simulation of sequential batch reactor (sbr) operation for simultaneous removal of nitrogen and phosphorus

1. Simulation of sequential batch reactor (SBR) operation
for simultaneous removal of nitrogen and phosphorus
Ho Nam Chang, Ra Kyung Moon, Byung Geon Park, Seong-Jin Lim, Dong Won Choi,
Woo Gi Lee, Seok Lyong Song, Yong Hee Ahn
Abstract Modeling of the operation of sequential batch
reactor (SBR) was performed to ®nd out optimum design
parameters for simultaneous removal of nitrogen and
phosphorus in a small-scale wastewater treatment plant.
The models were set up with material balances on SBR
operation and Monod kinetics. The model parameters
were obtained to best ®t the experimental results in a small
scale SBR. The models were useful in optimizing hydraulic
retention time (HRT) and successfully simulated opera-
tions of SBR in a larger scale. Especially the model pre-
dicted well the reactions occurring in the ®lling period as
well as the effect of dilution, and evaluated the perfor-
mance of SBR process under diverse operating conditions.
1
Introduction
A concept on wastewater has become more speci®c and
diverse with advances of industrial development and in-
tensi®cation of urban population. Accordingly, research
on wastewater treatment has developed into a more sys-
tematic and specialized process. Especially, the increase of
nitrogen and phosphorus discharged into a river from
sewage and industrial wastewater has made self-cleaning
function of the river ineffective and promoted abnormal
increase in algae growth resulting in rapid eutrophication.
It was known that sequential batch reactor (SBR) is suit-
able for treating wastewater containing high nitrogen and
phosphorus in a small and medium-size city of high
population density. The SBR has following advantages in a
small-scale system: ¯exibility in operation, low construc-
tion and maintenance costs, and simultaneous removal of
nitrogen and phosphorus.
In order to improve the quality of ef¯uents in SBR plant
it is necessary to understand the mechanism of nutrient
removal, upon which we can evaluate the process perfor-
mances with con®dence. We believe that a mathematical
model on complex biochemical phenomena occurring in
SBR operation can provide a better insight in the design
and operation of SBR plants under various process con-
ditions. An especially common characteristic in Korean
wastewater is low in carbon content resulting in a lower
C/N ratio. This creates a dif®culty in applying various
American or European processes to the treatment of Ko-
rean wastewater. We carried out experimental studies in a
small-scale SBR system of 30 l to de®ne important pa-
rameters affecting the process performances. The current
simulation study will help us understand the system be-
havior and play an important role in improving the system
performances eventually.
1.1
Biological removal of nitrogen and phosphorus
Nitrogen in wastewater can be removed by physico±
chemical methods such as break-point chlorination,
stripping by ammonia gas desorption, and ion exchange
by clinoptilolite column packed with zeolite [1], but bi-
ological methods receive more attention because it is
more stable in operation and cheaper. Also it has an
advantage of simultaneous removal of nitrogen and
phosphorus. Nitrogen compound can be metabolized into
a protein, but its amount is not much signi®cant. How-
ever, a large amount of nitrogen can be removed by
Nitrosomonas and Nitrobacter oxidizing ammonia to ni-
trite and nitrate, followed by denitri®cation of hetero-
trophs that can utilize the nitrate and nitrite as electron
acceptor [2, 3].
The kinetics of nitri®cation is represented by multiple
Monod kinetics and the limiting nutrients are ammonia
and oxygen concentrations. When organic nitrogen is
present in wastewater, the kinetics is limited by the
conversion of organic nitrogen to ammonia [3, 4]. Ex-
ternal factors affecting nitri®cation are known to be
dissolved oxygen concentration, pH and temperature.
Denitri®cation is carried out mainly by facultative het-
erotrophs such as Pseudomonas aeruginosa, P. stutzen,
Bacillus cereus, which require the presence of organic
carbon as electron donor. Nitrate and nitrite serve as
electron acceptor and is used partly as nitrogen source.
