Biodegradability potential of settleable dqo

Biodegradability and DenitrificationPotential of Settleable Chemical Oxygen

Demand in Domestic Wastewater

Didem Okutman Tas1*, Ozlem Karahan1, Guclu _Insel1, Suleyman Ovez1,Derin Orhon2, Henri Spanjers3

ABSTRACT: The effect of settling on mass balance and biodegradation

characteristics of domestic wastewater and on denitrification potential was

studied primarily using model calibration and evaluation of oxygen

uptake rate profiles. Raw domestic wastewater was settled for a period of

30 minutes and a period of 2 hours to assess the effect of primary settling on

wastewater characterization and composition. Mass balances in the system

were made to evaluate the effect of primary settling on major parameters.

Primary settling of the selected raw wastewater for 2 hours resulted in the

removal of 32% chemical oxygen demand (COD), 9% total Kjeldahl

nitrogen, 9% total phosphorus, and 47% total suspended solids. Respiro-

metric analysis identified COD removed by settling as a new COD fraction,

namely settleable slowly biodegradable COD (XSS), characterized by a

hydrolysis rate of 1.0 day21 and a hydrolysis half-saturation coefficient of

0.08. A model simulation to test the fate and availability of suspended (XS)

and settleable (XSS) COD fractions as carbon sources for denitrification

showed that both particulate COD components were effectively removed

aerobically at sludge ages higher than 1.5 to 2.0 days. Under anoxic con-

ditions, the biodegradation of both COD fractions was reduced, especially

below an anoxic sludge retention time of 3.0 days. Consequently, modeling

results revealed that the settleable COD removed by primary settling could

represent up to approximately 40% of the total denitrification potential of the

system, depending on the specific configuration selected for the nitrogen

removal process. This way, the results showed the significant effect of

primary settling on denitrification, indicating that the settleable COD fraction

could contribute an additional carbon source in systems where the denitri-

fication potential associated with the influent becomes rate-limiting for the

denitrification efficiency. Water Environ. Res., 81, 715 (2009).

KEYWORDS: biodegradation, chemical oxygen demand fractions, de-

nitrification potential, domestic wastewater, primary settling, respirometric

modeling, settleable chemical oxygen demand.

doi:10.2175/106143009X425942

IntroductionAssessment of biodegradation characteristics of different organic

fractions in wastewaters should be considered as one of the major

research milestones in environmental science and technology.

Because of the pioneering work of Dold et al. (1980) and Ekama

et al. (1986), chemical oxygen demand (COD) has been adopted as

the main parameter to quantify organic carbon. The biodegradable

COD conveniently establishes the electron balance between

substrate used, biomass generated, and electron acceptor (dissolved

oxygen in aerobic systems) consumed. Substantial research has

been conducted to identify and assess the biodegradation character-

istics of different COD fractions in domestic wastewater (Henze,

1992; Orhon et al., 1997; Sollfrank and Gujer, 1991) and different

industrial wastewaters (Kabdasli et al., 1993; Orhon et al., 1995;

Orhon, Tasli, and Sozen, 1999). Respirometry has been a significant

asset for the experimental assessment of these fractions (Spanjers

and Vanrolleghem, 1995), which are now incorporated as model

components to major activated sludge models (Gujer et al., 2000;

Henze et al., 1987, 1995).

Particle size is an integral component of COD fractionation.

Wentzel et al. (1999) stated that the biodegradation potential defined

for different COD fractions would have a correlation with physical

categorization, in terms of particle size and physical state in waste-

water. In wastewater characterization, one particle size (0.45-lm

membrane or 1.2-lm glass-fiber filter size) is commonly used to

roughly differentiate soluble and particulate ranges. Soluble inert

COD (SI), readily biodegradable COD (SS), and rapidly hydrolyz-

able COD (SH) are associated with the soluble range, while slowly

biodegradable COD (XS) and particulate inert COD (XI) are evalu-

ated within the particulate range. Recently, a settleable COD frac-

tion (XSS) with a slower biodegradation rate was defined within the

particulate COD for domestic wastewater (Okutman et al., 2001). In

a study involving detailed particle size analysis, Dulekgurgen et al.

(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.

A detailed characterization is a prerequisite for understanding and

interpreting the fate of pollutants in wastewaters. It should cover,

aside from COD and its significant fractions, all relevant parameters

and especially particulate solids—total suspended solids (TSS) and

volatile suspended solids (VSS)—in a way to establish basic mass

balances defining useful ratios, such as COD/VSS, COD/N/P, and

VSS/TSS, for treatment system design and operation (Orhon et al.,

1997; Rossle and Pretorius, 2001). From a practical standpoint,

accurate assessment of influent composition is essential for the

design and operation of biological treatment units. In this respect,

the removal rate in plain settling sets is another important factor

within the particulate range, to estimate the fate of different COD

fractions and other parameters before biological treatment.

