United States
Environmental Protection
Agency
Stateipaf Environmental Research
Cincinnati OK 45268
July 1979
and' Development
Laboratory
Evaluation of
Methods to
Separate Fine
Grained Sediment
from Stormwater
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution-sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-79-076
July 1979
LABORATORY EVALUATION OF METHODS TO SEPARATE
FINE GRAINED SEDIMENT FROM STORMWATER
by
L. M. Bergstedt, J. M. Wetzel, and J. A. Cardie
St. Anthony Falls Hydraulic Laboratory
University of Minnesota
Minneapolis, Minnesota 55414
Grant No. R 803579
Project Officer
Richard P. Traver
Storm and Combined Sewer Section
Wastewater Research Division
Municipal Environmental Research Laboratory (Cincinnati)
Edison, New Jersey 08817
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its
components require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem
solution and it involves defining the problem, measuring its impact, and
searching for solutions. The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the prevention,
treatment, and management of wastewater and solid and hazardous waste
pollutant discharges from municipal and community sources, for the
preservation and treatment of public drinking water supplies and to minimize
the adverse economic, social, health, and aesthetic effects of pollution.
This publication is one of the products of that research; a most vital
communications link between the researcher and the user community.
The deleterious effects of construction site stormwater runoff upon
the nation's waterways have become of increasing concern in recent times.
This report presents the results of a laboratory testing program of an
inclined tube settler and Discostrainer for the removal of erosion
sediment.
Francis T. Mayo
Director
Municipal Environmental
Research Laboratory
111
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ABSTRACT
A literature survey had been conducted by the St. Anthony Falls Hydraulic
Laboratory to assess various methods for separation of sediment from storm-
water at construction sites. Two methods have shown some promise in this
application, and a research program was initiated with the objective of eval-
uating the effectiveness of the methods in removing fine grained inorganic
solids from water.
Experimental facilities were set up to test full-scale units of an in-
clined tube settler and a Discostrainer in an environment approximating that
in the field. These units were tested for removal efficiencies of inorganic
solids with sizes less than 100f and influent concentrations of about
2000 mg/1. Measurements were made of the influent and effluent concentra-
tions for various flow rates through the systems.
Results indicated that the installation of an inclined tube settler im-
proved the efficiency of a sedimentation tank by about 20 percent at the high-
est overflow rate tested of about 200 Ipm/m (5gpm/ft ) for an average re-
moval efficiency of about 60 percent. The inclined tube settler also
reduced the sensivity of the overflow rate on the efficiency of sediment re-
moval. Limited tests with alum added to the influent to increase flocculation
indicated about a 6 percent improvement in removal efficiency.
The Discostrainer was found to be extremely sensitive to influent solids
concentration. Thirty percent solids removal was the maximum attained for
the tests conducted. Higher removal percentages may possibly be obtained by
reducing the flow rate or influent concentration.
This report was submitted in fulfillment of Grant No. R803579 by the St.
Anthony Falls Hydraulic Laboratory of the University of Minnesota under the
sponsorship of the U.S. Environmental Protection Agency. This report covers
a period from October 1, 1976 to July 1, 1978, and work was completed as of
December 15, 1978.
iv
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CONTENTS
Disclaimer ii
Foreword iii
Abstract iv
Figures vi
Tables vi
Acknowledgements vii
1. Introduction 1
2. Conclusions 2
3. Recommendations 3
4. The Test Material 4
Soil Selection 4
Slurry Preparation 5
5. The Inclined Tube Settler 8
General Considerations 8
Facility and Procedures 10
Experimental Results 14
6. The Discostrainer 19
General Considerations 19
Facility and Procedures 21
Experimental Results 21
7. Requirements for a Hypothetical Construction Site 29
References 33
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FIGURES
Number Page
1 Size Distribution of Test Sediment 6
2 Typical Module of an Inclined-Tube High Rate Settler 9
3 Facility Schematic for Inclined-Tube Settler Evaluation 11
4 Sedimentation Tank Details 12
5 Solids Removal as a Function of Overflow Rate, Inclined-Tube
Settler 17
6 Variation of Concentration with Depth in Sedimention Tank 18
7 Construction and Operating Features of Discostrainer 20
8 Facility Schematic for Discostrainer Evaluation 22
9 Photo of Discostrainer Set-up 23
10 Solids Removal for Discostrainer 26
11 Headless Across Discostrainer Screens 27
12 Conceptual Design of a Debris Basin Fitted with Inclined
Tube Settlers 30
TABLES
Number Page
1 Summary of Test Results Inclined Tube Settler 15
2 Summary of Results for Discostrainer 24
vi
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ACKNOWLEDGMENTS
Mr. Richard P. Traver, Project Officer for the Storm and Combined Sewer
Section of the U.S. Environmental Protection Agency, provided valuable
guidance throughout the execution of the project. The assistance of Mr.
Robert T. Manwaring and Mr. Thomas E. Rehder of the HYCOR Corporation in
providing a Discostrainer unit for evaluation is gratefully acknowledged.
vii
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SECTION 1
INTRODUCTION
An increase in urban construction, such as shopping centers and large
housing developments, has led to a stripping of vegatative cover and in-
creased exposure of the underlying soil to erosion. Following a heavy or
even moderate rainfall, the soil is eroded and the sediment laden runoff
eventually finds its way to a natural stream or lake. This type of pollution
is becoming of considerable concern, and efforts are being made to reduce its
impact. Two alternatives appear to be possible; reduce the amount of erosion
occuring by limited or localized stripping of the vegatative cover and pro-
viding protection for the exposed soil, or collect and subsequently process
the runoff to substantially remove the sediment before discharge to the
natural bodies of water. The St. Anthony Falls Hydraulic Laboratory has been
engaged in a study to investigate various methods by which sediment can be
removed from runoff.
The first part of the study consisted of a literature survey to deter-
mine the applicability of available solid separation devices or methods to
this particular problem. The results have been previously reported (1).
