Biotechnology Techniques, Vol 12, No 10, October 1998, pp. 737–741
Dual hydrolysis model of the slowly biodegradable substrate in activated sludge systems Derin Orhon*, Emine Ubay Çokgör and Seval Sözen Environmental Engineering Department, Istanbul Technical University, I.T.U. Insaat ¸ Fakültesi, 80626 Maslak, Istanbul, Turkey *e-mail:
[email protected] Experimental assessment of the hydrolysis rate coefficients for both domestic sewage and a number of industrial wastewaters was performed with emphasis on two different hydrolysis mechanisms associated with the readily and slowly hydrolyzable COD fractions. The adopted dual hydrolysis model was justified on the basis of significantly different rate constants. The hydrolysis rate of particulate COD occurred at such a slow rate that would significantly interfere with endogenous decay.
Nomenclature bH fES fEX Kh, khs, khX KS KX, KXS, KXX SO SS SH SP XH XS XT1 YH mˆ H
endogenous decay coefficient for active biomass, [T21] fraction of endogenous mass converted into soluble inert products inert fraction of biomass hydrolysis rate constants, [T21] half saturation coefficient [M COD L23] saturation coefficients for particulate COD [M COD(M cell COD)21] oxygen, [ML23] readily biodegradable substrate, [M COD L23] soluble slowly biodegradable substrate, [M COD L23] soluble residual product, [M COD L23] active heterotrophic biomass, [M cell COD L23] slowly biodegradable substrate, [M COD L23] total initial biomass, [M VSS L23] heterotrophic yield coefficient [M cell COD(M COD)21] maximum heterotrophic growth rate [T21]
Introduction Recognition and identification of substrate fractions with different biodegradation rates should be regarded as one of the most significant achievements for the understanding of the activated sludge behaviour as a biological treatment process. All activated sludge models for organic carbon and nutrient removal now adopt chemical oxygen demand (COD) as the major parameter as it sets an electron balance between substrate, biomass and the electron acceptor. However, the use of this parameter requires COD fractionation as an integral part of wastewater characterization, mainly to identify inert and biodegradable COD fractions © 1998 Chapman & Hall
and further to differentiate readily and slowly biodegradable COD components (Orhon and Ubay Çokgör, 1997). Currently, all activated sludge models for organic carbon and nutrient removal involve a hydrolysis step to quantify the fate of slowly biodegradable substrate as a model component. As the hydrolysis mechanism is significantly slower than heterotrophic growth, it assumes a dominant role, as the rate limiting step, for the accurate assessment of the substrate utilization, the demand for electron acceptor under aerobic and anoxic conditions, the composition of activated sludge and the generation of excess sludge. The slowly biodegradable COD fraction covers the bulk of the organic content of domestic and industrial wastewaters, as shown in Table 1. Consequently, it is conceivable that it may be difficult and sometimes misleading to characterize this major COD fraction by a single hydrolysis rate, as it is likely to cover a wide array of compounds with different composition and biodegradation patterns. In this context, this paper intends to set the conceptual framework of dual hydrolysis by means of model evaluation of the OUR profiles; it addresses the question of organics with different hydrolysis rates by identifying readily and slowly hydrolyzable COD fractions; it evaluates and interprets the kinetics of dual hydrolysis for domestic sewage together with tannery and textile effluents. Conceptual framework It is now commonly accepted that the slowly biodegradable substrate is adsorbed/enmeshed in the activated sludge and broken down to readily biodegradable substrate Biotechnology Techniques ⋅ Vol 12 ⋅ No 10 ⋅ 1998
