Treatment of olive mill wastewaters byozonation, aerobic degradation and thecombination of both treatmentsF Javier Benitez,* Jesus Beltran-Heredia, Joaquin Torregrosa and Juan L AceroDepartamento de Ingenierıa Quimica y Energetica, Universidad de Extremadura, 06071 Badajoz, Spain

Abstract: The degradation of the pollutant organic matter present in olive oil mill wastewaters (OMW)

is carried out by a single ozonation, a single aerobic degradation, and the combination of two

successives steps: an ozonation followed by an aerobic degradation, and an aerobic degradation

followed by an ozonation. In both single processes, the removal of this contaminant load is followed by

means of global parameters which are directly related to the concentration of organic compounds in

those ef¯uents: chemical oxygen demand and total aromatic and phenolic contents. In the ozonation,

an approximate kinetic study is performed which leads to the evaluation of the apparent kinetic

constants for the aromatic reduction, kA. In the aerobic degradation, the kinetic study is conducted by

using the Grau model, which is applied to the experimental data, and leads to the determination of the

kinetic parameters of this model, K2 and n. In the combined processes, a higher COD global reduction

is obtained by the successive stages, and an improvement in the removal of the organic material during

the second treatment of both processes due to the pretreatment conducted is also observed. This

enhancement is shown by an increase of the kinetic parameters (K2 and n in the aerobic degradation of

the pre-ozonated wastewaters; the apparent constant kA in the ozonation of the wastewaters

preliminary fermented aerobically), in relation to the values obtained for them in the single processes

carried out at the same operating conditions.

# 1999 Society of Chemical Industry

Keywords: olive mill wastewaters; ozonation; aerobic degradation; combined ozonation-aerobic degradation;kinetics

NOTATIONA Aromatic content, absorbance at 254nm

BOD Biological Oxygen Demand (gdmÿ3)

CA* Ozone equilibrium concentration

(moldmÿ3)

COD Chemical Oxygen Demand, measurement

of the substrate concentration (gdmÿ3)

CODnb Non-biodegradable concentration of

substrate (gdmÿ3)

kA Apparent kinetic constant for the ozonation

process (dm3molÿ1hÿ1)

k'A Apparent kinetic constant of pseudo-®rst

order for ozonation (hÿ1)

K2 Kinetic constant for aerobic degradation

de®ned by eqn (7) (g Sg VSSÿ1 dayÿ1)

n Parameter de®ned by eqn (7)

p Exponent of the [OHÿ]

q Speci®c substrate decomposition rate

(g Sg VSSÿ1 dayÿ1)

S Biodegradable substrate concentration

(gdmÿ3)

TOC Total Organic Carbon (gdmÿ3)

TP Total Phenolic content (g of caffeic acid

dmÿ3)

TS Total Solids concentration (gdmÿ3)

TSS Total Suspended Solids concentration

(gdmÿ3)

VSS Volatile Suspended Solids concentration

(gdmÿ3)

X Biomass concentration measured as VSS

(gdmÿ3)

XA Total aromatic compounds conversion (%)

XCOD COD conversion (%)

1 INTRODUCTIONThe extraction and manufacture of olive oil in the

Mediterranean countries are carried out in numerous

Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 74:639±646 (1999)

* Correspondence to: F Javier Benitez, Departamento de Ingenierıa Quimica y Energetica, Universidad de Extremadura,06071 Badajoz,SpainE-mail: javben@unex.esContract/grant sponsor: Comision Interministerial de Ciencia y Tecnologia (CICYT); contract/grant number: AMB97-339Contract/grant sponsor: Junta de Extremadura; contract/grant number: IPR98A014(Received 2 March 1998; revised version received 28 February 1999; accepted 13 March 1999)

# 1999 Society of Chemical Industry. J Chem Technol Biotechnol 0268±2575/99/$17.50 639

small plants which operate seasonally and generate

each year more than 30million m3 of liquid ef¯uents.1

These generate the so-called `olive mill wastewaters'

(OMW) that have considerable pollution potential

because of their high organic load, which includes

sugars, tannins, polyphenols, polyalcohols, pectins,

lipids, etc,2 and the seasonal operation, which requires

storage, often impossible in small plants.