1 Department of Environmental Sciences and Engineering, IstanbulTechnical University, Istanbul, Turkey.

2 Turkish Academy of Sciences, Ankara, Turkey.

3 Lettinga Associates Foundation, Wageningen, Netherlands.

* Department of Environmental Sciences and Engineering, Istanbul TechnicalUniversity, 80626 Ayazaga, Maslak, Istanbul, Turkey; e-mail:[email protected].

July 2009 715

Biodegradation characteristics of different COD fractions also

provide essential information for activated sludge systems designed

for biological nutrient removal. In fact, in these systems, an impor-

tant role is attributed to organic carbon. Basically, denitrification

relies on the same principles as organic carbon removal under

aerobic conditions, except for the final different electron acceptors

utilized. However, the two systems have totally opposite objectives

when evaluated in terms of the corresponding treatment processes;

in conventional aerobic activated sludge systems, in which the sole

purpose is the organic carbon removal, the typical approach is to

reduce the organic load by means of primary settling, which

removes, aside from suspended solids, the settleable fraction of the

influent COD. In denitrification, which occurs under anoxic condi-

tions, the main objective is to remove the final electron acceptor—

nitrate—and system design should ensure that a stoichiometrically

sufficient amount of biodegradable COD is present for the reduction

and removal of nitrate in the anoxic reactor. In this respect, organic

carbon assumes a different function as an essential ingredient for

denitrification.

The function of biodegradable COD is best evaluated in terms

of the denitrification potential (NDP), a parameter that reflects

the nitrogen equivalent that may potentially consume nitrate

under anoxic conditions. Basic process stoichiometry and process

modeling are commonly used for the assessment of NDP (Artan

et al., 2002; Ekama and Marais, 1984; Sozen et al., 2002). The

efficiency of denitrification greatly depends on the balance between

NDP and the extent of available nitrate (NA) introduced to the anoxic

reactor volume. The merit of the removing settleable COD by

primary settling requires serious consideration and reevaluation,

especially in cases where NDP becomes rate-limiting and additional

organic carbon becomes a significant asset. Obviously, the magni-

tude of the settleable COD fraction alone is not enough for this

evaluation, which basically depends on the biodegradation charac-

teristics of all COD fractions involved.

In this context, the main objective of the study was to establish

a conceptual basis for the assessment of the biodegradability and

denitrification potential of settleable COD. The conceptual approach

was illustrated using data derived from the domestic wastewater at

Atakoy, Istanbul, Turkey. Detailed characterization, emphasizing

the effect of primary settling on COD fractionation, and model

simulation were also carried out as the necessary tools to conduct

this examination.

Materials and MethodsSurvey Site. The study was conducted as part of a comprehen-

sive survey on the treatability-oriented characterization of domestic

wastewater within the metropolitan area of Istanbul, Turkey, now

housing more than 12 million residents. The Istanbul Metropolitan

Area is located on the northern coast of the Marmara Sea and lies on

both sides of the Bosphorus strait, connecting the Black Sea to the

Marmara Sea. The area has been the subject of similar studies as the

major polluting source in the Marmara basin (Gorgun et al., 1996;

Orhon et al., 1997; Orhon, Sozen, and Ubay, 1994; Orhon, Uslu,

Meric, Salihoglu, and Filibeli, 1994). The study was mainly carried

out at the Atakoy treatment plant, serving a population equivalent of

45 000 residents, now under extension for nutrient removal, where

a statistically significant number of composite samples (i.e., more

than 30 samples) were collected from the influent of the plant. The

composite samples were collected in summer (sampling from 8 a.m.

until 5 p.m. each day), during dry weather, for a period of 3 months

(July to September). The sampling sequence was arranged to

characterize all days of the week. In addition to the summer

characterization, five grab samples also were taken and analyzed

in December, to compare the summer and winter wastewater com-

positions. The study also included evaluation of wastewater samples

collected during the same summer period from the Baltalimani

treatment plant, another significant wastewater discharge station

within the metropolitan area along Bosphorus.Analytical Measurements. All analyses were performed

according to Standard Methods (APHA et al., 1998). As prescribed

in current activated sludge modeling, COD was used to characterize

wastewater organic matter. The soluble and particulate organic

matter was differentiated by filtration, using 0.45-lm cellulose

acetate membrane filters. The COD measurements were performed

as described in ISO6060 (International Organization for Standard-

ization, 1986). Whatman (Kent, England) GF/C glass-fiber filters

(1.2 lm) were used for TSS and VSS measurements. In addition,

0.45-lm cellulose acetate membrane filters were used to quantify

TSS and VSS in domestic wastewater, to compare the results with

the soluble parameters that are defined as filtrate from 0.45-lm

cellulose acetate membrane filters.Respirometric Modeling and Performance Simulation.

Oxygen uptake rate (OUR) measurements were conducted with a

Manotherm RA-1000 continuous respirometer (Nazareth, Belgium)

with a personal computer connection (Orhon and Okutman, 2003).

Three sets of composite samples collected from the influent of

Table 1—Conventional characterization of domestic wastewater.

