on-site wastewater treatment systems · an on-site wastewater treatment system, there- fore, assume...
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THE UNIVERSITY OF TENNESSEE AGRICULTURAL EXTENSION SERVICE
PB 1472
On-Site Wastewater Treatment systems
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On-Site
Tr ea t m en t systems
Timothy N. Burcham, former Assistant Professor, Agricultural Engineering
Agricultural Extension Service,
and C. Roland Mote, Professor, Agricultural Engineering
Agricultural Experiment Station
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ince there is a limited supply of water on earth, it must be continuously recycled. Even domestic waste- water must be reused. Fortunately, nature provides mechanisms
which continually recycle and clean this
finite water supply. This water recycling
process is commonly called the hydro-
logic cycle.
As water progresses through the
hydrologic cycle, it accumulates for vari-
ous periods of time (e.g., minutes, days,
years) in three types of reservoirs. It can
be found in surface water reservoirs (e.g.,
streams, lakes and oceans), ground water
reservoirs and the water vapor reservoir in
the atmosphere (clouds). Water moves
from clouds as precipitation, which falls to
the earth where it is stored in surface
reservoirs. Water from surface reservoirs
moves through the soil into ground water
reservoirs or evaporates into the air, form-
ing clouds. Hence the process begins
again.
Eventually, all water used in our
homes (water from sinks, toilets, showers,
washing machines, etc.) finds its way into
oneof these reservoirs. Theendless move-
ment of water through the hydrologic cycle
guarantees that all water coming into your
home can be termed “used.” The chal-
lenge facing us is to remove waste (feces,
hair, soap, etc.) from the water before it
returns to a reservoir of the hydrologic
cycle.
If we fail to meet this challenge
and allow waste to enter one of the reser-
voirs, the reservoir will begin to change
and may become something that can no
longer be enjoyed and safely used by
people. Nutrients such as nitrogen and
phosphorus that are naturally present in
domestic wastes can accelerate growth of
algae and similar organisms in lakes. Abun-
dant growth of such organisms may ad-
versely influence a lake’s beauty, limit its
ability to sustain fish and wildlife and can
reduce its suitability for other beneficial
uses. Domestic wastes may also contain
pathogens or disease-causing organisms.
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Pathogens in a reservoir, either ground or sur-
face water, reduce its suitability for water-con-
tact recreation and as a safe drinking water
source.
To ensure that each citizen has the
privilege of access to clean water, each of us
must take proper measures to remove our
waste materials from domestic wastewater be-
fore it returns to a reservoir of the hydrologic
cycle. Thus, as long as we use water to trans-
port waste materials away from our houses, we
must have properlyfunctioning domestic waste-
water treatment systems.
There are two basic types of domestic
wastewater treatment systems: municipal or
communitysystems and on-sitesystems. Tech-
nically they differ only in size and degree of
mechanical complexity. They each rely upon
physical, chemical and biological processes to
remove waste from water. Community systems
are larger because they renovate wastewater
from many homes and businesses, and more
complex because they strive to control natural
processes so treatment is achieved in the small-
est area at the greatest practical rate. The most
notable difference between the two types of
systems lies in the degree of homeowner in-
volvement in the treatment of the wastewater.
With acommunitysystem, the homeowner has
no direct involvement. Trained operators make
sure that treatment processes proceed around
the clock, and laboratory technicians verify that
only properly treated wastewater is returned to
a reservoir of the hydrologic cycle. On the other
hand, homeowners with on-site systems typi-
cally have no community-paid trained operator
at their disposal. They do not have the benefit
of an inspector and an analytical laboratory to
verify proper system performance. Owners of
an on-site wastewater treatment system, there-
fore, assume a great deal of personal respon-
sibility for the quality of water in their commu-
nity. Homeowners should understand and ap-
preciate the fundamentals of on-site domestic
wastewater treatment to properly meet this
responsibility.
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AI building sites d I
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properties. These sites have enormous
capacity for treating wastewater. Systems
of simple design can be effective on these
sites. Other sites, however, have soils that
are shallow and/or have poor treatment
properties. Such sites have limited capac-
ityfortreating wastewater and require sys-
tems with a more complex design to best
utilize the properties of the soil.
ome sites have abundantly deep
soils with excellent treatment
As in all other aspects of life,
more complex designs only come about
with additional investments of capital, en-
ergy and management resources. All soils
are not created equal. Homeowners and
builders must learn toaccept afact learned
long ago by farmers - the more shallow and
hard the soil, the greater the required in-
puts for satisfactory performance from a
piece of land.
