HomeMy WebLinkAboutEngineer's OWTS Design Report 04.17.2020April 17, 2020
Dylan Mace
88 Cedar,
Carbondale, CO 81623
"dylan mace" <dylawnmace@gmail.com>
RE: Design Report, On-site Wastewater Treatment System (OWTS), Mace Residence, 88 Cedar
Street, Lots 15-16, D Blk 9, Cooperton TWNSTE Carbondale, CO
SE Job No. 19219, Parcel No: 2393-284-09-015.
Dear Dylan:
This letter report presents the applicable findings in regard to the design and construction of an Onsite
Wastewater Treatment System (OWTS) and residential site improvements at the above referenced Site, in
support of your building permit application to Garfield County. The proposed improvements include
minor grading, potable water service, OWTS, electric and other utilities to serve the proposed residence
shown on the Site Plan prepared by SE in coordination with the property owner.
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 proposed site specific improvements have been designed in
accordance with County Regulations and general engineering guidelines regarding grading, drainage,
utilities and construction best management practices. Based on the physical site features, soils, and
proposed building layout, we have prepared civil site and OWTS design plans with construction details for
building permit approval. The design and construction recommendations with specifications are delineated on
the civil design plans, C 1 -SITE PLAN and C2-OWTS Details, dated 4-16-2020, by Sopris Engineering.
Existing & Pronosed Site Conditions
The site is located on Cedar Street, Carbondale, in Section: 28 Township: 7 Range: 88 Subdivision: Cooperton
Twnste. Block: 9 Lot: 15 Thru:- Lot: 16 and D, Garfield County, Colorado. The proposed site improvements will
include: a single family home with a detached shop/garage, driveway improvements, and site landscape features.
The site comprises 0.3 acres and is located on Cedar Street in Satank northwest of the Town of Carbondale. The
site is situated on a flat terrain with slope grades of 1% to 3%. The existing driveway access is from Cedar Street
on the East side of the property. The site vegetation consists of grasses, and scattered shrubs. The site has water,
electric, gas and phone services available.
Drainage Erosion control
Best management practices will be utilized to manage onsite drainage. The existing site has vegetated
areas, over very porous terrain. The existing conditions provide for natural storm water treatment and
attenuation with infiltration. The proposed site will be landscaped to improve the existing vegetation
conditions to provide continued attenuation and management of onsite drainage. Appropriate best
management practices will be installed and utilized during construction and maintained until vegetation is
established.
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
SOPRIS ENGINEERING • LLC
civil consultants
OWTS Design and Construction
Mace Residence
SE Job No. 19219.01
April 17, 2020
Page 2
OWTS Desigix 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 residential facility will include a
2 -bedroom dwelling to be served by the new OWTS system. 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 3 -bedroom design flow capacity.
Design recommendations include the installation of a three compartment 1,250 gallon 2 -compartment concrete
septic tank that will be equipped with an Orenco Systems simplex high head pump in a Biotube vault installed in the
secondary chamber with associated piping and controls. 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 450 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 38 feet by 12 feet sand filter bed.
Design Calculations OWTS
Installation of an OWTS with a minimum 3 -bedroom capacity.
From County adopted State Regulation 43:
Minimum population based on 3 bedrooms = 6 persons.
First 3 bedrooms (2 persons/bedroom) Over 3 bedrooms (1 person per bedroom)
Gallons per day per person =75 gal/person/day, No peaking factor
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qd = 6*75=450 gpd
Septic Tank Design: Use a 1250 gallon, two compartment septic tank with a bio -tube dosing assembly in the second
chamber. .
Sub Surface Conditions and Testing
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering on April
6, 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 hydraulic loading rate and to confirm the
appropriate absorptive surface level suitable for an intermittent sand filter treatment system in native soils. The soils
below nil to 3 inches of topsoil consist of medium dense gravelly loamy sand to sandy loam material to 0.3 to 4 feet
below the surface. The soil appears to be consistent across the site.
The native soils sampled from 1-2 feet below the surface are characterized as a soil type 1 consisting of loamy sand
to sandy loam texture with moderate granular structure. Soils below 2 feet are increasingly rocky becoming R-1
soils with more than 30% rock content smaller than 3/4". This soil has an effective loading rate for conventional
soil treatment of 0.8 Gal/S.F./day for a level 1 conventional treatment system. Due to limited area, rock content and
OWTS Design and Construction
Mace Residence
SE Job No. 19219.01
April 17, 2020
Page 3
to provide optimal treatment an intermittent sand filter is recommended. No free water was encountered in
excavations on site. 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 that will be fully pressure dosed and
utilize 24 -inches of washed concrete sand. An additional soil evaluation will be performed during construction to
verify the soils in the proposed field excavation and to confirm that appropriate sand filter media is installed.
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 Wastewater Treatment Level 1; loading rate for sand filter is (0.8 gal./S.F./day) for the minimum 24"
depth of secondary sand treatment media;
Loading factors; (Table 10-2, pressure dosed application bed = 1.0 )
A(sf)= Qd.: A=Area;
LTAR
A(sf) = 450 gpd x 1.0 = 562.5 S.F.
0.8 Gal/S.F./day
LTAR = 1.0 Gal/S.F./day for secondary sand, ASTM -33, " Concrete sand"
Qd = flow (gal/day) = 450 gpd
L.F.1=1.0 pressure dosing
Excavate a single 47' by 12' bed to a maximum 3.5' depth below finish grade. Backfill with 24 -inches of ASTM -33,
concrete sand. Install 3 rows of custom perforated 1.25 -inch distribution lateral on top of sand layer in the bed and
cover with 10 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 near a corner 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 tank.
The pump system shall incorporate a single high head pump. 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
OWTS Design and Construction
Mace Residence
SE Job No. 19219.01
April 17, 2020
Page 4
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.
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 3/16 -inch orifices drilled
every 2.0 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 wall mounted exterior control panel in direct line of site with the tank.
Automatic float switches for on, off 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 Design and Construction
Mace Residence
SE Job No. 19219.01
April 16, 2020
Page 5
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
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
NNN
Yancy Nichol, PE
Principal