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Home My WebLink About1979-06-16 Septic System Design Report• SPECIFICATIONS FOR PROPOSED ON -SITE SEWAGE DISPOSAL SYSTEM SHALLOW TRENCH: DISTRIBUTION THROUGH GRAVITY FLOW LOCATION: Lot 6, Block 1 of David Nitz subdivision Orono, Minnesota DATE: May 8, 1979 I. SEWAGE TANK - TWO 1000-gallon LIQUID CAPACITY tanks, IN SERIES, manufactured and installed in compliance with the following specifications. See Figure 1 for location. 1. MATERIALS A. General All tanks, regardless of material or method of construction shall be: (1) Water tight (2) So designed and constructed as to withstand all lateral earth pressures under saturated soil conditions with the tank empty. (3) So designed and constructed as to withstand a minimum of seven isfeet of saturated earth cover above the tank top. U In addition, the following specifications apply to specific mater- ials as indicated. B. Precast Concrete Tanks (1) r-ecast concrete tanks shall be constructed of at least a 4,000 psi. (pounds per square inch) core test, air o-+trained concrete mix. (2) Precast concrete tanks shall be cured to at least a seven --day strength before installation. C. Fiberglass and Plastic Tanks Fiberglass and plastic tanks shall be designed and constructed to meet the requirements in Section I - a. above. -1- D. Metal Tanks Metal tanks shall have a wall thickness of at least eight gauge corrosion resistant alloys and shall be coated inside and out with at least one coat of bitumastic seal, Koppers 300-M, or by other equivalent means. Tanks shall be carefully installed so as not to puncture or damage the seal Coating. E. Block or Sectional Tanks (1) Rectangular Blocks (a) Rectangular blocks shall be laid on an approved bottom. }'. Block shall be at least eight inches thick, constructed ,� of a 4,000 psi core test concrete mix, and cured to a ' 28-day strength before installation. r= (b) Rectangular blocks shall be laid either with mortar joints or filled cores. 4 (c) There shall be at least two coats of 1/4 inch thick concrete plaster placed on the inside of the block tank. There shall be at least two hours elapsed time between coats. The plaster coat shall start at the bottom course and work up. (2) Circular Block Tanks (a) Circular blocks shall be carefully placed on an approved bottom. Circular blocks shall be at least two inches thick and constructed of a 4,000 psi core test concrete mix, cured to a 28-day strength before installation. • (b) There shall be at least two coats of 1/4 inch thick concrete plaster placed on the insida of the block tank. There shall he at least two hours elapsed time between coats. The plaster coat shall start at the bottom course and work up. (3) Sectional Tanks (a) 11. sectional tank shall have an entire section cast as an integral unit. (b) A sectional tank shall have two or more Lections and shall have at least one joint below the level of the outlet in- vert of the tank. C: (c) Sectional tanks shall have an integrally cast bottcm, or shall be laid on an approved bottom. -2- (d) The joint or joints shall be permanently sealed so as to be watertight. (4) Bottoms (a) Any tank not having an integrally cast bottom shall not be installed when the water table is closer than three inches to the bottom of the excavation.. (b) If the bottom is poured -in -place concrete, it shall be at least four inches thick and extend to at least four inches outside the tank wall. A concrete mix of at least one part cement, two parts washed sand and three parts washed rock s1lia.11 be mixed before placement in the excavation. (c) If a starter ring is used, it hall be constructed of a 4,000 psi core test concrete M__., cured to a seven-day strength before installation. It shall extend at least one inch beyond the tank wall, inside and out. A bottom shall be poured of concrete mix defined in Section (b) above. It shall extend at least two inches above the starter ring and shall be at least four inches thick. ►? (d) If a precast bottom is used, it shall be made of G 4,''00 psi core test concrete mix, cured to a seven-day strength before installation. It shall extend at least one inch beyond the tank wall, or be cast integrally with the bottom section. (e) A sectional tank not having an integrally precast bottom shall have a four -inch thick bottom poured within the tank walls. (5) Covers (a) Any tank cover not integrally cast Zaith the tank shall be so installed that the joint with the to sidewall is permanently sealed so as to be watert 'it. (b) All .rovers shall be so designed and constructed as to withstand a minimum of seven feet of saturated earth cover above the tank top. F. Inlet and Outlet Device Materials (1) Inlet and outlet devices shall be sanitary tees, or baffles constructed of acid resistant concrete, acid resistant fiberglass or plastic. -3- (2) Sanitary Lees shall be glued to the inlet or outlet pipes. Baffles shall be integrally cast. with the tank, affixed with a permanent waterproof adhesive or affixed with stanless • steel connect.ors, top and bottom. 