Guide To Understanding Mound Septic Systems, Sand Filtering Low Down


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The Mound Septic System For Low Permeable Soils

A burrow? Buried treasure? An ancient crypt?  Mounds or in better perspective, mound septic systems aren’t as intriguing as our imagination can allure.

We, at GroundStone Wastewater Services, know mound septic systems very well because we have built many of them across British Columbia beside homes, schools and businesses. Our mounds don’t contain treasure (sorry folks), but they do contain an important 1940’s invention: the aptly named Mound Septic System.  

Certainly, when it comes to septic systems, “out of sight, out of mind” is most people’s ideal. However, this can’t always be the case. In certain locations and environmental situations, your septic system will need to be above ground.

And that’s okay because there are many advantages to having a Mound (or “Above Ground”) Septic System instead of an underground equivalent. We’ll get to these advantages shortly, as well as to the workings and maintenance of these kinds of Septic Systems. But first…

What are Mound Septic Systems?

Mound Septic Systems (also known as an Above Ground Septic System or a Sand Mound Septic System), is drain field that, true to its name, sits above ground in an engineered mound.

Developed in the 1940’s at the North Dakota College of Agriculture and originally called the “NODAK disposal system” after its place of origin,  Mound Septic Systems were a breakthrough alternative to traditional underground septic systems and drain fields.

The system was developed to overcome three environmental conditions: (1) permeability of the soil is too slow or fast, (2) a shallow soil cover quickly gives way to creviced or porous bedrock, or (3) the water table is especially high.

An area with any one of these conditions or a combination of the three can’t support a traditional septic system. So, enter the Mound Septic System, which allows optimum conditions to be created above the ground.

In the original NODAK system, distribution pipes ran the length and width of a gravel mound. The most common type of Mound Septic Systems used today is a modified version of the NODAK, designed in the 1970’s at the University of Wisconsin-Madison.

The “Wisconsin Mound” consists of gravel trenches set into a mound of sand (hence we sometimes call it a Sand Mound Septic System). The distribution pipes are placed in the gravel trenches and the wastewater is fed out of these by a method called pressure-dosing.

A soil covering protects the mound from weather events and can be seeded with certain types of vegetation to prevent erosion.

How does a Mound Septic System work?

So, we know what many of the mounds around British Columbia contain. Now let’s have a peek inside to see how a Mound Septic System works.

An Above Ground or Mound Septic System comprises a septic tank, a dosing chamber, and a mound. When you flush the toilet or drain the kitchen sink, the waste makes its way into your septic tank, which will usually be underground.

Here, the solids (or “sludge”) settle to the bottom, while the effluent passes through to a second tank – the dosing chamber.

Similarly to most septic systems, the sludge in your septic tank of your Mound Septic Systems will need regular pumping.

mound septic system

Into the mound

From the dosing chamber, the effluent gets pumped to the mound at a metered rate (or in, as the name suggests, doses).

This is important because your Mound Septic System would not be able to clean the effluent properly if it arrived all at once.

The wastewater is partly treated as it makes its way through the mounded sand. Finally, the wastewater arrives in the mound, where it is fully treated and dispersed.

Up and out

A Mound Septic System is all about the layers. The first layer actually starts before we get to the mound – it’s a tilled layer of soil where the mound is going to be located.  A lite layer of sand is mixed with the tilled soil to create a transition of the dispersed effluent into the ground.

The second layer is the sand that partially treats the wastewater as it passes from the dosing chamber to the mound. Inside the mound is a layer of gravel and piping, and it’s here that the final treatment and distribution occurs.

The effluent is pumped through the gravel, cover fabric protects any intrusion of the top cover of soil, finally, to the top layer of soil. And so the effluent is recycled back into the environment, the grass that has been seeded over the mound to prevent erosion.   

When would you use a Mound Septic System?

 The soil structure around your lot, home or business will different from region to region, depending on your location. In certain locations and situations, a traditional conventional septic system just won’t work. Instead, you’ll need to consider a different type of septic system.

