U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Consumer Protection and Environmental Health Service
Environmental Control Administration
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Safety Considerations
It is particularly important that proper safety precautions be taken
when percolation holes or larger'excavations are dug to install septic
tanks or seepage pits. Means should be provided to prevent the side-
walls from collapsing while workmen are in the hole. A common
method of affording proper protection to the workmen is through the
use of sheeting formed by semicircular sections of corrugated metal,
braced with semicircular compression rods which are bolted on the
inside with expansion bolts. In another type of seepage pit construc-
tion, the walls are made of precast reinforced concrete sections with
slotted holes. For deep seepage pits, or where there is any danger of
caving, the sections are installed as the excavation progresses, and are
used TLS the necessary protective sheeting. During non-work periods
holes should be covered with boards that cannot be easily removed or
the hole should be surrounded with a fence that cannot be easily
entered. Any open hole is dangerous and should be filled in when the
work is completed. Fatal accidents have occurred when these basic
safety measures have not been observed.
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Manual of
SEPTIC - TANK
PRACTICE
^S / J ' C ' r •// //
<=~Lseveiepefi m \^^&ope'cAlton with, Ike
l&int (^ontHtittee on v^ata-l
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
PUBLIC HEALTH SERVICE
Consumer Protection and Environmental Health Service
Environmental Control Administration
Rockvifle, Maryland 20852
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Public Health Service Publication No. 526
First Printed 1957
Reprinted 1963
Revised 1967
Reprinted 1969
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20201 - 35 cents
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Foreword
In the preparation of this manual, the Public Health Service was
fortunate in having the advisory assistance of the Joint Committee on
Rural Sanitation. This committee is composed of specialists from gov-
ernmental and other agencies in the field of rural sanitation. Their
comments and suggestions based on the long experience of the mem-
bers were invaluable in the preparation of the manual. The following
individuals and organizations constitute the current Committee
membership:
U.S. DEPARTMENT OF AGRICULTURE:
Agricultural Research Service
Harry J. Eby, Agricultural Engineer, Agricultural Engineering
Research Branch.
Farmers' Home Administration
Earl R. Bell, Agricultural Engineer.
Federal Extension Service
W. T. Cox, Agricultural Engineer.
Forest Service
H. A. Smallwood, Division of Engineering, Consultation, and
Standards.
Soil Conservation Service
William G. Shannon, Chief, Water Supply Forecasting Branch,
Engineering Division.
AMERICAN PUBLIC HEALTH ASSOCIATION:
Professor John E. Kiker, Jr., College of Engineering, University of
Florida.
U.S. COAST GUARD:
Captain James H. Le Van, Chief Sanitary Engineer Officer.
CONFERENCE OF MUNICIPAL PUBLIC HEALTH ENGINEERS:
William H. Gary, Jr., Associate Director for Environmental Health,
D.C. Department of Public Health.
FEDERAL HOUSING ADMINISTRATION:
William K. Rodman, Special Assistant for the Technical Studies
Program.
FEDERATION OF SEWAGE AND INDUSTRIAL WASTES ASSOCIATIONS:
David B. Lee, Director, Bureau of Sanitary Engineering, Florida
State Board of Health.
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U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE:
Office of Education
John L. Cameron, Acting Director, Division of Facilities De-
velopment.
Public Health Service
Malcolm C. Hope, Sanitary Engineer Director.
Joseph P. Schock, Public Health Engineer.
U.S. DEPARTMENT OF THE INTERIOR:
U.S. Geological Survey
Clyde S. Conover, District Chief, Water Resources Division.
TENNESSEE VALLEY AUTHORITY:
F. E. Gartrell, Assistant Director of Health.
VETERANS HOUSING ADMINISTRATION:
D. J. Guthridge, Construction and Valuation Specialist.
INDUSTRY ADVISORS:
John G. Hendrickson, Jr., Portland Cement Association.
James J. Spear, Spear Water and Sewerage Supplies
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Preface
Population movement within the United States continues to be from
rural to metropolitan areas. Because of the difficulty of providing ade-
quate sewerage systems for this new growth, individual septic tank - soil
absorption systems continue to be an important method of sewage
disposal where they are acceptable. Accurate figures are not available,
but it is estimated that at present 49 million persons are served by 15
million individual sewage-disposal systems in the United States. Of even
more importance, roughly one-fourth of the new homes are being con-
structed with these systems.
In 1946, the Public Health Service, in cooperation with Federal
agencies concerned with housing, undertook a 5-year study on septic
tank — soil absorption systems, seeking to develop a factual basis on which
they could be designed, installed and maintained. These studies are
described in detail in three technical reports: Studies on Household
Sewage Disposal Systems, Parts I, II, and III.
Subsequent studies on septic tanks and soil absorption systems have
been conducted by the Public Health Service, Federal Housing Admin-
istration, Universities and other organizations.
This manual is a revision of PHS Publication No. 526, Manual o/
Septic Tank Practice, issued in 1957, and reprinted in 1963 with
Addendums on "Serial Distribution Systems" and "Seepage Beds." The
updating reflects changing trends in the problems of individual sewage
disposal systems and includes new information in this field.
The decision on the suitability of septic-tank installations must be
based on many factors outside of those covered in the manual. It is
emphasized, however, that connection to an adequate public sewerage
system is the most satisfactory method of disposing of sewage. Every
effort should be made, therefore, to secure public-sewer extensions.
Where connection to a public sewer is not feasible, and when a con-
siderable number of residences are to be served, consideration should
be given next to the construction of a community sewerage system and
treatment plant. Specific information on this matter should be obtained
from the local authority having jurisdiction.
Individuals proposing to construct individual sewage-disposal systems
should consult the officials having jurisdiction over such installations in
their area. A number of States and localities have developed require-
ments which have been incorporated in their official regulations, in
many cases soundly based on conditions peculiar to those areas and
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adequately representing good practice there. The recommendations
contained in this manual should be considered as supplemental to such
local requirements. Builders, homeowners, and others interested in
septic-tank systems should seek advance guidance from the local authori-
ties prior to land acquisition, in order to have the benefits of their
experience as well as their approval of plans and construction.
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Contents
PART I. SEPTIC TANK - SOIL ABSORPTION SYSTEMS FOR PRIVATE RESIDENCES
Page
Introduction 1
Definitions 2
Suitability of Soil 3
Percolation Tests 4
Procedure for Percolation Tests Developed at Robert A.
Taft Sanitary Engineering Center 4
Soil Absorption System 8
Absorption Trenches 10
Construction Considerations 12
Seepage Beds 14
Construction Considerations 15
Distribution Boxes 16
Serial Distribution 16
Fields in Flat Areas 17
Fields in Sloping Ground 17
Deep Absorption Trenches and Seepage Beds 20
Seepage Pits 20
Sample Calculations 23
Construction Considerations 26
Selection of a Septic Tank 26
Functions of Septic Tanks 26
Removal of Solids 27
Biological Treatment 27
Sludge and Scum Storage 27
Location 27
Effluent 29
Capacity 29
Specifications for Septic Tanks 29
Materials 29
General 30
Inlet 30
Outlet 30
Tank Proportions 32
Storage Above Liquid Level 33
Use of Compartments 34
vii
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General Information on Septic Tanks 35
Cleaning 35
Grease Interceptors 38
Chemicals 38
Miscellaneous 38
Inspection 39
PART II. SEPTIC TANK-SOIL ABSORPTION SYSTEMS FOR INSTITUTIONS,
RECREATIONAL AREAS, AND OTHER ESTABLISHMENTS
Introduction 41
Estimates of Sewage Quantities 42
Estimates of Soil Absorption Areas 45
Building Sewers 48
Collection Systems 49
Grease Traps 50
Location 50
Construction Details 51
Capacity 52
Operation 53
Septic Tanks for Institutional Systems 53
Capacities 54
Dosing Tanks 55
Sand-Filter Trenches 56
Construction Features 60
Subsurface Sand Filters 62
Construction Features 62
Superficial Sand Filters 64
Chlorination 66
APPENDICES
Introduction to Appendices 71
A. Soil Absorption Capacity 72
Guide for Estimating Soil Absorption Potential 72
Soil Maps 72
Clues to Absorption Capacity 72
Texture 72
Structure 73
Color 73
Depth or Thickness of Permeable Strata 73
Swelling Characteristics 73
Evapotranspiration 74
Curtain Drains for Absorption Trench Systems 74
viii
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Percolation Test Holes 75
Other Percolation Tests 75
B. Suggested Ordinance 77
C. Engineering Information Forms 81
D. Drainage Fixture Unit Values 85
E. Suggested Specifications for Watertight Concrete 86
F. Industrial Waste Treatment 88
BIBLIOGRAPHY 91
ix
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Parti
Septic Tank - Soil Absorption
Systems for Private Residences
INTRODUCTION
A major factor influencing the health of individuals where public
sewers are not available is the proper disposal of human excreta. Many
diseases, such as dysentery, infectious hepatitis, typhoid and para-
typhoid, and various types of diarrhea are transmitted from one person
to another through the fecal contamination of food and water, largely
due to the improper disposal of human wastes. For this reason, every
effort should be made to prevent such hazards and to dispose of all
human waste so that no opportunity will exist for contamination of
water or food.
Safe disposal of all human and domestic wastes is necessary to protect
the health of the individual family and the community and to prevent
the occurrence of nuisances. To accomplish satisfactory results, such
wastes must be disposed of so that:
I. They will not contaminate any drinking water supply.
2. They will not give rise to a public health hazard by being acces-
sible to insects, rodents, or other possible carriers which may come into
contact with food or drinking water.
3. They will not give rise to a public health hazard by being acces-
sible to children.
4. They will not violate laws or regulations governing water pollu-
tion or sewage disposal.
5. They will not pollute or contaminate the waters of any bathing
beach, shellfish breeding ground, or stream used for public or domestic
water supply purposes, or for recreational purposes.
6. They will not give rise to a nuisance due to odor or unsightly
appearance.
These criteria can best be met by the discharge of domestic sewage
to an adequate public or community sewerage system. Where the instal-
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lation of an individual household sewage disposal system is necessary,
the basic principles outlined in this manual on design, construction,
installation, and maintenance should be followed. When these criteria
are met, and where soil and site conditions are favorable, the septic
tank system can be expected to give satisfactory service. Experience has
shown that adequate supervision, inspection and maintenance of
all features of the system are required to insure compliance in this
respect. Underground portions of the system should be inspected before
being covered, so necessary corrections can be made.
DEFINITIONS
Absorption Trench—A trench not over 36" in width with a minimum
of 12" of clean, coarse aggregate and a distribution pipe, and covered
with a minimum of 12" of earth cover.
Standard Absorption Trench—A trench 12" to 36" in width con-
taining 12" of clean, coarse aggregate and a distribution pipe, covered
with a minimum of 12" of earth cover.
Building Drain—That part of the lowest piping of a drainage system
which receives the discharge from soil, waste, and other drainage pipes
inside the walls of the building and conveys it to the building sewer
beginning three feet outside the building wall.
Building Sewer—That part of a drainage system which extends from
the end of the building drain and conveys its discharge to a public
sewer, private sewer, individual sewage disposal system or other point
of disposal.
Cesspool—A lined and covered excavation in the ground which re-
ceives the discharge of domestic sewage or other organic wastes from a
drainage system, so designed as to retain the organic matter and solids,
but permitting the liquids to seep through the bottom and sides.
Drainage Fixture Unit Value—A common measure of the probable
discharge into a drainage system by various types of plumbing fixtures.
This value for a particular fixture depends on its volume rate of drain-
age discharge, on the time duration of a single drainage operation, and
on the average time between successive operations.
Effective Size—That size of sand of which 10% by weight is smaller.
Individual Sewage Disposal System—A single system of sewage treat-
ment tanks and disposal facilities serving only a single lot.
Sand Filter Trenches—A system of trenches, consisting of perforated
pipe or drain tile surrounded by clean, coarse aggregate containing an
intermediate layer of sand as filtering material and provided with an
underdrain for carrying off the filtered sewage.
Scum—A mass of sewage matter which floats on the surface of sewage.
Scum Clear Space—Distance between the bottom of the scum mat
and the bottom of the outlet device.
Seepage Bed—A trench or bed exceeding 36" in width containing 12"
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a minimum of clean, coarse aggregate and a system of distribution pip-
ing through which treated sewage may seep into the surrounding soil.
Seepage Pit—A covered pit with lining designed to permit treated
sewage to seep into the surrounding soil.
Septic Tank—A water-tight, covered receptacle designed and con-
structed to receive the discharge of sewage from a building sewer, sep-
arate solids from the liquid, digest organic matter and store digested
solids through a period of detention, and allow the clarified liquids to
discharge for final disposal.
Serial Distribution—An arrangement of absorption trenches, seepage
pits, or seepage beds so that each is forced to pond to utilize the total
effective absorption area before liquid flows into the succeeding
component.
Sewage—Any liquid waste containing animal or vegetable matter in
suspension or solution, and may include liquids containing chemicals
in solution.
Sludge—The accumulated settled solids deposited from sewage and
containing more or less water to form a semi-liquid mass.
Sludge Clear Space—The distance between the top of the sludge and
the bottom of the outlet device.
Soil Absorption Field—A system of absorption trenches.
Soil Absorption System—Any system that utilizes the soil for subse-
quent absorption of the treated sewage; such as an absorption trench,
seepage bed, or a seepage pit.
Subsurface Sand Filters—A wide bed, consisting of a number of lines
of perforated pipe or drain tile surrounded by clean coarse aggregate,
containing an intermediate layer of sand as filtering material, and
provided with a system of underdrains for carrying off the filtered
sewage.
Subsurface Sewage Disposal System—A system for the treatment and
disposal of domestic sewage by means of a septic tank and a soil absorp-
tion system.
Uniformity Coefficient—A coefficient obtained by dividing that size
of sand of which 60% by weight is smaller, by that size of sand of
which 10% by weight is smaller.
SUITABILITY OF SOIL
The first step in the design of subsurface sewage disposal systems is
to determine whether the soil is suitable for the absorption of septic
tank effluent and, if so, how much area is required. The soil must have
an acceptable percolation rate, without interference from ground water
or impervious strata below the level of the absorption system. In gen-
eral, two conditions must be met:
(1) The percolation time should be within the range of those speci-
fied in Table 1, p. 8.
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(2) The maximum seasonal elevation of the ground water table
should be at least 4-feet below the bottom of the trench or seepage
pit. Rock formulations or other impervious strata should be at a depth
greater than 4-feet below the bottom of trench or seepage pit.
Unless these conditions can be satisfied, the site is unsuitable for a
conventional subsurface sewage disposal system.
PERCOLATION TESTS
Subsurface explorations are necessary to determine subsurface forma-
tions in a given area. An auger with an extension handle, as shown in
Figure 1 (p. 5), is often used for making the investigation. In some
cases, an examination of road cuts, stream embankments, or building
excavations will give useful information. Wells and well drillers' logs
can also be used to obtain information on ground water and subsurface
conditions. In some areas, subsoil strata vary widely in short distances,
and borings must be made at the site of the system. If the subsoil ap-
pears suitable, as judged by other characteristics described in Appen-
dix A, percolation tests should be made at points and elevations selected
as typical of the area in which the disposal field will be located.
The percolation tests help to determine the acceptability of the site
and establish the design size of the subsurface disposal system. The
length of time required for percolation tests will vary in different types
of soil. The safest method is to make tests in holes which have been
kept filled with water for at least 4 hours, preferably overnight. This
is particularly desirable if the tests are to be made by an inexperienced
person, and in some soils it is necessary even if the individual has had
considerable experience (as in soils which swell upon wetting). Percola-
tion rates should be figured on the basis of the test data obtained after
the soil has had opportunity to become wetted or saturated and has
had opportunity to swell for at least 24 hours. Enough tests should be
made in separate holes to assure that the results are valid.
The percolation test developed at the Robert A. Taft Sanitary Engi-
neering Center incorporates these principles. Its use is particularly
recommended when knowledge of soil types and soil structure is lim-
ited. When previous experience and information on soil characteristics
are available, some persons prefer other percolation test procedures,
such as those developed by Kiker and by Ludwig which are cited in
Appendix A.
Procedure for Percolation Tests Developed at Robert A. Taft Sanitary
Engineering Center
1. Number and location of tests.—Six or more tests shall be made in
separate test holes spaced uniformly over the proposed absorption field
site.
2. Type of test hole.—Dig or bore a hole, with horizontal dimensions
of from 4 to 12 inches and vertical sides to the depth of the proposed
absorption trench. In order to save time, labor, and volume of water
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Figure 1.—Auger and extension handle for making test borings.
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required per test, the holes can be bored with a 4 inch auger. (See
Fig. 2, page 7.)
3. Preparation of test hole.—Carefully scratch the bottom and sides
of the hole with a knife blade or sharp-pointed instrument, in order
to remove any smeared soil surfaces and to provide a natural soil inter-
face into which water may percolate. Remove all loose material from
the hole. Add 2 inches of coarse sand or fine gravel to protect the
bottom from scouring and sediment.
4. Saturation and swelling of the soil,—It is important to distin-
guish between saturation and swelling. Saturation means that the void
spaces between soil particles are full of water. This can be accomplished
in a short period of time. Swelling is caused by intrusion of water into
the individual soil particle. This is a slow process, especially in clay-
type soil, and is the reason for requiring a prolonged soaking period.
In the conduct of the test, carefully fill the hole with clear water to
a minimum depth of 12 inches over the gravel. In most soils, it is neces-
sary to refill the hole by supplying a surplus reservoir of water, possibly
by means of an automatic syphon, to keep water in the hole for at least
4 hours and preferably overnight. Determine the percolation rate 24
hours after water is first added to the hole. This procedure is to insure
that the soil is given ample opportunity to swell and to approach the
condition it will be in during the wettest season of the year. Thus, the
test will give comparable results in the same soil, whether made in a
dry or in a wet season. In sandy soils containing little or no clay, the
swelling procedure is not essential, and the test may be made as de-
scribed under item 5C, after the water from one filling of the hole has
completely seeped away.
5. Percolation-rate measurement.—With the exception of sandy soils,
percolation-rate measurements shall be made on the day following the
procedure described under item 4, above.
A. If water remains in the test hole after the overnight swelling pe-
riod, adjust the depth to approximately 6 inches over the gravel. From
a fixed reference point, measure the drop in water level over a 30 min-
ute period. This drop is used to calculate the percolation rate.
B, If no water remains in the hole after the overnight swelling pe-
riod, add clear water to bring the depth of water in the hole to approxi-
mately 6 inches over the gravel. From a fixed reference point, measure
the drop in water level at approximately 30 minute intervals for 4
hours, refilling 6 inches over the gravel as necessary. The drop that
occurs during the final 30 minute period is used to calculate the perco-
lation rate. The drops during prior periods provide information for
possible modification of the procedure to suit local circumstances.
C. In sandy soils (or other soils in which the first 6 inches of water
seeps away in less than 30 minutes, after the overnight swelling period),
the time interval between measurements shall be taken as 10 minutes
and the test run for one hour. The drop that occurs during the final
10 minutes is used to calculate the percolation rate.
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WHEN MAKING PERCOLATION
TESTS MARK LINES HERE AT
REGULAR TIME INTERVALS
WHEN MAKING PERCOLATION
TESTS MARK LINES HERE AT
REGULAR TIME INTERVALS
MEASURING STICK
MEASURING STICK
GUIDE LINE!