Parameter

This study Domestic wastewater in _Istanbul (Orhon et al., 1997)

Atakoya Baltalimania Kadikoy K.Cekmece

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.

Tas et al.

716 Water Environment Research, Volume 81, Number 7

the Atakoy treatment plant were used for the assessment of COD

fractions and evaluation of kinetic coefficients, as previously

described (Orhon et al., 2002). A 30-L composite sample was

collected from the influent of the treatment plant and subjected to

2 hours of gravity settling in a cylindrical reactor to simulate the

quality of fresh settled wastewater. Approximately 2 L of the settled

wastewater fraction was withdrawn from the bottom of the reactor

for the OUR measurements. The experiments were conducted at

room temperature (228C). The pH was kept in the range 7.0 to 8.0,

which is suitable for biological activity. Aeration was supplied

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

using the experimental OUR data. The COD fractionation and

soluble and particulate COD components of the wastewater were

determined according to the methods previously described (Ekama

et al., 1986; Insel et al., 2003; Orhon and Okutman, 2003; Orhon

et al., 2002).

A simulation study was performed to illustrate the biodegradation

characteristics of particulate slowly biodegradable COD (XS, XSS)

under different sludge ages (hX). The steady-state simulations were

performed using a conventional activated sludge system having

a single aerobic continuous stirred-tank reactor (CSTR) and a final

clarifier. A simulation study was conducted using average waste-

water characterization and influent COD fractionation. Sludge age

(hX) and hydraulic retention time (hH) were changed in parallel to

keep the mixed liquor suspended solids (MLSS) concentration at

approximately 4.0 kgSS/m3 in all simulations. The final clarifier

was assigned as a point settler, in which the mixed liquor is physi-

cally separated from the clarified effluent. The solids separation

efficiency of the final clarifier was assumed to be 100%. The return

activated sludge rate was adjusted to unity. The AQUASIM prog-

ram developed by Reichert et al. (1998) was used in all simulations.

The simulation results were processed to obtain the contributions

of different COD fractions to the denitrification potential (NDP) of

the system at different operating conditions and different anoxic

volume ratios (VD/V). The following simplified equation was used

to calculate the denitrification potential of both total and settleable

COD:

NDP ¼�CS

2:86ð1� YNHDÞ and YNHD ¼ YHD=ð1þ bHD�hXÞ ð1Þ

Where

CS 5 concentration of biodegradable COD (mg/L),

YNHD 5 net anoxic yield coefficient for heterotrophs (mgcell-

COD/mgCOD),

bHD 5 anoxic endogenous decay rate for heterotrophs (day21),

and

hX 5 total sludge age (days).

The anoxic endogenous decay rate (bHD) is calculated by reducing

the aerobic endogenous decay coefficient with the endogenous

decay correction factor (gE), as follows:

bHD ¼ bH �gE ð2Þ

Results and DiscussionConventional Characterization. Conventional characteriza-

tion results of the domestic wastewater investigated in this study

during the summer season are outlined in Table 1. The table in-

cludes previously reported literature data based on surveys con-

ducted on the domestic wastewater from important wastewater

discharge stations of Istanbul (Orhon et al., 1997) and representative

domestic wastewater characteristics for selected countries in Europe

(Pons et al., 2002). The results indicated that the average com-

position of domestic wastewater at the Atakoy treatment plant,

which served as the focal point of the study, could be expressed as

406 mg/L total COD, 190 mg/L TSS, 178 mg/L VSS, 41 mg/L total

Kjeldahl nitrogen, 27 mg/L ammonia nitrogen (NH3-N), and 8.3

mg/L total phosphorus. The average pH of the domestic wastewater

was 7.6. Slightly lower values were observed for all the parameters

in the Baltalimani treatment plant, the other station considered in

this study (Table 1). Both stations represent residential areas

generating municipal wastewater with no significant effect from

industrial activities. The mean values obtained for the Atakoy and

Baltalimani wastewaters represent typical domestic wastewater

quality, which has not changed significantly over time. The stronger

character of the Yenikapi wastewater reported in the previous study

indicates the significant effect of industrial discharges on wastewa-

ter quality. The appreciable quality spectrum observed among the

different wastewater characteristics given in Table 1 underlines the

specific nature of wastewater quality affected by local conditions.

The statistical distribution of major parameters for the Atakoy

treatment plant influent is plotted in Figure 1. The plots exhibit

a regular trend for all parameters with a range of 5 to 10% deviation

between mean and 70th-percentile values. The 70th-percentile

values, representing the upper threshold level of 70% of the samples

analyzed, are also indicated in Table 1. The 70th-percentile values

are generally recommended and adopted for design, as they could

Table 1—(Extended)

Parameter

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

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 — — — — — — —

Tas et al.

July 2009 717

provide more meaningful and useful information compared with

mean values (Kayser, 1989; Orhon et al., 1998). Therefore, in

this study, evaluations for different parameters presented in the

following sections are based on the 70th-percentile values.

