Pe~amon PII: S0043-1354(97)00209-1

Wat. Res. Vol. 32, No. 2, pp. 461-475, 1998 © 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0043-1354/98 $19.00 + o.oo

RESPIROMETRIC ANALYSIS OF ACTIVATED SLUDGE BEHAVIOUR~I. ASSESSMENT OF THE READILY

BIODEGRADABLE SUBSTRATE

E. U B A Y (~OKG(3ROt*, S. S(3ZEN ~t , D. O R H O N ~ t and M. HENZEO2

~Istanbul Technical University, Environmental Engineering Department, I.T.I~L In~aat FakOltesi, 80626 Maslak, Istanbul, Turkey and ~Department of Environmental Science and Engineering, Technical

University of Denmark, DK 2800 Lyngby, Denmark

(First received August 1996; accepted in revised form June 1997)

Abstract--Respirometric analysis of domestic sewage together with textile, dairy, meat processing, tan- nery and confectionery wastewaters were carried out for the experimental assessment of the readily bio- degradable COD. The accuracy and the reliability of the experimental procedure was tested using synthetic sewage and different wastewater mixtures. The merit of OUR and NUR measurements was investigated in parallel aerobic and anoxic batch reactors. The OUR data was analysed with specific emphasis to identify the lower plateau associated with the hydrolysis of slowly biodegradable substrate. The NUR data was corrected for nitrite formation to provide an accurate account of electron aeceptor consumption. The respirometric procedure and especially the effect of the initial substrate to biomass F/M (CTI/XTI) ratio was investigated using model simulation of the respirometry curve together with the collected experimental data. © 1998 Elsevier Science Ltd. All rights reserved

Key words--wastewater characterization, COD fractionation, biodegradation, readily biodegradable substrate, domestic sewage, industrial wastewaters

NOMENCLATURE INTRODUCTION

b H ffi endogenous decay coefficient O ' ' l ) Cs = total biodegradable COD (M-COD L -3)

Crt ffi total initial COD concentration (M-COD L -3) Crsl ffi settled COD concentration (M-COD L -3)

fa ffi fraction of endogenous mass converted into inert products

fx ffi COD equivalent of biomass (M-celI-COD(M-VSS) -t) Kh = hydrolysis rate constant (T "-1) Ks = half saturation coefficient (M-COD L -J) Kx = saturation coefficient for particulate COD

(M-COD(M-COD) -t) NO~-N =nitrite-nitrogen concentration (M-N L-3~ NO~-N = nitrate-nitrogen concentration (M-N L -°) Nox-N = oxidized nitrogen concentration (M N L -J)

NUR = nitrate uptake rate (M L -s T -t) OUR = oxygen uptake rate (M L -3 T "-t)

Sn = soluble inert COD concentration (M-COD L -3) So ffi oxygen concentration (M L -3) Sst = readily biodegradable COD concentration (M-COD L -3) S'rt ffi soluble COD concentration (M-COD L -3) XH ffi heterotrophic biomass concentration (M-VSS L -3)

Xm = heterotrophic biomass concentration in wastewater (M-COD L -s)

Xn = particulate inert COD concentration (M-COD L -3) Xp ffi particulate inert product (M-COD L -s) 3

Xst •slowly biodegradable COD (M-COD L- ) 3 XTt ffi total initial biomass concentration (M-VSS L- ) YH = heterotrophic yield coefficient (M-VSS(M-COD) -t)

YHD = anoxic yield coefficient (M-VSS(M-COD) -I) ANt = amount of e- acceptor consumed by growth on readily

biodegradable substrate (M-N L -s) #r~ = specific heterotrophie growth rate (T -t) ftr~ = maximum heterotrophic growth rate (T -'])

*Author to whom all correspondence should be addressed.

A significant development in the mechanistic understanding of the activated sludge process is the adoption of chemical oxygen demand (COD) as a model component for substrate and the ensuing concept of electron equivalence between substrate, active biomass and oxygen. This may be regarded as a turning point in ending the empirical and often misleading guesswork for substrate utilization and oxygen requirement calculations based on biological oxygen demand (BOD5). It was also very important in encouraging efforts for a better understanding of the nature and composit ion of substrate, leading to the concept of C O D fractionation. Biodegradable C O D fraction was introduced based upon the early observations that C O D also accounts for non-bio- degradable components (Eckhoff and Jenkins, 1967). Later, the bi-substrate model proposed by Dold et aL (1980) recognized the fact that the wide array of organics in wastewaters may be evaluated in two broad groups represented by markedly different rates of biodegradation. This approach was further elaborated for the identification and the mechanistic description of readily biodegradable and slowly biodegradable C O D components (Henze et al., 1987).

The correct assessment of the readily biodegrad- able C O D in wastewaters (Ssl) is of great theoreti- cal and practical significance, not only because this

461

462

parameter is the only substrate component directly related to microbial growth in current models, but also because it allows the calculation of the other important COD fraction, namely the slowly biode- gradable COD(Xsz), representing the bulk of the influent COD content and often the critical model component for the modelling and design of acti- vated sludge systems, especially for industrial waste- waters.

The objective of this part of the study was first to collect reliable experimental data on the readily bio- degradable COD content of domestic sewage and selected biologically treatable industrial wastewaters by means of the respirometric analysis of activated sludge behaviour in batch reactors. While COD fractionation of sewage has so far been investigated to a limited extent, practically no similar data exist for industrial effluents. Consequently, the results of the study are expected to provide essential infor- mation for the kinetic evaluation of such waste- waters. The second and equally important objective was to test the accuracy and the reliability of the adopted procedure by means of experiments carded out with synthetic sewage and wastewater mixtures. The merit of OUR and NUR measurements was investigated in parallel aerobic and anoxic batch reactors. Finally, kinetic interpretation of the col- lected experimental data were used for a better understanding and interpretation of the respiro- metric procedure.

