What should be the capacity of septic tank?
A septic tank used for pretreatment shall have an effective liquid capacity of not less than 50 percent of the estimated design flow. When garbage grinders are employed or the septic tank is used for sludge storage the effective liquid capacity shall be no less than 75 percent of the estimated design flow. When garbage grinders are employed and the septic tank is utilized for sludge storage, the effective liquid capacity shall be no less than 100 percent of the design flow. Multiple tanks are encouraged for tank sizes greater than 25,000 gallons.
What should be the Liquid Depth of Septic tank?
The liquid depth of the septic tank shall be a minimum of 4 feet. The septic tank may be rectangular, or square in plan, provided the distance between the outlet and the inlet of the tank is at least equal to the liquid depth of the tank.
How many compartments should be there?
Multi-compartment tanks with transverse baffles may be used for pretreatment. Septic pretreatment tanks may be installed in parallel provided the sewage flow is properly divided such that each tank receives an equal proportion of the total flow.
What are the construction features?
Septic pretreatment tanks shall be watertight (type WT) and shall be constructed of reinforced concrete. Tanks and covers shall be designed and constructed so as to withstand an H-20 wheel load. Any tank installed in a location where there is high ground water shall be weighted to prevent the tank from floating when emptied. Buoyancy calculations shall be included on the design plans for any tank with any portion installed below the anticipated high groundwater elevation.
What should be the MOC and depth of Tees?
Inlet and outlet tees shall be of cast-iron, SDR – 35 PVC, or cast-in-place concrete, and shall extend a minimum of 6 inches above the flow line of the septic tank and be on the centerline of the septic tank located directly beneath the clean out manhole. Any piping extending beyond 6 inches (15 cm) from a tank wall shall be properly supported. There shall be an air space of at least 3 inches (7.62 cm) between the tops of the tees and the inside of the tank cover, and the tops of the tees shall be left open to provide ventilation.
Depth of Tees – The inlet tee (baffles are not acceptable) shall extend a minimum of 12 inches (30 cm) below the flow line. The outlet shall be provided with a tee extending below the flow line in accordance with the following table:
Depth of Outlet Tee
Liquid Depth in Tank Below Flow Line
4 feet 14 inches
5 feet 19 inches
6 feet 24 inches
7 feet 29 inches
8 feet 34 inches
9 feet 39 inches
10 feet 44 inches
What are the other construction details?
Base – Septic tanks shall be installed on a level stable base that will not settle.
Materials – Septic tanks may be constructed of poured reinforced concrete or precast reinforced concrete.
Access Manholes – Septic tanks used for pretreatment shall be provided with at least two 24-inch (61 cm) diameter manholes (over inlet and outlet tees) with metal frames and covers at finished grade. Manhole covers shall be labeled and the type shall be specified in the specifications. Distance between access manholes shall not exceed 15 feet (4.57 m) on center.
Accessibility – Septic tanks shall be located so as to be accessible for servicing and cleaning
Invert Elevation –The invert elevation of the inlet of a septic tank shall be at least 2 inches (5.1 cm) above the invert elevation of the outlet.
Backfill – Backfill around the septic tank shall be placed in such a manner as to prevent damage to the tank and piping.
Groundwater – The invert elevation of the septic tank outlet shall be at least one foot above the maximum ground water elevation. In the case of segmented tanks all joints shall be at least one foot above the maximum ground water elevation.