Comparison of the wastewater composition in the summer and

winter seasons is shown in the following results, in terms of the

suspended solids and COD concentrations that can be expected to

change as a function of the seasonal activities. Because a significant

difference for nitrogen and phosphorus parameters was not expected

during summer and winter seasons under dry-weather conditions,

these two parameters were not investigated in the winter season.Effect of Settling and Filtration on Wastewater Characteristics.

The nature and size distribution of significant conventional parameters

have been studied for the following four indices associated with

different size thresholds: (1) 30 minutes of settling, (2) 2 hours of

settling, (3) 1.2-lm filtration, and (4) 0.45-lm filtration. The first two

have practical significance, as they illustrate the effect of primary

settling typically implemented before biological treatment of domestic

wastewater. Filtration through 0.45-lm filters is commonly used to

differentiate soluble and particulate fractions and roughly approx-

imates the effect of chemical treatment (Henze et al., 2000). Filtration

through 1.2-lm filters is also a common analytical technique for

wastewater characterization.

The results of settling and filtration experiments conducted on

all samples and displayed in Table 2 indicated that 74 to 78% of

the total COD could be considered of particulate nature based on

a 0.45-lm size threshold for both the summer and winter seasons.

In the summer samples, settling for 2 hours removed 32% of the

total COD or 43% of the particulate COD. After settling, 188 mg/L

of suspended or colloidal COD remained in the effluent, with

a different composition of 38% soluble COD and 62% particulate

COD. Thirty minutes of settling was roughly half efficient, only

providing 17% COD removal. The 0.45 to 1.2 lm size range was

observed to contain 7% of the COD content of domestic waste-

water, which is a statistically significant amount that should be

considered for the evaluation of wastewater characterization.

Similar results also were observed in the winter season.

The soluble fraction of the total nitrogen was assessed as 78%. A

similar fraction of 80% was calculated for total phosphorus. Two

hours of settling provided only 9% nitrogen removal, which cor-

responded to 42% of the initial particulate nitrogen. Similarly, 9%

of the total phosphorus and 44% of the particulate phosphorus was

removed. The removal efficiencies decreased to 5% for nitrogen

and 7% for phosphorus when the settling time was reduced to

30 minutes. The specific nature of wastewater characteristics was

confirmed, as the results of this study were only partially supported

by similar observations reported in the literature. Odegaard (1997)

indicated that the filtered fraction (1 lm) in raw wastewater samples

was typically 20 to 30% of the total COD, 30 to 40% of the total

phosphorus, and 75 to 85% of the total nitrogen. Tiehm et al. (1999)

stated that, in raw wastewater and primary effluent, 45% of

the COD and 35 to 80% of the phosphorus was associated with

suspended solids.

Similarly, in the summer samples, based on quantification with

membrane filters (0.45 lm), settling periods of 30 minutes and of

2 hours resulted in the removal of 33 and 47% TSS and 36 and 49%

Figure 1—Statistical distribution of characteristic parameters in Atakoy domestic wastewater: (a) COD, (b) suspendedsolids, (c) total Kjeldahl nitrogen, and (d) total phosphorus (¤ 5 total, � 5 30 minutes settled, � 5 2 hours settled, andu 5 soluble).

Tas et al.


VSS, respectively. Although measured concentrations were lower,

similar removal ratios were obtained based on quantification with

1.2-lm glass-fiber filters. In the winter season, although TSS con-

centrations were very similar to those measured in the summer, VSS

concentrations were relatively lower. Based on a 0.45-lm analytical

quantification, settling periods of 30 minutes and of 2 hours resulted

in the removal of 27 and 36% TSS and 18 and 36% VSS, respec-

tively. Thus, higher removal efficiencies for suspended solids were

obtained in the summer samples as opposed to the winter samples.

The difference in removal efficiencies can be attributed to different

characteristics of particulate pollutants during the wet season; to

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

for phosphorus (Orhon, Sozen, and Ubay 1994; Orhon et al., 1997).

Rossle and Pretorius (2001) have reported slightly higher removal

rates for South African wastewater. These results merit further

attention in terms of the settling properties of domestic wastewater

as it relates to the removal of a significant portion of the available

organic carbon source.Assessment of Characteristic Parameters. Interpretation of

conventional characterization, in terms of significant ratios of sel-

ected parameters, such as N/COD, P/COD, and VSS/COD, is quite

useful, as it could be used for prediction of the biodegradability of

domestic wastewater. As reported in Table 4, the N/COD ratio

for Atakoy was calculated to be in the range 0.07 to 0.15 mgN/

mgCOD, with an average value of 0.11 mgN/mgCOD. The cor-

responding value for Baltalimani was 0.10 mgN/mgCOD. These

values coincide with the average N/COD ratios of 0.10 to 0.11

mgN/mgCOD previously observed for Istanbul wastewater (Orhon

et al., 1997; Orhon, Sozen, and Ubay, 1994). They also are con-

sistent with the range 0.087 to 0.115 mgN/mgCOD reported for

different domestic wastewaters in Europe (Pons et al., 2002). The

N/COD ratio is an important index for predicting the efficiency of

denitrification in biological treatment. The overall mean N/COD

ratios measured in this study represent the limit value of 0.1 mgN/

mgCOD, below which, a single sludge predenitrification system has

the potential of providing high nitrogen removal efficiency (Orhon

and Artan, 1994). Pitman (1991) and Randall et al. (1992) reported

that, if the N/COD ratio is higher than 0.11 and the volatile fatty

Table 2—Significant parameters in domestic wastewaterafter settling and filtration (70% statistical values).a

