Pressure System with Pressure Mound: ***PICTURES BELOW***
The pressure mound is usually the type of system that most homeowners do not like to see in their yard. The pressure mound is that "big hump", or abnormal pile of material in the yard. That unattractive hump does serve a genuine purpose- to treat your effluent before it enters the natural water table.
The pressure septic system with pressure mound consists of a two compartment septic tank (minimum of 900 gallons) which then gravity feeds to a pump tank. The septic tank will most likely have a effluent filter in the outlet baffle or a mesh pump screen around the effluent pump in the pump tank. The pump tank will have a effluent pump which delivers effluent (liquid) to the pressure mound drain field. The pump selection depends on what distance the effluent has to travel, elevation between the pump and drain field, size of piping, friction loss in PVC fittings, etc.
The pump is controlled by a series of liquid level float switches that are wired into a time-dose control panel. Usually, there are either two, three, or four floats installed in the pump tank. The lowest float (near the bottom of the tank) is the "redundant off" float. Its purpose is to shut the pump off in the event the pump continues to pump due to electrical problems, other float problems, or if the control panel is set in the "manual" or "hand" position and left there. When this float is in the down position (if wired correctly) it will shut the pump off. The second float (higher up in the pump tank) is the "timer-on/off" float. Its basic function is to tell the timer in the control panel that there is or is not enough liquid in the tank to pump. When this float is in the up position or "activated", the timer, set at specific intervals will allow the pump to activate and dose (transfer liquid) to the drain field. The highest float in the system is a high level alarm. In the event that there is a pump problem, float problem, drain field problem, etc., the liquid level in the pump tank may rise to a higher than normal level activating this float. The float will signal the control panel to go in alarm mode which then the control panel will have a audible and visual alarm present.
The basic function of a time-dose control panel is to accurately allow the pump to deliver effluent to the drain field at specified intervals. These intervals are set as per design specifications and are set to not allow greater than designed effluent flows to the drain field. Let's just say you have a three bedroom home with a septic system that is designed for three bedrooms. The 24 hour daily load for you system is 360 gallons (120 gallons per bedroom), if your pump delivers (transfers/pumps) 30 gallons per minute, then you would typically set the timer to dose 2 minutes every 4 hours or 60 gallons, 6 times a day= 360 gallons. Ensuring proper dosing to the drain field is extremely important to the health and longevity of the septic system. Excess effluent delivered to the drain field can cause hydraulic overloading which will lead to very costly drain field failures in the future.
The pressure mound usually consists of 24" of ASTM-33 certified drain field sand as a bedding material, 9" of washed drain rock that is placed in a bed type pattern, and a PVC manifold and laterals (pipes) that equally distribute the effluent in the pressure mound. The laterals have holes or orifices drilled at specific intervals throughout the lateral piping system. The size of orifice, location, and quantity of orifices is calculated by the septic system designer. When the pump pressurizes the pressure mound, the effluent will get evenly distributed throughout the drain field. The effluent then gets "treated" above ground in the rock & sand before it enters the natural soil and eventually the natural water table.
The pressure mound will either have a "end manifold" or "center manifold". Either style is determined by the septic system designer. Usually, each lateral end will have a long-turn 90 degree fitting or flexible pipe to grade level for easy access to perform maintenance, cleaning, and testing of laterals.
Monitoring ports or stand-pipes (see pictures below) are usually installed in the drain field to visually inspect condition of the drain field. These pipes help allow a licensed inspector to determine the condition of the pressure mound by visually noting ponding or standing effluent in the drain field.
Below are some basic pictures of what a pressure mound looks like during and after installation:
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