1. Biodegradability and Denitrification
Potential of Settleable Chemical Oxygen
Demand in Domestic Wastewater
¨ ¨¸ ¨ _
Didem Okutman Tas1*, Ozlem Karahan1, Guclu Insel1, Suleyman Ovez1,
¨ ¨
2 3
Derin Orhon , Henri Spanjers
ABSTRACT: The effect of settling on mass balance and biodegradation et al. (1986), chemical oxygen demand (COD) has been adopted as
characteristics of domestic wastewater and on denitrification potential was the main parameter to quantify organic carbon. The biodegradable
studied primarily using model calibration and evaluation of oxygen COD conveniently establishes the electron balance between
uptake rate profiles. Raw domestic wastewater was settled for a period of substrate used, biomass generated, and electron acceptor (dissolved
30 minutes and a period of 2 hours to assess the effect of primary settling on
oxygen in aerobic systems) consumed. Substantial research has
wastewater characterization and composition. Mass balances in the system
were made to evaluate the effect of primary settling on major parameters.
been conducted to identify and assess the biodegradation character-
Primary settling of the selected raw wastewater for 2 hours resulted in the istics of different COD fractions in domestic wastewater (Henze,
removal of 32% chemical oxygen demand (COD), 9% total Kjeldahl 1992; Orhon et al., 1997; Sollfrank and Gujer, 1991) and different
nitrogen, 9% total phosphorus, and 47% total suspended solids. Respiro- industrial wastewaters (Kabdasli et al., 1993; Orhon et al., 1995;
metric analysis identified COD removed by settling as a new COD fraction, Orhon, Tasli, and Sozen, 1999). Respirometry has been a significant
namely settleable slowly biodegradable COD (XSS), characterized by a asset for the experimental assessment of these fractions (Spanjers
hydrolysis rate of 1.0 day21 and a hydrolysis half-saturation coefficient of and Vanrolleghem, 1995), which are now incorporated as model
0.08. A model simulation to test the fate and availability of suspended (XS)
components to major activated sludge models (Gujer et al., 2000;
and settleable (XSS) COD fractions as carbon sources for denitrification
showed that both particulate COD components were effectively removed
Henze et al., 1987, 1995).
aerobically at sludge ages higher than 1.5 to 2.0 days. Under anoxic con- Particle size is an integral component of COD fractionation.
ditions, the biodegradation of both COD fractions was reduced, especially Wentzel et al. (1999) stated that the biodegradation potential defined
below an anoxic sludge retention time of 3.0 days. Consequently, modeling for different COD fractions would have a correlation with physical
results revealed that the settleable COD removed by primary settling could categorization, in terms of particle size and physical state in waste-
represent up to approximately 40% of the total denitrification potential of the water. In wastewater characterization, one particle size (0.45-lm
system, depending on the specific configuration selected for the nitrogen membrane or 1.2-lm glass-fiber filter size) is commonly used to
removal process. This way, the results showed the significant effect of roughly differentiate soluble and particulate ranges. Soluble inert
primary settling on denitrification, indicating that the settleable COD fraction
COD (SI), readily biodegradable COD (SS), and rapidly hydrolyz-
could contribute an additional carbon source in systems where the denitri-
fication potential associated with the influent becomes rate-limiting for the able COD (SH) are associated with the soluble range, while slowly
denitrification efficiency. Water Environ. Res., 81, 715 (2009). biodegradable COD (XS) and particulate inert COD (XI) are evalu-
ated within the particulate range. Recently, a settleable COD frac-
KEYWORDS: biodegradation, chemical oxygen demand fractions, de-
tion (XSS) with a slower biodegradation rate was defined within the
nitrification potential, domestic wastewater, primary settling, respirometric
modeling, settleable chemical oxygen demand. particulate COD for domestic wastewater (Okutman et al., 2001). In
a study involving detailed particle size analysis, Dulekgurgen et al.
doi:10.2175/106143009X425942
(2006) reported that, for domestic wastewater, most of the COD was
in the size ranges above 0.45 lm, and only a relatively small part
was in the soluble range.
Introduction A detailed characterization is a prerequisite for understanding and
Assessment of biodegradation characteristics of different organic interpreting the fate of pollutants in wastewaters. It should cover,
fractions in wastewaters should be considered as one of the major aside from COD and its significant fractions, all relevant parameters
research milestones in environmental science and technology. and especially particulate solids—total suspended solids (TSS) and
Because of the pioneering work of Dold et al. (1980) and Ekama volatile suspended solids (VSS)—in a way to establish basic mass
balances defining useful ratios, such as COD/VSS, COD/N/P, and
1
Department of Environmental Sciences and Engineering, Istanbul VSS/TSS, for treatment system design and operation (Orhon et al.,
Technical University, Istanbul, Turkey. 1997; Rossle and Pretorius, 2001). From a practical standpoint,
¨
2
Turkish Academy of Sciences, Ankara, Turkey. accurate assessment of influent composition is essential for the
3
Lettinga Associates Foundation, Wageningen, Netherlands. design and operation of biological treatment units. In this respect,
* Department of Environmental Sciences and Engineering, Istanbul Technical
the removal rate in plain settling sets is another important factor
University, 80626 Ayazaga, Maslak, Istanbul, Turkey; e-mail: within the particulate range, to estimate the fate of different COD
okutmand@itu.edu.tr. fractions and other parameters before biological treatment.
July 2009 715

2. Tas et al.
Table 1—Conventional characterization of domestic wastewater.
This study Domestic wastewater in _
Istanbul (Orhon et al., 1997)
Atakoya
¨ Baltalimania ¨
Kadikoy K.Cekmece
¸
Parameter Mean 70% Range Mean 70% Range Mean Range Mean Range
COD (mg/L) 406 445 295 to 535 353 368 314 to 408 450 220 to 775 400 345 to 480
TKN (mg/L) 41 43 36 to 47 35 38 30 to 40 49 22 to 73 42 38.6 to 46.7
NH3-N (mg/L) 27 28.8 19 to 34 20.4 23 11 to 26.0 30.5 25 to 39 24.78 22.4 to 30.4
Total phosphorus (mg/L) 8.3 9.1 6.0 to 11.6 7.1 8.4 4.4 to 10.2 8.1 5.0 to 15 7.4 6.1 to 9.6
TSS (mg/L)b 190 210 122 to 247 184 195 151 to 262 310 140 to 930 200 165 to 270
VSS (mg/L)b 178 198 118 to 227 148 160 126 to 165 210 130 to 395 103 100 to 105
Total dissolved solids (mg/L) 2874 2920 2600 to 3300 4486 4510 4200 to 4630 474 425 to 495 616 520 to 680
pH 7.6 7.7 7.2 to 7.9 7.4 7.5 7.1 to 7.7 7.2 — 7.68 7.6 to 7.7
a
Summer season, composite samples.
b
1.2 lm.
