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Study of Competition for Ozone Between Soluble and Particulate Matter During Activated Sludge Ozonation
0263–8762/03/$23.50+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 81, Part A, October 2003
www.ingentaselect.com=titles=02638762.htm
STUDY OF COMPETITION FOR OZONE BETWEEN SOLUBLE AND PARTICULATE MATTER DURING ACTIVATED SLUDGE OZONATION ´ RIS2 , H. DEBELLEFONTAINE 1 , M. ROUSTAN 1 and E. PAUL 1 D. CESBRON1 , S. DE´LE 1
Department GPI – LIPE – INSA, Toulouse, France 2 Onde´o-Degre´mont, Croissy-sur-Seine, France
T
he use of ozone to attack soluble or solid recalcitrant compounds is often necessary to comply with environmental standards. In order to optimize ozone dosage its action must be properly targeted. Competition for ozone between soluble compounds and solid particles was studied using off-gas ozone concentration pro les. Deduced gas absorption enhancement for each matter type, alone and mixed, indicated non-classical competition. Results can be explained by the mass transfer lm concept in the presence of reactive particles. Besides classical in uential parameters (concentration and reactivity towards ozone), the temporary size distribution of solid particles related to the thickness of the effective liquid lm must be considered. Keywords: ozone; activated sludge; competitive reactions; gas absorption enhancement; particle size.
INTRODUCTION In wastewater treatment, processes combining partial oxidation such as ozonation with biological treatment are used to remove recalcitrant soluble and=or particulate inert organic materials: (1) Xenobiotic organics produced by modern industrial processes resist conventional biological wastewater treatment. Ozone pretreatment to improve biodegradation via partial oxidation is a potential solution for their removal (Alvares et al., 2001). (2) Most of the organics present in activated sludge are heterotrophic biomass and organic solid inert material, in the form of ocs. With respect to their recalcitrant nature and their physical state, subsequent microbial degradation of activated sludge compounds is not observed. The use of oxidative treatment (ozonation) improves digestion ef ciency by solubilizing and converting slowly biodegradable particulate organic material into low molecular weight, readily biodegradable compounds (Weemaes and Verstraete, 1998). Signi cant reduction of excess sludge production can then be achieved (De´le´ris, 2001). To minimize costs, the action of ozone should be targeted on either the soluble or solid recalcitrant compounds. However, in complex matrices such as wastewaters and activated sludges, these compounds and others such as soluble biodegradable by-products or inorganic soluble oxidant scavengers, will compete to react with ozone. Hoigne´ and Bader (1976) underline the competition
between particulate matter (micro-organisms for example) and dissolved species to react with OH radicals resulting from the decomposition of ozone during water treatment. Even at low solute concentrations, OH radicals that react very quickly with many types of dissolved species will be scavenged before they encounter a dispersed particle. Competition between soluble compounds obviously depends on their kinetic parameters and concentration. In the case of competition between soluble compounds and solid particles, competition may also be in uenced by mass transfer phenomena. In water and wastewater treatment, ozonation is usually performed by dissolving gaseous ozone in liquid in order to have it react with target compounds. Based on its low solubility, ozone mass transfer has been recognized to be controlled within the liquid lm immediately adjacent to the gas–liquid interface. Consequently, the overall mass transfer coef cient can be approximated by the local liquid mass transfer coef cient (Danckwerts, 1970). The overall rate of ozone mass transfer will be affected by different operating conditions, water quality and facility setup. It has also been recognized that the occurrence of rapid chemical reactions may enhance the mass transfer from the gaseous phase to the liquid phase (Danckwerts, 1970). Enhancement of Gas Absorpti on by Soluble Compounds Present in Wastewaters For highly concentrated wastewater, a fast kinetic regime was observed (Zhou and Smith, 2000) due to high chemical reaction rates, implying the absence of dissolved ozone in the bulk liquid (case b in Figure 1). All reactions are carried
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CESBRON et al.
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Figure 1. Ozone mass transfer schemes.
out in the proximity of the water–gas interface. The apparent rate of ozone mass transfer may even exceed the maximum rate of physical gas–liquid mass transfer. Then the enhancement factor E is de ned as the actual rate of mass transfer with the occurrence of chemical reactions divided by the maximum rate of physical absorption without any chemical reaction. In this case, the compound which reacts the fastest (due to high reactivity and=or high concentration) is preferentially attacked by ozone and controls the E value (Benbelkacem, 2002).
Enhancement of Gas Absorpti on by the Presence of Particles in Slurries Particles in slurry reactors may enhance the gas absorption for several reasons. In the case of activated sludge ozonation, there are two reasons for this (Beenackers and VanSwaaij, 1993): (1) particles in the mass transfer zone react with the transferred gaseous phase component; and (2) particles in the mass transfer zone are solubilized by ozone and the solubilized intermediate by-products react with the transferred gaseous phase component. The presence of particles can also in uence the resistance to mass transfer in the liquid phase at the gas–liquid interface. Therefore, any determination of the enhancement effect should take this
Figure 2. Experimental installation (gas ow rate (G)
factor into account. This requires careful de nition of ux enhancement due to the particles. E is de ned as the actual rate of mass transfer with particles divided by the maximum rate of physical absorption with the same particles but without any chemical reaction. If reactive particles have a diameter much smaller than the thickness of the mass transfer lm, and if suf cient particles are available in the lm, then enhancement of gas absorption due to chemical reactions may occur, provided that the speci c chemical conversion rate is high enough (Beenackers and VanSwaaij, 1993). In a mixed liquor like activated sludge, the competition between soluble compounds, as between soluble and particulate compounds may occur. Competition phenomena during real wastewater ozonation, particularly of activated sludge are dif cult to study because of the complexity of the mixed liquor. Beenackers and VanSwaaij (1993), in the case of gas absorption in slurries, pointed out the dif culty in distinguishing between the enhancement resulting from homogeneous liquid phase reactions, and that resulting from the presence of particles. These authors also showed the in uence of the solid particle diameter on the E value. The present work focuses on the characterization of the competition for ozone between soluble compounds with different reactivities, and solid particles of activated sludge with different sludge size distributions. MATERIALS AND METHODS To study the competitive reactions between soluble compounds and solid particles of activated sludge, solid and soluble fractions are rst oxidized alone, and then together. Ozone Contactor and Ozone Mass Balance The ozonation tests were performed in a lab-scale discontinuous reactor (2.1 l), perfectly mixed by stirring and bubbling an oxygen=ozone mixture into the reactor (Figure 2). The ozone generator (Ozone Technology, model OT 20) is fed with compressed oxygen. The off-gas ozone concentration was analysed by a UV spectrophotometer (Uvozon, model TLG 200). The delay in off-gas ozone concentration monitoring caused by the headspace between
30 NL=h, inlet gas ozone concentration (CGin)
30 or 50 mg O3=L, liquid volume (VL)