When COD/oxidized nitrogen (nitrate and nitrite) is 3.0±
3.5, denitri®cation can take place without a supply of
external carbon. Methanol is widely used as an external
Bioprocess Engineering 23 (2000) 513±521 Ó Springer-Verlag 2000
513
Received: 2 November 1999
Ho Nam Chang (&), Ra Kyung Moon, Byung Geon Park,
Seong-Jin Lim, Dong Won Choi, Woo Gi Lee
Department of Chemical Engineering and
Bioprocess Engineering Research Center,
Korea Advanced Institute of Science and Technology,
Taejon 305-701, Korea
Seok Lyong Song, Yong Hee Ahn
Department of Environment Research,
Hyundai Research Institute,
1, Cheonha-Dong, Dong-Ku, Ulsan, Korea
This research was supported in part by Hyundai Heavy Industries
as a part of Hyundai-KAIST cooperative research program.

2. carbon source because of its cheap price [2, 5] but ac-
etate is known to be best in the speed of denitri®cation
[6].
Phosphorus can be removed by repetitive operation of
anaerobic and aerobic steps. In an anaerobic condition
microorganisms take up carbon source and accumulate
PHB, and discharge phosphorus [7]. In an aerobic con-
dition Acinetobacter spp. take phosphorus excessively to
store it in the form of Poly-P. At this time PHB is de-
composed and used as carbon and energy source [8].
Organic carbon source taken up in an anaerobic condi-
tion is metabolized through acetyl-CoA by Embden-
Meyerhof/Entner-Doudoroff pathway, which is incorpo-
rated into PHB and used as sink for electrons and pro-
tons, and is used as energy source for ATP synthesis.
Poly-P is used as source of P for ATP synthesis. ATP is
decomposed to ADP for energy generation, and inorganic
P is discharged into a medium. The amount of phos-
phorus discharged in an anaerobic condition is known to
be proportional to the amount of short-chain volatile
fatty acid such as acetate. In summary, in an anaerobic
step poly-P is decomposed to generate enough energy to
take up organic carbon source and convert to store it as
PHB and liberated phosphorus is discharged into a so-
lution. Subsequently in an aerobic condition stored PHB
is decomposed to form ATP and phosphorus taken up
excessively is stored as poly-P.
1.2
SBR operation
SBR process consists of several time-oriented periodic
steps during which reactant is ®lled for a given time; re-
action is carried out batchwise; mixed liquor suspended
solids (MLSS) is precipitated; supernatant is discharged.
SBR can reduce number of reactors in the process and its
batch operation can improve the ef®ciency comparable to
that of plug ¯ow reactor. Thus SBR can control the dis-
tribution and physiological state of microorganisms which
are selected to grow in the reactor. Time-varying indi-
vidual components of incoming wastewater in each pro-
cess steps can have the microorganisms to be placed under
nutritional changes from feast to famine states. Thus ad-
justing operating holding time in each cycle-step can
maintain a wide variety of distributions in the population
of microorganisms. SBR process is known to save more
than 60% of expenses required for conventional activated
sludge process in operating cost.
2
Modeling
2.1
Previous work and assumptions
There was no modeling work on the simultaneous re-
moval of nitrogen and phosphorus on SBR, but we
could ®nd a similar work on activated sludge process
[9±11]. A limited model on the substrate removal in a
SBR was reported [3, 9, 12, 13]. Biological removal of
phosphorus can be found in literatures by Carucci et al.
[14], and Smolders et al. [15]. Several models on SBR
provide much information on the selection of state
variables and reaction kinetics for the modeling of SBR
[16±19]. Speci®cally for the modeling of nitrogen and
phosphorus removal the following relationships were
summarized [6, 20]:
1. The amount of phosphorus discharged in an anaerobic
condition increases in proportion to the food to mi-
croorganism ratio to some extent, but beyond which it
reaches plateau. The amount increases in the beginning
and declines later as external carbon source is ex-
hausted.
2. The amount of phosphorus which can be discharged
even in the presence of external carbon sources are
limited to 30±48% of the total phosphorus content of
the microorganisms.
3. The phosphorus release of microorganisms is very
dependent on the composition of external carbon
source until it reaches the limit.