Parameters (mg/L) Total

Supernatant

after settling Filtrate

30 minutes 2 hours 1.2 lm 0.45 lm

Organics and nutrients

Summer seasonb

COD 445 370 305 149 117

Total nitrogen 43 41 39 NM 33.5

Total phosphorus 9.1 8.5 8.3 NM 7.3

Winter seasonc

COD 510 350 300 150 115

TSS and VSS

Summer seasonb (1.2 lm)

TSS 210 144 114 NA NA

VSS 198 142 110 NA NA

Summer seasonb (0.45 lm)

TSS 230 153 123 NA NA

VSS 200 128 103 NA NA

Winter seasonc (1.2 lm)

TSS 195 168 129 NA NA

VSS 148 109 87 NA NA

Winter seasonc (0.45 lm)

TSS 207 152 132 NA NA

VSS 160 131 102 NA NA

a NA 5 not applicable, NM 5 not measured.b Composite samples.c Grab samples.

Figure 2—The mean values of mass balance for COD,nutrients, and suspended and volatile suspended solidsin the wastewater after primary settling (*0.45 lm).

Table 3—Removal ratios of the significant parameters inthe primary settling (2 hours).

Percent removal

COD

(%)

TKN

(%)

Total

phosphorus

(%)

TSS

(%)

Atakoy wastewaters

(this study)

32 9 9 47*

South African wastewaters

(Rossle and Pretorius, 2001)

40 15 to 20 15 to 20 60

Riva/Istanbul wastewater

(Orhon, Sozen, and Ubay, 1994)

26 7 15 —

Kadikoy/Istanbul wastewater

(Orhon et al., 1997)

33 9 25 63

ATV131 (2000) 33 9 11 64

* 0.45 lm.

Tas et al.

July 2009 719

acid (VFA) content is low (,50 mg/L), an external carbon source

should be used or prefermentation should be implemented. As ex-

pected, settling increased this ratio, as a result of significantly lower

nitrogen removal compared with COD. After 2 hours of settling, the

mean N/COD ratio was measured as 0.14 mgN/mgCOD for Atakoy

and 0.12 mgN/mgCOD for Baltalimani wastewater, which are both

above the limit for efficient predenitrification.

The mean value of the P/COD ratio for both the Atakoy and

Baltalimani wastewater was 0.02 mgP/mgCOD, which is also con-

sistent with the range 0.014 to 0.019 given for European wastewaters

(Pons et al., 2002). Settling for 2 hours resulted in an increase

of approximately 50%. Similarly, an increase has been reported in

the P/COD ratio from the range 0.015 to 0.025 to the range 0.02 to

0.03 in the settled sludge (Water Research Commission, 1984).

The VSS/SS ratio was 0.93 mgVSS/mgSS for Atakoy and 0.84

mgVSS/mgSS for Baltalimani, as summarized in Table 4, indicating

that 7 to 16% of the suspended solids were of an inorganic nature.

As expected, settling was more effective in removing the inorganic

suspended solids, as a result of higher settling velocities associated

with this fraction.

The ratios of particulate fractions of significant parameters are

also quite important for system design and process modeling (Gujer

et al., 2000). Table 5 outlines the values of two ratios, namely the

ratio of VSS to particulate COD, iX, and of particulate nitrogen to

particulate COD, iXN, for the Atakoy wastewater. In conventional

analysis, soluble and particulate components of the wastewater are

commonly differentiated using filters with pore sizes ranging from

0.45 to 1.8 lm (Henze et al., 2000). In this study, only a 0.45-lm

filter was used to quantify COD during the summer season,

whereas, during the winter season, both 0.45- and 1.2-lm pore-

sized filters were used for particulate COD assessment. As shown in

Table 5, iX was calculated as 0.66 mgVSS/mgCOD for both raw

and settled wastewater during the summer survey (0.45 lm). In the

winter season, this ratio was found to increase to 0.79 mgVSS/

mgCOD for the same type of filter and to 0.70 for a 1.2-lm filter. In

the winter season, particulate VSS/COD ratio ranged from 0.73 to

0.79 for the same type of filters (0.45 lm). According to the VSS

and COD measurements with 1.2-lm filters, the particulate VSS/

COD ratios were relatively small compared with the measurements

with 0.45-lm filters. Although no difference was observed in the iXlevel in summer, a significant decrease was observed as a function

of settling time in the winter samples. These values compare well

with the typical value of 0.75 mgVSS/mgCOD suggested for settled

wastewater in Activated Sludge Model No. 3 (ASM3) (Gujer et al.,

2000). The particulate N/COD ratio, iXN, was similarly calculated

as 0.03 mgN/mgCOD for both raw and settled wastewater for the

summer samples, a level consistent with the default value of 0.04

mgN/mgCOD of ASM3, after primary settling (Gujer et al., 2000).