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approaches to On-Site Wastewater Treatment: Conventional vs. fllternative
here are two groups of on-site wastewater treatment systems that need to be understood. The first group T has only one member - the conventional system. The other group, alternative systems, contains all systems
different from the conventional. The only difference between conventional and alternative systems is the method
used for distributing wastewater over an area of soil.
Since soil placed on-site by nature treats wastewater and conveys it into a reservoir of the hydrologic cycle, there
is actually nothing synthetic about an on-site domestic wastewater treatment system except a means for controlling and
distributing wastewater. Thus, differences between systems exist only in methods of wastewater distribution.
Conventional Conventional systems rely on soils with an abundant
treatment capacity to prevent waste from accompanying water into a ground water reservoir. Conventional systems employ a distribution mechanism of simple design which allows wastewater to flow in an almost uncontrolled fashion to the first available spot for infiltrating the soil. Such an approach guarantees that any soil area receiving wastewater receives a very large quantity per unit area. Conventional
alternative Alternative systems control wastewa-
ter flow and distribute it evenly over all the soil area allocated for the wastewater treatment system. They are required on sites with limited soil resources. These systems control waste- water loading at rates so that all of the available treatment capacity of a particular soil
systems are, therefore, not suited to sites where the soil does not have an abundant treatment capacity.
is utilized.
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Advantages
Relatively easy to install.
Reliable when properly sited and
installed.
Limitations
Seasonal ground water levels and bed
rock should be greaterthan 4 feet from
the bottom of the seepage trench.
Soil absorption rates should be no
greaterthan 75 minutes per inch (MPI).
Ground surface slopes should be no
greater than 30 percent.
The conventional system consists
of two primary components: a septic tank
and a series of disposal field seepage
trenches. A septic tank is a water-tight
container constructed of a durable mate-
rial, typically concrete. The functions of a
septic tank are to (1) receive wastewater
from the house, (2) liquefy a portion of the
solids and (3) separate the remaining sol-
ids from the liquid portion. Wastewater
received by septic tanks is a mixture of
water, dissolved and/or liquid organics
organic solids (e.g., soil),
and bacteria. Flow characteris-
ter. The wastewater leaving a sep-
tics of septic tanks are designed so
that wastewatermovesvery slowlythrough
the tank (usually two to three days from
entrance to exit). This permits very dense
solids to settle to the bottom and form a
sludge layer and lighter solids to rise to the
top and form a scum layer.
Microorganisms, which naturally
populate septic tanks, digest some of the
organic solids, thereby converting some of
the material into liquid. Not all solids are
converted-to liquid, however. All inorganic
solids and some organic solids accumu-
late in the septic tank forming sludge and
must be periodically removed by mechani-
cal pumping.
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tic tank, typically called effluent, is largely
a mixture of water, soluble organics and
bacteria. Wastewater exiting a septic tank
has not, therefore, been completely
cleaned or treated. Septic tanks serve as
conditioning or pre-treatment units to pre-
pare wastewater to be more readily ac-
cepted by soil for final treatment.
Seepage trenches (also called
leachfields), which receive septic tank ef-
fluent, are generally perforated distribu-
tion pipes surrounded by gravel and buried
beneath the soil surface. The distribution
pipe is generally made of plastic, has a
minimum diameter of 4 inches, and is
perforated with one-half inch holes every 6
inches. The maximum length of any single
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seepage trench should not exceed 100
feet unless specific site conditions require
a longer trench. Gravel or crushed stone
surrounding the perforated pipe should be
less than 2.5 inches in diameter. Burial
depths range between 24 and 48 inches.
Shallow burial depths are preferred, since
soil in the upper horizons typically has the
best treatment characteristics (i.e., it typi-
cally remains unsaturated, has plentiful
microorganisms and conductivity is higher).
