ONSITE WASTEWATER DISPOSAL
DISTRIBUTION NETWORKS FOR SUBSURFACE SOIL ABSORPTION SYSTEMS
Richard J. Otis, P.E,
Rural Systems Engineering
Madison, Wisconsin
Introduction
A. The manner in which wastewater is distributed over the Infiltrative
surface may be critical to the proper functioning and long term life
of the soil absorption system. Localized overloading from poor
distribution may result in inadequate treatment of the wastewater in
rapidly permeable soils and accelerated clogging in all soils (Bouma,
1975; Robeck, et.al, 1964; McGauhey and Winneberger, 1964),
B. Many different types and designs of distribution networks have been
tried in an effort to apply wastewater in an effective manner to all
parts of the soil absorption system and to reduce the degree of clogging.
Not all are satisfactory for all soil and site conditions because of
the different patterns of loading of the infiltrative surface which
result (Otis, et.al, 1978). Therefore, it is important first to decide
what pattern of loading would be acceptable for the qiven site and then
to select a network design which best provides that loading pattern.
Methods of Distribution
A. Gravity Flow
1. Description
The relative elevations between the pretreatment unit and the
soil absorption system are such that the wastewater can enter the
absorption system by gravity. This method is characterized by
"trickle flow" because wastewater is discharged into the system
as it is displaced from the pretreatment unit.
2. Performance
Distribution is very uneven, with localized overloading of the
infiltrative surface, Clogging occurs in these areas first and then
progresses through the system as the liquid seeks a more open surface.
Ultimately, the entire bottom of the system may become clogged,
resulting in continuous ponding of the infiltrative surface. This
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has the benefits of submerging the sidewalls, thereby increasing
the area of infiltrative surface exposed to the flow, and increasing
the hydraulic gradient across the clogging mat. However, these
benefits may be offset by ;nore severe clogging (Bendixen, etia^ 1950);
Wtnneberger, et.al, 196Q; Thomas, et.al, 1966; Univ. Wis,, 1978), or
by Inadequate treatment before the clogging mat is formed in
rapidly permeable soils (Univ, Wis, 1978},
B. Dosing
1. Description
The wastewater is collected after pretreatment for periodic
discharge into the soil absorption system via a pump or siphon.
In this manner, the absorption system receives wastewater in
slugs between which no loading occurs.
2. Performance
The wastewater is distributed over a larger portion of the
absorption system during each dose than is achieved with gravity
flow. The "resting" period between doses allows the infiltrative
surface to drain, exposing the clogging mat to air and drawing
air into the soil below the mat (Hills and Krone, 1971). This
promotes degradation of the clogging mat to maintain higher infil-
tration rates and to extend the life of the system (Univ. Wis., 1980).
In sands or coarser textured materials, rapid infiltration rates can
lead to bacterial and viral contamination of shallow groundwater.
Therefore, in these soils, doses should be more frequent and smaller
in volume. In finer textured soils where absorption is more of a
concern than treatment, larger, less frequent doses are more suitable.
See Table II. B-l for reconuiended dosing frequencies.
TABLE II. B-l
Recommended Dosing Frequencies for
Various Soil Textures (U.S. EPA, 1980)
Soil Texture
Dosing Frequency
Sand
Sandy loam
Loam
Silt loam; silty
clay loam
Clay
4 doses/day
1 dose/day
Frequency not critical
1 dose/day1
Frequency not critica1
1 Long-term resting provided by alternating fields is desirable
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C. Uniform Applicat ion
1. Description
Uniform application is similar to dosing except that the
network used to distribute the wastewater is pressurized to
control the application rate to all parts of the absorption
system. By carefully designing the network, the entire infil-
trative surface can be loaded uniformly and at a rate below the
saturated hydraulic conductivity of the soil. Pumps or siphons
are used for network pressurization.
2. Performance
The intent of uniform application is to load all parts of
the absorption system equally while maintaining unsaturated
conditions in the underlying soil for adequate treatment. Limited
experience indicates that the method is successful (Univ. of Wis.,
1978). Dosing frequencies presented in Table II. B-l are also
recommended for uniform application.
D. Selection of Distribution Method
The selection of the most appropriate method of distribution
depends on whether absorption or treatment is the most critical
concern. This is usually determined by the permeability of the soil
and the geometry of the infiltrative surface. Under some conditions,
the method of distribution is not critical, so selection is based
on simplicity and cost.
Methods of distribution into trenches and beds on level sites or
multiple trenches on sloping sites for various soil permeabilities
are listed in order of their preference in Table II. D-l. For
example, in very rapidly or rapidly permeable soils, ensuring adequate
treatment is the primary concern. Therefore, uniform application should
be employed. However, uniform application of wastewater into multiple
trenches on sloping sites is more difficult and, for that reason,
gravity distribution into serially loaded trenches may be more appro-
priate except in the very rapidly permeable soils. (Note that
conventional trenches and beds are not recommended for rapidly permeable
soils.)- In the moderately permeable soils, dosing seems to reduce the
degree of soil clogging but uniform application is not necessary since
the soil's fine texture will ensure adequate treatment. Therefore, for
level sites, dosing is the preferred method followed by gravity. Uni-
form application is third only because the cost may be greater, not
because it is less effective. On sloping sites, gravity methods using
serial distribution are recommended because dosinq and uniform applica-
tion into multiple trenches on sloping sites is difficult. Of the
latter two, uniform application can be designed more easily and is
preferred over dosing. It should be noted that uniform application,
unlike the other methods, is appropriate for all applications but design
difficulties and cost cause one of the other two to be preferred.
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TABLE IE. D-l
RECOMMENDED METHODS OF EFFLUENT DISTRIBUTION FOR VARIOUS
SYSTEM GEOMETRIES AND SOIL PERMEABILITIES1(OTIS, 1981)
Soil Permeability Trenches or Beds Multiple Trenches on
(Percolation Rate) on Level Site Sloping Sites (>5%)
Very Rapid2 Uniform application3 Uniform application
<1 tnin/in Dosing Gravity
(<0-04 cm/sec)
Gravity Dosing
Rapid Um form appl ication Gravity
1-10 min/in _o Dosing Uniform application
(4-0.4 cm/sec x 10 )
Gravity Dosing
Moderate Dosing Gravity
11-60 min/in , Gravity Uniform application
(4-0.7 cm/sec x 10 ) „
Uniform application Dosing
Slow Not Critical Not Critical
<»
60 min/in Uniform application
(>0.7 x 10"-^cm/sec)
*' Methods of application are listed in order of preference.
Conventional soil absorption systems not recommended for these soils.
Should be used exclusively in alternating field systems to ensure adequate
treatment
Preferred method for large flows.
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-5-
III. Design
A. Introduction
1. Types of Distribution Network Design
Several different distribution network designs are used in
subsurface soil absorption systems. Most use drainage tile or pipes
but small diameter pressure pipe is becoming used more frequently.
Large diameter perforated pipe networks
This type of network is used in most conventional systems to
permit gravity flow or dosing. In the past, short sections of
4-in. clay tile spaced 1/4 to 1/2 in. apart were used. Treated
building paper was placed over the top of each open joint to keep
the rock or soil from entering the pipe. This pipe has been
replaced by 4-in. bituminous fiber and plastic (rigid or flexible),
perforated pipe. One or t\-to rows of holes 1/2 to 5/8 in. in
diameter spaced 3 in. apart are conmon in this type of pipe. Uniform
distribution along the length of the pipe is not provided (Converse,
1974).
Small diameter perforated pipe networks
This type of network is used primarily in pressurized
networks to provide uniform application. The pipe diameter,
hole diameter and hole spacing are determined by each design.
A minimum pipe diameter of 1-in, and hole diameter of 1/4 in.
is recommended.
Other distribution designs.
Several other types of designs have been developed, many of
which do not use pipe. These are primarily proprietary in
nature and are discussed separately below.