1.5 L).
Trans IChemE, Vol 81, Part A, October 2003
STUDY OF COMPETITION FOR OZONE
the liquid and the analyser was taken into account. The offgas ozone concentration pro les shown in this paper come from a numerical reconstruction based on the measured concentration taking into account the hydrodynamic characteristics of the reactor and of the headspace. Ozone mass balance based on these pro les was used to calculate the transferred ozone during the experiments. The volumetric mass transfer coef cient of ozone (kLaO3) was determined by measuring kLaO2 using the non-steadystate method described by Zhou and Smith (2000). The enhancement factor E can be approximately determined by the following relation: Eˆ
N (kL a ¢ CL¤ ) ¢ VL
(1)
where N is the ozone transfer rate (mgO3 min¡1) and C¤L the saturated liquid ozone concentration which is determined by: CL¤ ˆ m ¢ Cgin
(2)
where m is the parting coef cient coming from Henry’s constant (m ˆ 0.25). The dissolved ozone concentration in the liquid phase is supposed to be nil assuming a fast kinetic regime. Compound Characteristics and Experimental Approach Different mixture ozonations were performed to show the in uence of three parameters on competition between particulate matter and soluble compounds: (1) soluble compound reactivity with ozone; (2) soluble compound concentration; and (3) particulate size distribution. For each parameter, two levels have been chosen (Table 1). Glucose and azo dye (Orange II) were chosen respectively for their low and high reactivity towards molecular ozone (0.45 M¡1 s¡1 for glucose and 1.23 £ 106 M¡1 s¡1 for Orange II). Two activated sludges (AS) from lab-scale pilot plants were used as solid particles. Sludge 1 came from a hybrid bio lm=AS reactor and sludge 2 came from a low loaded AS reactor. These two reactors had equivalent operating conditions (organic load, sludge retention time, in uent) leading to equivalent sludge characteristics (Table 2). Before ozonation, the original supernatant of the mixed liquor was removed after sludge settling. Thus
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Table 3. Granulometric distribution results (mm).
Sludge 1 Sludge 2
d10
d50
d90
13.5 42
41.5 166
103 460
the supernatant did not in uence the ozonation and did not affect the comparison between the two sludge particulate matters. These particulate matters differ in the size distribution of the solid particles. The granulometric distribution (Table 3) was measured with a Malvern Mastersizer 2000 laser beam diffraction granulometer, over a 0.5–900 mm range. Any particles <0.5 mm were ignored. For activated sludge 1, 10% of the total volume of particles had a diameter (d10) less than 13.5 mm, whereas for activated sludge 2, this corresponding diameter was 42 mm. Sludge 1 had more particles with a smaller diameter. Other Analysis Colour and TOC (total organic carbon) were analysed to monitor the effect of oxidation on colour loss and on particulate matter solubilization. The French standard method NFT 90 – 102 AFNOR was used for TOC and colour was determined by spectrophotometry at 483 nm. RESULTS Identi cation and Characterization of the Competition Between Organic Soluble and Particulate Sludge Compounds Figures 3 and 4 present respectively the off-gas ozone pro les and the ozone transfer rates for Orange II alone, activated sludge 2 alone and the mixture of the two. Dye: The ozone transfer rate, high at the beginning, decreases with time to stabilize (t ˆ 12 min) at a value of 6 mgO3 min¡1 corresponding roughly to E ˆ 1. At this E value, the reactions take place in the lm and in the bulk liquid. Sludge: The ozone transfer rate increases from the beginning of the ozonation to stabilize (t ˆ 6 min) at a
Table 1. Levels for each parameter. Parameter A 7 ‡
Glucose Azo dye
Parameter B ¡1
27 mg TOC l 270 mg TOC l¡1
Parameter C Sludge 1 Sludge 2
Table 2. Sludge characteristics.
¡1
COD (g O2 l ) COD (<8 mm) (mgO2 l¡1) TSS (g l¡1) VSS (g l¡1) pH
Sludge 1
Sludge 2
2.6 30 2.10 1.70 7.0
2.5 30 2.05 1.65 6.8
COD ˆ chemical oxygen demand TSS ˆ total suspended solids VSS ˆ volatile suspended solids
Trans IChemE, Vol 81, Part A, October 2003
Figure 3. Off-gas ozone concentration pro les for Orange II, activated sludge 2 and the mixture.
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CESBRON et al.
