The conventional septic system consists of two components, the septic tank and the drainfield. The septic tank is where the solids in the wastewater accumulate. The drainfield is where disposal of the wastewater occurs. The tank does provide some degree of treatment to the wastewater but most of the treatment occurs in the drainfield, specifically in the soil in which the drainfield is placed. In fact, the most important component of the system is the soil.
A conventional system can be installed completely below the soil surface if the soil conditions are good and the groundwater is deeper in the soil profile. These types of systems allow for gravity flow of the wastewater through the tank and into the drainfield.
A raised or mound system is required in areas where an elevated groundwater table is present. Mound systems can be unsightly and often create space that is essentially useless except to run a lawn mover over (drip systems can significantly reduce mound height and overall footprint). Conventional and mound systems can be constructed using materials such as rock and pipe or alternative drainfield products such as chambers or multi-pipe. All of these convey the wastewater across the bottom of the drainfield or absorption surface.
Every conventional system (in-ground, mounded or LPP) will fail at some point in time. Drainfield failure is the result of microbial activity that occurs while the “pollutants/contaminants/nutrients” are being broken down by bacteria in the soil. A bio-mat builds up over the absorption surface of the drainfield as a result of the microbial activity. This bio-mat severely impedes water movement into the soil. When water cannot move into the soil, a back-up into your house will result. If your system contains a pump, a “blowout” in your yard will usually occur when the drainfield is in failure.