Mound or Sand Mound Septic Systems are the best option when:

The permeability of the soil is too slow or fast: Soils with high permeability will not be able to adequately purify the wastewater before it reaches the water table line.

Using the wrong septic system in a highly permeable soil can result in contamination of nearby water sources or ecosystems. On the other hand, soils low in permeability will not be able to absorb wastewater fast enough. Using the wrong septic system in this case often gives rise to unsanitary conditions like surface pooling.

A shallow soil cover quickly gives way to porous bedrock: Where the soil cover over a creviced or porous bedrock is limited, there will also be limited space for wastewater to flow through the sand and soils. This means the wastewater will not receive a satisfactory level of treatment.

The water table is especially high: As with areas where the soil cover quickly gives way to creviced or porous bedrock, areas where the water table line is unusually high typically lack the amount of soil needed to sufficiently purify the water. This will be a challenge, even if the permeability of the soil is ideal.

mound septic systems

If your chosen drainage field site is an area with any one of these conditions or a combination of the three, an Above Ground or Mound Septic System will be your safest option. These systems create conditions above the ground that are able to perform the purifications that the soils below cannot.

How do you maintain  Mound Septic Systems?

At the most basic level (and this applies to pretty much every type of septic system), avoid pouring oil, grease and chemicals down the sink or flushing non-biodegradable items down the toilet.

These liquids or objects can block or damage your septic system. In the case of dangerous chemicals, these may hinder the natural breakdown of sludge and effluent in your septic system. These chemicals could also end up killing the grass on top of your mound or poisoning animals and people who pass by too close.

Avoid standing, walking or driving on the mound. These kinds of activities can cause erosion, compress the soil, and potentially damage your septic system. The more compact the soil, the less effective it will be at absorbing wastewater, which means your Mound Septic System will become less capable of handling the wastewater treatment needs of your household and property.

Finally, and importantly, regular maintenance performed by a septic system expert is essential. This is because Mound Septic Systems are more complex, fragile and costly than other types of septic systems. Plan a yearly inspection of your system and arrange for the septic tank and the dosing chamber to be pumped every three to five years.

This will help to ensure that your septic system is in good order and prevent an overspill, which can damage the mound and require the system to be reconstructed. 

sand mound maintenance

What are the advantages of a Mound Septic System?

  • Soil absorption levels will not affect your Mound Septic System, which means you can build this type of system on a property that would not be able to support a conventional septic system
  • Mound Septic Systems work in locations where the water table is unusually high, and where conventional systems would generally fail
  • A larger amount of effluent can pass through a Mound Septic System, without requiring more space
  • The amount of space needed to install a Mound Septic System is relatively small, so these systems are great for properties with minimal space
  • Because less space is needed to install a Mound Septic System, it is usually easier to meet building codes and health department regulations

What about the disadvantages of mound septic systems?

  • Since engineering and materials involved,  Mound Septic Systems can be more expensive and take longer to install than a traditional septic system or another above ground alternative
  • Mound Septic Systems are more complex than conventional systems, so they require extra attention, regular maintenance and may need repairs more often
  • The mound of your septic system will need to be placed on flat ground or low sloping grades, as too steep of a slope can lead to effluent runoff during times of high discharge
  • It can be difficult to incorporate the mound into your property, without it being unattractive and catching unwanted attention
  • A Mound Septic System is not gravity-fed, which means that if any of the components fail, you will very likely experience overspill, blockages or other problems
  • Once the system is installed, caution needs to be taken not to damage the mound, compact the soil or uproot the outer vegetation.

How much does a Mound Septic System cost?

Pricing can range depending on quite a few factors, soil conditions, size of lot, home size, the permeability of the soil, cost of sand and transportation (often very rural lots can be long distances from quarries and trucking costs can easily run high).  

Summarizing Video of the Sand Mound System

Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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