What Type Of Soil Is Best For A Septic Tank?

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The role of soil in a septic system is to act as a natural filter in the treatment of septic effluent. It removes any bacteria or pollutants that remain in the effluent. What Type Of Soil Is Best For A Septic Tank? The best soil for septic systems is neither too loose or dense and lies between clay and gravel. It has excellent percolation and drainage due to the mixture of both fine and coarse particles.

Septic systems are only as good as their foundation, so having a thorough understanding of your own site’s soil composition is key to ensuring that your septic system will meet all expectations. A soil analysis test will provide information on the type of soil, texture, structure, density and color. It helps determine how much septic effluent it can hold and how well it can drain.

How to perform a percolation test

Soil percolation tests help determine the permeability of the soil. This is important for determining the suitability of a site for septic system installation. Local council regulations require perc tests (soil percolation test) report before installation of a septic system. The tools needed to perform a percolation test include:

  • Ruler or measuring tape.
  • A shovel.
  • Stopwatch.
  • Book and pen, to record results.
  • A bucket of water.

You can easily carry out a percolation test by following these steps:

  • Make a 300mm x 300mm x 300mm deep hole.
  • Remove any debris, litter, and rocks from the hole.
  • Saturating the soil. Add water to the hole and measure how long it will take to drain using your stopwatch. Please measure in seconds.
  • Calculating the percolation rate. Add water to the test hole again up to the 300mm mark. Start your stopwatch when the water level reaches 225mm (3/4 full). Measure how long it takes to reach the 75mm (1/4 full) mark. You now have the time (in seconds) it takes for the water to drain by 150mm. Divide the number (time in seconds) by 150 to obtain the Vp Number which is the average time it takes for the water to drain by 1mm.
  • Repeat the test at least twice on different patches of the ground and different times of the day. Calculate the Average Vp number for all 3 tests.
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An ideal Vp number lies between 15 and 100. Vp number within this range means the soil’s percolation rate allows effluent to be sufficiently treated. A Vp number below 15 means the soil drains too quickly and would release untreated effluent into the ground and pollute ground water. A Vp over 100 means the effluent will not drain away fast enough.

What Type Of Soil Is Best For A Septic Tank: Soil Structure & Texture

Soil structure is the pattern in which soil particles are arranged. It affects the percolation and drainage ability of the soil. Soil consists of an equal amount of solid material and pore space. Pore space comprises of water and air. The solid material is made up of organic matter and the mineral particles : clay, sand and silt. Sand particles are big and gritty in texture. Clay particles are very small and get sticky when wet. Silt particles are bigger than sand particles but smaller than sand particles. The amount of mineral particles in soil affects its percolation and drainage rate.

What Type Of Soil Is Best For A Septic Tank: Soil Types

Loam, sandy soil, and clay are the three main types of soil. Their properties and suitability for septic use are as follows:

  • Montmorillonite and kaolinite are the two main types of clay. Montmorillonite clay swells when wet, reducing the pore space. It has been known to expand up to 150 times its normal size when wet. This makes montmorillonite clay soil unsuitable for a traditional or conventional septic system since it has poor permeability and drainage. It will not allow effluent to easily seep through. Montmorillonite clay also shrink when dry. The pressure from constant shrinking and swelling will eventually cause concrete septic systems to crack. It also causes plastic septic systems to buckle or cave in.
    On the other-hand, kaolinite clay swells slightly when wet. Kaolinite clay is suitable for a conventional septic system.
  • A traditional or conventional septic system will not function properly in sandy soil due to the high percolation rate. The fast percolation of sandy soil makes it impossible to efficiently treat septic effluent. There is a lot of documented research on effluent contamination in many beachfront septic systems. So what is the solution for beachfront properties and sandy soils? You need to install an aerobic septic system since it is more efficient at treating household waste than an anaerobic system. Furthermore, depending on the soil structure, you can either install a septic mound system or a raised bed septic system.
  • Loam soils are the most common type of soil found in the world. They contain a mixture of sand, silt, and clay particles. They are rich in organic material and have a good permeability rate. Loam soils are good for septic systems.
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What color soil is best for septic systems?

The color of soil has a huge impact on the performance of septic systems. The color of soil is not just about aesthetics, but it also impacts how well the septic system will function. Brightly colored soil provides excellent permeability and drainage compared to dull and grey-colored soil.

What is the best gravel for a leach field?

Gravel prevents sewage from muddying your yard. The most ideal type of gravel for a leach field is one that is made up of granite.

Worst soil for septic systems

Any type of soil that does not allow septic effluent to be treated is bad for septic systems. Sandy soil is fast draining and does not allow effluent to be treated naturally. This results in untreated effluent containing bacteria, pathogens, and pollutants being released in the ground. Soil such as clay is also bad for septic systems since it does not allow effluent to seep through. This causes backups in the drain field.

What Type Of Soil Is Best For A Septic Tank: Final Thoughts

One of the most important things to consider when designing and building a septic drain field is the soil around it. Soil affects many things – from what crops you plant on your farm to how well your septic system will function. Clay soils cause the drain field to backup since they do not allow effluent to easily ooze through. On the other hand, sandy soils have a quick or rapid drainage that results in untreated effluent being released into the ground where it contaminates ground water. The best soil for septic systems is a soil that lies in between gravel and clay.

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