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HomeMy WebLinkAboutOWTS ReportJune 17, 2021 502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313 S OPRIS E NGINEERING • LLC civil consultants Seth Kirby & Kasandralynn Horvath 710 Cedar Hills Road Silt CO 81652 "seth horvath" <seth.horvath@yahoo.com> RE: Design Report, On-site Wastewater Treatment System (OWTS), 710 Cedar Hills Road Silt, Garfield County, CO, Cedar Hills Ranch subdivision, Lot: 10 SE Job No. 30243 , Parcel No: 217901108010 . Previous EHD Permit #3879. 9-27-04 Dear Seth: This letter report presents the applicable findings in regard to the design and construction of an upgrade/replacement Onsite Wastewater Treatment System (OWTS) at the above referenced Site, A new OWTS construction permit is required for the proposed OWTS component upgrades, replacement soil treatment area (STA) with respect to current usage on the subject property. The OWTS upgrade design is based on our evaluation of the site conditions for use in obtaining a County OWTS construction permit. We have evaluated the existing and proposed site conditions with respect to the operation and installation of a upgrade/replacement OWTS in accordance with County OWTS Regulations and State of Colorado Regulation 43. The upgraded system meets all required setback requirements and operational demands for the current usage. Based on the physical site features, soils, and existing building layout, we have prepared a civil OWTS design plan with construction details for permit approval that was prepared with GIS, Survey Plat information and aerial mapping. The design criteria and system sizing information is summarized below. T he design and construction recommendations with specifications are delineated on the civil design plans, C1-OWTS Plan dated 07-05-20 22, by Sopris Engineering. OWTS Conditions and Design Criteria Existing OWTS Components: The current OWTS system serves the existing residential dwelling that currently has four bedrooms. The OWTS components consist of gravity service pipe, a 1,25 0-gallon septic tank, that gravity discharges septic tank effluent to 4 chamber trenches in a serial distribution configuration, providing approximately 530 S.F. absorption area consisting of 4 trenches approximately 50’' long by 3' wide with inspection ports installed on the ends and the middle of each trench.. The absorption field was installed in 2004; under County EHD permit no. 3879. The previous design was based on a regulatory design flow of 900 gpd. The existing absorption field was sized based on an average tested percolation rate of 24 minutes per inch that is equivalent to a soil type 2 silt loam. The absorption field sizing was calculated to require 882 S.F. with an applied 40% reduction for 530 S.F. of total absorption area required at the time of permitting. The existing trenches will be abandoned in place or removed as needed to install the new proposed OWTS Geomat beds in the general vicinity. Proposed OWTS System Replacement No increase over current usage for a 4-bedroom residence is proposed. The system will be constructed to provide for demand dosing of septic tank effluent to an engineered enhanced media soil treatment area (STA) pursuant to Regulation 43 design criteria. The OWTS will maintain the existing gravity sewer service pipe from the existing dwelling to the existing 1,250-gallon plastic 2-compartment septic tank. The tank lids risers and lids will be OWTS Upgrade Design and Construction 710 Cedar Hills Road SE Job No. 32159.01 July 12, 2022 Page 2 extended to the finish surface. A new 500 gallon plastic dosing tank will be installed equipped with a Highhead pump assembly to provide full pressure dosing of the replacement field. The upgraded STA system is designed based the current design criteria approved, on February 19, 2020, by CDPHE for the use of a pressure dosed geotextile sand filter (GSF) type system. Specifically the preferred treatment technology is Geomatrix System, LLC “Geomat” system approved by CDPHE for use as a proprietary distribution product for use in Colorado OWTS’s utilizing applicable multiplier loading rates for enhanced manufactured media. . The design is in compliance with area calculations based on the LTAR loading rate for the in-situ soils utilizing Geomat material installed over a 6-inch layer of ASTM-33 filter sand in shallow pressure dosed beds per the estimated design flow capacities The replacement system will include the installation of an Orenco System bio tube simplex effluent pump assembly for pressure distribution of septic tank effluent to a new Geotextile Sand filter (GSF) bed system consisting of Geomat enhanced manufactured media, filter sand and custom perforated distribution laterals. The existing septic tank will be maintained with a new outlet effluent pipe connection to the new dosing tank. The effluent discharge piping from the pump assembly will be installed to connect to an automatic distribution valve that will have individual distribution pipes connecting to a manifold pipes at the head of each Geomat treatment zoned bed each containing a pair of rows with custom perforated distribution laterals. Each paired bed row will have appropriate inspection port cleanouts and appurtenances. Design Calculations OWTS The existing residential structure has 4 bedrooms with a design flow based on residential usage. The upgrade design maintains the equivalent 4-bedroom single family residence design flow. The total peak residential population is 7 persons. From County adopted State Regulation 43,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 = 1 person for a total of 7 persons. 75 gal/person/day, No PF Equivalent 4-bedroom Design capacity = 7 x 2 = 14 persons @ 75 gal/person/day, Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day. Design flow Qd = 7 X 75 = 525 gpd Septic Tank Design: 2-Day detention time = 525 x 2 = 1,050 gallons required. Continue use of the existing 1,250 gallon, two-compartment primary septic tank and install a new 500 gallon, single compartment dosing basin with a bio-tube dosing pump assembly. Sub Surface Conditions and Testing Based on site observations, review of previous permit documentation and geotechnical evaluations the soils below nil to 4" of topsoil consist of sandy silt loam texture soils . A site assessment, OWTS inspection and Subsurface soil investigations were performed by SE on June 23, 2022. A shallow test pit was excavated to characterize the in situ soils along with investigation and sampling of soil conditions below the multiple inspection ports installed on the existing chamber trenches. The soils encountered in the pit and