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|موضوع: طرق معالجة الصرف الصحى للمنازل والتجمعات السكنية بيولوجيا/HOME AEROBIC WASTEWATER TREATMENT: AN ALTERNATIVE TO SEPTIC SYSTEMS السبت مارس 10, 2012 5:59 am|| |
HOME AEROBIC WASTEWATER TREATMENT:
AN ALTERNATIVE TO SEPTIC SYSTEMS
TECHNOLAB EL-BAHAA GROUP
these communities, central sewerage
systems are often not cost-effective, so
many homeowners rely on septic systems
or other systems that treat and dispose of
household wastewater onsite.
homes for which septic systems are not a
good option rely on individual home
aerobic systems for wastewater treatment.
Aerobic systems are similar to septic
systems in that they both use natural
processes to treat wastewater.
septic (anaerobic) treatment, the aerobic
treatment process requires oxygen.
Aerobic treatment units, therefore, use a
mechanism to inject and circulate air
inside the treatment tank.
requires electricity to operate.
For this reason, aerobic systems cost
more to operate and need more routine
maintenance than most septic systems.
However, when properly operated and
maintained, aerobic systems can provide
a high quality wastewater treatment
alternative to septic systems.
Why choose aerobic treatment?
This is due, in part, to the widespread use
of septic systems.
Septic tank-soil absorption systems are
relatively inexpensive and are easy to
They are the most common
onsite wastewater treatment systems used
in rural areas. However, there are many
households for which a septic system may
not be the best wastewater treatment
For example, septic systems are not
suitable for every lot.
In fact, approximately
two-thirds of all the land area in
the U.S. is estimated to be unsuitable for
the installation of septic systems.
homes may not have enough land area or
appropriate soil conditions to accommodate
the soil absorption drainfield.
some communities, the water table is too
high to allow the drainfield to give
adequate treatment to the wastewater
before it is returned to the groundwater.
Other site-related concerns include
homes located on wooded lots or on lots
close to a body of water.
wooded areas may not want to clear
enough land to install a septic tank and
drainfield, and the wastewater treated by a
septic system is often not of high enough
quality to be discharged very close to a
body of water.
But one of the most common reasons
that aerobic wastewater treatment units
are chosen by communities is to replace
failing septic systems.
HOW AEROBIC TREATMENT WORKS
Aerobic systems treat wastewater using
natural processes that require oxygen.
Bacteria that thrive in oxygen-rich
environments work to break down and
digest the wastewater inside the aerobic
Like most onsite systems, aerobic
systems treat the wastewater in stages.
Sometimes the wastewater receives
pretreatment before it enters the aerobic
unit, and the treated wastewater leaving the
unit requires additional treatment or
disinfection before being returned to the
Such a variety of designs exist for home
aerobic units and systems that it is
impossible to describe a typical system.
Instead, it is more practical to discuss how
some common design features of aerobic
systems work and the different stages of
Some aerobic systems include a
pretreatment step to reduce the amount of
solids in the wastewater going into the
Solids include greases, oils,
toilet paper, and other materials that are put
down the drain or flushed into the system
Too much solid material can clog the unit
and prevent effective treatment.
Some pretreatment methods include a
septic tank, a primary settling compartment
in the treatment unit, or a trash trap.
Pretreatment is optional but can greatly
improve a unit’s performance.
AEROBIC TREATMENT UNITS
The main function of the aerobic unit is to
collect and treat household wastewater,
which includes all water from toilets,
bathtubs, showers, sinks, and laundry.
Aerobic units themselves come in many
sizes and shapes—rectangular, conical, and
some shapes that defy classification.
Suspended Growth Units
The process most aerobic units use to
treat wastewater is referred to as suspended
These units include a main compartment
called an aeration chamber in
which air is mixed with the wastewater.
Since most home aerobic units are buried
underground like septic tanks, the air must
be forced into the aeration chamber by an
air blower or a compressor.
The forced air mixes with wastewater in
the aeration chamber, and the oxygen
supports the growth of aerobic bacteria that
digest the solids in the wastewater.
mixture of wastewater and oxygen is called
the mixed liquor.
The treatment occurring in the mixed
liquor is referred to as suspended growth
because the bacteria grow as they are
suspended in the liquid unattached to any
Unfortunately, the bacteria cannot digest
all of the solids in the mixed liquor, and
these solids eventually settle out as sludge.
Many aerobic units include a secondary
chamber called a settling chamber or
clarifier where excess solids
Other designs allow the sludge
to accumulate at the bottom of the tank
In aerobic units designed with a separate
settling compartment, the sludge returns to
the aeration chamber (either by gravity or
by a pumping device).
The sludge contains
bacteria that also aid in the treatment
Although, in theory, the aerobic
treatment process should eventually be
able to consume the sludge completely, in
practice, the sludge does build up in most
units and will need to be pumped out at
least once a year so that solids don’t clog
Attached Growth Units
An alternative design for aerobic
treatment is the attached growth system.
These units treat wastewater by taking a
surface made of material that the bacteria
can attach to, and then exposing that
surface alternately to wastewater and air.
