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Public Health - Seattle & King County

Types of septic systems

The common types of septic systems are gravity, pressure distribution, sand filter, and mound systems.

Gravity drainfields

As the name implies gravity drainfields work by letting gravity drain the effluent from the septic tank into a series of trenches. This means that a gravity drainfield area must be below the draining level of the septic tank. If this is not the case then a pump tank is necessary and it is called a pump to gravity system.

In conventional gravity systems the drainfield consists of a network 4 inch diameter perforated pipes laid in gravel-filled trenches (2-3 feet wide) in natural undisturbed soil. The bottom of the trench needs to be 3 feet above any restrictive layer, such as a hardpan, or water table. The soil between the bottom of the trench and the hardpan or water table is used in the final treatment of the septic tank effluent.

The soil below the drainlines filters effluent as it passes through the pore spaces. Chemical and biological processes treat the effluent as it percolates down through the soil. The treatment process cleans the effluent before it reaches the groundwater. This works best when the soil is somewhat dry, permeable, contains adequate amounts of oxygen and there is enough soil depth to complete the cleaning process.

The size of the drainfield depends on the estimated daily wastewater flow and soil conditions. The number of bedrooms and soil type determines the total number of square feet of drainfield area that is needed.

Pressure distribution drainfields

Pressure distribution systems are usually installed when there is less than optimal soil depth available for complete treatment of the effluent by a gravity system. Pressure distribution systems always have a pump and therefore they dose the drainfield with effluent and then let it rest until the pump tank accumulates enough effluent from the household for another dose. In addition a series of pressurized lines from the pump tank to the drainfield make sure the entire drainfield receives effluent at the same time. For more information, read the brochure Understanding and Caring for Your Septic Tank System (from the Washington State Dept. of Health.) Hard copies are available at the Eastgate Environmental Health office.

A pressure distribution system

Sand filter systems

When there is minimal soil available for treatment, a sand filter system is sometimes used. This consists of a sand containment vessel between the pump tank and the pressurized drainfield. The sand acts to treat the effluent before it enters the shallow soils on site. This effectively makes up for the lack of soil depth. The sand filter itself is a concrete or PVC-lined box filled with a specific sand material. A network of pressurized lines is placed in a gravel-filled bed on top of the sand. The septic tank effluent is pumped through the pipes in controlled doses to insure uniform distribution. As the effluent trickles down through the sand it is treated. A gravel underdrain collects and moves the treated wastewater to either a second pump chamber for discharge into a pressurized drainfield or the filter can sometimes drain into a gravity flow drainfield. The second pump chamber is commonly located within the sand filter. For more information, read the brochure Understanding and Caring For Your Septic Tank System (from the Washington State Dept. of Health.) Hard copies are available at the Eastgate Environmental Health office.

Sand filter anatomy
Sand filter anatomy

Sand filter on property
Sand filter on property

Sand filter system on property
Sand filter system on property

Mound systems

Another system that can be used when a site has inadequate soil depth is a mound. A mound is a drainfield raised above the natural soil surface with a specific sand fill material. Within the sand fill is a gravel bed with a network of pressurized pipes. Septic tank effluent is pumped through the pipes in controlled doses to insure uniform distribution throughout the bed. Treatment of the effluent occurs as it moves through the sand and into the natural soil.

Mound system on property
Mound system on property

Mound system
Mound system

Sub-surface mound system
Sub-surface mound system

Drainage around the mound site is critical if the system is to function properly. On sloping sites the downslope area below the mound must remain protected.

Technical review of these systems and others
  1. Gravity
    Consists of a septic tank which flows effluent by gravity to a distribution box (D-box); one or more connections to this D-box drain into trenches typically filled with gravel. Only a small part of the drainfield is used at any one time. Requires a minimum of 48 inches of permeable soil above a restrictive layer to be used on new construction. Least amount of maintenance required of any system type but requires the greatest soil depth in order to provide adequate treatment of the effluent.

  2. Gravity with Pump
    Gravity with the addition of a pump tank. Allows the drainfield to be located upslope from the septic tank. In addition, the drainfield is dosed which permits intermittent resting of the drainfield between doses. Additional maintenance to the pump tank and float switches are necessary.

  3. Pressure Distribution
    Consists of a septic tank and pump tank with pressurized lines leading to individual trenches. This allows dosing of the drainfield and use of the entire drainfield at once. Requires at least 30-36 inches of permeable soil above a restrictive layer to be used on new construction. Maintenance required to assure the orifices do not plug over time.

