OWTS/Septic System Types

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Types of Onsite Wastewater Treatment Systems (OWTS; also known as "Septic Systems")

 septic.example PD system (2) sandmoundEPA (2)


Why are there different types of Onsite Wastewater Treatment Systems (OWTS)?

OWTS (also known as "septic systems") treat domestic sewage/wastewater directly on site. The wastewater is eventually dispersed back to the land, and to protect groundwater it should be filtered and treated to a certain standard. There are project and site specific characteristics that will determine which OWTS will work best. These characteristics are identified during a process called a Site Evaluation. Based on these characteristics, there are two (2) main types of OWTS: a Standard System and an Alternative System.

There is also a third type of OWTS called a Non-discharging Wastewater Disposal Unit, which is a self-contained watertight container designed to hold wastewater until it is pumped and/or cleaned. However, these types are limited in use. For all types of OWTS and related requirements, please refer to the Yolo County OWTS Manual

Operation, Monitoring and Maintenance (OM&M) is necessary for all types of OWTS, even the simplest. Please see our OM&M webpage for more information.

What is the difference between a Standard OWTS and an Alternative OWTS?

  • STANDARD: This type of OWTS is the simplest, and is sometimes referred to as a "gravity system". It is comprised of a septic tank and a gravity-fed dispersal system which includes trenches installed in approved undisturbed native soil. Effluent from the tank will flow by gravity to a dispersal field (also known as "leach field") made up of rock-filled or chambered trenches, or may be pumped to the first distribution box of the dispersal field. 

Standard OWTS are designed by a licensed contractor or a Qualified Septic Designer; installed by a licensed contractor or a property owner; and ongoing operation, monitoring, and maintenance (OM&M) is the responsibility of the homeowner. Please click here for an OM&M handout.

  • ALTERNATIVE: This type of OWTS is necessary when the site and/or project will not allow for a standard OWTS. It consists of system components that provide higher quality effluent and/or a more efficient method of wastewater dispersal that results in enhanced wastewater treatment other than a standard system. Alternative systems include the following:
    • Supplemental Treatment Systems which are designed to produce a higher quality wastewater effluent and improved performance for effluent dispersal where a standard system is not suitable.
    • Dispersal fields that dispense effluent by means other than gravity distribution. This includes pressure distribution systems and subsurface drip dispersal. Subsurface drip dispersal is a method for treated wastewater only.
    • Other system types could be considered alternative due to their size, complexity, or advanced level of review/approval. 


    Due to the complex nature of alternative systems, they are required to be designed by a Qualified Septic Designer, installed by a licensed septic contractor, and an annual Operating Permit with ongoing monitoring and maintenance from an approved Service Provider may be required to assure ongoing maintenance and system performance. 

    What are some different types of Alternative OWTS?

    1. Supplemental Treatment Systems: These are alternative OWTS designed to produce a higher quality wastewater effluent prior to land dispersal.These systems are to mitigate soil and/or groundwater conditions which render a parcel inappropriate for a standard system. They require an annual Operating Permit. Some types of Supplemental Treatment Systems:
      • Proprietary Treatment Units: Approved manufactured or "package" units/components specifically designed to provide enhanced treatment producing effluent meeting a predetermined standard prior to dispersal to land. Most units fall into two categories: Aerobic Treatment Units (ATU) and Media Filters. (Some examples of proprietary units installed in Yolo County: AdvanTex, Hoot, Presby AES. This list of examples is not inclusive).
      • Intermittent Sand Filter (ISF): Consists of packed-bed filter of medium-grained sand, designed for single pass-through treatment of septic tank effluent. Sometimes referred to "single pass filter". These systems are rare in Yolo County. 
      •  Recirculating Sand Filter (RSF): Uses coarse-grained sand and a recirculation system that causes the effluent to pass through the sand media several times prior to final dispersal. These systems are rare in Yolo County.
      • Raised Sand Filter Bed: Also known as a "bottomless sand filter". Combines features of an ISF and a Mound System. This system is intended to be used only for repair or replacements where the site limits other options.

    2. Pressure Distribution (PD) SystemThis is a different method of wastewater dispersal to the land. It  is different from a standard gravity dispersal because a pump and small-diameter pressure piping is used to achieve broad, uniform distribution of wastewater for improved soil absorption and better treatment of percolating effluent. PD systems can be after a septic tank or after supplemental treatment, and may be installed in shallow depths or in standard trenches. PD Systems with supplemental treatment requires an Operating Permit. PD Systems without supplemental treatment may require an Operating Permit.

    3. Subsurface Drip Dispersal System: This is a method of wastewater dispersal to land that can be used only after supplemental treatment. It uses special drip tubing designed for this use. Usually placed 8-12 inches below ground surface, making use of the most biologically active soil zone. Effluent is conveyed under pressure to the laterals, normally with timed doses. Subsurface Drip Dispersal requires an Operating Permit because it will always be used with a supplemental treatment unit. 

    4. Mound Systems: These systems usually consist of an elevated sand bed with a gravel distribution bed covered by soil fill. They are intended to raise the soil absorption system above grade and provide further treatment of effluent before it reaches native shallow surface soils. Mound Systems may require an Operating Permit.


    For all types of OWTS and related requirements, please refer to the Yolo County OWTS Manual. For any questions, please contact our office at (530) 666-8646.

     advantex system
    Supplemental Treatment Units: Proprietary Media Filter Units
    used to "clean" effluent 
    prior to discharging to the land. 
    (Shown here: Advantex units).

    mound ...squirt 
    PD Mound after supplemental treatment: 
    Squirt test inspection - prior to backfill - to 
    verify orifice placement and correct pressure. 

    presby
    Supplemental Treatment Unit:
    Installation of a proprietary ATU (passive) with
    additional sand media filtration in the dispersal field. 
    (Shown here: Presby AES).

    drip field (2)
    Installation of Subsurface Drip Dispersal: 
    (black) drip tubing - look to right of (white)
    lateral pipe. The green tape is tracer wire 
    ribbon, so the lateral can be found in the future.

    orchard mounds (2)
    Mound System dispersal (background) after
    supplemental treatment.
    Valve (foreground) allows for automatic bed rotation.

    standard on hill (2)
    Standard System - installed on a hillside.
     Gravity dispersal from tank (foreground) to 
    dispersal field (background).

    pd pipes (2)
    Pressure Distribution piping on a mound prior to backfill.
    Orifice caps (shown) cover the holes drilled into pipe.

    squirt trenches (2)
    Pressure Dispersal into trenches 
    from septic tank/pump tank: 
    Squirt test inspection - prior to backfill - to 
    verify orifice placement and correct pressure.