CONCEPTUAL BASIS

At present, collecting the necessary information related to readily biodegradable COD, as an inte- gral part of a comprehensive package of other rel- evant kinetic and stoichiometric data, by means of

E. Ubay (~okg6r et al.

scientifically acceptable and technically reliable ex- perimental procedures, appears to be one of the challenging aspects of activated sludge modelling. Almost all the methods proposed for the determi- nation of readily biodegradable organic matter rely on respirometric analysis of activated sludge beha- viour. They either propose OUR measurements in dynamic continuous systems (Ekama et al., 1986) or OUR/NUR measurements in aerobic/anoxic batch reactors (Ekama et al., 1986; Sollfrank and Gujer, 1991; Kappeler and Gujer, 1992; Kristensen et al., 1992). Batch reactors are often preferred, as in the present study, because they are simpler to operate and problems and interferences inherently associ- ated with reactor hydraulics are avoided.

The aerobic batch test consists of obtaining and evaluating an oxygen uptake rate profile (OUR curve), with a preselected initial substrate to hetero- trophic biomass, (CTI/Xrl) , ratio. The following mathematical equation, derived in accordance with the endogenous model (Orhon and Artan, 1994) may be used to find Ss~ from the OUR curve:

z~S0 Ss~ = ~ (1)

1 --fxYH

In the current practice the term ASo, is defined as the area above the second plateau of the OUR pro- file, as indicated in Fig. 1 (Ekama et al., 1986).

The same parameter may also be calculated from the N profile in an anoxic batch reactor. In this test, the readily biodegradable COD fraction of the wastewater sample is detected as associated with the initial N utilization rate. The amount of N con- sumed due to the oxidation of S s l , corrected for the interference of the hydrolysed substrate as shown in Fig. 2, may be used to calculate the readily biode- gradable COD content of the sample, by means of

40

35

" " 30

""~ 25

15

10 4 I • I I 0 20 40 60 80 100

Fig. 1. The OUR test for the assessment of the readily biodegradable COD in domestic sewage (run D14).

Assessment of the readily biodegradable substrate 463

1- I O Nea-N lgO ~ ,, i- r', N O / - H

170

1 O 130

l t ~ t

A NO: - N + 0 ~ O / - N

120 0 50 100 I50 200 250

Fig. 2. The NUR test for the assessment of the readily biodegradable COD in domestic sewage (run D 15).

the following relationship:

2.86 Ss~ = 1 - A Y n ANl (2)

MATERIALS AND METHODS

All analyses were performed as defined in Standard Methods (APHA, 1989). Settled sewage was obtained as the supematant of a cylindrical settling column after a holding time of 2 h. The soluble (filtered) COD was defined as the filtrate through Whatman GF]C glass-fibre filters, also used in the determination of VSS and SS.

The respirometric procedure for the assessment of the readily biodegradable COD consisted of running l-litre batch reactors. In the OUR test, nitrification inhibitor (Formula 2533 TM, Haeh Company, IOWA, U.S.A.) was added to prevent any possible interference induced. The reactor was constantly aerated to maintain a dissolved oxygen concentration of 6--8rag litre -t, and the NUR reactor was kept anoxic by nitrogen gas bubbling. Both reactors were initially fed with the wastewater sample and seeded with appropriate biomass to start with a suitable initial CTI/,YTI (F/M) ratio. The biomass was previously acclimated to the same sample in a fill-and-draw reactor operated in a sequence of anoxie and aerobic conditions at a sludge age of 7-10 days. Aliquots were removed from reactors every 10--20rain for OUR and NUR measure- ments. OUR measurements were conducted with a DIGI, Weilheim, Germany, oxygen meter and recorder. Nox and NO~ tests were performed by means of a LAB, Essex, U.K. autoanalyser using the hydrazine reduction method. In the experiments the samples were adjusted to a pH of 7-8, a range suitable for biological activity; the tests were carried out at room temperature.

EXPERIMENTAL RESULTS

The respirometric measurements for the assess- ment of the readily biodegradable COD fraction were conducted both on domestic sewage and

industrial efliuents including textile, dairy, meat processing, tannery and two different confectionery wastewaters, exhibiting a wide spectrum of organic matter content and structure, reflected by COD concentrations varying from 990 nag litre -~ for tex- tile wastes to 3630mg litre - l for confectionery wastes. Table 1 outlines the conventional character- ization of the wastewater samples used in the exper- imental study.

Readily biodegradable COD fraction of domestic sewage

The assessment of readily biodegradable COD was carried out as part of a comprehensive charac- terization study on Istanbul sewage. Experimental evaluation was performed on 16 different samples from the Kadik6y region, over a period of more than a year, reflecting different environmental con- ditions that affect sewage composition and quality. Kadikty, located on the Asian side of the city is the collection point of around 24% of the sewage generated within the Metropolitan Area of Istanbul, representing a wastewater flow rate of 3.7 m3s -~ with a daily COD load of 120 t (Orhon, 1995). The COD concentration of the samples investigated var- ied within the range of 410-870 mg litre - l , with a mean value of 587mg litre -1, higher than 410- 430 mg litre -x ascertained as the average COD con- tent of the domestic sewage in Istanbul (Orhon et al., 1994ab, 1997). The average total soluble COD fraction (ST0 of the samples was 183 mg litre -I, accounting for 30% of the total COD. The readily biodegradable COD fraction, (Ss0 was calculated on the basis of OUR measurements. Similar calcu- lation were also performed with NUR measure- ments to provide a basis for comparative

464

o

o

c l

o

m ~ m ~

m ~

E. Ubay (~okg6r et al.

evaluation. The results obtained are outlined ifi Table 2. The statistical distribution of results were given in Fig. 3.