2. DESIGN All tanks, regardless of material or method of construction, shall conform to the following design criteria:_ A. The LIQUID DEPTH of any septic tank of compartment thereof shall be not less than 30 inches. A liquid depth greater than six and one-half feet shall not be considered in de- termining tank capacity. B. No tank or compartment thereof shall have an inside horizontal dimension less than 24 inches. C. Inlet and outlet connections of the tank thereof shall be sub- merged by means of vented, sanitary tees or baffled. D. The spare in the tank between the liquid surface and the top of the inlet and outlet baffles or sanitary tees shall be not less than 20 percent of the total required liquid capacity, except that in horizontal cylindrical tanks this space shall be not less than 15 percent of the total required liquid capacity. • E. The inlet baffle or sanitary tee shall extend at least six inches but not more than 20 percent of the total liquid depth below the liquid surface and at least one inch above the crown of the inlet sewer. • F. The outlet baffle or sanitary tee and the baffles or sanitary tees between compartments shall extend below the liquid surface a distance equal to 40 percent of the liquid depth except that the penetration of the indicated baffles or sanitary tees for horizontal cylindrical tanks shall be 35 percent of the total liquid depth. They also shall extend above the liquid surface as required in D. above. In no case shall they extend less than six inches above the liquid surface. G. There shall be at least one inch between the underside of the top of the tank and the highest point of the inlet and outlet devices. H. The inlet invert shall be not less than three inches above the outlet invert. -4- I. The inlet and outlet shall be located opposite each other along the axis of maximum dimension. The horizontal distance between ® the nearest points of the inlet and outlet devices shall be at least four feet. J. Sanitary tees shall be at least four inches in diameter. Inlet baffles shall be no less than six inches or no more than 12 inches measured from the end of the inlet pipe to the nearest point on the baffle. Outlet baffles shall be six inches meas- ured from beginning of the outlet pipe to the nearest point on the baffle. K. Access to the septic tank shall be as follows: (1) There shall be one or more manholes, at least 20 inch... least dimension, and located within six feet of all walls of the tank. The manhole shall extend through the cover to a point within 12 inches but no closer than six inches below finished grade. The manhole cover shall be covered with at least six inches of earth. (2) There shall be an inspection pipe of at least four inches diameter or a manhole over both the inlet and outlet devices. The inspection pipe shall extend through the cover and be capped flush or above finished grade. A downward projection of the center line of the inspection pipe shall be directly in line with the center line of the inlet or outlet device. iL. Multiple tanks (1) Where more than one tank is used to obtain the required liquid volume, the tanks shall be connected in series. (2) Each tank shall comply with all other provisions of Section 1.A. (3) No more than four tanks in series can b1p used to obtain the required liquid volume. MAINTENANCE SUGGESTIONS Inspect the tank annually and have the tank cleaned when the scum is 3 inches cr less above the outlet baffle and/or when the sludge level is 8 inches or less below the outlet baffle. 3. LOCATION A. The sewage tank shall be placed so that access is convenient for the removal of liquids and accumulated solids. B. The sewage tank shall be placed on firm and settled soil capable of bearing the weight of the tank and its contents. -5- • II. PUMPING A 600-gallon capacity tank, placed in series with the sewage tank, shall be manufactured and installed in compliance with the following specifications. See Figure 1 for location. 1. PUMPING CHAM; :R A. The pumping chamber shall be watertight and constructed of sound and durable--:terials not subject to excessive corro- sion or decay. B. .sere shall be one or more manholes, at least 20 inches least dimension and preferably located directly above the dosing device. The manhole shall extend through the dosing chamber cover to final grade and shall be so constructed as to prevent unauthorized entry. 2. PUMPS FOR PRESSURE DISTRIBUTION A. A pump shall deliver the dose to the soil treatment area. B. The pu.<tp discharge capacity shall be at least 450 gallons per hour for each 100 square feet of rock bed area but n•.3 more than 2700 GPH. • C. The pump shaii be cast iron or bronze fitted and with stain- ies: steel screws or constructed of other sound, durable and corrosion -resist -- materials. C. The quantity of iuent delivered for each pump axle shall be no more than 25 percent of one day's sewage flow. Use 150 gallons/bedroom to estimate daily flow. E. The pump discharge head shall be able°to overcome the pipe friction loss and the elevation difference between the pump and the dist-`.bution system. F. An alarm device shall be installed to warn of pump failure. Fo- -swellings, there should be reserve storage volume equal to .,ne day's sewage flow based on 150 gallons per bedroom. 