While most individual seepage
trenchesshould be no longerthan 1 OOfeet
for efficient wastewater distribution, the
necessary total length of seepage trench
is a function of the size of house to be
served and the ability of the soil to absorb
water. Necessary length increases with
increasing house size. It also increases
with decreasing soil-absorption rate. The
Tennessee Department of Environment
and Conservation’s publication, Reaula-
tions to Govern Subsurface Sewage Dis-
posal Systems’, explains the required pro-
cedures for determining soil-absorption
rates and appropriate seepage trench
lengths.
Multiple seepage trench networks
are usually required to provide the neces-
sarytotal length. Seepage trench networks
are usually connected in series with the far
end of trench one connected to the far end
of trench two and the near end of trench
two connected to the near end of trench
three. Such connection schemes ensure
that wastewater has a path for reaching all
portions of the seepage-trench network.
While the goal is even distribu-
tion of wastewater throughout the trench
system, in practice this never occurs. The
4-inch diameter pipe used in seepage
trenches is capable of handling flows in
excess of 100 gallons per minute, but most
effluent from septic tanks flows at rates
less than one gallon per minute - a mere
trickle in such a large pipe. The trickle of
effluent continues in the pipe only until it
encounters the first available perforation.
It then flows from the pipe, moves down
through the gravel and soaks into the soil.
All subsequent effluent follows the same
path, soaking into the soil at the same
location until the growth of biomass (sup-
ported by nutrients and energy in the efflu-
ent) slows the infiltration at that point.
Effluent then moves to the next location
where the process is repeated. This pro-
cess continues until soil in the bottom of ati
seepage trenches receives effluent. By
the time effluent reaches the end of the last
seepage trench, all of the other locations
have hydraulically failed and can not ab-
sorb additional effluent. Therefore, distri-
bution of effluent in conventional systems
occurs by a process of progressive hy-
draulicfailure. Once the last seepage trench
has hydraulically failed, wastewater will
either surface in the lawn or flow from a
drain or plumbing fixture inside the house.
ow 3rQssurQ 3ipe - fllternative
Advantages
Uniform distribution of septic tank
Unsaturated conditions in seepage
trenches promote maximum wastewa-
ter treatment.
Provides improved utilization of soil
with limited hydraulic properties and
depth.
Limitations
effluent.
Bedrock, impermeable soil and/or sea-
sonal groundwater levels should be
greater than 30 inches from the soil
surface (this can include up to 6 inches
of imported fill).
Soil percolation rates must be within
range of 10 to 120 minutes per inch
(MPI).
Mechanical components require regu-
Should not be used where grease
concentration exceeds 150 milligrams
per liter (mg/l) (e.g., small restaurants,
etc.).
lar inspection and maintenance.
The low pressure pipe (LPP) and
conventional systems have much in com-
mon. They both use a septic tank and
seepage trenches. The septic tank in an
LPP system is the same as in the conven-
tional system, but the seepage trenches in
an LPP system are significantly different.
In an LPP system, seepage trenches may
be as narrow as 6 inches and as shallow as
18 inches. Gravel used in LPP seepage
trenches is also smaller, having a maxi-
mum diameter in the range of 1/2 to 1 inch.
Seepage trenches in LPP systems can be
spaced as close as 5 feet on center.
Seepage trenches are not the
only thing different between conventional
and LPP systems. At the core of an LPP
system, the components that really set it
apart from the conventional system and
make it work are the small-diameter perfo-
rated pipe in the seepage trenches and the
dose tank that transfers septic tank efflu-
ent to them. The 4-inch pipe with its 1/2-
inch perforations used in a conventional
system is replaced in an LPP system by 1
to 1.5-inch diameter pipe with perforations
in the range of 5/32 to 7/32 of an inch. The
small diameter perforated distribution pipes
are fed by a pump or dosing siphon2 lo-
cated in the dose tank. The dose tank
collects water draining from the septic tank
until a volume is accumulated sufficient to
sustain a steady discharge into the distri-
bution pipes for a period of at least four
minutes. Discharge from the dose tank
then starts automatically and fills all of the
electric power supply
small diameter
In an LPP system all areas of the seepage trench are loaded uni- formly, as opposed to the spot loading that occurs in a conven- tional system.
clean outs
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distribution lines, pressurizing them slightly
(0.5 to 2 pounds per square inch). With all
linesfilledand pressurized tothesame low
level, each perforation discharges water at
the same rate. Thus, in an LPP system all
areas of the seepage trench are loaded
uniformly, as opposed to the spot loading
that occurs in a conventional system.
