NC State Extension

Designing Large Septic Systems

Aziz Amoozegar,
Soil Science Department,
Williams Hall, PO Box 7619,
Raleigh, NC 27695-7619,
(919) 515-3967

Kevin C. Martin,
Soils and Environmental Consultants, Inc.,
Raleigh, NC

Approximately 50% of the housing units in North Carolina use on-site wastewater treatment/disposal system. In rural areas and around large municipalities with no access to a public sewer system, small communities, businesses, schools, recreational, and other facilities with flow rates exceeding 1,000 gallons per day also use septic systems. In principal, the designer of a septic system for an individual residential dwelling or small business uses the pertinent soil and site information provided by a soil scientist to select an area suitable for drainfield within the boundary of the respective property. The soil scientist selects the type of septic system and determines the appropriate loading rate for the system based on the type of the soil in the drainfield area. Although one the of the objectives of the designer of any septic system is to avoid surfacing of untreated effluent in the disposal area, no specific evaluation of water or wastewater flow from the system through the soil in and around the disposal area is usually performed. For large systems, a large volume of wastewater (e.g., 5,000 gallons) may be applied to the drainfield. Depending on the type of the soil and septic system under consideration, the amount of wastewater applied to the drainfield area of a large septic system may be significantly larger than the annual precipitation over the area. Therefore, the traditional evaluation of soil and site for determining site suitability and designing a septic system does not seem to be appropriate for designing large systems, For these systems, a hydrologic analysis, including water flow from the trenches of drainlines, vertical water flow below the disposal lines, and water flow away from the drainfield area, is perhaps the best method to assure the success of the system.

The design of a large septic system is an iterative and multi-stage process. We recommend the following steps for designing large septic systems.

  1. Conduct a preliminary soil and site evaluation to determine the potential areas that can be used for the drainfield area (or subfields). This evaluation should address the requirements set by the respective regulations.
  2. Select a type of septic system (e.g., conventional, low-pressure pipe) and determine a design loading rate for it based on the type of soil using the guidelines given in the regulations.
  3. Prepare or obtain a topographic map of the site showing the contour lines, and lay the design of the system on the map. The map should show the positions of the drainlines, boundary of the drainfield area, boundary of the suitable area for a drainfield, and any natural or man-made drainage system on the property.
  4. Conduct a comprehensive soil characterization within an area covering the entire drainfield (or subfields). Use a few (3 to 6) observation pits inside and outside the drainfield area, hand-auger borings throughout the area, and perhaps deep borings at selected locations to determine soil properties such as texture and color, and depth to an impermeable or water table below the drainfield area. Measure saturated hydraulic conductivity (Ksat) of various horizons/depths in the unsaturated zone inside and immediately outside the drainfield area. Based on the measured Ksat values, determine the depth and thickness of the most hydraulically restrictive layer. If a water table exists below the drainfield area, measure the Ksat of the aquifer and determine or estimate the thickness of the aquifer, and the direction of ground water flow.
  5. Use the comprehensive evaluation of the soil in the drainfield area to determine if a septic system can be installed within the regulations. If you determine that the system will not function properly as designed, or if there are problems causing future failure of the system, then the site should be considered unsuitable. Subsequent to this, determine if modifications to the proposed system (e.g., changing the type and/or configuration of the system) or to the soil as permitted by regulations can be used to make the site provisionally suitable for the type of septic system under consideration.
  6. If you consider the site suitable for a septic system, then conduct a hydrologic analysis for the systems. First determine if the volume of wastewater that will be applied daily to the trenches (or the drip lines for drip systems) can infiltrate the soil within a 24-hour period. For this analysis, determine the loading rate based on the trench bottom area. Assume a porosity for the gravel in the trenches (approximately 30%) and calculate the average depth of wastewater in the trenches assuming that all the wastewater is applied to the trenches at once. Determine the maximum and minimum surface areas of infiltration to obtain an average surface area of infiltration. Calculate the amount of wastewater that can infiltrate the soil through this average surface area using Darcy’s equation with the minimum Ksat values measured for the horizon (or depth) near the bottom of the trenches and a unit hydraulic gradient. From the above calculation, determine the length of time required for the volume of wastewater to be applied to the trenches to infiltrate the soil. If the volume of wastewater that will be applied to the trenches daily does not infiltrate the soil in less than 24 hours, then the loading rate is too high or the trenches are too small. Determine if modifications can be made to allow complete infiltration of all the wastewater that will be applied to the trenches daily. A site may be rejected if the calculations show that continuous wastewater ponding will occur in the trenches. The second part of the analysis is to determine if wastewater can move vertically down through the unsaturated zone before reaching an impermeable layer, a slowly permeable layer, or a water table. For this, compare the areal loading rate of wastewater with Ksat of the most restrictive layer in the unsaturated zone. To accept the site, the Ksat of the most hydraulically restrictive layer should exceed the areal loading rate by a factor to be determined by the regulatory agency permitting the system. Finally, water flow away form the drainfield area must be assessed.

    The hydrologic analysis for water flow away from the drainfield area for a site with shallow water table differs from the evaluation for a site where an impermeable or slowly permeable layer exists in the unsaturated zone. For a water table below the drainfield area, a ground water mounding analysis can be performed. There are a number of mathematical models and computer programs that can be used to assess water flow through an aquifer under a septic system (or another source of water) system. Based on the ground water mounding analysis, determine if the rise in water table below the drainfield reduces the required unsaturated space below the bottom of the trenches. If no water table exists, then determine the lateral water flow over the slowly permeable layer and the potential vertical water flow through it. For lateral flow, use Darcy’s equation with measured Ksat data and the slope of the perched water table to determine the volume of water that can pass through one or more side of the drainfield such that the highest level of the perched water table does not reach the minimum required unsaturated distance below the trenches. For vertical flow, use a unit hydraulic gradient and the measured or estimated Ksat of the slowly permeable layer to assess the potential vertical flow of water through it.

  7. Based on the hydrologic analysis, the site can be accepted or rejected as proposed, or modifications can be required to assure proper functioning of the system. The analysis described above should be repeated every time the design parameters are changed.
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