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HomeMy WebLinkAboutOWTS Engineer's DesignSeptember 14, 2023 502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313 S OPRIS E NGINEERING • LLC civil consultants Caleb Waller 361 311 County Road Silt CO 81652 "Caleb Waller" <rivernestllc@gmail.com> RE: Design Report, On-site Wastewater Treatment System (OWTS), 361 County Road 311 Silt, Garfield County, CO, Section: 10, Township: 6, Range: 92 A TR IN LOT 9 OF SEC 10. SE Job No. 32128, Parcel No: 217910400516. Previous ISDS Permit #SEPT-11-13-3052. Dear Caleb: This letter report presents the applicable findings in regard to the design for a repair/upgrade to the existing Onsite Wastewater Treatment System (OWTS) at the above referenced Site, based on our evaluation of the site conditions. An OWTS repair/construction permit is required for the proposed OWTS component upgrades with respect to current and future usage on the subject property. We have evaluated the existing and proposed site conditions with respect to the operation and installation of an upgrade d 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, previous design drawings and aerial mapping. The design criteria and system sizing information is summarized below. The design and construction recommendations with specifications are delineated on the civil design plans, C1-OWTS Plan dated 09-12-2023, by Sopris Engineering. OWTS Conditions and Design Criteria Existing OWTS Components: The current OWTS system serves an existing plumbed structure that currently has a one -bedroom studio apartment, a RV space with sewer hookup. The OWTS components consist of gravity service pipe, two 1,000-gallon septic tanks in series with appurtenances for a dosing pump in the second chamber of the second tank installed in series. The installed OWTS components include a PVC pressure discharge transport pipe connected to a manifold pipe installed at the head of the bed to deliver effluent to a mounded sand filter bed consisting of 3 custom perforated distribution laterals installed with infiltrator chamber. The existing 26.5’ long by 12' wide sand filter bed provides approximately 318 S.F. absorption area and contains a minimum two foot depth of select filter media. The absorption field was installed in 2013; under County ISDS permit no. 3052. The previous design was based on a regulatory average design flow of 900 gpd. The existing absorption field was sized based on an average tested percolation rate of 20 minutes per inch based on typical infiltration capacity of select filter media and the underlying receiving basal area soils that is consistent with a soil type 2 silt loam. The absorption field sizing was calculated, under the former peaking factor flow and percolation rate equation design criteria, requiring 1690 S.F. with an applied 30% reduction for 1183 S.F. of total absorption that was acceptable and typical design criteria at the time of permitting. It is assumed that the smaller existing field was constructed to only serve the building usage at the time of OWTS Upgrade Design and Construction 361 County Road 311 SE Job No. 32128.01 September 14, 2023 Page 2 construction. The repair/upgrade design specifies an expansion of the existing bed to provide for a maximum 450 gallon per day capacity based on current regulatory design criteria. An additional matching field may be considered in the future to expand the capacity to 900 gpd. Proposed OWTS System Replacement The existing mounded treatment bed will be expanded to increase design capacity to 450 gpd in order to serve the current usage from an existing 1-bedroom apartment and single RV space and providing additional capacity to manage design flow for up to four no direct hook up campsites and 1 additional full bath. The system will be constructed to provide for demand dosing of septic tank effluent pursuant to Regulation 43 design criteria for Treatment Level-3 sand filters. The bed sizing is in compliance with area calculations based on the design flow and LTAR loading rate for a 24-inch layer of ASTM-33 secondary filter sand with respect to the underlying infiltrative capacity of the base and in-situ soil materials. The OWTS will maintain the existing gravity sewer service pipe from the existing dwelling/garage structure to the two existing 1,000-gallon concrete 2-compartment septic tanks. The tank lids risers and lids will be extended to the finish surface. A dosing pump assembly will be installed in the second tank’s second compartment to provide full pressure dosing of the expanded/refurbished treatment field. The upgraded system will include the installation of an Orenco System bio tube simplex effluent pump assembly for pressure distribution of septic tank effluent to the expanded sand filter bed system consisting of infiltrator chambers used as spray shields, filter sand and custom perforated distribution