2. Selection of Distribution Network Design
The choice of which network to use depends on the type and
geometry of the absorption system and the method of distribution
desired. Table III. A-l lists the most appropriate network designs
in order of preference for different system configurations. The
various network designs listed are described in the following sections.
B. Large Diameter Perforated Pipe Networks
1. Single Line
Description
Single line networks are used in trenches for gravity flow
or dosing. A single line in the center of each trench is used.
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TABLE III. A-l
DISTRIBUTION NETWORKS FOR VARIOUS SYSTEM DESIGNS AND APPLICATION METHODS3 (U.S. EPA, 1980)
Method
of
Appl i cati on
Single Trench
Mult1-Trench
(Fills, Drains)
On Level Site
Multi-Trench
{Drains)
On Sloping Site
Beds
(Fills, Drains)
Mounds
Gravi ty
Si ngl e 1 i ne
Drop box
Closed loop
Distribution box
Drop box
Relief line
Distribution box^
Closed loop Not applicabl
Distribution box
Dosi ng
Single line
Pressure
Uniform Pressure
Appli cation
Closed loop
Pressure
Distribution box
Pressure
Di stribution box
Pressure0
Closed loop
Pressure
Distribution box
Pressure
Not applicabl
Pressure
a Distribution networks are listed in order of preference,
b Use limited by degree of slope
c Because of the complexity of a pressure network on a sloping site, drop boxes or relief lines are
suggested.
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-7-
Design
The distribution line is 3 to 4 in. diameter perforated pipe
laid level or on a gradient of 1 to 2 in. per 100 ft. within
the porous media. If the pipe has one row of holes, the pipe
1s set such that the holes are at the invert. If the pipe has
two rows of holes, the pipe is laid such that the line bisecting
the acute angle between the holes intersects the pipe invert.
Two rows of holes are thought to be superior because of the
smal1^unobstructed channel left between the holes in the bottom of
the pipe. The end of the pipe terminates in an observation vent
or is capped.
Limitations
Traditionally, distribution lines have been limited to 100 ft.
in length because pipe breakage, root penetration or settling may
disrupt the flow within the pipe, These fears are largely
unfounded since most of the flow occurs in the gravel. However,
if lines do exceed 100 ft. in length, the inlet from the
pretreatment unit could be located near the center of the
line rather than at one end,
2. Closed Loop
Description
Closed loop networks are used for gravity flow or dosing
in trench or bed systems that have the entire infiltrative
surface at one elevation. The pipe is identical to that used
for single lines and is laid in a similar manner.
Design
More than one line is used and the ends of each are connected
to one another with ell, tee or cross fittings, as shown in
Figure III. B-l. In beds, the lines are laid parallel with 3 to
6 ft. spacings. A tee, cross or distribution box is used at the
inlet to the network.
3. Distribution Box
Description
Distribution hox networks are used for gravity flow or
dosing in multi-trench or bed systems which have independent
single lines. The single lines in the network extend from a
corunon watertight compartment called the distribution hox. The
purpose of the box is to divide the flow entering the box equally
between all laterals leaving the hox, In multi-trench systems, out-
lets to individual lines can be plugged to allow periodic resting
of selected trenches,
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FIGURE III, B-1
Closed Loop Distribution Network
Design
The distribution box is normally purchased prefabricated.
It is usually round or rectangular with a flat bottom
and an aoove grade removable cover. It has a single inlet, and
an outlet for each distribution line. The box must be set on
a dry, frost-proof footing with the outlet inverts at the same
elevation. The invert of the inlet should be at least 1 in. above
the outlet inverts. If dosing is to be employed or the slope of
the inlet pipe imparts a significant velocity to the influent, a
baffle must be placed in front of the inlet.to absorb the influent
energy and prevent short circuiting across the box. The slope
of the lines leaving the box should be laid at the same gradient
for at least 10 ft. beyond the box to ensure an even division of
flow (See Figure III. B-2).
Limitations
The use of distribution box networks should be restricted
to level or gently sloping sites where the system can be installed
so that the ground surface elevation above the lowest trench is
above the box outlets inverts elevation (Machmeier, 1981).
Tbis recommendation is made because it is difficult to maintain
the outlet inverts at the same elevation which is necessary for
equal, division of flow (Bendixen and Coulter, 1958). Carelessness
in placement or backfilling, uneven settling or frost heaving will
cause the misalignment. If the box were to settle unevenly so
that the lowest trench is overloaded, the trench will pond and
the flow will back up into the box where it can enter another
trench (See Figure III. B-2). However, if the ground elevation
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FIGURE III. B-2
Multi-Trench System on a Sloping Site Using a Distribution Box (Otis, et.al,
TMJKM AIOV1 MTUTI
ttttmmvnom bob
eiSTmiuTMN aoa
•fprto tam
FIGURE III. B-3
Use of a Distribution Box on a Level or Gently Sloping Site (Machmeier, 1981)
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over the trench is lower than the box outlets, the wastewater will
seep onto the ground surface and the remainder of the system may
never receive wastewater. On steeply sloping sites, drop boxes
or relief lines should be used unless the distribution box is set
carefully and its levelness maintained.
4. Drop Box
Description
Drop box networks are gravity flow networks used to serially
load multi-trench, systems, Each, trench has its own drop box which
accepts wastewater from the drop box upstream from it. The box
discharges the liquid into the distribution line of the trench
until the trench, is filled to capacity. At that point, the
drop box overflows into the next drop box downstream from it.
This network, is usually used on sloping sites since each drop
hox must be at a lower elevation than the one upstream from it,
but they may be used on level sites as well by lowering each
successive trench-
Design
A drop box, usually purchased prefabricated, is a circular box
with a watertight bottom and a removable cover. It has an inlet,
one or two outlets for the distribution lines and an overflow. The
distribution line outlets are located at or near the bottom of the
box. The invert of the overflow is located at the same elevation
as the crown of the outlet or 2 in. above it to flood the trench
to the top of the gravel. The inlet invert may be at the same
elevation as the overflow invert or a few inches above. See
Figure III. B-4.
Drop boxes may be located anywhere along the trench length.
An elevation difference of 1 to 2 in. between successive drop
boxes is all that is needed. Solid wall pipe is used between
each drop box.
Limitations
The only limitation of this system is that it can be used
only where gravity flow is appropriate. It is superior to all
other gravity flow networks because of its characteristics of
operation. Each trench is successively loaded to its full capacity.
Thus, only the portion of the system required to absorb the
wastewater is used. During periods of high flow or when evapo-
transpiration is low, more trenches are used. When flows are low
or evapotranspiration is high, the lower trenches will drain dry
and automatically go into a resting phase during which the
infiltrative surface is rejuvenated. The upper trenches may also
be rested by plugging the outlets to the distribution lines and
forcinq the liquid into the lower trenches. Another advantage
is that additional trenches can be added easily to the existing
system, 1f noc°ss^ry.
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-11
raiKMi
eoarev**
||M« 1 4««
FIGURE III. B-4
Drop Box Network
FIGURE III. B-5
Relief Line Network
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5. Relief Line
• Description
Relief line networks may be used in place of drop box networks
in multi-trench systems using gravity flow. Relief lines, used
in place of drop boxes, are simple overflow lines interconnecting
trenches such that one trench is filled before the wastewater
overflows into the ne*t trench.
• Design
A relief line is a solid wall pipe. The elevation of the
invert of the overflow section is set above the crown of
the distribution Tina, as shown in Figure III. B-5, Relief
lines between successive trenches should be staggered or
separated by 5 to 10 ft. to prevent short curcuiting.
• Limitations
Relief lines have the same limitations as drop box networks
but fewer of its advantages. There is less flexibility in
operation because individual trenches cannot be closed off.
Adding trenches to an existing system is also more difficult.
Construction costs are less than drop box networks, however.
C. Small Diameter Pipe Networks (Pressure Distribution)
1. Description
Small diameter pipe networks are used primarily in pressure
distribution networks. These networks are used to apply wastewater
uniformly over the entire infiltrative surface during each dose.