737
D. Orhon et al. Table 1 COD fractionation of domestic and industrial wastewaters used in this study. Organized Tannery Wastewaters Domestic (Orhon et al., 1998) Industrial DistrictSewage Predominantly (Orhon and Plain Chemical Textile Ubay Çokgor, ¨ settling settling (Orhon and Ubay 1997) effluent effluent Çokgor, ¨ 1997)
Parameters
Unit
Total COD, CT1 Total Soluble COD, ST1 Total Particulate COD, XT1
mg/l mg/l mg/l
450 155 295
mg/l %
18 4
Readily biodegradable COD, SS1 SS1/CT1
mg/l %
Rapidly hydrolyzable COD, SH1 SH1/CT1
2300 1300 1000
Textile Wastewaters (Babuna et al., 1998) Cotton Knit Fabric
Cotton and Polyester Denim Knit Fabric Processing
1100 1100 –
990 630 365
1470 1165 305
2400 1690 710
1910 1570 340
215 9.5
175 16
20 2
260 18
250 10
240 12.5
40 9
435 19
385 35
148 15
330 22
165 7
325 17
mg/l %
97 21.5
650 28.5
540 49
459 46
575 39
1275 53
1005 52.5
Slowly hydrolyzable COD, XS1 XS1/CT1
mg/l %
250 55.5
730 31.5
– –
363 37
288 20
598 25
340 18
Slowly biodegradable COD, SH11XS1 SH11XS1/CT1
mg/l %
347 77
540 49
822 83
863 59
1873 78
1345 70.5
Particulate inert COD, XI1 XI1/CT1
mg/l %
45 10
– –
– –
17 1
112 5
– –
COD components: Soluble inert COD, SI1 SI1/CT1
1380 60 270 11.5
through hydrolysis; the hydrolysis product can then be used for synthesis along with the influent readily biodegradable organics. From a process standpoint, hydrolysis is assumed to be the rate limiting step and the utilization of the slowly biodegradable substrate is defined by means of a surface-limited reaction kinetics (Dold et al., 1980; Henze et al., 1987):
dS0 1 2 YH SS 5 mˆ H X 2 (1 2 fEX 2 fES)bH XH (2) dt YH KS 1 SS H mˆ dSS SS SH / X H 52 H X 1 khs X 1 dt YH KS 1 SS H KXS 1 SH / XH H khX
dXS XS / X H 5 2kh X dt KX 1 XS / XH H
(1)
Hydrolysis kinetics are best investigated by means of the oxygen uptake rates, (OUR), in lab-scale aerated batch reactors. The lower plateau of the OUR profiles relates to the slower oxygen consumption indirectly controlled by the hydrolysis of the slowly biodegradable substrate. Evaluation of kinetic constants involves mathematical simulation of the OUR profile through the following rate expressions derived on the basis of an endogenous decay model (Orhon and Artan, 1994), and curve fitting with experimental data (Orhon et al., 1998). The set of differential equations equally apply to single or dual hydrolysis concepts, simply by omitting or including Equation 4 and correcting Equation 3 as appropriate. The OUR profile is evaluated by means of Equation 2 which reflects the oxygen utilization rate due to microbial growth induced by substrate generated through hydrolysis and to endogenous respiration corrected for the generation of soluble and particulate microbial products.
738
Biotechnology Techniques ⋅ Vol 12 ⋅ No 10 ⋅ 1998
XS / X H X KXX 1 XS / XH H
(3)
dSH SH / X H 5 2khs X dt KXS 1 SH / XH H
(4)
dXS XS / X H 5 2khX X dt KXX 1 XS / XH H
(5)
dXH SS 5 mˆ H X 2 b H XH dt KS 1 SS H
(6)
dSP 5 fESbHXH dt
(7)
dXP 5 fEXbHXH dt
(8)
The concept of dual hydrolysis has been proposed and tested before, using continuously fed lab-scale reactors and first order kinetics (Sollfrank and Gujer, 1991), giving a totally different interpretation of the hydrolysis mechanism as compared to surface-limited reaction kinetics.
