Watertight, structurally sound tanks are essential to the performance of onsite wastewater systems and to groundwater protection. Eric Ball's paper, "Watertight Tanks: No More Excuses" explains why watertight tanks are needed.
Poor soil conditions, high groundwater, and poor installation practices can all place unexpected structural loads on a septic tank, and many tanks on the market are unable to withstand these loads. At Orenco, we feel that all residential size septic tanks (up to 2000 gal. [7570 L]) should be able to pass two easily performed tests.
The first we call the "parking lot" test, in which the tank is filled completely with water while sitting on top of the ground with no external supports, then inspected for watertightness. Every Orenco tank undergoes this test after assembly. We also require installers to repeat the watertightness test once the tank is in the hole but before the final backfill. The tank must be 100% watertight with no visible deformation.
The second is a vacuum test. A vacuum of 6.5" Hg (equivalent to 3.2 psi, 7.4 ft of water pressure, or 0.22 bar) is pulled on the tank. This approximates the load on an empty tank buried four feet deep, with water to grade, and a 2500-lb (5500-kg) wheel load. We don't subject every tank to this test, but if you're evaluating a new brand of tank, it's a good way to see what might happen under the ground. Our tanks don't visibly deform at 6.5" Hg, but we've seen many others crumple.
When we do vacuum-test our tanks, we don't always stop at 6.5" Hg. Watch our video to see one of our 1000-gal. (3785-L) tanks undergoing destructive vacuum testing. The tank in the video was typical of the ones we've tested. It imploded at 11.5" Hg (5.6 psi, 0.38 bar) - substantially higher than the worst-case load described above.
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