United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA-600/S2-84-131 Sept. 1984
Project Summary
Hydraulic Characteristics of
Activated Sludge Secondary
Clarifiers
Jon Bender and Robert M. Crosby
The hydraulic characteristics of several
common types of full-scale activated
sludge secondary clarifiers were evalu-
ated. Attempts were then made to
modify and improve representative
examples. The tanks' characteristics
were inferred by the use of innovative
dye tracer techniques. The effects of
modifications were evaluated on the
basis of effluent quality.
The dominant hydraulic characteristic
of all clarifiers studied was density flow.
In most cases, the density flow had a
significant effect on effluent suspended
solids concentrations. When effluent
weirs were placed in the path of density
flow, effluent quality was generally
poor. Preventing density current forma-
tion by inlet modification was not nearly
as effective as interrupting flows at mid-
radius and near the weirs.
Problems also occurred with balancing
flows between parallel clarifiers. The
cause was improper application of
mixed liquor feed valves, poor splitter
box design, and inadequate flow mea-
surement. In addition, strong evidence
exists that flow transients are not at-
tenuated by upstream unit processes
and may significantly affect the solids
transport through clarifiers.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati. OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Theories of clarification and secondary
clarifier design standards for wastewater
treatment facilities assume that inlet and
outlet structures create certain patterns
of flow or hydraulic characteristics
within the clarifier. The major purpose of
this project was to determine whether
these hydraulic characteristics were
actually created within several types of
full-scale, activated sludge secondary
clarifiers. If the anticipated hydraulic
characteristics were not found, the
clarifier inlets and/or outlets were to be
modified, and the changes in hydraulic
characteristics and improvements in
effluent quality were to be measured.
The dye dispersion test has been used
in the past to measure the hydraulic
characteristics of clarifiers. This test may
indicate whether a clarifier has hydraulic
problems, but is useless for identifying
their causes. Another purpose of this
project was to evaluate a new dye tracer
technique that allows visualization of the
flow within full-size clarifiers.
These goals seemed straightforward at
the beginning of the project, but they had
to be modified after simple relationships
between inlet and outlet structures,
hydraulic characteristics, and effluent
performance were not found. The project
then evolved into a more comprehensive
evaluation of the factors believed to affect
clarifier performance at eight different
activated sludge secondary treatment
facilities.
Types of Clarifiers Studied
Activated sludge wastewater treatment
facilities use many different types of
secondary clarifiers. This study did not
attempt to identify all types or to
document their hydraulic behavior. Of the
eight clarifiers studied, five different
modifications were made on three tanks
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in an attempt to increase solids capture. It
was judged that some clarifiers included
in the study could not benefit from the
kinds of changes that were within the
budgetary scope of this project. Table 1
lists the general types of clarifiers that
were studied to some degree.
Clarifier Analytical Procedures
The dispersion of dye in a clarifier
following the instantaneous release of a
slug has been the most common test used
by clarifier analysis in the past. This study
needed to show the movement of fluid
from the clarifier inlet to the outlet and
the distribution of solids that results from
fluid motion. Since dispersion tests alone
could not accomplish this goal, other test
procedures were developed. Several of
these tests had not previously been used
in clarifier analysis.
Multi-point Dispersion Test
The multi-point dispersion test is a
variation of the usual point-of-discharge
dispersion test. A slug of dye is released
instantaneously upstream of the clarifier,
as before, but the multi-point version
measures dye concentration versus time
at several weir locations. The new test
was intended to reveal whether tank
throughflow assumed some preferred
directions or pathways.
Flow Pattern/Solids
Distribution Test
The flow pattern/solids distribution
test has two final results:
1) A visualization of the tank through-
flow route
2) A measurement of the distribution of
solids resulting from throughflow
The test is conducted by continuously
releasing dye upstream of the test
clarifier, then "instantaneously" sampling
25 or more places in a radial or longitudi-
nal section of the tank several times after
the start of dye injection. After the
samples have been analyzed for dye and
suspended solids, data are contoured 01
plotted as isolines of concentration of
the two parameters.
Flow patterns and the resulting solids
distributions were both found to be useful
tools in deciding on hydraulic modifica-
tions for the clarifiers.
Table 1. Types of Clarifiers Studied
Weir-Wall Solids Test
The weir-wall solids test measures the
suspended solids concentration versus
time near the effluent walls. Originally
intended to confirm a suspected wave of
solids being moved around circular tanks
by the rotating mechanisms, this test was
helpful in identifying time-varying effects
from other causes.
Sludge Dispersion/Sludge Jet
Test
The sludge dispersion and sludge jet
tests use a dye that readily adsorbs onto
the solids floe particles, but is insoluble in
water. The route and timing of solids
throughflow are thereby traced.