Figure 4. Ozone transfer rates for Orange II, activated sludge 2 and the mixture.
value of 11.5 mgO3 min¡1 corresponding to E > 1. The ozone transfer rate corresponding to sludge ozonation is higher than one corresponding to dye ozonation except during the rst 4 minutes. In the case of classical competition phenomena, it can be expected that pro les for the mixture t those of the most reactive fraction (sludge). Mixture: Contrary to expectations, pro les corresponding to the mixture tted rst (during 15 min) with dye pro les and then came closer to sludge pro les. Figures 5 and 6, respectively representing the solubilized organic carbon pro le and the remaining colour intensity pro le, clearly show that sludge begins to be attacked by ozone only when no more colour remains. The small decrease in the discoloration rate observed in the case of the mixture corresponds to ozone consumption by soluble compounds present in sludge 2. Therefore, though the reactivity to ozone is higher for sludge 2 than for Orange II, ozone reacts rst with Orange II soluble compound and then, when the colour is completely removed, with particulate matter. A non-classical kinetic competition is therefore indicated. In Figure 3, the time corresponding to the beginning of particulate solubilization can easily be determined because ozone transfer rate increases (or ozone concentration decreases). We named this point: P-point (Figure 3). P-point can be used to assess the
Figure 5. Particulate matter solubilization (cut-off for TOC sol
8 mm).
Figure 6. Discoloration pro les (l
483 nm).
competition intensity and the ozone loss when particulate matter attack is desired.
In uence of Different Parameters on Competition Ozonation of different mixtures was performed to study the in uence of three parameters on competition between particulate matter and soluble compounds: (1) soluble compound reactivity with ozone; (2) soluble compound concentration; and (3) particulate matter size distribution. The experimental conditions for each mixture ozonation are summarized in Table 4. Figures 7 and 8 show off-gas ozone concentration pro les obtained during these experiments. The beginning of particulate matter attack by ozone is identi ed by P-point determination. Particulate matter diameter in uence Comparisons between off-gas ozone pro les for mixtures of Orange II or glucose with sludge 1 (Figure 7) or 2 (Figure 8) enable us to study the effects of particulate size distribution on the competition between soluble and particulate matter for ozone. Indeed the two sludges have the same characteristics despite granulometric distribution. Sludge 1 has more particles with a smaller diameter. With sludge 1, the ozone concentration pro le decreases at the beginning of the ozonation process. The soluble compound does not create a screening effect for ozone action on particulate matter. Moreover, off-gas ozone concentration at the beginning of ozonation is lower in Figure 7 than in Figure 8 (Eini1 > Eini2) showing the initial particulate matter attack. On the other hand, for experiments 6 and 7 with sludge 2, P-point is identi ed at 8.5 and 4 min respectively after the beginning of ozonation (see Figure 8). Soluble compounds create a screening effect (experiments 6 and 7) whose intensity depends on the nature and concentration of the soluble compound. The intensity of the screening effect is de ned as the quantity of ozone transferred before the particulate matter attack identi ed by P-point determination. For experiments 4 and 8, dye concentration and the ozone transfer rate are so high that off-gas ozone concentration is nil for a long time. However this time value is higher for sludge 1 than for sludge 2. This is probably due to a Trans IChemE, Vol 81, Part A, October 2003
STUDY OF COMPETITION FOR OZONE
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Table 4. Experimental conditions.
Par A Par B* Par C
Exp 1
Exp 2
Exp 3
Exp 4
Exp 5
Exp 6
Exp 7
Exp 8
Glucose 27 Sludge 1
Dye 27 Sludge 1
Glucose 270 Sludge 1
Dye 270 Sludge 1
Glucose 27 Sludge 2
Dye 27 Sludge 2
Glucose 270 Sludge 2
Dye 270 Sludge 2
*Concentrations of glucose or dye in mg TOC l¡1.
simultaneous attack of particulate and soluble matter from the beginning of the ozonation for sludge 1. Soluble compound reactivity in uence For sludge 2, in experiment 6, 120 mg of O3 was transferred before the beginning of the solubilization compared to 50 mg of O3 in experiment 7. Despite a TOC concentration ten times lower, the dye created a screening effect (identi ed on Figure 8 by P-points) two times higher in intensity than glucose. Whatever the sludge, for the same TOC concentration, the off-gas ozone concentration at the beginning of the ozonation was lower with dye than with glucose. This was the consequence of a higher E value for dye due to its high reactivity with ozone. Therefore, the dye was oxidized at the beginning of the ozonation. For sludge 1, due to the presence of small particles and as previously demonstrated, the small particles were attacked at the beginning of the ozonation, simultaneously with dye. Soluble compound concentration in uence In Figures 7 and 8, the initial enhancement factor is higher at high concentrations for the dye. However, this is not the case for glucose. For sludge 2, the initial enhancement was the same and nearly equal to 1 whatever the glucose concentrations were. This is due to the low reactivity of ozone with glucose. For sludge 1 and 2, when glucose is in high concentration, the increase in the ozone transfer rate after the P-point is slower than when glucose is in low concentration. Glucose, despite a lower reactivity than particulate organic matter, slows down the increase in the ozone transfer rate due to the sludge attack. Therefore whatever the sludges, conventional competitive reactions between the particulate and the
Figure 8. Off-gas ozone concentration pro les for sludge 2 (CGin 50 mg O3=L).
soluble matter were observed. Similar competition reactions certainly occur between particulate matter and dye. INTERPRETATION Competition for ozone between soluble and particulate matter during activated sludge ozonation was observed. In addition to the classical effects of concentration and reactivity to ozone, the effect of particle size distribution on competition was determined. The mass transfer lm concept used in the case of gas absorption enhancement through the presence of reactive particles should be used to interpret our observed results. The consequence of ozone gas absorption enhancement is to x a new effective mass transfer lm (deff) (Figure 1). Beyond this lm, the dissolved ozone concentration is nil. If the reaction within the lm can be considered as quasihomogeneous, then it is possible to use the simple lm theory. The dimension of the effective mass transfer lm is: dL (3) E where dL is the dimension of the mass transfer lm. In our study of ozonation of activated sludges, the dimension of the mass transfer lm is approximately:
deff ˆ
dL ˆ
Figure 7. Off-gas ozone concentration pro les for sludge 1 (CGin 50 mg O3=L).
