Conventional Pressure Distribution: ***PICTURES BELOW***
Generally, a conventional pressure distribution 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 pressurized 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 drain field (final effluent disposal unit) usually consists of a pressurized gravel bed with a manifold and laterals of PVC piping. In some instances, gravel-less chambers can be used in lieu of gravel beds with health department approval. The size of drain field, piping, etc. is determined by the septic system designer. The laterals (pipes) have several holes or orifices drilled in them at specific intervals to allow effluent to discharge the pipe. The size of the hole is determined by various calculations by the septic system designer. When pressurized, the entire drain field gets dosed equally allowing proper distribution throughout the drain field. Monitoring ports or stand-pipes (see pictures below) are usually installed in the drain field to visually inspect condition of the drain field and to access lateral ends for maintenance and cleaning. Most conventional systems will be at grade or ground level at completion of installation.
Below are some basic pictures of conventional pressure distribution type systems:
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