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Residential Wastewater Treatment in Impermeable Clays

Residential Wastewater Treatment in Impermeable Clays

Hoping to find someone with experience in on-site residential wastewater design, that could provide some insight in the design of a residential wastewater system where the local ground consists of platy-clay type soil. The soil is fairly impermeable with a perc. rate of over 120 min/in. I'm also involved in another project where a weathered sandstone bedrock has been encountered at 3' deep. What type of systems are typically used for these kinds of ground conditions and where could I find a decent design manual or design source? Thanks for the help. Art

RE: Residential Wastewater Treatment in Impermeable Clays


The design of these systems is normally a specialised field in itself. But with many variables typically you either have to build a large soakage type trench which allows for the low percolation rates , or an evapo-transpiration trench , or you irrigate the effluent. The choices you have depend on the quality of the treatment , the size shape and slope of the land, climatic conditions, etc. Its possible your local regulatory authority, be it a Municipal Council or Planning and Building Department may actually publish some minimum guidelines or some sort of guidance notes on what is required. More than likely your design will require sign off by a specialist in the area anyway prior to approval.

Otherwise Google is your friend.

"Any water can be made potable if you filter it through enough money"

RE: Residential Wastewater Treatment in Impermeable Clays

Do a google search for mound systems.

RE: Residential Wastewater Treatment in Impermeable Clays

If I understand mound systems correctly, the sand is used to simply to distribute the waste water over a wider absorption area. The underlying base or native soils still need to have a percolation rate of less than 120 min/in. When dealing with clay or weather bedrock that has negligible percolation rates is there any way to still use a conventional two compartment septic tank system or does it become necessary to install a more complex (usually proprietary) tank treatment system that uses air and/or filter mediums? Would it be possible to use a conventional two compartment tank with an air pump and diffusers, essentially turning the system aerobic?

A design manual would be the most helpful, any recommendations would be great. I've seen the EPA manuals but haven't found them to be too helpful, other than covering the basics. Thanks again. Art

RE: Residential Wastewater Treatment in Impermeable Clays

Mounds were developed to overcome three natural conditions: (1) slow or fast permeable soils, (2) shallow soil cover over
creviced or porous bedrock, or (3) a high water table. A site that has any one of these three conditions (or a combination of them) is not suited for a conventional septic system. Because acceptable soil conditions are not always found below the
surface, the mound allows the conditions to be created above the surface.

If the property fails a percolation test, it doesn’t necessarily mean you can’t build anything on it. In many cases, you can get around this issue if you’re willing to spend more money on an engineered system and/or add a mound system to overcome to drainage issue (depending on what the Health Department is willing to allow).

The Purdue extension site has information on residential systems.

For a mound system:

For an advanced system:

In advanced or secondary wastewater treatment, septic tank effluent is purified to a level suitable for subsurface drip irrigation:

Would it be possible to use a conventional two compartment tank with an air pump and diffusers, essentially turning the system aerobic?
That is what the Orenco system does, but the Orenco also uses a cloth filter so that you can drip irrigate.

Before you go further, it would be prudent to visit the local Health Department to determine what the local practice is and what will be allowable.

RE: Residential Wastewater Treatment in Impermeable Clays

Thanks for taking the time to post. The information is very helpful.

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