The network is designed to discharge equal volumes of wastewater
from each perforation in the network. This is done by maintaining
a uniform pressure within all parts of the network. A pump or
siphon is used to pressurize the network.
2. Des i gn
Pressure distribution networks usually consist of a solid pipe
manifold which supplies wastewater to a number of perforated laterals.
To maintain uniform pressure throughout the network, the headlosses
incurred delivering the liquid to each perforation must be kept to
a minimum by properly sizing the manifold and lateral diameters in
relation to the size and number of perforations. This can be a long
and tedious process. To simplify the design, a method using graphs
and tables has been developed (Otis, 1981). If this method is used,
a maximum difference in discharge rates, between perforations through-
out the network will be 15 percent. The design orocedure is outlined
below:
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Step 1: Layout a network
For any absorption field, more than one configuration of
manifolds and laterals may provide uniform coveraqe of the in-
filtrative surface. The manifold may be located at one end,
in the center, or off-center of the laterals as the situation
demands. Central manifolds minimize lateral size because lateral
sizing is based in part upon the length of lateral between the
supply and distal ends. Central manifold inlets from the
pressurization unit minimize manifold sizing for the same reason.
For very long, narrow absorption areas, multiple manifolds may
be used as long as the pressures at each manifold inlet are equal.
To minimize leakage from the perforations nearest the manifold
at the start of each dose, the laterals can be mounted
aboye the manifold using tee-to-tee construction as shown
in Figure III, C-l. In this manner, the manifold will fill
before discharging into the laterals. However, provisions must
be made for draining the manifold in localities where freezing
is a concern. Where pumps are used for pressurization, the
manifold may be drained back into the dosing chamber. This
is impractical with large volume manifolds and impossible if
siphons are used, Tn such instances, the manifold should be
insulated and provisions made for manual draining if the system
is left idle for any extended period of time.
TOPSOIL
BACKFILL
~ BARRIER
"!T*5
o>
¦
LATERAL
__ DISTAL PERFORATION IN
CAP NEAR CROWN OF PIPE
O TEE
PERFORATIONS AT
LATERAL INVERT
REDUCER
Q> tee
MANIFOLD H?"-
FIGURE III. C-l: Tee-to-Tee Lateral/Manifold Construction (Otis, 1981
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Step 2: Select perforation size and spacing
Uniform distribution can be approached best by providing as
many uniformly spaced perforations as practically possible.
Perforation diameters of 1/4-in to 5/8-in are common. Smaller
diameter perforations permit more perforations per unit lenqth of
lateral to provide more uniform coverage, but holes smaller
than 1/4-in in diameter are more likely to clog. Larger spacinqs
between perforations permit longer laterals, but spacinqs too
great result in localized overloading of the soil's infiltrative
surface. Maximum spacings of 10 ft are sugqested here, but
spacings lesss than 5 to 6 ft are more desirable.
In bed systems, lateral spacings equal to the perforation
spacings are recommended. Perforations between any two laterals
should be staggered so that they lie on the vertices of equilateral
triangles. This arrangement will provide the most uniform
distribution of liquid.
Since the laterals drain between doses, air must be vented
from the laterals at the beginning of each dosing cycle. To
facilitate venting, the perforation at the distal end of each
lateral should be drilled horizontally in the end cap near the
crown of the pipe (See Figure III. C-l).
Step 3: Determine lateral pipe diameter
Use Figures III. C-2 through III. C-8 to determine the
appropriate lateral diameter for the selected perforation
diameter, perforation spacing and lateral length. These figures
were developed for plastic pipe using the Hazen-Wi 1 Hams equatiun
for closed conduit flow (Hazen-Wi11iams Coefficient of Ch = 150),
allowing no more than a 10 percent head loss from the supply end
to the distal end of the pipe (Otis, 1981).
Step 4: Calculate the lateral discharge rate
The lateral discharge rate is equal to the perforation
discharge rate times the number of perforations in the lateral
The perforation discharge can be obtained from Table III. C-l or
calculated using the orifice equation:
q = 11.79 d2hd%
where q is the perforation discharge rate in gpm, d the perfora-
tion diameter in inches, and h
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10
Perforation Diameter
1/4 - in.
(6.4 mmJ
a»
c
o
a
a
cn
1 1/4
1/2
c
o
o
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Lateral Length (ft.)
FIGURE III. C-2
Minimum Lateral Diameter for Plastic Pipe (C^ = 150) Versus Perforation
Spacing and Lateral Length for 1/4 in. Diameter Perforations (Otis, 1981)
10
Perforation Diameter:
5/16-in. (7.9 mmJ
a
_c
o
a
a
a
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Lateral Length (ft.)
FIGURE III. C-3
Minimum Lateral Diameter for Plastic Pipe (C^ = 150) Versus Perforation
Spacing and Lateral Length for 5/16 in. Diameter Perforations (Otis, 1981)
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16-
Perforat
10 20 30 4a 50 60 70 80 90 100 110 120 130 140 150
Lateral Length (ft.)
FIGURE III. C-4
Minimum Lateral Diameter for Plastic Pipe (C^ = 150) Versus Perforation
Spacing and Lateral Length for 3/8 in. Diameter Perforations (Otis, 1981)
Perforation Diameter-*
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Lateral Length (ft.)
FIGURE III. C-5
Minimum Lateral Diameter for Plastic Pipe (Ch = 150) Versus Perforation
Spacing and Lateral Length for 7/16 in. Diameter Perforations (Otis, (981 /
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10
Perforation Diameter-
1/2 - in.
(12.7 mm.)
1 1/4
O)
_c
u
es
a
(0
e
o
a
o
®
a.
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Lateral Length (ft.)
FIGURE III. C-6
Minimum Lateral Diameter for Plastic Pipe (C^ = 150) Versus Perforation
Spacing and Lateral Length for 1/2 in. Diameter Perforations (Otis, 1981)
10
Perforation Diameter:
9/16-in.
(14.3 mm J
O)
c
u
<0
a
V)
c
o
Q
h.
o
w
©
a
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Lateral Length (ft.)
FIGURE III. C-7
Minimum Lateral Diameter for Plastic Pipe (Ch = 150) Versus Perforation
Spacing and Lateral Length for 9/16 in. Diameter Perforations (Otis, 1981)
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-18-
10
Perforation Diameter:
5/8 - In.
(15.9 mm.)
o>
c
o
R3
OL
(/}
C
o
o
w
<0
&
Lateral Length (ft.)
FIGURE III. C-8
Minimum Lateral Diameter for Plastic Pipe (C^ = 150) Versus Perforation
Spacing and Lateral Length for 5/8 in. Diameter Perforations (Otis, 1981)
TABLE III. C-l
Perforation Discharge Rates versus
Perforation Diameter and In-Lme Pressure (Otis, 1981)
In-Line Perforation Diameter (in)
Pressure
(ft)
1/4
5/16
3/8
7/16
1/2
9/16
5/8
1.0
0.74
1.15
1.66
2.26
2.95
3.73
4.60
1.5
0.90
1.41
2.03
2.76
3.61
4.57
5.64
2.0
1.04
1.63
2.34
3.19
4.17
5.27
6.51
2.5
1.17
1.82
2.62
3.57
4.66
5.90
7.28
3.0
1.28
1.99
2.87
3.91
5.10
6.46
7.97
3.5
1.38
2.15
3.10
4.22
5.51
6.98
8.61
4.0
1.47
2.30
3.31
4.51
5.89
7.46
9.21
4.5
1.56
2.44
3.52
4.79
6.25
7.91
9.77
5-0
1.65
2.57
3.71
5.04
6.59
8.34
10.29
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-19-
variations in the elevation of perforations that will occur in
construction of the system. An elevation difference of -10
percent between perforation elevations will result in a 10 percent
difference in discharge rates between the highest and lowest
perforation. A minimum in-line pressure of 2.5 ft permits a
*3 construction tolerance. However, hj should not be excessive
because it affects the perforation discharge rate resulting in
increased junction losses and pump or siphon capacity.