The OUR measurements evaluated using equation 1 were found to yield an Ssl range of 23- 86 mg litre -~, with an average Ss~ concentration of around 53 mg iitre -1, corresponding to around 9% of the influent total COD (Ssl/CTI =0.088), 13% of the influent settled COD (Ssl/CTs=O.13) and 28% of the influent soluble COD (Ssl/STt=0.282). For the average COD content of Istanbul sewage (410- 430 mg litre-1), this ratio would correspond to a readily biodegradable COD fraction of around 40 mg litre -1.

Table 3 lists results of selected similar studies conducted on domestic sewage since the introduc- tion of the COD fractionation concept with the Task Group report in 1986 (Henze et al., 1987). The table shows that the findings of this study de- fine a somewhat stronger sewage with a lower readily biodegradable COD fraction than most others, explainable with the basic understanding that sewage quality and structure are quite site- specific and not amenable to numerical comparative interpretation.

Experimental results also showed that Ssl values obtained with NUR measurements were slightly but consistently higher than those computed with OUR tests, a difference that could be characterized by an average Ssl(NUR)/Ssl(OUR) ratio of I. 14.

Readily biodegradable COD fraction of industrial wastewaters

The results of OUR tests for the assessment of the readily biodegradable COD in selected indus- trial effluents are outlined in Table 4. It should be noted that the samples were prepared to represent the expected quality of biological treatment influent. Therefore, chemical settling effluents were collected as tannery samples and DAF effluent as meat pro- cessing and dairy samples respectively, because they are routinely subjected to these types of pretreat- ment before biological processes, whereas raw wastewater samples were used for the other indus- tries which do not normally require pretreatment. It was generally observed that industrial effluents tested contained a higher readily biodegradable COD fraction compared to domestic sewage, namely 0.13 for meat processing, 0.15 for textile and tannery, 0.23 for dairy and 0.44 for confection- cry (Sfzen, 1995; Ubay (~okgrr, 1997; Orhon et al., 1995). The corresponding Sst concentrations ranged from 140 mg litre -1 for textile effluents to 850 mg litre -t for confectionery discharges. It was also noted that the results were much more consistent than the ones obtained for domestic sewage, show- ing no appreciable variation between different samples for a given industry.

Assessment of the readily biodegradable substrate

Table 2. Evaluation of experimental results for domestic wastewaters (S6zen, 1995; Ubay (~okg6r, 1996)

465

Cxt/~'xl Sst Sst (rag-COD CT! I CTsl t ST1 (OUR) (NUR) Ssl (NUR)/ SsI/CTI SsI/CTsI SsI/STI

Run No. (rag-COD) -1) (rag litre- ) (rag litre- ) (rag litre -1) (rag litre -1) (rag litre -t) Ssl (OUR) (OUR) ( O U R ) (OUR)

D I 0.45 505 300 180 86 105 1.22 0.170 0.287 0.478 D2 0.24 500 - - 175 62 77 1.24 0.124 - - 0.354 D3 0.56 550 - - 160 24 30 1.25 0.044 - - 0.150 D4 0.86 580 - - 150 75 53 0.71 0.129 - - 0.500 D5 0.48 410 340 130 32 42 1.31 0.078 0.094 0.246 D6 0.48 440 350 140 - - 42 . . . . D7 0.48 425 - - 130 40 48 1.20 0.094 - - 0.308 D8 0.53 650 470 220 61 68 1.12 0.094 0.130 0.277 D9 0.43 630 - - 230 50 56 1.12 0.079 - - 0.217 D10 0.45 410 - - 125 - - 36 . . . . . D I I 0.54 840 585 220 80 92 1.15 0.095 0.137 0.364 D I 2 0.60 500 420 155 23 26 1.13 0.046 0.055 0.148 D 13 0.45 770 - - 265 42 44 1.05 0.055 - - 0.158 D 14 0.55 800 530 240 60 74 1.23 0.075 0.113 0.250 D I 5 0.73 525 450 185 45 50 1.11 0.086 0.100 0.243 D I 6 0.85 870 - - 230 58 64 1.10 0.067 - - 0.252 Mean 587 444 183 52.7 59.2 1.139 0.088 0.129 0.282 SD 156.3 98.43 45.05 19,74 22.44 0.138 0.034 0.072 0 A I 0 Range 410-870 300-585 130-265 23-86 26-105 0.71-1.31 0.044-0.17 0.055-0.287 0.148-0.5

E V A L U A T I O N O F R E S U L T S

Aside from data generation on the readily biode- gradable COD contents of domestic sewage and sig- nificant industrial effluents, the experiments also involved evaluations reflecting the reliability of the respirometric approach used in the study. The con- sistency of the procedure was tested with duplicate runs conducted under different experimental con- ditions. As outlined in Table 5, these runs per- formed with initial biomass acclimated under various conditions, and at different F /M ratios all yielded quite reproducible results.

The experimental accuracy was investigated by testing the ability of the procedure to recover and reflect known amounts of readily biodegradable

COD, using synthetic substrate, synthetic substrate- sewage mixtures and industrial--domestic waste- water mixtures.

