Cooperative Extension Service
West Lafayette, IN 47907
Almost 20 million housing units sheltering approximately 29 percent
of the U.S. population (1.7 million or 38 percent of
the homes in Indiana) dispose of domestic waste through
on-site disposal systems. Approximately 85 percent of these
systems are conventional septic-absorption fields, making
them by far the most common on-site disposal method.
Conventional septic systems, while common, are not suitable for all areas.
Among the limitations which might preclude installation of a conventional system
are: high groundwater tables;shallow limiting layers of bedrock or fragipan;
very slowly or rapidly permeable soils; topography; and lot size. ID-163
and ID-164 explain alternative wastewater systems that can overcome many of
Where suitable, however, conventional septic-absorption fields
will remain common for many years to come because of their
relatively low initial cost and low operating cost. They have proven
very satisfactory when properly located, designed, installed, and
maintained. This publication outlines the proper procedures
to ensure satisfactory performance of a conventional septic system.
Before arranging for installation, contact your county Extension
office or County Health Officer for the names of experienced
installers, and also consult homeowners who have had good
performance from their systems over a number of years for the
names of competent installers.
Conventional Septic Tank-Absorption Field System
The conventional septic tank-absorption field home sewage treatment
system is composed of two major element: the septic tank and the
soil absorption field (Figure 1). The septic tank (a settling and
decomposition chamber) allows the sewage solids to separate from the
liquid, undergo partial decomposition, and be stored as sludge at
the bottom of the tank. The effluent from the septic tank
then flows by gravity into the subsurface absorption field
where it infiltrates into the soil.
Figure 1. Typical conventional septic tank and soil absorption system
Septic systems require care in site selection,
design, and construction. Every individual
conventional septic tank-absorption field system
must be designed and constructed according
to State Board of Health regulation, 410
IAC 6-8, as explained in Residential Sewage
Disposal Systems (available through your
County Board of Health). A FACTS computer
program is available at your local county
Extension office which can make recommendations
for several types of on-site septic systems
for use with various site and soil conditions.
If a conventional system is suitable for your
situation, the program will provide the following
design information: estimates of the septic
gravel volume, total trench bottom area
required, and depth to the trench bottom. It
will also plot specific layouts of the septic field
for various trench widths that will best fit the
shape and size of the disposal area.
The following construction steps explain
the correct procedure for installation of a conventional
septic system. They will aid in
applying the design information provided by
the FACTS program. If the guidelines are
carefully followed, they should help insure
many years of troublefree operation.
Site Selection and Layout
All soils within the boundaries of the property
should be identified to evaluate their suitability
for an on-site soil absorption field by a
knowledgeable professional. The absorption
field should be located in the most suitable
soils and should even have location priority
over a new home. The absorption area should
also be one that sheds water. The long axis of
the soil disposal area should be oriented parallel
to the contours of the slope (i.e. lines 9f
equal elevation). Areas where the long axis
would run up and down the slope or where
wastewater movement would converge should
be avoided (Figure 2).
Figure 2. Proper orientation of trenches on complex slopes.
Traffic and construction must be avoided
over and immediately downslope from the soil
disposal area to prevent compaction and to
minimize frost penetration.
The locations of the septic tank, feedline
trench, and center line of all soil trenches
should be laid out and staked (Figure 3). Their
exact location may be dictated by minimum
distance requirements from water supplies,
structures, property lines, and bodies of water
as outlined by local and Indiana State Board
of Health regulations and listed in Table 1.
Some of these setback distances are also part
of the FACTS program's printed recommendations.
Figure 3. Before any construction on the lot, the spetic tank and distribution box should be staked and the absorption field should be laid out, staked, and fenced.
Table 1. Absorption field minimum distance requirement
Water supply source 50
Lake or reservoir 50
Stream, ditch, or 25
Dwelling or other 10
Side or rear lot lines 5
Front lot lines 5
Water lines continually 10
Suction water lines 50
When the center lines of the field trenches
are staked, the trenches should be spaced a
minimum of 7-½ feet center to center or as
specified by the computer program. As the
slope increases or the subsoil becomes more
impermeable, a greater spacing between
trenches is needed to keep from overloading
the soil around the lower trenches. This is
because wastewater entering the soil from
upslope trenches will tend to move downhill.