4. The amount of PHB storage increases gradually in an
anaerobic state. The amount of PHB increased in the
cell is in linear relationship with the decrease of car-
bohydrates (glycogen) in the cell.
Table 1. Reactions in the SBR
at the steady state Anaerobic Aerobic Anoxic
S1 Uptaken by X1 Uptaken by X1 Uptaken by X1
S2 Hydrolyzed by X3 Hydrolyzed by X3 Hydrolyzed by X3
P Released from X1 Overplus uptaken by X1 Overplus uptaken by X1 using NO
Poly-P Hydrolyzed by X1 Stored in X1 Stored in X1
PHB Stored in X1 S1 > 0: stored in X1 S1 > 0: stored in X1
S1 = 0: hydrolyzed by X1 S1 = 0: hydrolyzed by X1
NH4 Uptaken by X3 Nitri®ed to NO by X2 ±
Uptaken by X1, X3
NO ± Nitri®ed from NH4 Denitri®ed to N2
X1 Growing using S1 Growing using S1 Growing using S1
X2 Decayed Growing using NH4 Decayed
X3 Growing using S2 Growing using S2 Growing using S2
X1 = phosphorus removing microorganisms; X2 = nitrifying microorganisms; X3 = denitrifying
microorganisms; S1 = readily biodegradable substrate; S2 = slowly biodegradable substrate;
NH4 = ammonium; NO = oxidized nitrogen; poly-P = polyphosphate; PHB = poly hydro-
xybutyrate
514
Bioprocess Engineering 23 (2000)

3. 5. The total amounts of PHB produced and removed total
organic carbon (TOC) are in linear relationship with
the amount of phosphorus than can be discharged.
6. PHB is stored in an aerobic condition if external car-
bon source is suf®ciently present.
7. The uptake rate of phosphorus in an aerobic condition
depends on the unsaturated storage capacity of phos-
phorus in the cell and the extracellular phosphorus
concentration.
8. The limiting nutrients in the nitri®cation process are
ammonia and oxygen.
9. The limiting nutrients in the denitri®cation process are
external carbon source and nitrate ions.
2.2
Model equations
We formulated models based on scienti®c principles of
nitrogen and phosphorus removal, mass balances and
multiple substrate Monod kinetics (Table 1). The model
equations of Wentzel et al. [10] and Smolders et al. [16]
were modi®ed to ®t simultaneous removal of nitrogen and
phosphorus in SBR (Tables 2±6). In mixed culture systems
external environments such as pH, temperature and sub-
strate composition can in¯uence the distribution of mi-
Table 2. Model equations in anaerobic phase
dS1
dt
ˆ
q
V
…S1f S1† a…1†
