United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/007 Mar. 1985
c/ERA Project Summary
Technology Assessment of
Sequencing Batch Reactors
Robert L. Irvine
The innovative and alternative tech-
nology provisions of the Clean Water
Act of 1977 provide financial incentives
to communities using wastewater treat-
ment alternatives that reduce costs of
energy consumption over conventional
systems. To increase awareness and
implement such alternatives, the U.S.
Environmental Protection Agency's
(EPA) Water Engineering Research
Laboratory has initiated a series of
assessments intended to evaluate both
the current status and capabilities of
these technologies. This report provides
an analysis of one of these technologies,
the Sequencing Batch Reactor (SBR).
The SBR is a fill and draw activated
sludge system. Each tank in the SBR
system is filled during a discrete period
of time and then operated in a batch
treatment mode. If tank volumes and
aeration practices are properly designed,
the SBR can simulate any conventional
continuous flow activated sludge sys-
tem. In a cost and energy comparison,
the cost of SBR closely compared with
that of the oxidation ditch and was
roughly 20 percent less than that for the
conventional activated sludge systems
tested. As far as energy use is con-
cerned, the SBR was 13.5 percent more
efficient than the oxidation ditch and
was the equivalent of the conventional
activated sludge systems.
This Project Summary was developed
by EPA's Water Engineering 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).
Technology Description
The SBR, a type of periodic process, is a
fill and draw activated sludge system.
Each tank in the SBR system is filled, one
after the other, during a discrete period of
time and then operated in a batch treat-
ment mode. After treatment, the mixed
liquor is allowed to settle and the clarified
supernatant is drawn. The tank is then
refilled after the remaining tanks in the
SBR systems have been filled.
If the time required for a tank to fill is
very long when compared with the time
provided for batch treatment, the SBR
behaves like a conventional completely
mixed activated sludge facility. If the
opposite is true, the SBR behaves like a
nominal plug flow system. A properly
designed SBR can simulate any conven-
tional continuous flow activated sludge
system. Each SBR tank carries out the
functions of equalization, aeration, and
sedimentation in a time sequence rather
than in the conventional space sequence
of continuous flow systems, where these
functions are carried out in separate
tanks. Because the relative tank volumes
dedicated to, say, aeration and sedimen-
tation can be redistributed easily by
adjusting the mechanism that controls
the time (and, therefore, share of the total
volume) planned for either function, the
SBR is flexible. By working in time rather
than in space, the SBR can be either a
labor-intensive, low-energy, high-sludge-
yield system or a minimal-labor, high-
energy, low-sludge-yield system.
Each tank in an SBR system undergoes
one or more cycles (i.e., the time between
one filling and the next) during each day.
The cycle of a typical SBR tank is divided
into five discrete periods—FILL, REACT,
SETTLE, DRAW, and IDLE (Figure 1).
FILL: During FILL, either raw waste-
water (screened and degritted) or primary
effluent is added to the activated sludge
remaining in the tank from the previous
cycle. FILL ends either when the tank is
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Percent of:
Max Cycle
Volume Time
Influent
25
to
100
25
100
35
Purpose/Operation
Air
On/Off
Add
Substrate
Air
On/Cycle
Reaction
Time
100
20
Settle
Air
Off
Clarify
WO
to
35
15
Draw
Effluent
Air
Off
Remove
Effluent
35
to
25
Idle
Air
On/Off
Waste
Sludge
Figure 1. Typical SBR operation for a complete cycle in one tank.
full or when a maximum time for FILL is
reached. The wastewater flow is then
diverted to the next tank in the SBR
system. Although FILL time is shown to
be 25 percent of the total cycle time, a
range of 40% to 60% would be more
typical for a two-tank system and, in any
event, would more or less depend on the
extent of daily variations in the hydraulic
flow rate.