Biodegradation characteristics of different COD fractions also Materials and Methods
provide essential information for activated sludge systems designed Survey Site. The study was conducted as part of a comprehen-
for biological nutrient removal. In fact, in these systems, an impor- sive survey on the treatability-oriented characterization of domestic
tant role is attributed to organic carbon. Basically, denitrification wastewater within the metropolitan area of Istanbul, Turkey, now
relies on the same principles as organic carbon removal under housing more than 12 million residents. The Istanbul Metropolitan
aerobic conditions, except for the final different electron acceptors Area is located on the northern coast of the Marmara Sea and lies on
utilized. However, the two systems have totally opposite objectives both sides of the Bosphorus strait, connecting the Black Sea to the
when evaluated in terms of the corresponding treatment processes; Marmara Sea. The area has been the subject of similar studies as the
in conventional aerobic activated sludge systems, in which the sole major polluting source in the Marmara basin (Gorgun et al., 1996;
purpose is the organic carbon removal, the typical approach is to Orhon et al., 1997; Orhon, Sozen, and Ubay, 1994; Orhon, Uslu,
reduce the organic load by means of primary settling, which Meric, Salihoglu, and Filibeli, 1994). The study was mainly carried
removes, aside from suspended solids, the settleable fraction of the out at the Atakoy treatment plant, serving a population equivalent of
¨
influent COD. In denitrification, which occurs under anoxic condi- 45 000 residents, now under extension for nutrient removal, where
tions, the main objective is to remove the final electron acceptor— a statistically significant number of composite samples (i.e., more
nitrate—and system design should ensure that a stoichiometrically than 30 samples) were collected from the influent of the plant. The
sufficient amount of biodegradable COD is present for the reduction composite samples were collected in summer (sampling from 8 a.m.
and removal of nitrate in the anoxic reactor. In this respect, organic until 5 p.m. each day), during dry weather, for a period of 3 months
carbon assumes a different function as an essential ingredient for (July to September). The sampling sequence was arranged to
denitrification. characterize all days of the week. In addition to the summer
The function of biodegradable COD is best evaluated in terms characterization, five grab samples also were taken and analyzed
of the denitrification potential (NDP), a parameter that reflects in December, to compare the summer and winter wastewater com-
the nitrogen equivalent that may potentially consume nitrate positions. The study also included evaluation of wastewater samples
under anoxic conditions. Basic process stoichiometry and process collected during the same summer period from the Baltalimani
modeling are commonly used for the assessment of NDP (Artan treatment plant, another significant wastewater discharge station
et al., 2002; Ekama and Marais, 1984; Sozen et al., 2002). The within the metropolitan area along Bosphorus.
Analytical Measurements. All analyses were performed
efficiency of denitrification greatly depends on the balance between
according to Standard Methods (APHA et al., 1998). As prescribed
NDP and the extent of available nitrate (NA) introduced to the anoxic
in current activated sludge modeling, COD was used to characterize
reactor volume. The merit of the removing settleable COD by
wastewater organic matter. The soluble and particulate organic
primary settling requires serious consideration and reevaluation,
matter was differentiated by filtration, using 0.45-lm cellulose
especially in cases where NDP becomes rate-limiting and additional acetate membrane filters. The COD measurements were performed
organic carbon becomes a significant asset. Obviously, the magni- as described in ISO6060 (International Organization for Standard-
tude of the settleable COD fraction alone is not enough for this ization, 1986). Whatman (Kent, England) GF/C glass-fiber filters
evaluation, which basically depends on the biodegradation charac- (1.2 lm) were used for TSS and VSS measurements. In addition,
teristics of all COD fractions involved. 0.45-lm cellulose acetate membrane filters were used to quantify
In this context, the main objective of the study was to establish TSS and VSS in domestic wastewater, to compare the results with
a conceptual basis for the assessment of the biodegradability and the soluble parameters that are defined as filtrate from 0.45-lm
denitrification potential of settleable COD. The conceptual approach cellulose acetate membrane filters.
was illustrated using data derived from the domestic wastewater at Respirometric Modeling and Performance Simulation.
Atakoy, Istanbul, Turkey. Detailed characterization, emphasizing
¨ Oxygen uptake rate (OUR) measurements were conducted with a
the effect of primary settling on COD fractionation, and model Manotherm RA-1000 continuous respirometer (Nazareth, Belgium)
simulation were also carried out as the necessary tools to conduct with a personal computer connection (Orhon and Okutman, 2003).
this examination. Three sets of composite samples collected from the influent of
716 Water Environment Research, Volume 81, Number 7

3. Tas et al.
Table 1—(Extended)
Domestic wastewater in _
Istanbul (Orhon et al., 1997) Domestic wastewater in Europe (Pons et al., 2002)
Baltalimani Yenikapi France Austria Netherlands Sweden Norway Finland Germany
Parameter Mean Range Mean Range Mean Mean Mean Mean Mean Mean Mean
COD (mg/L) 340 265 to 645 680 280 to 1480 634 526 450 477 233 559 548
TKN (mg/L) 35 23.9 to 57 66 27 to 92 52 44 42 33.1 22 43.8 59
NH3-N (mg/L) 19.9 10 to 26.3 37.74 24 to 48.8 — — — — — — —
Total phosphorus (mg/L) 6.8 5 to 8.63 7 3.6 to 13 9.3 7.1 6.7 6.14 3 7.47 8
TSS (mg/L)b 140 85 to 318 480 110 to 820 302 — 237 243 143 378 208
VSS (mg/L)b 125 120 to 135 65 65 to 69 — — — — — — —
Total dissolved solids (mg/L) 435 335 to 537 — — — — — — — — —
pH 7.4 7.2 to 7.5 7.24 7.1 to 7.3 — — — — — — —
the Atakoy treatment plant were used for the assessment of COD
¨ YNHD 5 net anoxic yield coefficient for heterotrophs (mgcell-
fractions and evaluation of kinetic coefficients, as previously COD/mgCOD),
described (Orhon et al., 2002). A 30-L composite sample was bHD 5 anoxic endogenous decay rate for heterotrophs (day21),
collected from the influent of the treatment plant and subjected to and
2 hours of gravity settling in a cylindrical reactor to simulate the hX 5 total sludge age (days).