Experiments with synthetic wastes

In the first part of the experiments, glucose was utilized as substrate; OUR and N U R measurements were conducted on two parallel reactors seeded with glucose-acclimated biomass previously grown and sustained in a fill-and-draw system continu- ously operated at a sludge age of 10 days and started with an initial glucose concentration equiv- alent to 100 rag-COD litre -1. The results are plotted in Fig. 4. As shown in Fig. 4(a), OUR measure- ments exhibited an erratic profile before being

7,

"~ 0,1

D

S s i / S T I

0,01 ~ i ~ ) ) i ) I , 2 % I 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

Cmxda~e Prc~ali'F Fig. 3. Statistical distribution of Ssl, SSl/$TI and Ssl/Cxl in domestic wastewaters on the basis of

OUR measurements.

466 E. U b a y (~okg/ir e t al.

Table 3. Reported COD fractionation results for domestic wastewaters

CT~ o~; Crs~ (rag litre - l) Sn (%) Sst (%) Xsl (%) Xm (%) Xit (%) Reference

Raw South Africa 530 5 20 62 - - 13 Ekama et al. (1986) Denmark I - - 2 20 40 20 18 Henze (1992)

Primary South Africa 370 8 28 60 - - 4 Ekama et al. (1986) Switzerland 220 11 32 45 - - 11 Henze et aL (1987) Hungary 350 9 29 43 - - 20 Henze et al. (1987) Denmark II 515 8 24 49 - - 19 Henze et al. (1987) Denmark I - - 3 20 43 14 11 Henze (1992) Switzerland I 250 20 11 53 7 9 Kappeler and Gujer (1992) Switzerland II 430 10 7 60 15 8 Kappeler and Gujer (1992) Switzerland III 325 12 8 55 15 10 Kappeler and Gujer (1992) Switzerland IV 190 8 I0 56 - - 26 Siegrist et al. (1995) Switzerland V 250 8 10 58 - - 24 Siegrist et al. (1995) Switzerland V1 360 10 16 40 25 9 Sollfrank (1988) Spain 340 9 18 33 15 25 De la Soto et al. (1994) France I 450 10 33 44 - - 13 Lesouef et al. (1992) France I1 345 6 25 41 - - 8 Lesouef et al. (1992)

stabilized as a final plateau after 3 h. The fluctu- ation of the NUR data, corrected for NO2 accumu- lation, was more tempered at the initial stage, giving an appreciable change in slope after more than 4 h, as given in Fig. 4(b). The numerical in- terpretation of the data yielded Sst values of 110 and 116 mg litre -t with OUR and NUR profiles, re- spectively, corresponding to a recovery potential with an approximation of around 10%. Ss values were calculated by using the same yield coefficient of 0.45 g-VSS(g-COD) -l also adopted for domestic sewage, as the assumption of a similar yield coeffi- cient for simple carbonaceous, glucose and domestic

sewage is justifiable on the basis of energetic con- siderations.

Glucose has been used in many studies related to the biodegradation of simple substrates with the sti- pulation that the associated experimental procedure is straightforward and easily interpretable. The reported experience, however, has proven just the opposite due to the complex sequence of biochemi- cal reaction this compound undergoes. The OUR profile in Fig. 4(a) provides another proof that glu- cose is not fit for respirometric analysis. Therefore, in the second part of the experiments, a mixture consisting of acetic acid, propionic acid, ethanol,

Table 4. Evaluation of experimental results for selected industrial wastewaters

CTI STI CTI/XTI SSi (OUR} SS I/CTI SSI/STI Industry Set No, (rag litre - t ) (rag litre - t) (rag-COD (mg-VSS) -t) (rag litre-') (OUR) (OUR)

Textile I1 1075 710 0.80 133 0.124 0.187 12 935 470 1.00 135 0.144 0.287 13 785 560 0,47 149 0.189 0.266

Mean 932 580 139 0.152 0.247 Dairy 14 1815 1180 1,50 384 0.212 0. 325

1.28 410 0,226 0,347 15 1675 960 1.25 404 0.241 0.421

0.72 390 0.233 0.406 Mean 1745 1070 397 0.228 0.375 Tanneries I6 2500 1295 0.78 460 0.184 0.355

17 2410 1205 0.81 300 0.125 0.249 I8 2175 1300 0.52 340 0.156 0.262

Mean 2362 1267 367 0.155 0.289 Meat processing 19 2600 900 0.88 380 0.146 0.422

I10 2900 950 0.87 410 0.141 0.432 I11 3255 1145 0.57 385 0.118 0.336 112 2300 800 1.10 295 0.128 0.369

Mean 2764 949 368 0.133 0.390 Confectionary I

Confectionary II

Mean

I 13 3790 3070 I. I 0 1320 0,348 0.430 2. I0 1235 0.326 0.402

114 2540 2235 I. 14 I lO0 0.44 0.50 I 15 1950 1700 0.59 875 0,45 0.51 ll6 1800 1495 0,65 840 0.47 0.56 I17 1470 1230 0.72 600 0.41 0.49

1940 1665 854 0.44 0.52

Assessment of the readily

Table 5. The accuracy of the OUR experiments tested with dupli- cate runs under different conditions

Wastewater C'rl/X'rl Measured Type of used for (rag-COD Ssl wastewater acclimation (rag-COD) -I) (rag litre -I)

Domestic DI3 Domestic 0.45 42 Meat 0.45 46 processing

Domestic DI7 Dairy 0.97 75 Confectionary 2.1 74

Domestic DI4 Domestic 0.55 60 1.1 60

Confectionary I17 1.1 1320 2.1 1235

Domestic D7 raw settled

Confectionary

Domestic 0.48 40 0.48 42

ghitarnic acid and glucose, previously proposed to simulate the readily biodegradable COD of dom- estic sewage (Henze, 1992), was used as the syn- thetic waste in proportions indicated in Table 6. The batch aerobic and anoxic reactors were run with initial COD concentrations in the range of 57- 156 mg litre - l and with biomass acclimated to dom- estic sewage. The results outlined in Table 7 may best be evaluated in two groups. In the first group of experimental runs, the initial COD/VSS ratios (CT1/XTO set as 0.45-0.60 were clearly too high for the completion of the tests, so that the initial COD was only partially consumed at the end of the ex- periment, especially in the aerobic reactors. In the second group however, complete recovery of the added readily biodegradable substrate was achieved with an acceptable accuracy of 5-10%, by lowering the initial COD/VSS ratio to 0.13-0.22. In the duplicate sets of run $6, for example, the added COD concentration of 57 mg litre - l , was measured as 58 mg litre -1 in the first set, whereas the second set recovered 108rag litre - l COD as 117 and l l 0 m g litre -~ in the anoxic and aerobic reactors, respectively.

