January, 2004
Septic Systems Overview
Part Two of Three: Common "Alternative" Systems
With
so many types of septic systems out there it is easy to become confused when
distinguishing between them. In this series we're going through
several of the most common types of septic systems.
Now that we’ve covered how a traditional septic
system works, we’re going to cover alternative septic systems. There are many,
but for this article we’ll stick to those alternative systems encountered on
a more regular basis in the real estate industry.
Aerobic Systems (also called Aeration Systems)
Aerobic is a general term for a septic system that infuses air into the
wastewater while in the septic tank. There are many different types of
aerobic systems, differing greatly in appearance and performance; however,
generally all aerobic systems have the same basic functions. Those common
functions are going to be the focus here.
Aeration systems are generally used when the land around the house prevents
the use of a traditional system. Some aeration systems do not require a
drain field and instead utilize several methods of discharge, including sand
filters, evapotranspiration beds, or disinfecting the water and discharging
it via lawn sprinklers or directly into a nearby body of water, such as a
creek or stream.
Like
a traditional system, an aeration system starts with a septic tank. The tank
may look like a normal septic tank, but
it also comes in a variety of shapes and sizes. A more common design (shown
on the right) uses a two-chamber tank. Some systems pre-treat the water
before it enters the tank. Pre-treatment eliminates some solids prior to
entry into the tank, improving the efficiency of the system.
The main chamber, generally referred to as the aeration chamber, allows the
solids to settle at the bottom like a
traditional tank. In the aeration chamber, the solids are generally
tilled by a mixing rotor of some sort. Air, compressed by a pump, is fed
through the rotor and introduced into the wastewater. The main chamber also
prevents solids from entering the second chamber. The second chamber, which
also houses the outlet pipe, allows any remaining solids to settle and be
reintroduced into the first chamber, further breaking them down.
All septic systems utilize naturally occurring bacteria to break down the
solids. In a “traditional system” the bacteria, acting without the aid of
airflow (sometimes called anaerobic) only breaks down approximately 65 to70
percent of the solids. In an aerobic system, the breakdown rate is much
higher, approximately 90 to 90 percent. The quantity of water entering an
aeration system can affect the performance. Because of this, some systems
utilize additional components that restrict the flow of incoming wastewater,
such as an additional tank, which can be used for pre-treatment purposes.
Maintenance is required with an aeration system, just as with a traditional
system. Pump-outs are necessary about once a year. Also, since aeration
systems have moving parts, including an air compressor, parts can break
down. The maintenance rules for a
traditional septic system still apply to an aeration system.
Cesspool Systems
Cesspool
systems are usually found in older homes. They are only installed when other
options are not feasible, and usually require approval by the local health
authorities. A cesspool is an all-in-one, very low-tech system. It’s
basically just a hole in the ground with the walls lined by dry-stacked
stone or metal with holes in it and topped off with a lid. The wastewater
simply flows into the cesspool, the solids settle to the bottom, and the
effluent disperses into the ground through the walls and bottom of the pit.
Some of the systems, as they began to fail over time, were modified to
include an outlet pipe and leaching field.
Lagoon Systems (also called Ponds)
Lagoon systems (also known as ponds) are not commonly used as single-family
residential waste treatment systems in the United States because of the land
required to operate such a system. The systems that you do find are
typically located in smaller and more rural areas, as more land is required.
On the other hand, a lagoon system is relatively inexpensive to install and
maintain, so if it is affordable to sacrifice the land required, it’s worth
looking into.
Lagoons can differ in many ways. There are four fundamentally different ways
a lagoon can treat wastewater and four different ways a lagoon can handle
the discharge of the treated water. All lagoons utilize constructed bodies
of water resembling ponds, but these ponds can vary in depth and size
depending on the way it is intended to treat and discharge the wastewater.
The ponds are lined to prevent water from prematurely entering the ground,
which can detrimentally affect the treatment process.
Aerobic, Aerated, Anaerobic, and Facultative (combination of both aerobic
and anaerobic qualities) are the four different ways wastewater can be
treated in a lagoon. Aerobic lagoons tend to be shallower, allowing air and
sunlight access to more of the wastewater. Aerated lagoons utilize a system
that actively mixes oxygen into the wastewater. Anaerobic lagoons are
deeper, allowing the solids to separate like they would in a traditional
septic tank. A scum mat forms on the top, preventing air from gaining access
to the wastewater and allowing the anaerobic process to occur in the solids
that settle to the bottom. Last to be mentioned here, but what is actually
the most common system, is the facultative lagoon. The depth of the
facultative lagoon varies depending on the climate, for the system must be
shallow enough to prevent the formation of a scum mat and deep enough to
allow anaerobic conditions to occur in the bottom.
A lagoon can have continuous discharge, controlled discharge, hydrograph
controlled release discharge, or complete retention. A continuous discharge
lagoon releases water at a rate equivalent to the rate that water enters the
system. Controlled discharge, more commonly used in colder climates, allow
water to be released only at desired times. Hydrograph controlled release
systems employ equipment that measures several different factors and
discharges water when optimal conditions are present. Last, the complete
retention method requires a hot, dry climate to evaporate water out of the
lagoon.
Continuous discharge, controlled discharge, and sometimes hydrograph
controlled release discharge methods can require additional treatment.
Requirements for the quality of discharged wastewater vary by local
authority.
Maintenance of a lagoon system follows similar rules to that of a
traditional system. Of course
there are many things you shouldn’t flush, as with any system. But it varies
in that a lagoon does not require pumping out as often as a traditional
system. The sludge that accumulates at the bottom may require removal every
few years.
The growth of grass, weeds, aquatic plants, and the accumulation of scum,
trash, and leaves on the water are the biggest maintenance items.
Plant-growth in the water and around the edges can allow scum to accumulate
and provide areas for insects to breed. Scum accumulation will create
unpleasant odors and if it is a facultative or aerobic lagoon, it will
prevent proper treatment of the wastewater. Cutting the grass and removing
weeds, aquatic plant growth, scum, trash, leaves, tree branches, etc. from
the lagoon will help insure proper operation.
Click here for
the December 2003 article about traditional Septic Systems.
Click here for more information more about Septic Systems.
Contributed by Justin Gore
Quality Assurance Supervisor
U.S. Inspect, LLC
Other stories in this month's report:
Visit the Advisory
Report Archives