Model for slowly degradable substrates Materials and methods All analyses were performed as defined in Standard Methods (APHA, 1989). The soluble (filtered) COD was determined as the filtrate through Whatman GF/C glass fiber filters (effective pore size: 0.45m), which were also used for the assessment of volatile suspended solids. The inert COD fractions were measured using the methods defined by Germirli et al., (1991) and Orhon et al., (1994). The respirometric tests for the measurement of OUR were conducted on a 1-litre aerated batch reactor initially fed with the wastewater sample and seeded with acclimated biomass. Acclimation of the biomass to the wastewater sample tested was secured in a fill and draw reactor continuously operated at a sludge age of 10 days. Aliquots were removed from the reactor every 5–10 min for OUR measurements, conducted with a WTW OXI DIGI 2000 oxygen meter and recorder. Experimental evaluation The need for a dual hydrolysis approach is evidenced, as illustrated in Fig. 1, by the deviation of the model profile from the experimental OUR data, when the evaluation is made on the basis of a single hydrolysis mechanism
covering the entire slowly biodegradable COD. For such wastewaters, the experimental basis of the dual hydrolysis mechanism involves repeating the same batch OUR test in duplicate on soluble and total portions of the same sample, obtaining the representative hydrolysis kinetics (khs and KXS) on the soluble portion and reevaluating the total OUR profile with simultaneous dual hydrolysis approach to assess the coefficients related to the slowly hydrolyzable COD (khx and KXX). The dual hydrolysis model was tested on 5 domestic sewage samples, 6 primary effluent samples from tanneries as well as on single samples of different textile wastewaters. The results on domestic sewage, as summarized in Table 2, yielded average khs and KXS values of 3.1 d21 and 0.2 g COD(g COD)21 associated with the hydrolysis of SH and much lower values of 1.2 d21 and 0.5 g COD (gCOD)21 for XS. Fig. 2 illustrates the appreciable improvement of the discrepancy between model and experimental OUR profiles obtained with one domestic sewage sample, shifting from a single hydrolysis (kh 5 2.3 d21; KX 5 0.4) to dual hydrolysis mechanism (khs 5 3.5 d21, KXS 5 0.1; khx 5 1.8, KXX 5 0.4).
Figure 1 Discrepancy between model simulation with single hydrolysis and experimental data for a) tannery wastewaters b) textile wastewaters.
Table 2 Parameter estimation for domestic sewage sample using dual hydrolysis model.
Run No
khs d21
KXS g COD(g cell COD)21
khX d21
KXX g COD(g cell COD)21
Run I Run II Run III Run IV Run V Mean
3.5 2.1 3.0 2.5 4.5 3.1
0.10 0.13 0.20 0.20 0.30 0.20
1.75 0.75 1.50 0.75 1.0 1.2
0.40 0.18 0.50 0.50 0.80 0.50
Biotechnology Techniques ⋅ Vol 12 ⋅ No 10 ⋅ 1998
739
D. Orhon et al.
Figure 2 Model verification of experimental data for domestic sewage, using a) single hydrolysis model b) dual hydrolysis model. Table 3 Dual hydrolysis kinetics for tannery wastewaters, (Orhon et al., 1997). Dual Hydrolysis Model Rapidly Hydrolyzable COD, SH
Single Hydrolysis Model
Slowly Hydrolyzable COD, XS
Plain Settling Effluent
Chemical Settling Effluent
Run No.
Khs
KXS
KhX
KXX
Kh
KX
Kh
KX
1 2 3 4 5 6
0.7 1.4 1.6 0.6 1.2 1.1
0.1 0.1 0.4 0.1 0.2 0.1
0.3 0.1 0.2 0.2 0.6 0.25
0.1 0.1 0.1 0.1 0.5 0.2
0.6 0.5 1.3 0.7 1.0 0.8
0.15 0.15 0.4 0.1 0.2 0.2
1.0 0.6 1.6 0.8 1.3 1.2
0.15 0.1 0.5 0.1 0.2 0.1
Mean
1.1
0.2
0.3
0.2
0.8
0.2
1.1
0.2
Table 4 Dual hydrolysis kinetics for textile and tannery wastewaters. Dual Hydrolysis Model Rapidly Hydrolyzable COD, SH Category Organized Industrial DistrictPredominantly Textile Cotton Knit Fabric Cotton and Polyester Knit Fabric Denim Processing Tannery Plain Settling Effluent, (average) Chemical Settling Effluent
Slowly Hydrolyzable COD, XS
Single Hydrolysis Model
Khs
KXS
KhX
KXX
Kh
KX
2.5 3.0 3.0 0.8
0.40 0.05 0.05 0.05
0.1 1.0 1.0 0.15
0.5 0.5 0.2 0.15
2 –* –* 1.0
0.35 –* –* 0.15
1.1
0.2
0.3
0.2
0.8 1.1
0.2 0.2
* not compatible with single hydrolysis kinetics