S1
a…2† ‡ S1
Poly-P
a…3† ‡ Poly-P
X1 ‡ a…4†
dS2
dt
dS2
dt
ˆ
q
V
…S2f S2† a…5†
S2
a…6† ‡ S2
X3
dP
dt
ˆ
q
V
…Pf P† ‡ a…7†
S1
a…8† ‡ S1
Poly-P
a…9† ‡ Poly-P
X1
dPoly-P
dt
ˆ
q
V
…Poly-Pf Poly-P† a…10†
S1
a…8† ‡ S1
Poly-P
a…9† ‡ Poly-P
X1
dPHB
dt
ˆ
q
V
…PHBf PHB† ‡ a…11†
S1
a…2† ‡ S1
Poly-P
a…3† ‡ Poly-P
X1
dNH4
dt
ˆ
q
V
…NH4f NH4† a…12†
S2
a…6† ‡ S2
X3
dNO
dt
ˆ
q
V
…NOf NO†
dX1
dt
ˆ
q
V
…X1f X1† ‡ a…17†
S1
a…2† ‡ S1
X1 a…13†X1
dX2
dt
ˆ
q
V
…X2f X2† a…14†X2
dX3
dt
ˆ
q
V
…X3f X3† ‡ a…15†
S2
a…6† ‡ S2
X3 a…16†X3
dV
dt
ˆ q
Table 3. Model equations in aerobic phase (S1 0)
dS1
dt
ˆ
q
V
…S1f S1† b…1†
S1
b…2† ‡ S1
X1 ‡ b…3†
dS2
dt
dS2
dt
ˆ
q
V
…S2f S2† b…4†
S2
b…5† ‡ S2
X3
If Poly-P X1 0:04; then a ˆ 1:0; else a ˆ 0
dP
dt
ˆ
q
V
…Pf P† ‡ ab…6†
S1
b…7† ‡ S1
P
b…27† ‡ P
X1
dPoly-P
dt
ˆ
q
V
…Poly-Pf Poly-P† ab…8†
S1
b…7† ‡ S1
P
b…27† ‡ P
X1
dPHB
dt
ˆ
q
V
…PHBf PHB† ‡ b…9†
S1
b…2† ‡ S1
X1
dNH4
dt
ˆ
q
V
…NH4f NH4† b…11†
NH4
b…12† ‡ NH4
X2
b…28†
S1
b…2† ‡ S1
X1 b…29†
S2
b…5† ‡ S2
X3
If NH4 ˆ 0:0; then a ˆ 1:0; else a ˆ 0
dNO
dt
ˆ
q
V
…NOf NO† ‡ b…13†
NH4
b…12† ‡ NH4
X2
a b…28†
S1
b…2† ‡ S1
X1 ‡ b…29†
S2
b…5† ‡ S2
X3
dX1
dt
ˆ
q
V
…X1f X1† ‡ b…14†
S1
b…2† ‡ S1
X1 b…15†X1
dX2
dt
ˆ
q
V
…X2f X2† b…16†
NH4
b…12† ‡ NH4
X2 b…17†X2
dX3
dt
ˆ q…X3f X3† ‡ b…18†
S2
b…5† ‡ S2
X3 b…19†X3
dV
dt
ˆ q
Table 4. Model equations in aerobic phase (S1 = 0)
dS1
dt
ˆ
q
V
…S1f S1†
dS2
dt
ˆ
q
V
…S2f S2† b…4†
S2
b…5† ‡ S2
X3
If Poly-P X1 0:04; then a ˆ 1:0; else a ˆ 0
dP
dt
ˆ
q
V
…Pf P† ab…20†
PHB
b…21† ‡ PHB
P
b…27† ‡ P
X1
dPoly-P
dt
ˆ
q
V
…Poly-Pf Poly-P† ‡ ab…22†
PHB
b…21† ‡ PHB
P
b…27† ‡ P
X1
dPHB
dt
ˆ
q
V
…PHBf PHB† b…23†
PHB
b…24† ‡ PHB
X1
dNH4
dt
ˆ
q
V
…NH4f NH4† b…11†
NH4
b…12† ‡ NH4
X2
b…28†
S1
b…2† ‡ S1
X1 b…29†
S2
b…5† ‡ S2
X3
If NH4 ˆ 0:0; then a ˆ 1:0; else a ˆ 0
dNO
dt
ˆ
q
V
…NOf NO† ‡ b…13†
NH4
b…14† ‡ NH4
X2
a b…28†
S1
b…2† ‡ S1
X1 ‡ b…29†
S2
b…5† ‡ S2
X3
dX1
dt
ˆ
q
V
…X1f X1† ‡ b…25†
PHB
b…22† ‡ PHB
X1 b…15†X1
dX2
dt
ˆ
q
V
…X2f X2† ‡ b…16†
NH4
b…12† ‡ NH4
X2 b…17†X2
dX3
dt
ˆ
q
V
…X3f X3† b…19†X3
dV
dt
ˆ q
515
Ho Nam Chang et al.: Simulation of sequential batch reactor operation

4. crobial populations and their metabolism. The kinetic
parameters ranged within certain limits given in several
references and we chose the values of these parameters
that may ®t the current simulation models (Table 7).