11 � 0 10 C. Sewage tanks shall be set back the following di.stances from the features given below: Feature Minimum Setback Distances (feet) Water Supply Well and Buried 50 Water Suction Pipe Buried Pipe Distributing 10 Water Under Pressure Feature Minimum Setback Distances (feet Buildings Property Lines The ordinary High Water Mark of: t,ktural Environment Lakes and Rivers Recreational Development Lakes and Rivers General Development Lakes and Streams Wild River Scenic River Designated Tributaries Wild, Scenic, and Recreational Rivers 10 10 150* 75* 50* 150* 100* 75* 75* *Required fer new developments and where practicable, for exisl_ing developments. 4. SURFACE FLOODIIJ,G Sewage tanks shall not be placed in flood plains delineated by local ordinances adopted in compliance with the Statewide Standards for Management of Flood Plain Areas of Minnesota Minn. Reg. NR85-93), or in areas for which regional flood information is available from the Department of Natural Resources, except that in areas where ten year flood information is available from and/or approved by the Department of Natural Resources. III. DISTRIBUTION OF EFFLUENT • 1.. BY GRAVITY FLOW OVER THE SOIL TPEATMENT AREA A. revel Ground Where the elevation difference of the ground surface does not exceed 28 inches in any direction within the soil treatment area, the sewage tank effluent may be directed to the soil treatment area through a distribution box. (See Figure 2). (1) The box shall be watertight with a removable cover and st be constructed of durable materials not subject to ex.. sive corrosion or decay. The minimum inside dimen- sion shall be at least ten inches. (2) The inverts of all outlets shall at the same elevation as measured from a liquid surface, the bottom of the box. (3) The inlet invert s„all be at lei, .ne _ h above the outlet :,nverts . (4) the outlet inverts shall be at least four inches above the distribution box floor. (5) Each drainfield trench line shall be connected separately to the distribution box and shall not be subdivided. When sewage tank effluent delivered to the distribution box by pumo, either a baf--e a -hall be installed in the distribution box or the pump di arge shall be directed against a wail or side of the bu on which there is no outlet. The baffle shall be secured to the box and shall extend at least one inch above the crown of the inlet flow line. B. Sloping Ground Where the elevation difference of the ground surface exceeds 28 inches in any direction within soil treatment area a drop box shall be installed at the head end of each lateral line. Connections between drop boxes shall be by watertight pipes. (See Figure 3) (1) The drop box shall be watertight and constructed of durable materials not subject to excessive corrosion or decay. The minimum inside dimension shall be at least ten inches, unless the box is to be used as a valve box. Then each outlet shall have at least eight inches of sidewall space and the side of any outlet shall be e%t least six inches from any corner. U, L.. • (!) 'I'hei i►►vu► t ui tt►C, inlet. I,il►c! shall be .►t least une' inch hiyhur th,i►i thu invu►t of the c►utlCt I)ipC to thL: nUxt_ t rend►. (.i) The invert of the outlet pipe to the next. trench shall be at least two inches higher than the invert of the outlet pipe of the trench in which the box is located. (4) When sewage tank effluent is delivered to the drop box by a pump, the pump discharge shall be directed against a wall or side of the box on which there is no outlet. (5) The drop box shall have a removable cover either flush or above finished grade or covered by no more than six inches of soil. IV. DRAINFIELD - Final treatment and disposal of all sewage tank effluent shall be by means of soil treatment. 1. SIZING Indicated average percolation rate is 12.6 minutes r inch for the primary site and 11.6 mi..ates per inch for the a-ternate site. • A. Total drainfield area is = 1050 ft2 (Required soil treatment area based on a 4-bedroom house) B. Drainfield trench bottom width is 31. C. TOTAL , ►eal feet of drain the is 350. D. This requires 5 laterals of 70 feet in length. See Figure 1 for typical placement. E. , nimum spacing of laterals is 7.5 feet from center to center • 9- 1 i Lot 6, Block 1 • TABLE 1-A PERCOLATION RATES AND DEPTH TO THE WATER TABLE Hole No. Rate (minutes/inch) 1 10.8 2 10.8 3 15.6 4 13.2 5 10.5 6 12.7 Primary drainfield average percolation rate = 12.6 mpi (Holes 1,2,3,4) Secondary drainfield average percolation rate = 11.6 mpi (Holes 5,6) • Soil Boring A Depth 0-1' 1-4.5' 4.5-5.5' �5.5-6.5' 6.5-8' Soil Boring B Depth 0-1' 1-2.5' 2.5-3.5' 3.5-5' 5-6.5' TABLE 1-B Material Topsoil, dark brown, organic, moist Loamy clay, tan, wet Loamy clay, yellow, dry, friable, slightly mottled Loamy clay, yellow -tan, moist, mottled Loamy sand, tan, mottled Topsoil sandy dark brown, organic Loamy clay, moist, tan Sandy clay, tan, little moisture Sandy silt, yellow, friable, dry Sandy clay, tan, friable, dry obstruction at 6.5 feet, no mottling observed hi • TABLE 1-B (continued) SOIL LOGS Soil Boring C Depth 0-0.5' Topsoil, dark brown, organic 0.5-2' Silty clay, brown, moist 2-6, Silty sand, light brown, dry, friable, mottling at 5.5 feet 6-7' Sandy clay, yellow ` 7-8' Loamy clay, yellow, mottled • • 1 �. 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