These wastewaters are usually disposed of into

evaporation ponds or by discharge into rivers, so dis-

seminating the contaminants. Because of the resultant

environmental problems and potential hazards, many

countries have limited the discharge of OMW and

tried to develop technologies to reduce the pollutant

potential by removal of the main toxic organic

substances prior to discharge.

Of the technologies developed, anaerobic digestion

is the preferred process,3±5 but problems of high

toxicity and inhibition of biodegradation have been

encountered. This is because some bacteria, eg

methanogens, are particularly sensitive to the organic

compounds present,6 especially the phenolic com-

pounds, which limit the performance of anaerobic

digestion.7,8 Consequently, other treatments, such as

ozonation oxidations9 or aerobic biological degrada-

tions,10,11 have been investigated for OMW treatment,

and have been shown to be successful.

Ozone has many properties that suit it for use in

water treatment: it is a powerful oxidant capable of

oxidative degradation of many organic compounds

and is readily available, soluble in water and leaves no

by-products that need to be removed. The study of

OMW ozonation has thus become of interest.12,13

Similarly, aerobic fermentation seems to be an

effective process for the degradation of these wastes.14

These processes may also be used as pretreatment

stages to remove most of the phenolics, decrease the

toxicity for methanogenic bacteria and facilitate

subsequent anaerobic digestion.15

In the present work, chemical oxidation by ozone

and biological degradation by aerobic microorganisms

of OMW were studied separately, to provide data for

the removal of the total organic matter present. For

this purpose, the chemical oxygen demand (COD), the

total aromatic content (A) and the total phenolic

content (TP), were selected as criteria to monitor the

overall degradation process. Since the design of

equipment requires knowledge of the kinetics between

the oxidant agents and the substances to be degraded,

the apparent kinetic constants were determined for the

total aromatic content reduction during the ozonation,

and the kinetic parameters evaluated according to the

Grau model16 for the aerobic degradation. Two

combined processes consisting of ozonation followed

by aerobic degradation, and aerobic degradation

followed by ozonation, were carried out to establish

the total COD removal obtained by the consecutive

stages, and to determine the enhancement in the

ef®ciencies of the second treatments due to the

pretreatments conducted.

2 MATERIALS AND METHODS2.1 Olive mill wastewatersWastewaters were obtained from an olive oil produc-

tion plant located at the Extremadura Community

(south west Spain). In a ®rst step, the main physico-

chemical characteristics and features of the OMW

were determined: pH=4.8, BOD=52gdmÿ3,

COD=112gdmÿ3, TOC=16.0gdmÿ3, total phenolic

compounds TP=2.20 expressed as g caffeic aciddmÿ3

(determined by the Folin±Ciocalteau method17), total

solids concentration TS=90.97gdmÿ3, total sus-

pended solids TSS=5.14gdmÿ3 and volatile sus-

pended solids VSS=4.65gdmÿ3. Prior to

degradation experiments, the OMW were centrifuged

for 30min and ®ltered to remove suspended solids.

2.2 Ozonation processBecause of the high COD concentrations, OMW were

diluted with distilled water up to 10% (v:v) to give

initial COD concentrations of around 10gdmÿ3.

Experiments were conducted in a mixed batch reactor

(a 1200cm3 cylindrical Pyrex glass vessel) provided

with a cover containing inlets for bubbling the gas feed

and stirring, and outlets for sampling and venting. The

reactor was submerged in a thermostatic bath with the

necessary elements to maintain the temperature

constant within �0.2°C. For the ozone generation,

oxygen taken from a commercial cylinder was dried

with silica gel traps and introduced into an ozone

generator (Yemar, model HPA).