quality of fresh settled wastewater. Approximately 2 L of the settled
The anoxic endogenous decay rate (bHD) is calculated by reducing
wastewater fraction was withdrawn from the bottom of the reactor
the aerobic endogenous decay coefficient with the endogenous
for the OUR measurements. The experiments were conducted at
decay correction factor (gE), as follows:
room temperature (228C). The pH was kept in the range 7.0 to 8.0,
which is suitable for biological activity. Aeration was supplied bHD ¼ bH ÁgE ð2Þ
continuously during OUR measurements to maintain a sufficient
dissolved oxygen concentration. The OUR data were collected
online with a sampling frequency per minute. Experimental assess-
ment of the kinetic coefficients was performed by model calibration Results and Discussion
Conventional Characterization. Conventional characteriza-
using the experimental OUR data. The COD fractionation and
tion results of the domestic wastewater investigated in this study
soluble and particulate COD components of the wastewater were
during the summer season are outlined in Table 1. The table in-
determined according to the methods previously described (Ekama
cludes previously reported literature data based on surveys con-
et al., 1986; Insel et al., 2003; Orhon and Okutman, 2003; Orhon
ducted on the domestic wastewater from important wastewater
et al., 2002).
discharge stations of Istanbul (Orhon et al., 1997) and representative
A simulation study was performed to illustrate the biodegradation
domestic wastewater characteristics for selected countries in Europe
characteristics of particulate slowly biodegradable COD (XS, XSS)
(Pons et al., 2002). The results indicated that the average com-
under different sludge ages (hX). The steady-state simulations were
position of domestic wastewater at the Atakoy treatment plant,
¨
performed using a conventional activated sludge system having
which served as the focal point of the study, could be expressed as
a single aerobic continuous stirred-tank reactor (CSTR) and a final
406 mg/L total COD, 190 mg/L TSS, 178 mg/L VSS, 41 mg/L total
clarifier. A simulation study was conducted using average waste-
Kjeldahl nitrogen, 27 mg/L ammonia nitrogen (NH3-N), and 8.3
water characterization and influent COD fractionation. Sludge age
mg/L total phosphorus. The average pH of the domestic wastewater
(hX) and hydraulic retention time (hH) were changed in parallel to
was 7.6. Slightly lower values were observed for all the parameters
keep the mixed liquor suspended solids (MLSS) concentration at
in the Baltalimani treatment plant, the other station considered in
approximately 4.0 kgSS/m3 in all simulations. The final clarifier
this study (Table 1). Both stations represent residential areas
was assigned as a point settler, in which the mixed liquor is physi-
generating municipal wastewater with no significant effect from
cally separated from the clarified effluent. The solids separation
industrial activities. The mean values obtained for the Atakoy and
¨
efficiency of the final clarifier was assumed to be 100%. The return
Baltalimani wastewaters represent typical domestic wastewater
activated sludge rate was adjusted to unity. The AQUASIM prog- quality, which has not changed significantly over time. The stronger
ram developed by Reichert et al. (1998) was used in all simulations. character of the Yenikapi wastewater reported in the previous study
The simulation results were processed to obtain the contributions indicates the significant effect of industrial discharges on wastewa-
of different COD fractions to the denitrification potential (NDP) of ter quality. The appreciable quality spectrum observed among the
the system at different operating conditions and different anoxic different wastewater characteristics given in Table 1 underlines the
volume ratios (VD/V). The following simplified equation was used specific nature of wastewater quality affected by local conditions.
to calculate the denitrification potential of both total and settleable The statistical distribution of major parameters for the Atakoy ¨
COD: treatment plant influent is plotted in Figure 1. The plots exhibit
ÁCS a regular trend for all parameters with a range of 5 to 10% deviation
NDP ¼ ð1 À YNHD Þ and YNHD ¼ YHD =ð1 þ bHD ÁhX Þ ð1Þ
2:86 between mean and 70th-percentile values. The 70th-percentile
values, representing the upper threshold level of 70% of the samples
Where
analyzed, are also indicated in Table 1. The 70th-percentile values
CS 5 concentration of biodegradable COD (mg/L), are generally recommended and adopted for design, as they could
July 2009 717

4. Tas et al.
¨
Figure 1—Statistical distribution of characteristic parameters in Atakoy domestic wastewater: (a) COD, (b) suspended
solids, (c) total Kjeldahl nitrogen, and (d) total phosphorus (¤ 5 total, 5 30 minutes settled, Á 5 2 hours settled, and
u 5 soluble).
provide more meaningful and useful information compared with total COD or 43% of the particulate COD. After settling, 188 mg/L
mean values (Kayser, 1989; Orhon et al., 1998). Therefore, in of suspended or colloidal COD remained in the effluent, with
this study, evaluations for different parameters presented in the a different composition of 38% soluble COD and 62% particulate
following sections are based on the 70th-percentile values. COD. Thirty minutes of settling was roughly half efficient, only
Comparison of the wastewater composition in the summer and providing 17% COD removal. The 0.45 to 1.2 lm size range was
winter seasons is shown in the following results, in terms of the observed to contain 7% of the COD content of domestic waste-
suspended solids and COD concentrations that can be expected to water, which is a statistically significant amount that should be
change as a function of the seasonal activities. Because a significant considered for the evaluation of wastewater characterization.
difference for nitrogen and phosphorus parameters was not expected Similar results also were observed in the winter season.
