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  • Writer's pictureJustin Tahilramani

The "so what" about Soil Types and Septic Systems




In this post, I discuss the concepts of soil types and how they impact the feasibility of on-site septic systems. I will also cover the most common types of residential septic systems that are currently being utilized throughout the United States.


Soil Types and Septic Systems


Soil is an essential component of any septic system, as it provides the final treatment and disposal of the wastewater that flows out of the septic tank. Soil can remove contaminants such as microorganisms, organic matter, and nutrients from the wastewater through natural processes such as filtration, adsorption, and biological degradation. However, not all soils are suitable for septic systems, as they may vary in their texture, structure, depth, and permeability.


Soil texture refers to the relative proportion of sand, silt, and clay particles in the soil. Sand particles are the largest and coarsest, while clay particles are the smallest and finest. Silt particles are intermediate in size and shape. The texture of the soil affects how fast or slow water moves through it. Sandy soils have large pores that allow water to drain rapidly, while clayey soils have small pores that restrict water movement. Loamy soils are mixtures of sand, silt, and clay that have moderate drainage properties.

Soil structure refers to how the soil particles are arranged and held together to form aggregates or clumps. Soil structure affects how easily water can enter and move through the soil. Well-structured soils have stable aggregates that create pore spaces for water infiltration and aeration. Poorly structured soils have weak or no aggregates that result in compaction and reduced porosity.


Soil depth refers to the distance from the soil surface to a limiting layer that prevents wastewater treatment or dispersal. A limiting layer could be bedrock, gravel, hardpan, or water table. Soil depth determines the type of septic system that can be used on a property. The deeper the soil, the more suitable it is for conventional septic systems that rely on gravity to distribute wastewater to a drainfield. Shallow soils may require alternative septic systems that use pumps or advanced technologies to treat and disperse wastewater.

Soil permeability refers to the rate at which water moves through the soil under a given hydraulic gradient. Soil permeability is influenced by both soil texture and structure, as well as soil moisture and organic matter content. Soil permeability is measured in inches per hour or centimeters per hour and is used to estimate the loading rate or application rate of wastewater to the soil. The loading rate is the amount of wastewater applied per unit area of soil per unit time. The loading rate affects how long wastewater stays in contact with the soil and how much treatment it receives before reaching groundwater or surface water.


For purposes of septic system design, soils are classified into four broad categories according to their texture:


• Group I – Sandy Textured Soils: These soils have high permeability and low water-holding capacity. They drain rapidly and provide little treatment of wastewater. They are suitable for low-pressure pipe or drip distribution systems that apply small doses of wastewater at frequent intervals.


• Group II – Coarse Loamy Textured Soils: These soils have moderate permeability and moderate water-holding capacity. They drain well and provide adequate treatment of wastewater. They are suitable for conventional gravity or pressure distribution systems that apply larger doses of wastewater at longer intervals.


• Group III- Fine Loamy Textured Soils: These soils have low permeability and high water-holding capacity. They drain slowly and provide high treatment of wastewater. They are suitable for mound systems or aerobic treatment units that pre-treat wastewater before applying it to the soil.


• Group IV – Clayey Textured Soils: These soils have very low permeability and very high water-holding capacity. They drain poorly and provide excessive treatment of wastewater. They are unsuitable for most septic systems unless they are amended with sand or gravel to improve drainage.


To determine the soil type and characteristics on a property, a soil evaluation or site assessment is required by a qualified professional such as a soil scientist, engineer, or sanitarian. The soil evaluation involves digging test pits or boring holes in different locations on the property to observe and describe the soil profile, including its color, texture, structure, depth, permeability, and presence of limiting layers or seasonal water tables. The soil evaluation also involves conducting percolation tests or hydraulic conductivity tests to measure how fast water moves through the soil under controlled conditions. The results of the soil evaluation are used to select the most appropriate type and size of septic system for the property.


Types of Residential Septic Systems


A septic system is a decentralized wastewater treatment system that consists of two main components: a septic tank and a soil absorption system (SAS) or drainfield. A septic tank is a buried, watertight container that receives and partially treats raw domestic wastewater from the house. The septic tank separates the solids from the liquids and allows the solids to settle to the bottom as sludge and the lighter materials to float to the top as scum. The septic tank also provides anaerobic digestion of the organic matter in the wastewater, reducing its volume and odor. The septic tank should be pumped out regularly to remove the accumulated sludge and scum and prevent clogging of the outlet pipe.


A soil absorption system or drainfield is a network of pipes or chambers that distribute the wastewater from the septic tank to the soil for final treatment and disposal. The wastewater infiltrates into the soil and percolates downward until it reaches groundwater or surface water. The soil acts as a biological filter that removes pathogens, organic matter, and nutrients from the wastewater through physical, chemical, and biological processes. The type and design of the soil absorption system depends on the soil type, site conditions, and local regulations.


