Raised Treatment Mound Septic Systems
A mound system consists of a septic tank and soil absorption bed. The soil absorption bed is used to distribute the effluent (partially clarified water from septic tank) into the soil. Each mound contains ASTM C-33 sand, 1.5″ washed stone, a distribution network, synthetic geotextile fabric, and a top-soil cap. The amount of sand needed in a mound is determined by the soil test.
The bottom of the absorption bed must be three feet higher than the limiting factor i.e. bedrock, ground water, or impermeable soil. For example, a soil test reveals bedrock at 24 inches. The mound must have 12 inches of sand to get 3 feet of separation. The absorption bed consists of stone, pipe, and synthetic fabric.
First, 6 inches of stone is laid down on a flat sand bed. The distribution network is laid on top of the stone. The distribution network is created by drilling equally spaced holes in pipe. Another 2 inches of stone is placed on top of the distribution network. Synthetic fabric is placed over the stone to protect the stone from dirt and roots. Sand is than placed around the absorption bed. Finally, a top soil cap is placed over the entire mound area. The mound must have a 3:1 slope to prevent erosion and to divert water away from the mound. The entire mound is planted with grass seed.
After the initial treatment in the septic tank, the effluent travels to the mound component. A pump is used to control the amount of doses delivered to the mound and to pressurize the distribution network in the mound. After every dose, the mound has a “rest period” which will result in superior pathogen and nutrient removal. Also, a pressurized distribution network reduces the chances for localized clogging.
The size of the mound is dependent on the slope of the site in which the mound will be built on, gallons produced per day by a residence or business, and the loading rate of the sand used underneath the absorption bed. The higher the slope of a mound site the bigger the mound.
This is due to the requirement to maintain a 3:1 slope on the mound. The gallons/day produced by a residence is based on the number of bedrooms in the house. Each bedroom is 150 gallons/day. The loading rate of ASTM C-33 mound sand is 1.0. The square footage of the absorption bed must equal to or greater than the total gallons/day produced by a residence/business divided by the loading rate of the sand. Thus, a three bedroom house produces 450 gallons/day divided by the loading rate of ASTM C-33 sand, 1.0.
450/1.0 = 450 square feet
The square footage of the bed must equal 450 square feet. Common bed sizes are 6 feet by 75 feet or 5 feet by 90 feet. Generally speaking, beds that are longer and narrower are better functioning mounds. However, constraints like lot size and code setbacks can limit mounds to shorter, wider mounds.
The final treatment of the effluent occurs in the soil. The soil beneath the mound component removes pathogens, organic matter, reduction of contaminants by aerobic microorganisms and ion bonding to negatively charged clay particles. The soil serves as a fixed porous medium on which beneficial aerobic microorganisms grow. These organisms feed on organic matter present in the wastewater and help eliminate pathogens. Unless pre-treatment is used, a mound requires 6 inches to 36 inches of suitable soil.