Sequencing batch reactor treatment of tannery wastewater for carbon and nitrogen removal.
ABSTRACT The paper evaluates the organic carbon and nitrogen removal performance of the sequencing batch reactor (SBR), technology for tannery wastewater. For this purpose, a pilot-scale SBR was installed on site to treat the plain-sealed tannery effluent. The study involved wastewater characterization, start-up and operation of the reactor for carbon and nitrogen removal and model evaluation of system performance. Its removal efficiency was compared with that of the existing continuous-flow activated sludge system providing full treatment to wastewater from the Istanbul Tannery Organized Industrial District.
- SourceAvailable from: Christopher N. LylesNew Horizons in Biotechnology, Edited by Ashok Pandy, Christian Larroche, Carlos Ricardo Soccol, Claude-Gilles Dussap, 01/2008: chapter 16: Shrimp Production and Biological Treatment of Shrimp Wastewater in the United States.: pages 235-252; Asiatech., ISBN: 8187680199/9788187680192
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ABSTRACT: This paper presents experimental results from the implementation of two measures aimed at reducing the nitrogen concentration in a tannery wastewater. Specifically, this research has focused on the wastewater from the deliming/bating operations. The proposed measures are the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. The experimental study covered different wastewater pretreatment alternatives and experiments with two membranes (with different separation properties): one in the range of microfiltration (MF) and one in the range of the ultrafiltration (UF). Results of the pretreatment study indicated that neither settling nor protein precipitation were feasible. Only a security filtration prior to membrane filtration was recommended. The tested MF membrane was selected due to the higher flux (around 25 L/(m2 h)) in comparison with the UF membrane. The MF permeate was successfully reused in the deliming/bating process. The delimed leather quality was excellent according to both visual and organoleptic inspection from process technicians and phenolphthalein test, confirming the technical feasibility of the proposal. Globally, the implementation of the above mentioned two measures resulted in 53% total nitrogen reduction.Chemical Engineering Research and Design 02/2013; 91(2):369–376. · 2.28 Impact Factor
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ABSTRACT: In this study, sludge treatment and dewatering alternatives for a selected leather tanning wastewater treatment plant were evaluated by evaluating the best available technology for sludge management. In addition, different combinations of sludge treatment alternatives were compared on the basis of investment and maintenance costs.
Sequencing batch reactor treatment of tannery wastewater
for carbon and nitrogen removal
S. Murat*, E. Ates ¸ Genceli**, R. Tas ¸lı**, N. Artan** and D. Orhon**
* Istanbul Organized Leather Tanning Industrial District Aydınlı, 81464 Tuzla/Istanbul
** Environmental Engineering Department, I˙stanbul Technical University, I˙.T.Ü. I˙ns ¸aat Fakültesi, 80626
Maslak, I˙stanbul Turkey
Abstract The paper evaluates the organic carbon and nitrogen removal performance of the sequencing
batch reactor (SBR), technology for tannery wastewater. For this purpose, a pilot-scale SBR was installed
on site to treat the plain-settled tannery effluent. The study involved wastewater characterization, start-up
and operation of the reactor for carbon and nitrogen removal and model evaluation of system performance.
Its removal efficiency was compared with that of the existing continuous-flow activated sludge system
providing full treatment to wastewater from the Istanbul Tannery Organized Industrial District.
Keywords Characterization; COD fractionation; modelling; nitrogen removal; sequencing batch reactor;
The sequencing batch reactor (SBR) technology is well studied in terms of its potential for
simultaneous carbon and nutrient removal (Wilderer et al., 2001). Its performance is now
fully interpreted in terms of basic principles incorporated into recent activated sludge mod-
els (Artan et al., 2001, 2002). For such evaluations, the process offers the advantage of
observing and modelling concentration transients for selected key parameters such as
COD, NH3-N, NOX-N, etc. It also provides the necessary flexibility of operation to trans-
mit the outputs of kinetic evaluations into application, by appropriate adjustment of cycles
and manipulation of aerated and non-aerated phases.