during summer and winter seasons under dry-weather conditions, The soluble fraction of the total nitrogen was assessed as 78%. A
these two parameters were not investigated in the winter season. similar fraction of 80% was calculated for total phosphorus. Two
Effect of Settling and Filtration on Wastewater Characteristics. hours of settling provided only 9% nitrogen removal, which cor-
The nature and size distribution of significant conventional parameters responded to 42% of the initial particulate nitrogen. Similarly, 9%
have been studied for the following four indices associated with of the total phosphorus and 44% of the particulate phosphorus was
different size thresholds: (1) 30 minutes of settling, (2) 2 hours of removed. The removal efficiencies decreased to 5% for nitrogen
settling, (3) 1.2-lm filtration, and (4) 0.45-lm filtration. The first two and 7% for phosphorus when the settling time was reduced to
have practical significance, as they illustrate the effect of primary 30 minutes. The specific nature of wastewater characteristics was
settling typically implemented before biological treatment of domestic confirmed, as the results of this study were only partially supported
wastewater. Filtration through 0.45-lm filters is commonly used to by similar observations reported in the literature. Odegaard (1997)
differentiate soluble and particulate fractions and roughly approx- indicated that the filtered fraction (1 lm) in raw wastewater samples
imates the effect of chemical treatment (Henze et al., 2000). Filtration was typically 20 to 30% of the total COD, 30 to 40% of the total
through 1.2-lm filters is also a common analytical technique for phosphorus, and 75 to 85% of the total nitrogen. Tiehm et al. (1999)
wastewater characterization. stated that, in raw wastewater and primary effluent, 45% of
The results of settling and filtration experiments conducted on the COD and 35 to 80% of the phosphorus was associated with
all samples and displayed in Table 2 indicated that 74 to 78% of suspended solids.
the total COD could be considered of particulate nature based on Similarly, in the summer samples, based on quantification with
a 0.45-lm size threshold for both the summer and winter seasons. membrane filters (0.45 lm), settling periods of 30 minutes and of
In the summer samples, settling for 2 hours removed 32% of the 2 hours resulted in the removal of 33 and 47% TSS and 36 and 49%
718 Water Environment Research, Volume 81, Number 7

5. Tas et al.
Table 2—Significant parameters in domestic wastewater Table 3—Removal ratios of the significant parameters in
after settling and filtration (70% statistical values).a the primary settling (2 hours).
Supernatant Percent removal
after settling Filtrate
Total
Parameters (mg/L) Total 30 minutes 2 hours 1.2 lm 0.45 lm COD TKN phosphorus TSS
(%) (%) (%) (%)
Organics and nutrients
Summer seasonb Atakoy wastewaters
¨ 32 9 9 47*
(this study)
COD 445 370 305 149 117
South African wastewaters 40 15 to 20 15 to 20 60
Total nitrogen 43 41 39 NM 33.5
(Rossle and Pretorius, 2001)
¨
Total phosphorus 9.1 8.5 8.3 NM 7.3
Riva/Istanbul wastewater 26 7 15 —
Winter seasonc (Orhon, Sozen, and Ubay, 1994)
COD 510 350 300 150 115 Kadikoy/Istanbul wastewater 33 9 25 63
(Orhon et al., 1997)
TSS and VSS
ATV131 (2000) 33 9 11 64
Summer seasonb (1.2 lm)
TSS 210 144 114 NA NA * 0.45 lm.
VSS 198 142 110 NA NA
Summer seasonb (0.45 lm)
TSS 230 153 123 NA NA rates for South African wastewater. These results merit further
VSS 200 128 103 NA NA attention in terms of the settling properties of domestic wastewater
Winter seasonc (1.2 lm) as it relates to the removal of a significant portion of the available
TSS 195 168 129 NA NA organic carbon source.
VSS 148 109 87 NA NA Assessment of Characteristic Parameters. Interpretation of
Winter seasonc (0.45 lm) conventional characterization, in terms of significant ratios of sel-
TSS 207 152 132 NA NA ected parameters, such as N/COD, P/COD, and VSS/COD, is quite
VSS 160 131 102 NA NA useful, as it could be used for prediction of the biodegradability of
a
domestic wastewater. As reported in Table 4, the N/COD ratio
NA 5 not applicable, NM 5 not measured. for Atakoy was calculated to be in the range 0.07 to 0.15 mgN/
¨
b
Composite samples.
c mgCOD, with an average value of 0.11 mgN/mgCOD. The cor-
Grab samples.
responding value for Baltalimani was 0.10 mgN/mgCOD. These
values coincide with the average N/COD ratios of 0.10 to 0.11
VSS, respectively. Although measured concentrations were lower, mgN/mgCOD previously observed for Istanbul wastewater (Orhon
similar removal ratios were obtained based on quantification with et al., 1997; Orhon, Sozen, and Ubay, 1994). They also are con-
1.2-lm glass-fiber filters. In the winter season, although TSS con- sistent with the range 0.087 to 0.115 mgN/mgCOD reported for
centrations were very similar to those measured in the summer, VSS different domestic wastewaters in Europe (Pons et al., 2002). The
concentrations were relatively lower. Based on a 0.45-lm analytical N/COD ratio is an important index for predicting the efficiency of
quantification, settling periods of 30 minutes and of 2 hours resulted denitrification in biological treatment. The overall mean N/COD
in the removal of 27 and 36% TSS and 18 and 36% VSS, respec- ratios measured in this study represent the limit value of 0.1 mgN/
tively. Thus, higher removal efficiencies for suspended solids were mgCOD, below which, a single sludge predenitrification system has
obtained in the summer samples as opposed to the winter samples. the potential of providing high nitrogen removal efficiency (Orhon
The difference in removal efficiencies can be attributed to different and Artan, 1994). Pitman (1991) and Randall et al. (1992) reported
characteristics of particulate pollutants during the wet season; to that, if the N/COD ratio is higher than 0.11 and the volatile fatty
higher temperatures in the summer season, which may decrease the
fraction of XSS in the sewer because of the faster hydrolysis; and to
variation in the sampling method, which was composite sampling in
the summer season and grab sampling in the winter season.
In general, filtration through 0.45-lm filters is used to differen-
tiate soluble and particulate fractions. Although the use of 1.2-lm
filters is a common analytical technique, especially for the quantifi-
cation of suspended solids, it could be more appropriate to use
0.45-lm filters for all quantifications, to compare all the parameters
on the same basis.