BARER BOARD
OR OTHER FIXED
REFERENCE POINT
rx2" BATTER BOARD
KEEP MEASURING
STICK WITHIN
GUIDE LINES ON
BATTER BOARD
jWHEN EACH
[READING is
TAKEN
T*
WATER SURFACE
LEAVE BATTER BOARD IN PLACE
BE CAREFUL NOT TO MOVE IT
DURING TEST.
Figure 2.—Methods of making percolation tests.
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Table 1.—Absorption-area requirements for individual residences (a)
[Provides for garbage grinder and automatic clothes washing machines]
Percolation rate (time
required for water to
fall one inch,
in minutes)
1 or less
2
3
4
5
Required absorp-
tion area, in
sq. ft. per
bedroom (b),
standard trench
(c), seepage beds
(c), and seepage
pits (d)
70
85
100
115
125
Percolation rate (time
required for water to
fall one inch,
in minutes)
10
15
30 (e)
45 (e)
60 (e), (f)
Required absorp-
tion area in sq. ft.
per bedroom (b),
standard trench
(c), and seepage
beds (c), and
seepage pits (d)
165
190
250
300
330
(a) It is desirable to provide sufficient land area for entire new absorption system
if needed in future.1
(b) In every case sufficient land area should be provided for the number of bed-
rooms (minimum of 2) that can be reasonably anticipated, including the unfinished
space available for conversion as additional bedrooms.
(c) Absorption area is figured as trench-bottom area and includes a statistical al-
lowance for vertical side wall area.
(d) Absorption area for seepage pits is figured as effective side wall area beneath
the inlet.
(e) Unsuitable for seepage pits if over thirty.
(f) Unsuitable for absorption systems if over sixty.
1 Section 5.1(b) (2) (A) Page 20 of Recommended State Legislation and Regulations:
Urban Water Supply and Sewerage Systems Act and Regulations, Water Well Con-
struction and Pump Installation Act and Regulations, Individual Sewerage Disposal
Systems Act and Regulations. U.S.D.H.E.W., Public Health Service, July 1965.
SOIL ABSORPTION SYSTEM
For areas where the percolation rates and soil characteristics are
good, the next step after making the percolation tests is to determine
the required absorption area from Table 1 or Figure 3 (page 9), and
to select the soil absorption system that will be satisfactory for the
area in question. As noted in Table 1, soil in which the percolation
rate is slower than 1 inch in 30 minutes is unsuitable for seepage pits,
and that slower than 1 inch in 60 minutes is unsuitable for any type
of soil absorption system.
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350
300
250
200
150
100
50
7
>r
10 20 30 40 50
PERCOLATION RATE IN MINUTES PER INCH
Figure 3.—Absorption area requirements for private residences.
When a soil absorption system is determined to be useable, three
types of design may be considered: Absorption trenches, seepage beds.
and seepage pits. A modification of the standard absorption trench is
discussed on page 20 giving credit for more than the standard 12 inches
of gravel depth in the trench.
The selection of the absorption system will be dependent to some
extent on the location of the system in the area under consideration.
A safe distance should be maintained between the site and any source
of water supply. Since the distance that pollution will travel under-
ground depends upon numerous factors, including the characteristics
of the subsoil formations and the quantity of sewage discharged, no
specified distance would be absolutely safe in all localities. Ordinarily,
of course, the greater the distance, the greater will be the safety pro-
vided. In general, location of components of sewage disposal systems
should be as shown in the following table.
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Table 2.—Minimum distance between components of sewage disposal system
Component
of
System
Building sewer
Septic tank
Disposal field and
Seepage Bed
Seepage Pit
Cesspool (b)
Horizontal Distance (feet)
Well or
suction
line
50
50
100
100
150
Water
supply
line
(pressure)
10 (a)
10
25
50
50
Stream
50
50
50
50
50
Dwelling
5
20
20
20
Property
line
10
5
10
15
(a) Where the water supply line must cross the sewer line, the bottom of the
water service within 10 feet of the point of crossing, shall be at least 12 inches
above the top of the sewer line. The sewer line shall be of cast iron with leaded or
mechanical joints at least 10 feet on either side of the crossing.
(b) Not recommended as a substitute for a septic tank. To be used only when
found necessary and approved by the health authority.
Seepage pits should not be used in areas where domestic water sup-
plies are obtained from shallow wells, or where there are limestone
formations and sinkholes with connection to underground channels
through which pollution may travel to water sources.
Details pertaining to local water wells, such as depth, type of con-
struction, vertical zone of influence, etc., together with data on the
geological formations and porosity of subsoil strata, should be consid-
ered in determining the safe allowable distance between wells and sub-
surface disposal systems.
Absorption Trenches
A soil absorption field consists of a field of 12 inch lengths of 4 inch
agricultural drain tile, 2 to 3 foot lengths of vitrified clay sewer pipe,
or perforated, nonmetallic pipe. In areas having unusual soil or water
characteristics, local experience should be reviewed before selecting
piping materials. The individual laterals preferably should not be over
100 feet long, and the trench bottom and tile distribution lines should
be level. Use of more and shorter laterals is preferred because if some-
thing should happen to disturb one line, most of the field will still be
serviceable. From a theoretical moisture flow viewpoint, a spacing of
twice the depth of gravel would prevent taxing the percolative capacity
of the adjacent soil.
Many different designs may be used in laying out subsurface disposal
fields. The choice may depend on the size and shape of the available
10
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disposal area, the capacity required, and the topography of the disposal
area.
Typical layouts of absorption trenches are shown in Figures 4, 6,
pages 11, and 17.
To provide the minimum required gravel depth and earth cover, the
depth of the absorption trenches should be at least 24 inches. Addi-
tional depth may be needed for contour adjustment, extra aggregate
under the tile, or other design purposes. The maintenance of a 4 feet
separation between the bottom of the trench and the water table is
required to minimize ground water contamination. In considering the
depth of the absorption field trenches, the possibility of tile lines freez-
ing during prolonged cold period is raised. Freezing rarely occurs in a
carefully constructed system kept in continuous operation. It is impor-
tant during construction to assure that the tile lines are surrounded by
gravel. Pipes under driveways or other surfaces which are usually
cleared of snow should be insulated.
•r-4" DRAIN TILE / /
^^ mm^^^^^m ^^p^^^^^H •M^^^MMI r^^ffj •MBW ^•••••••B mm^mmm
L :7 r--i' : _ -
S ' I WATERTIGH-
yr~
DASHED LINES INDICATE EXTENT
OF COARSE AGGREGATE
WATERTIGHT JOINTS AT SEND
DASHED LINES INDICATE EXTENT
OF COARSE AGGREGATE
SECTION A-A
Figure 4.—Typical layout of absorption trench.
11
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The required absorption area is predicated on the results of the soil
percolation test, and may be obtained from column 2 or 4 of Table 1
(page 8), or Figure 3 (page 9). Note especially that the area require-
ments are per bedroom. The area of the lot on which the house is to
be built should be large enough to allow room for an additional system
if the first one fails. Thus for a 3 bedroom house on a lot where the
minimum percolation rate was 1 inch in 15 minutes, the necessary
absorption area will be 3 bedrooms X 190 sq. ft. per bedroom, or 570
sq. ft. For trenches 2 feet wide with 6 inches of gravel below the drain
pipe, the required total length of trench would be 570 + 2, or 285 feet.
If this were divided into 5 portions (i.e., 5 laterals), the length of each
line would be 285 •*- 5, or 57 feet. The spacing of trenches is generally
governed by practical construction considerations dependent on the
type of equipment, safety, etc. For serial distribution on sloping ground,
trenches should be separated by 6 feet to prevent short circuiting.
Table 2, page 10, gives the various distances the system has to be kept
away from wells, dwellings, etc.
In the example cited, trenches are 2 feet wide X 5 trenches - 10 feet
plus 6 feet between trenches X 4 spaces — 24 feet. The total width of
34 feet x 57 feet in length = 1,938 square feet, plus additional land
required to keep the field away from wells, property lines, etc.
•SUI1ABIE PERVIOUS BARRIER
OPENINGS AT JOINTS
JOINT COVBtlNG
IZ1
J
LONGITUDINAL
SECTION
GRAVEL OR
BROKEN 5IONE
LATERAL OF
DRAIN THE (SHOWN)
OPfN JOINTED SEWER PIPE
O« PERFOKAKO PIPE
"iOtt Dt-W* Tin tAlB **"* JOINTS OKNcD
fKJW I 8 TO I •« INGH. SPtC!»L COUAiS
v*Y if IKED IF HSlttD.
Figure 5.—Absorption trench and lateral.
Construction Considerations.—Careful construction is important in
obtaining a satisfactory soil absorption system. Attention should be
given to the protection of the natural absorption properties of the soil.
Care must be taken to prevent sealing of the surface on the bottom
12
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and sides of the trench. Trenches should not be excavated when the
soil is wet enough to smear or compact easily. Soil moisture is right
for safe working only when a handful will mold with considerable
pressure. Open trenches should be protected from surface runoff to
prevent the entrance of silt and debris. If it is necessary to walk in the
trench, a temporary board laid on the bottom will reduce the damage.
Some smearing and damage is bound to occur. All smeared or com-
pacted surfaces should be raked to a depth of 1 inch, and loose mate-
rial removed, before the gravel is placed in the trench.
The pipe, laid in a trench of sufficient width and depth, should be
surrounded by clean, graded gravel or rock, broken hard burned clay
brick, or similar aggregate. The material may range in size from i/2 inch
to 2i/2 inches. Cinders, broken shell, and similar material are not rec-
ommended, because they are usually too fine and may lead to prema-
ture clogging. The material should extend from at least 2 inches above
the top of the pipe to at least 6 inches below the bottom of the pipe.
If tile is used, the upper half of the joint openings should be covered,
as shown in Figure 5, page 12. The top of the stone should be covered,
with untreated building paper, a 2 inch layer of hay or straw, or similar
pervious material to prevent the stone from becoming clogged by the
earth backfill. An impervious covering should not be used, as this inter-
feres with evaportranspiration at the surface (see Appendix A, page 74).
Although generally not figured in the calculations, evapotranspiration
is often an important factor in the operation of horizontal absorption
systems.
Drain tile connectors, collars, clips, or other spacers with covers for
the upper half of the joints are of value in obtaining uniform spacing,
proper alignment, and protection of tile joints, but use of such aides
is optional. They have been made of galvanized iron, copper, and
plastic.
It has been found that root problems may be prevented best by using
a liberal amount of gravel or stone around the tile. Clogging due to
roots has occurred mostly in lines with insufficient gravel under the
tile. Furthermore, roots seek the location where moisture conditions are
most favorable for growth and, in the small percentage of cases where
they become troublesome in well designed installations, there is usually
some explanation involving the moisture conditions. At a residence
which is used only during the summer, for example, roots are most
likely to penetrate when the house is uninhabited, or when moisture
immediately below or around the gravel becomes less plentiful than
during the period when the system is in use. In general, trenches con-
structed with 10 feet of large trees or dense shrubbery should have
at least 12 inches of gravel or crushed stone beneath the tile.
"If trees are near the sewage disposal system, difficulty with roots
entering poorly joined sewer lines can be anticipated. Lead-caulked
cast iron pipe, a sulfur base or bituminous pipe joint compound, me-
13
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chanical clay pipe joints, copper rings over joints and lump copper
sulfate in pipe trenches have been found effective in resisting the en-
trance of roots into pipe joints. Roots will penetrate into the gravel in
tile field trenches rather than into the pipe. About 2 or 3 pounds of
copper sulfate crystals flushed down the toilet bowl once a year will
destroy roots the solution comes in contact with, but will not prevent
new roots from entering. The application of the chemical should be
done at a time, such as late in the evening when the maximum contact
time can be obtained before dilution. Copper sulfate will corrode
chrome, iron and brass, hence it should not be allowed to come into
contact with these metals. Cast iron is not affected to any appreciable
extent. Some time must elapse before the roots are killed and broken
off. Copper sulfate in the recommended dosage will not interfere with
operation of the septic tank." '
The top of a new absorption trench should be hand tamped and
should be overfilled with about 4 to 6 inches of earth. Unless this is
done, the top of the trench may settle to a point lower than the sur-
face of the adjacent ground. This will cause the collection of storm
water in the trench, which can lead to premature saturation of the
absorption field and possibly to complete washout of the trench.
Machine tamping or hydraulic backfilling of the trench should be
prohibited.
Where sloping ground is used for the disposal area, it is usually
necessary to construct a small temporary dike or surface water diver-
sion ditch above the field, to prevent the disposal area from being
washed out by rain. The dike should be maintained or the ditch kept
free of obstructions until the field becomes well covered with vegetation.
A heavy vehicle would readily crush the tile in a shallow absorption
field. For this reason, heavy machinery should be excluded from the
disposal area unless special provision is made to support the weight.
All machine grading should be completed before the field is laid.
The use of the field area must be restricted to activities which will
not contribute to the compaction of the soil with the consequent reduc-
tion in soil aeration,
Seepage Beds
Common design practice for soil absorption systems for private resi-
dences provides for trench widths up to 36 inches. Variations of design
utilizing increased width are being used in many areas. Absorption sys-
tems having trenches wider than 3 feet are referred to as seepage beds.
The design of trenches is based on an empirical relationship between
the percolation test and the bottom area of the trenches. The use of seep-
age beds has been limited by the lack of experience with their perform-
ance and the absence of design criteria comparable to that for trenches.
Joseph A. Salvato, "Environmental Sanitation," page 214.
14
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Studies sponsored by the Federal Housing Administration have dem-
onstrated that the seepage bed is a satisfactory device for disposing of
effluent in soils that are acceptable for soil absorption systems. The
studies have further demonstrated that the empirical relationship be-
tween the percolation test and bottom area required for trenches is
applicable for seepage beds.
There are three main elements of a seepage bed: absorption surface,
rockfill or packing material, and the distribution system. The design
of the seepage bed should be such that the total intended absorption
area is preserved, sufficient packing material is provided in the proper
place to allow for further treatment and storage of excess liquid, and
a means for distributing the effluent is protected against siltation of
earth backfill and mechanical damage. Construction details for a con-
ventional seepage bed are outlined in the following material in such a
way that these principal design elements are incorporated. Tabulation
of construction details for the conventional seepage bed is not intended
to preclude other designs which may provide tne essential features in a
more economical or otherwise desirable manner. Specifically, there may
be equally acceptable or even superior methods developed for distrib-
uting the liquid than by tile or perforated pipe covered with gravel.
The use of seepage beds results in the following advantages:
1. A wide bed makes more efficient use of land available for absorp-
tion systems than a series of long narrow trenches with wasted land
between the trenches.
2. Efficient use may be made of a variety of modern earth moving
equipment employed at housing projects for other purposes such as
basement excavation and landscaping, resulting in savings on the cost
of the system.
Construction Considerations.—When seepage beds are used, the fol-
lowing design and construction procedures providing for rockfill or
packing material, an adequate distribution system, and protection of
the absorption area, should be observed:
1. The amount of bottom absorption area required shall be the same
as shown in Table I, page 8.
2. Percolation tests should be conducted in accordance with pages 4-8.
3. The bed should have a minimum depth of 24 inches below natu-
ral ground level to provide a minimum earth backfill cover of 12 inches.
4. The bed should have a minimum depth of 12 inches of rockfill or
packing material extending at least 2 inches above and 6 inches below
the distribution pipe.
5. The bottom of the bed and distribution tile or perforated pipe
should be level.
6. Lines for distributing effluent shall be spaced not greater than 6
feet apart and not greater than 3 feet from the bed sidewall.
15
-------
7. When more than one bed is used: (a) there should be a minimum
of 6 feet of undisturbed earth between adjacent beds; and (b) the beds
should be connected in series in accordance with the section concerning
serial distribution, below.
8. Applicable construction considerations for standard trenches on
pages 10 through 14 should also be followed.
Distribution Boxes
The Final Report to the Federal Housing Administration on the
study to: Determine if Distribution Boxes can be Eliminated Without
Inducing Increased Failure of Disposal Fields by the Public Health
Service reached the following conclusions:
1. Distribution boxes can be eliminated from septic tank-soil absorp-
tion systems in favor of some other method of distribution without
inducing increased failure of disposal fields. In fact, evidence indicates
that distribution boxes as presently used may be harmful to the system.
2. Data indicates that on level ground, equal distribution is not nec-
essary if the system is designed so that an overloaded trench can drain
back to the other trenches before failure occurs.
3. On sloping ground a method of distribution is needed to prevent
excessive build-up of head and failure of any one trench before the
capacity of the entire system is utilized. It is doubtful that distribution
boxes as presently used give equal distribution. Rather, they probably
act as diversion devices sending most of the liquid to part of the system.
For the above reasons it is recommended that distribution boxes not
be used for individual sewage disposal systems.
Serial Distribution
Serial distribution is achieved by arranging individual trenches of
the absorption system so that each trench is forced to pond to the full
depth of the gravel fill before liquid flows into the succeeding trench.
Serial distribution has the following advantages:
1. Serial distribution minimizes the importance of variable absorption
rates by forcing each trench to absorb effluent until its ultimate capacity
is utilized. The variability of soils even in the small area of an indi-
vidual absorption field raises doubt of the desirability of uniform dis-
tribution. Any one or a combination of factors may lead to nonuniform
absorptive capacity of the several trenches in a system. Varying physical
and chemical characteristics of soil, construction damage such as soil
interface smearing or excessive compaction, poor surface drainage, and
variation in depth of trenches are some of the factors involved.
2. Serial distribution causes successive trenches in the absorption
system to be used to full capacity. Serial distribution has a distinct
advantage on sloping terrain. With imperfect division of flow in a
parallel system, one trench could become overloaded, resulting in a
surcharged condition. If the slope of the ground and elevation of the
distribution box were such that a surcharged trench continued to re-
16
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ceive more effluent than it could absorb, local failure would occur
before the full capacity of the system was utilized.
3. The cost of the distribution box is eliminated in serial distribu-
tion. Also, long runs of closed pipe connecting the box to each trench
are unnecessary.
Fields in Flat Areas.—Where the slope of the ground surface does not
exceed six inches in any direction within the area utilized for the
absorption field, the septic tank effluent may be applied to the absorp-
tion field through a system of interconnected tile lines and trenches in
a continuous system. The following specific criteria should be followed:
1. A minimum of 12 inches of earth cover is provided over the gravel
fill in all trenches of the system.
2. The bottom of the trenches and the distribution lines should be
level.
3, One type of a satisfactory absorption system layout for "level"
ground is shown in Figure 6, below.
4. Construction considerations for standard trenches, pages 10 through
14, should be followed.
Fields in Sloping Ground.—Serial distribution may be used in all
EXTENT OF COARSE AGGREGATE
— B
ElEV.
100.4
..'• •';-
100.0
A-A
B-B
Figure 6.— Absorption-field system for level ground.
-------
situations where a soil absorption system is permitted and and should
be used where the fall of the ground surface exceeds approximately 6
inches in any direction within the area utilized for the absorption field.
The maximum ground slope suitable for serial distribution systems
should be governed by local factors affecting the erosion of the ground
used for the absorption field. Excessive slopes which are not protected
from surface water runoff or do not have adequate vegetation cover to
prevent erosion should be avoided. Generally, ground having a slope
greater than one vertical to two horizontal should be investigated care-
fully to determine if satisfactory from the erosion standpoint. Also, the
horizontal distance from side of the trench to the ground surface should
be adequate to prevent lateral flow of effluent and breakout on surface
and in no case less than two feet.
In serial distribution, each adjacent trench (or pair of trenches) is
connected to the next by a closed pipe line laid on an undisturbed
section of ground, as shown in Figure 7, page 19. The arrangement is
such that all effluent is discharged to the first trench until it is filled.
