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Four Tips for Designing a Drainfield for a Residential Septic System

Posted on by Cameron King

So, you have a soils report in front of you and are looking to draft up a design for a residential septic drainfield. To do so, there are some key bits of information in that report that will be important in the design process. Whether you are a novice or a pro, here is a handy checklist outlining the design process and explaining each system component.

Daily Design Flow: Daily design flow, not to be confused with actual flow, is measured in gallons per day (GPD). In a residential application, this flow is based on the number of bedrooms in the home and varies by local health department. Daily design flow incorporates a safety factor to prevent a system from being undersized and reduces the risk of overloading the drainfield.

Seasonal High Water Table (SHWT)/Restrictive Layer: Before we look at our available soils, we need to know where the water table is. The bottom of the system will need to maintain a minimum vertical separation (MVS) from the SHWT or restrictive layer. The reason for this is that most of the treatment occurs in the soil beneath the drainfield. Maintaining this separation allows adequate time for that treatment before re-entering groundwater. The MVS can be found in your local code.

Soil Infiltration Loading Rate (SILR): Now that we know the depth of our system, we can select the soil that will be underneath it. Select the soil based on the MVS and find the corresponding loading rate in the local code or product manufacturer’s design manual. The loading rate, measured in GPD per square foot, describes the effluent volume that passes through a set area over the course of a day. A smaller SILR will result in a larger required area, because it will take longer for the effluent to infiltrate the soil.

Site Restrictions: In some cases, restrictions like property lines, structures, roadways, wells, or bodies of water may affect the dimensions of your system. Keep these in mind and adjust your plan accordingly.

Having found the daily design flow, SHWT or restrictive layer, and the SILR in the soils report, as well as any specific site restrictions, you now have the basic information required to draft up a drainfield design for your septic system. The final step requires some quick and painless math. Simply divide the daily design flow by the SILR and be left with the minimum area required for the drainfield.  

View State-Specific Design Manuals for Infiltrator Products

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About the Author

Cameron King, CAD Draftsman 

Cameron joined Infiltrator Water Technologies in 2019 as a CAD Draftsman after graduating with his B.A. in Mathematics from the University of Delaware. He spends his time at Infiltrator working on decentralized wastewater system designs, CAD schematics, and research and development initiatives.