The effect of primary settling, approximated by 2-hour settling
experiments, on the removal of significant parameters, is outlined in
Table 3, with similar results in the literature. A mass balance for
these parameters associated with primary settling is schematically
illustrated in Figure 2. The removal efficiencies are generally com-
patible with the results of similar studies in the Istanbul area, except Figure 2—The mean values of mass balance for COD,
for phosphorus (Orhon, Sozen, and Ubay 1994; Orhon et al., 1997). nutrients, and suspended and volatile suspended solids
Rossle and Pretorius (2001) have reported slightly higher removal
¨ in the wastewater after primary settling (*0.45 lm).
July 2009 719

6. Tas et al.
Table 4—Effect of settling and filtration on significant ratios for domestic wastewater.
¨
Atakoy Baltalimani
Mean Standard deviation Range Mean Standard deviation Range
N/COD Total 0.11 0.02 0.07 to 0.15 0.10 0.01 0.09 to 0.11
30 minutes settled 0.13 0.02 0.1 to 0.2 0.12 0.01 0.12 to 0.13
2 hours settled 0.14 0.02 0.1 to 0.21 0.12 0.02 0.1 to 0.15
Soluble (0.45 lm) 0.31 0.05 0.2 to 0.41 0.17 0.02 0.15 to 0.2
P/COD Total 0.021 0.004 0.017 to 0.028 0.020 0.006 0.013 to 0.027
30 minutes settled 0.026 0.005 0.013 to 0.037 0.025 0.006 0.017 to 0.032
2 hours settled 0.029 0.004 0.021 to 0.037 0.027 0.006 0.022 to 0.033
Soluble (0.45 lm) 0.067 0.018 0.027 to 0.089 0.012 0.036 0.015 to 0.043
VSS/TSS Total 0.93 0.09 0.57 to 0.98 0.84 0.19 0.51 to 0.96
30 minutes settled 0.97 0.01 0.94 to 0.99 0.86 0.16 0.58 to 0.96
2 hours settled 0.97 0.02 0.92 to 0.99 0.89 0.16 0.61 to 0.96
acid (VFA) content is low (,50 mg/L), an external carbon source commonly differentiated using filters with pore sizes ranging from
should be used or prefermentation should be implemented. As ex- 0.45 to 1.8 lm (Henze et al., 2000). In this study, only a 0.45-lm
pected, settling increased this ratio, as a result of significantly lower filter was used to quantify COD during the summer season,
nitrogen removal compared with COD. After 2 hours of settling, the whereas, during the winter season, both 0.45- and 1.2-lm pore-
mean N/COD ratio was measured as 0.14 mgN/mgCOD for Atakoy ¨ sized filters were used for particulate COD assessment. As shown in
and 0.12 mgN/mgCOD for Baltalimani wastewater, which are both Table 5, iX was calculated as 0.66 mgVSS/mgCOD for both raw
above the limit for efficient predenitrification. and settled wastewater during the summer survey (0.45 lm). In the
The mean value of the P/COD ratio for both the Atakoy and ¨ winter season, this ratio was found to increase to 0.79 mgVSS/
Baltalimani wastewater was 0.02 mgP/mgCOD, which is also con- mgCOD for the same type of filter and to 0.70 for a 1.2-lm filter. In
sistent with the range 0.014 to 0.019 given for European wastewaters the winter season, particulate VSS/COD ratio ranged from 0.73 to
(Pons et al., 2002). Settling for 2 hours resulted in an increase 0.79 for the same type of filters (0.45 lm). According to the VSS
of approximately 50%. Similarly, an increase has been reported in and COD measurements with 1.2-lm filters, the particulate VSS/
the P/COD ratio from the range 0.015 to 0.025 to the range 0.02 to COD ratios were relatively small compared with the measurements
0.03 in the settled sludge (Water Research Commission, 1984). with 0.45-lm filters. Although no difference was observed in the iX
The VSS/SS ratio was 0.93 mgVSS/mgSS for Atakoy and 0.84 ¨ level in summer, a significant decrease was observed as a function
mgVSS/mgSS for Baltalimani, as summarized in Table 4, indicating of settling time in the winter samples. These values compare well
that 7 to 16% of the suspended solids were of an inorganic nature. with the typical value of 0.75 mgVSS/mgCOD suggested for settled
As expected, settling was more effective in removing the inorganic wastewater in Activated Sludge Model No. 3 (ASM3) (Gujer et al.,
suspended solids, as a result of higher settling velocities associated 2000). The particulate N/COD ratio, iXN, was similarly calculated
with this fraction. as 0.03 mgN/mgCOD for both raw and settled wastewater for the
The ratios of particulate fractions of significant parameters are summer samples, a level consistent with the default value of 0.04
also quite important for system design and process modeling (Gujer mgN/mgCOD of ASM3, after primary settling (Gujer et al., 2000).
et al., 2000). Table 5 outlines the values of two ratios, namely the The inorganic portion of the suspended solids, called fixed solids,
ratio of VSS to particulate COD, iX, and of particulate nitrogen to XFS, also is an important parameter for the accurate estimation of
particulate COD, iXN, for the Atakoy wastewater. In conventional
¨ biomass under different operating conditions. The effect of settling
analysis, soluble and particulate components of the wastewater are on XFS and on the fixed solids fraction, fXFS, for the Atakoy ¨
¨
Table 5—Particulate nitrogen and COD fractions as a function of settling time in the Atakoy domestic wastewater
treatment plant.
ix (g-VSS/g-COD) ixN (g-N/g-COD)
Total 30 minutes settled 2 hours settled Total 30 minutes settled 2 hours settled
Summer season
0.45 lma 0.66 0.66 0.03 0.03 0.03
Winter season
0.45 lmb 0.79 0.76 0.73 — — —
1.2 lmb 0.70 0.68 0.63 — — —
Literature
ASM3 (Gujer et al., 2000) 0.75 0.04
a
Composite samples.
b
Grab samples.
720 Water Environment Research, Volume 81, Number 7

7. Tas et al.
¨
Table 6—The ratios of fixed solids in the Atakoy domestic wastewater treatment plant as a function of settling time,
where XFS15 [mg TSS/L - mg VSS/L] and fXFS5 [mg TSS/L - mg VSS/L]/[mg TSS/L].
