More and more homeowners in the Puget Sound region are investigating above-ground septic systems, also known as mound systems. Although we generally have favorable soil conditions here, some locations do not. These lots have poor absorbing soils, high clay content, soil with a shallow cover over porous bedrock, while other lots are simply too wet with the high seasonal water table.
When local soils or conditions do not have the capacity to absorb effluents properly, a mound septic system, or above ground, is a viable alternative.
Mound Septic System Architecture
A mound septic tank system follows the same architecture of an underground system, with a modification. For above-ground septic systems, there are typically two tanks or one tank with two chambers. The first tank, or chamber, retains solid waste. The second tank, or chamber, collects liquid waste. For mound septic tanks, there is an additional sump pump, pumping liquid waste to the above ground absorption mound.
This above-ground absorption mound septic system contains several layers. The first layer is a tilled up layer of soil on-site. Then, sand is spread over the soil. Then, a level of gravel is added over the sand. After that, the gravel is covered with construction fabric. Finally, another layer of soil is added to the top. Between all of this, a sump-pump moves the effluent from the second chamber to the above-ground mound.
Mound Systems Are Practical and Ecological
When the sink is drained or toilet is flushed, the waste flows directly into the septic tank. The solid waste remains in the first chamber; water flows to the second chamber. This liquid waste (effluent) is then pumped to the above-ground tank. Through the piping, the liquid waste flows down through to the gravel and sand layers. As a bonus, this waste is a natural fertilizer, nurturing the top layer of the soil. As with an underground septic system, the mound system also needs regular pumping from the solid waste tank.
Above ground septic tanks do require additional room and expertise. If you live on a lot where conditions do not permit a traditional septic system, then contact us. You may be pleasantly surprised at the viable and affordable alternatives that exist. You can still build on your dream lot.