A significant point of interest reflected by the ex- perimental results is the observation that the group of organics assumedly behaving like the readily bio- degradable COD fraction of domestic sewage is not reduced at a single overall rate but at appreciably different rates. The storage of a part of readily bio- degradable organics may also exert some effect in the observation of different sequential removal rates. The rate difference for selected organics is depicted as significant diversions in the slope of the N profile in the NUR experiments (Fig. 5a), or as intermediate plateaus in the OUR experiments (Fig. 5b). The various COD fractions so identified in the NUR experiments are indicated in Table 7. This observation also provides an explanation for the discrepancy between added and measured COD concentrations in the first group of experiments. In fact, in this group, most runs were stopped after 2- 3 h as illustrated in Fig. 6, with the identification of a distinct change in the N U R or OUR profile,

biodegradable substrate 467

assuming that the endogenous respiration zone was reached. In reality, this was only an indication of the COD fraction with a lower biodegradation rate.

Experiments with synthetic waste-sewage mixtures

Another group of experiments testing the accu- racy of the adopted method consisted of running parallel respirometric tests with domestic sewage and mixtures with known amounts of synthetic sub- strate. Table 8 outlines the evaluation of the exper- imental results. It is noted that in the majority of experiments, the respirometric tests were able to recover the readily biodegradable substrate as the total of the measured Ssl in the sewage and the added COD by the synthetic waste. In test D11, the recovery was only partial mainly because the test duration was 3 h, too short to consume the entire readily biodegradable COD due to differential bio- logical breakdown explained above. In run D16, the discrepancy between measured and calculated COD values was more attributable to the high value of the initial COD/VSS ratio (CTt/XTI ratio), requiring a longer reaction time for the completion of the test.

Experiments with domestic-industrial wastewater mixtures

Together with experiments on industrial waste- waters, additional tests were also run in parallel on domestic sewage and selected volumetric mixtures of domestic sewage and industrial wastewaters. The reactors were started with biomass previously accli- mated to the industrial and domestic wastewater samples. The results are given in Table 9. A point of interest to be noted was difficulty in performing OUR measurements with meat processing effluents in aerated reactors due to excessive foaming. Some of the tests could not be completed for these oper- ational problems and could only be complemented with NUR data. Generally, the sum of the individ- ual Ssl fractions could be recovered as the total readily biodegradable COD of the mixture with an approximation of 3-10%, with slightly higher observed levels especially for mixtures involving dairy and confectionery wastes. It should be noted that mixing two wastewaters of different character is likely to create a new environment enhancing hy- drolysis reactions to generate additional readily bio- degradable COD or a broader microbial consortium for which a bigger fraction of the COD will be readily degradable. In this framework the results obtained should best be regarded as the accurate experimental assessment of the COD frac- tionation in the mixture, rather than analytical dis- crepancies.

Kinetic structure of NUR experiments

A major issue to be considered in connection with NUR measurements is the assumption that the heterotrophic yield remains the same under aerobic

468 E. Ubay (~okg6r et al.

35

3O

25

2O

10

1 i i t

50 1 O0 150 200

Time, (minutes)

a) the OUg test

D

!

250

30

25 0

300

O Nor N

0 NOf-N

A NOr- N+0~NO,'- N

~ 20

z

10

5

0 0

!

50 350

0 0 0

0 °

0

0 [] O 0

I I ) I I '

1 ~ 150 2 ~ 250 300

T~, (nmmes)

b) the NUR test Fig. 4. The respirometric tests for the assessment of the readily biodegradable COD of glucose:

(a) the OUR test, (b) the NUR test.

and anoxic conditions. This is also the case in the assessment of the readily biodegradable substrate, as illustrated in equation (1) and (2). There is, how- ever, conceptual evidence that the yield coefficient is

Table 6, COD composition of the synthetic wastewater

Component Fraction (% COD)

Acetic acid 41 Propionic acid 17 Ethyl alcohol 8 Glutamic acid 17 Glucose 17

bound to be significantly lower for the anoxic growth on the basis of the energetics of related metabolic processes (Orhon et al., 1996). A lower anoxic yield coefficient, YHD induces a positive error of

1 -fx Y.o 1 - A Y H

in the calculation of Sst on the basis of NUR data, by means of equation (2). In fact, Sst for domestic sewage calculated using NUR measurement were observed to be systematically higher than the ones computed using OUR data, with an average value

Assessment of the readily biodegradable substrate

Table 7. Readily biodegradable COD results with synthetic wastewater

469

S e t n o .

