Why Use Pressure Distribution?

Uniform distribution of effluent is crucial for optimal treatment
Why Use Pressure Distribution?

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In our discussions of mound design principles, I indicated that pressure distribution in above-ground systems is needed to avoid problems with localized hydraulic overloading, leading to leakage or seepage out the toe of the system. Here’s a closer look at pressure distribution, not only for mounds but other types of systems as well.

Using pressure distribution methods, the daily flow is spread more evenly over time and space during the day.

Time is important because in most households there are two times during the day with heavier flows and periods in between with little or no flow coming from the house. In the morning as everyone gets ready for work and school and in the evening after everyone is back home are the typical periods of increased water use.

The heavy periods of water use can actually put quite a bit of stress on the system in terms of handling the peak flows. When using pressure distribution controlled by a timer, flow is spread more evenly throughout the entire day, relieving the pressure and allowing more time for treatment to occur before the next batch of effluent shows up.

In pressure distribution, every time the pump turns on, effluent is distributed over the entire area (space) system. This avoids having areas within the system where effluent is not treated as well until the biomat forms and restricts flow through the soil. In gravity systems we rely on the formation of the biomat to form over time, providing the distribution of effluent and helping in the treatment processes. In fine textured soils with cracks, effluent can penetrate deep into the soil until the biomat forms, shutting off those flow routes. This is often referred to as preferential flow.

With equal distribution this problem is avoided across the entire area from day one. If effluent moves through too quickly in sandy, coarse-textured soils, treatment will not occur. Using pressure distribution helps control the rate of water movement through the soil, which allows the time for treatment to take place.

Uniform distribution is achieved through a pressure distribution system. The systems are designed so the volume of septic tank effluent flowing out of each hole of the distribution pipe is nearly identical. Pipe diameters and hole diameters must be carefully sized to achieve this distribution.

A conventional pressure distribution system such as those found in a mound, at-grade or shallow trench system consists of five parts:

  • Pump tank to collect and store the septic tank effluent.
  • Pump to pressurize the system.
  • Manifold and main connected to laterals.
  • Lateral pipes with equally spaced holes drilled in the invert of the pipe.
  • Controls and power supply to operate the pump.

About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

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