laterals. The effluent discharge piping from the pump assembly will connect to a common manifold pipe with connections to each custom perforated distribution lateral. Each distribution lateral will have appropriate inspection ports and appurtenances. Design Calculations OWTS The existing plumbed structure has a 1 bedroom apartment and a single RV space with sewer hookup. The facility will likely include the addition of 1 additional full bath to serve guest occupying up to 4 campsites without water/sewer hookups. Design capacity is equivalent to 3 bedroom residential usage. Design Flow Criteria: Minimum population based on 2 people per 1 bedroom studio apartment, 1 existing RV site with hookup and number of campsites without water or sewer hookups. 1 Bedroom apartment = 2 persons @ 75 gal/person/day, RV space with sewer hookup = 1 @ 100 gal/space /day 4 campsites @ 50 gal/campsite/day Alternative: 1 additional 1-bedroom studio apartment and 1 campsite without sewer or water hook up. Design flow Qd = (2 x 75)+ (1 x 100) + (4 x 50) = 450 gpd Septic Tank Design: 2-Day detention time = 450 x 2 = 900 gallons required. There are two existing 1000 gallon concrete Copeland tanks in series. The sec ond chamber of the second tank will serve as a dosing basin to be equipped with a bio-tube simplex high head pump dosing assembly. OWTS Upgrade Design and Construction 361 County Road 311 SE Job No. 32128.01 September 14, 2023 Page 3 Sub Surface Conditions and Testing The permitted Design was for a typical pressure dosed mound system with top of the specified filter media and base fill installed above the limiting layer that is the base floodplain elevation. Previous design criteria and soil infiltration rates, established by Gamba and Associates in 2015, were based equivalent percolation rates for specified imported sand and gravel base materials to be placed over the native shallow silt loam overlying typical alluvial soils. The soils in the area of the proposed absorption field have very shallow topsoil underlain by alluvium soils in a shallow silt loam matrix. The imported gravel pit based materials specified to to create the mound foundation are assumed to have an equivalent percolation rate of 20 mpi for basal area design purposes. Gravelly sand and sand filter materials typically have equivalent percolation rate of 5 to 15 mpi. This previous design criteria is consistent with typical mound systems placed over alluvial floodplains. Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering in July of 2022. 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 below nil to 3 inches of topsoil consist of medium dense gravelly silt loam material to 1 to 2 feet below the surface. The soils become increasingly rocky with depth. The soil appears to be consistent across the designated field envelope. The native soils observed 1.5 feet below the surface are characterized as a soil type 2 soil consisting of sandy gravelly silt loam texture with fine 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 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 have an effective loading rate of 0.8 Gal/S.F./day for a treatment level-1 system. To be consistent with the previous design criteria with respect to limiting layers the expansion and refurbishment of the existing an intermittent sand filter is recommended to provide for a treatment level TL3 mounded sand filter system. No free water was encountered in shallow excavation on site at the time of exploration. Normal Groundwater levels are expected to be below 3-4 feet below existing surface grades, seasonal high groundwater levels are expected to be 2 foot below existing surface grades and the 100-year flood plain base flood elevation has previously been determined to at elevation 5425+/- approximately 1 foot below the existing surface grade. The underlying soils are suitable for the installation of the sand filter treatment bed system consisting of a minimum 24" depth of secondary sand Media placed at over class -3 base material overlying the basal are consisting of R1 with Type 2 silt loam matrix soils. Additional observations and soil evaluation will be performed, by Sopris Engineering, during construction prior to placement of the base course class-3 and enhanced sand filter media materials in the area of the proposed mounded treatment/absorption field to confirm native soil interface and installation of select fill material. If any adjustments to design or installation recommenda tions are necessary notice will be made to County EHD and applicable revisions to design/construction plans and report will be submitted. All conditions and installations will be included in the As-Constructed documentation. OWTS Upgrade Design and Construction 361 County Road 311 SE Job No. 32128.01 September 14, 2023 Page 4 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 bed system utilizing pressure