Measured Sm (rag [itre -j)

CTI/XT1 Added COD (mg-COD (mg-VSS) -I) (rag lit.re -I) (NUR) (OUR)

SI $2 $3 $4 $5 $6

$7 $8 $9 SI0

0.60 118 13;50 53 0.45 58 10;36 35 0.45 115 53 46 0.45 58 19;54 18 0.50 117 64;112 55 0,13 57 28;58 - -

108 35;I 17 110 0.20 62 36;55 50 0+22 72 29;63 - - 0.22 156 16;77;135 125 0.19 60 - - 67

25 O O

" - ~ " ~ ' ~ - O & NO3"- N + 0~'NO~'- N

o o ~ . . . . . + e . ~ ° - ~ ' ~ " o o o 20

0 0 15

4"

rt 0

ra I0

0 0 1:3

I I I I ' I

0 50 I00 150 200 250 300

T~me, (nmu~)

a) ~ ~ - l t test

4O

30

10

I l ! J l l +

50 100 150 200 250 300 :$50

T, rne, (nmutm)

400

b) the OUR test Fig. 5, Identification o f different biodegradation rates for synthetic waste (run $7): (a) the N U R test,

(b) the O U R test.

470 E. Ubay (~okgtr et al.

4 0 '

O

35

3O

2~ I I I

0 50 100 150

Tim, (mimms) Fig. 6. Incomplete NUR test for the assessment of Ssl for synthetic waste (run $2).

200

of 1.14 for the above ratio, as shown in Table 2. Since a YH value of 0.45 g-VSS (g-COD)-' was adopted in the study, the corresponding YI~D value can be calculated as 0.37g-VSS (g-COD) -~, or 0.53 g-cell-COD (g-COD) -t , from this ratio. This value is in excellent agreement with a YHD of 0.50 g-cell-COD (g-COD) -~, theoretically calculated for domestic wastewater on the basis of energetic considerations (Orhon et al., 1996). Data on indi- vidual industrial effluents are too few to draw simi- lar conclusions.

An equally important issue is the assessment of the accurate amount of electron acceptor during the NUR test. Measurements of Nox-N or NO~-N do not yield acceptable results when coupled with ap- preciable NO~ accumulation in the course of the ex- periment. In such cases, the electron equivalence of the readily biodegradable COD consumption is best satisfied by the following equation (Henze, 1986; S6zen and Orhon, 1996; S6zen, 1995).

N = NO~ - N + 0.6NO 2 - N

NUR experiments in this study provided irrefutable proof for the necessity of NO~" correction in the in- terpretation of experimental data. As shown in Figs 2, 4(b) and 5(a), the erratic nature of the data could only be rectified to linear trends after NO~- correc- tion.

Significant feature of the NUR data, linearized with appropriate NO~- correction, is the ability to depict differential biodegradation rates for portions of what is basically conceived as readily biodegrad- able COD. This was observed in most experiments with synthetic waste, where 30-50% of the C O D

content was found to be consumed at a rate ap- proximately twice higher that than the remaining portion, as illustrated in Fig. 5(a). A similar rate differentiation within readily biodegradable COD was also noticed for domestic and industrial waste- waters.

Table 8. Readily biodegradable C O D results with synthetic wastewater-scwagc mixtures

Synthetic C O D CTI/XTI added

Set no. Sample type ( r ag -COD (mg-VSS) - I ) (rag litrc - I )

Measured Calculated Ssl (mg litrc - I ) Ssl (mg litre -~)

( N U R ) ( O U R ) ( N U R ) ( O U R )

D9 Domest ic 0.43 - - 56 50 - - - - Mixture 0.50 67 31; 118 98 123 l 17

0.57 117 80;191 162 173 167 D L0 Domest ic 0,45 - - 36 - - - - - -

Mixture 0.45 236 80;306 276 272 - - D 11 Domest ic 0.54 - - 92 80 - - - -

Mixture 0.54 60 85 85 152 140 D l 2 Domest ic 0.60 - - 26 23 - - - -

Mixture 0.50 116 144 144 142 139 D 15 Domest ic 0.73 - - 50 45 - - - -

Mixture 0.73 97 132 130 147 142 D 16 Domest ic 0.85 - - 61 58 - - - -

Mixture 0.85 110 110 100 174 168

Assessment of the readily biodegradable substrate 471

o

I

o o o

g o

o

o o o o

o o

~ m m ~

0 o o

Kinetic structure o f OUR experiments

The aerobic batch experiments rely on the assumption that the high OUR level in the first phase of the test relates to the utilization of the readily biodegradable COD initially present in the wastewater; with the depletion of SSh the OUR is expected to drop to a lower plateau only correlated to the hydrolysis of the slowly biodegradable COD, Xst (Ekama et al., 1986). The current mechanistic explanation of the activated sludge process, how- ever, involves simultaneously occurring microbial processes, as described by the endogenous decay or the death-regeneration models, the two major mod- elling approaches accepted today (Henze et al., 1987; Orhon and Artan, 1994).

A thorough understanding of the OUR exper- iment is only possible through the evaluation and interpretation of the OUR curve by means of model simulation. For this purpose, the mass-bal- ance equations of the endogenous decay model were solved for the set of kinetic and stoichiometric data given in Table 10, previously ascertained as part of wastewater characterization for Istanbul (Orhon et al., 1994b). A brief explanation about the multi-component endogenous decay model is given in the Appendix. The evaluation related to the assessment of Ssl is illustrated in Fig. 7. The figure shows separately the theoretical OUR profiles as- sociated with Ssl (OUR 1) and Xsl (OUR 2) frac- tions alone, and the true OUR profile (OUR 3) calculated for domestic sewage, reflecting the com- bined effect of Ssl and Xsl. All three curves also include the 02 consumption due to endogenous res- piration. As the figure clearly shows, the experiment can only yield the overall OUR profile (OUR 3) and defines Ss~ in proportion to the area between the OUR 3 and OUR 2 curves. The overall OUR profile is distinctly different from the OUR curve induced by Ssl (OUR l) and yet it provides an accurate means of evaluating this COD fraction. Model outputs in Fig. 7. may be used to bring a logical explanation to this statement. Comparison of OUR I and OUR 3 profiles clearly shows that the presence of Xsl or the additional effect of