Those considering installing an
LPP system can find particular design and
installation details in the Tennessee De-
partment of Environment and
Conservation’s publication, Reaulations to
Govern Subsurface Sewage Disposal Svs-
terns'. Plans for an LPP system should
include, from the beginning, provisions for
regular inspection and maintenance to sus-
tain proper system performance. Required
maintenance is not extensive, but it is
essential. Pump-intake screens will have
to be cleaned periodically to maintain free
flow to the pump. The perforated distribu-
tion lines will also need periodic cleaning.
Passage of a properly sized bottle brush
will remove biological growth from perfora-
tions and adequately clean distribution
lines. Screens and distribution lines will
probably need cleaning once or twice a
year. In addition to regular cleaning of
screens and lines, mechanical devices
such as pumps and float switches will wear
and eventually require repair or replace-
ment
Advantages
Applicable in areas where soil is too
shallow for either a conventional or
an LPP system.
Mounds can be used if there is at
least 20 inches of soil between the
surface and a restrictive layer.
Limitations
Should not be used on slopes ex-
ceeding 12 percent. Sites with per-
colation rates of 61 through 120
MPI should not exceed 6 percent
slope.
Should not be used where grease
concentration exceeds 150 milli-
grams per liter (mg/l) (e.g., small
restaurants, etc).
Installation must be done with ex-
acting care for proper operation
(must avoid smearing and compact-
ing of natural soil during construc-
tion).
Mechanical components require
regular inspection and mainte-
nance.
Mounds, sometimes called “sand
mounds” or “Wisconsin mounds,” can be
very effective in treating domestic waste-
water. They are particularly well suited for
areas with shallow soil. Mounds differ from
conventional subsurface sewage disposal
in three ways: (1) mounds provide uniform
distribution of effluent, (2) they have dos-
ing and resting cycles and (3) they incor-
porate above-ground construction. While
mounds are classed as a subsurface dis-
posal system, they are constructed on top
of existing soil by “mounding” medium-
textured sand over existing natural topsoil.
The sand fill material should not contain
silt or clay. A shallow distribution bed is
excavated into the mound and filled with
small gravel that has been washed to
remove any small particles that might clog
the wastewater’s path. The original topsoil
is lightly tilled and left intact, therefore
preserving all of the available soil depth for
maximum wastewatertreatment. Like LPP
systems, mounds require pressurized dis-
tribution (pump or siphon) to evenlydistrib-
Ute wastewater throughout the mound.
Mounds can be located on land
up to 12 percent in slope, but require more
complex construction on steeper slopes.
Mounds perform better when located on
the tops of slight ridges, since this maxi-
mizes the spread of effluent in all direc-
tions. They should not be placed in de-
pressions where rainfall may collect. Since
wastewater is applied to a mound under
pressure, the mound can be located at an
elevation greater than that of the septic
tank.
Mounds are constructed in four
layers: (1) the layer of fill material (sand at
least 1 foot thick); (2) the distribution bed
(crushed stone at least 9 inches thick); (3)
a clay cap installed above the distribution
bed (1 foot thick at the center tapering to 6
inches at the mound edges); and (4) a
layer of non-clayey, fertile soil) (at least 6
inches thick) capable of supporting plant
growth.
The distribution bed should be at
least 9 inches thick, with a minimum of 6
inches of crushed rock or gravel below the
distribution pipe and 2 inches above for
proper dosing. Proper construction is es-
sential for maximum wastewater treatment.
The pressurized distribution sys-
tem for a mound is similar to the LPP
system described earlier. It consists of a ~
dosing chamber, a supply line, a manifold -~
and distribution laterals. Septic tank efflu-
ent is pumped under slight pressure to the
pipe) and subsequently to the laterals (usu-
ally 1, 1-1/4 or 1-1/2 inch PVC pipe). Lat-
eral lines should not exceed 50 feet for 1 -
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distribution manifold (usually 2-inch PVC .~ ~
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inch laterals, while larger diameter laterals
allow additional lateral length. Laterals
should be connected to the manifold in an
“H” configuration so that the distance from
the supply line to the ends of the laterals is
minimized. Small holes from 5/32 through
1/4 of an inch are drilled in the laterals at
predetermined spacing (typically 30 to 36
inches) to allow effluent to exit the pipe.
