The mound septic system was originally developed in the 1940s in North Dakota. Known as the “NODAK disposal system”, it was designed in response to the state’s harsh climate and variable site conditions. This septic system featured a septic tank that pumped wastewater to an above-ground gravel mound with distribution pipes running its length and width. The concept is still used today in places with especially slow or fast permeable soils, shallow soils over porous rock, or a high-water table.
Mound systems are recommended when a traditional “in-grade” systems cannot effectively treat home wastewater. This type of system has three main components: 1) a septic tank, 2) a pump chamber, and 3) the elevated mound drainfield.
The Septic Tank is a large, buried container, made from concrete, fiberglass, or polyethylene. Wastewater flows from the home into the tank where heavy solids (sludge) settle to the bottom and lighter solids (scum) rises to the top. Wastewater leaves the septic tank and flows to the pump chamber. Wastewater leaving the tank is called effluent.
The Pump Chamber is a second buried container that collects the septic tank effluent. It contains a pump, a pump control float, and a high-water alarm float. When the effluent rises to the float, the pump delivers a set amount of liquid to the mound. Most pump chambers have an alarm float to warn of pump or system problems.
The Mound is a drainfield that is raised above the natural soil surface. It is composed of sand fill, a gravel-filled bed, and a network of small diameter pipes known as the distribution system. From the pump chamber, effluent is pumped through the pipes in small doses, so it is distributed evenly. The effluent trickles through small holes in the pipes downward through the gravel bed into the sand, where treatment occurs.
If a mound system has been properly designed and maintained, it will last for many years. It is the homeowners’ responsibility to maintain their septic system. Pumping the septic tank every 3 to 5 years is the most important way to prevent malfunction and failure.
Signs of septic system failure include slow or backed up drains, ponding or smells in the yard, lush growth over the drainfield, and excessive algae in adjacent waterbodies. If you are experiencing any of these conditions, have your septic system inspected.
Mound septic system are not used in every situation, but where conditions are limiting, they offer an excellent alternative for home wastewater treatment.
Pictured top: Photo of a drainage tile laid in a bed of gravel before covered with a porous soil in a mound drainfield.
This information is part of the Upper Maple River Watershed Restoration Project funded through Michigan Department of environmental Quality’s Nonpoint Source Program by the United States Environmental Protection Agency.