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TO: John R. Gerhardson, Public Works Coordinator <br />FROM: Michael P. Gaffron, Septic System Inspector <br />DATE: October 19, 1981 <br />SUBJECT: Septic System, Police Building 1"5- _Ct) gcvc-« 4 <br />As you are aware, the septic system at the police station has been <br />backing up on occasion this oast year. At your request I have <br />reviewed the situation and wish to make the following comments: <br />1. ESTIMATED WATER USAGE <br />2 toilets, 2 sinks, 1 urinal <br />Assume 3 flushes per day per employee, at 4 gallons <br />per flush = 12 gallons/day/employee. Number of <br />employees - Mel, Jack, Secretary, average 3 officers <br />Frank, Jim, Dale, Steve, John = 8 employees (day only) <br />plus 3 x 3 shifts = 9 police = 17 employees <br />12 gallons/day x 17 employees = 204 gallons per day <br />PLUS Daily Public Use - Assume 200 gallons per day to <br />be safe. Total is - 400 gallons estimated water use. <br />2. SOIL CAPABILITIES <br />According to the Soil Survey of Hennepin County, the <br />soil type on the site is Anoka loamy fine sand. I <br />made a confirmation boring on Wednesday, October 14, <br />1981 and found essentially a loamy sand down to the <br />5' depth where the boring terminated. Mottling, <br />indicating a seasonal zone of soil saturation, occured <br />at about this 5' depth, so trenches should be no more <br />than 24" deep to maintain the 3' required separation <br />distance. The loamy sand on this site would be <br />expected to have an average percolation rate of faster <br />than 15 minutes per inch (although no testing was done). <br />3. SYSTEM DESIGN AND SIZING <br />Based on the Orono On -Site Code, a septic tank serving <br />an establishment other than a single family residence <br />must provide a detention time of 36 hours (1.5 days); <br />therefore the minimum tank size should be 400 gpd x 1.5 days <br />= 600 gallons. The code also specifies a minimum tank <br />size of 1000 gallons. I would recommend a 2 -compartment <br />1500 gallon precast tank be used, divided 1000-500, since <br />space is severely limited for this site. The size of <br />drainfield required is based on percolation rate and <br />water use. With a 6 to 15 mpi perc rate, 1.27 sf of <br />bottom area per gallon per day is required. 1.27 x 400 <br />= 508.00 500 sf. <br />