Mound systems, conventional "septic" systems, in-ground pressure systems, at-grade systems, trench systems, etc. are all types of systems used to treat wastewater from structures not served by public sewer. They all do basically the same thing, they are just designed differently to compensate for differing lot constraints and soil conditions.
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Mound Systems vs. Conventional Systems
A MOUND SYSTEM refers to a system in which the soil absorption part of the system must be designed and installed literally on top of the existing grade to assure the wastewater is properly treated on sites that have high groundwater or bedrock.
In all Privately Owned Wastewater Treatment Systems (POWTS), including both mound and conventional systems, the wastewater exits the house and enters a septic tank where the water is partially treated. In a septic tank the inlet and outlet of the tank are both near the top of the tank, allowing the larger solids to settle to the bottom of the tank. A portion of the solids accumulating on the bottom of the tank are eventually "broken down" by anaerobic bacteria, and the balance pumped out when you have your system periodically pumped. Greases, oils, and lighter solids which float on the top of the water in the tank are prevented from exiting (which leads to clogging of the soil absorption bed), by baffles which cover both the inlet and the outlet of the tank, allowing only relatively cleaner water near the middle of the tank to exit.
Remember: both conventional septic systems and mound systems must be periodically pumped for proper maintenance.
After the partially treated water exists the septic tank the water may follow gravity into a conventional (below grade) soil absorption bed, or flow into a pump tank to eventually be dosed into a mound system under pressure.
"What determines if I need a mound or a conventional septic system?"
A certified soil tester must complete a detailed analysis of your soil characteristics (and other site limitations i.e. set backs, slopes, etc.) to determine the type of system your lot requires. The most important item the soil tester is looking for is depth to a "limiting factor" such as high groundwater or high bedrock. If that same soil test showed the limiting factor at only 10 inches below the ground surface, how would you design a system to achieve the minimum required inch vertical separation? By artificially creating the bottom of the soil absorption system above the ground by placing medium washed sand over the top of the ground and then installing your distribution bed of stone and piping on top of the sand. When this system is covered up with topsoil it creates a mound, because it's designed from the ground up instead of below the surface of the ground.
contact a local septic system company for more details.