Industrial wastewater advanced oxidation. Part 1. UV radiation in the presence and absence of hydrogen peroxide

Pergamon PII: S0043-1354(97)00077-8

War. Res. Vol. 31, No. 10, pp. 2405-2414, 1997 © 1997 ElsevierScienceLtd. All fights reserved Printed in Great Britain 0043-1354/97 $17.00 + 0.00

I N D U S T R I A L W A S T E W A T E R A D V A N C E D OXIDATION. PART 1. UV R A D I A T I O N IN THE PRESENCE A N D A B S E N C E OF H Y D R O G E N P E R O X I D E F E R N A N D O J. BELTR/f,N @*, M A N U E L G O N Z , A L E Z and J U A N F. G O N Z / ~ L E Z Departamento de Ingenieria Quimica y Energ&ica, Universidad de Extremadura, 06071 Badajoz, Spain

(Received January 1995; accepted in revised form February 1997)

Abstract--The oxidation of two wastewaters collected from distillery and tomato processing plants with UV radiation (254 nm) alone and combined with hydrogen peroxide has been investigated. Distillery wastewaters are refractory to UV radiation but the presence of hydrogen peroxide leads to different COD reductions which indicates that the process is mainly due to the action of radicals. Tomato wastewaters, on the other hand, show a higher reactivity even with UV radiation alone. The quantum yield for tomato wastewaters with an initial COD of 930 mg O21-t was found to be 0.7 mol O: photon -~ and decreased with reactio~a time. The combined effect of UV radiation and hydrogen peroxide at a 0.01 M concentration on tomato wastewaters leads to about 25% COD reduction while TOC was unchanged. The contribution of radical n,~actions in this process was higher than 60%. © 1997 Elsevier Science Ltd

Key words---distillery wastewaters, tomato wastewaters, advanced oxidation technologies, wastewater treatment, UV radiation

NOMENCLATURE CH = COD = Con = FcoD= /7. = I0 = /COB= kr = L= rl = t= TOC =

concentration of hydrogen peroxide (M) chemical oxygen demand (mg 021 -~ or M) concentration of hydroxyl radicals (M) fraction of radiation the matter present in wastewater absorbs (dimensionless) fraction of radiation hydrogen peroxide absorbs (dimensionless) flux of incident radiation (Einstein 1-~ s -~) rate constant of the reaction between hydroxyl radicals and the matter present in wastewater (M-' s-') apparent p,;eudo first-order rate constant defined by equatiorL (5) (s -I) effective path of radiation through the photoreactor (cm) rate of initiation of hydroxyl radicals (M s -~) reaction time (min or s) total organ:Lc carbon (mg C 1-~)

Greek letters E. = extinction coefficient of hydrogen peroxide at 254 nm, ba:~e 10 (M -~ cm -~) ~coo = extinction coefficient of matter present in wastewater at 254 nm, base 10 (M -~ cm -1) (l)i = quantum yield of wastewater at 254 nm (tool 02 photon - ~) #r = total absorlaance of wastewater (dimensionless) INTRODUCTION Chemical oxidation is normally applied in the treatment of wastewaters for disinfection purposes, to *Author to whom .'Ill correspondence should be addressed [Fax: 34 24 271204].

eliminate toxic or hazardous compounds and can be combined with biological oxidation to reduce the total organic content of wastewaters or improve biodegradability. In order to strengthen oxidising power there is much interest nowadays in the use of advanced oxidation technologies (AOTs). It is generally accepted that the oxidant species of these A O T s is the hydroxyl radical which can be generated in water through different combinations of oxidants, like ozone and hydrogen peroxide, or of a single oxidant and U V radiation (Glaze et al., 1987). What is fundamental about these systems is the very high oxidising capacity of the hydroxyl radical (redox potential = 2.8 V; Masten and Davies, 1994), making it able to react very rapidly with most of the organic and inorganic compounds in water (Buxton et al., 1988). So far, the application of A O T s has mainly focused on laboratory prepared aqueous solutions containing model compounds of different natures (pesticides, VOCs, etc.) (Kearney et al., 1987; Kusakasabe et al., 1993; Hayasi et al., 1993; Beltrfin et al., 1993b, 1994). Regarding advanced oxidation o f wastewaters, the few works reported in the literature show the promise of these systems in the area of water treatment (Munter et al., 1993; Murphy et al., 1993). This paper reports the first part o f our work aimed at studying the advanced chemical oxidation of two industrial wastewaters obtained from distillery and tomato processing plants. These wastewaters contain, among other chemicals, phenols and unsaturated compounds (Beltr~in et al., 1992, 1993a) which are