Step 5: Calculate manifold diameter
If the manifold diameter is to be uniform throughout its
length, Table III. C-2 can be used. Based on the manifold
location and the lateral size and spacing, the manifold diameter
can be read from the table. The diameter obtained is that
necessary to maintain no more than a 5 percent headloss from
the manifold inlet to distal end.
To reduce pipe costs in larger systems, the manifold may be
telescoped in size, decreasing in diameter as the flow decreases
along its length. Using the following equation, F-j values are
calculated for each lateral segment:
Ft = 9.8 x 10"^ "5
where Qi is the flow in the ith manifold segment in gpm. The
F-j values represent empirical friction factors for each manifold
segment, derived from the Hazen-Williams equation. See Otis (1981)
for the derivation. The manifold diameter, D^, then can be computed
using:
n -
r ./,f lr~ M =
0.21
hoJ cr
~>
J /
where M is the number of manifold segments, L-j is the length of
the ith segment (lateral spacing) in ft, f is the fraction of the
total headloss desired in that manifold segment or series of
manifold segments, and hj is the distal pressure in the lateral
in ft. To maintain less that a 10 percent headloss, f must be
less than or equal to (0.1/number of manifold segments) x number
of manifold segments in the section under design.
Step 6: Determine the dose volume
In systems where the laterals are long in comparison to the
manifold, dosing volumes should be made large in relation to the
pipe volume to reduce the significance of the leakage. Five to
ten times the network pipe volume is suggested. However, this
volume should not be larger than the required dose volume
calculated by dividing the average daily flow by the desired
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TABLE III. C-2.
Maximum Manifold Length {ft) for Various Manifold Diameters Given
the Lateral Discharge Rate and Lateral Spacing (Otis, 1981
liurit
Olulurji btl
—tanlfold
Ktnifold Dliuter • 1 VI'
Kant fold Dliaettr • i'
Nidi fold OUnUr • ]'
Manifold Dliaettr « | 1/4'
Bint fold Dlinfldr • 4'
Ibnlffttd Dtueler • A'
litcrtl Sptclnq (ft)
Ut«r«l Sptdnj (ft)
liliril Spicing (ft)
l«tar«l Spicing (ft)
l»C
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-21-
dosing frequency (See Table II. B-l). The nomograph in Figure
III. C-9 can be used to calculate the pipe volume. If the crown
of the manifold lies below the lateral inverts, the manifold
pipe volume does not need to be included. If this is not the
case, the nomograph also can be used to determine the manifold
volume. If the minimum dose based on pipe volume is larger than
the required dose based on dosing frequency, a different network
may have to be designed to reduce the pipe volume.
Step 7: Determine minimum pump or siphon discharge rate
This is calculated by summing the perforation or lateral
discharge rates.
Step 8: Calculate the total friction losses
The total friction losses are the sum of the losses in the
delivery pipe and the network losses. The friction loss in
the delivery pipe between the dosing chamber and the network
inlet is determined by using the minimum discharge rate
computed in Step 7. This can be calculated using the Hazen-
Williams equation:
where is the length,in ft, of the delivery pipe from the dosing
dosing chamber to the network inlet, is the pipe diameter in
inches, Qm the discharge rate in gpm and Ch is the Hazen-Williams
friction factor equal to 150 if the pipe is plastic. The total
network losses are equal to 1.31 times the distal pressure
selected for the network, h^. (See Otis, 1981).
Step 9: Select the pressurization unit.
Pump selection is based on the pumping head and discharge
rate required for the network. The static lift or the difference
in elevation between the low water level in the dosing chamber
and the lateral inverts must be added to the friction losses
computed in Step 8 to obtain the total pumping head. Using
the head-discharge curves supplied by the manufacturer, a pump
able to efficiently discharge the minimum rate or greater from
Step 7 at the total pumping head is selected.
Siphons are selected based on the manufacturer1s stated
average discharge rate. This rate must be equal to or greater
than the minimum discharge rate of the network. To function
properly, the siphon discharge invert must be elevated above
r
3.55 ]1,85
Friction loss =
-------
tl
JC
u
c
c
o
«
0)
10 -
£ "
30
40
so
60
TO
ao
90
100
ISO
200-
350
s
-I
£>
E
30-
25
20-
IS
10
3-
1-J
000
700
BOO
500
400
300
200
FIGURE III. C-9
Nomograph for Determining the Total Pipe Volume Given the Diameter,
Length and Number of Laterals (Manifolds) (Otis, 1981)
100
90
eo
70
60
50
40
30
20
10
-------
-23-
the lateral inverts a distance equal to or greater than the
friction losses estimated in Step 8. The delivery pipe from
the siphon should be one nominal size larger than the siphon
to facilitate air venting unless the slope of the delivery
line from the siphon is great enough that the pipe does not
fill until the network is pressurized.
Step 10: Size of the dosing chamber
The dosing chamber is a crucial component of the network
design. It must discharge the appropriate volume at the required
rate with each dose. Also, appurtances must be selected carefully
to insure proper and reliable operation.
The volume of the chamber is determined by the dosing
volume computed in Step 6. A reserve capacity above the active
dosing volume equal to one day's average flow should be provided
if single pumps are used. This will provide one day or more
for repairs with normal water fixture use in case of pump failure.
A reserve volume is not needed if siphons are used because
overflows by-passing the siphon are provided.
Necessary appurtances include level controls and high water
alarm switches for pump systems, and suitable access to the
pressurization unit for servicing. A typical pumoinq chamber
is shown in Figure III. C-10. Switch selection and installation
are extremely important because the most frequent cause of
pump failure is a switch malfunction. The switches should be
sealed from the corrosive atmosphere in the chamber and all
electrical contacts and relays must be mounted outside the
chamber. Provisions should be made to prevent gases in the
chamber from following the electrical conduits into the control
box. The high water alarm switch should be located 2-3 in
(5-8-cm) above the pump activation level. This switch
must be on a separate circuit from the pump level controls.
Access for maintenance is best provided by a manway located
over the pump or siphon.
Siphons or siphon breakers must be used in networks where
the low water level in the dosing chamber is above the lateral
inverts. If a pump without a siphon breaker is used in such an
instance, a natural siphoning of liquid out the chamber will
occur. A simple siphon break can be merely a small hole drilled
in the discharge line at the highest point in the dosing chamber.
3. Example: Design a pressure distribution network for a mound system
to be constructed for a 3-bedroom home. The absorption area within
the mound is to be 6 ft x 65 ft. The distribution laterals will be
approximately 8 ft above the pump elevation. The distance from the
pump to the mound is about 75 ft.
Step 1: Network layout
A central manifold network with 4 laterals, 2 on either side
of the manifold is selected. The laterals are to be 32 ft long
-------
RELAY IN WEATHERPROOF
MANHOLE COVER
ENCLOSURE
VENT
HANGER PIPE FOR
/~ PUMP REMOVAL
w
INFLUENT
QUICK DISCONNECT
SLIDING COUPLER
RESERVE'CAPACITY
AFTER ALARM SOUNDS
HIGH WATER.
ALARM SWITCH
EFFLUENT
' 3
START- LEVEL 2
LEVEL CONTROL SWITCH
SHUT_-_OFF_ LEVEL y
LEVEL CONTROL SWITCH
ODD
FIGURE III. C-10
TYPICAL PUMPING CHAMBER FOR DOSING SOIL ABSORPTION SYSTEMS (U.S. EPA, 1980)
-------
-25-
and spaced 3 ft apart.
Step 2: Perforation size and spacing
One-quarter inch perforations spaced 30 in apart are selected.
Step 3: Lateral pipe diameter
From Figure III. C-2 a lateral diameter of 1 1/4-in is
requi red.
Step 4: Lateral discharge rate
Using 2.5 ft distal in-line pressure, 1/4-in perforations
will discharge 1.17 gpm (Table III. C-l).
