = Qd x LF 1 = A = Area;
LTAR
A(sf) = 675 gpd x 1.0 = 840 S.F.
0.8 Gal/S.F./day
LTAR = 0.8 Gal/S.F./day for secondary sand, ASTM-33, " Concrete sand"
Qd = flow (gal/day) = 675gpd
LF1=1.0 pressure dosing
Excavate a single 70' by 12' bed to a maximum 3.5' depth below finish grade. Backfill with 24-inches of ASTM-33,
concrete sand. Insta11 3 rows of custom perforated 1.25-inch distribution laterals on top of sand layer in the bed and
cover with 16 Quick-4 chambers per row for use as pressure dose spray shields, as delineated on the plan.
The standard Quick 4 chamber spray shields shall be installed over the top of the sand on 4 foot centers and
backfilled with a minimum 12-inch depth of cover. Filter fabric shall be placed over the chambers and the entire
extents of the sand filter bed. The slightly mounded field shall be constructed with final surface grading at a
minimum 6-inches above the up gradient surface grades with a minimum 3-inch topsoil layer placed over the field
surface.
The chamber rows shall be completed with standard end caps installed at both ends. 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.
Effluent Distribution System
The OWTS field shall be fully pressure dosed along the entire length of the distribution rows. An Orenco Systems
Bio Tube pump vault system will be installed with an effluent screen in the second compartment of the
septic/dosing tank. The pump system shall incorporate a single high head pump with a 1.5" discharge assembly. The
simplex pump assembly will demand dose effluent through a single 1.25-inch diameter PVC transport pipe from the
pump assembly and connect to manifold pipe that connects to the three custom perforated pipes at the head of the
bed. The 1.25" discharge transport pipe shall be installed with a minimum 2.5% continuous slope to allow for
drainage of the pipe.
OWTS Design and Construction
RFLANDCO LLC
SE Job No. 13201.01
December 9, 2020
Page 4
The effluent will be equally pressure dosed through each 1.25-inch custom perforated distribution lateral running
the length of the treatment bed installed on top of the sand filter layer. The pipe will have 5/32-inch orifices drilled
every 3.0 feet oriented upward to spray against the interior chamber unit that will broadcast the effluent onto the
sand surface. The first and last orifice will be drilled and oriented downward with an orifice shield to allow
drainage of the line. Individual 1.25" ball valves will be installed at the end of each perforated distribution lateral
centered below one of the pair of end cap vertical inspection port penetrations at the terminal end of the field. Two
vertical 4-inch inspection ports will be installed from the pair of end cap inspection port penetrations at the terminal
end of the field and one 4-inch inspection port will be installed from either end cap penetration at the head of the
field. All inspection ports shall extend a minimum 6-inches above the finish grade and be topped with a treaded cap.
Pump operations shall be controlled by a remote pedestal or exterior wall mounted control panel in direct line of
site with the tank. Automatic float switches for off, redundant off, on and high water with alarm activation shall be
installed on a dedicated PVC float tree. Two separate circuits from the electrical circuit panel and/or junction box
shall be routed to the control panel for separate power to the controls and pump.
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 -built 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.
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 property Owner. The owner shall
retain the services of qualified personnel to inspect the OWTS and to perform all maintenance and repairs necessary
OWTS Design and Construction
RFLANDCO LLC
SE Job No. 13201.01
December 9, 2020
Page 5
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 OWTS 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
Paul E. Rutledge
Design Engineer
Yancy Nichol, PE
Principal