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F.J. Beltr~n et al. Table 1. Main characteristicsof wastewaters~

COD (ragO., I-') BOD (mg 021 -I) TOC (mg C 1-~) pH SS (mg 1-') Absorbance (254 nm) (cm-')

Hanau low pressure mercury vapour UV lamp emitting mainly at 254 nm with an input of 15 W (according to the manufacturer) and an incident radiation flux of 3.8 x 10-6 Einsteins 1-* s-', calculated by actinometry of hydrogen peroxide (Nicole et al., 1990). For more details see Beltr~m et aL (1993b). Wastewaters were collected from distillery and tomato processing plants of the province of Badajoz (Spain). Their main characteristics are given in Table 1. The oxidation was followed via determination of global parameters like chemical oxygen demand (COD), total organic carbon content (TOC) and absorbance at 254 nm (/lr). The initial pH of wastewaters was also varied in some runs using known amounts of sodium hydroxyde and phosphoric acid. The analytical methods used have been described elsewhere (Beltr~in et al., 1992). Also, the experimental procedures were similar to others described in previous papers (Belmin et al., 1992, 1993b).

Distillery Tomato wastewaters wastewaters 750-3000 250-960 250-1000 125-540 230-930 280-815 3-5 6-7.5 320-1280 103-120 0.4-2.3 0.2-0.8

•Original distillery wastewaterscollectedfrom plant were at least 60 times more concentrated.

prone to attack by oxidants like ozone. U V radiation c o m b i n e d with hydrogen peroxide or ozone or a c o m b i n a t i o n o f these two oxidants are applied in this work. The objectives o f the work (in two parts) were to study the chemical oxidation rates with the above systems and to determine global kinetic parameters o f use in modelling the process. Research is now in progress to model the advanced oxidation and study the c o m b i n a t i o n o f chemical and biological oxidation o f wastewaters.

RESULTS AND DISCUSSION Since investigating the chemical oxidation rates o f wastewaters was the principal objective o f this work, C O D was the main parameter to follow. U V radiation

As U V radiation can sometimes be a complementary m o d e o f organic c o m p o u n d degradation with advanced oxidation systems, a few experiments on direct photolysis were first carried out. They were used as reference or blank experiments for the U V

EXPERIMENTAL

Experiments were carried out at room temperature (18 _+ 3°C) in batch mode, in a 850 cm3 glass vessel annular photoreactor equipped with inlets for feeding the wastewater, sampling and introducing a quartz well for a 17 cm

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Industrial wastewater advanced oxidation. Part 1

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Fig. 1. Influence of UV radiation on (a) COD and (b) gr of tomato wastewater. Conditions: I 0 = 3 . 8 x 10-6 Einstein I-~s -~, 18°C, pH7. I--1, COD0=927mg021 -L, /~r=0.78cm -~, +, CODo = 105 mg O51-I, pr = 0.19 cm -I.

radiation in combiination with hydrogen peroxide or ozone (see also the following paper; Beltnin et al., 1996) in order to check any improvements promoted by the combined system. The application of UV radiation to these industrial wastewaters was a priori thought to be a possible way of pollution elimination due to their absorbance values (see Table 1). Distillery wastewaters. Regarding distillery wastewaters, however, parameters such as COD and TOC remained practically constant after 2 h of UV radiation experiments. Hence, this type of wastewater is refractory to the individual action of 254 nm UV radiation. Tomato wastewaters. Tomato wastewaters, on the other hand, showed a different response when irradiated at 254 nm. Figure 1 presents the variation of COD and 254 nm absorbance (/~T) versus time, respectively, corresponding to UV radiation experiments with two different initial COD values. As can be seen from this tigure, the effect of direct photolysis is high dependent on the initial content of the water. Thus, it is observed that the higher the initial COD

is, the higher the degradation rate observed. As the photolysis rate of any substance is proportional to its concentration (see later kinetic study section), the results obtained are a logical consequence of the UV radiation kinetics. From Fig. 1 it is also seen that for the highest initial COD, /~r increases with reaction time up to a plateau value at about the maximum reaction time studied (2 h). It is evident from these results that during the reaction strongly UV absorbing compounds are generated. The presence of these compounds can alter the kinetic treatment of experimental results because parameters like quantum yields may not be constant throughout the reaction time as a result of the different photochemical reactions developed. However, direct photolysis or UV radiation does not seem to be a useful method for degrading tomato wastewaters of high concentration since the reduction of COD was only about 18% after 2 h. Wastewaters with low initial COD (105 mg O5 i -~) showed even lower photolysis rates, about 4% COD reduction, as seen in Fig. 1. Finally the, TOC variations (not shown) were negligible.

