This page contains LOTS of information about septic system types, their history, etc. If you want to jump to a certain section, please select from the following list:

  • If you would like to become a semi-professional septic consultant yourself, continue reading this entire page to learn LOTS of details!


    Septic System History

    Back in the “old days”, the standard method of liquid waste disposal was to install a pipe from the house to the closest water body and let it all go down hill. After discovering this method of wastewater disposal pollutes our waterways, causes disease and can be lethal to humans and animals, newer methods were created.

    To help solve these initial problems, “standard gravity” soil treatment systems were introduced. This type of a system allowed the solids to settle out in a tank and the liquids to be filtered by gravity through the soil. Putting the liquid waste into the ground instead of straight into our surface water was a good first step, but other problems ensued.

    Sites with poor draining soils were found to sometimes “perch” a water table near the surface in the wintertime. Having a drainfield in a “perched” water table would oftentimes cause the system to back up into the house or cause pre-mature failure. On sites with excessively well drained soils (fractured rock, etc). There wasn’t enough filter material (dirt) to filter the effluent before the pollutants made it into the aquifers. This caused contamination of our ground water which was also found to be a problem.

    Today’s regulators have studied many of these problems previously experienced and changed the sewage regulations to help increase the life of our systems and also try and protect the public from contamination of surface and ground water. The rules are not perfect, but most have been created based on documented problems and scientific research.


    How they Work - General

    The septic system is a natural method of treating and disposing liquid household waste. The first component of all septic systems is the tank. Most tanks are split into two compartments and have pipe baffles and an outlet filter to ensure the solids stay in the tank.

    The biologic process begins in the tank where the effluent separates into layers and begins the process of decomposition. Bacteria, which are naturally present in all septic systems, begin to digest the solids that have settled to the bottom of the tank, transforming a large percentage of these solids into liquids and gases. When liquids within the tank rise to the level of the outflow pipe, they enter the next part of the treatment system (pre-treatment device, distribution box, pump chamber, etc, depending on the type of system). Final treatment of the effluent always occurs in the soil where additional microbes break down the waste and the “clean” water is put back into the ground thereby recharging the aquifers.

    Wastewater contains several undesirable pollutants. Pathogens such as viruses or bacteria can enter drinking water supplies creating a potential health hazard. Nutrients and organic matter entering waterways can lead to tremendous growth in the quantity of aquatic microorganisms. Metabolic activity of these microbes can reduce oxygen levels in the water causing aquatic life to suffocate. Septic system regulations attempt to reduce the chance of these pollutants from having a negative impact on people and animals.

    For further technical information from the State of Washington, check out this document:


    Types of Systems - General

    As you know by now, there are nearly as many types and sizes of septic systems as there are cars on a new car lot. In Washington, the systems are divided up into three basic categories:

    Standard Gravity
    Pressure Distribution
    Advanced Treatment

    The first two types (standard gravity and pressure distribution) are relatively straightforward, non-propitiatory system types. Standard gravity systems require three feet of "good" soil under the trenches while pressure distribution sytems only require two feet. Advanced Treatment systems however are a much bigger animal and can be used where there is only one foot of "good" dirt beneath the trench bottom. They come is many makes, models and sizes. Some are proprietary, name brand systems and other’s are not. If you would like to know a brief history of why we have so many types of systems, please see our history section above.

    Most systems today include pumps, control panels, graveless infiltration chambers and effluent filters. Some systems even include textile filters, aerobic digestion and ultraviolet disinfection!

    Standard Gravity

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







    When you send new liquid waste into the septic tank, an equal amount of liquid (called effluent) comes out the outlet side of the tank. An “outlet filter” is installed in the tank to help prevent solids from escaping the tank.

    After passing through the outlet filter, the effluent flows through a distribution box (d-box) which diverts the flows to multiple pipes.

    The effluent leaves the d-box under the power of gravity and flows downhill to each of the underground drainage trenches. The trenches are made up of either: perforated pipe over drainrock or graveless chambers. Each type effectively does the same thing…they allow the effluent to “perc” into the ground at the bottom of the trench. By design, Standard gravity systems are progressively failing as the effluent works it's way down the trench.

    The bottom of the trench needs to be 3 feet above any restrictive layer, such as a hardpan, water table or excessively permeable soil (rock). Chemical and biological processes treat the effluent as it percolates down through the required three feet of soil. The size of the drainfield depends on the estimated daily wastewater flow and the soil conditions at your site. (ie- how much water is used per day and how fast will the soil take it?)