Mounds can be landscaped to
add beauty to any home site. In fact, plants
growing on the mound help renovate some
of the wastewater. When plant roots take
in wastewater, the plant separates the
waste from the water. It traps the nutrients,
which occur naturally in waste, and uses
them as food to help it grow. Then clean
water is released into the air through tiny
pores in its leaves called stomata.
If your soil is shallow and has
limited hydraulic properties, a mound may
provide the additional treatment capacity
needed. Contact your local environmental
specialist, usually located at your county
health office. He or she will determine the
suitability of a mound system at your par-
ticular building site or current residence.
xida t ion Lagoons- 0 fll terna tive
Advantages
Can be installed where percolation
rates are greater than 120 MPI.
Provide good wastewater treatment
throughout the year.
treatment is not an option.
Allow treatment where subsurface
Limitations
Minimum depth to bedrock is 5 feet.
Lot must be at least 5 acres in size.
Maximum slope for installation is 8
percent.
Lagoon must be fenced and clearly
Lagoon surface must be accessible
to prevailing winds to enhance
evaporation.
Lagoons are difficult to disguise and
may present aesthetic problems.
marked.
A lagoon system is a viable alter-
native for wastewater treatment when un-
derground disposal cannot be imple-
mented. Facultative bacteria, algae
(waterborne plants) and evaporation are
utilized to treat the incoming wastewater.
Lagoons are simply small ponds specifi-
cally constructed to treat wastewater. They
renovate wastewater year-round and re-
quire little or no maintenance.
A lagoon is typically conical in
shape with a liquid depth of 4 feet. The
interior sides of the lagoon are lined with
naturally occurring impermeable soil to
prevent seepage. Wastewater from the
septic tank enters the lagoon after passing
through a step-down region in the delivery
pipe (this forms a natural trap to prevent
odors from coming into the house). The
inlet into the lagoon is, thus, typically sub-
merged 24 inches below the water sur-
face.
Once wastewater enters the la-
goon, facultative bacteria begin to break
down the organic solids. As they consume
the organic material, they release carbon
dioxide (CO,). The release of carbon diox-
ide, combined with light, nitrogen and phos-
phates, provides all of the necessary in-
gredients for excellent algae growth. Al-
gae produce oxygen, which, in turn, feeds
the aerobic bacteria. Near the bottom of
the lagoon there is very little oxygen (i.e.,
it is anaerobic). This can result in the
formation of hydrogen sulfide (the sub-
stance responsible for the “rotten egg”
smell), but fortunately, aerobic sulfide oxi-
dizers in the upper layer of the lagoon
convert much of ittoelemental sulfur(which
doesn’t have an unpleasant odor).
The major limitation for a lagoon
system is its appearance. It is difficult to
hide a lagoon, and, to make matters worse,
it must be fenced and clearly marked
“Waste Lagoon.” Since lagoons utilize
some evaporation, they should be located
so that prevailing winds can move easily
across the lagoon surface. Therefore, no
trees or tall plants can be planted nearby to
help hide the system. Another major limita-
tion is lot size. The lot must be at least five
acres in size to utilize a septic tankhagoon
wastewater treatment system.
Subsurface sewage disposal is
preferable, but if soil conditions on your lot
prohibit subsurface sewage disposal and
your lot is at least five acres in size, a
lagoon may be a viable alternative for on-
site wastewater treatment. Contact your
local environmental specialist at the county
health department for more details on oxi-
dation lagoons.
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While conventional septic/
leachfield wastewater treatment systems
can effectively meet many on-site treat-
ment needs, sometimes the nature of the
soil prohibits the use of the conventional
system. Where soil resources are limited,
alternative domestic wastewatertreatment
systems must be installed to meet on-site
treatment needs.
Proper siting, installation and
maintenance of on-site domestic waste-
water treatment systems will help protect
water resources that must sustain both
present and future generations.
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This material is based upon work supported by the U. S. Department of Agriculture, Extension Service,
under special profect number 90-EWQI-1-9238.
- PB1472-15M-10/93 R12-2042-10-002-93
__ A State Partner in the Cooperative Extension System
The Agricultural Extension Service offers its programs to all eligible persons regardless of race, color, age, national origin, sex or
COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS The University of Tennessee Institute of Agriculture, U.S. Department of Agriculture,
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