Table 10. Kinetic and stoichiometric data used in the model simu- lation

Parameter Value

Csl 240 mg-COD litr¢ -I Xul 470 mg-COD litre -t Csl/Xm 0.51 mg-COD(mg-COD) -t Ssl 40 rag-COD litre -I Xsl 200 mg-COD litre -I YH 0.67 mg-COD(mg-COD) -l

fE 0.20 mg-COD(mg-COD) -~ ~a 3.5 d -1 Kh 2.0 d -~ Kx 0.20 mg-COD(mg-COD) -I b~ 0.24 d -I

472 E. Ubay ~okgrr et al.

35

30

~ , 25

10

5 10 15 20 25 30 35- 40 45 50 55 60

"rime, (minm) Fig. 7. Impact of Ssl and Xsl on the OUR profile of domestic sewage.

65 70

hydrolysis does not affect the initial portion of the overall OUR curve dictated by the maximum growth rate. Hydrolysis of Xs~ only provides a slight OUR increase in the following stages of the experiment. In other words, in the early phase of the experiment, Ss generated through hydrolysis cannot be totally degraded due to kinetic limi- tations imposed on microbial activity and an Ss surplus is gradually formed; consequently, the area under the OUR 2 (area 1) curve may be interpreted as the amount of Ss~ not utilized at this stage, due to additional input through hydrolysis. Model simu- lation shows that the Ss surplus is gradually reduced in the later stages of the experiment where the substrate utilization potential of the biomass exceeds the hydrolysis rate and the equilibrium is established at the Xsl plateau after the depletion of all remaining SSl. In this sense, the area between overall OUR profile and SSl curve (area 2) is also equivalent to the Ss excess or the remaining Sst fraction and this way, it is identical to the area under OUR 2 (area I).

The accuracy in the evaluation of SSl relates to the way in which the initial OUR value starting the lower plateau is detected. The extention of the tran- sition area before the second plateau is likely to cre- ate significant experimental errors. The CTI/.~TI (F/ M) ratio largely affects the shape of the OUR curve and the observation duration of the lower plateau. As illustrated by simulated and observed OUR curves for domestic sewage plotted in Fig. 8, it becomes increasingly difficult to identify the hy- drolysis zone from the transition area at high F/M values, imparting a smoother shape to the OUR profile.

CONCLUSIONS

In this study, observations of scientific and prac- tical significance, related to the assessment of readily biodegradable COD in domestic and indus- trial wastewaters by aerobic and anoxic respirome- try lead to the following conclusion.

The average readily biodegradable COD fraction was determined as 9% of the total COD and 13% of the settled COD content of Istanbul domestic sewage, corresponding to an SSl concentration of around 50 mg litre -l. These ratios compared with similar data reported for different countries show that sewage character with respect to COD fraction- ation is quite site-specific.

The readily biodegradable COD fraction of the industrial wastewaters tested were found to be higher than that of domestic sewage, varying from 13% for meat processing to 44% for confectionery effluents. The experimental results indicate that the readily bio- degradable, the slowly biodegradable and the inert COD fractions in relative to total COD are a useful characterization of a strong industrial waste.

Parallel tests run at different F/M ratios and under different biomass acclimation conditions, together with experiments conducted using synthetic waste and domestic-synthetic waste mixtures, pro- vided acceptable proof that the respirometric approach adopted in the study was quite reliable.

The respirometric analysis under anoxic con- ditions should be evaluated with appropriate NO~- correction to provide an accurate account of elec- tron acceptor consumption and with the consider- ation that the anoxic yield (Yrm) is likely to be appreciably lower than its aerobic counterpart, a fact overlooked in the current modelling practice.

Assessment of the readily biodegradable substrate 473

70

6O

50 d "

~ 4 0

30

20

10

0

gO

"• " " " " F / M = 0 , 3 3

• ' F/M : 0,~

. , F / M = I

. . . . F / M - 2

I • , . t , - - t ' I . . . . .

0 50 100 150 200 250 300

(minuS)

70

6O

4O

2O

10

F / M - 1.1

F / M - 0 . 5 5

20 40 60 gO

b) rmats

; . . . . ,

100 120 140

Fig. 8. The effect of F/M ratio on the OUR profile: (a) model simulation, (b) experimental results (run D14).

Also, the nature of the NUR test does not allow high sensitivity for the assessment of readily biode- gradable COD at low concentration, as was the case for domest ic sewage.

Experiments under aerobic conditions are best interpreted by model simulation of the OUR curve using applicable kinetic and stoichiometric data; as such, the experimental assessment of the readily biodegradable COD should only be conceived as part of a comprehensive investigation of wastewater characterization and treatability. The results showed that the observation of a distinct lower plateau was

essential for an accurate evaluation, and this was only possible with the choice of an appropriate F / M ratio and test duration.

R E F E R E N C E S

APHA (1989) Standard methods for the exarainatioa o f water and wastewater, 17th American Public Health Association, Washington, D.C.

De |a Sota A., Larrea L., Novak L., Grau P. and Heaze M. (1994) Performance and model calibration of R-D-N process in pilot plant. War. Sci, TeehnoL 30 (6), 355- 364.

474 E. Ubay ~okgtr et al.

Dold P. L., Ekama G. A. and Marais G.v.R .(1980) A general model for the activated sludge process. Prog. Wat. TechnoL 12, 47-77.

Eckhoff D. W. and Jenkins D. (1967) Activated sludge systems-kinetics of the steady and the transient states. SERL Report 67-12. Univ. of California, Berkeley.