The trenches should not be longer than 100
feet to ensure relatively uniform application of
wastewater. The area of trench bottom is
based on the estimated volume of wastewater
from the home (number of bedrooms x 150
gal/bedroom/day) and the average loading
rate of the 24-inch depth of soil below the proposed
depth of the trench bottom.
The total area required for the soil absorption
field plus an additional distance of at
least 50 feet downslope should be fenced. This
will prevent soil disturbance, scalping, or compaction
by vehicular traffic or construction
equipment. All traffic should be prohibited
from the area before, during, and after installation
of the system to prevent damage to the
If the trenches are constructed in the side
or at the base of a slope, a diversion ditch
and/or subsurface curtain drain should be
installed. It should be backfilled with gravel
and extend to the limiting soil layer or at least
2 feet deeper than the elevation of the proposed
trench bottom (Figure 4). It should be
located above the system to keep upslope run-
off and seepage water away from the system.
Curtain or perimeter subsurface drains generally
are necessary to help control the water
table beneath the field. Any subsurface drains
around the absorption field should be kept at
the distance specified by the health department.
Figure 4. Locations of subsurface and surface drains.
Septic Tank and
Distribution Box Installation
Excavation depths for the septic tank and
distribution box are determined largely by
what is necessary to obtain gravity flow in the
sewer from the point where it leaves the house
(Figure 5). A 2 percent slope is required from
the house sewer pipe to the septic tank, while a
1 percent slope is sufficient for the pipe carrying
septic effluent to the distribution box. To
ensure that absorption field trenches are
located in the more permeable soil horizons,
the septic tank and distribution box should be
installed at as shallow a depth as possible,
with a minimum soil cover of at least 6 inches.
The FACTS program will compute this
automatically for the site by assuming a 1 percent
slope for the effluent supply line to the
distribution box once the depth to the septic
tank outlet depth is specified.
Figure 5. Inlet and outlet slope of septic tank sewer lines
Both the septic tank and distribution box
should be carefully leveled for proper operation
after their installation. Access must be provided
to the tank and distribution box for
future inspection and maintenance.
Tank seams and inlet and outlet connections
should be sealed with an appropriate
material against groundwater seepage and tree
root intrusions. Four-inch minimum diameter
sewer pipe (PVC: ASTM-D-2665, 3033, 3034 or
other health department approved piping) with
watertight connections between the house and
septic tank and between the septic tank and
the distribution box should be used.
Soil backfill should be crowned over the
tank and distribution box to a height of 6
inches to allow for settling and to divert surface
runoff. Footing and roof drains must not
be connected to the septic system.
The trenches should only be excavated
when the soil is dry and friable. Smearing
and compaction due to construction in a wet
soil decrease the ability to absorb waste
water. If a sample of the soil at the trench
bottom depth forms a ribbon (e.g. 1/8-inch diameter)
when rolled between the palms of the
hands, the soil is too wet to excavate. If the
soil crumbles, excavation may proceed (Figure
6). This pre-excavation investigation is essential
to help ensure proper operation of the system.
Figure 6. Construction must not take place if soil is too wet.
The lateral trenches should be excavated to
the design depth and width recommended by
the FACTS program. The bottom elevation
should be checked with an engineer's level to
ensure that the trench bottoms are level over
the length of each trench. After most soil is
removed, an excavating bucket with teeth
should be used to rake each trench bottom to
final bottom elevation (Figure 7).
Figure 7. Keep backhoe bucket perpendicular to trench bottom to minimize compaction.
The sides of the lateral trenches should also
be raked to a depth of 1 inch to expose the
natural soil structure and to remove any
smeared and compacted soil surface caused by
the excavating bucket. This can be done by
attaching fabricated raker teeth to each side of
the bucket (Figure 8). Foot traffic on the excavated
trench bottom should be minimized to
prevent further compaction. If foot traffic is
necessary, planks should be used to spread out
the workers' weight. For both level and sloping
sites the trenches should be connected individually
to the distribution box by a header or
manifold trench. Distal ends of the trenches
may be tied together on level sites to further
improve wastewater application.