No. perforations/lateral = 32 ft lateral = 13 perforations
2.5 ft spacing
Lateral
discharge = 1.17 gpm/perforation x 13 perforations/lateral
rate
= 15 gpm/lateral
Step 5: Manifold diameter
The manifold length is the distance between the first and
last lateral in the network. In this case it is 3 ft. Table
III. C-2 indicates that a central manifold network discharging
15 gpm into laterals spaced 3 ft apart can use a 1 1/2-in manifold
with a maximum length of 3 ft or a 2-in manifold with a maximum
length of 6 ft. The 1 1/2-in manifold is sufficient.
Step 6: Dose volume
A mound should be dosed 2 to 4 times daily. However, the
minimum dose volume should be 5 to 10 times the network pipe
volume.
Required gp^/bdrm x 3 bdrm = 115 qal/dose
ao*e " 4 doses/day
volume J
Minimum
dose = 10 x total pipe volume
volume
In this case, the manifold will be located below the lateral
inverts so only the lateral volume need be considered. However,
to prevent freezing, the manifold and delivery line are to be
-------
26-
dramed back to the dosing chember. Therefore, the dose
volume must be increased by the volume of water which will
drain back. The nomograph in Figure III. C-9 can be used
to determine these volumes quickly. To determine the lateral
volume, place a straight edge at 32 ft on the "Lateral Length"
scale and connect it with the 1 1/4-in mark on the "Lateral
Diameter" scale. The straightedge crosses the "Lateral Volume"
scale at about 2 gal/lateral. Rotate the straightedge at about
this point on the "Lateral Volume" scale and align it with 4
on the "Number of Laterals" scale. Where it crosses the "Total
Volume" scale, read the total lateral volume. In this case, it
is less than 10 gal and off the scale, so multiply 2 gal/lateral
times 4 laterals to obtain the total volume.
Total lateral volume = 2 gal/lateral x 4 laterals = 8 gal
Minimum dose volume = 10 x 8 gal =80 gal => Use 115 gal/dose
Using the same procedure, the manifold and delivery line
volume can be determined. A 3-in delivery line, 75 ft long, is
to be used. The manifold is 1 1/2-in diameter, 3 ft long.
Manifold/delivery line volume = 1 gal + 27 gal = 28 gal
Therefore,
Total dose volume = 115 gal/dose + 28 gal ® 145 gal/dose
Step 7: Pump capacity
Discharge rate = 4 laterals x 15 gpm/lateral = 60 gpm
HOLE SPACING
30 In.
GRAVEL BED
11/4 In. LATERALS
FIGURE III. C-ll
Pressure Distribution Network for Design Example
-------
-27-
Step 8: Total friction losses
Network losses =1.31 x distal in-line pressure
= 1.31 x 2.5 ft = 3.3 ft
Delivery losses
(3-in delivery
pipe)
= L,
3.55 Qm
ChDd2"63
75 ft
1 8 S
3.55 x 60 qpm
150 x (3 m)'63
= 0.7 ft
Step 9: Pump selection
Total pumping _ Network + Delivery + Static
head ~ losses losses lift
= 3.3 ft + 0.7 ft + 8 ft = 12 ft
Therefore, a pump capable of pumping 60 gpm against a head
of 12 ft is selected.
Step 10: Dosing chamber volume
Chamber volume = Dose volume + Reserve
= 115 gal/dose + 200 to 400 gal reserve
= 325 to 500 gal
4. Limi tations
Pressure distribution networks have the advantages over other
networks of providing dosing in addition to more uniform application,
permitting irregular field configurations, providing equal division
of flow between multiple trenches and simultaneous application of
effluent over large absorption areas. However, they are not well
suited to sloping sites. The static heads between laterals installed
at different elevations will vary thereby affecting the perforation
discharge rates. If this is not properly compensated for, unequal
distribution between the various infiltrative surfaces will result.
-------
-28-
If the infiltrative surfaces are restricted to only two different
elevations of equal loading, two separate distribution networks can
be used with each network receiving alternate doses through the
use of alternating pumps, valves or siphons. Each network is
designed separately. This arrangement provides the best assurance
of an equal division of flow. If the infiltrative surface areas
are not equal or or more than two levels are necessary, a single
network can be designed by taking into account the differences in
the total heads within each lateral.
D. Other Distribution Designs
Several other distribution designs are used occasionally. Most of
these are proprietary in nature. Little performance data is available.
1. Inverted Network
Inverted networks are similar to conventional gravity flow
systems except that the perforated pipe is laid with the holes at
or near the crown of the pipe. This arrangement is designed to
provide more uniform distribution of wastewater over a larae area,
and to prolong the life of the field by collecting any settleable
solids passing out of the septic tank in the bottom of the pipe.
Water-tight sumps are located at both ends of each inverted line
to facilitate periodic removal of the accumulated solids. Limited
testing indicates distribution is not improved substantially over
conventional gravity flow networks (Converse, 1974).
2. Case System
The Case System is a network of specially treated concrete blocks
mortared together to form a sealed conduit. The block conduit is
set in a trench and backfilled eliminating the need for distribution
pipe and porous media. The septic tank effluent flows through the
block and diffuses through the porous block walls and into the soil.
3. Gravel-less System
Similar to the Case System, large diameter drainage pipe
10-in to 12-in in diameter enclosed in drainage fabric is
buried without porous media. The volume in the pipe acts as
the storage volume while the liquid seeps out into the soil.
4. Leaching Chambers
This method employs open bottom chambers, in place of
perforated pipe and qravel for distribution and storaqe of
the wastewater. The chambers interlock to form an underground
cavern over the soils' infiltrative surface. The wastewater
is discharged into the cavern through a central weir, trough
or splash plate and allowed to flow over the infiltrative surface
in any direction. Access holes in the roof of the chamber allow
visual inspection of the soil surface and maintenance as necessary.
-------
-29-
IV. Construction
A. Materials
Three to 4-in diameter pipe or tile is typically used for
nonpressurized networks. Either perforated pipe or 1 ft lengths
of suitable drain tile may be used. The perforated pipe comnonly
has one or more rows of 3/8 to 3/4 in diameter holes. Hole spacing
is not critical. Table IV. A-l can be used as a guide for acceptable
materials for nonpressurized networks.
Plastic pipe is used for pressure distribution networks because
of the ease of drilling and assembly. Either PVC Schedule 40
(ASTM D 2663) or ABS (ASTM 2661) pipe may be used.
B. Large Diameter Perforated Pipe Placement
To insure a free flow of wastewater, the distribution pipe
should be laid level or on a qrade of 1-in to 2-in per 100 ft.
To maintain a level or uniform slope, several construction techniques
can be employed. In each case a tripod level or transit is used to
obtain the proper grade elevations. Hand levels are not adequate.
The rock is placed in the excavation to the elevation of the pipe
invert. The rock must be leveled by hand to establish the proper
grade. Once the pipe is laid, more rock is carefully placed over the
top of the pipe. Care must also be taken when flexible corrugated
plastic pipe is used, because the pipe tends to "float" up as rock is
placed over the top of the pipe. One method is to employ special holders
which can be removed once all the rock is in place.
C. Small Diameter Perforated Networks
Pressure distribution networks are usually fabricated at the
construction site. This may include drilling holes in distribution
laterals. The holes must be drilled on a straight line along the
length of the pipe. This can be accomplished best by using 1-in by 1-in
angle iron as a straight-edge to mark the pipe. The holes are then
drilled at the proper spacing. Care must be used to drill the holes
perpendicular to the pipe and not at an angle. All burrs left around
the holes inside the pipe should be removed. This can be done by sliding
a smaller diameter pipe or rod down the pipe to knock the burrs off.
Solvent weld joints are used to assemble the network. The laterals
are attached to the manifold such that the perforations lie at the bottom
of the pipe. The rock is placed in the absorption area first, to the
elevation of the distribution laterals. The rock should be leveled
by hand, maintaining the same elevation throughout the system, before
laying the pipe. After the pipe is laid, additional rock is placed
over the pipe.