The inorganic portion of the suspended solids, called fixed solids,

XFS, also is an important parameter for the accurate estimation of

biomass under different operating conditions. The effect of settling

on XFS and on the fixed solids fraction, fXFS, for the Atakoy

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



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



Table 5—Particulate nitrogen and COD fractions as a function of settling time in the Atakoy domestic wastewatertreatment 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.

Tas et al.


wastewater is shown in Table 6. The fixed solids fraction, fXFS, was

calculated as 0.09 for summer and 0.16 for winter samples. These

values are relatively lower than the 0.2 to 0.3 range suggested by

Gujer and Kayser (1998). Settling induced a significant decrease in

this ratio, to a level of 0.02 in summer and to 0.12 to 0.14 in winter.

Similar to these results, Orhon et al. (1997) reported a decrease in

the fixed solids from 30 to 15% after 2 hours of settling.Effect of Settling on Chemical Oxygen Demand Fractionation.

The results of this study indicated that more than 30% of the COD

content of the Atakoy domestic wastewater could be removed by

primary settling. While this removal could be quite beneficial in

decreasing the organic load of biological treatment, it also may pose

the problem of reducing the available organic carbon necessary for

denitrification, where applicable. An accurate evaluation of the merit

of COD removal by primary settling can only be made in terms of

specific biodegradation characteristics of settled COD. This requires

COD fractionation primarily in relation with particle size ranges

(Dulekgurgen et al., 2006) and experimental assessment of the bio-

degradation characteristics of different COD fractions (Henze et al.,

2000; Okutman et al., 2001).

In this study, calibration of the OUR profile, now a widely

accepted and tested experimental instrument, was used for the

assessment of COD fractions and corresponding biodegradation

kinetics. The ASM1, modified for the endogenous decay process

and for the separate identification and hydrolysis of rapidly

hydrolyzable COD (SH) and settleable COD (XSS), was adopted

as the basis for model calibration (Orhon, Cokgor, and Sozen, 1999;

Orhon et al., 2002). The schematic description of the modified

ASM1 in a matrix format is given in Table 7. The experiments were

carried out with a composite sample taken from the Atakoy treat-

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.

Table 7—Matrix representation for endogeous decay model.

Processes SO SNO SNH SS SH XS XSS XH XA SP XP Reaction rate

Heterotrophs

Hydrolysis of SH 1 21 khSH=XH

KX þ SH=XHXH

Hydrolysis of XS 1 21 khXSXS=XH

KXXS þ XS=XHXH

Hydrolysis of XSS 1 21 khXSSXSS=XH

KXXSS þ XSS=XHXH

Aerobic growth �1� YH

YH

2iXB � 1

YH

1 lH

SS

KS þ SS

XH

Anoxic growth � 1� YHD

2:86�YHD

2iXB � 1

YHD

1 g� lH

SS

KS þ SSXH

Aerobic endogenous decay 2(12fES2fEX) 21 fES fEX bHXH

Anoxic endogenous decay �ð1� fES � fEXÞ2:86

21 fES fEX g�bHXH

Autotrophs

Aerobic growth � 4:57� YA

YA

1

YA�iXB �

1

YA

1 lA

SNH

KNH þ SNHXA

Aerobic endogenous decay 2(12fES2fEX) iXB(12fES2fEX) 21 fES fEX bAXA

Tas et al.

July 2009 721

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 slowlybiodegradable 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

et al., 1996; Moser-Engeler et al., 1998; Munch and Koch, 1999).

In a recent study involving a detailed investigation of the potential

of primary sludge fermentation for the generation of readily bio-

degradable substrate, Cokgor et al. (2006) reported that un-

controlled fermentation converted 22% of the initial VSS in the

Figure 3—Model simulation of the OUR profiles: (a)filtered wastewater (0.45 lm), F/M ratio 5 0.05 gCOD/gVSS; (b) settled wastewater, F/M ratio 5 0.07 gCOD/gVSS; (c) raw wastewater, F/M ratio 5 0.1 gCOD/gVSS;and (d) settled COD, F/M ratio 5 0.2 gCOD/gVSS.

Tas et al.


sludge into soluble biodegradable COD, and approximately 85% of

this soluble COD was associated with the formation of short-chain

VFAs.Effect of System Design on the Biodegradation of Settleable

Chemical Oxygen Demand. In ASM models, the degradation of

particulate organic matter is described using surface-saturation-type

hydrolysis kinetics, as shown in Table 7. Basically, the hydrolysis

rate is controlled by two kinetic parameters—(1) the maximum

hydrolysis rate (kh), and (2) the half-saturation coefficient for

hydrolysis (KX). The type of reaction becomes zero-order (kh�XH)

when the substrate (XS or XSS) is abundant in the bulk compared

with the active biomass (XH). On the other hand, the reaction rate

can be expressed as first-order (r 5 kh�XS) at low substrate levels

(low XS/XH), where kH represents the kh/KX ratio.

