United States	Office of Water
Environmental Protection Washington, D C
EPA 832-F-00-031
September 2000
Decentralized Systems
Technology Fact Sheet
Aerobic Treatment
Natural treatment of biological waste has been
practiced for centuries However, engineered
aerobic biological treatment of wastewater has been
practiced in the United States, on a large scale, for
only a few decades In fact, in 1925, 80 percent of
all cities in the United States with populations of
over 100,000 had no treatment systems at all
(Linsley 1972) The basic aerobic treatment
process involves providing a suitable oxygen rich
environment for organisms that can reduce the
organic portion of the waste into carbon dioxide
and water in the presence of oxygen With the ever
increasing development of land, both suburban and
rural, large central sewerage systems have not
always been cost-effective or available Many
homeowners still rely on individual septic tank or
other systems to treat and dispose of household
wastewater onsite
Historically, aerobic treatment was not feasible on
a small scale, and septic tanks were the primary
treatment device, but recent technology advances
make individual aerobic treatment systems efficient
and affordable Aerobic systems are similar to
septic systems in that they both use natural
processes to treat wastewater But unlike 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 Because aerobic
systems use a higher rate process, they are able to
achieve superior effluent quality The effluent can
be discharged to the subsurface as in a septic tank
leach field or, in some cases, discharged directly to
the surface
Current Technologies
Individual aerobic systems have been in place since
the 1950's, however, these early systems consisted
of little more than an aerator placed in a traditional
septic tank They were prone to noise, odor and
maintenance complaints, and were used only where
standard septic tanks were not feasible The newer
aerobic treatment units are pre-engineered and
operate at a high level of efficiency The demand
for these units and the desire for direct surface
discharge of the treated waste stream has led to a
certification process by the National Sanitation
Foundation (NSF) This certification (NSF Standard
40 for Individual Wastewater Treatment Plants)
applies to plants with capacities of up to 1,500
gallons per day, and leads to approval as a Class I
or Class II plant A Class I certification indicates
performance to EPA Secondary Treatment
Guidelines for three parameters BOD, suspended
solids and pH Noise levels, odors, oily films and
foaming are also measured The Class II catena
require that not more than 10% of the effluent
CBOD5 values exceed 60 mg/L and that TSS not
exceed 100 mg/L
As of June 2000, 15 manufacturers carry NSF 40
Class I Certification with available capacities
ranging from 1514 2 Liters/day to 5,678 1
Liters/day (400 to 1,500 gallons per day) Table 1
provides a list of the certified manufacturers, the
number of models available, and the range of flows
treated It is importanttonotethattheNSF certified
Product Listing is continually changing The NSF
should be contacted directly to confirm the status of
the listing provided in Table 1 Table 2 shows the
NSF Class I effluent performance limits

Number of
Flow Range
Alternative Wastewater Systems, Inc
Batavia, IL
American Wastewater Systems, Inc
Duson, LA
Aquarobic International
Front Royal, VA
Shawnee, KS
Clearstream Wastewater Systems, Inc
Beumont, TX
Consolidated Treatment Systems, Inc
Franklin, OH
Delta Environmental Productss
Denham Springs, LA
H E McGrew, Inc
Bossier City, LA
Hydro-Action, Inc
Beaumont, TX
Jet, Inc
Cleveland, OH
Microseptec, Inc
Laguna Hills, CA
Natonal Wastewater Systems, Inc
Lake Charles, LA
Nordbeton North Amenca, Inc
Lake Monroe, FL
Norweco, Inc
Norwalk, OH
Thomas, Inc
Sedro Woolley, WA
* As of June 19, 2000 This list is continually changing Please contact NSF to confirm the status of any listing
Source National Sanitation Foundation, 2000
S30mg/L (2 504 x 10"7 lb/gal)
(Monthly Average)
6 0-9 0 Units
15 Units
<60dbA @20
Source NSF Evaluaton of JET Model J-500 (1998)
Although there have been small scale "home
aerobic systems" in the United States for more than
50 years, their use has been fairly limited, in part,
because of the widespread use of septic systems,
which are relatively inexpensive and easy to
maintain They are the most common onsite
wastewater treatment systems in rural areas
However, many households may not be well suited
for septic systems
For example, septic systems are not suitable for all