The sludge dispersion test consists of
an instantaneous release of an emulsion
of mixed liquor and Oil Red "0" dye and
subsequent sampling in the sludge return
to define the residence time of the sludge
in various parts of the tank.
The sludge jet test allows a visualiza-
tion of the sludge flow route as it seeks
the blanket. This procedure was conduc-
ted only once during the study. An instan-
taneous emulsion of Oil Red "0" and
mixed liquor was released upstream of
the clarifier. After a time defined by the
sludge dispersion test, 30 samples were
taken instantaneously at mid-radius of
the circular tank in a plane parallel to the
tank periphery near the sludge blanket.
Lines of equal concentration of dye and
suspended solids were then plotted. The
two sludge tracer tests helped to confirm
the existence of inlet jets that had been
only minimally dispersed by inlet baffling.
Typical Hydraulic
Characteristics of the Clarifiers
and Modifications
Implemented
The hydraulic characteristics of the
clarifiers in this study were inferred from
the results of all tests conducted. First the
clarifier's baseline hydraulic behavior
was measured at one or more flow rates.
These data were used to determine
whether modifications could be made
within the budget of the project. In the
three plants to which clarifier modifica-
tions were made, subsequent hydraulic
tests were followed by long-term evalua-
Clarifier
Type
Rectangular
Center Feed - Peripheral Overflow
Peripheral Feed - Peripheral Overflow
Total Number
Studied
2
5
1
Total Number
Modified
1
2
0
tion of the effluent quality of the modified
unit compared with that of an unmodified
parallel clarifier receiving the same flow
and mixed liquor.
Rectangular Clarifiers
This study evaluated two different
rectangular secondary clarifiers. One is a
relatively small, shallow unit, and the
other is longer and deeper. Characteris-
tics were very different in the two units.
The small rectangular clarifier was part
of a 4540 mVday (1.2 mgd) activated
sludge facility. The plant had two
secondary clarifiers 19.3 m (65 ft) long
and 4.9 m (16 ft) wide with a 2.7 m (9 ft)
sidewater depth. Mixed liquor flowed
from the aeration basins to an open
channel at the head of the clarifiers
where it splits to the two tanks. Mixed
liquor then enters each secondary
clarifier through four square inlet ports
at the surface. A chain and flight sludge
scraper mechanism transports settled
sludge to a sump at the inlet end of each
clarifier.
Figure 1 shows a flow pattern that was
measured in this clarifier before any
modifications. Flow moves downward
from the inlet to the sludge sump, then
proceeds in a horizontal layer along the
top of a sludge blanket. At the weir end of
the tank, throughflow turns upward. This m
flow pattern entrained flow particles and
carried excessive concentrations to the
weirs.
The clarifier was modified with a
reaction baffle and several other flow-
modifying structures at the inlets. Figure
2 shows the final version of the baffle.
Figure 3 shows that the flow pattern in
the clarifier following all inlet modifica-
tions resulted in very little improvement
in flow pattern. Sludge sump scouring
was reduced, however. This change alone
resulted in a 13.8% reduction in effluent
total suspended solids (TSS).
At this plant, very large flow transients
were induced at the point of secondary
clarifier discharge by raw sewage lift
stations in the city. As a second modifica-
tion, a stop-gate was placed in a channel
upstream of the aeration basins to reduce
transient amplitude. This modification
was tested on a 1-week-on, 1-week-off
basis because it affected the performance
of both clarifiers. As a result, the effluent
quality of the newly baffled clarifier with
the stop-gate in place was 31.5 % lower
in effluent TSS than the unmodified
clarifier without the stop-gate.
The second, larger rectangular clarifier
is 62.8 m (206 ft) long and 22.8 m (75 ft)
wide with a sidewater depth of 3.9 m
(12.9 ft). Four inlet ports direct the mixed
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Reaction Baffle
Effluent Weirs
Inlet Ports
New Raction Baffle
• Sludge Sump
Figure 1. Original flow pattern for small rectangular clarifier. Distribution of the dye
concentration. Note: Vertical scale is exaggerated.
liquor flow downward with the help of a
deflection baffle. Longitudinal effluent
weirs covering about half of the clarifier's
surface area are located at the opposite
end of the basin. A traveling bridge with
air-lift pumps removes the sludge only
during the part of the cycle in which the
bridge is moving from the weir end to the
inlet end of the tank.
Figure 4 shows a flow pattern for this
clarifier. The hydraulic characteristic is
very different from the smaller tank
described earlier. The narrow horizontal
flow layer that existed in the inlet region
changed to a more uniform flow at the
outlet. The flow distribution recommended
by design standards appears to have been
achieved in this clarifier. Effluent TSS
concentrations were typically 10 mg/L or
less.
This study did not determine the exact
reasons for this clarifier's exceptional
hydraulic characteristic. Gas bubbles
observed near the inlet have led to one
theory. The solids distribution tests
Inlet Pon
Figure 2.