1 Anaerobic stage (in the absence of oxidized nitrogen
and oxygen as electron acceptor)
Phosphorus-removing X1 takes up readily biodegradable
substrate S1 to metabolize and store it as PHB by de-
composing poly-P to use its energy and release Pi. Aerobic
nitrifying X2 becomes inactive but facultative denitrifying
X3 grows by hydrolyzing slowly biodegradable substrate
S2 (Table 2).
2 Aerobic stage (in the presence of oxidized nitrogen and
oxygen as electron acceptor)
Phosphorus-removing X1 takes up phosphorus to store it
as poly-P to a certain level. If carbon source is present,
the substrate is used to carry out poly-P production as
well as to store PHB, but PHB is used to produce poly-P
in the absence of the substrate. Nitrifying X2 oxidizes
ammonia to oxidized nitrogen using oxygen as electron
acceptor and ammonia as energy source and denitrifying
X3 grows using S2 as substrate. Table 3 is applied in the
presence of the substrate and Table 4 in the absence of
the substrate.
3 Anoxic stage (in the presence of oxidized nitrogen as
electron acceptor)
Phosphorus-removing X1 uses its substrate in the presence
of carbon source or PHB in the absence of substrate in
taking up excess phosphorus. This time oxidized nitrogen
is used as electron acceptor. Denitrifying X3 denitri®es
oxidized nitrogen (NO3 and NO2 ) to N2 in the presence of
substrate (carbon source) and otherwise decays. Table 5 is
applied in the presence of substrate and Table 6 in the
absence of substrate.
3
Materials and methods
3.1
Apparatus
SBR system consists of reaction vessel, control unit, con-
stant temperature bath, various sensors and pumps. The
reactor is made of transparent acrylic polymer through
which settlement of sludge can be observed from outside
and its volume is 30 l. The control unit controls closing
and opening of solenoid valves for air to maintain anaer-
obic and aerobic condition with a proper cycle time. Also
Table 5. Model equations in anoxic phase (S1 0)
dS1
dt
ˆ
q
V
…S1f S1† c…1†
S1
c…2† ‡ S1
NO
c…27† ‡ NO
X1
‡ c…3†
S2
c…5† ‡ S2
X3
dS2
dt
ˆ
q
V
…S2f S2† c…4†
S2
c…5† ‡ S2
NO
NO ‡ c…27†
X3
If Poly-P X1 0:04; then a ˆ 1:0; else a ˆ 0
dP
dt
ˆ
q
V
…Pf P† ac…6†
S1
c…7† ‡ S1
P
c…24† ‡ P
X1
dPoly-P
dt
ˆ
q
V
…Poly-Pf Poly-P† ‡ ac…8†
S1
c…7† ‡ S1
P
b…24† ‡ P
X1
dPHB
dt
ˆ
q
V
…PHBf PHB† ‡ c…9†
S1
c…10† ‡ S1
X1
dNH4
dt
ˆ
q
V
…NH4f NH4†
dNO
dt
ˆ
q
V
…NOf NO† ‡ c…11†
NO
c…12† ‡ NO
S2
c…5† ‡ S2
X3
c…25†
S1
c…2† ‡ S1
NO
NO ‡ c…27†
X3
dN2
dt
ˆ
q
V
…N2f N2† ‡ c…29†
NO
c…27† ‡ NO
S1
c…2† ‡ S1
X1
dX1
dt
ˆ
q
V
…X1f X1† ‡ c…13†
S1
c…2† ‡ S1
NO
c…12† ‡ NO
X1 c…14†X1