-------
-30-
TABLE IV. A-l
PIPE MATERIALS FOR NONPRESSURIZED DISTRIBUTION NETWORKS (U.S. EPA, 1980)
Type of Materiai
Clay Drain Tile
Clay Pipe
Standard and Extra-
Strength Perforated
Bituminized Fiber Pipe
Homogeneous
Perforated
Lami nated-Wal 1
Perforated
Concrete Pipe
Perforated Concrete
PI astic
Acryloni tri1e-
Butadi ene-
Styrene (ABS)
Polyvi nyl
Chloride (PVC)
Speci fi cation
ASTM C-4
ASTM C-211
A STM £7-2312
ASTM D-2313
ASTM C-44 (Type 1
or Type 2)
ASTM 0-2751*5
ASTM 0-2729b
D-3033b D-3034&
Class
Standard Drai n Ti 1 e
Standard
ASTM C-14^
Styrene-Rubber ASTM D-2852b
Plastic {SR) D-3298&
Polyethylene (PE)
o Straight Wall ASTM 0-1248b
o Corrugated (Flexible) ASTM F-405-76'3
a Must be of quality to withstand sulfuric acid.
b These specifications are material specifications only. They do not
give the location or shape of perforations.
-------
-31-
D. Distribution Boxes
If used, distribution boxes should be installed level and placed
in an area where the soil is stable and remains reasonably dry. To
protect the box from frost heaving, a 6-in layer of 1/2 to 2-1/2 in
rock should be placed below and around the sides of the box. Solid
wall pipe should be used to connect the box with the distribution
laterals. Separate connections should be made for each lateral. To
insure a more equal division of flow, the slope of each connecting
pipe should be identical for at least 5 to 10 ft beyond the box.
E. Dosing Chambers
Monolithic concrete, fiberglas or plastic tanks should be used
as dosing chambers. Steel is not recommended because of the corrosive
nature of the waste. The tank must be watertight to avoid groundwater
infiltration. Waterproofing consists of adequately sealing all joints
with asphalt or other suitable material. Coating the outside of the
tank prevents groundwater from seeping into the tank. Asphalt coating
the inside and outside of steel tanks helps retard corrosion.
Application of 4-mil plastic to the wet asphalt coating protects the
coating when back filling. At high water table sites, flotation
collars should be used so the chamber does not float out of position
due to hydrostatic pressures on a near-empty tank. This is not
normally a problem for concrete tanks, but for the lighter-weight
materials, such as steel or fiberglas, it could present a problem.
The manhole riser pipe should be a minimum of 24-in in diameter and
should extend 6-in above ground level to keep surface water from
entering the chamber. A cast iron pipe sleeve or other suitable device
can be slipped over the plastic pipe extending from the tank to unexca-
vated soil to provide protection from breakage due to backfilling or
settling.
V. Operation and Maintenance
A. Routine
Routine maintenance is required in alternating field systems
and 1n systems employing dosing, uniform application or distribution
boxes.
1. Alternating Fields
On a regular schedule, usually annually, an operating field
is taken out of service to "rest" and a "rested" field is put
into service. The switch should be done in later spring or early
summer when the soil temperatures are warm so that a good biological
mat can develop quickly to ensure adequate treatment.
-------
-32-
2. Systems Employing Dosing or Uniform Application
Systems employing pumps for dosing should be inspected monthly
for proper switch and pump operation. On and off switches as well
as the high water alarm switch should be tested. Periodically, the
pump discharge rate should be checked by timing the period it takes
the pump to empty the chamber. If the time has increased signifi-
cantly, the pump should be removed and inspected for wear, cloqging
or impellor damage. The distribution network should also be checked
for obstructions.
Siphons should be observed semi-annually for proper operation.
The bell and any bell vents should be flushed each year.
3. Distribution Boxes
Systems employing distribution boxes should be inspected semi-
annually. The levelness of the box should be checked in the fall
after the first heavy frost and again in the spring after the frost
has left the ground. In areas with no frost, annual inspections
should be sufficient. The box should be leveled, if necessary.
B. Other
Improper maintenance of the pretreatment unit may result in
plugging of the distribution network. Rodding of the pipe may be
necessary. Pressure distribution networks can be flushed, if
necessary, by cutting the ends of the laterals and activating the
pump or siphon.
VI. Questions
1. What factors influence the selection of a distribution method?
2. Under what conditions should drop boxes not be used? Pressure
distribution?
3. What special considerations should be taken in the placement
and operation of siphons?
4. How could dosing be employed on sloping sites? Would other methods
be more satisfactory?
-------
-33-
VII. References
Bendixen, T.W. and J.B. Coulter. 1958. Effectiveness of the distribution
box. U.S. Public Health Service, Washington, D.C.
Bendixen, T.W., M. Berk, J.P. Sheehy and S.R. Weibel. 1950. Studies on
household disposal systems. Part II. Federal Security Agency. U.S.
Public Health Service, Environmental Health Center. Cincinnati, Ohio.
Bouma, J. 1975. Unsaturated flow during soil treatment of septic tank
effluent. Journal of Environmental Engineering Division. American
Society of Civil Engineers. 101:957-983.
Converse, J.C. 1974. Distribution of domestic waste effluent in soil
absorption beds. Transactions of the American Society of Agricultural
Engineers. 17:299-309.
Hills, D.J. and R.B. Krone. 1971. Hydraulically ventilated underground
filter. Journal of the Sanitary Engineering Division. American Society
of Civil Engineers. 97:851-866.
McGauhey, P. And J.T Winneberger. 1964. Studies of the failure of septic
tank percolation systems. Journal Water Pollution Control Federation.
36:593-606.
Machmeier, R.E. 1981 . Town and country sewage treatment. Bulletin 304.
University of Minnesota Agricultural Extension Service. St. Paul,
Minnesota.
Otis, R.J., J.C. Converse, B.L. Carlile and J.E. Witty. 1978. Effluent
distribution. In: Home Sewage Treatment Proceedings of the second
national home sewage treatment symposium. American Society of Agricultural
Engineers. Publication 5-77. St. Joseph, Michigan.
Otis, R.J. 1981. Design of pressure distribution networks for septic tank-
soil absorption systems. Small Scale Waste Management Project. University
of Wisconsin, Madison.
Robeck, G.C., T.W. Bendixen, W.A. Schwartz and R.L. Woodward. 1964. Factors
influencing the design and operation of soil systems for waste treatment.
Journal Water Pollution Control Federation. 36:971-983.
Thomas, R.E., W.A. Schwartz, and T.W. Bendixen. 1966. Soil chemical
changes and infiltration rate reduction under sewage spreading. Soil
Science Society of America Proceedings. 30:641-646.
University of Wisconsin, 1978. Management of small waste flows. U.S.
Environmental Protection Agency. EPA-600/2-78-173. Cincinnati, Ohio.
Winneberger, J.H., L. Francis, J.A. Klein and P.H. McGauhey. 1960.
Biological aspects of failure of septic tank percolation systems. Sanitary
Engineering Research Laboratory, University of California, Berkeley.
-------
-34-
VIII. Problems
A. Statements
1. Pressure Pistribution: Level Site
A multi-trench system is to be constructed on a level site
in a sandy loam soil. It is to receive an average flow of 250 qpd.
The system is to consist of 5 trenches, each 3 ft x 40 ft and spaced
9 ft on center.
Design a pressure distribution network for this system. The
dosing chamber is to be located 50 ft from the first lateral.
2- Pressure Distribution: Large Bed
A subsurface soil absorption system is to be used for wastewater
disposal in a small community. Three beds, each designed for
15,000 gpd, are to be used. The beds will be 100 ft x 130 ft.
Design a pressure distribution network for one of the beds.
The dosing chamber will be located 200 ft from the network inlet.
If siphons are used for pressurization, what should be their
elevation in relation to the lateral inverts?
