The NoMound® onsite system makes it possible to use a conventional septic system drainfield or trench in a shallow water table setting.

It uses a below ground "barrier system" (usually 30 mil PVC geomembrane) constructed around the sides and over the top of the drainfield into which air at low pressure is circulated. 1-2 psi pressure is often sufficient to lower the water table beneath the drainfield and maintain a suitable thickness of unsaturated soil.

The fresh air circulation also provides oxygen to maintain aerobic biological activity in the drainfield.

Visualize a septic system with a conventional drainfield absorption area and pumped dosing. To this, the additional components of the NoMound® system are:

  • a perimeter barrier and capping arrangement of geomembrane and/or natural materials such as bentonite-soil slurry or clay;
  • a very small (17 Watt) air supply pump; and
  • 2-inch diameter groundwater monitoring well with a high level indicator probe.

After construction of the facilities, during start-up, air is pumped into the drainfield enclosure pressurizing the void spaces in the unsaturated soil. As the air pressure is increased, the water table is lowered to achieve the design requirement.

Each 1.0 psig of air pressure in the soil voids inside the NoMound geomembrane enclosure offsets the static pressure of 2.3 feet (28 inches) of naturally-occurring groundwater in the soil voids outside the enclosure [2.3 ft X 1.0 ft² X 62.4 lb/ft³ = 1.0 psi X 144 in² ].

If, for example, 3 feet of unsaturated thickness below the gravel or “gravelless” drainfield's distribution system is the regulatory requirement (and, therefore, the basis of design), a pressure of 2 psig will maintain 4.6 feet of unsaturated soil below the cap level. This satisfies the 3 foot requirement and allows one foot for the distribution system thickness plus a half-foot for water level movement in response to barometric pressure changes and dynamic headloss requirements (groundwater recharge mounding).

This example is fairly typical; and, most NoMound® systems will operate at 2 psig or less.

From a biological treatment process design standpoint, the system is set-up to mimic the natural soil environment of a conventional absorption field. For example, built into the system is a feature which will control an intentional small leakage of air from a location deep within the unsaturated zone created by the NoMound® barrier enclosure. This leakage causes the fresh air exchange that will accomplish the oxygen transfer which normally occurs by barometric pumping and diffusion effects between the atmospheric air and the soil gas in the unsaturated zone soils.

For residential systems of a typical size (300-500 gpd), one to two pore volumes of fresh air per week supplies the oxygen requirements of the soil organisms which reduce the BOD and nitrify the ammonia component of the septic tank effluent. For larger systems, the oxygen demand requirements should be calculated specifically for such a project.