State code specifications determine septic tank sizing to treat an estimated amount of water used in the home. Additional bedroom or water-using fixtures can change the volume of the tank needed to properly treat waste water. Undersized septic tanks cannot retain waste water for long enough periods of time to fully treat the water before it flows on to the soil dispersal system. Hydraulically overloaded septic tanks allow solids to move through, overwhelming drain-fields and potentially clogging pipes. When an existing tank is too small for the waste stream flow you can add an additional tank.
Locate the additional septic tank between the existing tank and the drain field. The tank can be butted up to the existing tank or be located farther away.
Excavate a hole for the new tank to be set with an excavator using the dimensions from the tank's manufacturer.
Remove the soil over the outlet side of the existing septic tank with the excavator. Excavate the material around the outlet hole with a shovel and clean any remaining soil from the concrete with a wire brush.
Insert the 4-inch pipe that will connect the two tanks into the second tank's inlet hole before lowering the tank into the excavation site.
Position the tank truck close to the excavation site and lower the tank into the hole.
Butt the tanks next to each other and insert the 4-inch pipe into the outlet hole in the existing tank. Insert the pipe so that it hangs over the interior of both tanks about 2 inches.
Refill the septic tank hole with the excavated soil. Compact the soil in 6- to 8-inch increments with a vibratory soil compactor.