dX2
dt
ˆ
q
V
…X2f X2† c…15†X2
dX3
dt
ˆ
q
V
…X3f X3† ‡ c…16†
S2
c…5† ‡ S2
NO
c…12† ‡ NO
X3 c…17†X3
dV
dt
ˆ q
Table 6. Model equations in anoxic phase (S1 = 0)
dS1
dt
ˆ
q
V
…S1f S1†
dS2
dt
ˆ
q
V
…S2f S2† c…4†
S2
c…5† ‡ S2
NO
c…12† ‡ NO
X3
If Poly-P X1 0:04; then a ˆ 1:0; else a ˆ 0
dP
dt
ˆ
q
V
…Pf P† ac…18†
PHB
c…19† ‡ PHB
P
c…24† ‡ P
X1
dPoly-P
dt
ˆ
q
V
…Poly-Pf Poly-P† ‡ ac…20†
PHB
c…22† ‡ PHB
P
b…24† ‡ P
X1
dPHB
dt
ˆ
q
V
…PHBf PHB† c…21†
PHB
c…22† ‡ PHB
NO
c…12† ‡ NO
X1
dNH4
dt
ˆ
q
V
…NH4f NH4†
dNO
dt
ˆ
q
V
…NOf NO† c…11†
NO
c…12† ‡ NO
S2
c…5† ‡ S2
X3
c…25†
PHB
c…2† ‡ PHB
NO
NO ‡ c…27†
X1
dN2
dt
ˆ
q
V
…N2f N2†
dX1
dt
ˆ
q
V
…X1f X1† ‡ c…23†
PHB
c…22† ‡ PHB
NO
c…12† ‡ NO
X1
c…14†X1
dX2
dt
ˆ
q
V
…X2f X2† c…15†X2
dX3
dt
ˆ
q
V
…X3f X3† c…17†X3
dV
dt
ˆ q
516
Bioprocess Engineering 23 (2000)

5. it controls the reaction conditions such as mixing rpm and
pumping rates of in¯uent and ef¯uent streams. The con-
stant temperature bath maintains the reaction vessel
temperature and pH controller keeps pH within a limit.
Decanter is made of stainless ®lter and its height is ad-
justed in accordance with the amount of sludge. It also
prevents the discharge of scum. The electrodes for dis-
solved oxygen and pH were installed at the wall of the
reactor vessel.
3.2
SBR operation
The reactor was operated with a working volume of 30 l of
wastewater and 7±10 l of precipitated sludge volume. The
time for ®lling period was about 10±20 min and the dis-
charging time through the decanter was 30 min±1 h. The
Table 7. Parameters in the model equations
Anaerobic Aerobic Anoxic
a(1) = 0.43 b(1) = 0.24 c(1) = 0.15
a(2) = 48.0 b(2) = 1.8 c(2) = 1.8
a(3) = 0.8 b(3) = 0.6 c(3) = 0.7
a(4) = 0.8 b(4) = 0.07 c(4) = 0.06
a(5) = 0.04 b(5) = 5.0 c(5) = 5.0
a(6) = 5.0 b(6) = 0.011 c(6) = 0.007
a(7) = 0.012 b(7) = 4.0 c(7) = 4.0
a(8) = 9.0 b(8) = 0.018 c(8) = 0.023
a(9) = 10.0 b(9) = 0.007 c(9) = 0.011
a(10) = a(7)1.8 b(10) = 2.0 c(10) = 1.8
a(11) = 0.045 b(11) = 0.016 c(11) = 0.05
a(12) = 0.003 b(12) = 1.0 c(12) = 2.7
a(13) = 0.0017 b(13) = 0.013 c(13) = 0.013
a(14) = 0.0021 b(14) = 0.03 c(14) = 0.0017
a(15) = 0.01 b(15) = 0.0017 c(15) = 0.0021
a(16) = 0.0024 b(16) = 0.009 c(16) = 0.04
a(17) = 0.02 b(17) = 0.0021 c(17) = 0.0024
b(18) = 0.01 c(18) = 0.007
b(19) = 0.0026 c(19) = 1.8
b(20) = 0.007 c(20) = 0.03
b(21) = 1.3 c(21) = 0.0026
b(22) = 0.02 c(22) = 8.0
b(23) = 0.021 c(23) = 0.0185
b(24) = 8.0 c(24) = 3.0
b(25) = 0.03 c(25) = 0.0034
b(27) = 3.0 c(26) = 0.0034
b(28) = 0.01 c(27) = 2.7
b(29) = 0.01 c(28) = 0.009
c(29) = 2.7
Table 8. Operating conditions of SBR
Temp.
