System Site Plans: Soil Evaluation, Loading Determination and Treatment Train Components

Identifying critical information during the review and bidding process can help avoid problems and make for a smooth installation

System Site Plans: Soil Evaluation, Loading Determination and Treatment Train Components

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Soil and site evaluation report

Design plans might come with a soil and site evaluation report attached to the plans. The installer should have at least minimum knowledge of soil colors and texture to differentiate between topsoil, subsoil and parent materials. Additionally, the installer should be able to differentiate between fine-textured soils (clays and silts), medium-textured soils (sandy loams, silt loams), and coarse-textured soils (loamy sand, sand). 

Soil morphology reports help the installer verify during installation that the bottom of the soil treatment area is in the soil horizon specified in the design. The soil in the soil treatment area, particularly at the infiltrative surface, is significant to the treatment and acceptance capabilities of system. If the actual soil in the field varies from what is shown in the soil evaluation, it is likely the system will not function as intended. With experience and continuing education, installers can gain knowledge on where the seasonal high-water table resides in a soil profile. 

Another important aspect of the soil and site report to consider is the hydrology of the site. It is important for the installer to understand how the groundwater moves within the site and if the design has elements that are going to modify or impede groundwater movement. For example, a site may have mainly clay soil and a shallow groundwater table. If a driveway is located down-gradient from the groundwater movement, traffic from the cars is going to compact the soil and slow down or impede groundwater flow. In this case, groundwater will rise up-gradient from the driveway and the soil treatment area may be compromised, potentially even to the point that effluent could come to the soil surface. In-ground pools or retaining walls are other examples of common structures that may impede groundwater flow.

Hydraulic and organic loading

The design hydraulic load to the onsite wastewater treatment system is an important parameter that can be found on the design plan. The hydraulic loading rate is the amount of wastewater per day that the system is designed to accept, typically expressed in gallons per day. It can be a prescribed number of gallons per bedroom per residence, flow per capita, a value based on average flow for a similar facility or a number derived specifically for the facility. The installer should be familiar with these numbers. 

The peak flow may also be specified in the design plans. The peak flow is the highest occurring flow for a given period of time. The peak flow is typically site specific. For example, a church might have a design flow of 100 gallons per day but a peak flow of 400 gallons per day on Sundays. If a peak flow is given on the plan it should be documented as well. 

Not only do all onsite wastewater treatment systems have a hydraulic loading capacity, but they also have limitations as far as organic loading. Ideally the design will indicate the organic loading rate that the system will be able to handle, particularly if the system is serving a facility other than a residence. Residential systems are typically assumed to have a biochemical oxygen demand (BOD) and total suspended solids (TSS) coming out of a septic tank of 170 mg/L and 60 mg/L, respectively. If concentrations of either constituent are considered beyond typical residential strength wastewater, special considerations in the design need to be in place for higher strength wastewater. (For more on understanding high-strength wastewater, you can watch this webinar.) The important thing to keep in mind is that a system that is not designed to handle the organic load will eventually malfunction when consistently loaded with high-strength waste.  

Treatment train components

The design plan should include all details of the onsite wastewater treatment system. If the design is for innovative and alternative technologies, even greater attention must be given to the treatment train. If the installer understands the basics of each technology, they can determine if the system is set up appropriately and whether the components complement each other to meet the treatment goal. If the plan is older or the designer specifies obsolete technologies, the installer should check if the technologies called for in the design plan are available. 

There are many things to consider when evaluating the treatment train on the design. Component specification should include details such as dimensions, capacity and location. In addition, it is essential that all components in the treatment train be maintainable. A component should never be designed or installed in a manner that it cannot be maintained.  

One of the most important aspects of treatment train evaluation to the installer is whether the system is constructible as designed. There are many reasons a system’s constructability might be limited. For example, the components specified may not be locally available. This could be the case for sand used for single pass, recirculating, mound or bottomless sand filters. Such sand is an engineered media that may not be available in the area where the system is to be built and importing the material may not be economical. Constructability might also be limited by topography or existing structures that restrict access by the necessary equipment. On steep slopes, fill may be required, or structural walls may need to be built to contain the soil treatment area. 

A design is full of useful information when evaluating and bidding a job. Identifying some of these critical items during the review and bidding process can help avoid problems and make for a smooth installation. 

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to

This article is part of a series on site planning:


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