Tanneries generate a strong wastewater, characterized by a high nitrogen (TKN) content
of around 200 mgl–1and yet a similarly high COD/N ratio. In the raw wastewater a COD/N
ratio of 16 was measured after homogenization; plain sedimentation was observed to drop
this ratio to 10.5 due to significant COD removal; chemical settling induced a further
decrease in this ratio to 5.5, a level clearly unfavorable for simultaneous carbon/nitrogen
removal without external carbon source (Orhon et al., 1999). Aside from a suitable COD/N
ratio, nitrogen removal from tannery effluents and especially nitrification require specific
consideration for the impact of sulfide, chromium and chloride inherently present in the
wastewater. Previous studies have shown that the high chloride content of the wastewater,
which fluctuates around 5,000 mgl–1, should be the major concern for the inhibition of nitri-
fication, thus acting as the limiting step for nitrogen removal (Orhon et al., 2000).
The objective of the study is to highlight the major features of the SBR technology
for carbon and nitrogen removal, applicable to tannery effluents. It basically involves
(i) observation of system performance for COD and nitrogen removal, as practically
achievable with the SBR; (ii) evaluation of the fate of nitrogen on the basis of fundamental
stoichiometry for nitrification and denitrification and (iii) interpretation of system
performance by means of model calibration of the concentration profiles for the main
parameters, such as NH3-N, NOX-N and soluble COD, observed during the cyclic opera-
tion of the SBR at steady state.
Water Science and Technology Vol 46 No 9 pp 219–227 © 2002 IWA Publishing and the authors
Materials and methods
The study was carried out operating a pilot-scale SBR unit installed on site, at the treatment
facilities of the Istanbul Organized Leather Tanning Industrial District, located in
Tuzla/Istanbul. The industrial district, originally planned for a capacity equivalent to a
wastewater flow rate of 36,000 m3d–1, presently operates with 110 tanneries processing
both cattle hide and sheep skin and generates a daily wastewater flow of around
The SBR unit used in the study is a cylindrical PVC tank of 1 m diameter, with an effec-
tive volume of around 844 l. The unit is equipped with a PLC control sustaining the desired
sequence of anoxic/aerobic phases. Two 30 cm diameter membrane diffusers, connected to
a compressor through a solenoid valve, provide air and mixing for the aerobic phase.
Mixing in the anoxic phase is secured by a combination of mechanical mixer and internal
recirculation using a submersible pump. The unit is fed from the primary settling tank out-
let of the treatment plant serving the industrial district. The plain-settled effluent is first
introduced into a 1m3volume, completely mixed PVC tank serving as a homogenizing unit,
and then fed into the SBR with an adjustable speed dosing pump. The volume of wastewater
feeding is also controlled by means of a level sensor connected to the dosing pump. A 1 inch
pipe operated with a solenoid valve provides the wastewater discharge at the end of the
treatment and settling sequence.
All analyses for conventional characterization were performed as defined in Standard
Methods (APHA, 1995). The soluble (filtered) COD was defined as the filtrate through
Whatman GF/C glass fiber filters with an effective pore size of 1.3 µm, also used in the analy-
sis were volatile suspended solids (VSS) and suspended solids (SS). Oxygen uptake rate
(OUR) measurements were conducted with a Manotherm RA-100 continuous respirometer
with a PC connection. The OUR experiments were used for the assessment of the readily
biodegradable COD, SS, and the evaluation of major kinetic and stoichiometric coefficients,
namely, the maximum heterotrophic growth rate, µH, the endogenous decay coefficient, bH
and the heterotrophic yield coefficient, YH. The respirometric procedure for the assessment of
SSand the other coefficients involved using 1-l aerated batch reactors. The OUR test was con-
ducted with an inhibitor (Formula 2533TM, Hach Company, Loveland Colorado)for the pre-
vention of possible interference induced by simultaneous nitrification. The reactor was
initially fed with the wastewater sample and seeded with biomass previously acclimated to
the sample in a fill-and-draw reactor continuously operated at a constant sludge age of
approximately 10 days. In the experiments, pH was kept in the range of 7.0–8.0, suitable for
biological activity. The readily biodegradable COD, SS, was determined according to the
method defied by Ekama et al. (1986). For the assessment of the maximum heterotrophic
growth rate, µH, OUR reactors were typically run in duplicate, one started at a moderate F/M
ratio of 0.2 to 0.8 g COD(g VSS)–1, and the other started a higher F/M ratio of around 4–5 g
COD(g VSS)–1as recommended by Kappelar and Gujer (1992). The heterotrophic yield, YH,
was evaluated by comparing the OUR and COD profiles obtained on the sample according to
the method proposed by Ubay Çokgör (1997). Determination of the endogenous decay coeffi-
cient, bH, involved removing an activated sludge sample and aerobically digesting it over a
period of several days (Ekama et al., 1986). The hydrolysis rate coefficients, khand KX, were
determined through model calibration of the OUR data for the most appropriate values. The
residual COD components, XIand SIwere computed using the procedure by Germirli et al.