Excess liquid is then carried by means of a closed line to the next suc-
ceeding or lower trench. In that manner, each portion of the subsurface
system is used in succession. When serial distribution is used, the
following design and construction procedures should be followed:
1. The bottom of each trench and its distribution line should be
level.
2. There should be a minimum of 12 inches of ground cover over
the gravel fill in the trenches.
3. The absorption trenches should follow approximately the ground
surface contours so that variations in trench depth will be minimized.
4. There should be a minimum of 6 feet of undisturbed earth be-
tween adjacent trenches and between the septic tank and the nearest
trench.
5. Adjacent trenches may be connected with the relief line or a drop
box arrangement, Figure 7, page 19, in such a manner that each trench
is completely filled with septic tank effluent to the full depth of the
gravel before effluent flows to succeeding trenches. (The Figure shown
does not preclude the use of other arrangements to provide serial
distribution.)
a. Trench connecting lines should be 4 inch, tight-joint sewers
with direct connections to the distribution lines in adjacent
trenches or to a drop box arrangement.
b. Care must be exercised in constructing relief lines to insure
an undisturbed block of earth between trenches. The trench for
the relief pipe, where it connects with the preceding absorption
trench, shall be dug no deeper than the top of the gravel. The
relief line should rest on undisturbed earth and backfill should be
carefully tamped.
c. The relief lines connecting individual trenches should be as
18
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far from each other as practicable in order to prevent short
circuiting.
6. Invert of the overflow pipe in the first relief line must be at least
4 inches lower than the invert of the septic tank outlet, Figure 7.
> FROM SEPTIC TANK
L
• ALTERNATE DROP BOX METHOD AS SHOWN IN
SECTION BB MAY BE UStD.
NOTE:
INVERT OF THE OVERFLOW PIPE
MUST BE AT LEAST 4" LOWER THAN
2J INVERT OF THE SEPTIC TANK OUTLET
UNDISTURBED
EARTH
• •:
UNDISTURBED EARTH
6' MINIMUM
SECTION B-B
(ALTERNATE CONSTRUCTION)
• DIFFERING GROUND SLOPES OVER SUBSURFACE DISPOSAL FIELD
MAY REQUIRE USE OF VARIOUS COMBINATIONS OF FITTINGS.
Figure 7.—A relief line arrangement for serial distribution.
IS
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7. All other construction features of the disposal field are the same
as recommended on pages 10 to 14.
Deep Absorption Trenches and Seepage Beds
In cases where the depth of filter material below the tile exceeds the
standard six inch depth, credit may be given for the added absorption
area provided in deeper trenches with a resultant decrease in length of
trench. Such credit shall be given in accordance with Table 5 which
gives the percentage of length of standard absorption trench (as com-
puted from Table 1), based on six inch increments of increase in depth
of filter material.
Table 3.—Percentage of length of standard trench 1
Depth of
Gravel Below
Pipe in Inches*
12
18
24
30
36
42
Trench
width
12"
75
60
50
43
37
33
Trench
width
18"
78
64
54
47
41
37
Trench
width
24"
80
66
57
50
44
40
Trench
width
36"
83
71
62
55
50
45
Trench
width
48"
86
75
66
60
54
50
Trench
width
60"
87
78
70
64
58
54
1 The standard absorption trench is one in which the filter material extends two
inches above and six inches below the pipe.
*For trenches or beds having width not shown in Table 3, the percent of length
of standard absorption trench may be computed as follows:
Percent of length standard trench =
w
w
1
2d
X 100
Where w = width of trench in feet
d = depth of gravel below pipe in feet
To use this table, consider the example on page 12. Using a trench 2 feet wide
with 6" of gravel under tile, 285 feet are required. If the depth of gravel is
increased to 18", keeping trench width at 2 feet, only 66% of 285 feet is required,
or 188 feet. If 4 laterals are used, the length would be 188 divided by 4 — 47 feet.
The space between lines for serial distribution on sloping ground is 6 feet x
3 spaces = 18 feet, plus 4 lines x 2 feet = 8 feet. Total land required is 26 feet
in width x 47 feet in length = 1,222 square feet, plus additional area required to
keep the field away from wells, property lines, etc.
Seepage Pits
Seepage pits, as with all soil absorption systems, should never be used
where there is a likelihood of contaminating underground waters, nor
where adequate seepage beds or trenches can be provided. When seep-
age pits are to be used, the pit excavation should terminate 4 feet above
the ground water table.
In some States, seepage pits are permitted as an alternative when
20
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PERVIOUS
STRATA
2" LAYER OF GRAVEL
Figure 8.—Deep percolation test for seepage pit.
absorption fields are impracticable, and where the top 3 or 4 feet of
soil is underlaid with porous sand or fine gravel and the subsurface
conditions are otherwise suitable for pit installations. Where circum-
stances permit, seepage pits may be either supplemental or alternative
to the more shallow absorption fields. When seepage pits are used in
combination with absorption fields, the absorption areas in each system
should be pro-rated, or based upon the weighted average of the results
of the percolation tests.
It is important that the capacity of a seepage pit be computed on
the basis of percolation tests made in each vertical stratum penetrated.
The weighted average of the results should be computed to obtain a
design figure. Soil strata in which the percolation rates are in excess of
30 minutes per inch should not be included in computing the absorp-
tion area. As will be apparent from Figure 8 (above), adequate tests
-------
DWELLING
SEPTIC TANK
4" BELL AND SPIGOT PIPE
(TIGHT JOINTS)
D SHOULD BE AT LEAST 3 TIMES DIAMETER OF SEEPAGE PIT
MINIMUM D AT LEAST 20 FT. FOR PITS OVER 20 FT. IN DEPTH
Figure 9.—Disposal system using two seepage pits.
-------
for deep pits are somewhat difficult to make, time-consuming, and
expensive. Although few data have been collected comparing percola-
tion test results with deep pit performance, nevertheless the results of
such percolation tests, while of limited value, combined with compe-
tent engineering judgment based on experience, are the best means of
arriving at design data for seepage pits.
Table 4.—Vertical wall areas of circular seepage pits
[In Square Feet]
Diameter of
seepage pit
(feet)
3
4
5
6
7
8
'J
10
11
12
Effective strata depth below flow line (below inlet)
1 foot
9.4
12.6
15.7
18.8
22.0
25.1
28.3
31.4
34.6
37.7
2 feet
19
25
31
38
44
50
57
63
69
75
3 feet
28
38
47
57
66
75
85
94
104
113
4 feet
38
50
63
75
88
101
113
126
138
151
5 feet
47
63
79
94
110
126
141
157
173
188
6 feet
57
75
94
113
132
151
170
188
207
226
7 feet
66
88
110
132
154
176
198
220
242
264
8 feet
75
101
126
151
176
" 201
226
251
276
302
9 feet
85
113
141
170
198
226
254
283
311
339
10 feet
94
126
157
188
220
251
283
314
346
377
Example: A pit of 5 foot diameter and 6 foot depth below the inlet has an
effective area of 94 square feet. A pit of 5 foot diameter and 16 foot depth has an
area of 94 -f 157, or 251 square feet.
Table 1 or Figure 3 (page 9) gives the absorption area requirements
per bedroom for the percolation rate obtained. The effective area of
the seepage pit is the vertical wall area (based on dug diameter) of the
pervious strata below the inlet. No allowance should be made for im-
pervious strata or bottom area. With this in mind, Table 4 may be
used for determining the effective side-wall area of circular or cylindri-
cal seepage pits.
Sample Calculations.—Assume that a seepage pit absorption system is
to be designed for a 3 bedroom home on a lot where the minimum
percolation rate of 1 inch in 15 minutes prevails. According to Table I,
3 X 190 (or 570) square feet of absorption area would be needed.
Assume also that the water table does not rise above 27 feet below the
ground surface, that seepage pits with effective depth of 20 feet can be
provided, and that the house is in a locality where it is common prac-
tice to install seepage pits of 5 feet diameter (i.e., 4 feet to the outside
walls, which are surrounded by about 6 inches of gravel). Design of
the system is as follows:
23
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Let d = depth of pit in feet; D = pit diameter in feet:
TrDd = 570 square feet.
3.14 X 5 X d = 570 square feet
Solving for d depth of pit = 36 feet (approx.)
In other words, one 5 foot diameter pit 36 feet deep would be needed,
but since the maximum effective depth is 20 feet in this particular
location, it will be necessary to increase the diameter of the pit, or
increase the number of pits, or increase both of these. This is illustrated
in the example below:
(a) Design for 2 pits with a 10 foot diameter; d = depth of each pit.
2 X 3.14 X 10 X d = 570 square feet
d = 9.1 feet deep
NOTE: REMOVE PLUG FOR INSPECTION
' EARTH COVER
FILL IN HOLE
VARIABLE WITH MORTAR .
ENTRY PIPE . / / PRECAST REINFORCED CONCRETE
/ SLAB NOT RESTING ON L
^
t I
SECOND LAVE* OF BRICK
. DISTANCE TO GROUND WATER
LEVEL A FT. MIN.
. NOTE SECOND AND REMAINING
/ LAYHS ARE LAID END TO END
AND AT RIGHT ANGLES WITH FIRST
LAYER OF BRICK
PLACE 6" COARSE AGGREGATE
i'/j" TO I"! AROUND UNMORTARED
MASONRY
BRICKS OVERLAP ON EACH LAYER
SSCONO LAYER
THIRD LAYER
FIRST LAYER
SECTION A-A
FIRST LAYER OF BRICK
SECTION B-B
Figure 10.—Seepage pit.
24
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Use 2 pits 10 feet in diameter and 9.1 feet deep.
(b) Design for 2 pits with a 5 foot diameter; d — depth of each pit.
2x 3.14 x 5 X d = 570 square feet
d = 18 feet (approximately)
Use 2 pits 5 feet in diameter and 18 feet deep.
Experience has shown that seepage pits should be separated by a
distance equal to 3 times the diameter of the largest pit. For pits over
20 feet in depth, the minimum space between pits should be 20 feet
(See Fig. 9, page 22). The area of the lot on which the house is to be
built should be large enough to maintain this distance between the
pits while still allowing room for additional pits if the first ones should
fail. If this can be done, such an absorption system may be approved;
if not, other suitable sewerage facilities should be required.
ROOf TERMINAL
.
FIXTURES TO BE
'PERLY TRAP£
VENTED
V CHOUSE SEWER (A]
TO IE LAID ON WELL-COMPACTED EARTH
SEPTIC TANK [81
COMPACT EARTH AROUND TANK
Figure 11 .—Septic-tank sewage-disposal system.
-------
Construction Considerations.—Soil is susceptible to damage during
excavation. Digging in wet soils should be avoided as much as possible.
Cutting teeth on mechanical equipment should be kept sharp. Bucket
augered pits should be reamed to a larger diameter than the bucket.
All loose material should be removed from the excavation.
Pits should be backfilled with clean gravel to a depth of one foot
above the pit bottom or one foot above the reamed ledge to provide a
sound foundation for the lining. Preferred lining materials are clay or
concrete brick, block, or rings. Rings should have weep holes or notches
to provide for seepage. Brick and block should be laid dry with stag-
gered joints. Standard brick should be laid flat to form a four inch wall.
The outside diameter of the lining should be at least six inches less
than the least excavation diameter. The annular space formed should
be filled with clean, coarse gravel to the top of the lining as shown in
Figure 10.
Either brick dome or flat concrete covers are satisfactory. They
should be based on undisturbed earth and extend at least 12 inches
beyond the excavation and should not bear on the lining for structural
support. Bricks should be either laid in cement mortar or have a two
inch covering of concrete. If flat covers are used, a prefabricated type
is preferred, and they should be reinforced to be the equivalent in
strength of an approved septic tank cover. A nine inch capped opening
in the pit cover is convenient for pit inspection. All concrete surfaces
should be located with a protective bitumastic or similar compound to
minimize corrosion.
Connecting lines should be of a sound, durable material the same as
used for the house to septic tank connection. All connecting lines
should be laid on a firm bed of undisturbed soil throughout their
length. The grade of a connecting line should be at least two percent.
The pit inlet pipe should extend horizontally at least one foot into the
pit with a tee or ell to divert flow downward to prevent washing and
eroding of the sidewalls. If multiple pits are used, or in the event
repair pits are added to an existing system, they should be connected
in series.
Abandoned seepage pits should be filled with earth or rock.
SELECTION OF A SEPTIC TANK
Assuming that the lot will be large enough to accommodate one of
the types of absorption systems, and that construction of the system is
permitted by local authority, the next step will be selection of a suit-
able septic tank.
Functions of Septic Tanks
Untreated liquid household wastes (sewage) will quickly clog all but
the most porous gravel formations. The tank conditions sewage so that
it may be more readily percolated into the subsoil of the ground. Thus,
the most important function of a septic tank is to provide protection
26
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for the absorption ability of the subsoil. Three functions take place
within the tank to provide this protection.
1. Removal of Solids.—Clogging of soil with tank effluent varies
directly with the amount of suspended solids in the liquid. As sewage
from a building sewer enters a septic tank, its rate of flow is reduced
so that larger solids sink to the bottom or rise to the surface. These
solids are retained in the tank, and the clarified effluent is discharged.
2. Biological Treatment.—Solids and liquid in the tank are sub-
jected to decomposition by bacterial and natural processes. Bacteria
present are of a variety called anaerobic which thrive in the absence
of free oxygen. This decomposition or treatment of sewage under anae-
robic conditions is termed "septic," hence the name of the tank. Sewage
which has been subjected to such treatment causes less clogging than
untreated sewage containing the same amount of suspended solids.
3. Sludge and Scum Storage.—Sludge is an accumulation of solids at
the bottom of the tank, while scum is a partially submerged mat of
floating solids that may form at the surface of the fluid in the tank.
Sludge, and scum to a lesser degree, will be digested and compacted
into a smaller volume. However, no matter how efficient the process is
a residual of inert solid material will remain. Space must be provided
in the tank to store this residue during the interval between cleanings;
otherwise, sludge and scum will eventually be scoured from the tank
and may clog the disposal field.
If adequately designed, constructed, maintained, and operated, septic
tanks are effective in accomplishing their purpose.
The relative position of a septic tank in a typical subsurface disposal
system is illustrated in Figure 11 (page 25). The liquid contents of the
house sewer (A) are discharged first into the septic tank (B), and finally
into the subsurface absorption field (C).
The heavier sewage solids settle to the bottom of the tank, forming
a blanket of sludge. The lighter solids, including fats and greases, rise
to the surface and form a layer of scum. A considerable portion of the
sludge and scum are liquefied through decomposition or digestion.
During this process, gas is liberated from the sludge, carrying a portion
of the solids to the surface, where they accumulate with the scum. Ordi-
narily, they undergo further digestion in the scum layer, and a portion
settles again to the sludge blanket on the bottom. This action is re-
tarded if there is much grease in the scum layer. The settling is also
retarded because of gasification in the sludge blanket. Furthermore,
there are relatively wider fluctuations of flow in small tanks than in
the large units. This effect has been recognized in Table 5 (page 29),
which shows the recommended minimum liquid capacities of household
septic tanks.
Location.—Septic tanks should be located where they cannot cause
contamination of any well, spring, or other source of water supply.
Underground contamination may travel in any direction and for con-
27
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siderable distances, unless filtered effectively. Underground pollution
usually moves in the same general direction as the normal movement
of the ground water in the locality. Ground water moves in the direc-
tion of the slope or gradient of the water table, i.e., from the area of
higher water table to areas of lower water table. In general, the water
table follows the general contour of the ground surface. For this reason,
septic tanks should be located downhill from wells or springs. Sewage
from disposal systems occasionally contaminate wells having higher
surface elevations. Obviously, the elevations of disposal systems are
almost always higher than the level of water in such wells as may be
located nearby; hence, pollution from a disposal system on a lower
surface elevation may still travel downward to the water bearing stra-
tum as shown in Figure 12, below. It is necessary, therefore, to rely
Figure 12.—Pollution of well from sources with lower surface elevations.
upon horizontal as well as vertical distances for protection. Tanks
should never be closer than 50 feet from any source of water supply:
and greater distances are preferred where possible.
The septic tank should not be located within 5 feet of any building,
as structural damage may result during construction or seepage may
enter the basement. The tank should not be located in swampy areas,
nor in areas subject to flooding. In general, the tank should be located
where the largest possible area will be available for the disposal field.
Consideration should also be given to the location from the standpoint
of cleaning and maintenance. Where public sewers may be installed at
a future date, provision should be made in the household plumbing
system for connection to such sewer.
2B
-------
Effluent.—Contrary to popular belief, septic tanks do not accomplish
a high degree of bacteria removal. Although the sewage undergoes
treatment in passing through the tank, this does not mean that infecti-
ous agents will be removed; hence, septic tank effluents cannot be
considered safe. The liquid that is discharged from a tank is, in some
respects, more objectionable than that which goes in; it is septic and
malodorous. This, however, does not detract from the value of the
tank. As previously explained, its primary purpose is to condition the
sewage so that it will cause less clogging of the disposal field.
Further treatment of the effluent, including the removal of patho-
gens, is effected by percolation through the soil. Disease producing
bacteria will, in time, die out in the unfavorable environment afforded
by soil. In addition, bacteria are also removed by certain physical forces
during filtration. This combination of factors results in the eventual
purification of the sewage effluent.
Capacity.—Capacity is one of the most important considerations in
septic tank design. Studies have proved that liberal tank capacity is not
only important from a functional standpoint, but is also good economy.
The liquid capacities recommended in Table 5 allow for the use of all
household appliances, including garbage grinders.
Table 5.—liquid capacity of tank (gallons)
[Provides for use of garbage grinders, automatic clothes washers, and
other household appliances]
Number of bedrooms
2 or less
3
41
Recommended
minimum tank
capacity
750
900
1,000
Equivalent
capacity
per bedroom
375
300
250
1 For each additional bedroom, add 250 gallons.
Specifications for Septic Tanks
Materials.—Septic tanks should be watertight and constructed of
materials not subject to excessive corrosion or decay, such as concrete,
coated metal, vitrified clay, heavyweight concrete blocks, or hard
burned bricks. Properly cured precast and cast-in-place reinforced con-
crete tanks are believed to be acceptable everywhere. Steel tanks meet-
ing Commercial Standard 177-62 of the U. S. Department of Commerce
are generally acceptable. Special attention should be given to job built
tanks to insure water tightness. Heavyweight concrete block should be
laid on a solid foundation and mortar joints should be well filled. The
interior of the tank should be surfaced with two 14 inch thick coats of
29
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portland cement-sand plaster. Some typical septic tanks are illustrated
in Figure 13 (page 31). Suggested specifications for watertight concrete
are given in Appendix E.
Precast tanks should have a minimum wall thickness of 3 inches,
and should be adequately reinforced to facilitate handling. When pre-
cast slabs are used as covers, they should be watertight, have a thickness
of at least 3 inches, adequately reinforced. All concrete surfaces should
be coated with a bitumastic or similar compound to minimize corrosion.
General.—Backfill around septic tanks should be made in thin layers
thoroughly tamped in a manner that will not produce undue strain on
the tank. Settlement of backfill may be done with the use of water,
provided the material is thoroughly wetted from the bottom upwards
and the tank is first filled with water to prevent floating.
