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,

and county governments cooperating in furtherance of Acts of May 8 and June 30, 1914. Agricultural Extension Service

Billy G. Hicks, Dean

handicap and is an Equal Opportunity Employer. -