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F.J. Beltr~.net al.

UV radiation~hydrogen peroxide oxidation

hydroxyl radicals are mainly formed by direct photolysis of hydrogen peroxide (reaction (1) below) (Baxendale and Wilson, 1957) when its initial concentration is higher than 0.01 M since hydrogen peroxide absorbs most of the radiation at these conditions through reaction (1):

Distillery wastewaters. Distillery wastewaters with high initial COD (t> 3000 mg 021 -~) did not suffer any appreciable oxidation with the combination of UV radiation and hydrogen peroxide. For lower initial COD, only concentrations of hydrogen peroxide higher than 10 -2 M exerted a definite effect on the COD disappearance rate, as shown in Fig. 2(a). Note that in all cases the pH did not vary more than 0.5 units. The null effect observed with high COD (3000 mg 021 -~) can be justified taking into account the molar absorptivity of hydrogen peroxide (19 M -~ cm-~; Nicole et al., 1990) and the absorbance of the starting wastewaters (2.3 and 0 . 4 c m - ' for distillery wastewaters of 3000 and 600 mg 021-', respectively). Thus, high COD wastewaters absorb most of the incident UV radiation, while for lower COD wastewaters hydrogen peroxide is the main absorber, especially when its concentration is higher than 0.01 M. For example, wastewaters with about 6 0 0 m g l -~ COD have 0.4 cm -~ absorbance while 0.01 and 0.1 M hydrogen peroxide concentrations present absorbance values of 0.19 and 1.9 era-', respectively. In summary, according to the results obtained it is likely that

hv

H202 --* 2OH.

(1)

The increase of COD disappearance rate is consequently due to the action of these hydroxyl radicals. As seen in Fig. 2(c) the decrease of pT is mainly observed when hydrogen peroxide is present at the highest concentration applied, 0.1 M. In any case, however, this AOT does not seem suitable as a single oxidation treatment step for distillery wastewaters for two main reasons: the high concentration of hydrogen peroxide needed and the low TOC conversion achieved (only 6% in 2 h of oxidation with 0.1 M initial hydrogen peroxide concentration; see Fig. 2(b)). The consumption of energy in 2 h was 5.1 x 105Jm-L Tomato wastewaters. The combined effect of UV radiation and hydrogen peroxide on the oxidation of tomato wastewater was similar to that presented above for distillery wastewaters as far as COD is

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Industrial wastewater advanced oxidation. Part 1 concerned, although the effect of radical reactions is more pronounced. Thus, Fig. 3 shows the variation of COD and /~r Of these wastewaters when treated with UV radiation and hydrogen peroxide at two different concentrations. In this case a positive effect of hydrogen peroxJLde concentration on the variation rate of/~T was also observed (Fig. 3(b)). The presence of radical reactions can be deduced at 10-3M hydrogen peroxid,: concentration because of the existence of an i~duction period of about 90 min (when the rate of disappearance of COD is very low). Then, at higher reaction times, the oxidation rate is enhanced, probably due to the presence of a high concentration of hydroxyl radicals. According to these results, oxidation does proceed (the reduction of COD was 23% after 2 h reaction time with 0.01 M initial hydrogen peroxide concentration) but, as TOC (not shown) was unchanged, mineralization is not achieved with this AOT. Kinetic study

From the complex content of wastewaters and the different compounds that could form via oxidation, one can expect a priori that kinetic studies of this type

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are not appropriate. However, as shown in previous papers for ozonation of wastewaters (Beltrfin et al., 1995a, b) kinetic studies can also be accomplished for model oxidation in a similar way to that applied to single compounds. Precisely because of the unknown nature of wastewater constituents, the kinetics of these oxidations was studied following the evolution of COD with time. In fact, among other parameters that characterize the water, COD gives an appropriate magnitude of oxidation, better than TOC (Preis et al., 1988; Beltrfin et al., 1992), since only the former gives a real measure of the oxygen spent on oxidation during a given time. Thus, from the kinetic standpoint, COD represents a measure of the actual wastewater concentration. U V radiation kinetics Tomato wastewaters. Since distillery wastewaters are not sensitive to UV radiation the kinetic study focused only on the tomato wastewater degradation. In a photolytic process the main kinetic parameter is the quantum yield, ~ , which represents the amount of matter decomposed per photon absorbed. This parameter is specific for any substance so that, in this