Of particular interest were devices or methods that were inexpensive, con-
sumed little or no energy for operation, could be left unattended for long
periods of time, required little maintenance, could be readily moved from
one site to another, and had the capability of removing high percentages of
inorganic particles in sizes of less than 100 y. . On the basis of the above
requirements, two devices were selected for further investigation. A commer-
cially available unit was an inclined tube settler manufactured by Neptune
Microfloc, Inc. which has been used in some sewage treatment plants for
effluent clarification. Another commercially available device having success
in a somewhat parallel application (combined sewer overflow and sanitary
sewage treatment) was a disc strainer, and a small prototype unit was pro-
vided by the manufacturer, HYCOR Corporation, for evaluation.
The following report describes the results of the tests with the above
mentioned units.
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SECTION 2
CONCLUSIONS
Two commercially available solid separation devices have been tested in
the laboratory to evaluate their capability of removing inorganic solids with
sizes less than 100 y. from water. Results of these tests indicated that if
an inclined tube settler were installed in a sedimentation basin, it would
reduce the sensitivity of the efficiency of solid removal to overflow rate.
The tube settler increased the performance of the basin by about 20 percent
at the highest overflow rate tested for a total combined removal efficiency
of 60 percent. Addition of alum to the influent increased the solids removal
by about 6 percent, primarily due to the increase in flocculation.
Tests with a small Discostrainer equipped with screens of 45 fi openings
indicated that the unit was extremely sensitive to the solids concentration
of the influent. Thirty percent removal of solids was attained at influent
concentrations of 1300 mg/1 and decreased to less than 10 percent -at concen-
trations of 2100 mg/1. The limited tests have shown that the efficiency may
possibly be improved by decreasing the flow rate through the unit or by de-
creasing the influent solids concentration.
It was found that the solid particles of interest were extremely diffi-
cult to separate from water utilizing physical unit processes. The results
with the two devices tested were somewhat _discouraging, and their use at a
construction site to remove solid particles in the clay and colloidal size
range which generally constitute substantial portions of the sediment fraction
found in construction site runoff may not be economically justifiable.
A conceptual design was made of a debris basin fitted with inclined tube
settlers for a small hypothetical construction site of about 10 hectares
(25 acres). For a rainfall of about 1.3 cm/hr (0.5 in./hr), the estimated
minimum cost to process the storm runoff from the site and remove up to 60 per-
cent of the solids was about $5/lpm ($18/gpm) or $5000/hectare ($2000/acre).
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SECTION 3
RECOMMENDATIONS
The inclined tube settler has shown some capability for removing fine,
inorganic solids from water. As its use does reduce the size of the required
sedimentation basin, the tube settler may be considered for application to
construction sites where large basins are not feasible. The soil conditions
of a particular construction site should be carefully examined, and if large
percentages of clay and colloidal sized particles are not present, the units
may have some benefits. The desired quality of the effluent must also be
considered.
Although the performance of the Discostrainer was marginal in the limited
tests conducted with a slurry of extremely fine, inorganic solids, further
investigations should be carried out. The trend of the test data indicated
that an improvement in solids removal efficiency may be possible at low
inflow rates with high solids concentration, or high flow rates with low
solids concentration. Additional tests should be initiated to verify this
possibility. Also, it is suggested that tests be conducted in an attempt to
optimize performance with organic or fibrous additives in the slurry, and to
examine other screen configurations.
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Section 4
THE TEST MATERIAL
SOIL SELECTION
The first objective of this study was to locate sediment material which
could be used to evaluate sediment removal techniques. As erodible mater-
ials at a construction site may vary over a wide size range, it was decided
to conduct the tests using extremely small particles which are known to be
the most difficult to remove from water. The following criteria for the
test material were established:
1. It should be typical of a wide range of possible applications
and locations.
2. It should have a size distribution which covers the range of silt
and clay sizes, ie, 1.0 to 100H .
3. It should be primarily inorganic.
4. It should be readily available in quantities large enough to test
prototype scale devices.
5. The properties should be sufficiently well known so that the
various removal techniques can be properly analyzed. The proper-
ties must be constant so test results are reproducible.
6. The material should be ready-to-use to avoid costly material
preparation procedures.
After an extensive search and with the cooperation of soil authorities
in the region, a local material was found which had most of the desirable
characteristics. Consideration initially was given to using several standard
materials having a wide range of physical and electrochemical properties, one
of which would be typical of the sediment materials at any given construction
site. However, the magnitude of effort required to establish and prepare
these standard materials was considered far beyond the scope of this study.
In addition, because of the wide variation in properties of different soil
types and the strong influence of water quality on the characteristics of
sediment suspensions, it was felt that no standard material used in a labora-
tory environment could simulate field conditions sufficiently well to reliably
permit transferring the data to any specific construction site. The material
-------
chosen for this study was readily available locally, had the desirable par-
ticle size characteristics, and was usable without any special processing.
A size distribution analysis of the selected material is shown in Figure 1.
SLURRY PREPARATION
In order to determine the effect of dilution water on the stability of
the sediment suspensions, six hydrometer size-distribution analyses were
carried out on the selected sediment material using untreated distilled water,
city water, and river water. Upon comparison of the results of these exper-
iments with the original size distribution, it was found that the fine sedi-
ment particles had a very definite tendency to flocculate and settle
rapidly out of solution. Thus, the size distribution of the sediment in the
settling tank could not be predicted. The distribution would depend on the
amount of mixing, water temperature, dilution water quality, and other para-
meters, many of which could not be controlled. A series of experiments were
carried out to determine whether a reasonably small dosage of some chemical
could be added to the dilution water which would prevent this unpredictable
flocculation from occurring in the settling tank. An attempt was made to
stabilize the suspension by raising the pH by adding sodium hydroxide; there
was a partial stabilization but the results were not satisfactory. It was
known that sodium hexametaphosphate added to the suspension would keep the
particles dispersed if added in sufficient quantities. A series of tests
were run to find the minimum dosage of sodium hexametaphosphate which would
be required to insure complete dispersion of the particles. The test results
indicated that, in order to be confident that no flocculation was occurring,
it would be necessary to add several hundred pounds for each run. This was
not practical because of the cost and also such high concentrations of
chemicals added to the water may alter the physical properties of the diluted
water. In order to insure that the sediment initially was completely and
uniformly dispersed, sodium hexametaphosphate buffered with sodium carbonate
was used as a dispersing agent in the concentrated slurry tank. When the
concentrated slurry was diluted to the test conditions with river water, the
effect of the dispersing agent was eliminated and the material flocculated as
before. Therefore, the size distribution in the settling tank was not pre-
dictable.