    Pressure Distribution

    Pressure distribution systems are usually required when there is less than optimal soil depth available for complete treatment of the effluent by a gravity system. A minimum of two feet of properly drained soil is required under the trenches. The tank and drainfield size are normally the same as a standard gravity system, but the method by which the effluent is distributed to the soil is different.

    A pump (or sometimes a siphon) is used to pressurize the effluent into a small underground pvc pipe which transports it to the drainfield. The drainfield itself consists of pipe and rock, graveless chambers or drip irrigation tubing. Unlike a standard gravity system, a pressure distribution system wets the entire length of the trench each time the pump turns on. This allows the effluent to be spread over a larger area and receive better treatment from the soil.













    Advanced Treatment Systems:

    As mentioned above, advanced treatment systems come in MANY makes and varieties. Some are built in a factory and some are built on site. Some are proprietary and some are public domain. These systems are required when shallow soils exist on the site (12-30 inches). The basic function of these systems is to clean the wastewater prior to the final disposal into the ground. Most of the time, these treatment systems are followed by pressure distribution drainfields (trenches or drip tube).

    The most common types of pre-treatment systems used in Clark County include the following:
    • Sand Filters
    • Sand Mounds
    • Aerobic Treatment
    • Textile Filters (AdvanTex)
    • Glendon Pods

    For a detailed description and pictures of each, continue reading…or jump down to our advanced treatment system comparison charts.

    Sand Filter – After the septic tank, a pump sends pressurized effluent to a large underground box which is full of sand and drainrock. Effluent is spread evenly over the surface of the sand via a pressurized pipe network (similar to a pressure distribution system). The dirty effluent filters through the sand and collects in a sump at the bottom. A second pump sends the relatively clean effluent to the drainfield where the final treatment occurs and the effluent is disposed in the soil.

    Sand Mounds – Another system suited for sites with shallow soils is a sand mound. A mound is a drainfield raised above the natural soil surface with a 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 downward through the sand and directly into the natural soil beneath.





    Aerobic Treatment – Another method of treating wastewater involves an “aerobic” process by which air is injected into the effluent in a specific manner. The increased levels of oxygen in the effluent allow the microorganisms to thrive and digest the biological nutrients. A small air compressor is used to inject air into the effluent.

    EGSD does not typically recommend these systems for residential projects. They do however have a place in commercial applications with high-strength waste. Although we do not recommend aerobic systems for residential projects, you may have heard about many aerobic systems being installed for residential projects in Clark County. Most of these systems are specified by designers who are also associated with selling this product.









    Textile Filter (AdvanTex) – Considered to be one of the higher quality treatment systems available, AdvanTex filters are becoming more and more popular in Washington State. Similar to a sand filter, the AdvanTex filter uses media to filter out the contaminants in the effluent. Instead of sand however, the AdvanTex uses a textile product.

    The AdvanTex system has the following benefits:
    • Easily replaceable media (no need to dig up the whole yard if the media gets plugged)
    • Constant re-circulation of the effluent to keep all biological processes working and ensure consistent treatment
    • 24/7 monitoring with an advanced control panel
    • Much smaller footprint than a sandfilter

    Although the up front cost of the AdvanTex is more than a sandfilter, it’s lifetime cost is typically lower
    (see the system comparison section)

    To learn more about this product, please visit Orenco’s website at:


    Glendon® Biofilters – Glendon Bilfilters are used on the really tough sites, typically where seasonal groundwater or significant rock is found at 12-18 inches below the surface. The system consists of different layers of sand and gravel placed in a watertight box built into the soil with a sand fill placed over the top of the entire area. 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 and into the soil (effluent remains under the cover sand). Typically one “pod” is used for each bedroom in the residence. After installation, grass can be planted over the mounds.

    To learn more about this product, please visit Glendon’s website at:


    Advanced Treatment Comparison

    The following tables and graphs are intended to help you better analyze and compare the top advanced treatment technologies used in SW Washington. The numbers and data provided are approximate and should not be considered absolute.

    The systems compared are:
    • AdvanTex Textile Filter
    • Whitewater Aerobic System
    • Sand Filter
    • Glendon Pods







    *NOTE: This graph includes the APPROXIMATE cost of: installation, inspections, pump and blower replacement, tank pumping, electricity costs, filter cleaning/replacement, jetting pressurized distribution laterals and a 3.0% inflation rate. All prices are approximate and based on multiple sources.

    **ALSO NOTE: This graph assumes the sandfilter will need to be replaced after 20 years and the Glendon pods after 30 years. Newer design methodologies along with systematic system inspections and maintenance may allow sandfilters to last indefinitely. Many older sandfilters are failing after 10 years or less. Glendon systems have not been around long enough to know their life expectancy. Actual costs will vary.