Septic Drain Field Design and installation
In this article, we will discuss the top 10 considerations for drain field design and typical construction.
Table of Contents
Overview
Septic systems are a a key component for homes and buildings that are not connected to a municipal sewer system.
Most septic systems have three major sections, the septic tank, the distribution / piping and the drain field.
All these components are typically located in the ground.
The septic tank is required for the storing & breaking down wastewater, and the septic drain field is required to disperse the treated water back into the surrounding soil.
Septic drain fields, also known as leach fields or absorption fields, are a vital part of a septic system, designed to treat and disperse the effluent, or treated wastewater, that comes from the septic tank.
The goal of a drain field is to slowly release the treated water into the surrounding soil, where it can be naturally filtered and cleansed before entering the groundwater.
Septic Drain Field Design Considerations
When designing a septic system drain field, several factors must be considered.
1) The soil type / percolation
2) The number of occupants in the household
3) The amount of water being used
4) The perforated components in the trenches
Soil Type
The soil composition on the property is one of the most important factors to consider when designing a septic drain-field.
The soil has to absorb the waste water (effluent) and filter out any remaining contaminants before this biologically treated water enters the groundwater table.
The ideal soil type for a drain field is sandy loam or loamy sand, which allows for proper drainage and filtration.
Soils that are too clayey or too sandy are not suitable for a drain field.
Occupancy
The size of the drain field will be proportional to the load, and that is influenced by the number of occupants in the household.
Larger households require a larger drain field to treat the additional effluent that will be produced. As the family grows, more water is used by each individual. Teenagers use the most water, followed by the adults. The elderly and infants use the least water. If there are babies and toddlers in the household, make sure to NEVER put disposable diapers in the toilet.
Amount of Water Being Used
With more water moving through the system, a larger drain field will need to be used to distribute the treated water into to the ground.
Activities such as flushing the toilet, washing laundry, taking showers or baths, and running the dishwasher all contribute to the amount of wastewater being produced.
See our Water Use Calculator on the Septic Pumping services page.
Size of the Drain Field
A rule of thumb to size a conventional septic drainfield is 1 square foot of area per gallon of wastewater produced per day.
A household of four people producing a total of 400 gallons of water per day would require a drain field measuring at least 400 square feet. Note that occupancy based estimates like number of occupants and bedrooms are just a couple of the inputs that are required to generate a proper design, and should not be used alone.
Location, Location, Location
One of the first things to look for when purchasing a parcel of land is the suitability for how you want to use the property. If the use case will include a septic system, you need to know if the soil is going to “perc” OK. Perc is short for the ability of the soil to absorb water, or Percolate.
Most rural lots need both, an on-site septic and a source of good drinking water (like a well)… these are important considerations for the price of a building lot.
You may want to pull in a couple of pros to help you make an informed decision before making the property purchase. You may want to contact a Soil Scientist to help you gauge the soil quality in the area. A local Septic Installer may share some have first-hand information about the local soil conditions and may provide a ballpark estimate of the cost to install a typical system based on the local conditions.
Note: Water treatment systems can be expensive, that goes for drinking water treatment as well as the septic waste water treatment.
The least expensive septic design is the in-ground gravity system where the soil has an ideal percolation rate and enough level area to handle the absorption rate requirement with some margin.
Site Evaluation
The site evaluation cannot be done on paper, or aerial photographs or topographical maps or soil classification surveys. You have to go on-site to identify the most favorable place to put the septic and reserve areas because of the scale of the typical system and the tight specifications for the design.
- The area should be level and free from trees or other obstructions.
- There should be at least 100 feet to any well or body of water.
- The ground should not be rocky or too wet during seasonal rains.
There should be 10-15 feet from the dwelling and property lines. Be sure to check with the local authorities on the zoning and setbacks and clearances.
You may be required to provide a septic reserve area in addition to the main drain field area. This is to plan for the future in case there is a system failure or a need to relocate the major septic components.
The slope and contour of the area needs to be considered because surface water can cause the system to slow down since there is an increase in the water to be absorbed by the ground.
Percolation Testing
Percolation testing involves digging several test holes in the area where the drain field will be installed, and observing how quickly the soil absorbs water.
If the soil is not suitable to perc a drain field, a raised bed (mound) system or an alternative treatment systems may need to be considered. These systems can add tens of thousands of dollars to the cost of your project.
Once the soil has been tested and approved for a drain field, the design process and specification of materials and components can continue.
Component Installation
Proper construction and maintenance of the septic drain field is critical to ensuring that the system functions properly and efficiently.
Drain fields should be installed by a licensed professional that is familiar with the local regulations and codes.
Preventative Maintenance
Proper sizing, design and placement of the septic drain field is critical to system performance. Once installed, regular maintenance and inspection of the drain field are important to identify and address any issues before they become major problems.
Clogging
Clogging can cause the drain field to fail and can result in backups in the septic system. This can happen if the pipes or gravel become blocked with solids or if the soil becomes compacted.
Regular pumping of the septic tank can help prevent clogging by removing solids before they have a chance to enter the drain field.
Excessive traffic on the drain field can cause soil compaction, like hooved animals, driving heavy vehicles over the area etc.
Overloading
Another issue that can occur with septic drain fields is overloading. Overloading can cause the drain field to become saturated and can eventually lead to backups in the septic system.
Surface water can be a factor, especially during very rainy weather or in the case where there are sprinklers, poor contouring, settling, or a negative slope towards the drain field.
Overloading can also happen if the drain field is not large enough to handle the amount of wastewater being generated by the household. Having extra guests or excessive water consumption can cause the issue.
Summary
The design and construction of a septic system is a balancing act that requires careful consideration of many factors.
- Number of occupants
- Water usage habits
- Type of soil on your property
- Percolation rate of the soil
- Slope of the property
- Layout of the drain field
- Choice of gravel or plastic domes in the trenches
Proper construction and maintenance of the septic drain field are critical to preventing problems and having a system that will operate safely and effectively for many years to come.
Additional technical information for each state is available at the EPA website for onsite wastewater treatment products by state.