As a result of the above preliminary investigations, the following
standard procedure was adopted 'for the preparation of the sediment slurry:
1. The soil was tested for moisture content and a quantity of soil
required to achieve the desired sediment concentration was
added to a slurry tank.
2. The dispersing agent, sodium hexametaphosphate buffered with
sodium carbonate, was added and city water was introduced to
provide the desired concentration of 100 g/1.
3. The slurry mixture was mixed briefly and allowed to stand
approximately 12 hours.
-------
100
I I TT
1 I IT
T I I
80
£ 60
5
S
u
20
I
I III
.001
.01
Figure 1.
.1
1
Particle Size, nun
Size Distribution of Test Sediment.
10
-------
4. The slurry was then mixed thoroughly for two hours before
the test began.
5. The concentrated 100 g/1 slurry was injected into the river water
influent line to achieve the desired influent concentration by
turbulent mixing. Contact time before entering the settling
tank was less than 10 sees.
This procedure yielded a sediment suspension which began to flocculate slowly
at an unknown rate as soon as the dilution began. Therefore, the settling
properties of the particles in the influent were not known and the analysis
of the test results were more difficult. However, it was found early in the
test program that the moderate amount of flocculation that did occur did not
seriously affect the test results because approximately 95 percent of the
flocculated material still passed through the settling tank.
-------
SECTION 5
THE INCLINED TUBE SETTLER
GENERAL CONSIDERATIONS
In a conventional settling basin, separation of solid particles from the
water occurs naturally by gravity. The time required for such separation is
dependent on the particle fall velocity and the distance that it must fall to
reach the bottom. For very small particles with very low fall velocities,
the settling time is extremely long. As an example, it requires about 230
days for clay particles of O.I/* diameter to settle 0.3m (1 ft) in water of
10 C (50 F). Several schemes have been employed to reduce the detention
time, and generally involve a reduction in fall distance. Typical of these
schemes involves the passage of flow through stacked plates. A more practi-
cal scheme has been to pass the flow through inclined tubes; the slope of
the tube contributes to self cleaning of the settled material on the bottom
of the tube. The inclined tube concept has been utilized in clarification
of effluent from waste treatment plants with some success (2). It has been
found that the tube settlers also permit a higher flow rate through the basin
while maintaining good efficiency of particle removal.
Several commercial units of inclined tube settler are available. A
typical module, as manufactured by Neptune Microfloc, Inc., is shown in
Figure 2. This module is constructed of a lightweight plastic and consists
of passageways 5 x 5 cm (2 x 2 in.) in cross section with a length of 61 cm
(24 in.) inclined at an angle of 60° to the horizontal. In use, the module
is submerged to a shallow depth below the water surface. The particle laden
water enters the module from the bottom, passes upward through the tubes, and
exits at the top. Average flow velocity through the tubes is very low, less
than 3 mm/sec. (0.01 fps), and the larger particles settle to the tube invert,
where they eventually drop out of the tube to the bottom of the settling tank.
The deposited material may be periodically collected and removed.
For application to removing sediment from runoff of construction sites,
it was envisioned that a number of tube modules would be installed at the
downstream end of a settling basin. The larger solid particles would be
settled in the basin itself, and a large percentage of the finer particles
would be removed by the inclined tube modules. It should be noted that the
tubes would still require a settling basin, although the surface area of the
basin may be reduced to about 1/3 the area required without the tubes.
-------
Flow
Figure 2. Typical Module for an Inclined-Tube High Rate Settler
from (2).
-------
The success of such a scheme is dependent on the capability of the tube
settler in removing fine, inorganic solids. As previously noted, good per-
formance has been observed in removing particles which may have been largely
organic. Some carefully controlled laboratory tests have been made to
evaluate the variables which influence the performance of a single inclined
tube in removing turbidity in water (3). Attractive percentages of turbidity
removal were reported. The problem addressed in the current study was the
evaluation of a full-scale inclined tube settler module exposed to essential-
ly inorganic solids of less than 100 ^ in size in an environment similar to
that expected in the field.
FACILITY AND PROCEDURES
The tube settler evaluation test facility is shown schematically in
Figure 3. The sediment material was first measured and mixed in the slurry
tank by the procedure outlined in Section 4 above. The slurry was mixed with
a propeller mixer powered by a variable speed drive. The mixing speed was
set at the maximum of about 60 rpm, which could be tolerated without spilling
the slurry mixture in order to minimize the sorting of particle sizes in the
tank. The concentrated slurry was pumped from the tank with a lOOgpm centri-
fugal pump and discharged into the clear water line leading to the settling
tank. The clear or dilution water was taken from a supply line fed by
gravity from the Mississippi River. Both the water supply line and the
slurry feed line had orifice meters and control valves installed to regulate
the discharge and sediment concentration entering the sedimentation tank.
The settling tank was designed to be a simplified version of what might
be used in the field. Construction details of the experimental set-up are
shown in Figure 4. It was basically a rectangular tank 1.83 m (6 ft) deep,
0.76 m (2.5 ft) wide, and 6.1 (20 ft) long". One side of the tank consisted
of a transparent glass wall to allow for visual observation of particulate
settling characteristics. The mixed slurry and river dilution water in-
fluent entered an inlet chamber at one end of the tank. A slotted wall
separated the inlet chamber from the rest of the tank to distribute and
unify the flow. The tube settler selected for the study was manufactured by
Neptune Microfloc, Inc. and was supported at the downstream end of the tank
near the water surface. A divider wall was installed in the top half of the
tank at the upstream end of the tube settler to prevent short-circuiting of
the flow in the tank. The water level in the tank was controlled by an
adjustable weir located at the downstream end.