(1993). The slowly biodegradable COD components SHand XHwere calculated from mass
balance. Testing of the experimental data was performed by means of model simulation,
using the AQUASIM computed program developed by Reichert (1994). ASM1 (Henzeet al.,
1987) modified for endogenous decay (Orhon and Artan, 1994) and ASM2 (Henze et al.,
1995) were used for model evaluations.
S. Murat et al.
Results and evaluations
Performance of the SBR for COD and nitrogen removal
The SBR operation for carbon and nitrogen removal was designed for one cycle a day (A
total cycle time, TC= 1 day), where 22 h was devoted to the process phase, TPand the
remaining 2 h for settle, draw and idle phases (TS + TD + TI= 2 h). The anoxic phase, TDN
was selected as 3 h, with continuous wastewater feeding (TF=TDN) and the reactor was aer-
ated for 19 h (TA =19 h), during the remaining portion of the process phase. The system was
ultimately adjusted to a total sludge age, θXof 15 days to ensure complete nitrification. A
fill volume, VFof 281 l was selected, leaving 562 l for the stationary volume, V0and corre-
sponding to V0/VF= 2. Under these operating conditions, the nominal hydraulic retention
time, θhof the system was set to 3 days.
The pilot SBR unit was operated for 6 months, from the beginning of December 2000 to
the end of May 2001. The plain-settled tannery effluent that served as the feeding stream to
the SBR exhibited the typical characteristics of the tannery wastewater with an average
COD of 2,514 mgl–1and a soluble (filtered) COD/total COD ratio of 0.62. Average VSS
and SS concentration were calculated as 463 and 621 mgl–1. The corresponding fixed solids
concentration was 158 mgl–1, an important parameter for the MLSS level that will be sus-
tained in the SBR at steady state. The main feature of the influent was, as expected, the high
nitrogen content, with average total Kjeldahl nitrogen (TKN) concentration of 224 mgl–1,
incorporating 126 mgl–1of NH4-N and the very low total P content, below the minimum
requirements for biological processes. The influent was also characterized by a total
chromium concentration of 52 mgl–1, representing approximately 0.08% of the SS content,
a sulfide concentration of 45 mgl–1and a chloride concentration of 5,760 mgl–1. Results of
the analytical survey conducted during the operation of the SBR are outlined in Table 1. As
shown in the Table, the data are in agreement with the general character of the primary set-
tling effluent of the corresponding full-scale treatment plant (Orhon et al., 1999).
The first 2 months of the SBR operation were basically devoted to system start-up. The
operation was initiated with a biomass seeding from the activated sludge units of the main
treatment plant providing a MLSS level of around 1,500 mgl–1. The unit was operated fully
aerobic and excess sludge withdrawal was adjusted for the necessary biomass build-up in
the reactor. The full aerobic operation was maintained at steady state with the design sludge
age of 15 days for one month, before the introduction of the anoxic phase for nitrogen
removal. After a new adjustment period, the system performance was maintained and
observed at steady state for 50 days, for different wastewater temperatures gradually
increasing from 15 to 23°C. The average characteristics of the SBR performance for COD
and nitrogen removal are outlined in Table 2.
At steady state for θx= 15 days, an average MLVSS concentration of 3,150 mgl–1was
maintained in the reactor. The corresponding MLSS concentration was 4,030 mgl–1. The
S. Murat et al.
Table 1 Influent wastewater characterization
RemarksTotal Soluble TKN NH3-NTotal PSS VSSS–2
Total Cr Cl–1
(mgl–1)(mgl–1)(mgl–1)(mgl–1) (mgl–1) (mgl–1)(mgl–1)(mgl–1)(mgl–1)
This study start-up period
Orhon et al. (1999)
2255 13002141645.9768467 364054007.9
average total COD and soluble COD concentrations in the treated effluent were calculated
as 250 mgl–1and 164 mgl–1respectively, corresponding to an overall COD removal
efficiency of 90%. An activated sludge with good settling characteristics could be
continuously maintained with no bulking and foaming and an average SVI level lower than
90–100 mlg–1. The average effluent quality in terms of SS and VSS concentration were 28
mgl–1and 23 mgl–1respectively, with a VSS/SS ratio of 0.82, slightly higher than that in the
reactor. The COD equivalent of the VSS in the effluent could be calculated as 3.74 mg
particulate COD(mgVSS)–1. This high ratio may be taken as an indication that a significant
portion of the organic particulate matter was small enough to pass through 1.3 µm filters
and be considered as soluble COD.