(°C)
pH DO
(mg/l)
MLSS
(mg/l)
SRT
(days)
HRT
(h)
rpm
25 27 6.5 7.5 (Anaerobic) 0.2
(Aerobic) 4.0
3500 4000 20 6±10 280
Table 9. Composition of synthetic wastewater
Sodium acetate According to BOD loading
Glucose According to BOD loading
(NH4)2SO4 3.50 g/20 l
KH2PO4 0.51 g/20 l
MgSO4á7H2O 1.00 g/20 l
FeCl3á6H2O 0.075 g/20 l
CaCl2 0.05 g/20 l
MnSO4áH2O 0.10 g/20 l
NaHCO3 2.05 g/20 l
Fig. 1. Pro®les of simulated phosphorus during sludge acclima-
tion period
Fig. 2. Pro®les of simulated cell mass during sludge acclimation
period (When surplus carbon is not available.) (X1 Phosphorus
removing heterotrophs, X2 Nitri®er, X3 Denitri®er, X4
Anaerobes, X5 Aerobes)
Fig. 3. Pro®les of simulated cell mass during sludge acclimation
period (When surplus carbon is available.) X1 Phosphrous re-
moving heterotrophs, X2 Nitri®er, X3 Denitri®er, X4 Anaerobes,
X5 Aerobes)
517
Ho Nam Chang et al.: Simulation of sequential batch reactor operation

6. reaction was carried out for 6±10 h of hydraulic retention
time (HRT) using time controller. In an anaerobic step
nitrogen was introduced to lower oxygen concentration
below 0.2 ppm and in an aerobic step oxygen concentra-
tion was maintained over 4 ppm by introducing air.
Samples were taken every 1±3 h to be ®ltered with glass±
®ber ®lter (Whatman) and analyzed. By varying pH,
temperature, HRT, and composition of synthetic waste-
Fig. 5. Simulation results according to the BOD loading
Fig. 4. Comparison of experimental data and simulational results.
(Anaerobic-Aerobic-Anoxic: 2.5±3±3 h)
b
Fig. 6 a Simulated results according to HRT (BOD ˆ 100 ppm).
b Simulated results according to HRT (BOD ˆ 200 ppm)
518
Bioprocess Engineering 23 (2000)

7. water, optimal values of operating parameters on SBR in
maximizing nitrogen and phosphorus removal rates were
obtained. The sludge acclimation conditions were con-
trolled at 25 °C and pH of 7±8. A steady-state experimental
condition is given in Table 8.
3.3
Influent wastewater
Synthetic wastewater was used in this experiment and its
composition is given in Table 9.
3.4
Analysis
Concentration of NO2 , NO3 , NH‡
4 and PO4 were measured
with eutrophication meter (HC-1000, Central Kagaku, Ja-
pan). Total nitrogen and phosphorus was measured by the
methods described in Standard Methods [21].
4
Results and discussion
4.1
Simulation in the start-up period
The microbial concentration responsible for the removal
of nitrogen and phosphorus is generally low in the start-up
period. Thus the simulation shows that the distributions
and cell density of the microorganisms would approach
those of desired ones with the treatment progressing. We
de®ned the ®ve different microorganisms categories as
phosphorus-removing X1, nitri®er X2 (autotrophs), den-
itri®er X3, anaerobic heterotrophs X4 and aerobic het-
erotrophs X5. The material balances of PHB, ammonia,
oxidized nitrogen, growth and deactivation of microbial
cells, and discharge and uptake of phosphorus were rep-
resented with Monod kinetics in anaerobic, aerobic and
anoxic conditions of oxygen.
Figure 1 shows clearly that discharge and uptake of
phosphorus become stabilized as the sludge becomes ac-
climated. Figures 2 and 3 show that the microorganisms
for phosphorus removal (X1), nitri®er (X2) and denitri®er
(X3) become dominant as the treatment time progresses.