dosed distribution laterals and gravelless chamber units as spray shields installed over a 24-inch layer of filter sand (ASTM-33, "washed ASTM-33 Concrete sand"). Long Term Acceptance Rate (LTAR) Considering the application of regulation 43 design criteria of unlined sand filters receiving septic tank effluent, pressure dosed sand filter results in the applied effluent to be treated to Wastewater Treatment Level 3 quality prior to infiltration into native soils: The design basal area for a treatment level 3 STA is sized per the maximum LTAR of (1.1 gal./S.F./day) for type 2 receiving soils or the maximum LTAR of 0.8 gal/S.F./day for the minimum 24" depth of secondary sand treatment media whichever results in the larger area; The maximum Loading rate for this design is therefore 0.8 gal./sq.ft./day The STA is designed utilizing a loading factor of 1: (Table 10-2, pressure dosed application bed = 1.0) A(sf) = Qd x L.F. A = Area; LTAR LTAR = 0.8 Gal/S.F./day for secondary sand, ASTM-33, " Concrete sand" A(sf) = 450 gpd x 1.0 = 562.5 S.F. Qd = flow (gal/day) = 450 gpd 0.8 Gal/S.F./day L.F.1= 1.0 pressure dosed bed Existing A(sf) = 26.5*12 = 318 S.F. 562.5 – 318 = 244.5 S.F. additional filter area required. Treatment Bed Design The single 26.5' by 12' mounded sand filter bed with a minimum 24-inch depth of ASTM-33, concrete sand will be extended to 47' of length over shallow native soils and imported Class-3 pit run material placed over native soil to the specified depth at the bottom of sand. The 12-feet width of sand media will be contained on the mounded base material with extended sideslopes by placement of an embankment consisting of native soil and/or imported Class-3 pit run material placed over the native soil along the down gradient perimeter of the bed to the specified height of 12-inches above the top of sand level. Install 3 rows of custom perforated 1.5 inch distribution laterals on top of sand layer and cover with 8 Quick-4 chambers per row for use as pressure dose spray shields, as delineated on the plan. Cover chambers and top of sand with filter fabric and backfill entire bed with native soil, free of rocks, 8 to 12 inches above the top of chambers. The top of the sand filter absorptive surface to be set at a minimum 2.5 feet above the existing surface grades and a minimum 3-feet above the limiting BFE. OWTS Upgrade Design and Construction 361 County Road 311 SE Job No. 32128.01 September 14, 2023 Page 5 Effluent Distribution System Filtered effluent will discharge to the trench laterals via a 1.5 inch diameter transport pipe from the high head pump biotube discharge assembly to distribution piping manifold between the septic/dosing tank and the head of the sand filter bed distribution lateral rows. The distribution system will be installed to allow for effluent to be equally distributed across the absorption surface via individual custom perforated full pressure distribution laterals connected to the distribution manifold at the head of each perforated pipe installed on the native soil surface along the entire length of the bed. The transport pipe tee fitting connection on the manifold pipe to be set at a highpoint invert approximately 2” above the invert of the manifold pipe to insure drainage into the distribution lateral and back to the tank. The discharge assembly will have an anti-siphon orifice to allow for complete drainage of pipes after each dosing cycle. The effluent will be equally pressure dosed through each 1.5-inch custom perforated distribution lateral running the length of the treatment bed installed on top of the sand filter layer. The lateral pipe will have 5/32-inch orifices drilled every 2.5 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. 5” 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. A vertical 4-inch inspection port will be installed from the 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 with push on cap will be installed below the surface, housed in a 6" valve box with lid set 1" above surface grades. 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 Additional assessments and recommendations will be made by the engineer during initial stages of construction to direct installation in accordance with OWTS design, verify the soil conditions and location of the OWTS components. Prior to construction of the permitted system the engineer should be contacted by the contractor to provide adequate time to discuss the system components, answer questions, resolve any conflict issues and schedule inspection site visits based on construction progress. A pre-OWTS construction meeting with the engineer, the contractor, electrician and the qualified installer is essential and required prior to installation of the OWTS components. Pre-construction consultation, installation and startup by SI Valley Pre Cast, Inc. or Sopris Engineering will be coordinated by the contractor.