REACT: Reactions begun during FILL
are completed during REACT. Although
the liquid level appears to remain maxi-
mum, sludge wasting can take place
during REACT as a simple means to
control sludge age, e.g., sludge age in
days would be equal to the reciprocal of
the fraction of the maximum liquid volume
wasted each day. The total cycle time
dedicated to REACT can actually vary
from just greater than zero to more than
50 percent.
SETTLE: During SETTLE, solids and
liquids separate. The time should be
between 0.5 and 1 hour so that the sludge
blanket remains below the withdrawal
mechanism during DRAW and does not
rise (because of gas formation) before
DRAW is completed.
DRAW: During DRAW, the treated
wastewater is removed. The percent of
cycle time can range from 5 to more than
30. DRAW cannot be overly extended,
however, because of possible problems
with rising sludge.
IDLE: The IDLE period can be used to
waste sludge from the system. Otherwise
IDLE is simply that time that must be
waited after DRAW for the last tank in the
SBR system to be filled before the tank in
question can be refilled, thus beginning a
new cycle.
An SBR system consists of the head-
works, one or more tanks, an aeration
device, a mechanism to withdraw waste-
water, and a control system. The influent
sequences from one tank to the next and
may be either pumped in or allowed to
flow in by gravity. When using gravity
flow, some device such as an adjustable
weir or automatic valve must be used to
divert the flow to one tank or the other.
Theoretically, there's no limit to the
size of each tank, or the number of tanks
used in the SBR system. The tank may be
an earthen ditch, an oxidation ditch, a
rectangular basin, or any concrete or
metal structure. Virtually any aeration
system (e.g., diffused, floating, mechan-
ical, or jet) can be used although a system
that separates mixing from aeration (e.g.,
as for the jet) and one that is not clogged
by having mixed liquor settle on it once
each cycle would likely be best. The
withdrawal mechanism may be as simple
as a pipe fixed at some predetermined
level (with the flow regulated by either an
automatic valve or a pump depending on
the hydraulic grade line of the system) or,
preferably, an adjustable or floating weir
at or just beneath the liquid surface. As
with the fixed mounted pipe, discharge
from the weir can be regulated by an
automatic valve or a pump. Level sensors
and a timing device provide overall control
of the SBR.
Because REACT, SETTLE, and DRAW
take place after the flow has been di-
verted, the SBR system with just one tank
would be quite unusual for a municipal
waste situation but not all that uncommon
for day schools, amusement parks, or
industries that operate for 8 to 1 5 hours
each day with little or no flow generated
during the remaining hours. In a two or
more tank system, the time for REACT,
SETTLE, and DRAW in one tank must be
less than or equal to the time required to
fill the other tanks An SBR with three
tanks is probably the practical limit for
such systems. Although the total volume
(sum of all tanks used) of an SBR de-
creases with the number of tanks em-
ployed, the incremental reduction is
minimal for greater than three. In addi-
tion, the complexity of operation increases
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as the number of tanks increases. As a
result, except under quite unusual circum-
stances, only one, two, or three tanks
would be recommended.
Status of the
Developed Technology
The retrofit plant in Culver, Indiana,
served as the first full-scale demonstra-
tion plant for SBR treatment of domestic
wastewater in the United States. At least
four domestic waste plants are in various
stages of design or construction: Juneau,
Alaska; Sabula, Iowa; LeClaire, Iowa; and
Poolesville, Maryland. An SBR-like facility
was started up in July 1983 in Grundy
Center, Iowa.
On the industrial side, Alphenrose Dairy
owns and operates a two-tank fill and
draw system in Portland, Oregon. A
single-tank SBR was reportedly used in
Ada, Oklahoma, to treat wastewaters
from a vehicle maintenance area for a
utility company. A single-tank SBR for a
hazardous waste disposal site owned and
operated by CECOS International, Niagara
Falls, New York, began operation in June
1984, and Occidental Chemical Corpora-
tion has designed a similar system to
treat its landfill leachates in the Niagara
Falls area.