The results of the dual hydrolysis kinetics for tannery wastewaters are summarized in Table 3. A significant observation was that the magnitude of the maximum specific hydrolysis rates for both SH and XS portions were appreciable lower than their counterparts for domestic sewage; in fact the average values defining khs and khx for
740
Biotechnology Techniques ⋅ Vol 12 ⋅ No 10 ⋅ 1998
tannery wastewaters were calculated as 1.1 d21 and 0.3 d21 respectively. The important feature of the results related to textile wastewaters, as displayed in Table 4, was their specific character and their variation from one textile category to
Model for slowly degradable substrates another; as the Table shows, khs was found to vary from 0.8 d21 for denim processing to 3.0 d21 for cotton finishing. A similar variation was also depicted for khx, ranging from 0.15 to 1.0 mg COD(mg COD)21, within the same categories. Conclusions The following issues, derived on the basis of collected experimental data, are estimated to deserve specific emphasis: (a) The slowly biodegradable substrate constitutes the major fraction of the COD content of different wastewaters; this fraction was experimentally assessed as 77% for domestic sewage and was found to range from 50 to 83% for different industrial effluents. This study provided strong indication that the wide array of organics within this fraction could not be represented by a single hydrolysis model; in fact, the rate constants for the hydrolysis of the rapidly hydrolizable COD, SH, were observed to be significantly higher than the ones characterizing particulate biodegradable COD, XS. (b) Estimation of the single hydrolysis kinetics by evaluation of the OUR profile yielded values very close to constants associated with the soluble (rapidly hydrolyzable) portion of the slowly biodegradable substrate. Consequently, the single hydrolysis model was observed to involve a significant risk of overestimating the biodegradation of particulate COD. (c) For the majority of the industrial effluent samples, the maximum specific hydrolysis rate of the particulate slowly biodegradable COD was computed to be at the same level as the endogenous decay constant of the biomass, generally reported as 0.24 d21 (Henze et al., 1987). This observation challenges the validity of this value and that of routine aerobic digestion tests used for the measurement of the endogenous decay constant,
likely to be seriously affected by the interference of the particulate slowly biodegradable substrate (Avcıoglu ˘ et al., 1998). Acknowledgment This study was conducted as part of the sponsored research activities of the Environmental Biotechnology Center of the Scientific and Research Council of Turkey. It was also supported by the Research and Development Fund of Istanbul Technical University. References APHA, (1989). Standard Methods, 17th Edition. American Public Health Association, Washington D.C. Avcıoglu, ˘ E., Orhon, D., Sözen, S., (1998). Proceedings, 9th Biennial Conference, International Association on Water Quality, vol.2, pp. 85–93, Vancouver, Canada. ˙ Babuna Germirli, F., Orhon, D., Ubay Çokgor, E., Insel, G., th Yapraklı, B., (1998). Proceedings, 9 Biennial Conference, International Association on Water Quality, vol. 5, pp. 9–16, Vancouver, Canada. Dold, P. L., Ekama, G. A. and Marais, G. v. R., (1980). Prog. Wat. Tech., 12, 6: 47–54. Germirli, F., Orhon, D., and Artan, N., (1991). Wat. Sci. Technol,. 23, 4–6: 1067–1074. Henze, M., Grady, C. P. L. Jr., Gujer W., Marais, G. v. R. and Matsuo, T., (1987). Activated Sludge Model No.1, International Association on Water Pollution Research and Control, Scientific and Technical Report No.1, London. Orhon, D. and Artan, N., (1994). Modelling of Activated Sludge Systems., Lancaster PA., Technomics. Orhon, D., Artan, N., and Ate¸s, E., (1994). J. Chem.Tech. Biotechnol., 61, 73–80. Orhon, D., Ubay Çokgör, E., (1997). J. Chem. Tech. Biotechnol., 68, 294–302. Orhon, D., Genceli Ate¸s, E., Ubay Çokgör, E., (1998). Water Environ. Res., (in press). Sollfrank, U. and Gujer, W., (1991). Wat. Sci. Technol., 23, 4–6: 1057–1066
Received: 15 June Revisions requested: 24 June Revisions received: 10 August Accepted: 11 August
Biotechnology Techniques ⋅ Vol 12 ⋅ No 10 ⋅ 1998
1998 1998 1998 1998
741