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Fig. 2. Variation of (a) COD, (b) TOC and (c) yx with time during the oxidation of distillery wastewaters with a combination of UV radiation and hydrogen peroxide. Conditions: I0 = 3.8 x 10-4 Einstein 1-~ s-~, 18°C, pH4. I~, CH=I0-~M, COD0=589mgO21 -~, /tx=0.39cm -I, TOCo=190mgC1 i, +, CH=10-ZM, COD0=709mgO21 -t, px=0.56cm -I, TOC0=200mgCl -I, O, CH=I0-1M, COD0 = 850 mg O_,1-I, pT = 2.3 cm i, TOC0 = 192 mg C 1-I. case, it is reasonable to assume that the overall quantum yield of the wastewater varied with time as a result of the changing nature of compounds present in solution at any time. The kinetics of direct photolysis can be conveniently described by the following equation (Leifer, 1988; Beltr~in et al., 1993b): dC dt = I00[1 -- exp(--2.303L/~T)],

(2)

where C represents the concentration of the compound studied, I0 the incident flux of radiation and L the effective path of radiation through the wastewater inside the photoreactor, In the case of wastewaters C is substituted by COD. Then equation (2), where C and /~x are known at any time, was solved numerically to determine ~ . Table 2 shows some of the results obtained. It can be seen that diminishes with time when COD is higher than 100 mg Oz i -~ (for lower values of COD, @ remains practically constant). The direct photolysis of tomato wastewaters was especially important in the first 30 min, during which ~ and COD decrease 87 and 16%, respectively. Another important point to highlight is

the high value of • during this initial period compared to others reported in the literature for single compounds (Draper, 1985; Dulin et al., 1986; Beltr~in et al., 1993).

UV radiation~hydrogen peroxide oxidation kinetics This advanced oxidation is based on the formation of hydroxyl radicals from the direct photolysis of hydrogen peroxide (see reaction (1); Baxendale and Wilson, 1957). In a process of this nature the rate of disappearance of COD can be expressed as follows: dCOD dt - I0~[1 - exp(-2.303L/~T)] + koHCoHCOD, (3) where the first and second terms of equation (3) represent the contributions of direct photolysis and hydroxyl radical attack to the degradation of wastewaters. Also, in equation (3) ko. and Co. are the rate constant of the reaction between hydroxyl radicals and the dissolved matter and the concentration of these radicals, respectively. Distillery wastewaters. Since direct photolysis is negligible in the treatment of distillery wastewaters,

Industrial wastewater advanced oxidation. Part l equation (3) reduces to the following: dCOD dt = koaCo, COD.

(4)

This equation can further be simplified if one takes into account that the concentration of hydroxyl radicals is constant assuming the steady state situation for the net formation rate of these intermediates. Thus, the rate of disappearance of COD due to the combination of hydrogen peroxide and UV radiation is finally: dCOD dt

krCOD,

(5)

where kr represent:~ an apparent pseudo first-order radical rate const~mt of the oxidation. Given the changing nature of substances formed during the reaction time, equation (5) was solved numerically at each time by calculating the rate of disappearance of COD, and kr was determined at different times. For so doing, C O D - t data was fitted to polynomials by least squares analysis (correlation coefficients were always higher than 0.98). From L, the concentration

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of hydroxyl radicals, Con, was also calculated, assuming that ko. (see equations (4) and (5)) is 5 x 109 M -~ s -~, which is a reasonable value if one considers that rate constants for hydroxyl radicals and compounds in water range between 109 and 10 t° M -j s -I (Buxton et al., 1988). Some of the results obtained for kr and Cou are given in Table 3. As can be seen from Table 3, both parameters increase with increase in hydrogen peroxide concentration, confirming the importance of radical reactions. Thus, at 60min of reaction, Co. goes from 3.8 x 10 -~6 to 1.1 x 10 -~4 M when the hydrogen peroxide concentration is varied from 10 -3 to 10 -I M, respectively. When the hydrogen peroxide concentration is lower than 0.1 M, Con sharply decreases after the first 60 min of reaction. Then the oxidation rate is inhibited, which could suggest that hydrogen peroxide has been consumed. Only at 0.1 M hydrogen peroxide concentration does Co. seem to reach a steady state situation over the reaction period investigated. The high oxidation rate obtained with these conditions indicates a negligible effect of hydrogen peroxide with respect to consuming