Adequate access must be provided to each compartment of the tank
for inspection and cleaning. Both the inlet and outlet devices should
be accessible. Access should be provided to each compartment by
means of either a removable cover or a 20 inch manhole in least
dimension. Where the top of the tank is located more than 18 inches
below the finished grade, manholes and inspection holes should extend
to approximately 8 inches below the finished grade (see Figure 14,
page 32), or can be extended to finished grade if a seal is provided
to keep odors from escaping. In most instances, the extension can be
made using clay or concrete pipe, but proper attention must be given
to the accident hazard involved when manholes are extended close to
the ground surface. Typical single and double compartment tanks are
illustrated in Figures 15 and 17, pages 33 and 35.
Inlet.—The inlet invert should enter the tank at least 3 inches above
the liquid level in the tank, to allow for momentary rise in liquid
level during discharges to the tank. This free drop prevents backwater
and stranding of solid material in the house sewer leading to the tank.
A vented inlet tee or baffle should be provided to divert the incom-
ing sewage downward. It should penetrate at least 6 inches below the
liquid level, but in no case should the penetration be greater than
that allowed for the outlet device. A number of arrangements com-
monly used for inlet and outlet devices are shown in Figure 16
(page 34).
Outlet.—It is important that the outlet device penetrate just far
enough below the liquid level of the septic tank to provide a balance
between sludge and scum storage volume; otherwise, part of the
advantage of capacity is lost. A vertical section of a properly operating
tank would show it divided into three distinct layers; scum at the top,
a middle zone free of solids (called "clear space"), and a bottom layer
of sludge. The outlet device retains scum in the tank, but at the
same time, it limits the amount of sludge that can be accommodated
without scouring, which results in sludge discharging in the effluent
from the tank. Observations of sludge accumulations in the field, as
30
-------
.
VII
Figure 13.—Typical septic-tank shapes.
31
-------
MANHOLE COVER
BE EITHER
FLUSH WITH SURFACE
OR 8" BELOW SURFACE
\
G*S SEAL MADE SI FILLING TOP
Of MANHOLE WITH SAND OR BY PLASTERING
(I" MIN.I WITH STUCCO WIRE REINFORCED
OM TOP Of WOOD PLATFORM
SECTION
MANHOLE (WITHOUT GAS SEAL;
SECTION
MANHOLE (WITH GAS SEAL)
NOTf: GAS SEAL iSANDI IS REMOVED TO
GAIN ACCESS TO TANK. AND REPLACED
TO REFORM SEAL
MANHOLC COf£8
PLAN
NOTt. USE STANDARD MANHOLE RING AND COVER
FOR HEAVY TRAFFIC DUTY—LIGHT DUTY MANHOLE
RING AND COVER FOR LIGHT TRAFFIC DUTY
MANHOLE 'WITH TRAFFIC PROTECTION SLAB!
Figure 14.—Design of manholes.
reported in Section B, Part III, of "Studies on Household Sewage
Disposal Systems" (bibliography reference, page 92), indicate that the
outlet device should generally extend to a distance below the surface
equal to 40 percent of the liquid depth. For horizontal, cylindrical
tanks, this should be reduced to 35 percent. For example, in a horizon-
tal cylindrical tank having a liquid depth of 42 inches, the outlet
device should penetrate 42 X -35 = 14.7 inches below the liquid level.
The outlet device should extend above the liquid line to approxi-
mately one inch from the top of the tank. The space between the top
of the tank and the baffle allows gas to pass off through the tank into
the house vent.
Tank Proportions.—The available data indicate that, for tanks of a
given capacity, shallow tanks function as well as deep ones. Also, for
tanks of a given capacity and depth, the shape of a septic tank is
unimportant. However, it is recommended that the smallest plan
dimension be at least 2 feet. Liquid depth may range between 30
and 60 inches.
32
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INLET
MAKE BAFFLES OF PRECAST REIN-
FORCED CONCRETE OR EQUIVALENT
MATERIAL I'/i" OR 2" THICK
[T
10"
J_
' 1_ _
1 1
1 1
-i;--i
• OUTLET
20"
PLAN
SLOT IN WALL
MANHOLE COVER
•**•!*I ^^*^* **^B*^ F"!" T
4 MAXIMUM FOR MANHOLE COVfR AS SHOWN.
' FOR GREATER DISTANCES BELOW GROUND
PROVIDE EXTENSION COLLARS
-A. —-
»" TO 4"
INLET
0.2 D MIN.-(2O7, OF LIQUID DEPTH! f
OUTLET
NOTE: MAKE INLET AT LEAST
3" AJOVE OUTLET.
PENETRATION OF OUTLET
BAFFLE GENERALLY 40% OF LIQUID DEPTH
FOR RECTANGULAR TANKS (SEE TEXT)
SECTION ON
CENTER LINE
Figure 15.—Household septic lank,
Storage Above Liquid Level.—Capacity is required above the liquid
line to provide for that portion of the scum which floats above the
liquid. Although some variation is to be expected, on the average
about 30 percent of the total scum will accumulate above the liquid
line. In addition to the provision for scum storage, one inch is usually
provided at the top of the tank to permit free passage of gas back to
the inlet and house vent pipe.
For tanks having straight, vertical sides, the distances between the
top of the tank and the liquid line should be equal to approximately
20 percent of the liquid depth. In horizontal, cylindrical tanks, area
equal to approximately 15 percent of the total circle should be pro-
vided above the liquid level. This condition is met if the liquid depth
(distance from outlet invert to bottom of tank) is equal to 79 percent
of the diameter of the tank.
33
-------
TEE
4" CAST IRON SOIL PIPE T BRANCH
4" CAST IRON SANITARY T BRANCH
4' VITRIFIED CLAY OR CONCRETE
T BRANCHES
PLACE INLET AND OUTLET TEE
IN NOTCH AND FILL WITH MORTAR
PACK MORTAR
AROUND TEE
STRAIGHT BAFFLE
POURED IN PLACE OR PREFABRICATED
AND DROPPED IN SIDES OF TANK
Bfisam ft=B^^s3
SEMI-CIRCULAR BAFFLE
PREFABRICATED CONCRETE CLAY
TILE OR STEa
NOTE: 'A' SHOULD BE NO LESS THAN 6'
AND NO GREATER THAN B
B PENETRATION OF OUTLET DEVICE
GENERALLY 40% OF LIQUID DEPTH FOR
TANKS WITH VERTICAL SIDES & 35% FOR
HORIZONTAL CYLINDERS TANKS.
SECTION D-D
Figure 16.—Types of inlet and outlet devices.
Use of Compartments.—Although a number of arrangements are
possible, compartments, as used here, refer to a number of units in
series. These can be either separate units linked together, or sections
enclosed in one continuous shell (as in Figure 17, page 35, with water-
tight partitions separating the individual compartments.
A single compartment tank will give acceptable performance. The
available research data indicate, however, that a two compartment tank,
with the first compartment equal to one-half to two-thirds of the
total volume, provides better suspended solids removal which may be
especially valuable for protection of the soil absorption system. Tanks
with three or more equal compartments give at least as good per-
formance as single compartment tanks of the same total capacity.
-------
Each compartment should have a minimum plan dimension of 2 feet
with a liquid depth ranging from 30 to 60 inches.
An access manhole should be provided to each compartment. Vent-
ing between compartments should be provided to allow free passage
of gas. Inlet and outlet fittings in the compartmented tank should be
proportioned as for a single tank. (See Figure 16, page 34). The same
allowance should be made for storage above the liquid line as in a
single tank.
r ~ 11
i n
M
- •
4 •- !--,
I
:
- - — - —
^
. •
-
_]]
1
H07C: ALL FITTINGS 4" V.C.
OUTLET FITTING SET 3"
BELOW INLET FITTING
PLAN
-VENT
VENT
LIQUID CAPACiTI
750 GALS.
LONGITUDINAL
SECTION
SECTION A-A
Figure 17.—Precast septic tank.
Genera/ Information on Septic Tanks
Cleaning.—Septic tanks should be cleaned before too much sludge
or scum is allowed to accumulate. If either the sludge or scum ap-
proaches too closely to the bottom of the outlet device, particles will
be scoured into the disposal field and will clog the system. Eventually,
when this happens, liquid may break through to the ground surface.
and the sewage may back up in the plumbing fixtures. When a dis-
posal field is clogged in this manner, it is not only necessary to clean
the tank, but it also may be necessary to construct a new disposal field.
The tank capacities given in Table 5 on page 29 will give a reason-
:s
-------
able period of good operation before cleaning becomes necessary. There
are wide differences in the rate that sludge and scum will accumulate
from one tank to the next. For example, in one case out of 20, the tank
will reach the danger point, and should be cleaned, in less than 3 years.
Tanks should be inspected at least once a year and cleaned when
necessary.
Although it is difficult for most homeowners, actual inspection of
sludge and scum accumulations is the only way to determine definitely
when a given tank needs to be pumped. When a tank is inspected, the
depth of sludge and scum should be measured in the vicinity of the
outlet baffle. The tank should be cleaned if either: (a) The bottom
of the scum mat is within approximately 3 inches of the bottom of
the outlet device; or (b) sludge comes within the limits specified in
Table 6 (see Figure 18, page 37).
Table 6.—Allowable sludge accirmu/ofion
Liquid capacity
of tank,
gallons *
750. .
900
1,000
2i/£ feet
Distance from 1
5
4
4
Liquk
3 feet
>ottom of outlet
6
4
4
I depth
4 feet
device to top c
10
7
6
5 feet
>f sludge, inches
13
10
8
• Tanks smaller than the capacities listed will require more frequent cleaning.
Scum can be measured with a stick to which a weighted flap has
been hinged, or with any device that can be used to feel out the
bottom of the scum mat. The stick is forced through the mat, the
hinged flap falls into a horizontal position, and the stick is raised
until resistance from the bottom of the scum is felt. With the same
tool, the distance to the bottom of the outlet device can be found (see
Figure 18, page 37).
A long stick wrapped with rough, white toweling and lowered to the
bottom of the tank will show the depth of sludge and the liquid depth
of the tank. The stick should be lowered behind the outlet device
to avoid scum particles. After several minutes, if the stick is carefully
removed, the sludge line can be distinguished by sludge particles cling-
ing to the toweling.
In most communities where septic tanks are used, there are firms
which conduct a business of cleaning septic tanks. The local health
department can make suggestions on how to obtain this service.
-------
Cleaning is usually accomplished by pumping the contents of the tank
into a tank truck. Tanks should not be washed or disinfected after
pumping. A small residual of sludge should be left in the tank for
ceding purposes. The material removed may be buried in uninhabited
places or, with permission of the proper authority, emptied into a
sanitary sewer system. It should never be emptied into storm drains
or ducharged directly into any stream or watercourse. Methods of dis-
posal should be approved by the health authorities.
When a large septic tank is being cleaned, care should be taken
to enter the tank until it has been thoroughly ventilated and
;ases have been removed to prevent explosion hazards or asphyxiation
the workers. Anyone entering the tank should have one end of a
:out rope tied around his waist, with the other end held above ground
MlEl
-T
6
SCUM CLEAR SPACE A
SLUDGE CLEAR SPACE B
COVER
OUTLET
WHITE TOWELING WILL BE BLACKENED-
BY SLUDGE
j- MEASURING
/ DEVICES
FOR SCUM-
MEASURING STICK
FOR SLUDGE -
TURKISH TOWELING
WEIGHT
NOIE: MAKE MEASURING STICKS ABOUT 6 LONG
HINGE
NOTE: CLEAN WHEN A IS 3" OR LESS, AND
WHEN B IS WITHIN THE LIMITS SPECIFIED
IN TABLE 6.
Figure T8.-Devices for measuring sludge and
scum.
-------
by another person strong enough to pull him out if he should be
overcome by any gas remaining in the tank.
Grease Interceptors.—Grease interceptors (grease traps) are not ordi-
narily considered necessary on household sewage disposal systems. The
discharge from a garbage grinder should never be passed through them.
The septic tank capacities recommended in this manual are sufficient
to receive the grease normally discharged from a home.
Chemicals.—The functional operation of septic tanks is not improved
by the addition of disinfectants or other chemicals. In general, the addi-
tion of chemicals to a septic tank is not recommended. Some proprie-
tary products which are claimed to "clean" septic tanks contain sodium
hydroxide or potassium hydroxide as the active agent. Such compounds
may result in sludge bulking and a large increase in alkalinity, and
may interfere with digestion. The resulting effluent may severely dam-
age soil structure and cause accelerated clogging, even though some tem-
porary relief may be experienced immediately after application of
the product.
Frequently, however, the harmful effects of ordinary household
chemicals are overemphasized. Small amounts of chlorine bleaches,
added ahead of the tank, may be used for odor control and will have
no adverse effects. Small quantities of lye or caustics normally used in
the home, added to plumbing fixtures is not objectionable as far as
operation of the tank is concerned. If the septic tanks are as large as
herein recommended, dilution of the lye or caustics in the tank will
be enough to overcome any harmful effects that might otherwise occur.
Some 1,200 products, many containing enzymes, have been placed
on the market for use in septic tanks, and extravagant claims have
been made for some of them. As far as is known, however, none has
been proved of advantage in properly controlled tests.
Soaps, detergents, bleaches, drain cleaners, or other material, as nor-
mally used in the household, will have no appreciable adverse effect
on the system. However, as both the soil and essential organisms might
be susceptible to large doses of chemicals and disinfectants, moderation
should be the rule. Advice of responsible officials should be sought
before chemicals arising from a hobby or home industry are discharged
into the systems.
Miscellaneous.—It is generally advisable to have all sanitary wastes
from a household discharge to a single septic tank and disposal system.
For household installations, it is usually more economical to provide
a single disposal system than two or more with the same total capacity.
Normal household waste, including that from the laundry, bath, and
kitchen, should pass into a single system.
Roof drains, foundation drains and drainage from other sources pro-
ducing large intermittent or constant volumes of clear water should
not be piped into the septic tank or absorption area. Such large volumes
of water will stir up the contents of the tank and carry some of the
38
-------
solids into the outlet line; the disposal system following the tank will
likewise become flooded or clogged, and may fail. Drainage from garage
floors or other sources of oily waste should also be excluded from the
tank.
Toilet paper substitutes should not be flushed into a septic tank.
Paper towels, newspaper, wrapping paper, rags, and sticks may not
decompose in the tank, and are likely to lead to clogging of the plumb-
ing and disposal system.
Waste brines from household water softener units have no adverse
effect on the action of the septic tank, but may cause a slight shorten-
ing of the life of a disposal field installed in a structured clay type soil.
Adequate venting is obtained through the building plumbing if the
tank and the plumbing are designed and installed properly. A separate
vent on a septic tank is not necessary.
A chart showing the location of the septic tank and disposal system
should be placed at a suitable location in dwellings served by such a
system. Whether furnished by the builder, septic tank installer, or the
local health department, the charts should contain brief instructions
as to the inspection and maintenance required. The charts should assist
in acquainting homeowners of the necessary maintenance which septic
tanks require, thus forestalling failures by assuring satisfactory opera-
tion. The extension of the manholes or inspection holes of the septic
tank to within 8 inches of the ground surface will simplify maintenance
and cleaning.
Abandoned septic tanks should be filled with earth or rock.
INSPECTION
After a soil absorption system has been installed and before it is used
the entire system should be tested and inspected. The septic tank
should be filled with water and allowed to stand overnight to check
for leaks. If leaks occur, they should be repaired. The soil absorption
system should be promptly inspected before it is covered to be sure
that the disposal system is installed properly. Prompt inspection before
backfilling should be required by local regulations, even where approval
of plans for the subsurface sewage disposal system has been required
before issuance of a building permit. Backfill material should be free
of large stones and other deleterious material and should be overfilled
a few inches to allow for settling.
39
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Part II
Septic Tank - Soil Absorption
Systems for Institutions,
Recreational Areas, and
Other Establishments
INTRODUCTION
The septic tank system is utilized in providing sewage treatment and
disposal for many types of establishments, such as schools, small insti-
tutions, motels, rural hotels and restaurants, trailer parks, housing
projects, large private estates, and camps, where larger quantities of
sewage are involved than are discharged from an individual home. In
general, the usefulness of the septic tank system decreases as the size
of the establishment served increases. Lack of competent sanitary engi-
neering advice in the development of such systems generally will lead
to failures, excessive costs, and a multitude of troubles. The soundest
advice available to anyone contemplating such a system is the early
retention of competent sanitary engineering consultation, whose first
determination should be the suitability of this method of sewage dis-
posal for the proposed establishment.
Any institutional septic tank system must incorporate appurtenances
and supplemental features of design to meet the requirements of the
establishment and varying site conditions. These can be generally suc-
cessful when appropriate experience, study, and planning are employed
in the choice and development of such a system. This manual cannot
present all of the results of experience gained in the design and opera-
tion of such systems, but describes the most generally successful proce-
dures and practices as a guide to engineers designing them.
While many effluents from institutional septic tanks are disposed of
by soil absorption methods, others are discharged to available water-
courses after suitable treatment. When soil absorption systems are con-
templated, it is essential, as described in Part I, to determine the char-
acteristics and suitability of the soil as the first step toward design. As
a matter of fact, the wise builder of an establishment will explore this
feature on a contemplated site before the site is purchased. After the
41
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percolation tests are completed, the quantity of sewage to be treated
must be estimated to determine the size and capacities of the disposal
units.
ESTIMATES OF SEWAGE QUANTITIES
Where there are water meters in existing buildings, the quantity of
sewage may best be estimated from the recorded meter readings.
In using water meter readings for estimating the quantity of sewage
to be contributed, some allowance should also be made for maximum
conditions that may not be readily apparent from the readings. For
example, water consumption by an ordinary family of four in an apart-
ment building may average 48 gallons of water per person per day over
a period of 5 months, but actually range from perhaps 30 gallons per
person on certain days to something in excess of 80 gallons per person
on days when water consumption is heaviest, as on washdays. Besides
these peak loads, some allowance should be made for the sewage con-
tributed by occasional guests. Therefore, when computing sewage flows
from average meter readings, a minimum factor of safety gf about 25
percent should be allowed to cover the range of variations. Accordingly,
the design of a disposal system for the apartment house referred to,
where the average usage is 48 gallons per person per day, should be
based upon a computed maximum usage of at least 60 gallons per
person per day.
Conversely, unusually high meter readings may be caused by lawn
sprinkling or by leakage of water that does not enter the disposal sys-
tem. Due allowances should be made for abnormalities of this kind.
Where measurements of water consumption are not possible, as
where water meter records are not available, or where disposal facilities
are being planned for a new building, it is necessary to use other
methods of estimating the amount of sewage to be discharged. One way
is to base the estimated flow on the number of bedrooms, as in Part I.
Another way is to compute the flow on the basis of the number and
kinds of plumbing fixtures. If the building is used as a restaurant, the
number of patrons or the number of meals served may be the best cri-
terion. The competent designer will base his estimates upon a combina-
tion of the various influencing factors. He will consider each case on
its own merits, especially when disposal facilities are being designed for
a large institution where the cost of construction will amount to a con-
siderable sum. If definite information and accurate water measurements
are not available, the quantity of sewage may be estimated from experi-
ences at establishments similar to that for which the new sewage dis-
posal facilities are intended. Table 7 (page 43) may be helpful in such
cases.
The quantities listed in the table are merely the best averages avail-
able at this time, and they should be modified in localities or estab-
lishments where experience indicates a need for so doing.
42
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It is sometimes economical and advisable to construct separate dis-
posal systems for different types of wastes at a given establishment. The
decision as to the number of disposal systems may be influenced by
conditions of terrain, topography, and locations of the buildings con-
tributing to the wastes. At large camps, for example, and at some re-
sorts, kitchens and central dining facilities may be located at appre-
ciable distances from the barracks or cottages and cabins. Under such
circumstances, the kitchens may be provided with separate disposal
systems, including facilities for the removal of grease ahead of the
septic tank.