The temperature and pH (by adding orthophos-

phoric acid and sodium hydroxide) of the diluted

OMW were adjusted to the desired values. The ozone±

oxygen gas stream was then fed into the reacting

medium through a bubble gas sparger with a constant

¯ow rate of 40dm3hÿ1 (at 1atm and 20°C). The

ozone partial pressure in the mixture was kept constant

at a value of 1.77kPa (= 1.41g hÿ1). Each experiment

lasted for 8h, and samples were taken at intervals to

analyse the residual COD concentration, as well as the

concentrations of total aromatics and phenolics, par-

ameters directly related to two groups of organic

compounds which are the most important contribu-

tors to the total pollutant load of these ef¯uents. Total

phenolic compounds were evaluated as described

above17 and the aromatic compounds were deter-

mined by measuring the absorbance of the samples at

254nm, which corresponds to the absorbance maxi-

mum of these organics.

In order to evaluate the removal of organic matter

during the ozonation process the COD its conversion

was de®ned in the form:

XCOD � COD0 ÿ CODf

COD0

� 100 �1�

and a similar expression was used to determine the

conversion of the total aromatic compounds, XA.

640 J Chem Technol Biotechnol 74:639±646 (1999)

FJ Benitez et al

2.3 Aerobic degradation processSince OMW lack microorganisms capable of aerobic

degradation, it was necessary for bacterial ¯ora

sampled from other wastes to be adapted to this

substrate.18 For this, the reactor was inoculated with

activated sludge taken from a municipal wastewater

treatment plant. In the ®rst instance, the bioreactor

was loaded with suf®cient inoculum to obtain a

biomass concentration of 1g of VSSdmÿ3, and the

culture was completed with the OMW having an initial

COD in the reactor of 5gdmÿ3. The digester was

stirred and aerated for 7 days. After this time, and after

a settlement period of 1 day, the biomass was

separated by ®ltration from the supernatant liquid.

This procedure was repeated in the subsequent

experiments, in which successive additions of OMW

with gradually increasing initial concentrations of

COD (5, 10, 15 and 20gdmÿ3 respectively) were fed

to the reactor. Finally, three experiments were con-

ducted with OMW containing an initial COD of

20gdmÿ3: the biomass acclimatization was achieved

because a similar removal of COD was obtained after

these experiments.

After the adaptation phase, the degradation

experiments were conducted in the completely mixed

batch reactor described above. The air ¯ow was fed to

the reacting medium through a bubble gas sparger

with a constant ¯ow rate of 60dm3hÿ1 (at 1atm and

20°C), and the temperature was kept constant at

28°C.

Prior to each run, the OMW was diluted in order to

attain the required initial concentration of COD. The

digester was then inoculated with the adapted bio-

mass, as described above, and was charged with

1000cm3 of the diluted OMW. Amounts of

K2HPO4, (NH4)2SO4, NaNO3 and MgCl2.6H2O

were also added as nutrients in order to maintain the

ratio COD:N:P at around 100:5:1, this ratio being

adequate to keep satisfactory microbial activities in the

inoculum culture.10,11 These experiments lasted be-

tween 5 and 7 days during which time samples were

withdrawn daily for analyses of COD, biomass (X,

expressed as gVSSdmÿ3) and the total aromatic and

phenolic compounds.

In the aerobic process, it was also necessary to

determine the non-biodegradable organic matter

present in OMW. For this, batch processes were

performed in which the reactor was loaded with

wastewaters of known substrate concentration and

degraded with adapted ¯ora until no additional

removal of COD was observed. Fermentation was

continued for two more weeks, and analysis of COD

was made: this latter value corresponded to the

amount of non-biodegradable substrate, mainly re-

calcitrant organic compounds such as polyphenols,

which are not degraded by the microbial ¯ora as

several authors have reported.10,11 The average value

of non-biodegradable organic matter (CODnb) ob-

tained from those experiments was 10% of the initial

COD.

