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What is an OSS?
If you have any questions about septic systems
or operations and maintenance inspections you can email them to:
omseptic@co.cowlitz.wa.us
What is an
OSS?
An OSS is an On-site
Sewage System. OSS are more commonly known as septic systems.
They are referred to in some states as ISDS or Individual Sewage
Disposal Systems.
One type of a simple OSS
What is the purpose of an OSS?
OSS are designed to treat sewage and dispose
of it without causing health or environmental issues. Originally
OSS were viewed as temporary solutions until large municipal sewage
treatment systems could reach the area. In the 1980's the EPA
re-evaluated this idea and realized that it was too cost prohibitive for
municipal plants to reach all developed properties. The EPA
realized that OSS needed to be designed to last longer and better treat
sewage.
There have been many changes to OSS laws
over the last 30+ years. Many universities have done extensive
studies on OSS and their ability to effectively treat sewage.
Washington State has used this research to implement a comprehensive law
that protects human health and the environment. You can access this law,
WAC 246-272A,
HERE.
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What are the parts of an OSS?
OSS typically contain three main components.
These components can take on many different types. The three main
components are:
Septic tank
This is the component most people are
familiar with. Many people assume this is the whole septic system.
The purpose of the septic tank, in an OSS, is to trap solids and scum
and keep them from entering the disposal component or drainfield.
The tank also offers the first part of the treatment of sewage by
allowing separation and settling of the solids and scum and some
bacterial action.
There are many types of tanks installed in
Cowlitz County. The most common type of tank is a two compartment
concrete tank, similar to the one pictured above. Tanks have in
the past also been made of steel. If you have a steel tank it is
very important to have it inspected frequently since these tanks tend to
decay and deteriorate quickly. Tanks can also be made of plastic
(polyethylene) or fiberglass (FRP), though these are less common.
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Treatment Component
Both the tank and the drainfield accomplish
some treatment. In areas with significant soil depth, the tank and
the drainfield may be enough treatment to protect human health. In other areas, where there is not
enough soil, additional treatment is required. There are many
different types of treatment available. Some of these treatment
types are public technologies that are NOT restricted by copyright
or patent.
Other treatment technologies are patented and licensed by companies, we
refer to these as proprietary treatment devices.
All of these treatment devices provide extra
treatment to make up for shallow soils.
Some
examples of public technologies are:
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Sand Filter
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Sand Mound
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Sand lined trenches
Some
common proprietary treatment devices are:
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Advantex
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Whitewater
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Glendon
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Biorobics
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Disposal Component
Two main types of
distribution:
The disposal component is commonly referred
to as the drainfield or leech field. This is the part of an OSS
that disposes of the sewage. The disposal component also provides
the final treatment depending on the depth of "good" soil under the drain
line.
Traditionally the disposal component was a
rock and gravel system as pictured above on the bottom. In the mid
1990's a new product was introduced which is the gravelless chamber
system pictured on the top above. Both of these types are
available today. Each one has its own list of advantages and
disadvantages.
There are several other types of disposal
components that are not as common. One of them is the sand mound
which incorporates extra treatment along with disposal. Subsurface
drip is another common type of distribution system.
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How does an OSS treat sewage?
With help from our
little friends.
The key to any OSS is microorganisms and
oxygen.
Mainly we need the cooperation of bacteria, like those pictured on the
top left. Other microorganisms such as amoeba (top right) or
nematodes (bottom) also pitch in to an extent.
Our small friends eat what we dispose of in
our sewage. The bacteria can accomplish this in two ways.
First, they can function in environments with little or no oxygen called
anaerobic. When they are in these anaerobic environments they
process sewage much more slowly. The septic tank is mostly an
anaerobic environment. That is why there is some treatment
occurring in the septic tank, but not complete treatment.
The second way our friends process sewage is
in environments with oxygen known as aerobic. When there are
aerobic conditions all three types of our little friends can get
involved. In oxygenated environments the bacteria and other
organisms can work much more quickly and efficiently to break down our
waste. Many
treatment components provide
aerobic conditions to help speed up the treatment of waste.
Sewage is also treated in the soil in the
disposal component. In the soil there is dissolved oxygen
available to the microorganisms as long as there is not too much water.
The diagram below shows soil pores in a drainfield.
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Pores are formed over time as water
infiltrates, soil settles, and plant roots make channels. Because
of this, disturbed soils do not provide very much or any treatment and
are generally not suitable for OSS. The exception to this is some
types of sand.
Soil pores can be disturbed by compaction or
can be plugged when solids from the septic tank make it to the
drainfield. This is one of the reasons that you should check your
tank regularly and have it pumped before the solids or scum get too
deep.
When there is too much fluid going to a
drainfield the pores fill completely with fluid. When the pores
fill completely with fluid this is called saturated flow. During
saturated flow oxygen is not readily available causing anaerobic
conditions which limit treatment.
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