A model simulation has been conducted to verify the effect of

system design on the biodegradation of the hydrolyzable COD

fractions in a conventional activated sludge system. The simulation

was performed for a CSTR with the operating conditions, namely

for different sludge age (hX, days) and hydraulic retention time (hH,

hours) couples. The hX/hH ratio was kept constant during the

simulations. The sludge age and hydraulic retention time couples

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

written as follows:

dXSS

dt¼ 0 ¼ Q�XSSin �

V �XSS

hX

� V �khXSS

XSS

KXXSS�XH þ XSS

XH ð3Þ

or

0 ¼ XSSin �hH�XSS

hX� hH�khXSS

XSS

KXXSS�XH þ XSS

XH ð4Þ

Where

Q 5 influent flowrate (L/d),

V 5 volume of biological reactor (L), and

XH 5 active heterotrophic biomass (mgcellCOD/L).

Thus, to calculate the overall removal efficiencies by correcting for

the accumulation effect resulting from sludge age, the particulate

biodegradable COD concentrations (XS, XSS) shown in Figure 6

were multiplied with the ratio hH/hX.

As shown in Figure 6a, significant removal of slowly biodegrad-

able COD (XS, XSS) can be obtained for hX levels above 2 days. The

hydrolysis reaction can be assumed to be first-order, because the

hydrolyzable COD concentration (XS, XSS) can be neglected com-

pared with the active biomass concentration (XH), where the kh and

KX parameters both influence the hydrolysis rate. Under these

conditions, the removal efficiency was found to be approximately

90%, both for XS and XSS. At higher hX values, the corresponding

Figure 4—Schematic representation of COD fractionation and size distribution in domestic wastewater.

Figure 5—The mean values of the COD fractions in thewastewater after primary settling (reported values are themean values of the three sets of experiments—one set inthis study and two sets from Okutman et al., 2001 andOrhon et al., 2002).

Tas et al.

July 2009 723

kh/KX ratios exhibit an effect on the hydrolysis of XS and XSS at

nearly similar orders of magnitude.

The differences in removal efficiencies with respect to XS and XSS

are much more pronounced when the system is operated at lower hX

values (,2 days). As shown in Figure 6a, the XSS accumulation was

much higher than the XS level, because the low kh parameter played

a dominant role in the overall hydrolysis rate. For example, the

COD removal efficiencies for XS and XSS corresponded to 60 and

Figure 6—Steady-state simulation of the biodegradationof particulate slowly biodegradable COD fractions under(a) aerobic conditions and (b) anoxic conditions.

Table 8—Kinetic and stoichiometric coefficients used for the modeling of aerobic activated sludge systems.

Set no.

bH

(day21) fE

kh

(day21) KX

khXS

(day21) KXXS

khXSS

(day21) KXXSS KS

lHmax

(day21)

YH

(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).

Figure 7—Steady-state simulation of the soluble biodegrad-able COD components and the fate of total soluble CODunder (a) aerobic conditions and (b) anoxic conditions.

Tas et al.


35%, respectively, at 0.5 days of hX. A lower maximum hydrolysis

rate (kh 51.0 day21) of XSS caused more accumulation in the

reactor, despite the close influent COD levels of XS and XSS. The

simulation also indicates that both XS and XSS became completely

hydrolyzed and removed beyond the sludge age of 2.0 days. Figure

6b shows the simulation results for the removal of biodegradable

particulate COD fractions under anoxic conditions. The removal of

slowly biodegradable COD (XS, XSS) of above 95% can be obtained

for hX levels above 3 days. As presented in the simulation results

under aerobic and anoxic conditions, the degradation rate of slowly

biodegradable COD fractions is relatively slower under anoxic

conditions, as a result of the reduced rates of hydrolysis processes.

The anoxic hydrolysis rates are 60% lower than the aerobic

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

to different hX and hH couples, under both aerobic and anoxic

conditions, respectively. The dashed line indicates an SI level of

32 mg/L. The simulation study showed that nearly complete

degradation of soluble biodegradable COD can be achieved for

hX.2 days under both aerobic and anoxic conditions. Considering

the soluble inert COD baseline, the difference of 10 mg/L COD is

the result of the contribution of rapidly hydrolyzable COD (SH) with

inert soluble microbial products (SP). Total effluent soluble COD

(ST) rapidly increased as a result of the incomplete degradation

of rapidly hydrolyzable COD- SH for hX levels below 1.0 day. The

total effluent soluble COD (ST) and effluent rapidly hydrolyzable

COD (SH) are slightly higher under anoxic conditions, as a result of

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

organic carbon under anoxic conditions. From a modeling standpoint,

it is controlled by the readily biodegradable COD fraction, as nitrate

demand is associated with processes related to biomass growth and

decay. Consequently, hydrolysis of slowly biodegradable COD

compounds is the rate-limiting step in the development of NDP, with

18 significant operating parameters related to the anoxic phase, such

as the sludge age (hX) and the anoxic volume ratio (VD/V).