In terms of nitrogen components, SBR operation with the selected pre-denitrification
mode was observed to be quite efficient, producing an effluent with an NOX-N concen-
tration of 25 mgl–1and an NH4-N concentration of 0.6 mgl–1at 23°C. As expected,
nitrification was quite sensitive to temperature effects but full nitrification could be secured
at temperatures higher than 18°C.
The performance of the SBR was further investigated by measuring the concentra-
tion profiles of relevant parameters, mainly to allow for model calibration and evaluation,
within a selected complete cycle during the steady state operation. Characteristics of this
observation are outlined in Tables 1 and 2. Table 2 also gives performance data related to the
operation of the full-scale plant where the pilot SBR unit was installed and operated. The
results indicate compatible performance of the two systems in terms of COD removal with a
slight effect of improved activated sludge settling at full-scale, but clearly a more efficient N
removal with the SBR unit, as the full-scale unit is arbitrarily operated for this purpose.
Fundamental relationships for the evaluation of system performance
Understanding of the SBR behavior has quite evolved from the level where manipulation of
the operating parameters on a trial and error basis was the only way for the observation of
related performance patterns. Mechanisms for carbon and nutrient removal are now well
described. Process kinetics and stoichiometry can be interpreted through mass balance to
derive fundamental relationships for performance prediction and system design. The relat-
ed conceptual approach and procedure are reported in detail in the literature (Orhon and
Artan, 1994; Tasli et al.,2001; Artan et al., 2001, 2002). They will be used here to illustrate
the prediction potential of the appropriate mass balance expressions, mainly for the inter-
pretation of system performance outlined above. Such an evaluation requires a number of
appropriate kinetic and stoichiometric coefficients. Table 3 lists the coefficients used in
S. Murat et al.
Table 2 Effluent quality in biological treatment
Parameters (mg l–1) Steady state operationComplete cycle Full-scale plant
NH3-N T = 15°C
T = 18°C
T = 23°C3.0
this study. A few were experimentally determined as part of the study, others were taken
from similar experiments on the same wastewater and the rest were adopted as suggested
default values in ASM1 or ASM2.
Biomass in the reactor. A total sludge age of θX= 15 days was set for the SBR operation at
steady state. It corresponds to an effective sludge age, θXEof 13.8 days. The fundamental
parameter defining mass balance for carbon and nitrogen removal is the net heterotrophic
yield, YNH. Using YH= 0.64 g cell COD(gCOD)–1experimentally determined for the tan-
nery wastewater, YNHmay be computed as follows:
110 1213 8
For the observed complete cycle at steady state, the influent biodegradable COD, CS1is cal-
culated as 2000 mgl–1. With the assumption that CS1is totally removed, the following
expression yields the excess sludge per daily volume of wastewater treated, PX/Q:
The basic mass balance between PX/Q and the hydraulic retention time, θhmay be used to
compute the biomass concentration in the reactor:
This value is in good agreement with the observed average MLVSS concentration of
3150 mgl–1associated with the SBR operation at steady state.
Nitrification. The extent of nitrification achieved in the SBR, as in all activated sludge sys-
tems, relies on the selection of an appropriate sludge age for the authotrophic microorgan-
isms, θXA. The following expression defines the basic relationship between the ammonia
nitrogen concentration in the effluent, SNHand θXA. In this expression, the limiting param-
eter is the maximum growth rate for the authotrophs, µA, which should be specifically