Table 7.—Quantities of sewage flaws
Type of Establishment Gallons Per Person Per Day
7r (Unless Otherwise Noted)
Airports (per passenger) 5
Apartments—multiple family (per resident) 60
Bathhouses and swimming pools 10
Camps:
Campground with central comfort stations 35
With flush toilets, no showers 25
Construction camps (semi-permanent) 50
Day camps (no meals served) 15
Resort camps (night and day) with limited plumbing 50
Luxury camps 100
Cottages and small dwellings with seasonal occupancy ... 50
Country clubs (per resident member) 100
Country clubs (per non-resident member present) 25
Dwellings:
Boarding houses 50
additional for non-resident boarders 10
Luxury residences and estates 150
Multiple family dwellings (apartments) 60
Rooming houses 40
Single family dwellings 75
Factories (gallons per person, per shift, exclusive
of industrial wastes) 35
Hospitals (per bed space) 250+
Hotels with private baths (2 persons per room) 60
Hotels without private baths : 50
Institutions other than hospitals (per bed space) 125
Laundries, self-service (gallons per wash, i.e., per
customer) 50
Mobile home parks (per space) 250
Motels with bath, toilet, and kitchen wastes
(per bed space) 50
Motels (per bed space) 40
Picnic Parks (toilet wastes only) (per picknicker) 5
Picnic parks with bathhouses, showers, and flush toilets 10
Restaurants (toilet and kitchen wastes per patron) 10
43
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Table 7.—Continued
Type of Establishment
Gallons Per Person Per Day
(Unless Otherwise Noted)
Restaurants (kitchen wastes per meal served)
Restaurants additional for bars and cocktail lounges
Schools:
Boarding
Day, without gyms, cafeterias, or showers
Day, with gyms, cafeteria, and showers
Day, with cafeteria, but without gyms, or showers
Service stations (per vehicle served)
Swimming pools and bathhouses :
Theaters:
Movie (per auditorium seat)
Drive-in (per car space) ,
Travel trailer parks without individual water and
sewer hook-ups (per space)
Travel trailer parks with individual water and
sewer hook-ups (per space)
Workers:
Construction (at semi-permanent camps)
Day, at schools and offices (per shift)
3
2
100
15
25
20
10
10
5
5
50
100
50
15
Separate systems may also be used for community bathhouses. When
this is done, the total per capita flow must be broken down into its
component parts, and some allowance should be made for the amount
of sewage tributary to the different disposal systems. Table 8 (below)
illustrates how this may be done where there are no definite data as to
the exact distribution of flow.
Table ft.—Estimated distribution of sewage flows, in gallons per day per person
Type of Waste
Volume, gallons per day per person
Total Flow (gallons)...
Kitchen wastes
Toilet wastes
Showers, washbasins, etc..
Laundry wastes
30
10
15
15
J0
40
7
15
18
10
50
10
20
20
75
10
25
25
15
100
15
30
35
20
1 No wastes from these uses.
Example: In a household contributing 75 gallons of sewage per day per person, as
shown in column 4, an average breakdown for each of the four types of wastes
listed might be about 10 gallons per day per person for kitchen wastes; 25 gallons
-------
per day per person for toilet wastes; 25 gallons per day per person for shower wastes,
bathtubs, and washbasins; and about 15 gallons per day per person for laundry
wastes.
For certain types of new establishments, the designing engineer may
be unable to obtain from his clients accurate estimates as to the num-
ber of patrons to be served by the disposal facilities. This is particularly
true in the case of restaurants and at recreational places, such as picnic
areas, country clubs, and the like. In such cases, computations and esti-
mates may best be made from the number of plumbing fixtures in-
stalled. Table 9 indicates average values for quantities of sanitary
wastes per fixture at country clubs with modern plumbing.
Table 9.—Sewage flow from country dubs
Type of fixture
Showers
Baths ...
Lavatories
Gallons per
day per
fixture
500
300
100
Type of fixture
Toilets
Urinals
Sinks
Gallons per
day per
fixture
150
100
50
Estimates of sewage quantities from golf clubs should be checked and
calculations based on the weekend population. Allowances of 10 gallons
per person for showers and 7 gallons per person for toilet and kitchen
wastes, both for the average weekend population, have been found
reasonable.
Table 10 shows one method used in estimating the amount of sewage
discharged hourly during the hours when public parks are open. Simi-
lar figures may be used for fairgrounds, carnivals, ball parks, etc.
Table 10.—Sewage flow at public parks
[During hours when park is open]
Type of fixture
Flush toilets
Urinals
Gallons per
hour per
fixture
36
10
Type of fixture
Showers
Faucets
Gallons per
hour per
fixture
100
15
ESTIMATES OF SOIL ABSORPTION AREAS
With information from percolation tests and with due consideration
to the results of test borings or subsurface explorations, as explained in
45
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Part I and Appendix A, the rate at which sewage may be applied to a
soil absorption system may be taken from Table 11 or from the corre-
sponding curve in Figure 19 (see page 47).
Table 11.—Allowable rate of cewage application to a toil absorption system
Percolation rate (time
in minutes for water
to fall 1 inch)
1 or less
2
5
4
5
Maximum rate of
sewage application
(gallons per square
foot per day)1 for
absorption
trenches,* seepage
beds, and seepage
pits 3
5.0
35
2.9
2-5
22
Percolation rate (time
in minutes for water
to fall 1 inch)
10
15
30*
45*
60**
Maximum rate of
sewage application
(gallons per square
foot per day)1 for
absorption
trenches,* seepage
beds, and seepage
pits3
16
1 3
09
08
06
1 Not including effluents from septic tanks that receive wastes from garbage
grinders and automatic washing machines.
* Absorption area is figured as trench bottom area, and includes a statistical allow-
ance for vertical sidewall area.
'Absorption area for seepage pits is effective sidewall area.
* Over 30 unsuitable for seepage pits.
s Over 60 unsuitable for absorption systems.
Table 11 and the curves in Figure 19 do not allow for wastes from
garbage grinders and automatic washing machines. Discharge from
these appliances to an institutional septic tank system calls for extra
capacity of 20 and 40 percent, respectively, over the calculated absorp-
tion area values. If both of these appliances are installed, the absorption
area should be increased by about 60 percent over the calculated value.
Part I allowed for these types of waste in giving specifications for small
household systems, but they may never occur in institutional effluents,
such as those from factories and offices. Other institutions, such as a
country club or drive-in theatre with eating facilities, may contribute
wastes from garbage grinders, but not from automatic washing ma-
chines. As previously emphasized, all institutions should have their
systems designed by an engineer who is competent to place proper
evaluations on the kind of wastes to be contributed.
Use of Table 11 is demonstrated in the following examples, which
illustrates the design for the two types of soil absorption systems. For
each example assume: 5,000 gallons of sewage per day to be disposed
of; percolation rate, 1 inch in 5 minutes.
-------
2
**•
= 5
Q Yf' F°r stondord trenches or seepage pits
0 10 20 30 40 50 60
PERCOLATION RATE IN MINUTES PER INCH (t)
Figure 19.—Graph showing relation between percolation rate and allowable rate at
sewage application.
Example I.—Absorption Trenches:
Standard trench, 2 feet width.
5,000 gal./ day -f- 2.2 gal./sq. ft./day — 2,270 square feet of absorp-
tion area required.
2,270 -H 2 sq. ft/linear ft. = 1,135 feet of trench required.
With garbage grinder only—1,135 -f- .20 (1,135) = 1,362 linear feet.
With automatic washing machine only—1,135 4- .40 (1,135) — 1,589
linear feet
With garbage grinder and automatic washing machine—1,135 -\- .60
(1,135) = 1,816 linear feet.
Example II.—Seepage Pits:
Seepage pit 10 foot diameter; depth of effective absorption area 25
feet; let d — effective depth of pit in feet; D equal pit diameter in feet.
Effective sidewall area equals total area needed.
7rD(d) = 2,270 sq. ft.
3.14 (10) (d) - 2,270.
Solve for d, effective depth of pit — 72 ft.
Obviously, more than one pit is required.
Design for 3 pits, 10 ft. in diameter.
72 ~ 3 = 24 ft.
Use 3 pits, 10 ft. in diameter with an effective depth of 24 ft.
47
-------
With garbage grinder only—3.14 (10) (d) = 2,270 + 2,270 (.20) = 2,724
sq. ft.
Solve for d = 86.4 ft. Use 4 pits, 10 ft. in diameter with an effective
depth of 21.6 ft.
With automatic washing machine only—3.14 (10) (d) = 2,270 + 2,270
(.40) = 3,178 sq. ft. Solve for d = 101 ft. needed. Use 5 pits, 10 ft.
in diameter with an effective depth of 20.2 ft.
With garbage grinder and automatic washing machine—3.14 (10) (d) =
2,270 + 2,270 (.60) = 3,632 sq. ft. required. Solve for d = 115.7 ft.
required. Use 5 pits, 10 ft. in diameter with an effective depth of
23.1 ft.
BUILDING SEWERS
The building sewer or house sewer is the pipeline carrying sewage
from the building or house drain to a private sewer, public sewer,
septic tank, or other point of disposal. It usually extends from a point
3 feet outside the building, where it is connected to the building drain.
The building sewer should be constructed of cast iron, vitrified clay,
concrete, bituminized fiber, asbestos cement, or other durable material.
AH joints in the sewer line should be watertight and rootproof. The
grade of the building sewer should be at least 2 percent (2 foot fall per
100 feet, or 14 inch per foot) except for the 10 feet immediately pre-
ceding the septic tank, where it should not exceed 2 percent. Buildings
should be planned so that a proper slope of the building sewer can be
obtained. Where the terrain is extremely flat, however, it may be ad-
visable to allow a slope of only 6 inches per 100 feet, or 0.5 percent
(see Table 12, page 49).
The size of the pipeline to a septic tank is generally dependent upon
its ability to allow objects to pass, and upon its capacity to conduct
high flows for short periods, rather than upon its capacity in relation
to the average flow. No building sewer serving water closets should be
less than 4 inches in size. The relationship of minimum sizes to the
slopes and to the number of fixture units is indicated in Table 12.
Cleanouts should be provided at the junction of the building drain
and building sewer and at each change in direction of the building
sewer greater than 45° They should also be provided at intervals of
not more than 100 feet. That portion of the sewer line within 50 feet
of any well or suction line from a well, or within 10 feet of any
drinking water supply line under pressure or within 5 feet of any
basement foundation should be durable, corrosion resistant, root proof,
and so installed as to remain water tight. Cast iron or other high
strength pipe should be required wherever the line crosses under drive-
ways with less than 3 feet of earth cover. While no general statement
can be made to cover all cases, Table 2 (page 10) should be followed
in locating components of the sewage disposal system. Where sewer
lines are in the vicinity of trees or dense shrubs, and are not con-
45
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Table 12.—Maximum number of drainage fixture units1 served by building sewer of
indicated size and slope
Minimum pipe diameter,
inches
4
5
6
8 , ..
10
12
Me
1,400
2,500
3,900
Pipe slope i
ys
180
390
700
1,600
2,900
4,600
inch per foot)
y*
216
480
840
1,920
3,500
5,600
%
250
575
1,000
2,300
4,200
6,700
1A drainage fixture unit is a quantity in terms of which the load producing
effects of different plumbing fixtures on the plumbing system are expressed on some
arbitrarily chosen scale. For example, a small lavatory is rated as 1 fixture unit; a
kitchen sink, 2; a bathtub, 2; a tank operated water closet, 4; and a valve operated
water closet, 6. A table of ratings from the United States of America Standards Insti-
tute National Plumbing Code is given in Appendix D, page 85.
structed of cast iron as indicated above, the joint should be made of
special, root resistant construction.
Bends ahead of the septic tank should be limited to 45° or less
wherever possible. If 90° bends cannot be avoided, they should be
made with two 45° ells, or a long sweep quarter curve or bends.
COLLECTION SYSTEMS
At institutions where there are a number of buildings, it may be
advisable to provide a system of sewers for the collection, transporta-
tion, and possibly the pumping of sewage to the septic tank. In general,
the collection system should be designed for a capacity expected to be
adequate for at least one decade, and preferably two. It is usually ad-
visable to design the sewers with capacities, when running full, of not
less than 10 times the average estimated flow of sewage.
Institutional sewers carrying raw or untreated sewage should be at
least 6 inches in diameter and have slopes of at least ys inch to the
foot (1.0 percent). In very small installations, 4 inch sewers at 14 inch
per foot (2 percent) may be used to carry raw sewage, and 4 inch sewers
with slopes of y8 inch per foot are acceptable for carrying settled sew-
age. All sewers should be designed and constructed with hydraulic
slopes sufficient to give mean velocities, when flowing full, of not less
than 2.0 feet per second, based on Kutter's or Manning's formula using
an "n" value of 0.013. Use of other practical "n" values will be per-
mitted by the plan reviewing agency for longer pipe sections if deemed
justifiable on the basis of research or field data presented.
Under exceptional conditions, if full and justifiable reasons are given,
49
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and if special arrangements are available or will be provided for
flushing, velocities as low as 1.6 feet per second may be permitted. In
general, the following minimum grades should be provided:
Percent
8 inch sewers 0.40
10 inch sewers 058
12 inch sewers 0.22
The use of a larger sewer to take advantage of the lesser minimum
slope, when there is not enough sewage to fill the sewer nearly one-half
full, is generally not advisable. The use of asbestos cement pipe or
enamel lined or cement lined cast iron pipe is suggested under such
conditions.
Sewer lines must be laid on straight alignment and uniform slope
between manholes. Manholes should be placed on sewers at all points
of change of slope or alignment, at the upper ends of all sewer lines,
and otherwise at intervals not greater than 400 feet. If the topography
is very uneven and frequent changes in alignment and slope are neces-
sary, a limited number of lampholes may be substituted for manholes
at such points of change. Not more than one lamphole should be
placed between two successive manholes.
There must be no physical connection- between a public or private
potable water supply system and a sewer, sewage disposal system, or
appurtenances thereto, which would render possible the passage of any
sewage or polluted water into the potable water supply.
GREASE TRAPS
Premature failures of septic tank systems are sometimes due to accu-
mulations of grease within the tanks. Sewer lines frequently clog be-
cause of grease collections. Where grease traps are used, the grease must
be removed at frequent intervals. Grease traps may be used on kitchen
wastelines from institutions, hotels, restaurants, schools with lunch-
rooms, and other places where the volume of kitchen wastes is large.
After passing through the grease trap, the kitchen wastes must still be
treated in the septic tank before being discharged to the disposal area.
Wastes from garbage grinders should not be discharged to a grease
trap. (Grease traps have been used with some degree of success for the
removal of lints and fines from laundry wastes, but better results may
be obtained by increasing the size of the septic tanks receiving such
wastes, or by the use of rock filters to remove the lint.)
location
Grease traps should be placed at a location that is readily accessible
for cleaning, and close to the fixture discharging greasy wastes.
so
-------
Construction Details
The most important considerations for design of grease traps are:
1. Capacity of the trap.
2. Facilities for insuring that both the inlet and outlet are properly
trapped.
3. Ease and convenience with which the traps can be cleaned and
the accumulated grease removed,
4. Inaccessibility of the traps to insects and vermin.
5. Distance between inlet and outlet, which should be sufficient to
allow gravity differential separation of the grease, so that it will not
escape through the outlet.
The principles are illustrated in Figure 20.
INLET
PLAN
(TOP REMOVED) COVER OF REINFORCED
•CONCRETE, ALUMINUM
OR CAST IRON
INLET
OUTLET
4
f
SECTION
CONCRETE BOX
PLAN
(TOP REMOVED)
OUTLET
CLAY TILE
PIPE
,
/SEALED IN CONCRETE
SECTION
CLAY TILE PIPE
2" INLET FROM
KITCHEN SINK
ALUMINUM OR C.I. COVER
• CLEANOUT
4" OUTLET TO
MAIN SEWER
Figure 20.—Typical grease traps.
-------
Flow control fittings should be installed on the inlet side of smaller
traps to protect against overloading or sudden surges from the sink or
other fixtures. Venting is not necessary in the large, outdoor traps,
where siphonage of the contents can be prevented by providing outlets
of liberal size. Where efficient removal of grease is very important, use
has been made of an improved 2 chamber trap which has a primary
(or grease separating) chamber and a secondary (or grease storage)
chamber. By placing the trap as close as possible to the source of wastes,
where the wastes are still hot, the separating grease at the surface may
be removed by means of an adjustable weir and conveyed to the sep-
arate secondary chamber, where it accumulates, cools, and solidifies.
Capacity
Selection of size of traps should be based on verified efficiency ratings
and flow capacities. The required flow capacities should be based upon
the number and kind of sinks or fixtures discharging to the trap. In
addition, a grease trap should be rated on its accumulated grease ca-
pacity, which is the amount of grease (in pounds) that the trap can
hold before its average efficiency drops below 90 percent. It is com-
monly regarded that the grease retaining capacity in pounds should
equal at least twice the flow rating in gallons per minute. That is to
say, a trap rated at 20 g.p.m. should retain at least 90 percent of the
grease discharged to it until it holds at least 40 pounds of grease. Most
manufacturers of commercial traps rate their products in accordance
with this procedure.
Recommended minimum flow rate capacities of traps connected to
different types of fixtures is given in Table 13.
Table 13.—Recommended ratings for commerciaf grease traps
Type of fixture
Restaurant kitchen sink
Single compartment scullery sink ...
Double compartment scullery sink
2 single compartment sinks
2 double compartment sinks
Dishwashers for restaurants:
Up to 30 gal. water capacity
30 to 50 gal. water capacity
50 to 100 gal. water capacity
Rate of
flow, in
g.p.m.
15
20
25
25
35
15
25
40
Grease
retention
capacity
rating, in
pounds
30
40
50
50
70
30
50
80
Recommended
maximum
capacity of
fixture con-
nected to trap,
in gallons
37.5
50.0
62.5
62.5
87.5
37.5
62.5
100.0
52
-------
A net capacity of 2.i/2 gallons per person has been found large enough
to hold the flow from one meal long enough to accomplish proper
grease separation. The minimum allowable capacity should be about
125 gallons for small installations serving up to 50 people with propor-
tionately larger capacities for larger populations. As with septic tanks,
however, it is generally good economy to build grease traps somewhat
oversize. For installations too small to justify a 125 gallon grease trap,
best results can usually be obtained by applying available funds to an
oversized and properly fitted septic tank.
Operation
In order to be effective, grease traps must be operated properly and
cleaned regularly to prevent the escape of appreciable quantities of
grease into the septic tank. The frequency of cleaning at any given
installation can best be determined by experience based on observation
over selected typical periods of use. Generally, cleaning should be done
when 75 percent of the grease retention capacity is filled with accumu-
lated grease.
Grease traps must be kept tightly covered to prevent odor nuisances
and to exclude insects and vermin. Grease removed from the traps is
disposed of by burial. (It may be used for making soap and glycerine,
and for other manufacturing purposes, when economically feasible or
because of shortages of such materials during wartime.)
SEPTIC TANKS FOR INSTITUTIONAL SYSTEMS
As explained in Part I, septic tanks of two compartments give better
results than single compartment tanks. Although single compartment
tanks are acceptable for small household installations, tanks with two
or more compartments should be provided for the larger institutional
systems. Tanks with more than two compartments are installed
infrequently.