decentralized wastewater treatment applications In
fact, approximately two-thirds of all land area in the
United States is estimated to be unsuitable for the
installation of septic systems (Linsley 1972) Some
homes may not have enough land area or
appropriate soil conditions to accommodate the soil
absorption drainfield In 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
Homeowners in wooded areas may not want to
clear enough land to install a septic tank and
drainfield, and wastewater treated by a septic
system is often not of high enough quality to be
discharged near a body of water
One of the most common reasons to select aerobic
wastewater treatment units is to replace failing
septic systems, which are a major source of
groundwater pollution in some areas If a failed
septic system needs to be replaced or if a site is
inappropriate for a septic system, aerobic
wastewater treatment may be a viable option
	Can provide a higher level of treatment than
a septic tank
	Helps protect valuable water resources
where septic systems are failing
Provides an alternative for sites not suited
for septic systems
May extend the life of a drainfield
May allow for a reduction in drainfield size
	Reduces ammonia discharged to receiving
	More expensive to operate than a septic
	Requires electricity
	Includes mechanical parts that can break
	Requires more frequent routine
maintenance than a septic tank
 Subject to upsets under sudden heavy loads
or when neglected
May release more nitrates to groundwater
than a septic system
On-site aerobic processes typically produce a higher
degree of treatment than septic tanks, but periodic
carryover of solids due to sludge bulking, chemical
disinfection addition, or excessive sludge buildup
can result in substantial variability of effluent
quality Regular, semi-skilled operation and
maintenance are required to ensure proper
functioning of moderately complex equipment
Inspections every two months are recommended
Power is required to operate aeration equipment and
pumps Absorption beds are dependent upon site
and soil conditions, and are generally limited to
sites with percolation rates less than 2 4
minutes/millimeter (60 minutes/inch), depth to
water table or bedrock of 0 61 to 1 2 meters (2 to 4
feet), and level or slightly sloping topography
Two aerobic primary systems have been adapted for
onsite use suspended growth and fixed film In
suspended growth systems, the microorganisms
responsible for the breakdown of wastes are
maintained in a suspension with the waste stream
In fixed film systems, the microorganisms attach to
an inert medium V ery few commercially produced
fixed film systems are available for onsite
application, and they include a variety of
proprietary devices, making it difficult to prescribe
design guidelines In many cases, however, design
guidelines for fixed film systems are similar to
those applied to suspended growth systems
Most aerobic treatment units designed for
individual home application range in capacity from
1514 to 5678 Liters (400 to 1,500 gallons), which
includes the aeration compartment, settling
chamber, and in some units, a pretreatment
compartment Based upon average household
flows, this volume will provide total hydraulic
retention times of several days

Some aerobic units provide a pretreatment step to
remove grease, trash and garbage gnndings
Pretreatment devices include trash traps, septic
tanks, comminutors, and aerated surge chambers
Hie use of a trash trap or septic tank before the
extended aeration process reduces problems with
floating debris in the final clanfier, clogging of
flow lines, and plugging of pumps Pretreatment is
required m fixed film systems to prevent process
Flow Mode
Suspended growth aerobic treatment plants may be
designed as continuous or batch flow systems The
simplest continuous flow units provide no flow
equalization and depend upon aeration tank volume
and/or baffles to reduce the impact of hydraulic
surges Some units use more sophisticated flow
dampening devices, including air lift or float-
controlled mechanical pumps to transfer the
wastewater from aeration tank to clanfier Still
other units provide multiple-chambered tanks to
attenuate flow The batch (fill and draw) flow
system eliminates the problem of hydraulic
variation This unit collects and treats wastewater
over a period of time (usually one day), then
discharges the settled effluent through pumping at
the end of the cycle Fixed film treatment plants
operate on continuous flow
Method of Aeration
Oxygen is transferred to the waste stream by
