Billingsley Engineering
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This type of septic system became known as a mound due to the fact that the entire absorption field is above the ground which forms a mound of dirt in a yard. As you can see from the graphic to the right, the mound consists of nothing more than sand, washed stone, and top soil. Characteristics of a mound, such as length, width, and height, are determined by the soil test performed on the site. The length and width of the mound are determined by the hydraulic rating of the soil. The height of a mound varies based on the depth of the sites limiting factor for the soil. A limiting factor is the elevation in which either a high ground water table exists or bedrock is found. To
Mound Septic System Click here to see a detail of the septic tank Click here to see a detail of a pump tank Click here for a detail of a mound

learn more about limiting factors please see the Soil Test page of this web site. The approximate size of a mound for a three bedroom house is 25' wide, 90' long, and 4' tall.

Now that you know what determines the physical characteristics of a mound; lets go over the functionality of the system. As the effluent (wastewater) leaves the house, it first passes through two tanks. The first of these tanks is the septic tank. The septic tank (shown above) is know as a double compartment tank and is pre-cast, watertight concrete. For a three-bedroom home, it is typically six feet wide, eight feet long, and five feet tall, with 1000 gallons in capacity. The size and volume will change depending on the number of bedrooms in the house.

The primary purpose of the septic tank is to act as a temporary reservoir that allows time for the solids to settle to the bottom creating a sludge layer and fats to float to the top creating a soft layer of crust. As the effluent is retained in the tank, natural bacteria biodegrade the majority of the solids leaving a partially clarified liquid. Inside the septic tank is a baffle (wall) that retains the majority of the solids and fats in the first compartment. In the second compartment there is a filter. With the combination of a double compartment tank and a filter, the probability of keeping solids from flowing down to the pump tank is greatly increased. The filter must be cleaned periodically to keep it from becoming plugged.

After the effluent has been partially treated and filtered in the septic tank, it flows down to the second tank, the pump tank. The pump tank, like the septic tank, is constructed of pre-cast concrete and is watertight. However, it does not have two compartments. It typically is six feet wide, seven feet long, five feet tall, with a volume of at least 770 gallons. It may be larger depending on individual site restrictions.

Inside the pump tank is an effluent pump operated by two separate floats. The first float controls the pump on and off settings. The second float is an alarm float, which is connected to the alarm box that is mounted inside the house. This alarm will sound, audible and visual, if there is any malfunction with the effluent pump

A water resistant junction box is mounted on the pump tank riser. THis is where the electrical connections for the pump and alarm are made inside a junction box. All risers that are above the ground surface have warning labels and are locked with a cable and padlock. It is important to ensure that the risers are locked at all times to avoid accidental entry that could result in injury or death.

The absorption cell, commonly known as the "mound", is where the effluent treatment is finalized before it reenters the underground water table. The mound consists of a layer of sand on the bottom, a cell of washed stone placed in the middle, and then covered with a layer of topsoil that will support vegetation. Typically, two laterals (PVC pipe) 1 ½" in diameter are placed in the gravel cell with 3/16" holes drilled in the bottom. When the effluent pump in the pump tank turns on, the water is pumped into the laterals and is dispersed evenly through the holes in the laterals and into the gravel. As the water percolates through the stone and sand, it is naturally filtered leaving all of the bacterial and harmful substances behind before entering the water table.

Billingsley Engineering LLC.
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Click on picture to see detail
Mound Septic System