Sizing an Onsite Treatment Area is All About the Soil

Soil identification mistakes lead to systems that are hard to fix and difficult to manage
Sizing an Onsite Treatment Area is All About the Soil

Interested in Onsite Systems?

Get Onsite Systems articles, news and videos right in your inbox! Sign up now.

Onsite Systems + Get Alerts

A colleague of mine has often said that the most common mistake he has seen around installation of onsite systems is misidentifying soil characteristics, which results in the wrong type of system or the wrong-size system being installed.

Here I will briefly explore a few of the common soil identification problems and how they impact system performance. Remember, this is in the context of being able to manage the system into the future.

Two major ways the soil affects the system are determining the size of the final soil dispersal and treatment system, and the location of the system in the soil. I call this the soil treatment unit; depending on soil characteristics, this unit can consist of gravity-fed trenches installed in the soil, in-ground pressure distribution systems including drip distribution, or above-ground systems such as mound and pressure-distributed at-grade systems. Sizing the system is a big part of defining where in the soil profile the system should be located.

It has been demonstrated that soil is very effective at treating septic tank or aerobic system effluent. The key is having enough soil between where the effluent enters the soil through the system and any type of limiting layer that interrupts the flow and treatment processes. Limiting soil layers include the presence of seasonally saturated zones, dense soil layers, any significant change in texture, and presence of bedrock, either creviced or hard.

Where I worked, the magic number for separation distance was 3 feet. Bear in mind, this 3 feet must be unsaturated and well aerated to provide treatment. If there is a limiting layer exactly 3 feet beneath the infiltrative surface, you should recognize when effluent is added to the soil daily there will probably be less than 3 feet of separation because the water will build up above this layer before it moves sideways or downward.

The bottom line is that establishing the depth to a limiting layer is key, and staying above it is critical to long-term operation and performance. If there is not adequate separation, management will need to be different than if there is separation. Management changes might include installing a media filter or aerobic tank rather than a septic tank so effluent is cleaner. Even this might not be enough to allow the system to function hydraulically, depending on the actual separation distances and soil permeability.

This leads to the second major impact of soil on the type and location of the treatment unit: the soil-sizing factor, which coupled with estimated sewage flow determines the size of the system. Soil characteristics involved in this assessment include texture, structure and consistence. If the site evaluator, designer or installer is not well versed in these characteristics it is easy to select the wrong sizing factor, resulting in the wrong-size soil treatment unit. If this type of mistake is made, management would have to incorporate flow reduction from the residence, or replace or enlarge the system. Either would be very costly to the homeowner.

Mistakes here can negate a lot of good operation and maintenance procedures. Mistakes in soil identification in general are very hard to fix and almost impossible to manage.

About the author 
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program and is an emeritus professor in the university’s Department of Soil Water and Climate. Send him questions about septic system maintenance and operation by email to


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.