Trans IChemE, Vol 81, Part A, October 2003
DG ˆ 17 mm kL
(4)
where kL ˆ 1 £ 10¡4 m s¡1 and DG ˆ 1.74 £ 10¡9m2 s¡1 corresponding to typical values. The thickness of this lm decreases when E increases. In the case of the enhancement due to homogeneous liquid phase reactions, the E value is xed by the concentration and the reactivity towards ozone of soluble compounds and can reach very high values. Indeed experimental results con rm that, for dye, initial enhancement is
CESBRON et al.
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concentration dependent, and that, for the same TOC concentration, the off-gas ozone concentration at the beginning of the ozonation is lower with dye than with glucose whatever the sludge. The initial enhancement obtained with dye (E ˆ 2 for experiment 6) xes an effective mass transfer lm with a smaller thickness (deff ˆ 8.5 mm) than with glucose (deff ˆ 17 mm for experiments 5 and 7) at the beginning of ozonation. In the case of the enhancement due to the presence of reactive particles, the E value depends on the particle diameter (Beenackers and VanSwaaij, 1993). Experimental results show that the off-gas ozone concentration at the beginning of the ozonation is lower with sludge 1 than with sludge 2 (Eini1 > Eini2). Small particles of sludge 1 (d10 ˆ 13.5 mm) have more chances to penetrate the effective mass transfer lm than particles of sludge 2 (d10 ˆ 42 mm) and can therefore react with the dissolved gas nearer the gas–liquid interface. Several authors (Beenackers and VanSwaaij, 1993) show the existence of a particle-free layer at the interface. The thickness depends on the particle diameter (dp) and varies depending on the studies (deff ˆ dp or deff ˆ dp=4). To this minimum effective lm corresponds a maximum enhancement factor: E max ˆ
dL min deff
(5)
As a consequence, E increases when particle diameters are smaller. During activated sludge ozonation, these two kinds of enhancement can occur. At the beginning of the ozonation, the soluble fraction xes the thickness of an effective mass transfer lm. If some of the particles are small enough to access the lm (dp < deff) (sludge 1), ozone reacts with particles simultaneously with soluble compound. On the other hand, if the particles have a greater diameter than the thickness of the lm (sludge 2), no reaction between ozone and particles will occur until the thickness of the effective mass transfer lm reaches a value which makes this reaction possible. In the last case, the thickness increases because the concentration in highly reactive molecules decreases and the products formed are less reactive with ozone. When the soluble fraction is poorly reactive with ozone (very low reaction rate constant like glucose), even for high soluble COD (chemical oxygen demand) concentrations, the thickness of the mass transfer lm is not decreased (deff ˆ 17 mm). As a consequence, the initial enhancement factor remains equal to 1 if particles have a diameter greater than the thickness of the lm (sludge 2 in Table 3). In this case, at the beginning of ozonation, ozone reacts with soluble and particulate compounds that are in the bulk liquid. When ozone begins to react with particles, activated sludge ocs are disintegrated into smaller particles which can access the lm and increase the ozone transfer rate. In parallel, ozone reacts with soluble compounds. There is competition between the two kinds of reaction. If
the soluble compound concentration is high the increase in the ozone transfer rate is slower. CONCLUSION The competition for ozone between soluble and particulate matter during activated sludge ozonation is a nonclassical competition phenomenon. For certain conditions, ozone can react rst with the soluble fraction of the activated sludge and after attacks (solubilization) the particulate fraction, despite the differences of reactivity of each fraction: the soluble fraction has a screening effect on the particulate matter attack by ozone. Key parameters for this competition between soluble and solid matter are: (1) the mass transfer lm thickness imposed by compounds (soluble or solid) small enough to penetrate the lm. This thickness depends on the concentration and reactivity of these compounds, and it increases as the enhancement factor decreases; and (2) the temporary size distribution of solid particles related to the thickness of the effective liquid lm. This study determines key parameters involved in targeting the action of ozone on recalcitrant soluble COD and=or solid particle compounds (activated sludge ocs) according to the desired applications, by acting for example, on particle diameter. REFERENCES Alvares, A.B.C., Diaper, C. and Parsons, S.A., 2001, Partial oxidation by ozone to remove recalcitrance from wastewaters, Environ Technol, 22: 409–427. Beenackers, A.A.C.M. and VanSwaaij, W.P.M., 1993, Mass transfer in gas– liquid slurry reactors, Chem Eng Sci, 48(18): 3109–3139. Benbelkacem, H., 2002, Mode´lisation du transfert de matie`re couple´ avec une re´action chimique en re´acteur ferme´ . Application au proce´de´ d’ozonation. The`se INSA no. 662. Danckwerts, P.V., 1970, Gas–Liquid Reactions (McGraw-Hill, New York, USA). De´le´ris, S., 2001, Re´duction de la production de boue lors du traitement des eaux re´siduaires urbaines, Analyse du traitement combine´: ozonation et traitement biologique, The`se INSA no. 621. Hoigne´, J. and Bader, H., 1976, The role of hydroxyl radical reactions in ozonation processes in aqueous solutions, Wat Res, 10: 377–386. Weemaes, M.P.J. and Verstraete, W.H., 1998, Evaluation of current wet sludge disintegration techniques, J Chem Technol Biotechnol, 73: 83–92. Zhou, H. and Smith, D.W., 2000, Ozone mass transfer in water and wastewater treatment: experimental observations using a 2D laser particle dynamics analyzer, Wat Res, 34(3): 909–921.
ADDRESS Correspondence concerning this paper should be addressed to Dr. E. Paul, Department GPI – LIPE – INSA, 135 avenue de Rangueil, 31077 Toulouse, France. E-mail: [email protected] The paper was presented Chemical Engineering held 2003. The manuscript was publication after revision 15
at the 9th Congress of the French Society of in Saint-Nazaire, France, 9–11 September received 10 March 2003 and accepted for September 2003.