Raw wastewater 30 minutes settled wastewater 2 hours settled wastewater
This study XFS1 fXFS XFS1 fXFS XFS1 fXFS
Summer season
0.45 lma 15 0.09 6 0.03 5 0.02
1.2 lma 11 0.09 4 0.03 3 0.02
Winter season
0.45 lmb 52 0.16 25 0.14 20 0.14
1.2 lmb 36 0.16 20 0.15 11 0.12
Literature
Gujer and Kayser (1998) (1.2 lm) 0.2 to 0.3
Orhon et al. (1997) (1.2 lm) 100 0.3 — — 17 0.15
a
Composite samples.
b
Grab samples.
wastewater is shown in Table 6. The fixed solids fraction, fXFS, was specific biodegradation characteristics of settled COD. This requires
calculated as 0.09 for summer and 0.16 for winter samples. These COD fractionation primarily in relation with particle size ranges
values are relatively lower than the 0.2 to 0.3 range suggested by (Dulekgurgen et al., 2006) and experimental assessment of the bio-
Gujer and Kayser (1998). Settling induced a significant decrease in degradation characteristics of different COD fractions (Henze et al.,
this ratio, to a level of 0.02 in summer and to 0.12 to 0.14 in winter. 2000; Okutman et al., 2001).
Similar to these results, Orhon et al. (1997) reported a decrease in In this study, calibration of the OUR profile, now a widely
the fixed solids from 30 to 15% after 2 hours of settling. accepted and tested experimental instrument, was used for the
Effect of Settling on Chemical Oxygen Demand Fractionation. assessment of COD fractions and corresponding biodegradation
The results of this study indicated that more than 30% of the COD kinetics. The ASM1, modified for the endogenous decay process
content of the Atakoy domestic wastewater could be removed by
¨ and for the separate identification and hydrolysis of rapidly
primary settling. While this removal could be quite beneficial in hydrolyzable COD (SH) and settleable COD (XSS), was adopted
decreasing the organic load of biological treatment, it also may pose as the basis for model calibration (Orhon, Cokgor, and Sozen, 1999;
the problem of reducing the available organic carbon necessary for Orhon et al., 2002). The schematic description of the modified
denitrification, where applicable. An accurate evaluation of the merit ASM1 in a matrix format is given in Table 7. The experiments were
of COD removal by primary settling can only be made in terms of carried out with a composite sample taken from the Atakoy treat-
¨
Table 7—Matrix representation for endogeous decay model.
Processes SO SNO SNH SS SH XS XSS XH XA SP XP Reaction rate
Heterotrophs
SH =XH
Hydrolysis of SH 1 21 kh XH
KX þ SH =XH
XS =XH
Hydrolysis of XS 1 21 khXS XH
KXXS þ XS =XH
XSS =XH
Hydrolysis of XSS 1 21 khXSS XH
KXXSS þ XSS =XH
1 À YH 1 ^ SS
Aerobic growth À 2iXB À 1 lH XH
YH YH KS þ SS
1 À YHD 1 ^ SS
Anoxic growth À 2iXB À 1 gÁ lH XH
2:86ÁYHD YHD KS þ SS
Aerobic endogenous decay 2(12fES2fEX) 21 fES fEX bHXH
ð1 À fES À fEX Þ
Anoxic endogenous decay À 21 fES fEX gÁbHXH
2:86
Autotrophs
4:57 À YA 1 1 ^ SNH
Aerobic growth À ÀiXB À 1 lA XA
YA YA YA KNH þ SNH
Aerobic endogenous decay 2(12fES2fEX) iXB(12fES2fEX) 21 fES fEX bAXA
July 2009 721

8. Tas et al.
ment plant. Soluble and particulate COD fractions were determined
by means of model calibration and simultaneous evaluation of four
different OUR profiles obtained from raw, settled, filtered waste-
water and settled COD derived from the composite sample. Two
parallel tests were carried out with two different initial food-to-
microorganism (S0/X0) ratios for all types of samples. The con-
ceptual basis of the evaluation procedure was previously explained,
in detail, by Orhon et al. (2002). As illustrated in Figure 3, four
different OUR profiles could be successfully calibrated with the
same set of model coefficients. Modeling results indicated that the
Atakoy wastewater also was quite typical, in terms of COD frac-
¨
tionation, as it involved 77% biodegradable COD (CS), with a
soluble inert COD fraction (SI) of 7% and a particulate inert COD
fraction (XI) of 16%. As expected, a small fraction—only 9%—was
readily biodegradable (SS). The rest of the biodegradable COD was
composed of 13% rapidly hydrolyzable COD (SH), 26% suspended
slowly biodegradable COD (XS), and 29% settleable slowly bio-
degradable COD (XSS). The correlation between conventional size
distribution and COD fractionation is better visualized in Figure 4.
The figure shows that the sum of SI 1 SS 1 SH does not extend
beyond the 0.45 to 1.2 lm range, justifying the term soluble slowly
biodegradable commonly adopted to define SH. Figure 4 also
indicates that the settled COD fraction for this particular experiment
was 37%, which is slightly higher than the average of all samples
tested. Based on the COD fractionation for settled COD, settleable
slowly biodegradable COD (XSS0) and settleable inert COD (XI0)
represented 29 and 8% of the raw domestic wastewater COD,
respectively. These results agree with previous observations, where
approximately 180 mg/L of the particulate slowly biodegradable
COD was of a settleable nature (Orhon et al., 2002). Thus, only
78% of the COD removed by primary settling was biodegradable
in nature.
Mass balance and the effect of primary settling on the COD
fractions are shown in Figure 5. The concentrations and corre-
sponding percent fractions reported in this figure are the average
values of three different sets of experiments—one in this study
and the other two previously reported by Okutman et al. (2001) and
Orhon et al. (2002) for the same wastewater. The kinetic and
stoichiometric coefficients used for modeling for the three sets of
experiments are given in Table 8. The results in this table show
good agreement between the OUR tests conducted on different
samples of the Atakoy wastewater and provide a clear indication
¨
for the need of differentiating three slowly biodegradable COD
fractions—the first in the soluble range, SH, the second of a
suspended particulate nature, XS, and the third within the settled
COD, XSS—associated with clearly different hydrolysis rates of
3.5, 1.7, and 1.0 day21, respectively. The biodegradation kinetics of
the settled COD fraction also show that the carbon source directly
derived from primary settling may not be very attractive for both
nitrogen and phosphorus removal processes. Fermentation of this
settled COD fraction, in sewers with long retention times or in the
anaerobic zone of the wastewater treatment plant, or in prefer-
menters, if properly controlled, is recognized as one of the cheapest
ways of generating additional readily biodegradable COD by con-
verting slowly biodegradable COD into more easily biodegradable
components (Bannister and Pretorius, 1998; Hatziconstantinou
Figure 3—Model simulation of the OUR profiles: (a) et al., 1996; Moser-Engeler et al., 1998; Munch and Koch, 1999).