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t i m e , rain Fig. 3. V a r i a t i o n o f (a) C O D a n d (b) pT with time d u r i n g the o x i d a t i o n o f t o m a t o w a s t e w a t e r s with U V r a d i a t i o n c o m b i n e d with h y d r o g e n peroxide. C O n d i t i o n s : 10 = 3.8 x 10 -6 Einstein 1-' s - ' , 18°C, p H 7. I-1, Without hydrogen peroxide, COD0 = 105 m g O_, 1-', /~T = 0.19 c m -t, +, CH = 10 -3 M, COD0 = 118 m g 021 -~, /IT = 0.39 c m - ' , Q , C , = 10 -2 M, COD0 = 201 m g O21 -~, /tr = 0.51 c m - ' .

hydroxyl radicals through the following reactions (Christensen et al., 1982): k=2.7

x 107 M - 1 s - I

H202 + OH

~

HO2 + H20

(6)

H02 + O H - .

(7)

and k=7.5

HO2 + OH

radicals are formed from the direct photolysis of one molecule of hydrogen peroxide. This reaction has a quantum yield of 0.5mol O2(photon)-' (Baxendale and Wilson, 1957), so that r~ can be expressed as follows:

x 109 M - I s - I

~

Finally, the rate of initiation of hydroxyl radicals, r, corresponding to reaction (1) was also calculated. As observed from the stoichiometry, two hydroxyl

r, = IoFH[1 -- exp(--2.303L~)],

where F , is the fraction of the incident radiation Table 3. UV radiation/hydrogen peroxide oxidation of distillery wastewaters: kinetic parameters

Table 2. Values of quantum yields • in the UV radiation of tomato wastewaters~

Time (rain)

Time (min)

0 60 120 0 60 120 0 60 120

0 60 120 0 60 120

COD (mg 021- ')

qb (mol 02 Einstein -~)

927 780 761 105 105 100

0.683 0.09 I 0.013 0.005 0.005 0.006

qo = 3.8 x 10-6 Einstein I-~ s-k

(8)

COD (mg O21 -I )

Ca (M)

589 583 581 709 641 639 850 705 510

0.001 0.001 0.001 0.01 0.01 0.01 0.1 0.1 0.1

k, x 106 (s-') 4.0 1.9 0.8 42.1 4.2 2.8 56.2 59.7 83.4

Co, (M) 8.1 3.8 1.6 8.5 8.4 5.7 1.1 1.2 1.7

x x × x x x × x x

I0 -'6 I0 ,6 I0 -'6 10-~ I0 -'~ 10 -16 I0 -~4 I0 ~4 10-.4

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Table 4. UV radiation/hydrogenperoxide oxidationof tomato wastewaters: kineticparameters Time COD CH kr x 105 Con x 10t4 r~x 106 % (rain) (mgOzI-') (M) (s-t) (M) (M s-') free radicalway 0 I 18 0.001 0.6 0. I 1.9 61 0 201 0.01 5.7 1.1 1.8 83

absorbed by hydrogen peroxide: F. =

£H C H

/~T

,

(9)

E. and CH being the extinction coefficient (19 M -~ cm-'; Nicole et al., 1990) and the concentration of hydrogen peroxide, respectively. Equation (6) can be simplified to equation (10) below, since its exponential term was always found to be higher than 2 (Nicole et al., 1!)90): (10)

r, = l o F , .

Equation (10), however, could only be applied to the start of oxidation because hydrogen peroxide concentration further along the reaction time was unknown (this cc,ncentration could not be determined due to the interference of organic peroxides in the analytical method). Thus, at t = 0, values of r~ were 1.94 x 10 -7, 1.29 x 10-6 and 3.15 x 10-6M s -~ with 10-3, 10-2 and 0.1 M hydrogen peroxide concentration, respectively. T o m a t o wastewaters. The kinetics of advanced oxidation of tomato wastewaters is somewhat more complicated since both contributions, direct photolysis and hydroxyl radical attack, are important for eliminating the matter present in water. Thus, equation (3) was used after applying the steady state situation for hydroxyl radicals to obtain the following rate equation for COD: dCOD dt - I0¢'Fcoo[1 -- exp(--2.303L/~r)] + krCOD,

(ll)

where Fcoo is the fraction of incident radiation the matter present in water absorbs: Fcoo = /~

(12)

and /~ the absorbance due to wastewater without hydrogen peroxide. Equation (11) can also be simplified to equation (13) as in the case of distillery wastewaters: dCOD dt = I0~FcoD + krCOD.