diffused air, sparged turbine, or surface entrainment
devices When diffused air systems are used, low
pressure blowers or compressors force the air
through diffusers on the bottom of the tank The
sparged turbine uses a diffused air source and
external mixing, usually from a submerged flat-
bladed turbine The sparged turbine is more
complex than the simple diffused air system A
variety of surface entrainment devices are used in
package plants to aerate and mix the wastewater
Air is entrained and circulated in the mixed liquor
through violent agitation from mixing or pumping
Oxygen transfer efficiencies for these small
package plants are normally low (3 4 to 16 9 kg
02/MJ or 0 2 to 1 0 lb 02/hp/hr) as compared with
large-scale systems which may transfer 50 7 kg
02/MJ or more (3+ lbs 02/hp/hr) This difference is
pnmanly due to the high power inputs to the
smaller units Normally, there is sufficient oxygen
transferred to produce high oxygen levels In an
attempt to reduce power requirements or enhance
nitrogen removal, some units use cycled aeration
periods Care must be taken to avoid developing
poor settling biomass when cycled aeration is used
Mixing the aeration tank contents is also an
important consideration in the design of oxygen
transfer devices Rule of thumb requirements for
mixing in aeration tanks range from 0 465 to 0 931
kW/m3 (0 5 to 1 hp/1,000 ft3 ) depending upon
reactor geometry and type of aeration or aeration
system configuration Commercially available
package units are reported to deliver mixing inputs
ranging from 0 005 to 2 8 kW/m3 (0 2 to 3 hp/1,000
ft3) Solids deposition problems may develop in
units with lower mixing intensities
Biomass Separation
The clanfier is critical to the successful
performance of the suspended growth process A
majority of commercially available package plants
provide simple gravity separation Weir and baffle
designs have not been given much attention in
package units Weir lengths of at least 12 in (30
cm) are preferred and sludge deflection baffles
(Stamford baffles) should be included as a part of
the outlet design The use of gas deflection barriers
is a simple way to keep floating solids away from
the weir area
Upflow clanfier devices have been used to improve
separation, but hydraulic surges must be avoided in
these systems Filtration devices have also been
employed in some units, but they are very
susceptible to clogging
Controls and Alarms
Most aerobic units are supplied with some type of
alarm and control system to detect mechanical
breakdown and to control the operation of electncal

components They do not normally include devices
to detect effluent quality or biomass deterioration
These control systems are subject to corrosion
because they contain electrical components All
electrical components should be waterproofed and
regularly serviced to ensure their continued
Additional Construction Features
Typical onsite extended aeration package plants are
constructed of noncorrosive materials, including
reinforced plastics and fiberglass, coated steel, and
reinforced concrete The unit may be buned as long
as there is easy access to all mechanical parts,
electrical control systems, and appurtenances
requiring maintenance such as weirs, air lift pump
lines, etc Units may also be installed above
ground, but should be properly housed to protect
against severe climatic conditions Installation
should be in accordance with the manufacturers
Appurtenances for the plant should be constructed
of corrosion-free materials including polyethylene
plastics Air diffuser support legs are normally
constructed from galvanized steel or an equivalent
Large-diameter air lift units should be used to avoid
clogging problems Mechanical units should be
waterproofed and/or protected from the elements
For fixed film systems, synthetic packing or
attachment media are preferred over naturally
occurring materials because they are lighter, more
durable, and provide better void volume-surface
area characteristics
Since blowers, pumps, and other pnme movers are
abused by exposure to severe environments, lack of
attention, and continuous operation, they should be
designed for heavy duty use They should be easily
accessible for routine maintenance and tied into an
effective alarm system
In extended aeration package plants, long hydraulic
and solids retention times (SRT) are maintained to
ensure a high degree of treatment at minimum
operational control, to hedge against hydraulic or
organic overload to the system, and to reduce
sludge production Since waste of accumulated
solids is not routinely practiced in many of these
