SECTION 7: Septic Tanks - Onsite Sewage Treatment Program ...

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SECTION 7: Septic Tanks 7-5capacity must be at least 50 percent greater than that required in subpart 1 and mustinclude either multiple compartments or multiple tanks. In addition, an effluentscreening device is recommended (7080.1930 Subp. 2).The use of garbage disposals adds unnecessary solids to the waste stream and encourageshomeowners to introduce food scraps that result in increased BOD and FOGs. If garbagedisposals are used, more frequent solids removal may be needed.The same tank capacity requirements apply if sewage is pumped from a sewage ejector orgrinder pump from a dwelling to a septic tank (7080.1930 Subp. 3). This is requiredbecause the discharge line cannot be restrictedby a valve and because theFIGURE 7.5 Ejector Pump in Basementpumping rate is so rapid, a great deal ofagitation will take place in the first tank.The second compartment/tank is necessaryfor a quiet zone to exist for propersolids separation. The pump dosevolume from the ejector pump is recommendedto be limited to less thenCleanoutfive percent of the volume of the firsttank or compartment tank as shown in4" building sewer Effluent screen Figure 7.5.Dose should be 1-5%of septic tank capacityEjector PumpFIGURE 7.6 Effective Volume of a New Tank+50% capacity, multiplecompartments, and effluentscreen recommendedSubpart 4 states that, if conditionsin both subparts 2 and 3 apply to adwelling, the mitigative requirementsof either subpart 2 or 3 apply; the requirementsof both subparts 2 and 3need not be additive (7080.1930).For systems serving multiple dwellingswith a common septic tank, the liquidcapacity must be determined by addingthe capacities for each dwelling as determinedin Table 7.2.Determining Tank CapacitiesNew tanks have a rated volume basedupon the depth from the bottom of thetank to the invert of the outlet (Figure7.6). Over time, the rated volume staysthe same, but the effective volume is decreasedas solids and scum accumulate.Septic tanks should be large enough toretain the wastewater in a calm, turbulence-freeenvironment for sufficientsettling. The average length of time thatthe wastewater spends in the tank iscalled detention time. Detention timeis a function of the effective volume andthe rate of flow into the tank.
7-6 SECTION 7: Septic TanksLiquid volume is calculated by using the surface area and the liquid depth as establishedby the bottom of the outlet pipe. The minimum recommended detention time to allowadequate settling of solids is two days. The tank should also have sufficient volume toaccommodate storage of sludge and scum. Finally, if wastewater is recirculated from anadvanced treatment device back to a treatment tank, the increased hydraulic load and reducedhydraulic detention time must be addressed in tank sizing. An additional 50% canhelp in these conditions.For example, if a tank has inside dimensions of 4 feet 6 inches by 9 feet 6 inches, the surfacearea is equal to 4.5 x 9.5 = 42.75 ft 2 and the volume is 42.75ft 3 per foot of depth. If aseptic tank is to contain 1,500 gallons, this is equivalent to 200 cubic feet of liquid (1,500gal.÷ 7.5 gal/ft 3 ). If each foot of depth contains 42.75ft 3 of liquid, then 200.0 ÷ 42.75 =4.68 feet of depth. This is equivalent to 56 inches of tank depth between the bottom ofthe tank and the bottom of the outlet pipe.There will need to be additional volume in the tank to allow for floating scum storage.Sewage tanks should be placed so as to be accessible through the manhole for the removalof liquids and accumulated solids. They should be placed on firm and compacted soilcapable of bearing the weight of the tank and its contents. Sewage tanks should not beplaced in areas subject to flooding.Multiple TanksOne advantage of using multiple tanks is when very warm effluent is being dischargedsuch as from a high temperature dishwasher. In these cases, multiple tanks will have moresurface area in contact with the native soil to help cool the effluent. Another reason toconsider using multiple tanks versus a large tank with compartments is based on localavailability. Often large tanks may not be readily available. From MN Rules 7080.1940,if more than one septic tank is used to obtain the required liquid septic tanks mustbe connected in series or employ multiple collection systems. If tanks are connectedin series, each tank or compartment must contain at least 25 percent of the requiredtotal liquid capacity. The use of multiple tanks allow delivery and placement of smaller,lighter tanks and reduces the overall excavation width. The disadvantage of using multipletanks is the installation and maintenance of the 2” drop in subsequent tanks. A multipletank configuration is shown in Figure 7.7.
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SECTION 7: Septic Tanks - Onsite Sewage Treatment Program ...

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