¨
filtered wastewater (0.45 lm), F/M ratio 5 0.05 gCOD/ In a recent study involving a detailed investigation of the potential
gVSS; (b) settled wastewater, F/M ratio 5 0.07 gCOD/ of primary sludge fermentation for the generation of readily bio-
gVSS; (c) raw wastewater, F/M ratio 5 0.1 gCOD/gVSS; degradable substrate, Cokgor et al. (2006) reported that un-
and (d) settled COD, F/M ratio 5 0.2 gCOD/gVSS. controlled fermentation converted 22% of the initial VSS in the
722 Water Environment Research, Volume 81, Number 7

9. Tas et al.
Figure 4—Schematic representation of COD fractionation and size distribution in domestic wastewater.
sludge into soluble biodegradable COD, and approximately 85% of or
this soluble COD was associated with the formation of short-chain hH ÁX SS X SS
VFAs. 0 ¼ X SSin À À hH ÁkhXSS XH ð4Þ
Effect of System Design on the Biodegradation of Settleable hX K XXSS ÁX H þ X SS
Chemical Oxygen Demand. In ASM models, the degradation of Where
particulate organic matter is described using surface-saturation-type
hydrolysis kinetics, as shown in Table 7. Basically, the hydrolysis Q 5 influent flowrate (L/d),
rate is controlled by two kinetic parameters—(1) the maximum V 5 volume of biological reactor (L), and
XH 5 active heterotrophic biomass (mgcellCOD/L).
hydrolysis rate (kh), and (2) the half-saturation coefficient for
hydrolysis (KX). The type of reaction becomes zero-order (khÁXH) Thus, to calculate the overall removal efficiencies by correcting for
when the substrate (XS or XSS) is abundant in the bulk compared the accumulation effect resulting from sludge age, the particulate
with the active biomass (XH). On the other hand, the reaction rate biodegradable COD concentrations (XS, XSS) shown in Figure 6
can be expressed as first-order (r 5 khÁXS) at low substrate levels were multiplied with the ratio hH/hX.
(low XS/XH), where kH represents the kh/KX ratio. As shown in Figure 6a, significant removal of slowly biodegrad-
A model simulation has been conducted to verify the effect of able COD (XS, XSS) can be obtained for hX levels above 2 days. The
system design on the biodegradation of the hydrolyzable COD hydrolysis reaction can be assumed to be first-order, because the
fractions in a conventional activated sludge system. The simulation hydrolyzable COD concentration (XS, XSS) can be neglected com-
was performed for a CSTR with the operating conditions, namely pared with the active biomass concentration (XH), where the kh and
for different sludge age (hX, days) and hydraulic retention time (hH, KX parameters both influence the hydrolysis rate. Under these
hours) couples. The hX/hH ratio was kept constant during the conditions, the removal efficiency was found to be approximately
simulations. The sludge age and hydraulic retention time couples 90%, both for XS and XSS. At higher hX values, the corresponding
have been selected such that the hX/hH ratio maintains an average
MLSS concentration of 4.0 kgMLSS/m3 in the reactor.
Figures 6 and 7 show the simulation outputs for the particulate
biodegradable COD fractions (XS, XSS) and the effluent quality
(total soluble COD, ST), with respect to different hX and hH couples
under both aerobic and anoxic conditions, respectively. In Figures 6
and 7, the x-axis defines, with the sludge age, the corresponding
hydraulic retention time, expressed in terms of hours, as explained
previously. The raw wastewater characteristics (as influent) and
model parameters used in the simulations were adopted from Figure
4 and Tables 8 and 9. Under steady-state conditions, a mass-balance
equation on XSS in a conventional activated sludge system can be Figure 5—The mean values of the COD fractions in the
written as follows: wastewater after primary settling (reported values are the
mean values of the three sets of experiments—one set in
dX SS VÁX SS X SS this study and two sets from Okutman et al., 2001 and
¼ 0 ¼ QÁX SSin À À VÁkhXSS X H ð3Þ
dt hX K XXSS ÁX H þ X SS Orhon et al., 2002).
July 2009 723

10. Tas et al.
Table 8—Kinetic and stoichiometric coefficients used for the modeling of aerobic activated sludge systems.
bH kh khXS khXSS lHmax YH
Set no. (day21) fE (day21) KX (day21) KXXS (day21) KXXSS KS (day21) (gcellCOD/gCOD)
This study 0.2 0.2 3.2 0.04 1.4 0.28 1.0 0.10 3 3.5 0.67
1a 0.2 0.2 1.6 0.05 — — 0.7 0.05 3 4.2 0.67
2b 0.2 0.2 3.8 0.20 1.9 0.18 1.2 0.10 6 3.5 0.67
Mean — — 3.5 0.12 1.7 0.2 1.0 0.08 — — —
a
Data taken from Orhon et al. (2002).
b
Data taken from Okutman et al. (2001).
kh/KX ratios exhibit an effect on the hydrolysis of XS and XSS at values (,2 days). As shown in Figure 6a, the XSS accumulation was
nearly similar orders of magnitude. much higher than the XS level, because the low kh parameter played
The differences in removal efficiencies with respect to XS and XSS a dominant role in the overall hydrolysis rate. For example, the
are much more pronounced when the system is operated at lower hX COD removal efficiencies for XS and XSS corresponded to 60 and
Figure 6—Steady-state simulation of the biodegradation Figure 7—Steady-state simulation of the soluble biodegrad-
of particulate slowly biodegradable COD fractions under able COD components and the fate of total soluble COD
(a) aerobic conditions and (b) anoxic conditions. under (a) aerobic conditions and (b) anoxic conditions.