Trans IChemE, Vol 81, Part A, October 2003
www.ingentaselect.com=titles=02638762.htm
STUDY OF COMPETITION FOR OZONE BETWEEN SOLUBLE AND PARTICULATE MATTER DURING ACTIVATED SLUDGE OZONATION ´ RIS2 , H. DEBELLEFONTAINE 1 , M. ROUSTAN 1 and E. PAUL 1 D. CESBRON1 , S. DE´LE 1
Department GPI – LIPE – INSA, Toulouse, France 2 Onde´o-Degre´mont, Croissy-sur-Seine, France
T
he use of ozone to attack soluble or solid recalcitrant compounds is often necessary to comply with environmental standards. In order to optimize ozone dosage its action must be properly targeted. Competition for ozone between soluble compounds and solid particles was studied using off-gas ozone concentration pro les. Deduced gas absorption enhancement for each matter type, alone and mixed, indicated non-classical competition. Results can be explained by the mass transfer lm concept in the presence of reactive particles. Besides classical in uential parameters (concentration and reactivity towards ozone), the temporary size distribution of solid particles related to the thickness of the effective liquid lm must be considered. Keywords: ozone; activated sludge; competitive reactions; gas absorption enhancement; particle size.
INTRODUCTION In wastewater treatment, processes combining partial oxidation such as ozonation with biological treatment are used to remove recalcitrant soluble and=or particulate inert organic materials: (1) Xenobiotic organics produced by modern industrial processes resist conventional biological wastewater treatment. Ozone pretreatment to improve biodegradation via partial oxidation is a potential solution for their removal (Alvares et al., 2001). (2) Most of the organics present in activated sludge are heterotrophic biomass and organic solid inert material, in the form of ocs. With respect to their recalcitrant nature and their physical state, subsequent microbial degradation of activated sludge compounds is not observed. The use of oxidative treatment (ozonation) improves digestion ef ciency by solubilizing and converting slowly biodegradable particulate organic material into low molecular weight, readily biodegradable compounds (Weemaes and Verstraete, 1998). Signi cant reduction of excess sludge production can then be achieved (De´le´ris, 2001). To minimize costs, the action of ozone should be targeted on either the soluble or solid recalcitrant compounds. However, in complex matrices such as wastewaters and activated sludges, these compounds and others such as soluble biodegradable by-products or inorganic soluble oxidant scavengers, will compete to react with ozone. Hoigne´ and Bader (1976) underline the competition
between particulate matter (micro-organisms for example) and dissolved species to react with OH radicals resulting from the decomposition of ozone during water treatment. Even at low solute concentrations, OH radicals that react very quickly with many types of dissolved species will be scavenged before they encounter a dispersed particle. Competition between soluble compounds obviously depends on their kinetic parameters and concentration. In the case of competition between soluble compounds and solid particles, competition may also be in uenced by mass transfer phenomena. In water and wastewater treatment, ozonation is usually performed by dissolving gaseous ozone in liquid in order to have it react with target compounds. Based on its low solubility, ozone mass transfer has been recognized to be controlled within the liquid lm immediately adjacent to the gas–liquid interface. Consequently, the overall mass transfer coef cient can be approximated by the local liquid mass transfer coef cient (Danckwerts, 1970). The overall rate of ozone mass transfer will be affected by different operating conditions, water quality and facility setup. It has also been recognized that the occurrence of rapid chemical reactions may enhance the mass transfer from the gaseous phase to the liquid phase (Danckwerts, 1970). Enhancement of Gas Absorpti on by Soluble Compounds Present in Wastewaters For highly concentrated wastewater, a fast kinetic regime was observed (Zhou and Smith, 2000) due to high chemical reaction rates, implying the absence of dissolved ozone in the bulk liquid (case b in Figure 1). All reactions are carried
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Figure 1. Ozone mass transfer schemes.
out in the proximity of the water–gas interface. The apparent rate of ozone mass transfer may even exceed the maximum rate of physical gas–liquid mass transfer. Then the enhancement factor E is de ned as the actual rate of mass transfer with the occurrence of chemical reactions divided by the maximum rate of physical absorption without any chemical reaction. In this case, the compound which reacts the fastest (due to high reactivity and=or high concentration) is preferentially attacked by ozone and controls the E value (Benbelkacem, 2002).
Enhancement of Gas Absorpti on by the Presence of Particles in Slurries Particles in slurry reactors may enhance the gas absorption for several reasons. In the case of activated sludge ozonation, there are two reasons for this (Beenackers and VanSwaaij, 1993): (1) particles in the mass transfer zone react with the transferred gaseous phase component; and (2) particles in the mass transfer zone are solubilized by ozone and the solubilized intermediate by-products react with the transferred gaseous phase component. The presence of particles can also in uence the resistance to mass transfer in the liquid phase at the gas–liquid interface. Therefore, any determination of the enhancement effect should take this
Figure 2. Experimental installation (gas ow rate (G)
factor into account. This requires careful de nition of ux enhancement due to the particles. E is de ned as the actual rate of mass transfer with particles divided by the maximum rate of physical absorption with the same particles but without any chemical reaction. If reactive particles have a diameter much smaller than the thickness of the mass transfer lm, and if suf cient particles are available in the lm, then enhancement of gas absorption due to chemical reactions may occur, provided that the speci c chemical conversion rate is high enough (Beenackers and VanSwaaij, 1993). In a mixed liquor like activated sludge, the competition between soluble compounds, as between soluble and particulate compounds may occur. Competition phenomena during real wastewater ozonation, particularly of activated sludge are dif cult to study because of the complexity of the mixed liquor. Beenackers and VanSwaaij (1993), in the case of gas absorption in slurries, pointed out the dif culty in distinguishing between the enhancement resulting from homogeneous liquid phase reactions, and that resulting from the presence of particles. These authors also showed the in uence of the solid particle diameter on the E value. The present work focuses on the characterization of the competition for ozone between soluble compounds with different reactivities, and solid particles of activated sludge with different sludge size distributions. MATERIALS AND METHODS To study the competitive reactions between soluble compounds and solid particles of activated sludge, solid and soluble fractions are rst oxidized alone, and then together. Ozone Contactor and Ozone Mass Balance The ozonation tests were performed in a lab-scale discontinuous reactor (2.1 l), perfectly mixed by stirring and bubbling an oxygen=ozone mixture into the reactor (Figure 2). The ozone generator (Ozone Technology, model OT 20) is fed with compressed oxygen. The off-gas ozone concentration was analysed by a UV spectrophotometer (Uvozon, model TLG 200). The delay in off-gas ozone concentration monitoring caused by the headspace between
30 NL=h, inlet gas ozone concentration (CGin)
30 or 50 mg O3=L, liquid volume (VL)