units, SRT increases to a point where the clarifier
can no longer handle the solids, which will be
uncontrollably wasted in the effluent Treatment
performance (including nitrification) normally
improves with increasing hydraulic retention time
and SRT to a point where excessive solids build-up
will result in high suspended solids washout This
is one of the biggest operational problems with
these extended aeration units, and is often the
reason for poor performance
Dissolved oxygen concentrations in the aeration
tank should exceed 2 mg/L (1 669 x 10"8
pounds/gallon) to insure a high degree of treatment
and a good settling sludge Normally, onsite
extended aeration plants supply an excess of
dissolved oxygen due to minimum size restnctions
on blower motors or mechanical drives An
important element of aeration systems is the mixing
provided by the aeration process Package units
should be designed to provide sufficient mixing to
ensure good suspension of solids and mass transfer
of nutrients and oxygen to the microbes
Wastewater characteristics may also influence
performance of the process Excess amounts of
certain cleaning agents, grease, floating matter, and
other detritus can cause process upsets and
equipment malfunctions
Process efficiency may also be affected by
temperature, generally improving with increasing
The clarifier is an important part of the treatment
process If the biomass cannot be properly
separated from the treated effluent, the process will
fail Clarifier performance depends upon the
settleability of the biomass, the hydraulic overflow
rate, and the solids loading rate Hydraulic surges
can result in serious clarifier malfunctions As
mentioned previously, high solids loadings caused
by accumulation of mixed liquor solids result in
eventual solids carryover Excessively long
retention times for settled sludges in the clarifier
may result in gasification and flotation of these
sludges Scum and floatable material not properly

removed from the clanfier surface will also impair
effluent quality
Generally, extended aeration plants produce a high
degree of nitrification since hydraulic and solids
retenti on times are high Reducti ons of phosphorus
are normally less than 25 percent The removal of
indicator bactena (fecal cohforms) in onsite
extended aeration processes is highly variable and
not well documented Reported values of fecal
cohforms appear to be about two orders of
magnitude lower in extended aeration effluents than
in septic tank effluents
Aerobic units can achieve higher BOD5 removals
than septic tanks, but suspended solids removals,
which are highly dependent on solids separation
methods, are similar Nitrification is normally
achieved, but little reduction in phosphorus is
accomplished NSF studies indicate that suspended
growth units can provide from 70 to 90 percent
BOD5 and SS reductions for combined household
wastewater, yielding effluent BOD5 and suspended
solids concentrations as low as 20 mg/1
General Plant Operation
The activated sludge process can be operated by
controlling only a few parameters, the aeration tank
dissolved oxygen, the return sludge rate, and the
sludge wasting rate For onsite package plants,
these control techniques are normally fixed by
mechanical limitations so that very little operational
control is required Dissolved oxygen is normally
high and cannot be practically controlled except by
"on or off' operation Experimentation with the
process may dictate a desirable cycling arrangement
using a simple time clock control that results in
power savings and may also achieve some nitrogen
The return sludge rate is normally fixed by pumping
capacity and pipe arrangements Return sludge
pumping rates often range from 50 to 200 percent
of the incoming flow They should be high enough
to reduce sludge retention times in the clanfier to a
minimum (less than one hr), yet low enough to
discourage pumping of excessive amounts of water
with the sludge Time clock controls may be used
to regulate return pumping
Sludge wasting is manually accomphshed in most
package plants, usually during routine maintenance
Through experience, the technician knows when
mixed liquor solids concentrations become
excessive, resulting in excessive clanfier loading
Usually 8 to 12-month intervals between wasting is
satisfactory, but this vanes with plant design and
wastewater charactenstics Wasting is normally
accomplished by pumping mixed liquor directly
from the aeration tank Wasting of approximately
75 percent of the aeration tank volume is usually
satisfactory Wasted sludge must be handled
