OF SEPTIC SYSTEMS
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!
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
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
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:
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:
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
Most systems today include pumps, control panels, graveless infiltration
chambers and effluent filters. Some systems even include textile
filters, aerobic digestion and ultraviolet disinfection!
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.
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 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.
(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.
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
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
– 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.
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.
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
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)
more about this product, please visit Orenco’s website at:
– 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.
more about this product, please visit Glendon’s website at:
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
TABLE OF APPROXIMATE COSTS AND FEATURES
APPROXIMATE LIFETIME COSTS*
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.