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HomeMy WebLinkAboutOWTS Design 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. Install 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