724 Water Environment Research, Volume 81, Number 7

11. Tas et al.
35%, respectively, at 0.5 days of hX. A lower maximum hydrolysis Table 9—Kinetic and stochiometric parameters used in
rate (kh 51.0 day21) of XSS caused more accumulation in the the calculation of NDP under anoxic conditions.
reactor, despite the close influent COD levels of XS and XSS. The
simulation also indicates that both XS and XSS became completely Parameters Symbol Value Unit
hydrolyzed and removed beyond the sludge age of 2.0 days. Figure Anoxic yield coefficient YHD 0.54 mgcellCOD/mgCOD
6b shows the simulation results for the removal of biodegradable Endogenous decay rate* bH 0.20 day21
particulate COD fractions under anoxic conditions. The removal of Anoxic growth correction
slowly biodegradable COD (XS, XSS) of above 95% can be obtained factor gg 0.80 —
Anoxic hydrolysis rate
for hX levels above 3 days. As presented in the simulation results
correction factor gH 0.60 —
under aerobic and anoxic conditions, the degradation rate of slowly Endogenous decay
biodegradable COD fractions is relatively slower under anoxic correction factor gE 0.48 —
conditions, as a result of the reduced rates of hydrolysis processes.
The anoxic hydrolysis rates are 60% lower than the aerobic * Converted from death regeneration concept.
hydrolysis rates because of the anoxic hydrolysis rate correction
factor gH (Barker and Dold, 1997; Henze et al., 2000).
Figure 7 illustrates the effluent quality simulations, with respect of XSS is achieved. Thus, the maximum contribution of the
to different hX and hH couples, under both aerobic and anoxic settleable biodegradable COD (XSS) to the total NDP generated in the
conditions, respectively. The dashed line indicates an SI level of system can be as high as 40% for the domestic wastewater studied.
32 mg/L. The simulation study showed that nearly complete The maximum contribution of XSS is dependent on the design and
degradation of soluble biodegradable COD can be achieved for operating conditions. While the NDP of XSS is maximized at a total
hX.2 days under both aerobic and anoxic conditions. Considering sludge age of 6 days for a VD/V of 0.5, it can only be maximized if
the soluble inert COD baseline, the difference of 10 mg/L COD is the system is operated at total sludge ages higher than 15 days and
the result of the contribution of rapidly hydrolyzable COD (SH) with
at a VD/V of 0.2. In addition, it should be emphasized that, in the
inert soluble microbial products (SP). Total effluent soluble COD
presence of a primary settler as a system component, shorter settling
(ST) rapidly increased as a result of the incomplete degradation
times would introduce some XSS to the system, resulting in an
of rapidly hydrolyzable COD- SH for hX levels below 1.0 day. The
increase of NDP, depending on the settling efficiency and system
total effluent soluble COD (ST) and effluent rapidly hydrolyzable
conditions. This may be needed for cases where NDP acts as the
COD (SH) are slightly higher under anoxic conditions, as a result of
limiting factor for the denitrification efficiency.
the slower degradation of SH, as given in Figure 7b.
Effect of Settleable Chemical Oxygen Demand on
Denitrification Potential. The denitrification potential (NDP) is
a measure of the electron acceptor (nitrate) demand of the available Conclusions
organic carbon under anoxic conditions. From a modeling standpoint, The results of the respirometric evaluations in this study enabled
it is controlled by the readily biodegradable COD fraction, as nitrate the establishment of a mass balance for the significant COD
demand is associated with processes related to biomass growth and fractions of domestic wastewater. In this context, the effect of
decay. Consequently, hydrolysis of slowly biodegradable COD primary settling was assessed, not only as overall COD removal, but
compounds is the rate-limiting step in the development of NDP, with also in terms of a new COD fraction, the settleable COD. This
18 significant operating parameters related to the anoxic phase, such fraction, removed by primary settling, was identified as a slowly
as the sludge age (hX) and the anoxic volume ratio (VD/V). biodegradable substrate with a low hydrolysis rate of 1.0 day21,
The model simulation results under anoxic conditions were used providing a clear differentiation from the other particulate slowly
to evaluate the relative contribution of the settleable biodegradable biodegradable COD components.
COD fraction to the overall NDP of the system. The results have The efficiency of the primary settling is expected to be site-
shown that the denitrification potential (NDP) increases with in- specific. Accordingly, the results related to the fate of conventional
creasing VD/V ratios, as shown in Figure 8a. No significant increase parameters, reported in this study, relate only to the selected
in NDP is observed above an anoxic sludge age of 3 days (i.e., total domestic wastewater. However, the proposed approach introduced
sludge age of 6 days for VD/V 5 0.5), where almost all biode- a generally applicable new concept of evaluating the settled COD
gradable COD is consumed by heterotrophic biomass. The gradual as a separate entity, which is well-defined in terms of its bio-
increase in NDP with increasing sludge ages above an anoxic sludge degradation characteristics. The settleable (biodegradable) COD
age of 3 days is the result of a similar increase in electron acceptor (XSS) was incorporated to a multicomponent model as a new model
consumption of decaying biomass (because the concentration of component, with its hydrolysis kinetics. The fate of this COD
biomass in the system increases with increasing sludge age). How- fraction could then be evaluated by means of model simulation,
ever, for single sludge systems, denitrification at low sludge ages which indicated that it would be totally hydrolyzed and removed
will not be possible, because the nitrifiers will be washed out from at a sludge age of 2 days. Model simulation, accounting for the
the system under such short aerobic sludge ages. In addition, the biodegradation kinetics respirometrically determined for XSS, with
system would be limited by the available amount of biodegradable similar characteristics of other COD components, identified a new
COD at anoxic sludge ages less than 3 days, as a result of incom- dimension of the settleable COD fraction as a possible source of
plete hydrolysis of XSS. The NDP generated by the degradation of additional organic carbon that could directly contribute to the
XSS for different sludge ages and VD/V ratios is presented in Figure denitrification potential of the system, if the total sludge age and
8b. The figure shows that the NDP contribution of the XSS com- the VD/V are adjusted to secure an anoxic sludge age of over 2 days.
ponent increases with increasing VD/V ratios and an increasing The result challenges the function of primary settling, especially in
anoxic sludge age of 3 days, where almost complete biodegradation activated sludge systems, where the denitrification potential asso-
July 2009 725

12. Tas et al.
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