The test system was set up and the flow calibrations were carried out.
A set of operating curves were computed relating the slurry concentration,
slurry flow rate, and clear water flow rate to the total influent flow rate
and sediment concentration. The particle size and sediment concentration
distribution in the slurry tank were checked to verify that there was no
stratification.
The following standard test procedure was followed:
10
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Variable
Speed ~\
Mixer
cb
t Water from
Mississippi
River
Slurry
Tank
Flow Metering
Orifices
Inclined Tube Settler
(Neptune Microfloc, Inc)
Slurry
Pump
Sedimentation
Tank
Effluent
Figure 3. Facility Schematic for Inclined-Tube Settler Evaluation.
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"Tube Settler Assembly
T
0.76 m
(2.5')
© i
i
1.83 m
(6.0')
Influent
Line
0.76 m
(2.5')
Slotted
Wall
^3,
JL
Water
Surface
Cut-off Wall
/- uut-orr
.3 m
A
Sta. A
PTVr
/ » O.SS.m
Sta. B1
.3 m
(I1)
1m
(3.3')
3.05 m (10')
6.10 m (20')
Figure 4. Sedimentation Tank Details.
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1. Soil, water, and dispersing agent were added to the slurry tank,
mixed briefly and allowed to stand overnight. The slurry tank
was then mixed thoroughly for several hours before beginning
the test.
2. The river water supply channel was flushed for a period of time
to obtain uniform influent water temperatures.
3. Water and slurry flow rates were set at predetermined values
using the operating curves to establish the desired influent
concentration and overflow rate.
4. The settling tank was filled and allowed to stabilize at the set
operating conditions for the particular test run to be accomplished.
5. Samples were taken of the slurry and effluent at 10 minute intervals
during the run. The temperature in the settling tank was monitored
continuously during each run.
In addition to the inflow and outflow data, the temperature and sediment
concentration distributions in the settling tank were measured during two
runs.
The samples taken during the tests were evaluated using the following
procedure:
1. Slurry samples
a. Determine total volume of the sample.
b. Pour sample through a #325 sieve (44 \t openings) and measure
the quantity retained on the sieve.
c. Determine the concentration of material passing the #325 sieve
by evaporating 20 ml of the sample and weighing the residue.
(Corrections were made for the dissolved solids present in
the water.)
d. Determine suspended solids concentration from steps a through
c.
2. Effluent and settling tank samples
The same procedure was used except that there was no need for
passing the sample through the #325 sieve.
Initially the turbidity of the effluent and settling tank samples was
measured. However, the low removal effectiveness on suspended matter had
little effect on the turbidity and the turbidity measurements were abandoned.
The efficiency of removal was calculated as follows:
13
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~ c ff
% Removal = 100 | -- —
Cin
where C. and C ff are the concentration by weight of the influent and
effluent, respectively. The effluent concentration used was an average of
all effluent concentration measurements made.
For each test the design concentration of the influent was 2000 mg/1.
The actual concentration was determined by three alternate methods:
1. Samples were taken of the slurry at the beginning and end of the
test to determine an average slurry concentration. The concentra-
tion was calculated based on this average slurry concentration and
the known flow rates of slurry dilution water.
2. An average concentration of material passing the #325 sieve in
the slurry was determined from the two slurry samples. Calcula-
tions of influent concentration were than based on this concentra-
tion.
3. All the material added to the slurry tank was assumed to be in
suspension thus producing a slurry of known concentration. The
influent concentration was then calculated based on this concen-
tration.
Of the three methods above, the first was found to give the most con-
sistent results. All of the subsequent analysis and discussion will refer
to removal efficiencies determined using the slurry concentration as deter-
mined in alternate one.
The first tests were conducted without the tube settler in place to
determine the performance of the settling tank. The tube settler bundle was
installed and the tests were repeated to check the effect of the tube set-
tler. The last two runs were made with and without the tube settler in
place and with the addition of alum to the slurry tank to evaluate the effect
of coagulants on the removal efficiency. No dispersing agents were added to
the slurry tank during the test with the coagulant. Comparisons were also
made of the distribution of sediment concentration in the settling tank with
and without the coagulant added.
EXPERIMENTAL RESULTS
The results of the tests with the inclined tube settler are summarized
in Table 1. The overflow rate has been calculated based on the total dis-
charge through the system divided by the top surface area of the settler
tube's. As previously noted, the target influent concentration was 2000 mg/1
for all tests, and the values listed in the Table were the actual measured
values. Examination of the removal efficiencies attained indicates considerable
14
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TABLE 1. SUMMARY OF TEST RESULTS INCLINED TUBE SETTLER
Run
No.
2
3
4
5
6
7
8
9
10
11
12
13
14
15(T)
16(T)
17 (T)
18 (T)
19 (T)
20 (T)
21 (T)
22 (T)
23 (T)
24 (T)
26 (T)
Alum*
27
Alum*
Overflow Rate,,
Ipm/ni (gpm/ft )
81.4
81.4
81.4
81.4
122.1
122.1
122.1
162.8
162.8
203.5
40.7
40.7
81.4
81.4
81.4
122.1
122.1
40.7
40.7
162.8
162.8
203.5
203.5
81.4
81.4
Note: (T)
*
(2)
(2)
(2)
(2)
(3)
(3)
(3)
(4)
(4)
(5)
(1)
(1)
(2)
(2)
(2)
(3)
(3)
(1)
(1)
(4)
(4)
(5)
(5)
(2)
(2)
Water Temp.