Figure 2 shows the distribution of the microbial popula-
Table 10. Operating strategy and removal ef®ciencies of 2 cycles/day (h)
Fill period Reaction period Period Removal rate (%)
Anaerobic Aerobic Anoxic Anaerobic Aerobic Anoxic Settle Drain TP TN
Time # 6 3 1 2 ± ±
#1 3 3 0 0 1 2 1 2 77 87
#2 0 6 0 0 0 3 1 2 88 89
#3 2 2 2 0 2 1 1 2 92 96
Table 11. Operating strategy and removal ef®ciencies of 3 cycles/day (h)
Fill period Reaction period Period Removal rate (%)
Anaerobic Aerobic Anoxic Anaerobic Aerobic Anoxic Settle Drain TP TN
Time # 4 2.5 0.5 1 ± ±
#1 2 2 0 0 1 1.5 0.5 1 87 83
#2 0 4 0 0 0 2.5 0.5 1 95 94
#3 1 2 1 0 1 1.5 0.5 1 92 94
Fig. 7. Simulation results of 2 cycles/day operations in Table 10
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8. tions in the absence of periodic supply of external carbon
source while Fig. 3 shows those in the presence of the
external carbon. In both cases simple anaerobic (X4) and
aerobic (X5) microorganisms decay with time while X1, X2
and X3 become dominant. Hence we did not included X4
and X5 in the following simulation programs applicable to
the acclimated sludge.
4.2
Measured and simulated concentrations
of N and P in the SBR system
The measured values of nitrogen and phosphorus con-
centrations in the SBR system are in good agreements with
those predicted by the simulation study (Fig. 4). An initial
phosphorus concentration of 6 ppm increases to 11 ppm
and decreases to 3 ppm at the end of cycle time when 2.5 h
of anaerobic, 3 h of aerobic, and 3 h of anoxic cycles are
applied. The ®tness of the experimental results to those by
the simulation justi®es the validity of the various param-
eters assumed in the simulation study to some extent.
4.3
Efficiency of phosphorus removal with BOD loading
Figure 5 shows the removal ef®ciency of P with BOD
loading. When the BOD loading is only 100 ppm, the ®nal P
concentration drops from 6 to 2 ppm. But when the BOD
loading is raised to 350 ppm, the ®nal P level can drop to
nearly zero. When the initial BOD loading is not high, then
extent of uptake and discharge of P becomes smaller. This
will result in less ef®cient removal of P. Thus this simula-
tion result shows that the in¯uent wastewater should have a
minimum level of BOD for the ef®cient removal of P.
4.4
Variation of HRT
Varying HRTs with the BOD loadings of 100, 200 ppm
could yield optimized removal rates of N and P within a
shorter cycle time (Fig. 6). Six hours of cycle time with
1±3±2 h (anaerobic±aerobic±anoxic) at 200 ppm BOD
loading was found to have best removal rates for N and P
(Fig. 6b).
4.5
Simulation of full scale SBR process system
An advantage of large-scale SBR plant is to have ¯exible
®lling time. During the ®lling period aerobic, anaerobic
and anoxic condition can be imposed to carry out desired
reactions. Also the precipitated sludge can function as
dilution reservoir to prevent shock loading. In a large-
scale SBR ®lling period can take 25±75% of the total cycle
time and reaction period can range from 0 to 50%. The
time for precipitation is relatively constant as 0.5±1.5 h
depending on the sludge volume index (SVI). The dis-
charge time may take 5±30% of the total cycle time.
We have simulated 3000 ton/day SBR plant using 2 and
3 cycles/day operations. The conditions are as follows:
2 cycles/day:
HRT = 12 h,
V (working volume) = 1500 ton
V0 (sludge volume) = 500 ton
Q (in¯uent ¯ow rate) = 167 ton/h
Q¢ (ef¯uent ¯ow rate) = 500 ton/h
3 cycles/day:
HRT = 8 h,
V (working volume) = 1000 ton
V0 (sludge volume) = 500 ton
Q (in¯uent ¯ow rate) = 125 ton/h
Q¢ (ef¯uent ¯ow rate) = 500 ton/h
The results of these simulations are given in Tables 10, 11
and Figs. 7, 8. Having 3 cycles/day operation could have a
much smaller reactor size than that of 2 cycles/day. This
simulation for a large-scale plant can be a valuable tool for
the optimization of the reactor size and performance of
SBR system.
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