Ekama G. A., Dold P. L. and Marais G.v.R (1986) Procedures for determining influent COD fractions and the maximum specific growth rate of heterotrophs in activated sludge systems. Wat. ScL TechnoL 18 (6), 91- 114.

Henze M. (1986) Nitrate versus oxygen utilization rates in wastewater and activated sludge systems. Wat. Sci. TechnoL 18 (6), 115-122.

Henze M. (1992) Characterization of wastewater for mod- elling of activated sludge processes. Wat. Sci. Technol. 25 (6), 1-15.

Henze M., Grady C. P. L. Jr, Gujer W., Marais G.v.R and Matsuo T. (1987) Activated sludge model No. 1. IAWPRC Sci. and Technol. Report No. 1. IAWPRC, London.

Kappeler J. and Gujer W. (1992) Estimation of kinetic parameters of heterotrophic biomass under aerobic conditions and characterization of wastewater for acti- vated sludge modelling. Wat. ScL Technol. 25 (6), 125- 139.

Kristensen G. H., Jorgensen P. E. and Henze M. (1992) Characterization of functional microorganism groups and substrate in activated sludge and wastewater by AUR, NUR and OUR. War. ScL Technol. 25 (6), 43-57.

Lesouef A., Payraudeau M., Rogalla F. and Kleiber B. (1992) Optimizing nitrogen removal reactor configur- ations by on-site calibration of the IAWPRC-activated sludge model. War. Sci. TechnoL 25 (6), 105-124.

Orhon D. (1995) Scientific basis for wastewater treatment and disposal in Istanbul. Wat. Sci. Technol. 32 (7), 191-198.

Orhon D. and Artan N. (1994) Modelling o f activated sludge systems, Technornic Press, Lancaster, PA.

Orhon D., Uslu O., Merit S., Saliho~lu I. and Filibeli A. (1994a) Wastewater management for Istanbul: basis for treatment and disposal. Environ. Poll. 84, 167-178.

Orhon D., S6zen S. and Ubay E. (1994b) Assessment of nitrification-denitrification potential of Istanbul dom- estic wastewater. Wat. Sci. Technol. 30 (6), 21-30.

Orhon D., Yddiz G., Ubay (~okg6r E. and S6zen S. (1995) Respirometric evaluation of the biodegrad- ability of confectionary wastewater. War. Sci. Technol. 32 (12), 11-19.

Orhon D., Atef E., Stzen, S and Ubay ~okgtr E. (1997) Characterization and COD fractionation of domestic wastewaters. Environ. Poll. 95, 191-204.

Orhon D., S6zen S. and Arran N. (1996) The effect of het- erotrophic yield on the assessment of the correction fac- tor for the anoxic growth. Wat. Sci. Technol 34 (5-6), 67-74.

Siegrist H., Krebs P., B~hler R., Purtschert I., Rtck C. Rufer R. (1995) Denitrification in secondary clarifiers. War. Sci. Technol. 31 (2), 205-214.

Sollfrank U. (1988) Bedeutung Organischer Fraktionen in Kommunalem Abwasser im Hinblick auf die Mathematische Modellierung yon Belebtsehlamm- systemen. Dissertation No 8765. ETH-Zurich, Switzerland.

Sollfrank U. and Gujer W. (1991) Characterization of domestic wastewater for mathematical modelling of the activated sludge process. Wat. Sci. Technol. 23, 1057- 1066.

S6zen S. (1995) Experimental evaluation of nitrification and denitrification kinetics. PhD thesis. Istanbul Technical University.

S6zen S. and Orhon D. (1996) The effect of nitrite correction on the evaluation of the rate of nitrate utilization under anoxic conditions. J. Chem. Tech. (submitted, 1995).

Ubay ~okg6r E. 1997. Respirometric evaluation of pro- tess kinetic and stoichiometry for aerobic systems. PhD thesis. Istanbul Technical University.

APPENDIX

The endogenous decay model as defined by Orhon and Artan (1994) and used in this paper is schematically illustrated by the flow diagram in Fig. A1. The model includes growth, hydrolysis and endogenous decay processes and describes the kinetic and stoichiometric relationships for seven components; Sb X~, Ss, Xs, XH, Xp, So. The first four components Sb Xz, Ss, Xs define the organic nature of the wastewater. S~ and X~, influent inert COD fractions, do not participate in biochemical processes. The readily biodegradable organic matter (Ss) is considered to be the only component that can be directly used by growth. The slowly biodegradable substrate (Xs) produces readily biodegradable substrate through hy- drolysis, which is a slower mechanism as compared to the growth. The next component (Xa) reflects viable heterotrophic biomass, whereas Xp is defined as a particulate inert product generated through endogenous decay. Dissolved oxygen concentration So is involved in two major biochemical processes, growth and endogenous decay where the latter is stipu- lated to consume endogenous mass at the expense of oxygen utilization. The mechanistic explanation of the model can be outlined in the following matrix format as given in Table AI.

© @ @ ®

Fig. AI. Process scheme for the endogenous decay model.

Assessment o f the readily biodegradable substrate 475

Table AI. Process kinetic and stoichiometry for carbon removal involving endogenous decay model

Component 1 2 3 4 5 6 7 Process rate Process SI Xx Ss Xs XH Xp So (ML -3 T -I)

Growth I (1 - Yn) Ss ~,~ ~ X H Yn l Yn (Ks + Ss)

XS/XH v Hydrolysis l -1 Kh (Kx ~ ' ~ s / ,VH) A H

Decay -1 fe I - fE b~xXH Parameter (ML -3) COD COD COD COD Cell COD COD 02