New reaction baffles for small
rectangular clarifier.
New Reaction Baffles
Figure 3. Flow pattern after modification of small rectangular clarifier with new reaction
baffles. Distribution of the dye concentration. Note: Vertical scale is exaggerated.
indicated that the traveling bridge piles
sludge near the inlet without completely
removing it. The sludge then turns septic
and produces fine rising bubbles that
appear to help distribute the throughflow
vertically. No modifications were attempted
on this clarifier.
Center-Feed, Peripheral
Overflow, Circular Clarifiers
Most activated sludge facilities in the
United States use center-feed, peripheral
overflow, circular clarifiers. Five of these
tanks were included in the study. Two
modifications were made on one and a
third modification was made on another
one.
Figure 5 shows the flow pattern in a 24-
m-diameter (80 ft), 3.1-m-sidewater
depth (10 ft) clarifier in a pure oxygen
activated sludge plant. A very thin, high-
velocity layer of horizontal throughflow
rebounded off the peripheral wall,
inducing excessive floe particles to
approach the weirs. In addition, a weir-
wall solids test and a multi-point disper-
sion test suggested excessive rotational
speed for the sludge removal mechanism.
The first modification slowed the
sludge riser pipe mechanism to 56% of its
previous speed by means of a sprocket
change. This modification reduced efflu-
ent TSS by 10.5%. After completing this
modification on both secondary clarifiers,
a cylindrical ring baffle/flocculation
chamber was installed at mid-radius of
the test tank (Figure 6). The baffle extends
from mid-depth to just off the bottom and
rotates with the mechanism.
Figure 7 shows the considerable effect
of the baffle on the clarifier's flow pattern.
After an extended test of comparative
performance, efffluent TSS was found to
be 37.5% lower in the newly baffled
secondary clarifier.
A 39.6-m-diameter (130 ft), 40-m-
sidewater depth (13 ft) secondary clarifier
in an air activated sludge facility received
another modification - a baffle placed just
beneath the weirs at the periphery.
Figure 8 shows the baffle. The post-
modification flow pattern appears in
Figure 9. As expected, the new baffle
decreased the tendency for density flow
induced upflow at the periphery.
In a 52-day, side-by-side comparison of
effluent TSS from the modified and
unmodified parallel secondary clarifiers,
the newly baffled tank showed effluent
TSS that was 38.3% lower.
Peripheral-Feed, Peripheral-
Overflow, Circular Clarifier
A peripheral-feed, peripheral-overflow,
circular clarifier was evaluated at 65%,
3
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Effluent Weirs
Deflection Baffle
Inlet Ports
M.
III
-- '"~----?0 '5 10 ?
- ,2S 20»,=.„,_ || I i
-.' ........ '"" i'i >ii
,. . . -i i;
.. ;;_-_;-----<="=Sfc?- j
Figure 4. Flow pattern for the large rectangular clarifier. Distribution of the dye concentration.
Note: Vertical scale is exaggerated.
Inlet Feed Well
Effluent Weirs
—60-
Figure 5. Original flow pattern for a center-feed, peripheral-overflow, circular clarifier.
Distribution of the dye concentration. Note: Vertical scale is exaggerated.
Figure 6. Ring Baffle/Flocculation Chamber modification to a center-feed, peripheral-
overflow, circular clarifier.
100%, and 150% of design overflow rate.
The secondary clarifier, in a pure oxygen
activated sludge process, was one of 12
identical units. The tank is 42.7 m (140 ft)
in diameter with a 4.3-m(14-ft)sidewater
depth.
Figure 10 shows a typical flow pattern
with the clarifier operating at a design
overflow rate of 26.48 m /m2 per day
(650 gal/ft2 per day). The initial flow
pattern was similar to other clarifiers,
with flow moving across the sludge
blanket from the inlet region. In this type
of clarifier, however, the flow converges
at the center rather than meet an obstacle
such as an end or peripheral wall. Later
"snapshots" in the flow pattern time
series confirmed that upflow was gradual
and relatively uniform over the surface
area.
Other Factors Thought To
Affect Clarifier Performance
The investigators considered factors
both within and upstream of secondary
clarifiers—factors thought to affect the
effluent quality of the tanks. Although the
study did not subject their observations to
rigorous proof, facility designers should
at least consider them.
Sludge Removal Mechanism
Except for the small, rectangular
secondary clarifier discussed, all clarifiers
included in this study had so-called
hydraulic sludge removal mechanisms.
All appeared to have some degree of
problem in uniformly removing sludge.
Reasons varied from plugging of individu-
al riser pipes or orifices with sludge or
debris to operator uncertainty about how
to adjust them.
The rotational speed of mechanisms
might be greater than needed to maintain
sludge quality. At one facility, a 10.5%
reduction in effluent TSS resulted from a
reduction to 56% of design speed.