(13)

For similar reasons to those explained for the case of distillery wastewaters,/=COD was only calculated at the start of oxidation. Quantum yields corresponding to these conditions were extrapolated from results obtained in the direct photolysis. In this way, the contribution of direct photolysis during this AOT, the first term on the right-hand side equation (13), was determined. Then, kr and Con were also

calculated following the procedure described above for the oxidation of distillery wastewaters. Table 4 presents some of the results obtained. It can be observed that these two parameters increased with increasing hydrogen peroxide concentration as expected (hydroxyl radical concentration increases by one order of magnitude when hydrogen peroxide concentration increases 10 units), although the rate of initiation was practically constant, probably due to the different initial COD. As the contribution of direct photolysis was as high as 39%, the oxidation was mainly due to radical reactions. CONCLUSIONS The following conclusions can be drawn from the direct photolysis and UV/HzOz oxidation of distillery and tomato wastewaters. The application of 254 nm UV radiation alone is not appropriate to reduce the contaminant content of distillery wastewaters. However, in the case of tomato wastewaters, a slight decrease of COD is observed. Quantum yields of direct photolysis for this latter case vary from about 0.7 to 0.005 mol Ozphoton -t for initial COD values of 907 and 105mgO21 -~, respectively. Advanced oxidation of these wastewaters with a combination of UV radiation and hydrogen peroxide leads to reductions of COD in both cases, although distillery wastewaters of more than 3000 mg 021 -I are refractory to this kind of oxidation. Reductions of COD for distillery wastewaters can only be as high as 38% when the initial COD is 850 mg O~ 1-~ and the initial concentration of hydrogen peroxide is 0.1 M. In the case of tomato wastewaters, the contribution of hydroxyl radical reactions was higher than 60% in all cases. Hydrogen peroxide seems to act only as an initiator of hydroxyl radicals, which is the most significative difference with respect to UV/H202 oxidation of single compounds in waters (in such cases hydrogen peroxide, at concentrations lower and higher than 10-2 M, acts as an initiator and inhibitor of hydroxyl radicals through reactions (1), (10) and (11), respectively). The main disadvantage observed with this AOT was the negligible reduction of TOC. From the results obtained, it is evident that application of UV radiation alone or combined with hydrogen peroxide for the treatment of the wastewaters studied is not recommended as the only treatment method due to the slight reductions achieved for COD. Thus, it is likely that experimental studies on the combination of this treatment with biological oxidation are of more interest, since chemical oxidation usually improves water

F. J. Beltrhn et al.

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biodegradability. This kind of treatment will be the subject of forthcoming work in which BOD will be the main parameter to follow. In the present work the main point under investigation was chemical oxidation rate which gives the extent of oxidation of both non-biodegradable and biodegradable compounds in wastewater. Kinetic results show important quantum yields for direct photolysis of tomato wastewaters of high concentration (with an initial C O D of about 1000 mg 021-~). During UV/H202 oxidation of the wastewaters studied, the concentration of hydroxyl radicals is usually kept lower than l0 -~3 M, a very low concentration to consider this process as the only treatment step apart from primary treatment operations. This conclusion agrees with experimental results on the variation of C O D with time. It can also be concluded that modelling of this type of oxidation can be developed through the use of global kinetic parameters like apparent quantum yields and radical rate constants in spite of the complexity of the wastewater content. Application of this information to model oxidation treatments in reactors of larger size has already been carried out for the case of ozonation (Beitr~in et al., 1995a, b) so that in future work we may be able to apply the kinetic information obtained here to model the oxidation, in order to predict/simulate the behaviour of the process. F r o m a practical point of view, it should be noted that lamp costs, maintenance and energy consumption can also be bars to applying this mode of chemical oxidation, so that an economic study, that lies outside the scope of this paper, is also recommended to investigate any possible application of the treatment presented here. Acknowledgements--The authors thank the CICYT of Spain for the economic support through grant AMB96/ 1042.

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