For any given site, the soil loading rate is a function of the soil texture, structure and consistence; wastewater characteristics; and site characteristics. Permeability is related to all the soil characteristics. The SLR is the variable that quantifies the area needed to both accept and treat effluent. In general, finer-textured soils cannot accept as much effluent as coarser-textured soils, but on the other hand, finer soil is more effective in treatment. Soils with better-developed structure can accept more effluent than massive or weak grade structured soils.
The goal is to design a soil treatment area that can provide the needed treatment and acceptance of the effluent from the septic tank or other advanced treatment system. The shallower and narrower we design and install the system, the more oxygen will be available.
In addition, sites with more separation before a water table will have increased oxygen availability. An STA with even distribution and a rest cycle (pump or siphon) will also allow the soil to re-aerate between doses.
The loading rates found in most state or local codes assume that the soil is “original.” Original soil means naturally occurring soil that has not been cut, filled, moved, smeared, compacted, altered or manipulated to the degree that the key soil properties have been impacted. The following is a discussion of the key soil and wastewater characteristics that affect STA design.
- Soil texture is the quantity of various inorganic particle sizes present. The inorganic particles are grouped together into sand-, silt- and clay-sized particles. Soil texture influences how fast water moves into and through the soil. A soil texture analysis is required to estimate the size of the STA and is typically done with the “feel method.” Coarser-textured soils are the right size for treatment of septic tank effluent while fine-textured soils are sized for hydraulic acceptance.
- Soil structure refers to the arrangement of soil separates into units called soil aggregates, consisting of solids and pore space. Natural soil structures are formed by wetting and drying, freezing and thawing, microbial activity that aids in the decay of organic matter, activity of roots and soil animals, and adsorbed cations. Good soil structure shape and grade will provide for more rapid acceptance of septic tank effluent and require less area.
- Soil consistence is the degree of cohesion and adhesion or resistance to deformation upon rupture of a given soil. In the field, resistance of the soil structure to rupture is used to determine consistence. The level of cementing of the soil has an influence on movement of liquids through soil. In coarse-textured soils, cementation is less desirable because liquids can move readily in these porous conditions. With a fine-textured soil, a moderate level of cementation results in optimal soil conditions for liquid movement.
The soil texture, structure and consistence are best described by a soil pit or other method, as long as undisturbed structure can be observed. Several soil observations are commonly conducted across the proposed area to adequately assess soil variations. The depth of excavation is to the depth of the periodically saturated layer, bedrock or other limiting condition, or typically 2 to 4 feet below the proposed depth of the system.
- Permeability is the rate of water movement through a saturated soil in inches per hour. The percolation test measures only the rate of the drop of water in a test hole of a specific diameter and does not measure the rate of movement of water through the soil. However, the relative values obtained by the percolation test will give some idea of the ability of soil to transmit water. A very slow permeability also indicates that a soil is relatively high in fine material such as silt and clay, and thus may need extreme care during the installation of the STA. Slowly permeable layers occur in soils due to many geologic or soil-forming events. They may be layers cemented by translocation and deposition of iron, calcium or clay. Dense layers can be formed by the weight of glacial ice over soil parent material or by heavy construction equipment. There are other hydraulic tests that can be conducted, such as a constant head permeameter (the Amoozemeter is one type). Percolation tests are also useful in providing a better understanding of site impacts (e.g. compaction, fill, etc.). The percolation rates should be compared to the soil texture to verify the SLR. The most conservative SLR (largest number) should be used from the combination of both techniques.
- The linear or contour loading rate is the potential horizontal and vertical flow pattern in the soil. On many sites all the effluent does not travel straight down, but travels laterally. Lower CLRs should be chosen for fine-textured soils or near surface soil; limiting conditions exist. Typical CLRs range from 2 to 12 gallons per foot. The 2-gallon per foot minimum accounts for nearly all-horizontal flow of effluent. This minimum should be used for a system limited by impermeable bedrock or very heavy clay soils, or in any situation where horizontal movement of contaminants is a concern. The 12-gallon per foot loading rate (the maximum) would be used when water moves down through the soil much faster than it moves sideways, as in a sandy soil profile. Design values should be somewhere between these two. For a “typical” soil horizon made up of a variety of soil textures, a CLR of 4 to 12 gallons per foot is typically used. Using the soil texture, structure and percolation rate, if available, a CLR should be chosen based on the most limiting condition. Sites with steep slopes may also want to consider lower CLRs, as horizontal movement is more likely.
If the sewage is from domestic sources, such as dwellings, the hydraulic soil loading found in your local code will be used for septic tank effluent. Some codes allow for a reduction in the area required following a secondary pretreatment system. However, with commercial properties you will need to consider the organic loading rate. This can be done by installing a pretreatment system to reduce the effluent to the STA down to domestic-strength levels, or size the system using organic loading rates. The details this design option can be found here.
About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to firstname.lastname@example.org.