absorptive surface of the existing field consist of silt loam and scattered gravels and small boulders. Previous percolation testing at the time of permit resulted in an average percolation rate of 24 minutes per inch that is consistent with a soil type 2. The previous and recent soils evaluation did not indicate the presence of any limiting soil or hydrological conditions. OWTS Upgrade Design and Construction 710 Cedar Hills Road SE Job No. 32159.01 July 12, 2022 Page 3 This soil type 2 has an effective loading rate for conventional soil treatment of 0.6 gal/S.F./day for a level 1 conventional treatment system. Therefore for design purposes the soil underlying the proposed soil treatment area (STA) will be considered type 2. In order to provide optimal treatment a shallow pressure dosed STA utilizing enhanced manufactured media will be installed to provide for a treatment level TL-1 system. No free water is expected to be encountered within the field excavations. Additional observations and soil evaluation will be performed during construction, by Sopris Engineering in the area of the proposed absorption field and tank to confirm the previous soil evaluation and design criteria. Sopris Engineering will observe new profile pits in the area of the proposed absorption field. The soils will be sampled and characterized by application of the USDA soil texture methods. The soils will be further evaluated to determine the presence of any limiting soil or hydrological conditions, appropriate hydraulic loading rate, absorptive surface level capacity and to verify treatment level design for the in-situ native soils. If soil conditions encountered require adjustments to design criteria or the design plan notice will be made to County EHD and applicable revisions to design/construction plans and report will be submitted. The results of the additional soil evaluation and soil characterization will be included in the As-Constructed documentation. Normal Groundwater levels are expected to be below 10 feet from the existing surface grades. GeoMat™ Treatment Unit/ Absorption System Design The treatment/absorption fields are designed based on utilizing the effective Long Term Acceptance Rate (LTAR) for a geotextile sand filter (GSF) treatment system utilizing pressure dosed distribution laterals. The new treatment systems will consist of pressure dosed GeoMat sand filter treatment beds with 6-inches of washed filter sand (ASTM-C33, "Concrete sand") below 2 rows of 39-inch wide GeoMat model 3900 manufactured treatment/distribution media, in accordance with Regulation 42 sections (43.10(C), 43.13.E.1.d, Table 10-2 and Table 10-3 as approved by CDPHE for this technology. Long Term Acceptance Rate (LTAR) Considering the allowable application of the OWTS regulation 43 and CDPHE approved proprietary Distribution Product for Geomatrix Systems, GeoMat product: Receiving septic tank effluent, the treatment level-1 for GeoMat requires a minimum 6" depth of filter sand med ia with the bottom of the sand layer defined as the infiltrative surface being at least 4-feet above a limiting layer (43.8, Table 7-2.4); Maximum hydraulic loading rate for TL1 effluent is the long term acceptance rate of the receiving soil Type 2 is 0.6gal./S.F./day) (Table 10-1 &10-1A.). A(sf) = Qd x L.F.1 x L.F.2 .: A = Area; LTAR LTAR = 0.6 gal/S.F./day for silt loam Qd = flow (gal/day) for OWTS = 525gpd L.F.1=1.0 pressure dosed bed application L.F.2=0.7 Enhanced Manufactured Media OWTS: A(sf) = 525 gpd x 1.0 x 0.7 = 613 S.F. 0.6 Gal/S.F./day Excavate each bed to a minimum 20” depth below finish grade. Scarify surface, remove any unsuitable materials and backfill with 6-inches of ASTM-C33, "Concrete sand". Install the specified length of GeoMat rows with the specified custom perforated distribution laterals and manufacturer supplied orifice shields below the specified orifices on top of the GeoMat material. The distribution laterals to be installed per manufacturer’s recommendations below the top geofabric cover. Backfill the completed geo mat beds with suitable excavated OWTS Upgrade Design and Construction 710 Cedar Hills Road SE Job No. 32159.01 July 12, 2022 Page 4 materials to finish top of bed grade slightly mounded approximately 3 to 6 inches above the surrounding surface grades. A piezometer monitoring well pipe riser with threaded cap will be installed at the southwest corner of each field and be extended down to the excavated surface depth at the bottom of the placed sand. The treatment field will consist of three 32 feet long by 6.5 feet wide pressure dosed Geomat sand filter beds each with of 6208 square feet of absorption treatment area. The 1624 S.F. treatment field will be installed with 3 zones that will be alternately pressured dosed by use of an automatic distribution valve (ADV). Each zone will be constructed with 6-inches of washed filter sand below 2 rows of 39 -inch wide GeoMat model 3900 manufactured treatment/distribution media. An ADV assembly will be installed at a high point between the septic/dosing tank and the field provide for equal pressure distribution of effluent across each treatment/absorption bed zone. The septic tank effluent will be dosed across each zone via 2 individual 1.5” custom perforated pressure distribution pipes running the length of each bed zone centered on each 39” GeoMat material row. Effluent Distribution System Filtered effluent will discharge to the Geomat field laterals via a high head pump biotube discharge assembly to an automatic distribution valve between the septic/dosing tank and the head of the STU beds. The distribution system will be installed to allow for effluent to be alternately and equally distributed across the absorption surface in each pair of bed rows via individual custom perforated full pressure distribution laterals connected to the manifold pipe at the head of each pair of geo mat rows installed on the enhanced manufactured media surface along the entire length of the individual beds. A ball valve will be installed on the terminal end of each distribution lateral centered below and housed in a 10" valve box with lid set 2" above surface grades. The minimum slope of the transport pipe from the tank's dosing compartment to the ADV shall be installed at a minimum 2.5% upward slope to allow for drain back to the tank. The individual distribution pipes from the ADV shall be installed at a minimum 1% downward slope and will connect to the distribution lateral manifold at the head of each paired Geomat row. 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 dosing tank. All 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-Constructed drawing.