The compartments should be separated by walls containing ports or
ell fittings at proper elevations as discussed in Part I, pages 30 and 32.
In the selection of these elevations, the same general principles apply
as in the design of outlet fittings. Vents for the passage of gases from
one compartment to another should also be included.
In a two compartment tank, the compartment nearest the outlet is
used both as a settling basin and as an observation well. The relative
absence of solid material in the second compartment serves as an indi-
cation that the tank is functioning properly, although it may not serve
as an index of conditions within the first compartment, and it is some-
times advisable to clean the first compartment before accumulations in
the second become appreciable. In a two compartment tank, the first
compartment should have a capacity of at least two to three times the
capacity of the second compartment.
Whatever the number of compartments, means should be provided
for gaining access to each compartment, and for measuring the depths
53
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of sludge and scum at the outlet device as illustrated in Figure 18
(page 37). Unless tops of septic tanks are near the surface of the ground,
as in Figure 15 (page 33), manholes should be provided as required to
facilitate ingress and egress. A single manhole above a dividing wall
may be used for gaining access to two compartments in a multicom-
partment tank. Manholes are also desirable over the inlet and outlet
devices.
Alternate designs for manholes are shown in Figure 14 (page 32). The
design selected will depend on whether or not the manhole cover must
be flush with the surface, and on the anticipated traffic loadings over
the area.
The effect of a multiple compartment tank can be accomplished by-
using two or three tanks in series. A better construction arrangement,
particularly for medium or large installations, is to connect special tank
sections together into a unit having single end-walls and two compart-
ments. A unit of four precast tank sections forming two compartments
is shown in Figure 21.
7- DIA. IMi"KTlON HOU
Figure 21.—Battery of four precast reinforeed-concrete septic-tank sections.
Capacities
The net volume or effective capacity below the flowline of a septic
tank, for flows up to 500 gallons per day, should be at least 750 gallons.
For flows between 500 and 1,500 gallons per day, the capacity of the
tank should be equal to at least H/2 days' sewage flow. With flows
greater than 1,500 gallons per day, the minimum effective tank capacity
should equal 1,125 gallons plus 75 percent of the daily sewage flow; or
V = 1,125 + 0.75Q
where V is the net volume of the tank in gallons and Q is the daily
sewage flow in gallons. If garbage grinders are used, additional volume
for extra sludge storage may be desired to minimize the frequency of
cleaning.
-------
Recommended tank capacities for flows up to 14,500 gallons per day
may also be obtained from Figure 22 (below). For higher flows of
up to about 100,000 gallons per day, Imhoff tanks may be more satis-
factory than septic tanks for primary treatment. For flows above
100,000 gallons, still other types of sedimentation tanks may be more
economical. These fall within the category of design for municipal
systems, and are beyond the scope of this manual.
DOSING TANKS
When the quantity of sewage exceeds the amount that can be dis-
posed of in about 500 lineal feet of tile, a dosing tank should be used
in conjunction with the septic tank, in order to obtain proper distribu-
1J.UUU
CO
o
_l
CD
^
1^
G
«c
a£
i—
D
LU
s
oc
1 .000
,'
/
/
/
/
/
V = i
5 Q
/
,/'
/
/
>
/
/
/
/
•
-/
/
V = 1
/
25 t (
/
.75 Q
1000 2000 3000 4000 5000 6000 7000 8000 9000 10.00011,00012.00013.00014.000
SEWAGE FLOW, "Q". IN GALLONS PER DAY
Figure 22.—Septic-tank capacities for sewage flows up to 14,500 gallons per day.
tion of sewage throughout the disposal area and give the absorption
bed a chance to rest or dry out between dosings. Rest periods are always
advantageous, especially when the soil is dense. The dosing tank
should be equipped with an automatic siphon which discharges the
tank once every 3 or 4 hours. The tank should have a capacity equal
to about 60 to 75 percent of the interior capacity of the tile to be
dosed at one time.
-------
Where the total length of tile lateral exceeds 1,000 feet, the dosing
tank should generally be provided with two siphons dosing alternately
and each serving one-half of the tile field. The proper design of dosing
devices involves hydraulic problems and head losses which must be
carefully considered. Dosing siphons require operating heads ranging
from about 5 feet down to a minimum of 1 or 2 feet, and repeated
studies of different layouts may be necessary in order to arrive at an
economical design in which available head is conserved and best use is
made of existing conditions of topography. Specal siphons are manu-
factured for low head losses. For details and capacities, the manufac-
turers' bulletins should be consulted.
Special care must be taken in the installation of siphons. All dimen-
sions and elevations should be carefully followed, and no changes
should be made without prior approval by the engineer or manufac-
turer. Tests should be made to determine that there are no air leaks,
either in the piping or bell casting, and if leaks are found, they should
be repaired. Leaks may be detected by filling the dosing tank with
water, to a point where the bell and piping connections are covered,
and observing the water surface for rising bubbles. New siphons should
be primed by filling with water when the system is first started anyway,
and the tests for leakage can be made at that time.
When properly installed, automatic dosing siphons will give satis-
factory service. They contain no moving parts and are more foolproof
than any dosing device that depends upon moving parts for its opera-
tion. Figure 23 (page 57) shows a septic tank and dosing compartment
with alternating siphons, as illustrated in suggested plans and designs
developed by a State health department.
A typical layout plan or "general plan" of a system using alternating
siphons is illustrated in Figure 24 (page 58).
Separate siphon compartments already equipped with siphons are
made by some manufacturers. Duplex siphon compartments equipped
with two siphons which operate alternately are also prefabricated.
If the head available is inadequate to permit the use of a siphon,
intermittent pump operation may be used to obtain proper dosing of
the field, as shown in Figure 25 (page 60). The pump discharge lines
may be inter-connected to obtain flexibility and to facilitate operations
when one pump is down for repairs.
SAND FILTER TRENCHES
In permeable soil, absorption trenches of the type illustrated in
Figure 5 (page 12), seepage beds, or seepage pits as illustrated in Figure
10 (page 24), provide the best underground method of disposing of
the effluent from a septic tank. The absorption trench is preferred, and
is required in some areas where seepage pits are prohibited. In soil
that is relatively impermeable, however, neither absorption trenches,
seepage beds nor seepage pits are satisfactory. They become more ex-
56
-------
SEPTIC
TANK
l! II
DOSING
CHAMBER
ALTERNATING
SIPHONS
PLAN
GRADE
INLET
OVERFLOW
DISCHARGE
SECTION (NOT TO SCALE)
Figure 23.—Septic tank and dosing tank with alternating siphons,
pensive as the soil permeability decreases, and may not be economically
feasible when the percolation time exceeds 30 or 40 minutes per inch.
The exact point at which they are no longer desirable will depend
largely upon the relative costs of labor and material in the locality
where the disposal system is to be constructed. The decision as to
whether to use absorption trenches, seepage beds or seepage pits may
also depend upon the area available. Where land is at a premium, ab-
sorption trenches become less attractive. They begin to require exces-
sive areas when the percolation time exceeds 20 minutes per inch.
When absorption trenches, seepage beds, and seepage pits are imprac-
tical for institutional or similar use, the possibility of treating the
tank effluent in subsurface sand filter trenches may be considered.
The filter trenches are somewhat similar to absorption trenches, the
major difference being that the filter trenches are deeper, generally
somewhat wider, contain an intermediate layer of sand as filtering ma-
terial, and are provided with underdrains for carrying off the filtered
sewage. The tank effluent is not absorbed, except to a limited extent.
It is simply filtered and must be disposed of when it leaves the trench.
-------
WELL 6 DIA.-140 DEEP THROUGH SAND AND LIMESTONE
NEAREST OTHER WELL 200 YDS. NORTH
153
HOTEL
26 RMS; 25 BATHS, 25 TOILETS
y/1 /— 6" PIPE
f
4" KITCHEN WASTE LINE
\
GREASE TRAP (150 GAL.)
\
GARBAGE GRINDER LINE
SLOP! 1%
MANHOLE
NEAREST ADJOINING WELL 375
"TEST HOLE"-(T.H.)
1" DROP IN 5%
MINUTES
4050 GAL. OR
LARGER SEPTIC TANK
DOSING TANK
PARTITION
80
ALTERNATING "Sf /
SIPHONS-—J/ y'
DIVERSION BOX -
V ' I*-
150
Figure 24.—Typical layout plan of a subsurface sewage-disposal system.
-------
DESIGN DATA FOR DISPOSAL ILLUSTRATED IN FIGURE 24
Hotel
26 rooms total
20 rooms with private baths x 2 persons per room
X 60 gal. per person per day = 2,400 gal. per day
6 rooms with connecting baths x 2 persons per room
X 50 gal. per person per day = 600 gal. per day
5 daytime employees x 25 gal. per person per day r= 125 gal. per day
2 night employees x 25 gal. per person per day — 50 gal. per day
Kitchen wastes: 60 persons x 3 meals per day
X 4 gal. per meal = 720 gal. per day
Total daily sewage flow = 3,895 gal. per day
Required septic tank capacity (from Fig. 22) = 4,046 gal.
(In this case, consideration should be given to using a 5,000 gal. capacity tank
which will require less frequent cleaning.)
Required grease trap capacity 60 persons x 2i/£ gal. = 150 gal.
Percolation test: 6 min. per inch drop.
Allowable rate of sewage application (from Fig. 19) = 2.1 gal. per sq. ft.
per day
Trench area required for normal wastes = = 1,855 sq. ft.
Plus 20% additional for wastes from garbage grinder 371 sq. ft.
Total trench area required = 2,226 sq. ft.
Desired trench width — 24 inches (2.0 ft.)
Total trench length = -?HH1 - 1,113 ft
As stated in the section on Dosing Tanks, where the length of trench1 exceeds
1,000 feet a dosing tank should be used with alternating siphons, each serving one
half of the absorption field.
Total trench length 111 s ... , r
T — - — 557 Lineal ft.
Number of separate absorption fields 2 j trench
per field.
80 ft. lengths can best be worked into available disposal area with room for future
additions if needed.
XK7
— 7 lines of (4") laterals required for each field.
80 \ / i
Preferred dosing tank capacity = 75 percent of interior capacity of tile to be dosed
at one time.
4" tile = 3.14 x 22 X !2 = 151 cu. in. per ft.
151 cu. m./ft. x 0-75 _ 049 gai./ft. required dosing tank capacity (say 0.5)
231 cu. in./gal.
Thus the capacity of the dosing tank should be 0.5 gat./ft, x ~< X 80 ft. — 280
gallons.
For this reason, filter trenches are properly regarded as a means of
sewage treatment, and not sewage disposal. They accomplish a high
degree of purification, however, and the sewage effluent coming from a
filter trench that is properly designed can sometimes be disposed of
without further treatment, by discharging into ditches, small streams,
or dry streambeds. If the receiving stream contributes to a source of
water supply, shellfish growing area, or recreational area, chlorination
should be required as hereinafter explained. In some jurisdictions,
chlorination is required for all effluents from sand filters.
59
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1
-
t
II
1
-—, I'-ll
| seme TANK
1
li
: ij
1 '
'',1
1 1
HAN
SECTION
(NOT TO SCALE)
Figure 25.—Septic tank and pump-dosing chamber.
Approval should be obtained from the State or local health depart-
ment before filter trenches are installed. This method of sewage treat-
ment becomes less economical and less desirable as the quantity of
sewage and the required area increase. Subsurface sand filters may be
needed for larger installations.
Construction Features
Typical details of an underdrained sand filter trench are shown in
Figure 26 (page 61). For all year service, filter trenches should be de-
signed for filtration rates as given in Table 14.
Table 14.—Loading rates for subsurface fitters
Type of service
With(
With
With
With
>ut garbage grinder or automatic washer
garbage grinder
automatic washing machine
both garbage grinder and automatic washer
Area Requirements
Gallons
per acre
per day
50.000
41,500
50,000
41,500
Gallons per
square feet
per day
1.15
0.95
1.15
0.95
The filtering material should be clean, coarse sand, all passing a
screen having four rneshes to the inch.
The sand should have an effective size between 0.25 and 0.6 mm
60
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FILL MATERIAL
UNTREATED BLDG. PAPER
"_2'/j" GRAVEL O«
CRUSHED STONE
FINE GRAVEL
4 PERFORATED P
OR DRAIN TILE
WITH OPEN JOINTS
4' PERFORATED PIPE
OR DRAIN THE
WITH OPEN JOINTS
UNDISTURBED EARTH
"-2'/>" GRAVEL OR
CRUSHED STONE
CROSS SECTION
SECTION B-B
8' LENGTHS
PERFORATIONS
CONNECTING TILE OF
BELL AND SPIGOT PIPE-OPEN JOINTS
SECTION A-A
Figure 26.—Underdrained sand-filter trench.
and preferably 0.4 to 0.6 mm. The uniformity coefficient should be
less than 4.0.
Fine sand will lead to premature clogging and a need for replace-
ment. The sand should be not less than 2 feet deep.
The distributors and underdrain should be surrounded by coarse
screened gravel or crushed stone. All of the gravel or stone should pass
a 2y2 inch screen and should be retained on a % inch screen. Fine
gravel, down to 14 inch, may be used above and around the coarse
material, both at the distributors and at the underdrains.
The slope of the distributors should be about 0.5 percent where
6'
-------
dosing tanks are not used, and the slope of the underdrains about 0.5
to 1.0 percent. It is essential that the sand be thoroughly settled by
flooding or other means before the distributors are placed at the final
grade. The distributor and underdrains may be of agricultural tile or,
bell and spigot pipe, or perforated pipe.
SUBSURFACE SAND FILTERS
As indicated previously, filter trenches are not economical for large
installations. Subsurface sand filters are cheaper. They require less area,
and will do the job just as well as filter trenches. They require more
care in design and construction, but this is more than offset by savings
in cost. As a rough guide, filter trenches are impracticable when the
amount of sewage (and consequently the filter area or the length of tile
distributors) is enough to require the use of dosing siphons. Subsurface
sand filters are generally indicated when dosing siphons are needed in
conjunction with artificial filtration.
The principles of design of subsurface sand filters are similar to the
fundamentals involved in the layout of filter trenches. Both types of
installations have distributors and underdrains, with filter sand be-
tween. The essential difference in the two is that filter trenches are
smaller and the filter material is laid in trenches, as implied in the
name. When two or more trenches are used, natural earth is permitted
to remain between them. For subsurface sand filters, on the other hand,
the entire filtration area is filled in artificially. A typical plan and sec-
tion of a subsurface sand filter are illustrated in Figure 27 (page 63).
Construct/on Features
The sand in a subsurface filter should be of a quality at least equal
to that specified for a filter trench, with an effective size preferably
between 0.4 and 0.6 mm, and never less than 0.25 mm, and with a uni-
formity coefficient less than 4.0. The gravel or crushed stone should
likewise be equal to the quality previously specified. The same depth
of sand is used, and the required filter area is the same as given in
Table 14, with one exception: Where filters are constructed solely for
seasonal use and have long rest periods over the greater part of the
year, they may be expected to work satisfactorily at rates as high as
twice those given in Table 14, provided that the sand has an effective
size no smaller than 0.4 mm.
Dosing tanks should be provided where the total filter area exceeds
1,800 square feet and where the distributors exceed 300 lineal feet. The
size of the dose, or the net capacity of the dosing tank, should be 60 to
75 percent of the volume of the distributors dosed at one time. When
an installation has over 1,000 feet of distributors, best results can be
obtained by constructing the filter in two or more sections, dosed sep-
arately by alternating siphons.
The distributors and underdrains may be of agricultural tile, bell
-------
DOSING TANK
FROM SEPTIC TANK
ALTERNATING SIPHON
TILE PIPE WITH TIGHT JOINTS i
(SLOPE TO FIELD, 0.5%) — X| \ DIVERSION
i - l - . •
18" TO 36"
t
i !
18" TO 36"
...1, .^ BOXES *..
I
s
s"
MASONRY WALL
UNDERDRAIN PIPE
WITH OPEN JOINTS
DISTRIBUTION PIPE ON 03% SLOPE
.CHLORINATE HERE
IF NECESSARY
TO CHLORINE CONTACT TANK
UNDERDRAIN COLLECTOR
PIPE WITH BELL & SPIGOT JOINTS
PLAN
•6-
OPEN JOINT OR
•,.TI °"- -./PE«FORA1»;PIPE
*•'"'. V
6'±
GRADED GRAVEL
24 TO 30"
I
CLEAN COARSE SAND
GRADED GRAVEL
••4" FARMTILE OR VITRIFIEU PIPE
WITH OPEN BELL * SPIGOT JOINTS
SECTIONAL ELEVATION
Figure 27.—Typical plan and section of a subsurface sand filter.
and spigot pipe; or perforated pipe. Special joints of the precast, sleeve
coupling, and taper coupling types are available which insure align-
ment of the pipe lengths. In any event, the distributors should be laid
accurately to grade after the filter sand has been thoroughly settled by
flooding or other means. It may be necessary to surround the filter bed
with a masonry wall to retain the filter bed.
Where dosing tanks are used, the distributors should be laid to 0.3
percent grade; otherwise, they are laid to a grade of about 0.5 percent.
63
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SUPERFICIAL SAND FILTERS
In some areas, the ground water table is too near the surface to per-
mit construction or successful operation of any subsurface method of
sewage disposal. The construction of buildings and sewage disposal
systems should generally be avoided in such places. Sometimes, how-
ever, the elevation of the water table may be found to fluctuate over
a wide range unexpectedly. Through floods or other hydrologic condi-
tions, areas which, for decades, may have experienced a water table
appreciably below the ground surface may suddenly be faced with a
condition wherein the ground water approaches or even covers the sur-
face. Sometimes such occurrences are brought about artificially, as by
the construction of dams or dikes, and the conditions may be likely to
remain relatively permanent.
As previously explained, impervious soil may also preclude subsurface
methods of disposal, and the occurrence of solid rock at or just below
the surface of the ground may make subsurface treatment methods
uneconomical.
In these events, it may be necessary to forego completely the con-
struction or renovation of subsurface methods of sewage treatment and
disposal. The circumstances may require treatment and disposal by
superficial means above the surface of the ground. This frequently
necessitates pumping, but can be accomplished in superficial sand filters
constructed as shown in Figure 28 (page 65), except that the ends and
sides must be surrounded by a masonry wall or earth dike for retaining
the filter bed. The filters may be completely above the ground or only
partly above the surface, depending primarily upon the depth to
ground water or the depth to solid rock, as the case may be.
From the standpoint of operation, sewage filters do not have to be
covered, but in built-up areas a shallow earth cover of about 6 inches
is recommended, to prevent freezing in cold weather and to prevent
emanation of odors and other nuisances from the septic sewage in warm
weather. While open filters are accessible for cleaning and can be
operated at higher rates than those covered with earth, they ordinarily
require daily attention, and are not usually advisable for private instal-
lations where septic tanks are used. They are more suitable for munici-
palities and large institutions where sewage plant operators are em-
ployed. In such cases, the filters generally follow a conventional pri-
mary settling tank or secondary treatment and secondary settling tanks,
and the method of distributing sewage onto open filters is different
from that indicated for covered filters. Figure 28 (page 65) illustrates a
typical installation of open filter beds.