  4. Mound
    A treatment-based system consisting of pressurized lines lying in a sand bed mounded above the original soil surface. Requires at least 18 inches of permeable soil above a restrictive layer to be used on new construction. This system type has allowed construction on sites previously thought unsuitable due to lack of soil depth. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  5. Subsurface Mound
    A treatment-based system consisting of pressurized lines lying in a sand bed mounded above the original soil surface. Requires at least 18 inches of permeable soil above a restrictive layer to be used on new construction. This system type has allowed construction on sites previously thought unsuitable due to lack of soil depth. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  6. Sand Filter - Pressure Distribution
    Consists of sand placed in a watertight box built into the soil. Effluent is spread evenly over the surface of the sand via a pressurized network. The sand layer treats the effluent and is collected in the bottom of the filter box from which it is then pumped to a drainfield. The drainfield in this case is a pressure distribution system, which finishes the treatment process and then disposes of the wastewater. Requires at least 18 inches of permeable soil above a restrictive layer to be used on new construction. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  7. Sand Filter - Gravity
    Consists of sand placed in a watertight box built into the soil. Effluent is spread evenly over the surface of the sand via a pressurized network. The sand layer treats the effluent and is collected in the bottom of the filter box from which it then is pumped or gravity fed to a gravity drainfield. Requires at least 24 inches of permeable soil above a restrictive layer to be used on new construction. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  8. Sand Filter - Mound
    Consists of sand placed in a watertight box built into the soil. Effluent is spread evenly over the surface of the sand via a pressurized network. The sand layer treats the effluent and is collected in the bottom of the filter box from which it then is pumped to a mound. The mound finishes the treatment process and then disposes of the treated wastewater. Used in situations where minimal soil exists, at least 12 inches of soil is required on at least 5 acres. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  9. Sand Filter - Subsurface Mound
    Consists of sand placed in a watertight box built into the soil. Effluent is spread evenly over the surface of the sand via a pressurized network. The sand layer treats the effluent and is collected in the bottom of the filter box from which it then is pumped to a subsurface mound. The mound finishes the treatment process and then disposes of the treated wastewater. Used in situations where the soil condition does not permit adequate treatment of the wastewater before disposal. The addition of the sand filter may be used to help mitigate other shortcomings of a particular site. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  10. Sand Lined Bed
    A treatment-based system consisting of pressurized lines lying in a sand bed buried into the ground. The sizing of drainfield laterals is equal to a standard system. Used in situations where the soil is deep but very porous thus lacking treatment capability. This occurs in areas where soils are very gravelly or extremely gravelly in nature. The system is easier to construct than sand lined trenches but takes up more area than a subsurface mound. It may be the system of choice where there are soil inconsistencies such that a smaller subsurface mound may not have sufficient contact with the more gravelly soil portions of the site. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  11. Sand Lined Trench
    A treatment-based system consisting of pressurized lines lying in sand filled trenches. The sizing of drainfield laterals is equal to a standard system. Used in situations where the soil is deep but very porous thus lacking treatment capability. This occurs in areas where soils are very gravelly or extremely gravelly in nature. Can be used as an alternative to a mound or sand filter system. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  12. Sand Filter - Lined Bed
    Consists of a sand filter followed by a sand lined bed system. The addition of the sand filter may be used to help mitigate other shortcomings of a particular site. See #10 for more details.

  13. Sand Filter - Lined Trench
    Consists of a sand filter followed by a sand lined trench system. The addition of the sand filter may be used to help mitigate other shortcomings of a particular site. See #11 for more details.

  14. Upflow Sand Filter (Glendon Biofilter)
    Consists of different layers of sand and gravel placed in a watertight box built into the soil. Effluent is pumped into the bottom of the filter and allowed to wick itself up through the sand and over the rim of the box into the soil. Several boxes or pods may be used to accommodate varying site conditions and number of bedrooms. A splitter along with a timing device is used to assure even flow to all pods. This system can be used in situations where a lot has 18 inches of soil and only12 inches if 5 acres or greater. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  15. Aerobic Treatment Unit (ATU) – Gravity
    Consists of a watertight tank with an aeration chamber where sewage and microorganisms come in contact with each other in the presence of dissolved oxygen. Blowers, compressors or air pumps supply the air. The treated effluent is then drained into a conventional gravity drainfield. To meet the highest treatment standards a disinfection unit must be part of the device to reduce the bacteriological counts. With disinfection the system could be used on a site with only 24 inches of soil. Without disinfection the system would require at least 4 feet of soil. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  16. ATU - Mound
    Consists of a watertight tank with an aeration chamber where sewage and microorganisms come in contact with each other in the presence of dissolved oxygen. Blowers, compressors or air pumps supply the air. The treated effluent is then pumped into a mound system for final treatment and disposal. This system works well in combination as the ATU lowers waste strength but still produces high bacteriological counts, which are eliminated in the mound. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  17. ATU - Pressure Distribution
    Consists of a watertight tank with an aeration chamber where sewage and microorganisms come in contact with each other in the presence of dissolved oxygen. Blowers, compressors or air pumps supply the air. The treated effluent is then pumped into a pressure distribution system for final treatment and disposal. To meet the highest treatment standards a disinfection unit must be part of the device to reduce the bacteriological counts. With disinfection the system may be used with as little as 18 inches of soil. Without disinfection the system requires no less than 30 inches of soil. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  18. ATU - Sand Filter
    Consists of a watertight tank with an aeration chamber where sewage and microorganisms come in contact with each other in the presence of dissolved oxygen. Blowers, compressors or air pumps supply the air. The treated effluent is then pumped into a sand filter for further treatment. A pressure distribution system would then dispose of the treated wastewater. To meet the highest treatment standards a disinfection unit must be part of the device to reduce the bacteriological counts. With disinfection the system may be used with as little as 18 inches of soil. Without disinfection the system requires no less than 30 inches of soil. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.

  19. Composting Toilet
    Consists of a self-contained toilet with a chamber and venting system. The chamber contains sawdust or some other composting media, which when combines with the waste material to form compost over time. There is usually some method to turn the pile to assure an even mixture and complete composting. Once composting is complete the residue is removed manually from the chamber. These systems require a separate grey water discharge and disposal system. Most commonly used where water availability is an issue. The long-term proper operation of composting toilets depends on regular maintenance by the owner.

  20. Holding Tank
    Self-contained watertight wastewater tank with a high water alarm. Must be routinely pumped to prevent overflows or back-ups into the house. Most commonly used as a temporary measure to allow continued occupancy of a house until a more permanent fix can be arranged. Some schools use this system when sewers are not available and no suitable site exists for an on-site sewage system. Requires posting a bond for potential spills and being on contract with an approved sludge hauler. Management oversight is needed to prevent sewage overflows.