Balancing of Flows Between
Parallel Clarifiers—Inadequate
Flow Control and
Measurement Devices
Most municipal wastewater treatment
plants have several identical clarifiers
operating in parallel. Splitting the
influent equally among the clarifiers
should generally produce the best
effluent, but this project found that
achieving this balance of flows was
usually difficult if not impossible at all
flow rates.
The inability to balance the flows was
caused by inadequate flow control and a
lack of flow measurement devices. Gate
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Ring Baffle/Flocculation Chamber
•29
60-
80-
•100
Figure 7. Flow pattern after modification at a center-feed, peripheral-overflow, circular
clarifier with a Ring Baffle/Flocculation Chamber. Distribution of the dye concen-
tration. Note: Vertical scale is exaggerated.
Figure 8. Peripheral Baffle modification to a center-feed, peripheral-overflow, circular
clarifier.
Inlet Feed Well Peripheral Baffle
• 0 •
" " fffluent
Weirs
~t63
Figure 9. Flow pattern after modification of a center-feed, peripheral-overflow, circular
clarifier with a Peripheral Baffle. Distribution of the dye concentration. Note: Vertical
scale is exaggerated.
valves or slide gates were the only means
of flow control at some plants. These
devices are satisfactory for stopping i\ow,
but they are inappropriate for control
because their head loss characteristics
are a nonlinear function of flow rate.
Therefore, a flow split adjusted for one
flow rate will change with flow. Such
gates and valves also tend to collect
debris when operated partially closed.
Some splitter boxes also complicate the
problem.
The problem of splitting and balancing
flows between parallel clarifiers is
further exacerbated by a lack of flow
measurement devices to confirm a
balanced operation. Typically, underflow
is measured and mixed liquor feed and
effluent flow rates are not. Elaborate flow
measurement devices are not necessary,
but an operator should be able to
determine flow rates without installation
of special equipment.
Flow Variation and Solids
Transport
The general perception is that activated
sludge secondary clarifiers are not
significantly affected by flow variation
because tanks upstream of the second-
aries will equalize the flow. This is true
only in a very limited sense; flow
transients undergo very little attenuation
in a typical plant. A mathematical model
developed for this project indicates that
the amplitude and frequency of flow
variations may greatly increase solids
transport through the clarifier, thus
increasing effluent TSS.
Conclusions and
Recommendations
• None of the clarifiers evaluated in
this project had the hydraulic charac-
teristics called for in design standards
for wastewater treatment plants. All
of the clarifiers had some similar
hydraulic characteristics, character-
ized by a horizontal flow layer, prob-
ably caused by density flow. Perform-
ance suffered only in those clarifiers
where the effluent weirs were placed
in the path of the density currents.
The sludge jet effect may intensify
these density currents and their
effect on performance.
• Baffles at the inlets of rectangular
clarifiers did not prevent the forma-
tion of density currents. Breaking the
density flows up after they had
formed improved clarifier perform-
ance.
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Effluent Weirs
Peripheral Wall
Inlet Ports
Canter
of
Tank
Figure 10. Flow pattern for the peripheral-feed, peripheral-overflow, circular clarifier.
Distribution of the dye concentration Note.' Vertical scale is exaggerated.
• Clarifiers with effective control of
density currents may be capable of
producing acceptable effluents at
higher hydraulic loading rates than
those of conventional design.
• Sludge blanket level affects clarifier
hydraulic chactenstics. A moderate
blanket level just below the bottom of
the inlet baffling may be the worst
condition, producing high levels of
solids-transporting turbulence
• Balancing of flows between parallel
clanfiers is impossible without
proper control devices and some
form of flow measurement device on
each clarifier
• Wastewater treatment plant design-
ers and operators must be aware that
conventional treatment process
tankage does not greatly attenuate
flow transients and that any flow
variation induced in the system could
significantly increase solids trans-
port in the activated sludge secondary
clanfiers
• The flow pattern/solids distribution
test is an effective technique for
evaluating the hydraulic phenomena
occurring within full-scale, operating,
activated sludge secondary clanfiers.
The full report was submitted in
fulfillment of Contract No. 68-03-2782 by
Crosby, Young and Associates under the
sponsorship of the U S. Environmental
Protection Agency.
The EPA author, Jon Bender (also the EPA Project Officer, see below), is with the
Municipal Environmental Research Laboratory, Cincinnati, OH 45268; Robert
M. Crosby is with Crosby, Young and Associates, Piano, TX 75074.
The complete report, entitled "Hydraulic Characteristics of Activated Sludge
Secondary Clarifiers," (Order No. PB 84-229 665; Cost: $23.50, subject to
change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
*USGPO: 1984-759-102-10700
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Environmental Protection Information
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