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Home My WebLink About20 - Design Report - Lots 5 & 6 Gallatin Center PUD Ph 1 (Burke Street Car Wash) - Sewer Design Report - Page 2 of 5 INTRODUCTION The proposed development on Lots 5 & 6 of Gallatin Center Subdivision P.U.D. Phase 1 consists of a new carwash business with a drive-thru carwash, parking area, office space, car wash detail bays, and associated site landscaping and stormwater management features. The property is located within the Bozeman city limits and is currently zoned B-2 commercial. This project will require connection to existing City of Bozeman water and sanitary sewer systems. SANITARY SEWER SYSTEM An 8-inch PVC sanitary sewer line will be installed within a proposed 30-foot public utility easement. The proposed main and will flow southeast and connect to an existing manhole located along the eastern property line as shown on the drawings included herein. Both the carwash building on the western portion of the site and the office/detailing building on the northeastern portion of the site will connect to the proposed main via 4-inch and 6-inch sanitary sewer services. Figure 1: Sewer Main Plan DESIGN REQUIREMENTS The flow rates are based on the carwash design flows from the manufacturer and are shown on the table included herein. The automated car wash in the wash tunnel building operates almost exclusively on recycled water from the water reclaim pit. Although the flow rate into the tunnel Design Report - Page 3 of 5 during a car wash can be as high as 154 gpm, only an average of 40 gallons of new water will be used for each vehicle being washed. Therefore, only approximately 40 gallons of recycled water will be discharged from the reclaim pit to the sanitary sewer system for each vehicle washed. This rate of 40 gal/vehicle with an estimated 400 vehicles washed per day was used to calculate the average daily flow anticipated to be discharged to the sanitary sewer main from the wash tunnel. The maximum estimated number of vehicles to be washed per day is 1,000 vehicles. However, if for some reason the reclaim pit is out of service, then all wash water generated during the car wash (154 gpm) would be discharged to the sanitary sewer system because no recycled water would be used. This reclaim pit is out of service for a few hours to a day, once or twice a year, when the pump in the reclaim pit needs maintenance. The developer currently plans to have a back-up pump on hand for use during these times. However, in case the back-up pump is unable to be operated, the max possible (peak) flow to the sanitary sewer system from the wash tunnel is 154 gpm. The detail bay building is estimated to wash 12 vehicles per day, with an average wash time of 7 minutes / vehicle at a water flow rate of 4.5 gpm. Since this building is not connected to the reclaim pit, all of the water used in car washes for this building will be new water and all of it will be discharged to the sewer system. These estimates were used to determine the average daily flow to the sewer system from the detail building. However, while vehicles are being washed the maximum possible (peak) flow rate to the sewer system will be 4.5 gpm. The flow rates used for the employee bathrooms are based on the DEQ Circular 4, specifically section 3.1, table 3.1-1. An average daily flow of 12 gpd per employee was used. The peak flow rate from the employee bathrooms is calculated based on the proposed number of fixtures in the buildings as provided by the mechanical engineer for the project. The infiltration rate used for both the average and peak flows is based on the City of Bozeman DSSP rate of 150 gal per acre per day of infiltration to the sanitary sewer system. Design Report - Page 4 of 5 Average Design Flow Rate Employee Bathrooms (Based on # of Employees): (11 employees)(12 gpcd) = 132 gpd Wash Tunnel: (40 gal/vehicle)(400 vehicles) = 16,000 gpd Detail Bays: (4.5 gpm)(7 min/vehicle)(12 vehicles/day) = 378 gpd Infiltration: (150 gal/acre/day)(2.725 acres) = 408.75 gpd Average Design Flow Rate = 132 + 16000 + 378 + 408.75 = 16,919 gpd Peak Flow Rate (Reclaim Pit in Operation at All Times) Employee Bathrooms (Based on Fixture Count): 15.5 gpm Wash Tunnel: (40 gal/vehicle)(1,000 vehicles) = 40,000 gpd = 27.77 gpm Detail Bays: 4.5 gpm Infiltration: 408.75 gpd = 0.284 gpm Peak Flow Rate = 15.5 + 27.77 + 4.5 + 0.284 = 48.05 gpm = 0.107 cfs Peak Flow Rate (Reclaim Pit is Out of Service) Employee Bathrooms: 15.5 gpm Wash Tunnel: 154 gpm Detail Bays: 4.5 gpm Infiltration: 0.284 gpm Peak Flow Rate = 15.5 + 154 + 4.5 + 0.284 = 174.3 gpm = 0.388 cfs The capacity of an 8-inch main is checked using Manning’s Equation: Qfull = (1.486/0.013)AR2/3S1/2 For an 8-inch PVC main: Manning's n = 0.013 for PVC Pipe Minimum Slope = 0.004 ft/ft A = area = (3.14/4)d 2 = (3.14/4)(8/12)2 = 0.3491 ft2 P = perimeter = 2(3.14)r = 2(3.14)(4/12) = 2.0944 ft R = hydraulic radius = A/P = 0.34907/2.0944 = 0.16667 ft R2/3 = 0.30286 ft Design Report - Page 5 of 5 S = 0.004 ft/ft S1/2 = 0.0632 ft/ft Qfull = (1.486/0.013)(0.3491)(0.30286)(0.0632) = 0.764 cfs Percent of Flow Capacity for 8” PVC Sanitary Sewer Main: Peak Flow (Reclaim Operational at All Times) = Q/Qfull = 0.107 / 0.764 = 0.14 or 14% Peak Flow (Reclaim Not in Operation) = Q/Qfull = 0.388 / 0.764 = 0.508 or 51% Based on these calculations, an 8-inch sewer line has adequate capacity to carry the peak design flow, even if the reclaim system is not operational. The City of Bozeman Engineering Department has confirmed that there is adequate downstream capacity in their sanitary sewer system for both peak flow scenarios. However, it is recommended that a back-up pump be installed with the wastewater reclaim system to ensure that the reclaim pit will remain operational, even when the main pump is down for maintenance.