Process Capabilities
The only full-scale SBR performance
data currently available is from the EPA-
funded research project completed at
Culver, Indiana.* Between May 1980 and
May 1981, the Culver SBR produced
average 5-day biochemical oxygen de-
mand (BODs) and suspended solids (SS)
concentrations of less than 10 g/m3
each:
Raw Effluent
Waste- North South
water' Tank Tank
BOD5, g/m3 160(152) 9+(147) 10*(144)
SS, g/m3 130(153) 7 (258) 9 (258)
*( ) = number of observations
'Effluent BOD5 measurements were conducted on
prechlormation samples Post-chlormation BOD5
effluent averaged 5 g/m3 (143 observations)
Nitrification
Because nitrification requires that the
dissolved oxygen (DO) be greater than
approximately 0.5 g/m3, the aeration time
during FILL and REACT must be suffi-
ciently long and the DO must be suffi-
ciently high to allow both the enrichment
of nitrifiers and the completion of
am monia-mtrogen(NH4-N) oxidation. The
following data are from those 5 months of
the Culver study when nitrification was
achieved, August to Decembr 1 981 :
Raw
Waste- North
water Tank
South
Tank
NH4-N,g/m3 20(82) 11(79) 10(78)
SS, g/m3 150(94) 5 (94) 6 (94)
BOD5, g/m3 170(59) 11* (57) 10' (59)
"( ) = number of observations
'Post chlorinatron BOD5 effluent (August to December
19811 = 7 g/m3
Denitrification
Denitrification requires the DO to be
less than approximately 0.5 g/m3, the
presence of nitrite and/or nitrate nitrogen
(the sum of these two is reported in NOX-
N), and a carbon source for energy. A
mixing-only period of FILL with no oxygen
supplied and the organics in the waste-
water as the carbon source provides the
best conditions for denitrification until
the oxidized nitrogen supply, left in the
residual liquid after DRAW, is exhausted.
After all available NOX-N is depleted,
however, anaerobic reactions occur. To
prevent these conditions after achieving
denitrification, aeration during the later
part of FILL can be instituted. The follow-
ing denitrifcation data are also from
August to December 1 981 :
Raw
Waste-
water
Effluent
North
Tank
South
Tank
•Full-Scale Study of Sequencing Batch Reactors. R F.
Irvine and L. H. Ketchum, Jr., EPA/600/2-83/020,
NTIS No. PB83-183186, Municipal Environmental
Research Laboratory, U.S. Environmental Protection
Agency, Cincinnati, Ohio 45268.
NOx-N, g/m3 20(80)* 13(81) 10(81)
NH4-N +
NOx-N.g/m3 22* 2.4 23
*( ) = number of observations
'Sum of averages of NH4-N and NO.-N
Controlling Microorganism
Populations
The types of bacteria in SBR activated
sludge can be controlled by the treatment
plant operator who can easily relax or
eliminate some of the selective pressures.
For example, the treatment plant operator
at Culver, Indiana, modified the aeration
and mixing scheme in such a way as to
encourage biological phosphorus removal
and minimize nitrification and denitrifica-
tion. During the subsequent 10-month
period, effluent phosphorus concentra-
tions averaged less than 1 g/m3 without
addition of chemicals.
In every SBR cycle, microorganism
selection pressures are quite severe—the
mixed culture microorganisms are sub-
jected to feast and famine as well as high
and essentially zero DO conditions. Only
a limited number of microorganisms can
both survive and compete in this envi-
ronment. In a conventional continuous
flow activated sludge facility, population
dynamics is largely influenced by the
unsteady state nature of the influent
wastewater. By way of contrast, the
unsteady state nature of the SBR opera-
tion overwhelms variations in the waste-
water and, thus, results in a more control-
lable system.
