Mound Septic System:

The above mound system is ready to be covered with soil. It was designed by Meade Septic Design Inc. and is being installed by Quality Excavating of Goshen. The machine to the right is installing a Perimeter Drain. Click image for more mound pictures.

Septic systems are usually composed of two main components: the septic tank, and the absorption field. Some systems require a third component called a dosing tank or pump tank.

1. The septic tank is where raw wastewater goes initially. Two important things happen in the septic tank. First - the waste is partially treated and broken down by the bacteria in the tank. Second - The waste settles into three layers: The sludge, the liquid effluent, and the scum layer (makes you hungry doesn't it). A Baffle on the tank outlet allows only the liquid effluent (partially treated liquid sewage) to exit the septic tank and go to the absorption field, trapping everything else in the septic tank.
See our Septic System Care Page and our Septic Types and Terms Page.

2. The Absorption field is where the liquid effluent infiltrates the ground. A mound system is a type of absorption field, so are trenches, beds, trench chambers, and drip irrigation.

3. The Dosing tank is where the pump is located. Effluent from the septic tank enters the dosing tank and lifts the "on" pump float switch. Once on, the pump sends effluent through the delivery line to the mound system. The carefully designed volume and rate of this dose pressurizes the distribution system and effluent is sprayed out of the ¼" holes in the laterals. The sprayed effluent is baffled (slowed) by the gravel in the bed and soaks down through the medium sand and is incorporated into the original soil.

The floats in the dosing tank are set to pump about four times a day (1/4 of the homes wastewater flow). All pump systems are (or at least should be) set up with a high water alarm. In the event that the pump should fail, the water level in the tank will continue to rise until it trips the high water alarm - letting you know that you have a problem in the dosing tank and that your afternoon is ruined.

The best way to learn about a mound system is to review how one is constructed. The following is a general description. Your local septic code may be different. Contact your local Health Department for details - Here we go!!

• The area of the mound system is determined by the loading rate (how many gallons of effluent water per square foot the soil can absorb) of the soil. Heavier soils have lower loading rates and therefore require larger absorption fields. The absorption area for a mound system is often referred to as the Basal Area. The actual size of this area will vary. In Indiana, a typical mound system for a 3 bedroom house in clay soils can be expected to be about 85 feet x 40 feet. Preferably long and narrow, the mound must be designed parallel with the contour. Usually the maximum allowed slope for a mound system is 6%.
  
  
• After determining the proper site for the system, the area is plowed.
  
  
• The plowed area is then covered with a one-foot-deep layer of washed medium textured sand. The sand helps to distribute the liquid effluent throughout the absorption area and aids incorporation into the topsoil. This sand also functions as an attached media filter and helps in the treatment of the wastewater.
  
  
• Next, a gravel bed is installed on top of the sand area. The size of the gravel bed is determined by the loading rate of the underlying medium sand. The bed is much smaller than the basal area and might be 65 feet long x 6 feet wide. Don't be confused by the terminology, this is not a good place to spend the night.

• Distribution laterals are placed in the gravel bed. Perhaps they are sleepy? Laterals are laid parallel into the bed about 3 feet apart. Laterals are usually 1" - 1 ½" in diameter and made of PVC. ¼" holes are drilled on the bottom side of each lateral every three feet. The laterals are then manifolded together and the ends are capped.
  
  
• A delivery line connects the laterals and manifold to the pump in the dosing tank. This delivery line should slope back to the dosing tank so that the contents can drain back between doses.
  
  
• The gravel bed is then covered with a geotextile fabric (a coffee filter like fabric).
  
  
• The entire mound is covered with a 6-inch layer of clay and then followed by 6 more inches of topsoil.
  
  
• A thorough grass cover should be established on the mound. Don't plant trees or shrubs on the mound, however, because their roots can cause problems.

General Tips on Mound Design:

Be sure that your proposed mound system is designed correctly. If the hydraulics aren't calculated properly, the distribution laterals won't pressurize as needed. This will cause uneven distribution of effluent and can lead to premature pump failure.

Be sure that your pump is appropriate for the job. Effluent and Sewage pumps are made specifically for these types of applications and should last many years (see pump curves page). Don't let someone sneak you a regular basement sump pump. Sump pumps are for occasionally pumping clear water out of your basement and not for septic systems! They aren't designed for the consistent demands -- or the caustic environment -- of a dosing tank.

Delivery lines must be designed to drain back to the dosing tank. Incorrect design or installation of the delivery line could mean freezing and big problems.
Not something that you want to deal with on the coldest day of the year!

Visit our home page at Meade Septic Design, Inc. for access to our System Care, Types and Terms pages and a whole lot more!