°C (°F)
4
3
3
-
2
4
8
5
3
3
4
4
7
6
6
8
15
16
16
18
17
16
17
18
18
(39)
(38)
(38)
-
(35)
(40)
(47)
(41)
(38)
(38)
(40)
(40)
(45)
(43)
(43)
(47)
(59)
(61)
(61)
(64)
(63)
(61)
1 (63)
(65)
(65)
indicates inclined tube
Concentration 500 mg/1
Influent Effluent
Cone., mg/1 Cone., mg/1
2040
1944
2068
1862
1728
1891
1791
1880
1685
1754
1566
2549
1982
1808
1580
2002
2214
2230
2722
2068
1921
-
1930
1892
2017
settler
603
830
655
723
581
803
932
947
798
1031
946
1141
924
825
734
827
746
811
822
965
835
792
786
557
731
installed
Percent
Removal
70.4
57.3
68.3
61.2
66.4
57.5
48.0
49.6
52.6
41.2
39.6
55.2
53.4
54.4
53.5
58.7
66.3
63.6
69.8
53.3
56.5
-
59.3
70.6
63.8
15
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scatter in the data, which may be expected in tests of an installation of
this type. To show trends more clearly, the data have been plotted in
Figure 5 using an average value of the removal efficiency for a given over-
flow rate. The extreme ranges of data have also been shown for each point.
Several trends are to be noted. Without the tubes installed, approxi-
mately 50 percent of the solids have been settled in the tank. The decrease
in removal efficiency at the lowest overflow rate was probably caused by
"short circuiting" within the settling tank; this phenomenon was reduced con-
siderably by the addition of the settler. The efficiency also drops off
with increasing overflow rate. After the tubes were installed, the removal
efficiency was slightly higher in general, and efficiency was less dependent
on the overflow rate. At the highest overflow rate, the removal was increased
by nearly 20 percent by the presence of the inclined tubes. The addition of
500 mg/1 of alum to the influent with a 15 minute detention time increased
the efficiency of removal by about 6 percent, both with and without the
tube settler.
By referring to the sediment size distribution in Figure 1 ( p. 6)
it can be seen that the removal efficiencies of 60 to 70 percent achieved
with the tube settler imply that fines with sizes into the clay range were
removed both with and without the addition of coagulants. This indicates
that a considerable amount of natural flocculation was occurring in the in-
fluent as discussed in Section 4.
The effect of the tube settler on the vertical distribution of sediment
concentration at the longitudinal centerline of the settling tank is shown
graphically in Figure 6. Locations of the two measurement stations are
shown in Figure 4. The sediment concentration is plotted versus distance
from the water surface for runs made with alum added to the slurry tank and
with and without the tube settler in place. It can be seen that the use of
the tube settler increases the sediment concentration in the settling tank
while decreasing the concentration in the influent. The effect of the
higher removal rate within the tube settler can be clearly seen.
The temperature in the settling tank was carefully surveyed with a
thermistor temperature probe to determine whether there was any thermal
stratification in the settling tank which could significantly influence
particulate settling characteristics. However, the temperature was constant
throughout the tank and no stratification was observed.
16
-------
100
80
>
o
^ 60
(3
-------
oo
co
n
0)
cu
o
nt
Vi
M-l
0)
-------
SECTION 6
THE DISCOSTRAINER
GENERAL CONSIDERATIONS
The Discostrainer had been observed to perform very well in the treat-
ment of combined sewer overflow and sanitary flows, especially where there
was a high percentage of fibrous material present in the waste stream.
Based on these observations it was felt that the Discostrainer might have
potential for the treatment of construction site runoff. A test unit was
obtained from the manufacturer (HYCOR Corporation) and a series of tests
were run using the same material as discussed in Section 4.
The..D_iscostrainer is a disc straining device which, because of its de-
sign, is able to separate solids by a combination of straining, sedimentation,
and filtration. The general construction and operating details of the Disco-
strainer (4) are shown in Figure 7. The device consists of a series of
stainless steel mesh-covered discs in a specially designed chamber. Each
pair of discs forms a separation unit. The influent is routed into the
cavity between pairs of discs and flows outward through the wire mesh leaving
the captured solids behind. These trapped solids form a precoat material
which may aid the filtration effect. A spray system is used to flush the
trapped solids from the mesh back into the cavity between the discs. The
spray system can utilize the filtered water with a built-in recirculating
pump or an external water supply may be used. As the solids concentration
between the discs increases, the slurry in the downstream end of the cavity
becomes sufficiently thick so that the solids can be swept up by the rotating
discs and out through the solids discharge opening, or adjustable weir
window.
i
The Discostrainer is a compact and uncomplicated device which is capable
of handling a fairly wide range of discharges and influent solids concentra-
tions. It does not require a full-time operator and the power and head re-
quirements are quite low. It was felt that if the performance with inorganic
solids could be shown to be satisfactory, then it would be well suited for
use at construction sites.
The Discostrainer Model DS-110, evaluated in these tests, was a small
prototype unit with one pair of discs. It was equipped with a recirculating
spray system and stainless steel wire mesh with 45 M openings. The r^ted
capacity of the test unit was 568 1pm (150 gpm) and flux of 379 lpm/m
19
-------
SIDE VIEW
DISC ROTATION
SCREEN MESH SUPPORT STRUCTURE
FRONT VIEW
DISCS
INFLUENT PRECOAT
FORMATION
TOP VIEW DISCS
EFFLUENT
DISC SHAFT
SUSPENDED
SOLIDS
PRECOAT
SCREEN MESH
SUPPORT STRUCTURE
FLEXIBLE DISC SEAL
DISC RIM
FLEXIBLE SEAL
Figure 7. Construction and Operating Features of
Discostrainer from (4).
20
-------
2
(9.3 gpm/ft ) in raw sanitary sewage with suspended solids of 300 mg/1 or
less when equipped with a wire mesh with 200 H openings. The total open area
of both the mesh used for these tests and the rated mesh was approximately 30
percent, so any variation in rated discharge should be primarily due to the
differences in the concentration and physical properties of the suspended
solids.
FACILITY AND PROCEDURES
The Discostrainer was evaluated in the test facility shown in Figures
8 and 9. The test facility was similar to the one used previously for the
tube settler except for modifications in the slurry mixing and feed system.