1.5 L).
Trans IChemE, Vol 81, Part A, October 2003
STUDY OF COMPETITION FOR OZONE
the liquid and the analyser was taken into account. The offgas ozone concentration pro les shown in this paper come from a numerical reconstruction based on the measured concentration taking into account the hydrodynamic characteristics of the reactor and of the headspace. Ozone mass balance based on these pro les was used to calculate the transferred ozone during the experiments. The volumetric mass transfer coef cient of ozone (kLaO3) was determined by measuring kLaO2 using the non-steadystate method described by Zhou and Smith (2000). The enhancement factor E can be approximately determined by the following relation: Eˆ
N (kL a ¢ CL¤ ) ¢ VL
(1)
where N is the ozone transfer rate (mgO3 min¡1) and C¤L the saturated liquid ozone concentration which is determined by: CL¤ ˆ m ¢ Cgin
(2)
where m is the parting coef cient coming from Henry’s constant (m ˆ 0.25). The dissolved ozone concentration in the liquid phase is supposed to be nil assuming a fast kinetic regime. Compound Characteristics and Experimental Approach Different mixture ozonations were performed to show the in uence of three parameters on competition between particulate matter and soluble compounds: (1) soluble compound reactivity with ozone; (2) soluble compound concentration; and (3) particulate size distribution. For each parameter, two levels have been chosen (Table 1). Glucose and azo dye (Orange II) were chosen respectively for their low and high reactivity towards molecular ozone (0.45 M¡1 s¡1 for glucose and 1.23 £ 106 M¡1 s¡1 for Orange II). Two activated sludges (AS) from lab-scale pilot plants were used as solid particles. Sludge 1 came from a hybrid bio lm=AS reactor and sludge 2 came from a low loaded AS reactor. These two reactors had equivalent operating conditions (organic load, sludge retention time, in uent) leading to equivalent sludge characteristics (Table 2). Before ozonation, the original supernatant of the mixed liquor was removed after sludge settling. Thus
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Table 3. Granulometric distribution results (mm).
Sludge 1 Sludge 2
d10
d50
d90
13.5 42
41.5 166
103 460
the supernatant did not in uence the ozonation and did not affect the comparison between the two sludge particulate matters. These particulate matters differ in the size distribution of the solid particles. The granulometric distribution (Table 3) was measured with a Malvern Mastersizer 2000 laser beam diffraction granulometer, over a 0.5–900 mm range. Any particles <0.5 mm were ignored. For activated sludge 1, 10% of the total volume of particles had a diameter (d10) less than 13.5 mm, whereas for activated sludge 2, this corresponding diameter was 42 mm. Sludge 1 had more particles with a smaller diameter. Other Analysis Colour and TOC (total organic carbon) were analysed to monitor the effect of oxidation on colour loss and on particulate matter solubilization. The French standard method NFT 90 – 102 AFNOR was used for TOC and colour was determined by spectrophotometry at 483 nm. RESULTS Identi cation and Characterization of the Competition Between Organic Soluble and Particulate Sludge Compounds Figures 3 and 4 present respectively the off-gas ozone pro les and the ozone transfer rates for Orange II alone, activated sludge 2 alone and the mixture of the two. Dye: The ozone transfer rate, high at the beginning, decreases with time to stabilize (t ˆ 12 min) at a value of 6 mgO3 min¡1 corresponding roughly to E ˆ 1. At this E value, the reactions take place in the lm and in the bulk liquid. Sludge: The ozone transfer rate increases from the beginning of the ozonation to stabilize (t ˆ 6 min) at a
Table 1. Levels for each parameter. Parameter A 7 ‡
Glucose Azo dye
Parameter B ¡1
27 mg TOC l 270 mg TOC l¡1
Parameter C Sludge 1 Sludge 2
Table 2. Sludge characteristics.
¡1
COD (g O2 l ) COD (<8 mm) (mgO2 l¡1) TSS (g l¡1) VSS (g l¡1) pH
Sludge 1
Sludge 2
2.6 30 2.10 1.70 7.0
2.5 30 2.05 1.65 6.8
COD ˆ chemical oxygen demand TSS ˆ total suspended solids VSS ˆ volatile suspended solids
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Figure 3. Off-gas ozone concentration pro les for Orange II, activated sludge 2 and the mixture.
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CESBRON et al.