3. Pressure Distribution: Sloping Site
A multi-trench system is to be constructed on a sloping site.
It will serve a 4-bedroom home with a present average flow of
200 gpd. Five trenches will be used as shown in the figure below.
Because of large trees, the trenches will be of unequal lengths
as shown. The elevations of the distribution lateral inverts will
be as follows:
Invert
Lateral No. Elevation
1 873.0 ft
2 873.5 ft
3 874.0 ft
5 & 6 874.5 ft
6 & 7 875.5 ft
-------
-35-
^ — a*o
NG
PTOracrr line
lATOW. NUMBER
A6S0WT10M TBCWCH
.-"jiiroL^ «c^icnT NuM»ej\
Network Layout for a Trench System on a Sloping Site (Problem
-------
-36-
B. Solutions
1. Pressure Distribution: Level Site
Step 1: Layout network
Two layouts would be suitable for this system. The
distribution laterals can be fed either by an end or a central
manifold. With an end manifold, 5 laterals are required,
while a central manifold requires 10 laterals. An end
manifold will be used in this example.
seme tatik
POWN& dMAM&ER
A&&OVTION TFENOK
1
1
I
NETWORK.
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1 ,
. ^ i r* % x ,
\
! 1
PWOfATEP LA1^_X
1
»
1
E.NP MAMfOU?
•/ '< -*; "V -S *
1
,dENTR/\L f-WiirOLP
r;
f >c
i
i
i
1
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TT
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- <¦ > <.. - *
1 r -' ' 1 ' V
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End and Central Manifold Configurations for
a Trench System on a Level Site (Problem 1}
-------
-37-
Step 2: Select perforation size and spacing
Perforations 1/4-in in diameter spaced 2.5 ft will be used.
(Other combinations may be just as suitable).
Step 3: Select lateral diameter
To provide the most uniform effluent application over the
trench bottom, the first and last perforations in the lateral will be
located one-half the perforation spacing from either end of the trench.
Therefore,
Lateral Length = 40 ft - (1/2 x 2.5) = 38.75 ft
From Figure III. C-2 (for 1/4-in diameter perforations), the minimum
lateral diameter for a 38.75 ft lateral with a 2.5 ft perforation
spacing is 1 1/2-in.
Step 4: Calculate the lateral discharge rate
A minimum in-line pressure of 2.5 ft is desired. From Table
III. C-l, a 1/4-in perforation will discharge 1.16 gpm at this
pressure.
No. of Perforations/Lateral = 40 = 16 perforations
Lateral Discharge Rate = 16 x 1.16 gpm = 17.5 gpm
Step 5: Calculate the manifold size
The manifold diameter is to be uniform along its length to
simplify construction.
Manifold Length = 4 ft x 9 ft = 36 ft
From Table III. C-2, an end manifold with lateral discharge rates
of 17.5 gpm and lateral spacings of 9 ft can have a maximum length
of 20 ft for a 2-in diameter or 43 ft for a 3-in diameter. Therefore,
a 3-in diameter is necessary.
Step 6: Determine dose volume
The crown of the manifold is to be located below the lateral
inverts and the manifold drained back into the dosing chamber at the
end of each dose. Therefore, the minimum dose volume is based on
lateral pipe volume only. Using the nomograph in Figure III. C-9,
a straightedge is placed at 38.75 ft on the Lateral Length scale and
at 1 1/2-in on the Lateral Diameter scale. The straightedge crosses
the Lateral Volume scale at about 3.5 gal. Maintaining this point
-------
-38-
on the Lateral Volume scale, the straightedge is rotated to align
with 5 on the Number of Laterals scale. The straightedge crosses
the Total Pipe Volume scale at 17.5 gals. A minimum dose volume
of 5 to 10 times the total pipe volume or 90 to 175 gal should be
used.
The required dosing frequency taken from Table II. B-l is
1 dose/day for sandy loam. Therefore,
Required Dosing Volume
250 gpd = 250 gal/dose
1 dose/day
The minimum dose is less than the required dosing volume so the
network is satisfactory. Since the manifold will drain back to
the dosing chamber, the dose volume must be increased in volume
equal to that in the manifold and delivery line. If a 50 ft 3-in
diameter delivery line is used, the volume increase is equal to
50 ft + 36 ft or 86 ft of 3-in pipe. Using the nomograph in Figure
III. C-9, this volume is determined to be approximately 32 gals.
Therefore,
Dosing Volume = 250 gal + 32 gal = 282 gal
Step 7: Calculate the minimum discharge rate
Minimum Discharge Rate = 5 laterals x 17.5 gpm/lateral
Step 8: Calculate total friction losses
Network losses = 1.31 x distal in-line pressure
= 1.31 x 2.5 ft = 3.28 ft
Delivery losses = L^
3.55 Qm
_chDd
2.6 3
1.8 S
= 50 ft
3.55 x 87.5 qpm
1.8 5
= 0.9 ft
_ 150 x (3-in)2*6 3_
Total losses = Network losses + Delivery losses
= 3.28 ft + 0.9 ft = 4.2 ft
Step 9: Select pressurization unit
In this instance, a pump is to be used.
Total Pumping Head = Static Head + Friction Losses
-------
-39-
If the low water level in the dosing tank is 5 ft below the lateral
inverts, the total pumping head is:
5 ft + 4.2 ft (friction losses from Step 8) or 9 ft
Using head-discharge curves provided by manufacturers, a pump able
to discharge at least 80 gpm against 9 ft of head is selected.
Step 10: Size the dosing chamber
Since only one pump is to be used, a reserve volume equal to
one day's average flow is necessary in case of pump failure.
Therefore, a volume of 280 gal (dose volume) + 250 gal (average
daily flow) or 530 gal must be provided between the pump off switch
and the dosing chamber inlet invert. (The high water alarm switch
is located just above the pump on switch.)
2. Pressure Distribution: Large Bed
Step 1: Layout network
A central manifold configuration is selected as shown below.
i
9C9 rcAiMSTEA
MA/tifSLP SO*tNT KUMCA
•«o
Network Layout for Large Bed System (Problem 2)
-------
-40-
Step 2: Select perforation size and spacing
Perforations are to be 3/8-in diameter spaced 5 ft apart.
The perforations are to be staggered between laterals to provide
more uniform distribution (See figure).
Step 3: Select lateral diameter
From Table III. C-4: 2-in laterals required
Step 4: Calculate the lateral discharge rate
A minimum in-line pressure of 2 ft is used. From Table III. C-l
Perforation Discharge Rate =2.34 gpm
Perforations/lateral = 65 = 13
5~
Lateral Discharge Rate = 13 x 2.34 gpm = 30 gpm/lateral
Step 5: Calculate Manifold size
This network is too large to determine the manifold size from
Table III. C-2. Therefore, the F^ values are calculated.
Number of Manifold Segments = 100 -1=19 segments
7
Results of Calculations to Determine Manifold Segment Diameters
¦M
ft
f c
A
v-fe
Segment
i
-------
I I
f,
-1
I
f,
f /
/// ;-„l-
/
/ 'V:
Allowing O.l loss of head in the manifold, the necessary manifold
diameter can be calculated.
°M =
M
L I Fj
i=l
O.l hd
0.21
5 ft x 3184.89
0.1 x 2 ft
0.21
= 10.7-in or 12-in
By this method, a 12-in manifold would be required.
A uniform sized manifold is not necessary. To save expense and
to provide more uniform distribution by reducing the difference between
lateral entrance losses, the manifold should be telescoped to smaller
diameters downstream. The same method as above may be used to determine
the proper diameters for each segment if the allowable headloss in the
manifold is assumed to be linear along its length. Making this assumption,
each segment may account for (0.1 4 19)hd of the manifold function loss.
Calculated diameters of the even numbered segments appear in the table.