As can be seen in the schematic drawing (Figure 8), a high speed recircula-
ting jet was used to mix the concentrated slurry rather than the propeller
used in the previous tests. A variable-speed peristaltic pump was used to
meter the concentrated slurry into the mixing tube of the influent line.
The Discostrainer was operated with inflow rates of 189, 379, 473, and
568 1pm (50, 100, 125, 150 gpra), fluxes of 126, 253, 315, 379 Ipm/m (3.1,
6.2, 7.8, 9.3 gpm/ft ),'and inflow solids concentrations of 850 to 2270 mg/1.
The headloss through the screens was determined by measuring the difference
in elevation of the water level in the inlet chamber and the bottom edge of
the discs. The influent and effluent samples were taken at the inlet chamber
and effluent pipe, respectively. The samples were analyzed for solids concen-
tration using the same procedure as for the tube settler. Flow was estab-
lished in the Discostrainer by first setting the primary flow of clear water,
then starting the disc drive and sprayer pump. The slurry feed system was
then turned on to inject the concentrated sediment slurry into the influent
line.
EXPERIMENTAL RESULTS
Representatives of the local distributer of the unit, Northwestern
Power Equipment Company, were notified and observed the initial tests. The
initial runs with the Discostrainer were entirely unsuccessful. Immediately
after the slurry flow was started the mesh plugged or blinded, and the water
level in the inlet chamber increased until the water discharged through the
overflow outlet. The blinding continued as long as the flow was maintained,
even after the slurry feed line was turned off. The sprayer system cleared
the mesh as the discs rotated past them but the mesh blinded again as soon
as it re-entered the flow. There was sufficient material trapped between
the discs to cause the mesh to blind even when no additional slurry was
added. This rapid blinding was partially caused by a deficiency in the
slurry feed system which caused a surge of high sediment concentration to
enter the Discostrainer when the slurry flow was first turned on. The
slurry feed system was subsequently modified so the initial solids concentra-
tion could be controlled and so long as the inflow concentration was care-
fully monitored, the screens could be prevented from blinding. However, the
persistence of this problem illustrates the sensitivity of the square-mesh
screen to plugging by granular particles.
21
-------
to
N>
Dilution Water
i from
Mississippi River
Variable Speed
Peristaltic Slurry
Feed Pump
r
*
Slurry Feed Line
r-
/
Discostrainer
Influent -/
Line
O V
i v
/Q
f
Solids
Discharge
Screened
Effluent
Slurry
Tank
Slurry Mixing Pump
isO\J
^n
Drain
Line
Figure 8. Facility Schematic for Discostrainer Evaluation.
-------
A. Slurry Tank F.
B. Slurry Feed Pump G.
C. Slurry Mixing Pump H.
D. Dilution Water from River I.
E. Inflow Metering Orifice J.
Influent Line
Discostrainer
Effluent Line
Solids Discharge
Drain
Figure 9. Photo of Discostrainer Set-up.
23
-------
A tabulation of the essential test data shown in Table 2 gives the
average values of several measurements taken during each test after the
slurry feed system was modified. The run time for each test was typically
about 45 to 60 minutes during which time 6 to 8 samples of the inflow and
outflow were taken. The length of the runs was limited by the capacity of
the slurry tank.
TABLE 2. SUMMARY OF RESULTS FOR DISCOSTRAINER
Run
1
2
3
4
5
Flow Rate
1pm (gpm)
189
379
473
379
379
379
189
379
(50)
(100)
(125)
(100)
(100)
(100)
(50)
(100)
Flux
2 2
Ipm/m (gpm/ft )
126
253
315
253
253
253
126
253
(3
(6
(7
(6
(6
(6
(3
(6
.1)
• 2)
.8)
.2)
.2)
.2)
• 1)
.2)
5in
mg/1
1350
1570
1345
2100
1620
1630
1690
1300
Removal Head
Rate Loss
% cm (in.)
8.9
22.2
3.7
5.7
6.1
10.6
33.1
27.5
15
26
30
23
20
26
16
27
.5
.2
.7
.6
.3
.2
.5
.2
(6.1)
(10.3)
(12.1)
(9.3)
(8.0)
(10.3)
(6.5)
(10.7)
Spray
Source
Recirc.
Recirc.
Recirc.
Recirc.
Recirc.
City water
City water
City water
The removal rate was determined from the difference in suspended solids con-
centration between the inlet chamber and the effluent pipe. By referring to
the sediment size distribution shown in Figure 1 (p. 6) and assuming that
the particles suspended in the inlet box are primarily less than 100 H ,
the 45 \i mesh openings could be expected to remove about 12 percent of the
material. The minimum removal rate actually varied from about 6-10 percent.
This discrepancy may be due to the existence of a greater amount of settling
in the inlet chamber than assumed above. It is also possible that some of
the coarser sediment was forced through the mesh by the high pressure back-
flush spray. Visual observations indicated that the spray penetrated the
mesh and blasted across the cavity and impinged on the opposite side disc
with considerable force. There were not enough data available to determine
the exact cause of the discrepancy. The seals between the discs and the
housing were tight and it is very doubtful that they leaked significantly.
To remove the influence of the solids in the recirculated water spray on the
results, some brief tests were made using a city water spray source. Run
numbers 4 and 5, made with a low pressure clear water backflush using city
water, showed some tendency toward higher removal rates, probably because
the lower intensity spray jets did not force the sediment material through
the opposite side mesh.
24
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A plot of percent removal as a function of inflow solids concentration
for a given inflow rate is shown in Figure lOa. As the concentration of the
inflow increases, the percent removal decreases. In Figure lOb, data have
been plotted to show the influence of the rate of solids loading on the
screens, which is represented by the product of the inflow rate and concen-
tration. With the exception of one point, a general trend is established
showing that the removal efficiency decreases with an increase of solid load-
ing. Thus, the possibility is suggested that the higher efficiency may be
experienced with very low inflow rates of high solids concentration, or
higher flow rates with very low solids concentration. Project budgeting did
not permit an experimental verification of these conditions.