S. Murat et al.
Table 3 Model coefficients characterizing plain-settled tannery wastewater
Heterotrophic maximum growth rate
Half saturation coefficient for het. Growth
Autotrophic maximum growth rate (T = 20°C)µAmax
Half saturation coefficient for aut. Growth
Heterotrophic endogenous decay
Autotrophic endogenous decay
Maximum specific hydrolysis rate
Half saturation coefficient for hydrolysis
Heterotrophic yield coefficient
Autotrophic yield coefficient
COD equivalent of biomass
0.05 mg COD (mg cell COD)–1This study
0.64 mg COD (mg cell COD)–1Orhon et al., 1999
0.24mg COD (mg N)–1
1.42 mg cell COD (mg VSS)–1Orhon and Artan,
Orhon et al., 1999
Orhon et al., 2000
mg COD l–1
mg N l–1
Particulate inert fraction of biomass
Soluble inert fraction of biomass
N content of biomass
N content of XI
N content of SI
Anoxic correction factor for growth
Anoxic correction factor for hydrolysis
mg COD (mg cell COD)–1ASM1
mg COD (mg cell COD)–1Orhon et al., 1999
mg N (mg cell COD)–1
mg N (mg COD)–1
mg N (mg COD)–1
Orhon et al., 1999
Gujer et al., 1999
Gujer et al., 1999
E H XE
= 637 mgl VSS
Q TV V
(). (1 0 1 )1
assessed for the wastewater. For the plain-settled tannery wastewater, µAwas experimen-
tally determined as 0.27 d–1 at 20°C, a relatively low level, presumably due to the effect of
severe inhibitory action of components such as chloride and chromium (Orhon et al.,
2000). Under the operating conditions selected for the SBR, θXA= 11.9 days and SNHis
calculated as 1.6 mgl–1. This value matches well with the average effluent SNHof 1.0 mgl–1
characterizing the steady state SBR operation at 20°C and indicates the validity of the
selected coefficients in the above expression for the prediction of nitrification perform-
ance. The same equation yields SNH=0.6 mgl–1 for 23°C,using the temperature coefficient,
θ of 1.127 associated with the tannery effluents (Orhon et al., 2000), exactly coinciding
with the observed average level at the same temperature. Similarly, SNH= 5.1 mgl–1is
predicted for 18°C, slightly higher than the observed average value, presumably due to
a higher efficiency of the cyclic/transient SBR operation approximating plug flow con-
ditions. Calculations indicate 15°C as the lower temperature limit for the initiation of
nitrification for tannery wastewater.
Nitrogen balance and denitrification. Assessment of the fate of nitrogen compounds
involves setting the balance between the four major parameters, namely, the nitrogen con-
centration removed as part of the excess sludge, NX; the nitrogen concentration oxidized to
nitrate (and nitrite), NOX; the denitrification potential, NDPand the available nitrate, NA.
The basic stoichiometry enables the calculation of NX, in terms of the biodegradable COD
consumed in the reactor and the net heterotrophic yield:
NX=iNBMYNHCS1=0.085 ×0.32 ×2000 =55 mgl–1 N
NOX may then be calculated from simple mass balance:
NOX=CTKN1– iNXI XI1– iNSISI1– SNH– NX=167 – 10.6 – 0.6 –1.6 – 55 =99 mgl–1N
NDPreflects the oxidized (nitrate) nitrogen concentration that may be potentially removed
by denitrification during the anoxic phase. It may be conveniently calculated by means of
the following mass balance:
The performance of the SBR for nitrate removal depends upon the balance between NDP
and NA, which defines the concentration of oxidized nitrogen introduced or kept in the
anoxic phase for denitrification. As an inherent property of SBR, NAis basically the nitrate
remaining in volume V0at the end of the previous cycle and must be equal to NOX–SNO. If
NDP >NAas is clearly the case for the SBR operation in this study, NAbecomes the limiting
parameter for nitrate removal and the effluent nitrate concentration will essentially be a
function of NOXand the V0/VFratio:
=99/(1 +2) =33 mgl–1
In fact, NA=NOX– SNO=66 mgl–1and NA <NDP.
The above calculations clearly indicate that mass balance based on process stoichiometry
offers a reliable tool for the prediction of nitrogen removal performance, as it closely
S. Murat et al.
220 8 1
220 8 1
approximates the effluent nitrate concentration of 27 mgl–1observed at the completion of
the cycle. The difference may be attributed to the uncertainty introduced by the selected
values for certain coefficients such as fX, iNBM, iNXI, etc. It may also be due to a slight mag-
nesium ammonium phosphate (MAP) settling, under favorable pH conditions, utilizing a
portion of the excess P added for compensating the nutritional deficiency of the tannery
wastewater for biological treatment (Tünay et al., 1997).