Pnor to actual start-up, a dry checkout should be
performed to insure proper installation Seeding of
the plant with bactenal cultures is not required as
they normally develop within a 6 to 12-week
penod Initially, large amounts of white foam may
develop, but will subside as mixed liquor solids
increase Dunng start-up, it is advisable to return
sludge at a high rate Monitonng by qualified
maintenance personnel is desirable dunng the first
month of startup
Routine Operation and Maintenance
The maintenance process for suspended growth
systems is more labor-intensive than for septic
systems and requires semi-skilled personnel Based
upon field expenence with these units, 12 to 48
man-hours per year plus analytical services are
required to ensure reasonable performance Power
requirements are vanable, but range between 2 5 to
10 kWh/day (8,530 8 to 34,123 2 Btu/day)
Maintenance for fixed film systems is less labor-
intensive but still requires semi-skilled personnel
Based upon limited field expenence, 8 to 12 man-
hours per year plus analytical services are required
for adequate performance Power requirements
depend upon the device employed, but range from
1 to 4 kWh/day (3,412 3 to 13,649 3 Btu/day)
Maintenance for both types of aerobic treatment
units is usually completed through routine contract
services No chemicals are required for either

method unless chemical disinfection or additional
nutrient removal (N and P) is required for surface
Operational Problems
Major mechanical maintenance problems for onsite
treatment units include blower or mechanical
aerator failure, pump and pipe clogging, electrical
motor failure, corrosion and/or failure of controls,
and electrical malfunctions Careful attention to a
maintenance schedule will reduce these problems
and alleviate operational problems due to the
biological process upset Emphasis should be
placed on adequate maintenance checks during the
first 2 or 3 months of operation
Costs for both suspended growth and fixed film
systems of between 1,892 and 5,678 Liters/day (500
to 1,500 gallons per day) are typically in the $2,500
to $9,000 cost range, installed These costs have
been updated using the ENR construction cost
index (ENR=6076) These units need more
frequent maintenance than a traditional septic tank,
and quarterly servicing is recommended This
maintenance cost averages $350 per year Since
many of these systems are bemg installed to replace
failed septic systems, additional costs may be
incurred to account for site conditions and
additional piping
1	Barrett, Michael E and J F Malina, Jr
September 1, 1991 Technical Summary of
Appropriate Technologies for Small
Community Wastewater Treatment Systems
The University of Texas at Austin
2	Cheremisinoff, Paul N 1987 Wastewater
Treatment Pudvan Publishing Co
4	Cntes, R , G Tchobanoglous 1998 Small
and Decentralized Wastewater Management
Systems WCB McGraw-Hill, Inc Boston,
5	Jet Inc March 1998 Wastewater
Technology Report on the Performance
Evaluation of the Jet Inc Model J-500
Wastewater Treatment System NSF
International Ann Arbor, Michigan
6	Jet Inc Technical Manual 1500 Series
BAT Media Plants Jet Inc Cleveland,
7	Linsley, Ray K. and J B Franzim 1972
Water-Resources Engineering 2nd Ed
McGraw-Hill, Inc New York, New York
8	Multi-Flo Wastewater Treatment System ~
for Residential and Commercial Properties
Multi-Flo Franklin, Ohio
9	Norweco, Inc Engineering Data
Wastewater Treatment System for
Developments Beyond the Reach of Publicly
Owned Sanitary Sewers Norweco, Inc
Norwalk, Ohio
10	Pipeline Small Community Wastewater
Issues Explained to the Public Winter
1996 National Small Flows Clearinghouse
vol 7 no 1
11	US Environmental Protection Agency
1980 Design Manual Onsite Wastewater
Treatment and Disposal Systems EPA
Office of Water EPA Office of Research &
Development Cincinnati, Ohio EPA
Northbrook, Illinois	Mr Mike Price, Vice President
Norweco, Inc
Corbitt, Robert A 1990 Standard 220 Republic Street
Handbook of Environmental Engineering Norwalk, Ohio 44857
McGraw-Hill, Inc New York, New York

Mr William Neal, Vice President
JET, Inc
750 Alpha Dnve
Cleveland, Ohio 44143
Mr Mike Lynn
Onsite Solutions
PO Box 570
Nokesville, Virginia 20182
Mr Thomas Bruursema
NSF International
789 Dixboro Road
Ann Arbor, MI 48105
The mention of trade names or commercial
products does not constitute endorsement or
recommendationforuseby theU S Environmental
Protection Agency
For more information contact
Municipal Technology Branch
Mail Code 4204
1200 Pennsylvania Ave, NW
Washington, D C 20460
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