The rated discharge for this mesh and sediment material is considerably
lower than the published value of 568 1pm (150 gpm) with 70 mesh 200 M
screen. The maximum removal rates achieved were about 30 percent. This in
itself is not very impressive, however, the 30 percent removal rate implies
that the Discostrainer was removing material which was considerably smaller
(approximately 50 percent) than the openings in the mesh. This indicates
that some of the trapped solids were forming a pre-coat on the mesh. The
higher removal rates did not occur consistently or predictably and the re-
sults could not be repeated very well from one run to the next.
The relationship between headless and flow rate is shown in Figure 11.
The headless is the difference in elevation between the inlet chamber water
surface and the bottom edge of the discs and is a measure of the blockage in
the screens. The Discostrainer was operated with plain river water with a
very small percentage of suspended solids (mostly in the form of organic
matter and algae) at flow rates of up to about 946 1pm (250 gpm) and flux
631 Ipm/m (15.5 gpm/ft ) with not more than a few centimeters of headloss.
This was true even after long runs when the sludge was beginning to noticeably
thicken in the cavity between the discs. However, when the test sediment
was added to the river water, the headloss was very strongly dependent on
discharge and the Discostrainer could not be operated successfully a| flow
rates greater than 473 1pm (125 gpm) and flux 315 Ipm/m (7.8 gpm/ft ) with-
out causing blinding. There was no apparent relationship between the head-
loss and removal efficiency.
Several attempts were made to optimize the flow rate and the disc rota-
tional speed. The performance was too inconsistent to give any definite
results.
The solids discharge action was never observed due to the relatively
short run times and the low removal rates. However, before using this system
for the removal of purely inorganic waste, this feature would have to be
checked very carefully. If the trapped solids were primarily non-cohesive,
the thickened sludge would not be discharged as well as the organic and
cohesive material for which this unit was designed.
The Discostrainer is not very well suited for the removal of inorganic
fine grained sediments such as may normally be found in construction site
25
-------
60
I
T
> 40
§
O Recirculating Water Spray
^ Clear Water Spray
Q = 379 1pm (100 ppm)
g
o
!-i
0)
20
1200
60
to
> 40
0
0)
2°
I
1400
1600
1800
2000
2200
Influent Concentration C. , mg/1
f N in
(a)
2400
200 300 400 500 600 700
Q cin, g/1
(b)
Figure 10. Solids Removal for Discostrainer.
800
26
-------
40
30
o>
4-1
£
CO
CO
o
20
10
T
T
O Recirculating Water Spray
• Clear Water Spray
I
100
200
300 400
Flow Rate, 1pm
500
600
Figure 11. Head Loss Across Discostrainer Screens .
27
-------
runoff. The unit could probably be made to work fairly well over a very nar-
row range of inflow conditions, however, the tendency to "blind" when over-
loaded slightly by either high concentrations or high flow rates limits its
applicability for this purpose. The indications are that the performance
with organic or fibrous material is very good and where a high percentage of
fibrous material would be expected, the unit works well. The organic
material would form a pre-coat on the mesh and prevent blinding. No tests
were run in this program to try to optimize performance with organic or
fibrous additives to the slurry. A possible improvement might be to equip
the discs with mesh that has some other shape of opening, either slotted
holes or a non-woven fiber fabric. Such tests were not within the scope of
this investigation.
28
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SECTION 7
REQUIREMENTS FOR A HYPOTHETICAL CONSTRUCTION SITE
Each particular construction site has its own unique characteristics,
such as topography, underlying soil conditions, hydrologic exposure, etc.
Many of these items have been discussed in (1). It is therefore difficult
to arrive at a typical design for a solids removal scheme which can be
applied to a wide variety of situations. However, in the following a hypo-
thetical case has been assumed to illustrate some of the features of an in-
clined tube settler used, in conjunction with a debris basin. The case to be
considered is a small construction site with an area of 25 acres, which may
be typical for an industrial or commercial development area. The site will
be exposed to a rainstorm with an intensity of 0.5 in/hr.
Some general features of a possible runoff collection and treatment
scheme are shown in Figure 12. The basin would be located at the downstream
end of the construction site, and the excavation required is dependent on the
local topography. The minimum depth recommended for the basin is 8 ft when
tube settlers are used, and as discussed in (1), the basin should have a
length to width ratio of about 4. Baffles should be placed in the vicinity
of the inlet to the basin to reduce turbulence of the incoming stream. These
baffles may consist perhaps of material such as snow fencing. To further
reduce the effect of turbulence of the flow on the performance of the tube
settlers, the tube settlers are placed near the downstream end of the basin.
Following the general guidelines of the manufacturer, it is recommended that
1/4 to 1/3 of the basin area not be covered with the tube settlers. The tube
modules are supported on a light framework, and the top of the modules are
submerged 2 ft below the water surface. A solid barrier is shown at the up-
stream end of the modules which insures that all of the flow enters the tubes
from the bottom of the module.1 After the flow passes through the tube modules,
it is collected by a series of perforated pipes or passes over a weir and
discharged. If chemicals are to be added to increase flocculation, they
would be introduced upstream of the baffles. The scheme shown in Figure 12
will be designed to continually process the runoff from the storm. Following
the storm event, flow through the tube modules will cease, and the runoff will
be retained in the basin and particles allowed to settle by gravity. The
cleared water then can be gradually released through a small pipe drain, and
during dry periods the settled solids can be removed for proper disposal.
The material may require further dewatering after removal from the basin.
The runoff from the site may be approximated from the rational formula
(1)
29
-------
Flow Baffles
Inflow
Channel
Area Covered with
Tube Modules , ,- Collection Launders
I
J/ZJ
I
Plan
Chemicals
(if Required) r-Flow Baffles
Isolation Barrier
Inflow
Channel
( r I t f
Tube Module Supports
Section A-A
/ i / /
Effluent
Figure 12. Conceptual Design of a Debris Basin Fitted with
Inclined Tube Settlers.