Process Limitations
Two major limitations became apparent
during the developmental stages of the
SBR. (1) It is a noncontinuous flow system
with no real operating counterpart in the
United States. This significant liability
has been partially overcome with the
Culver demonstration. (2) SBR was per-
ceived to have value only in small sys-
tems. Although a limit of 18,925 m3/d
was considered reasonable for purposes
of cost analyses in this assessment, the
author believes that there are no theoret-
ical or technical reasons for any upper
limit. System selection should be based
on a cost-effectiveness analysis for each
specific application and the level of
reliability and consistency desired.
Limitations of more concern include
the freezing of scum (if present) during
winter operating conditions, the possibil-
ity of high effluent SS of high mass loaded
systems, and finally, the possibility of
developing through improper operation
an organism population that has a large
number of filaments.
SBR Cost and Energy
Cost Considerations
A modular design was used to estimate
costs for SBR's operating at four different
average daily flow rates (Table 1). A two-
tank system was used for the 379 m3/d
plant, and three-tank systems were as-
sumed for the remaining three daily flow
rates. In all cases, size selection was
based on a 50 percent draw-off volume
and on a cycle with zero time in IDLE
when using a peak flow, which is double
the average dry weather flow. A three-
tank system was used for the 18,925
m3/d flow with each tank composed of
four equal sized modules. In all other
cases, only one module was used for each
tank.
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Energy Requirements
Estimated energy requirements for
aeration and decanting for each of the
four SBR flow rates are:
Energy Requirements (kWh/yr)
Table 1.
Flow
Rate
(mVd) Aeration
Decanting Total
379 33.1 x103
1,893 12.4x10"
3,785 24.9x10"
18,925 12.4x106
4.1 x 103 37.2 x 103
37x10" 16.1x10"
6.6x10" 31.5x10"
3 7 x 106 16 1 x 106
Cost and Energy Comparisons
A cost and energy analysis comparing
the SBR with oxidation ditch and conven-
tional activated sludge systems for flow
rates of 379, 1,893, 3,785, and 18,925
mVd indicates similar costs for the SBR
and the oxidation ditch, with the SBR
being slightly less costly. Costs are
roughly 20 percent less for the SBR than
for the conventional activated sludge
systems compared. The energy analysis
showed the SBR to be 13.5 percent more
efficient than the oxidation ditch and
equally as efficient as conventional acti-
vated sludge. The unique fill and draw
feature of the SBR permits its energy
input to be widely varied without seriously
impairing the effluent quality.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3055, Roy F.
Weston, Inc., by Robert L. Irvine of the
University of Notre Dame, under the
sponsorship of the U.S. Environmental
Protection Agency.
SBR Copt Estimates tin's 1,000) for Four A vervfye Daily Flow Rates
Flow Rates (m3/d)
Process Unit,
Inlet control system'
Contact chamber baffle walls
Aerators
Excavation, concrete, handrail
Microprocessors
Level control/ monitor
Decant system
Subtotal (!)
. 379
$ 2
2
25
70
10
2
9
120
1.893
$ 3
, 4
50
150
10
4
16
237
3.785
$ 4
5
60
250
10
4
18
351
18.925'
$ 20
24
256
840
10
16
90
1.256
Noncomponent costs*
Subtotal (II)
Engineering, construction, supervision.
and contingencies^
Total installed capital
Annual operation and maintenance
Present worthy
30
59
88
314
150
45
195
13
329
296
89
385
24
632
439
132
571
40
983
1.570
471
2,041
148
3,564
* At 25 percent of subtotal (I), cost includes piping, electrical installations, instrumentation, andsite
preparation.
\A 130 percent of subtotal (II).
\Present worth computed at 73/n percent interest rate and 20-year life (PWF - 10.29213). Add
present worth O&M costs to Total Installed Capital costs.
Robert L. Irvine is with the University of Notre Dame, Notre Dame, IN 46556.
Jon Bender is the EPA Project Officer (see below).
The complete report, entitled "Technology Assessment of Sequencing Batch
Reactors, "(Order No. PB85-167 245/AS; Cost: $11.50. 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:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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