This is done either by rotating the surface
in and out of the wastewater or by dosing
the wastewater onto the surface.
The air needed for the
process is either naturally present or is
Attached growth systems, such as trickling
filters and rotating disks, are less
common than suspended growth systems,
but have certain advantages.
there is no need for mixing, and solids are
less likely to be washed out of the system
during periods of heavy household water
The way and the rate in which wastewater
is received by and flows through the
aerobic unit differs from design to design.
Continuous flow designs simply allow the
wastewater to flow through the unit at the
same rate that it leaves the home.
designs employ devices (such as pretreatment
tanks, surge chambers, and baffles) to
control the amount of the incoming flow.
Batch process designs use pumps or
siphons to control the amount of wastewater
in the aeration tank and/or to discharge
the treated wastewater in controlled
amounts after a certain period of time.
Controlling the flow of wastewater helps
to protect the treatment process. When too
much wastewater is flushed into the system
all at once, it can become overburdened,
soil or directly to a body of water.
health department is familiar with local
regulations and the treatment options that
are best in your area and for your property.
Soil absorption fields (or drainfields) are
the most common method of final
treatment used for septic systems.
aerobic system is being used in place of a
septic system or to replace a failing septic
system, a drainfield may not be an option.
However, an aerobic unit can sometimes
help to prolong the life of a drainfield.
The amount of dissolved oxygen
contained in wastewater from an aerobic
unit can help the growth of microorganisms
that treat the wastewater in the soil,
and can help prevent the pores in the soil
However, when aerobic
units malfunction, they can release solids
that can clog the drainfield, which may
cancel out any potential benefits.
Evapotranspiration beds are a less common
method of final treatment and use
vegetation and evaporation to naturally
treat the wastewater.
Drip irrigation is
another less commonly used method to
treat and dispose of wastewater.
Sand filters are sometimes used to treat
the wastewater from aerobic units.
wastewater is pumped evenly over several
layers of sand and gravel, which are
located either above or below ground.
with soil treatment systems, the purification
process is aided by bacteria that occur
naturally in the sand.
Disinfection is another method of
treatment commonly used with aerobic
Some units have the disinfection
process incorporated into the unit design.
In some cases, disinfection may be the
only treatment required of the wastewater
from an aerobic unit before the water is
released into the environment.
disadvantage of this method is the added
cost of the disinfectants, such as chlorine.
OTHER DESIGN CONSIDERATIONS
Controls and Alarms
Most aerobic units have controls that can
be switched on and off by the homeowner
in case of emergency.
Aerobic units also
are required to have alarms to alert the
homeowner of malfunctions.
on the design of the system, controls and
alarms can be located either inside or
and the quality of treatment can suffer.
disadvantages to mechanical flow control
devices are that, like all mechanical
components, they need maintenance and
run the risk of malfunctioning.
Homeowners can help their system’s
performance by conserving water.
faucets and running toilets should be
repaired, and washing machines and dishwashers
should be used only when full.
Installing water saving devices in toilets,
faucets, and showers can reduce water use
by up to 50 percent.
should try to space out activities requiring
heavy water use (like laundry) to avoid
overloading their systems.
FINAL TREATMENT AND
Although properly operated and
maintained aerobic units are very effective,
the wastewater leaving the units is not
ready to be returned to the environment
and must receive final treatment or
Methods for final treatment
include discharge to a soil absorption field,
a sand filter, or an evapotranspiration bed.
Sometimes, the wastewater receives
disinfection before being discharged to the
outside the home, and alarms can be
visible, audible, or both.
Homeowners should make sure that
controls and alarms are always protected
from corrosion, and that the aerobic unit is
turned back on if there is a power outage or
if it is turned off temporarily.
Aerobic units should be large enough to
allow enough time for the solids to settle
and for the wastewater to be treated.
size of most units range from 300 to 1,500
gallons per day, but local regulations often
require that the unit be at least large
enough to handle 500 gallons of wastewater
The needed size of an aerobic unit is
often estimated the same way the size of a
septic tank is estimated, by the number of
bedrooms (not bathrooms) in the house.
is assumed that each person will use
approximately 50 to 100 gallons of water
per day, and that each bedroom can accommodate
When calculated this
way, a three-bedroom house will require a
unit with a capacity of 300 to 600 gallons
Some health departments require that
aerobic units be sized at least as large as a
septic tank in case the aerobic unit
malfunctions and oxygen doesn’t mix with
In such cases, the aerobic
unit will work as a septic tank—which
will, at least, provide partial treatment for
Lower temperatures tend to slow down
most biological processes, and higher
temperatures tend to speed them up.
aerobic process itself creates heat, which,
along with the heat from the electrical
components, may help to keep the
treatment process active. However, cold
weather can have adverse effects on the
performance of aerobic units.
In one study of aerobic units, there were
problems when the temperature of the
wastewater inside some of the units fell
below 15 degrees Celsius (59 degrees
Problems can sometimes be
avoided by insulating around the units.
Your health department should know
whether aerobic systems are suitable for