There are many different types of septic systems that can be classified by different criteria, such as the location and soil conditions of the property, the level of treatment and disinfection of the wastewater, the use of oxygen and bacteria to break down the sewage, or the size and design of the system. Some common types of septic systems are gravity, pressure distribution, aerobic, anaerobic, conventional, alternative, and advanced. Each type has its own advantages and disadvantages, and the choice of the most appropriate system depends on the specific situation of the property.


Below are some of the most common types of residential septic systems used in the United States:


• Gravity System: This is a simple and low-cost system that relies on gravity to transport wastewater from the septic tank to a trench or bed subsurface wastewater infiltration system (SWIS), also known as a drainfield. The SWIS consists of perforated pipes laid in gravel-filled trenches or beds that allow wastewater to drain into the surrounding soil. The SWIS is usually located downhill from the septic tank and requires a minimum slope of 2% to ensure adequate flow. The SWIS should also be located at least 3 feet above any limiting layer or water table to provide sufficient unsaturated soil for treatment. A gravity system is suitable for properties with deep, well-drained soils and enough space for a large SWIS.


• Pressure Distribution System: This is a more complex and expensive system that uses a pump or siphon to distribute wastewater evenly throughout a SWIS. The pump or siphon is located in a dosing chamber or pump tank after the septic tank and is controlled by a timer or float switch. The dosing chamber or pump tank delivers small doses of wastewater at regular intervals to a network of small-diameter pipes with small holes that spray wastewater over a large area of soil. The SWIS can be located uphill or downhill from the septic tank and does not require a slope. A pressure distribution system is suitable for properties with shallow, poorly drained, or variable soils that need more uniform application of wastewater.


• Aerobic Treatment Unit (ATU): This is an advanced system that uses oxygen and aerobic bacteria to treat wastewater before it enters a SWIS. An ATU is a prefabricated device that consists of an aeration chamber, a clarifier chamber, and a disinfection chamber. The aeration chamber provides air to mix and aerate the wastewater, enhancing the breakdown of organic matter and nutrients by aerobic bacteria. The clarifier chamber allows the solids to settle out from the treated wastewater and return them to the aeration chamber for further digestion. The disinfection chamber uses chlorine or ultraviolet light to kill any remaining pathogens in the treated wastewater. An ATU can reduce biochemical oxygen demand (BOD), total suspended solids (TSS), nitrogen, phosphorus, and fecal coliforms by more than 90%. An ATU requires less space for a SWIS than a conventional system, as it can use smaller pipes with smaller holes and less gravel. An ATU also requires less depth of unsaturated soil for treatment than a conventional system, as it can be located closer to a limiting layer or water table. An ATU is suitable for properties with limited space or poor soils that need higher levels of treatment.


• Mound System: This is an alternative system that uses an elevated mound of sand and gravel to treat and disperse wastewater from a septic tank or an ATU. A mound system consists of a dosing chamber or pump tank that delivers small doses of wastewater at regular intervals to a network of small-diameter pipes laid on top of a layer of sand. The sand layer provides additional filtration and treatment of wastewater before it drains into a layer of gravel below. The gravel layer provides storage and dispersal of wastewater into the underlying native soil. A mound system is built above ground level using imported sand and gravel materials. A mound system requires a large area of land and a lot of sand and gravel to construct. A mound system is suitable for properties with shallow, rocky, or clayey soils that have a high water table or a limiting layer close to the surface.


• Drip Distribution System: This is an advanced system that uses a network of small-diameter tubes with tiny holes to apply wastewater slowly and uniformly to the soil. A drip distribution system consists of a septic tank or an ATU, a dosing chamber or pump tank, a filter, a pressure regulator, and a drip tubing network. The dosing chamber or pump tank delivers small doses of wastewater at frequent intervals to the filter, which removes any solids that could clog the drip tubes. The pressure regulator maintains a constant pressure in the drip tubing network, which is buried shallowly in the soil. The drip tubes emit wastewater at a low rate of 0.6 to 2.4 gallons per hour per emitter, allowing the soil to absorb and treat the wastewater without saturating it. A drip distribution system requires less space and depth for a SWIS than a conventional system, as it can use narrower trenches or beds with less gravel. A drip distribution system is suitable for properties with limited space or poor soils that need precise application of wastewater.


These are some of the most common types of residential septic systems that are currently being utilized throughout the United States. Each type has its own advantages and disadvantages, and the choice of the most appropriate system depends on the specific situation of the property. If you are interested in installing or upgrading your septic system, you should consult with a qualified professional who can perform a soil evaluation and recommend the best option for your needs.


I hope you found this blog post informative and helpful. If you have any questions or comments, please feel free to reach out to us at www.sandhillsREH.com. Thank you for reading!


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