-------
R = KP
where R = runoff in time span in inches/acre
K = runoff coefficient
P = average precipitation in inches over time span
The runoff coefficient, K, is dependent on the exposed material and
varies over a wide range. Runoff over a deep gravel bed may be essentially
zero, whereas impervious soils such as clay may yield nearly complete runoff.
For the present purpose, K will be assumed to be 0.5. Thus,-for the given
storm, the amount of runoff is 43560 x 0.5 x 0.5/12 = 908 ft /hr/acre of
watershed, which reduces to 0.25 cfs/acre or 113 gpm/acre. It is readily
apparent that the drainage and collection layout be such that handling of
water not associated with the site be minimized. In this example, only the
runoff from the 25 acre site will be considered.
The experimental data obtained in the Laboratory study indicated that
the percentage removal of fine particles did not change significantly with
overflow rate up to the maximum plan area tested of 5 gpm/ft . A slightly
more conservative value of 3 gpm/ft will be used as the design overflow
rate. The required plan area of the tube modules is therefore 113 x 25/3 =
942 sq. ft. As the standard tube modules are 10 ft long and 2.5 ft wide, an
arrangement of 8 units wide and 5 units long provides an area coverage of
1000 sq. ft. The total surface area of the basin with a length to width
ratio of 4 is 1600 sq. ft so that about 0.6 of the surface area is covered
with tubes.
The tube module support system should be designed for a surface loading
of 7.5 psf, with support members at each end of the module. As the installa-
tion is considered necessary only during the period of the project, the
support system could be fabricated using simple timber construction techniques.
Some rough cost figures have been assembled for construction of the
solids removal system. These costs are based on the assumption that about
750 cu. yds of excavation are required for the basin and heavy equipment is
available at the site, a four! man crew can construct the module support
framing in one week, install the modules in two days, and complete the
peripheral items such as baffles, drain lines, and overflow system, in about
five days. The costs are summarized below:
Excavation $ 2,500
Tube Modules 32,000
Support Framing 5,000
Module Installation 1,500
Baffles, drains, outlet 4,000
Contingencies 5,000
$50,000
31
-------
The total estimated minimum cost reduces to about $2000/acre of con-
struction site. The addition of chemicals to the influent would increase the
cost by about $3000 for equipment. The quantity of chemical to be added is
dependent on the characteristics of the suspended solids and the water it-
self. With the concentration of 500 mg/1 of alum used in the Laboratory
tests, the cost of alum at $.10 per pound would be about $72 per hour of
treatmment. If the basin site is to be returned to its initial condition
at the end of the construction program, additional costs would be incurred.
It should be emphasized that the above costs are only approximate, and
may vary quite widely, dependent on local conditions. With this system,
the Laboratory results indicate that about 60 percent of the influent
solids with concentrations of about 2000 mg/1 can possibly be removed down
into the clay size range.
32
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REFERENCES
1. Ripken, J.F., J.M. Killen, and J.S. Gulliver, "Methods for Separation
of Sediment from Storm Water at Construction Sites," EPA-600/2-77-033,
NTIS No. PB 262 782, Cincinnati, Ohio, 1976, 92 pp.
2. Gulp, G.L., K. Hsiung, and W.R. Conley, "Tube Clarification Process,
Operating Experiences," Journal of Sanitary Engineering Division, ASCE,
95(SA5): 829-847, 1969.
3. Yao, K.M., "Design of High Rate Settlers," Journal of Environmental
Engineering Division, ASCE, 99(EE5): 621-637, 1973.
4. HYCOR Corporation, Lake Bluff, 111., Bulletin No. DS1111-877.
33
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-79-076
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
LABORATORY EVALUATION OF METHODS TO SEPARATE FINE
GRAINED SEDIMENT FROM STORM WATER
July 1979 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7, AUTHOR(S)
L.M. Bergstedt, J.M. Wetzel, and J.A. Cardie
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
St. Anthony Falls Hydraulic Laboratory
University of Minnesota
Mississippi River and 3rd Ave. S.E.
Minneapolis, Minnesota 55414
10. PROGRAM ELEMENT NO.
1BC822. SOS #2. Task 18
11. CONTRACT/GRANT NO.
R 803579
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Gin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Richard P. Traver, Staff Engineer, Storm and Combined Sewer Section
8-340-6677 201-321-6677
16. ABSTRACT
A literature survey had been conducted by the St. Anthony Falls Hydraulic Labora
tory to assess various methods for separation of sediment from storm water at con-
struction sites. Two methods have shown some promise in this application, and a
research program was initiated with the objective of evaluating the effectiveness of
the methods in removing fine grained inorganic solids from water.
Experimental facilities were set up to test full-scale units of an inclined tube
settler and a Discostrainer in an environment approximating that in the field. These
units were tested for removal efficiencies of inorganic solids with sizes less than
100 li and influent concentrations of about 2000 mg/1. Measurements were made of the
influent and effluent concentrations for various flow rates through the systems.
Results indicated that the installation of an inclined tube settler improved the
efficiency of a sedimentation tank by about 20 percent at the highest overflow rate
tested of about 200 Ipm/m (7000 gpd/ft ). The inclined tube settler also reduced
the sensitivity of the overflow rate on the efficiency of sediment removal. Limited
tests with alum added to the influent to increase flocculation indicated about 6 per-
cent improvement in removal efficiency.
^The Discostrainer was found to be extremely sensitive to influent solids concen-
tration. Thirty percent solids removal was the maximum attained for the tests con-
ducted. Higher removal perppnt-agpR may nncc-fhl-tT f»o n^t-^-i-^aA T^T rediip-inp i-h^ -F1 OT.T -ra-t-
7°r influent concentration. KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Soil erosion, Stream pollution, Suspen-
ded sediments
Sediment separation
13B
3. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)'
Unclassified
21. NO. OF PAGES
42
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
34
ft U.S. GOVERNMENT PRINTING OFFICE: 1979 -657-146/5467
------- |