Open filters are operated intermittently and, thus, are known as
intermittent sand filters. They should always be divided into two or
more units. Sand specifications and depths are the same as for subsur-
face sand filters. Loading rates are influenced by temperature, effective
size and uniformity coefficient of the filtering medium, and the charac-
64
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rTIGHT JOINTS
3 CONTROL
IN CI. PIPE LINE
FILTER SAND OR GRAVEL OR CRUSHED ROCKj
II
TILE LINES WITH
OPEN JOINTS
TYPICAL SECTION AT A-A
[NOT TO SCALE)
TIGHT JOINTS IN ALL TILE LINES OUTSIDE BED -r—
— TO DISPOSAL-
CONCRETE SPLASH
WITH ROUGH TOP
FINISH
PLAN: TYPICAL FILTER BED WITH INFLUENT
PIPE BELOW LEVEL OF FILTER SAND
(NOT TO SCALE)
MASONRY RETAINING WALL
(ALL AROUND) WITH DIVIDING
WALL TO SEPARATE BEDS
=i=. its*;.
SLOPE BOTTOM OF CUT
TOWARD DRAIN LINES AS SHOWN
TO DISPOSAL
DRAIN LINES
WITH OPEN JOINTS
CONCRETE SPLASH BLOCK
WITH ROUGH
3 CONTROL VALVES
IN C.I. PIPE LINE
*
TOP FINISH^
4Mt*&^
TYPICAL FILTER BED WITH INaUENT
PIPE ABOVE LEVEL OF RLTER SAND.
(NOT TO SCALE)
Cl. PIPS
RLTER
SAND
CONCRETE
SPLASH BLOCK
AVERAGE SIZE
«'0" i «'0" « 4" THICK
IBELOW LEVEL OF FILTER SAND] (ABOVE LEVEL OF FILTER SAND)
DETAIL— TYPICAL DISCHARGE FOR INFLUENT PIPE
Figure 28.— Open-filter-bed layout.
teristics of the sewage to be treated. The last depends largely upon the
degree of pretreatment received by the influent to the filter, but, since
this manual is concerned only with septic tank systems, pretreatment
may be ruled out of consideration here. Table 15 gives recommended
maximum loading rates for the effluents from septic tanks which do
not receive wastes from garbage grinders or automatic washing
machines.
65
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Table 15.—Recommended loading rates of septic tank effluents on open sand filters with
uniformity coefficients not over 4.0
[Gallons per acre per day l]
Region
Southern U. S....
Northern U. S....
Effective Size of Sand
05mm.
100,000
80,000
0.3 mm.
130,000
100,000
0.4 ram.
160,000
120,000
0.5 mm.
180,000
140,000
0.6 mm.
200,000
160,000
1 Multiply rates by 0.83 if garbage grinders are used.
In addition to dosing by siphons or pumps, some open filters are
dosed by means of hand operated gates or valves. In some places, it has
been customary to dose about once a day. Other plants operate with as
many as 4 doses per filter per day, but some dose at long and irregular
intervals of several days. With fine sands, filtration efficiencies are in-
creased by loading twice daily. With coarse sand (0.45 mm and up),
efficiencies are increased by loading more frequently than twice a day,
but multiple loadings present a problem in obtaining adequate cover-
age of the bed. Spray nozzles and rotary distributors have been used
with reasonable success as dosing devices in some small installations,
but it has been found necessary to cover the surface of the sand with
several inches of pea gravel in order to prevent disturbance of the
surface sand.
Dosing tanks should have capacities that will provide for flooding the
beds to depths of 2 to 4 inches. Discharge from the dosing tanks should
at least equal the flow of the raw sewage.
Since various States have different requirements or policies regarding
open filter installations, the State or local health department should
always be consulted before filters of this type are designed.
CHLORINATION
Filter trenches and artificial sand filters that are properly designed
and operated will produce an effluent that is clear and sparkling, and
the filtration will remove a high percentage of the bacteria contained
in the sewage. Some of the remaining bacteria may still be capable of
producing disease, however, and the sewage effluent must be disinfected
if it is discharged to a ditch that is accessible to humans, or to a stream
that contributes to a nearby source of water supply used for drinking,
growing of shellfish, or swimming. Chlorine is the disinfectant most
commonly used.
Present custom is to apply chlorine in the amount necessary to obtain
a chlorine residual of 0.5 to 1.0 ppm after thorough mixing and 15
66
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minutes contact at peak flows. On an average, this requires a chlorine
dosage in the neighborhood of 6 ppm for sand filter effluents. This is
equivalent to i/2 pound of available chlorine for every 10,000 gallons
of sewage, or less than 1 ounce per 1,000 gallons. Most States require
a chlorine contact chamber large enough to retain the sewage for at
least 15 minutes. A typical design is illustrated in Figure 29 (page 68).
Chlorine may be obtained in steel cylinders in which the gas is com-
pressed to a liquid state. It may also be obtained from sodium hypo-
chlorite or calcium hypochlorite.
Sodium hypochlorite is obtainable only as a solution. Upon drying,
it decomposes. Solutions of hypochlorite are obtainable in strengths
ranging from 1 percent to 15 percent of available chlorine. This mate-
rial is corrosive to ordinary metals and should be handled in glass,
stoneware crocks or plastic containers.
Calcium hypochlorite is obtainable only in a dry form, of which
there are two principal types. Hypochlorite of lime (or "bleaching
powder") contains about 35 percent of available chlorine when manu-
factured, and high-test hypochlorite contains about 70 percent of
available chlorine.
The ordinary hypochlorite of lime is really a mixture of compounds
which may be largely a calcium oxychloride. It is not stable, and may
lose a large percentage of its chlorine content if stored for long periods.
When made into a water solution, it also has the undesirable character-
istic of forming a scale of calcium carbonate which tends to clog the
feeding apparatus. It is cheaper than high-test hypochlorite, however,
and it can be used satisfactorily with certain types of feeding equip-
ment, if large solution tanks are employed and if proper precautions
are taken in the preparation and utilization of the solution. The undis-
solved portion must be allowed to settle out of the solution, and only
the clear, supernatant liquid fed through the feeding apparatus.
High-test hypochlorite is a true calcium hypochlorite. It is more
stable and more convenient to handle than ordinary hypochlorite of
lime. It is available under several trade names, familiar to sanitary
engineers and sanitarians. When made up with soft water in solutions
containing 1 to 2 percent of available chlorine, calcium hypochlorite
does not give much trouble with calcium carbonate deposits, but pre-
caution should still be taken to feed only the clear, supernatant por-
tion of the solution through the feeding equipment. When hard water
is used to make up the solution, the addition of a small quantity of
sodium hexametaphosphate will help prevent deposits.
The decision between sodium hypochlorite or calcium hypochlorite
should depend upon the relative costs of the two materials in the local-
ity of the job. Information as to their costs can usually be obtained
from the suppliers of the hypochlorinators.
The decision as to whether to apply chlorine as a gas or in liquid
form as a hypochlorite is sometimes dependent on the availability of
67
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^— WOODEN IAFFUS -J
V
WIMfOdCtD CONCttlE
- MOS€ «O* CHLOSINATOT
PCMNT Of *m.'CA-
1ION Of CHiOflINt
Figure 29.—Chlorine-contact chamber.
power. Most manufactured hypochlorinators require either electric cur-
rent or clean water under pressure for their operation. In some remote
places, these are not available. In such cases, hypochlorinators using
chlorine in cylinders can be operated without any outside power source.
Usually, however, power is available and the decision as to whether
to use hypochlorite or chlorine gas is dependent upon their relative cost.
Gas is the cheaper of the two, but equipment for controlling it is much
more expensive than simple hypochlorite feeders.
In general, hypochlorite is preferred when the amount of chlorine
required is less than 2 or 3 pounds per day. In other words, hypochlo-
rite would normally be used for disinfecting the effluent from a sand
sewage filter for sewage flows up to about 50,000 gallons per day. For
chlorinating a septic tank effluent, the sewage flow limit for hypochlori-
nators would be about 10,000 gallons per day. Beyond these quantities,
savings in the cost of the chlorine gas will usually offset the more
expensive original investment in equipment.
Several satisfactory types of hypochlorinators are available. Various
types of chlorinators are also made. Both kinds of apparatus are being
gradually improved. Both require routine maintenance. Therefore, in
choosing the make of apparatus to be purchased, the first consideration
should be that of service. It is practically always advisable to purchase
a chlorinator from a manufacturer who has a fulltime serviceman trav-
eling through the area where it is to be used. Jobbers and agents han-
dling chlorinators are seldom qualified to furnish the best technical
assistance, and are usually unable to render the prompt service that is
necessary when a chlorinator gets out of order.
66
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Wherever chlorination is necessary, daily tests should be made to
determine the amount of chlorine residual after the requisite contact
period. Many health departments require monthly reports giving the
results of the tests.
The test is simple. It is usually made by adding a small amount of
standard orthotolidine solution to a sample of the effluent and com-
paring the color developed with color standards which may be obtained
with the solution from most laboratory supply houses. If the color of
the sample changes to an orange-yellow hue, it indicates that chlorine
is being applied in a proper amount.
69
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Appendices
Introduction To Appendices
Since conditions vary in different parts of the country, practices that
are suitable in one section may not be acceptable in another. Thus, it
would not be feasible to suggest one set of standards that should be
followed in all circumstances. In a sense, this may be considered fortu-
nate, as, to paraphrase Sedgwick, "Standards are devices to keep the
lazy mind from thinking." In public health work there can be no sub-
stitute for the use of judgment or discretion, more aptly termed by
Phelps as an exercise of "the principle of expediency."
Just as no individual design of a sewage disposal system is applicable
for universal adoption because of varying local conditions, so also no
single ordinance or code would be universally applicable. Though some
States and many local health departments have laws or regulations
governing the installation of individual sewage disposal systems, there
is still a growing need for reasonable regulations which will protect the
public from unsound practices leading to health hazards. The suggested
ordinance governing individual sewage disposal systems presented in
these appendices may be useful as a guide in the development of new,
or the revision of existing, regulations on this subject.
The purpose of these appendices, therefore, is to discuss additional
practices which have been found successful in some parts of the coun-
try, although their application may be necessarily limited in other parts,
and also to present some additional information considered useful to
workers in this field.
71
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Appendix A
Soil Absorption Capacity
GUIDE FOR ESTIMATING SOIL ABSORPTION POTENTIAL
A percolation test is the only known means for obtaining a quantita-
tive appraisal of soil absorption capacity. However, observation and
evaluation of soil characteristics provide useful clues to the relative
capacity of a soil to absorb liquid. Most suitable and unsuitable soils
can be identified without additional testing. When determined and
evaluated by trained or experienced soil scientists or soil engineers, soil
characteristics may permit further categorizing of suitable soils. This
has been done for some areas of the country and described in the soils
reports mentioned below.
Soil Maps.—The capacity of a soil to absorb and transmit water is an
important problem in agriculture, particularly in relation to irrigation,
drainage, and other land management practices. Through studies in
these fields, a variety of aids have been developed for judging the
absorption of water transmission properties of soils, which could be
helpful in the sewage field. Considerable information has been accu-
mulated by agricultural authorities on the relative absorption capacities
of specific soils in many areas of the United States. Much of this infor-
mation is included in Soil Survey Reports and Maps published by the
United States Department of Agriculture in cooperation with the vari-
ous State agricultural colleges. The general suitability of specific soils
for effluent disposal may often be interpreted from these reports and
maps.
Clues to Absorption Capacity.—Considerable information about rela-
tive absorption capacities of soils may also be obtained by a close visual
inspection of the soil. The value of such an inspection depends upon
some knowledge of the pertinent soil properties. The main properties
indicative of absorption capacity are soil texture, structure, color, depth
or thickness of permeable strata, and swelling characteristics.
Texture.—Soil texture, the relative proportion of sand, silt, and clay,
is the most common clue to water absorption capacity. The size and
distribution of particles govern the size and distribution of pores which,
in turn, govern the absorption capacity. The larger the soil particles,
the larger are the pores and the faster is the rate of absorption.
Texture can best be judged by the feel. The lighter or sandier soils
72
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have a gritty feel when rubbed between the thumb and forefinger; silty
type soils have a "floury" feel and, when wetted have no cohesion;
heavier, clay type soils are dense and hard when dry, and have a slick
greasy feel when wetted.
The use of texture as a clue to absorption qualities has its limita-
tions; it is primarily reliable in the sandier soils. In the heavier type
soils, including sandy soils containing appreciable amounts of silt or
clay, one must look for additional clues, such as structure and soil
color, as indicators of absorption capacity.
Structure.—Soil structure is characterized by the aggregation or
grouping together of textural particles, forming secondary particles of
larger size. Such secondary particles then tend to govern the size and
distribution of pores and, in turn, the absorption properties. Structure
can easily be recognized by the manner in which a clod or lump breaks
apart. If a soil has structure, a clod will break with very little force,
along well defined cleavage planes, into uniformly sized and shaped
units. If a soil has no structure, a clod will require more force to break
apart and will do so along irregular surfaces, with no uniformity in
size and shape of particles.
In general, there are four fundamental structure types, named accord-
ing to the shape of the aggregate particles: platy, prism-like, block-like,
and spheroidal. A soil without structure is generally referred to as
massive. Spheroidal structure tends to provide the most favorable ab-
sorption properties, and platy structure, the least. Although other fac-
tors, such as size and stability of aggregates to water, also influence the
absorption capacity, recognition of the type of structure is probably
sufficient for a general appraisal.
Color.—One of the most important practical clues to water absorp-
tion is soil color. Most soils contain some iron compounds. This iron,
like iron in a tool or piece of machinery, if alternately exposed to air
and to water, oxidizes and takes on a reddish-brown to yellow color.
Thus, if a soil has uniform reddish-brown to yellow oxidized color, it
indicates that there has been free alternate movement of air and water
in and through the soil. Such a soil has desirable absorption character-
istics. At the other extreme are soils of a dull gray or mottled coloring,
indicating lack of oxidizing conditions or very restricted movement of
air and water. These soils have poor absorption characteristics.
Depth or Thickness of Permeable Strata.—The quantity of water that
may be absorbed is proportional to the thickness or volume of the
absorbent stratum, when all other conditions are alike. In a soil having
a foot or more of permeable material above tight clay, absorption ca-
pacity is far greater than that of the same kind of material lying within
3 inches of tight clay. When examining soils or studying soil descrip-
tions, the depth and thickness, therefore, are important criteria of
absorption capacity.
Swelling Characteristics.—Most, but not all, clays swell upon the
73
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addition of moisture. There are many clays (in the tropics, in particu-
lar) that do not swell appreciably. There are also some soils in the
United States which do not swell noticeably. On the other hand, some
soils have a very high percentage of swelling, and these in particular
must be suspect. Relative swelling of different soils is indicated by rela-
tive shrinkage when dry, as shown by the numbers and sizes of cracks
that form. Those that shrink appreciably when dry are soils that may
give trouble in a tile field when they are wet.
Information obtained through inspection or from soil maps and re-
ports can be of particular value in preliminary appraisal of soils for
sewage disposal. For instance, in many cases, unsuitable soils may be
immediately ruled out on the basis of such information; in other cases,
selection of the best of several sites may be made on the basis of the
inspection. Absorption capacity information obtained in this manner is
relative. For quantitative information upon which to base specific
design, we still must depend on some direct measurement, such as a
water absorption rate as measured by a percolation test.
EVAPOTRANSPI RATION
In tight clay soils, where absorption is very limited, plant transpira-
tion has been employed with some degree of success in aiding in the
disposal of sewage effluent. By placing the tile lines near the top of the
trench, where they are not likely to become clogged with roots, or by
laying the lines practically at ground surface and covering with suitable
fill material, use may be made of the action of trees, shrubs, and grasses
in the absorption and subsequent release to the atmosphere of appre-
ciable quantities of moisture, through the process of evapotranspiration.
Climate is important, Evapotranspiration increases wth decreasng
humidity and increasing temperature and air turbulence. Length of day
and amount of sunshine are further influencing factors. Consequently,
evapotranspiration should be of most benefit to soil absorption systems
in the southern portion of the country, where the growing season is
longest. In the extreme North, where the growing season extends only
from May to September, evapotranspiration would be of minimum
value, but may be useful in summer resort areas.
Because of these variations, and equally great variations in the types
of plants grown in different parts of the country, as well as the differ-
ing transpiration rates in different plants, it would be hazardous to
generalize in making specific suggestions on design criteria for systems
dependent on evapotranspiration for successful operation.
CURTAIN DRAINS FOR ABSORPTION TRENCH SYSTEMS
Even though proper precautions are taken to ascertain the maximum
ground water table before the absorption trench method of sewage
effluent disposal is approved for a given area, instances occur where
the height of a water table rises above that indicated by past perform-
ances and observations. This may lead to failure of the disposal system.
74
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It is sometimes possible to rehabilitate such a system by constructing
around the disposal area a curtain drain designed to intercept the
superficial ground water and carry it away from the area. If the absorp-
tion field is on sloping ground, a single drain at the upper end of the
area may be sufficient. Where open curtain drains are not feasible, rock-
filled trenches above or around the disposal area may sometimes be
used. In either case, it is necessary to have a suitable outlet for the
ground water that is intercepted.
PERCOLATION TEST HOLES
When making percolation tests in some types of soil, it has been
found that the side walls of test holes have a tendency to cave in or
slough off, and settle to the bottom. The condition is most likely to
occur where the earth is initially dry and overnight soaking is necessary.
The caving can be prevented, and more accurate results obtained, by
placing in the test hole a wire cylinder surrounded by gravel of the
same size that is used in the tile field.
OTHER PERCOLATION TESTS
A Soil Percolation Test for Determining the Capacity of Soils for
Absorption Sewage Effluents, by John E. Kiker, Jr.1
1. Dig a hole about 1 foot square to the depth at which it is proposed
to lay the tile drain.
2. Fill the hole with water and allow the water to seep away. When
the water level falls to within 6 or 8 inches of the bottom of the hole,
observe the rate at which the water level drops.
3. Continue these observations until the soil is saturated and the
water seeps away at a constant rate. (Keep adding water until the rate
becomes constant.)
4. Compute the time required for the water to drop 1 inch after the
soil becomes saturated. This is the standard percolation time, t.
A Percolation Test for Determining the Capacity of Soils for Absorp-
tion Sewage Effluents, by Harvey F. Ludwig, Gordon W. Ludwig, and
John Stewart.
While the previously cited percolation tests include procedures gen-
erally familiar to workers in this field, the method proposed by Ludwig
et al., includes an additional step. The field data are further analyzed
graphically to obtain the percolation rate under saturated soil condi-
tions. The mathematical evaluation is based on the premise that the
ratio, time in minutes for the water level to drop 1 inch, is constantly
increasing during the progress of the test, but at a rate of increase
which is constantly decreasing. This hyperbolic relationship is utilized
minutes
to determine the maximum ratio -.—; which is the saturated
inch
1 Reprinted from Subsurface Sewage Disposal, by John E, Kiker, Jr., Bulletin No.
23, Florida Engineering and Industrial Experiment Station, 1948.
75
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condition under which water would seep away at a constant rate. A
discussion of this method with illustrations and an example is found
in the following articles: Improved Soil Percolation Test, Harvey F.
Ludwig and Gordon W. Ludwig, Water and Sewage Works Journal,
vol. 96, no. 5, May 1949, p. 192; Equilibrium Percolation Test for Esti-
mating Soil Leaching Capacity, Harvey F. Ludwig and John Stewart,
Modern Sanitation, vol. 4, no. 10, Oct. 1952.
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Appendix B
Suggested Ordinance
The following is suggested for consideration in drafting an ordinance
for local application to permit the exercise of appropriate legal con-
trols over individual sewage disposal systems.1 Persons utilizing this
draft as a guide are urged to familiarize themselves with the applicable
legal requirements governing adoption of ordinances of this kind in
their locality.