Figure 4. Ozone transfer rates for Orange II, activated sludge 2 and the mixture.
value of 11.5 mgO3 min¡1 corresponding to E > 1. The ozone transfer rate corresponding to sludge ozonation is higher than one corresponding to dye ozonation except during the rst 4 minutes. In the case of classical competition phenomena, it can be expected that pro les for the mixture t those of the most reactive fraction (sludge). Mixture: Contrary to expectations, pro les corresponding to the mixture tted rst (during 15 min) with dye pro les and then came closer to sludge pro les. Figures 5 and 6, respectively representing the solubilized organic carbon pro le and the remaining colour intensity pro le, clearly show that sludge begins to be attacked by ozone only when no more colour remains. The small decrease in the discoloration rate observed in the case of the mixture corresponds to ozone consumption by soluble compounds present in sludge 2. Therefore, though the reactivity to ozone is higher for sludge 2 than for Orange II, ozone reacts rst with Orange II soluble compound and then, when the colour is completely removed, with particulate matter. A non-classical kinetic competition is therefore indicated. In Figure 3, the time corresponding to the beginning of particulate solubilization can easily be determined because ozone transfer rate increases (or ozone concentration decreases). We named this point: P-point (Figure 3). P-point can be used to assess the
Figure 5. Particulate matter solubilization (cut-off for TOC sol
8 mm).
Figure 6. Discoloration pro les (l
483 nm).
competition intensity and the ozone loss when particulate matter attack is desired.
In uence of Different Parameters on Competition Ozonation of different mixtures was performed to study the in uence of three parameters on competition between particulate matter and soluble compounds: (1) soluble compound reactivity with ozone; (2) soluble compound concentration; and (3) particulate matter size distribution. The experimental conditions for each mixture ozonation are summarized in Table 4. Figures 7 and 8 show off-gas ozone concentration pro les obtained during these experiments. The beginning of particulate matter attack by ozone is identi ed by P-point determination. Particulate matter diameter in uence Comparisons between off-gas ozone pro les for mixtures of Orange II or glucose with sludge 1 (Figure 7) or 2 (Figure 8) enable us to study the effects of particulate size distribution on the competition between soluble and particulate matter for ozone. Indeed the two sludges have the same characteristics despite granulometric distribution. Sludge 1 has more particles with a smaller diameter. With sludge 1, the ozone concentration pro le decreases at the beginning of the ozonation process. The soluble compound does not create a screening effect for ozone action on particulate matter. Moreover, off-gas ozone concentration at the beginning of ozonation is lower in Figure 7 than in Figure 8 (Eini1 > Eini2) showing the initial particulate matter attack. On the other hand, for experiments 6 and 7 with sludge 2, P-point is identi ed at 8.5 and 4 min respectively after the beginning of ozonation (see Figure 8). Soluble compounds create a screening effect (experiments 6 and 7) whose intensity depends on the nature and concentration of the soluble compound. The intensity of the screening effect is de ned as the quantity of ozone transferred before the particulate matter attack identi ed by P-point determination. For experiments 4 and 8, dye concentration and the ozone transfer rate are so high that off-gas ozone concentration is nil for a long time. However this time value is higher for sludge 1 than for sludge 2. This is probably due to a Trans IChemE, Vol 81, Part A, October 2003
STUDY OF COMPETITION FOR OZONE
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Table 4. Experimental conditions.
Par A Par B* Par C
Exp 1
Exp 2
Exp 3
Exp 4
Exp 5
Exp 6
Exp 7
Exp 8
Glucose 27 Sludge 1
Dye 27 Sludge 1
Glucose 270 Sludge 1
Dye 270 Sludge 1
Glucose 27 Sludge 2
Dye 27 Sludge 2
Glucose 270 Sludge 2
Dye 270 Sludge 2
*Concentrations of glucose or dye in mg TOC l¡1.
simultaneous attack of particulate and soluble matter from the beginning of the ozonation for sludge 1. Soluble compound reactivity in uence For sludge 2, in experiment 6, 120 mg of O3 was transferred before the beginning of the solubilization compared to 50 mg of O3 in experiment 7. Despite a TOC concentration ten times lower, the dye created a screening effect (identi ed on Figure 8 by P-points) two times higher in intensity than glucose. Whatever the sludge, for the same TOC concentration, the off-gas ozone concentration at the beginning of the ozonation was lower with dye than with glucose. This was the consequence of a higher E value for dye due to its high reactivity with ozone. Therefore, the dye was oxidized at the beginning of the ozonation. For sludge 1, due to the presence of small particles and as previously demonstrated, the small particles were attacked at the beginning of the ozonation, simultaneously with dye. Soluble compound concentration in uence In Figures 7 and 8, the initial enhancement factor is higher at high concentrations for the dye. However, this is not the case for glucose. For sludge 2, the initial enhancement was the same and nearly equal to 1 whatever the glucose concentrations were. This is due to the low reactivity of ozone with glucose. For sludge 1 and 2, when glucose is in high concentration, the increase in the ozone transfer rate after the P-point is slower than when glucose is in low concentration. Glucose, despite a lower reactivity than particulate organic matter, slows down the increase in the ozone transfer rate due to the sludge attack. Therefore whatever the sludges, conventional competitive reactions between the particulate and the
Figure 8. Off-gas ozone concentration pro les for sludge 2 (CGin 50 mg O3=L).
soluble matter were observed. Similar competition reactions certainly occur between particulate matter and dye. INTERPRETATION Competition for ozone between soluble and particulate matter during activated sludge ozonation was observed. In addition to the classical effects of concentration and reactivity to ozone, the effect of particle size distribution on competition was determined. The mass transfer lm concept used in the case of gas absorption enhancement through the presence of reactive particles should be used to interpret our observed results. The consequence of ozone gas absorption enhancement is to x a new effective mass transfer lm (deff) (Figure 1). Beyond this lm, the dissolved ozone concentration is nil. If the reaction within the lm can be considered as quasihomogeneous, then it is possible to use the simple lm theory. The dimension of the effective mass transfer lm is: dL (3) E where dL is the dimension of the mass transfer lm. In our study of ozonation of activated sludges, the dimension of the mass transfer lm is approximately:
deff ˆ
dL ˆ
Figure 7. Off-gas ozone concentration pro les for sludge 1 (CGin 50 mg O3=L).