For example, the diameter for segment 2 is:
5 ft x 8.79
2 x x 2 ft
0.21
4.98-in or 6-in
From the table, the nominal manifold diameters are selected:
Manifold segments: 1-3 6-in
4-8 8-in
9-16 10-in
16-19 12-in
Step 6: Determine dose volume
The crown of the manifold is to be located below the lateral
elevation. A manual drain valve will be installed on the manifold
to drain the manifold when the network is out of service. From Fiqure
III. C-9:
Minimum Dose Volume = 10.5 gal/lateral x 40 laterals x (5 to 10)
= 2)00 to 4200 galfac u \ „v. ^
y -\r. i-u)-1'
From Table II. 8-1: L 1/ ,f ,«
15,000 qpd Lf-K'/i'
Required Dose Volume = 4 dose/day = 3750 gal /,.'(<•
! / /
This is satisfactory. ^ )/))C
-------
-42-
Step 7: Calculate the minimum discharge rate
Minimum Discharge Rate = 30 gpm/lateral x 40 laterals
= 1200 gprn
Step 8: Calculate total friction losses
Network Losses = 1.31 h^ = 1.31 x 2 ft = 2.62 ft
itwork
it
3.55 x 1200 qpm 19S
150 x (12-in)
1 .
= 0.55 ft
Step 9: Select pressurization unit
A 12-in siphon with a manufacturer's average discharge rating of
1200 gpm is selected. The discharge invert must be elevated a minimum
of 3.2 ft above the lateral inverts.
Step 10: Size the dosing chamber
A dosing volume of 3750 gal is to be used. The siphon has a
30-in draw. No reserve volume is necessary since the sipon has an
overflow.
Pressure Pistribution: Sloping Site
Step 1: Layout network
A layout as shown in the figure is to conform to the trench layout.
Step 2: Select the perforation size and spacing
Because the static heads in the laterals in each trench will vary,
either the perforation diameter or the perforation spacing must be
changed to maintain uniform application of effluent to each of the
infiltrative surfaces. It is most practical to change the spacinq,
since the perforation diameters normally can only change by nominal
drill bit sizes.
A perforation diameter of 1/4-in is selected with a maximum spacing
of 5 ft. Since the lateral at the lowest elevation will have the
highest oerforation discharge rate due to the greater static head, the
maximum spacing is to be used in this lateral.
To determine the spacing for the remaining laterals, it Is necessary
to compute the fraction of the dosage rate that is directed into each
lateral to provide uniform distribution. Knowing this and the in-line
pressure, the perforation discharge rates can be determined for each
lateral and thence, the perforation spacing.
-------
-43-
To calculate the lateral discharge rates, the discharge rate of
the lowest lateral must be calculated first based on the perforation
diameter and spacing selected. To do this, a minimum in-line pressure
in the uppermost lateral must be selected. Then the minimum in-line
pressure in the lowermost lateral is equal to the minimum in-line
pressure in the uppermost lateral plus the elevation difference between
the two laterals less the upstream manifold losses. Therefore, in
Lateral 1:
Minimum -
In-line = 2.5 + (875.5 - 873.5) - (fx 0.1) x (2.5) = 4.75 ft
Pressure
From the perforation discharge equation:
Perforation
Discharge = 11.79 (1/4-in)2 (4.75 ft)** = 1.60 gpm
Rate
Lateral *5 ft
Discharge = c f. x 1.6 gpm = 24 gpm
Rate 15 TZ
Knowing that the ratio of the lateral discharge rates to the total
trench loading in each trench must be equal to maintain uniform
distribution, the remaining lateral discharge rates, in-line pressures
and perforation discharge rates can be calculated (See accompanying
table). The perforation spacing is determined by first dividing the
lateral discharge rate by the perforation discharge rate to obtain the
number of perforations per lateral and then dividing this into the
trench length. The accompanying table presents the results of these
calculations.
Step 3: Select lateral diameter
Figure III. C-2 is used to select the later diameter. The diameters
obtained appear in the table. To reduce the number of different pipe
diameters, larger nominal diameters ultimately may be chosen. For
instance, laterals 1 through 4 could be 1 1/2-in pipe.
Step 4: Calculate the lateral discharge rate
This was done in Step 2. See the table.
Step 5: Calculate manifold size
The manifold is to be a uniform diameter throughout.
Manifold length = 4 segments x 10 ft = 40 ft
-------
-44-
Since the lateral discharge rates vary, F^ must be computed for
each segment.
Fj = 9.8 x KT-Qi1-85
Manifold n_ P_
Segment _j i_
1 24 gpm 0.35
2 40 gpm 0.90
3 64 gpm 2.15
4 99 gpm 4.82
Total 8.22
°M =
£ Li F-j
i = l
f hd
0.2 1
10 ft x 8.22
0.I x 2.5 ft
0.2 1
= 3.38 in => 4-in
Step 6: Determine dose volume
Since the manifold must fill entirely before the upper laterals
are filled, the lateral and manifold pipe volume must be included
in the calculation of the minimum dose volume. Figure III. C-9 is
used to make this calculation.
Lateral Volume:
Manifold Volume:
275 ft of 1 1/2-in
75 ft of 2-in
50 ft of 1-in
40 ft of 4-in
26 gal
12
2
26
66 gal
Five to 10 times the pipe volume gives a minimum dose volume of
330 to 660 gal/dose equal to about 1 dose per day. If a 330 gal dose
is used, at average daily flow 1 dose will occur every 1 1/2 days.
This is satisfactory.
-------
Determination of Lateral Diameters on a Sloping Site (Problem 3)
literal
Ho
Trench
Length
(ft)
Trench
V14th
(ft)
Loadlnq
Rate,
(gpd/ft*)
Trench
Loading
(gp<0
I Total
Loading
In-line
Pressure
ift)
Perforation
Dimeter
(In)
Per foritIon
Dltcharqe
Rate (qpe)
Lateral
Discharge No.
Kate {npa>Perforatloni
Perforation
Spacing
(ft)
Lateral
Length
(ft)
Literal
01 iseter
(In)
1
7S
}
O.J
112.5*
18 Bb
4 75
1/4
1 61
24
H
S 0
72. 5
1 1/2
2
SO
1
0.5
75.0
12.5
4.31
1/4
1 S3
16c
I0d
5.0e
47 Sf
1 1/4
J
7i
3
0.5
112.S
18.8
3.B8
1/4
1 45
24
17
4.4
72.8
1 1/2
1
75
3
O.S
112 S
IB 8
3.44
1/4
1.37
24
18
4.2
72.9
1 1/4
5
3S
1
0.5
S2.S
8.8
3 44
1/4
1 37
11
»
4.4
32.8
1
6
75
3
O.S
112 5
18.8
2.50
1/4
1.17
24
II
l.S
71.3
2
7
15
)
O.S
22. S
3.6
2.50
1/4
1.17
S
4
1.8
13.1
1
Toll!
400
•
-
600
100 3
-
-
-
128
r
•
-
' 15 ft I J ft i O.S gpd/ft* • 112.5 qpd d It I 1 S3 • 10 perforation!
b (112/5 • 600) > 100 • 16.SI ' iO i 10 - 5 ft
C (24 I 18 8| i IM • II gpa 1 50 - (5 t J) * 47.5 ft
-------
-46-
Step 7: Calculate the minimum discharge rate
This is the sum of the lateral discharge rates equal to
128 gpm from the table.
Step 8: Calculate total friction losses
Network losses = 1.31 x 2.5 ft = 3.3 ft
Delivery losses
3.55 Qm
*-h Dd
2£ 3
1.8 S
= 20 ft
3.55 x 128 gpm
150 x 4-m^
1 8 5
= 0.2 ft
Total losses = Network losses + Delivery losses
= 3.3 ft + 0.2 ft » 3.5 ft
Step 9: Select pressurization unit
In this case, a siphon can be used. It would be selected on the
basis of the average rated discharge. The discharge invert would be
set at a minimum of 3.5 ft above the uppermost lateral invert.
Step 10: Size the dosing chamber
The draw of the siphon and size of the dose selected, 330 gal,
is sufficient to size the dosing chamber.
------- |