Model evaluation of the cyclic operation. Observation of the concentration profiles
throughout a representative cycle is a powerful tool for the understanding of SBR behavior
under corresponding conditions. When properly interpreted, it may be used to indicate the
required operation adjustments for optimizing system operation. This approach is often
adopted for SBR treatment of different wastewaters, including tannery wastewater (Tasli et
al., 1999; Carucci et al., 1999). In this study, NH3-N and NOX-N concentration profiles
measured within a selected complete cycle during the steady state operation of the SBR are
given in Figure 1. The figure shows that NOX-N is rapidly consumed to depletion before the
end of the anoxic phase, as NDPis markedly higher than NAprovided by the selected V0/VF
ratio. It gradually builds up with the nitrification process in the aerobic phase to its final
level of 27 mgl–1for this cycle. The lag period in the initial part of the aerobic phase with
stagnant NOX-N levels should be attributed to observed low DO concentrations of
0.1–0.3 mgl–1 in the reactor during the anoxic/aerobic phase transition. Conversely, NH3-N
concentration increases during the entire anoxic fill period to be gradually oxidized to
0.6 mgl–1before the end of the aerobic phase. Review of the NH3-N and NOX-N profiles in
the framework of process stoichiometry indicates that (i) the anoxic phase is clearly over-
designed with an NDP excess, and (ii) the N removal performance of the system may be
improved simply by increasing NAto a level more compatible with NDP. A new operation
scheme was then designed, with two cycles a day (TC= 12 h) and a V0/VFratio of 5, to be
implemented for improved N removal. The results of this operation will be reported else-
Model evaluation of the experimental data was performed using the appropriate coeffi-
cients listed in Table 3 and the COD fractions experimentally determined for the tannery
influent at the beginning of the cycle, as given in Table 4. It should be mentioned that COD
fractionation assessed in this study reflects a character quite typical for the tannery waste-
water aside from the low soluble inert COD content mainly responsible for the low soluble
COD concentration in the process effluent. This observation is presumably due to the sea-
sonal property of leather processing in the district. Model simulation yields as shown in
Figure 1, a good fit with the experimental NH3-N and NOX-N profiles, providing further
proof for the validity of kinetic information used in the evaluation. Figure 2 illustrates an
S. Murat et al.
SNOX and SNH4 (mgl-1)
Figure 1 Modelling of NH3-N and NOX-N profiles during a complete cycle
equally acceptable model validation for the MLVSS level characterizing the selected cyclic
operation. Model evaluation of the soluble COD profile during the cycle is not however
very effective as shown in Figure 3. This is understandable in view of the fact that SBR is
also observed to achieve limited enhanced biological phosphorus removal, (EBPR), at the
expense of external P addition, after the depletion of NOX-N in the non-aerated phase. The
corresponding COD demand is not accounted for in ASM1 used in modeling.
The findings of the study may be reviewed to conclude that: (i) SBR is well suited to
tannery wastewater for effective COD and N removal. It lowers soluble COD to a level
essentially consisting of initial soluble inert COD and additional residual COD generated
as metabolic products. It offers the flexibility of adjusting the degree of N removal by
appropriate manipulation of the operating parameters. Compared to continuous-flow acti-
vated sludge, the V0/VFratio is the essential additional parameter for this purpose. (ii) Mass
balance based on process stoichiometry is an essential complement of the experimental
evaluation of process performance. It requires a minimum set of information, basically
limited to YH. In this study, mass balance provided an accurate and satisfactory confirma-
tion of system performance for nutrient removal. (iii) Experimental assessment of major
process coefficients is required for the modeling of the cyclic SBR performance. The study
has defined these coefficients for tannery wastewater and tested their validity by means of
model calibration of the experimental data.
This study was conducted as part of the sponsored research activities of The Environmental
Biotechnology Center of the Scientific and Research Council of Turkey. It was also sup-
ported by The Research and Development Fund of Istanbul Technical University.
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Table 4 COD Fractionation of plain settled tannery wastewaters
CT (mg l–1)
CS (mg l–1)
SH (mg l–1)
SI (mg l–1)
XI (mg l–1)
Orhon et al., 1999
Figure 2 Modelling of MLVSS profile
Figure 3 Modelling of the soluble COD profile
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