Trans IChemE, Vol 81, Part A, October 2003
DG ˆ 17 mm kL
(4)
where kL ˆ 1 £ 10¡4 m s¡1 and DG ˆ 1.74 £ 10¡9m2 s¡1 corresponding to typical values. The thickness of this lm decreases when E increases. In the case of the enhancement due to homogeneous liquid phase reactions, the E value is xed by the concentration and the reactivity towards ozone of soluble compounds and can reach very high values. Indeed experimental results con rm that, for dye, initial enhancement is
CESBRON et al.
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concentration dependent, and that, for the same TOC concentration, the off-gas ozone concentration at the beginning of the ozonation is lower with dye than with glucose whatever the sludge. The initial enhancement obtained with dye (E ˆ 2 for experiment 6) xes an effective mass transfer lm with a smaller thickness (deff ˆ 8.5 mm) than with glucose (deff ˆ 17 mm for experiments 5 and 7) at the beginning of ozonation. In the case of the enhancement due to the presence of reactive particles, the E value depends on the particle diameter (Beenackers and VanSwaaij, 1993). Experimental results show that the off-gas ozone concentration at the beginning of the ozonation is lower with sludge 1 than with sludge 2 (Eini1 > Eini2). Small particles of sludge 1 (d10 ˆ 13.5 mm) have more chances to penetrate the effective mass transfer lm than particles of sludge 2 (d10 ˆ 42 mm) and can therefore react with the dissolved gas nearer the gas–liquid interface. Several authors (Beenackers and VanSwaaij, 1993) show the existence of a particle-free layer at the interface. The thickness depends on the particle diameter (dp) and varies depending on the studies (deff ˆ dp or deff ˆ dp=4). To this minimum effective lm corresponds a maximum enhancement factor: E max ˆ
dL min deff
(5)
As a consequence, E increases when particle diameters are smaller. During activated sludge ozonation, these two kinds of enhancement can occur. At the beginning of the ozonation, the soluble fraction xes the thickness of an effective mass transfer lm. If some of the particles are small enough to access the lm (dp < deff) (sludge 1), ozone reacts with particles simultaneously with soluble compound. On the other hand, if the particles have a greater diameter than the thickness of the lm (sludge 2), no reaction between ozone and particles will occur until the thickness of the effective mass transfer lm reaches a value which makes this reaction possible. In the last case, the thickness increases because the concentration in highly reactive molecules decreases and the products formed are less reactive with ozone. When the soluble fraction is poorly reactive with ozone (very low reaction rate constant like glucose), even for high soluble COD (chemical oxygen demand) concentrations, the thickness of the mass transfer lm is not decreased (deff ˆ 17 mm). As a consequence, the initial enhancement factor remains equal to 1 if particles have a diameter greater than the thickness of the lm (sludge 2 in Table 3). In this case, at the beginning of ozonation, ozone reacts with soluble and particulate compounds that are in the bulk liquid. When ozone begins to react with particles, activated sludge ocs are disintegrated into smaller particles which can access the lm and increase the ozone transfer rate. In parallel, ozone reacts with soluble compounds. There is competition between the two kinds of reaction. If
the soluble compound concentration is high the increase in the ozone transfer rate is slower. CONCLUSION The competition for ozone between soluble and particulate matter during activated sludge ozonation is a nonclassical competition phenomenon. For certain conditions, ozone can react rst with the soluble fraction of the activated sludge and after attacks (solubilization) the particulate fraction, despite the differences of reactivity of each fraction: the soluble fraction has a screening effect on the particulate matter attack by ozone. Key parameters for this competition between soluble and solid matter are: (1) the mass transfer lm thickness imposed by compounds (soluble or solid) small enough to penetrate the lm. This thickness depends on the concentration and reactivity of these compounds, and it increases as the enhancement factor decreases; and (2) the temporary size distribution of solid particles related to the thickness of the effective liquid lm. This study determines key parameters involved in targeting the action of ozone on recalcitrant soluble COD and=or solid particle compounds (activated sludge ocs) according to the desired applications, by acting for example, on particle diameter. REFERENCES Alvares, A.B.C., Diaper, C. and Parsons, S.A., 2001, Partial oxidation by ozone to remove recalcitrance from wastewaters, Environ Technol, 22: 409–427. Beenackers, A.A.C.M. and VanSwaaij, W.P.M., 1993, Mass transfer in gas– liquid slurry reactors, Chem Eng Sci, 48(18): 3109–3139. Benbelkacem, H., 2002, Mode´lisation du transfert de matie`re couple´ avec une re´action chimique en re´acteur ferme´ . Application au proce´de´ d’ozonation. The`se INSA no. 662. Danckwerts, P.V., 1970, Gas–Liquid Reactions (McGraw-Hill, New York, USA). De´le´ris, S., 2001, Re´duction de la production de boue lors du traitement des eaux re´siduaires urbaines, Analyse du traitement combine´: ozonation et traitement biologique, The`se INSA no. 621. Hoigne´, J. and Bader, H., 1976, The role of hydroxyl radical reactions in ozonation processes in aqueous solutions, Wat Res, 10: 377–386. Weemaes, M.P.J. and Verstraete, W.H., 1998, Evaluation of current wet sludge disintegration techniques, J Chem Technol Biotechnol, 73: 83–92. Zhou, H. and Smith, D.W., 2000, Ozone mass transfer in water and wastewater treatment: experimental observations using a 2D laser particle dynamics analyzer, Wat Res, 34(3): 909–921.
ADDRESS Correspondence concerning this paper should be addressed to Dr. E. Paul, Department GPI – LIPE – INSA, 135 avenue de Rangueil, 31077 Toulouse, France. E-mail: [email protected] The paper was presented Chemical Engineering held 2003. The manuscript was publication after revision 15
at the 9th Congress of the French Society of in Saint-Nazaire, France, 9–11 September received 10 March 2003 and accepted for September 2003.
Trans IChemE, Vol 81, Part A, October 2003