Figure 8. Fabfricated raker teeth to reduce compaction and smearing of trench side walls.
Monitoring wells, sometimes installed to
permit evaluation of the performance of the
absorption field once it is in operation, can be
installed at this point in construction of the
system. These wells are usually 4-inch diameter
PVC pipe. Each monitoring pipe is perforated
with ½ inch holes over the lower 6-inch
length. The well should extend to the ground
surface and be covered with a friction-fit cap
or screw cap. If used, monitoring wells should
be located in several trenches, extending from
the trench bottom to the final surface grade
(Figure 9). This will provide a means of
evaluating the depth of ponding in a trench, a
measure of the system's performance.
Distribution Network Installation
Figure 9. Monitoring wells shown in completed trench and outside of absorption field.
Washed Indiana State Highway Specification
(Spec #5) aggregate or other trench fill
material that has been approved by the
County Sanitarian should be carefully placed
to a depth of 6 inches over the bottom of the
trenches. Finally, the aggregate should be leveled.
If trenches are constructed in a wooded
area, at least 12 inches of fill material should
be placed below the pipe or tile to discourage
root growth within the distribution line.
A 4-inch diameter perforated distribution
line of plastic or vitrified tile should be laid on
a maximum grade of 4 inches per 100 feet.
Pipe for the distribution lines must meet local
and state health department specifications.
Coiled perforated plastic tubing should not be
installed because the tubing will not maintain
grade or lay flat. All open joints in the distribution
lines should be covered with a non-
decomposing material to prevent the entry of
stone. Each lateral line for the absorption
field at a sloping or level site should be individually
connected to a leveled distribution
box by a nonperforated header pipe at the
same invert elevation to ensure equal distribution
to all the trenches. During the assembly
of the plastic piping in the trenches, it is
important to ensure that the perforations are
not on the invert side of the pipe (Figure 10).
Figure 10. Ensure outlet holes are at 4 and 8 o'clock positions and not at the invert position.
Backfilling the Trenches
The pipe network should be carefully
covered with additional washed Spec #5 gravel
or approved aggregate to a depth of at least 2
inches above the crown of the pipe.
Next, a backfill barrier such as a synthetic
fabric filter, 4-6 inches of marsh hay or straw,
or untreated building paper (red rosin) should
be placed over the aggregate cover.
The trenches should be backfilled with
excavated soil and compact slightly. The soil
should be mounded 4-6 inches over the top of
the trench to allow for settlement. Individual
trenches can be excavated and completed m
sequence for ease of construction.
Since settlement may take 6-12 months, the
construction area should be sodded or seeded
immediately, using grasses adapted to the
Maintaining the System
To maximize the uptake of water, good
grass or vegetative cover should be maintained
over the area. The homeowner should have a
layout diagram of the septic system, referenced
to the house and lot boundaries. This will
facilitate location of the tanks and absorption
field for future maintenance.
Sludge should be removed from the septic
tank every 3-5 years. This clean-out schedule
is important with standard gravity-fed systems
to avoid carryover of solids which can
plug the disposal trenches.
Homeowners should monitor the performance
of the septic system by routinely checking
the depth of water in the monitoring wells
during spring, summer, and fall. Any progressive
increase m ponding depth within the
trenches over time may be indicative of future
Water conservation measures in the home
help to ensure that the soil disposal area will
not be overloaded. It would be wise to install
faucet aerators and low-flow fixtures and
appliances when possible.
Operating and Maintaining the Home Septic System
(ID-142) is a Cooperative Extension publication written for homeowners.
This publication and the above recommendations should help homeowners better
understand the operation and maintenance of their on-site septic system for
many years of troublefree operation.
Cooperative Extension Work in Agriculture and Home Economics, State
of Indiana, Purdue University
and U.S. Department of Agriculture Cooperating. H.A. Wadsworth, Director
West Lafayette, IN. Issued
in furtherance of the Acts of May 8 and June 30, 1914. It is the policy
of the Cooperative Extension
Service of Purdue University that all persons shall have equal opportunity
and access to our programs