The model simulation results under anoxic conditions were used

to evaluate the relative contribution of the settleable biodegradable

COD fraction to the overall NDP of the system. The results have

shown that the denitrification potential (NDP) increases with in-

creasing VD/V ratios, as shown in Figure 8a. No significant increase

in NDP is observed above an anoxic sludge age of 3 days (i.e., total

sludge age of 6 days for VD/V 5 0.5), where almost all biode-

gradable COD is consumed by heterotrophic biomass. The gradual

increase in NDP with increasing sludge ages above an anoxic sludge

age of 3 days is the result of a similar increase in electron acceptor

consumption of decaying biomass (because the concentration of

biomass in the system increases with increasing sludge age). How-

ever, for single sludge systems, denitrification at low sludge ages

will not be possible, because the nitrifiers will be washed out from

the system under such short aerobic sludge ages. In addition, the

system would be limited by the available amount of biodegradable

COD at anoxic sludge ages less than 3 days, as a result of incom-

plete hydrolysis of XSS. The NDP generated by the degradation of

XSS for different sludge ages and VD/V ratios is presented in Figure

8b. The figure shows that the NDP contribution of the XSS com-

ponent increases with increasing VD/V ratios and an increasing

anoxic sludge age of 3 days, where almost complete biodegradation

of XSS is achieved. Thus, the maximum contribution of the

settleable biodegradable COD (XSS) to the total NDP generated in the

system can be as high as 40% for the domestic wastewater studied.

The maximum contribution of XSS is dependent on the design and

operating conditions. While the NDP of XSS is maximized at a total

sludge age of 6 days for a VD/V of 0.5, it can only be maximized if

the system is operated at total sludge ages higher than 15 days and

at a VD/V of 0.2. In addition, it should be emphasized that, in the

presence of a primary settler as a system component, shorter settling

times would introduce some XSS to the system, resulting in an

increase of NDP, depending on the settling efficiency and system

conditions. This may be needed for cases where NDP acts as the

limiting factor for the denitrification efficiency.

ConclusionsThe results of the respirometric evaluations in this study enabled

the establishment of a mass balance for the significant COD

fractions of domestic wastewater. In this context, the effect of

primary settling was assessed, not only as overall COD removal, but

also in terms of a new COD fraction, the settleable COD. This

fraction, removed by primary settling, was identified as a slowly

biodegradable substrate with a low hydrolysis rate of 1.0 day21,

providing a clear differentiation from the other particulate slowly

biodegradable COD components.

The efficiency of the primary settling is expected to be site-

specific. Accordingly, the results related to the fate of conventional

parameters, reported in this study, relate only to the selected

domestic wastewater. However, the proposed approach introduced

a generally applicable new concept of evaluating the settled COD

as a separate entity, which is well-defined in terms of its bio-

degradation characteristics. The settleable (biodegradable) COD

(XSS) was incorporated to a multicomponent model as a new model

component, with its hydrolysis kinetics. The fate of this COD

fraction could then be evaluated by means of model simulation,

which indicated that it would be totally hydrolyzed and removed

at a sludge age of 2 days. Model simulation, accounting for the

biodegradation kinetics respirometrically determined for XSS, with

similar characteristics of other COD components, identified a new

dimension of the settleable COD fraction as a possible source of

additional organic carbon that could directly contribute to the

denitrification potential of the system, if the total sludge age and

the VD/V are adjusted to secure an anoxic sludge age of over 2 days.

The result challenges the function of primary settling, especially in

activated sludge systems, where the denitrification potential asso-

Table 9—Kinetic and stochiometric parameters used inthe calculation of NDP under anoxic conditions.

Parameters Symbol Value Unit

Anoxic yield coefficient YHD 0.54 mgcellCOD/mgCOD

Endogenous decay rate* bH 0.20 day21

Anoxic growth correction

factor gg 0.80 —

Anoxic hydrolysis rate

correction factor gH 0.60 —

Endogenous decay

correction factor gE 0.48 —

* Converted from death regeneration concept.

Tas et al.

July 2009 725

ciated with the influent stream becomes rate-limiting for the desired

nitrogen removal efficiency.

The study also reports the effect of primary settling, in terms of

mass balances for significant conventional parameters. The conven-

tional characterization and mass balance were interpreted in terms

of significant ratios of selected parameters, which need to be incor-

porated to models for accurate prediction of the biodegradability of

domestic wastewater.

Submitted for publication July 17, 2007; revised manuscript

submitted December 22, 2008; accepted for publication January 23,

2009.The deadline to submit Discussions of this paper is October 15,

2009.

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