AN ORDINANCE GOVERNING INDIVIDUAL SEWAGE DISPOSAL SYSTEMS
An ordinance defining and regulating individual sewage disposal
systems; requiring minimum standards governing the design, construc-
tion, and installation of septic tank-soil absorption systems, privies, and
chemical-type toilets; authorizing the issuance of permits, and providing
for penalties for violations.
Section /—Definitions
For the purposes of this ordinance, the following words and phrases
shall have the meanings ascribed to them in this section.
1.1.1 Health officer shall mean the legally designated health au-
thority of the (name of political subdivision) or his authorized
representative.
1.1.2 Individual sewage disposal system shall mean a sewage dis-
posal system, other than a public or community system, which
receives either human excreta or liquid waste, or both, from
one premises. Included within the scope of this definition are
septic tank-soil absorption systems, privies, and chemical type
toilets, and such other types as may be prescribed in regu-
lations by the health officer.
1.1.3 Permit shall mean a written permit issued by the health
officer, permitting the construction of an individual sewage
disposal system under this ordinance.
1.1.4 Person shall mean any institution, public or private corpo-
ration, individual, partnership, or other entity.
1 Reference should also be made to Recommended State Legislation and Regula-
tions: Urban Water Supply and Sewerage Systems Act and Regulations, Water Well
Construction and Pump Installation Act and Regulations, Individual Sewerage Dis-
posal Systems Act and Regulations, U.S.D.H.E.W., Public Health Service, July 1965.
77
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Section //—Requirements for Individual Sewage Disposal Systems
2.1 The health officer of (name of political subdivision), in order to
protect the health and safety of the people of (name of political sub-
division) and of the general public, is authorized and directed, after
public hearing, to promulgate and amend, from time to time, regula-
tions establishing minimum standards governing the design, construc-
tion, installation, and operation of individual sewage disposal systems.
Such regulations shall establish such minimum standards as, in the
judgment of the health officer, will insure that the wastes discharged
to various individual sewage disposal systems:
1. Do not contaminate any drinking water supply.
2. Are not accessible to insects, rodents, or other possible carriers
of disease which may come into contact with food or drinking
water.
3. Do not pollute or contaminate the waters of any bathing beach,
shellfish breeding grounds,* or stream used for public or do-
mestic water supply purposes or for recreational purposes.
4. Are not a health hazard by being accessible to children.
5. Do not give rise to a nuisance due to odor or unsightly appear-
ance.
6. Will not violate any other laws or regulations governing water
pollution or sewage disposal.
2.2 The health officer is authorized to promulgate such additional regu-
lations as are necessary in his judgment to carry out the provisions of
this ordinance.
Section ///—Permits
S.I It shall be unlawful for any person to construct, alter, or extend
individual sewage disposal systems within the (name of political sub-
division) unless he holds a valid permit3 issued by the health officer
in the name of such person for the specific construction, alteration, or
extension proposed.
3.2 All applications for permits shall be made to the health officer, who
shall issue a permit upon compliance by the applicant with provisions
of this ordinance and any regulations adopted hereunder.
3.3 The health officer may refuse to grant a permit for the construction
of an individual sewage disposal system where public or community
sewerage systems are reasonably available.
3.4 Applications for permits shall be in writing, shall be signed by the
applicant, and shall include the following:
3.4.1 Name and address of the applicant.
Lot and block number of property on which construction,
alteration, or extension is proposed.
•Optional with locality.
*The permit issued by the Health Officer is in addition to the building permit
usually required and should be obtained prior to construction, alteration, and
extension of the residence or facility to be served.
78
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3.4.2 Complete plan of the proposed disposal facility, with sub-
stantiating data, if necessary, attesting to its compliance with
the minimum standards of the health officer.
3.4.3 Such further information as may be required by the health
officer to substantiate that the proposed construction, altera-
tion, or extension complies with regulations promulgated by
the health officer.
3.5 A complete plan for the purpose of obtaining a permit to be issued
by the health officer shall include:
3.5.1 The number, location, and size of all sewage disposal facili-
ties to be constructed, altered, or extended.
3.5.2 The location of water supplies, water supply piping, existing
sewage disposal facilities, buildings or dwellings, and adja-
cent lot lines.
3.5.3 Plans of the proposed sewage disposal facilities to be con-
structed, altered, or extended.
3.6 Any person whose application for a permit under this ordinance
has been denied may request and shall be granted a hearing on the
matter before the health officer within 30 days after receipt of the
request.
Sect/on IV—Inspections
4.1 The health officer is hereby authorized and directed to make such
inspections as are necessary to determine satisfactory compliance with
this ordinance and regulations promulgated hereunder.
4.2 It shall be the duty of the owner or occupant of a property to give
the health officer free access to the property at reasonable times for
the purpose of making such inspections as are necessary to determine
compliance with the requirements of this ordinance and regulations
promulgated hereunder.
Section V—Penalties
5.1 Any person who violates any provision of this ordinance, or any
provision of any regulation adopted by the health officer pursuant to
authority granted by this ordinance, shall, upon conviction, be punished
by a fine of not less than dollars nor more than
dollars, or by imprisonment for not less than days nor more than
Hays; and each day's failure to comply shall constitute a separate
violation.
Section W—Conflict of Ordinances, Effect on Partial Invalidity
6.1 In any case where a provision of this ordinance is found to be in
conflict with a provision of any zoning, building, fire, safety, or health
ordinance or code of this (name of political subdivision) existing on
the effective date of this ordinance, the provision which, in the judg-
ment of the health officer, establishes the higher standard for the pro-
motion and protection of the health and safety of the people shall
prevail. In any case where a provision of this ordinance is found to be
79
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in conflict with a provision of any other ordinance or code of the
(name of political subdivision) existing on the effective date of this
ordinance which establishes a lower standard for the promotion and
protection of the health and safety of the people, the provisions of this
ordinance shall be deemed to prevail, and such other ordinance or
codes are hereby declared to be repealed to the extent that they may
be found in conflict with this ordinance.
6.2 If any section, subsection, paragraphs, sentence, clause, or phrase
of this ordinance should be declared invalid for any reason whatsoever,
such decision shall not affect the remaining portions of this ordinance,
which shall remain in full force and effect; and, to this end, the pro-
visions of this ordinance are hereby declared to be severable.
Section VII-Effective Date
7.1 This ordinance shall be effective on and after the day
of , 19
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Appendix C
Engineering Information Forms
The pertinent information needed on individual sewage disposal
systems prior to approval by the health or other agency having juris-
diction is outlined in Appendix B. In those instances where subdivision
developments are involved, considerably more data are needed by the
health agency prior to approval of water and sewage service facilities.
The possibility of providing connections to existing municipal systems
or the construction of community water and sewage systems should re-
ceive first consideration. In the extreme situation where this is not
possible, and when subsoil conditions are favorable, information of the
type called for in the following "Statement of Information" has been
found satisfactory in one State. Unless real estate developments in non-
sewered areas are properly planned and designed under competent
sanitary engineering supervision, public health hazards and nuisances
may result, with attendant economic loss to the community.
STATE DEPARTMENT OF HEALTH
Statement of Information
Regarding Water and Sewerage Service for Realty Subdivisions
To the State Commissioner Health 19
Sir:
As required by the provisions of section of Article of the
Public Health Law, the following statement is made and submitted with
the plat of a proposed realty subdivision in ,
(State)
General Information
I. Name of subdivision
2. Owner ,
(State name of person, company, corporation, or association owning
the subdivision)
3. Business address__
4. Officers
(If organized, give names of officers)
5. Location
(Give name of incorporated village or town in which subdivision
is located)
6. Area of subdivision Number of lots
(Total size in acres)
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Families accommodated.
7. Do you intend to build houses on this subdivision?_
Do you intend to sell lots only?_
Do you intend to build on some lots and sell others without
buildings?
8. Is this subdivision or any part thereof located in an area under
the control of local planning, zoning, or other officials?
If so, have these plans been submitted to such authorities?
Have these plans been approved or disapproved by such governing
authority?
9. Nature of soil
(Describe to a depth of 10 feet if tile fields are to be used
for sewage disposal, or 20 feet if leaching pits are proposed, giving thickness
of various strata, such as top soil, clay, loam, sand, gravel, rock, etc.)
10. Topography
(State whether ground is flat, rolling, steep, or gentle slope, etc.)
11. Relative elevation of water table below ground surface
(Give maximum
and minimum, if there is any variation)
Water Service
12. Proposed method of supplying water_
(Describe in detail, giving name of
municipality, water district or company, if a public water supply is to be used.)
15. State approximate distance to nearest public water supply main of
municipal system .
(Give name of municipality, water district, or company)
Sewerage Service
14. Proposed method of collection and disposal of sewage.
(Give name of
municipality or sewer district, if public sewers are to be used)
15. State approximate distance to nearest public sewer main of munici-
pal system
Storm Water Drainage
16. State proposed method, if any, of disposing of surface water from
streets, roofs, land, and other areas .
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Subdivision Owners Who Intend to Build Homes Must Submit the
Following Additional Information:
Additional Information:
17. Cellar drainage: Are cellar or footing drains LO be installed?
If so, how will drainage be disposed of?
18. Laundry wastes: Are laundry tubs to be located in basement?
If so, how will wastes be disposed of?_
It is hereby agreed that if the attached plans dated
or any amendment or revision thereof, are approved by the State
Department of Health, installation of water supply and sewage
disposal facilities will be made in accordance with the details
thereof as shown on such approved plans. If the subdivided lands
shown on such plans are sold before such installations are made, it
is agreed that reference to said plans will be made in the deed or
contract of sale, and a covenant inserted therein requiring the
purchaser to install such facilities in accordance with such ap-
proved plans.
Signature
Official title__
This statement must be signed by the owner of the land platted
for subdivision or the responsible official of the company or
corporation offering the same for sale.
Important Note
This form must be accompanied by:
(1) One U. S. G. S. topographic map or other general map showing
exact location and approximate boundaries of subdivision.
(2) A print on cloth for filing with the State Department of Health,
together with such other tracings and prints (see below) as may be
necessary for filing with the county clerk and owner of the sub-
division, showing:
(a) subdivision layout, including streets, building lines, lot dimen-
sions, and other pertinent data;
(b) existing and proposed water mains, if available. If public
water supply is available, show existing and proposed water
mains for all lots, and submit a copy of the contract between
the developer and the waterworks officials, or a letter from
such officials stating that an agreement has been reached
regarding the supply of such facilities.
(c) existing and proposed sewers; if already approved by Depart-
ment give date of approval; or, if not approved, application
must be made and detailed plans of sewer extensions sub-
mitted by officials in charge of sewer systems, in accordance
with section of the Public Health Law.
(d) details of a typical lot arrangement showing general location
of well and septic tank, subsurface absorption devices, etc.
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(where either or both public water and sewerage service are
inaccessible), plus the following:
(1) development of well (giving sufficient detail to show how
the well will be developed and protected from pollution,
its depth, and strata penetrated).
(2) cross section of soil showing depth of various strata (unless
explained in detail under item 9).
(3) plan and section of all parts of sewage disposal system,
giving all dimensions and grades.
(4) actual field results of soil tests to determine absorptive
capacity of soil (may be submitted with correspondence).
Inasmuch as stamp of approval must be placed on face of plans, a
space 3 by 6 inches should be reserved for this purpose. This space
must be blocked out in white if blueprints are submitted.
Size of plans for filing with county clerk; 20 by 20 inches or 20 by 40
inches, tracing cloth or white prints on cloth.
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Appendix D
Drainage Fixture Unit Values
Type of Fixture or Group of Fixtures ____ Drainage Fixture Unit Value
Automatic clothes washer (2" standpipe) ......................... 3
Bathroom group consisting of a water closet, lavatory
and bathtub or shower stall:
Flushometer valve closet ............................................... 8
Tank type closet [[[ 6
Bathtub (with or without overhead shower) ................... 2
Combination sink and tray with food disposal unit ..... 4
Combination sink and tray with one li^" trap ............. 2 '
Combination sink and tray with separate \i/z" traps ... 3
Dental cup or cuspidor [[[ 1
Dental lavatory [[[ 1
Drinking fountain [[[ i/z
Dishwasher, domestic (gravity drain) ................................. 2
Floor drains with 2" waste ................................................. 3
Kitchen sink, domestic, with one Ii/£" waste ................... 2
Kitchen sink, domestic, with food waste grinder ........... 2
Lavatory with Ii4" waste [[[ 1
Laundry tray (1 or 2 compartments) ............................... 2
Show stall, domestic [[[ 2
Showers (group) per head [[[ 2
Sinks:
Surgeon's [[[ 3
Flushing rim (with valve) ................................................. 6
Service (trap standard) .................................................. 3
Service (P trap) [[[ 2
Pot, scullery, etc [[[ 4
Urinal, pedestal, syphon jet blowout ............................... 6
Urinal, wall lip [[[ 4
Urinal stall, washout [[[ 4
Urinal trough (each 6 ft. section) ..................................... 2
Wash sink (circular or multiple) each set of faucets ..... 2
Water closet, tank operated ............................................... 4
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Appendix E
Suggested Specifications for Watertight Concrete
1. Materials
Portland cement should be free of hard lumps caused by moisture
during storage. Lumps from dry packing that are easily broken in the
hand are not objectionable.1
Aggregates, such as sand and gravel, should be obtained from sources
known to make good concrete. They should be clean and hard. Particle
size of sand should range very fine to 14 inch. Gravel or crushed stone
should have particles from 14 inch to a maximum of \yz inches in size.
Water for mixing should be clean.
2. Proportioning
Not more than 6 gallons of total water should be used for each bag
of cement. Since sand usually holds a considerable amount of water,
not more than 5 gallons of water per bag of cement should be added
at the mixer when sand is of average dampness. More mixing water
weakens the concrete and makes it less watertight. For average aggre-
gates, the mix proportions shown in the table below will give water-
tight concrete.
Average Proportions for Watertight Concrete
Max. Size
Gravel (in.)
114
s/.
Cement
(volume)
1
1
Water »
(volume)
VA
/4
Sand
(volume)
O* y
o* /
Gravel
(volume)
3
1 Assuming sand is of average dampness.
3. Mixing and Placing
All materials should be mixed long enough so that the concrete has
a uniform color. As concrete is deposited in the forms, it should be
1 Type V portland cement may be used when high sulfate resistance is required.
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tamped and spaded to obtain a dense wall. The entire tank should be
cast in one continuous operation if possible, to prevent construction
joints.
4. Curing
After it has set, new concrete should be kept moist for at least seven
days to gain strength.
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Appendix F
Industrial Waste Treatment
Septic tank systems are not employed for the treatment of industrial
wastes from large manufacturing industries. In small industrial plants,
septic tank systems, assisted as needed by supplementary processes may
be used advantageously either for treating wastes to render them suit-
able for final disposal by one of the methods described in Part II, or
for conditioning wastes to render them acceptable for discharge to
sewer systems where they would otherwise be objectionable.
It is important to note that the design of septic tank systems for
treatment and disposal of industrial wastes is not sufficiently simple to
permit formulation of specific criteria for design. This is because indus-
trial wastes vary widely in their composition; in fact, rarely do any two
plants produce wastes of the same character and composition. The
design of an industrial waste treatment and disposal system is, there-
fore, a problem requiring specialized sanitary engineering consultation,
and this consultation usually includes special laboratory analyses and
studies of the particular wastes under consideration. With these pre-
cautions, however, it is possible, in many instances, to develop a work-
able septic tank system which is actually simple, effective, economical,
and within the means of the small industrial plant.
The following is intended to indicate only qualitatively some of the
methods applicable to a few industrial wastes. The particular design
for any given waste must be prepared by competent sanitary engineers.
LAUNDRY WASTES AND OTHER GREASY WASTES
Wastes derived from laundries and some other sources, which contain
soaps, oil, and greases in relatively low concentrations, may be success-
fully disposed of by one of the methods previously described, provided
the greasy substances are first removed. If greasy substances, including
soaps and oils, are discharged into absorption systems, they accumulate
and soon cause clogging of the soil pores.
The greasy substances contained in wastes are of two types: (1) the
floatable greases, which are nonemulsified and which, therefore, upon
quiescence or standing, will separate and rise to the surface of the
liquid to form a surface layer; and (2) the emulsified greases, which
do not separate upon standing. Both of these components may be
present in considerable concentrations, and both must be removed.
88
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Properly designed, conventional grease traps, such as those used for
restaurant wastes, will remove floatable grease, but not emulsified
grease. Emulsified grease may be removed in a practical manner, how-
ever, either (1) by passing the waste through a rock filter, or (2) by
chemically flocculating the waste.
Where relatively low concentrations of emulsified grease are present,
the rock filter process may be successfully used. The rock filter serves
to deemulsify the grease, changing it to floatable grease so that it may
be subsequently removed by simple trapping. Septic tank units may be
employed advantageously both for trapping the floatable grease and
to serve as the container for the rock filter. The servicing required for
this system includes removal of the accumulated floatable grease at
regular intervals and occasional replacement of the filter rock. The
sizes of the trap and rock filter units will depend upon the quantity of
the waste and its composition, as determined by laboratory analyses.
Where high concentrations of emulsified grease are present, a chem-
ical flocculation process may be needed. For all of these processes (trap-
ping, flocculation, and settling), septic tanks may be used as the most
economical means of obtaining the necessary capacities. Also required,
as accessories to the flocculation process, are a suitable electrically
operated stirring device and a volumetric type chemical feeder. Mainte-
nance of the system includes keeping the chemical feeder stocked with
chemicals and periodic pumping of accumulated sludge from the set-
tling basin. In this process, the grease particles are agglomerated by
the flocculating action into large particles which settle out in the
settling basin. The sizes of the various tanks, the amounts of flocculat-
ing chemicals needed, and the frequency of pumping required will
depend upon the quantity of the waste and its composition, as deter-
mined by laboratory analyses.
SLAUGHTERHOUSE, DAIRY, AND OTHER ORGANIC WASTES
These wastes are generally characterized by very high concentrations
of organic materials, up to (and even exceeding) 100 times the strength
of ordinary sanitary sewage. Part of the organic material is in true
solution, and this may sometimes be disposed of by simple under-
ground absorption, provided there is adequate area and provided a suit-
able schedule for resting the absorption areas is maintained. Much of
the organic material, including the emulsified greases, is in colloidal
suspension. Colloidal particles will clog underground absorption sys-
tems. They do not settle out on quiescence and, therefore, have to be
removed by some means, such as by flocculation and sedimentation,
or by application to trickling filters followed by settling. As in the pre-
ceding example, septic tanks may advantageously be used to provide
the necessary tank capacities for the trapping, flocculating, and set-
tling processes.
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MINERAL OR INORGANIC WASTES
Mineral and similar wastes present special problems generally re-
quiring chemical neutralization or other chemical treatment before
the wastes may be filtered or absorbed into the ground. Important
factors are the metallic and nonmetallic ions present, their concentra-
tions, and, for underground absorption the nature and importance of
the water basin lying below the area intended for absorption. Septic
tanks can sometimes be used advantageously in the solution of these
problems, as chemical reaction chambers, settling tanks, etc.
90
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Partial List of References on Septic Tank-
Soil Absorption Systems
Appendices to Study of Seepage Beds, James B. Coulter; Thomas \V. Bendixen; and
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to the Federal Housing Administration, Dec. 15, 1960.
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Report to the Federal Housing Administration, May 1, 1963.
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Coulter; and T. W. Bendixen; Report to the Federal Housing Administration,
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•& US. GOVERNMENT PRINTING OFFICE: 1969—O-363-353
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