2.4 Combined processesThe experimental procedure for the combined pro-

cesses (ozonation±aerobic degradation and aerobic

degradation±ozonation) was similar to the single

oxidation processes. Thus, in the ozonation±aerobic

process, a volume of the ozonated pretreated OMW

was loaded into the bioreactor for an aerobic experi-

ment. The initial COD concentration was the ®nal

COD concentration of OMW from the ozonation

stage. Similarly in the aerobic degradation followed by

ozonation, the aerobically pretreated wastewater was

charged to the ozonation reactor for the ozone

oxidation. In both combined processes, the COD

concentration was measured at the end of each stage.

3 RESULTS AND DISCUSSION3.1 OzonationData from investigations of ozonation of OMW, where

the temperature and the pH were varied are given in

Table 1. The initial substrate concentrations were

around 10gdmÿ3, and the ozone partial pressure of

1.77kPa in the gas mixture was kept constant. As

mentioned, the evolution of the organic matter

reduction was followed through each experiment by

measuring COD and aromatic and phenolic com-

pounds concentrations.

Table 1 also shows the initial COD and A values for

each experiment, and the conversions obtained de-

®ned by eqn (1). The conversions for COD ranged

between 17 and 28% depending on the operating

conditions. Those moderate conversions (XCOD)

could be attributed to the fact that, in general, the

organic compounds are very reactive with ozone, but

this ozonation process leads to partially oxidized and

less polluting compounds, which utilize ozone to

complete their oxidation and still demand oxygen.

Table 1 also shows a direct in¯uence of both variables

on the COD reduction. In addition, Table 1 depicts

the removal obtained for the aromatic compounds

with very similar values in all experiments: from them,

an average reduction of aromatics of 76% can be

proposed.

The COD concentration decreased continuously

with time as can be seen in Fig 1 which shows the

COD evolution in experiment O-6 taken as an

example. Figure 1 also shows the change in the

Table 1. Effect of temperature and pH on COD and absorbance (254nm)removal by ozonation of OMW

Experiment T (°C) pH

COD0

(gdmÿ3)

A0

(254nm)

XCOD

(%) XA (%)

O-1 20 5 10.14 0.765 17.1 68.8

O-2 10 7 9.88 1.159 17.0 68.5

O-3 20 7 9.74 1.171 19.9 73.5

O-4 30 7 9.33 1.039 24.9 73.7

O-5 40 7 12.70 1.458 27.5 77.0

O-6 20 9 12.16 1.486 21.5 76.2

J Chem Technol Biotechnol 74:639±646 (1999) 641

Treatment of olive mill wastewaters

absorbance at 254nm (a measure of aromatic com-

pound content), and the concentration of the total

phenolic compounds during the same ozonation

experiment which decreased rapidly. So, it could be

concluded that ozone is an excellent oxidizing agent in

the speci®c destruction of phenolic compounds of

OMW.

An approximate kinetic study on the ozonation of

OMW can be performed by using any of the global

parameters directly related to the organic load present

in the ef¯uent. Among these, the aromatic compounds

content seems to be the most suitable parameter for

representing the evolution of the organic matter in

these wastes, and therefore was selected for the kinetic

study.

The oxidation by ozone of the dissolved organic

matter contained in OMW, is a complex process.13

However, the total consumption of ozone by that

organic load can be evaluated by the decrease in this

total aromatic content of the wastewater and repre-

sented by a simple irreversible reaction in the form:

A0 �O3ÿ!Non-aromatic compounds �2�By assuming that this reaction follows a pseudo-®rst

order kinetics with respect to the aromatic compounds

concentration, eqn (3) is obtained:

ÿdA

dt� k0A � A �3�

which can be integrated between t =0 and t = t,yielding:

lnA0

A� k0A � t �4�

According to this expression, for each experiment a

plot of the ®rst term versus t must yield a straight line

whose slope is k'A, an apparent kinetic constant of

pseudo-®rst order which is a function of the equili-

brium ozone concentration in the solution, CA* :

k0A � kA � CA� �5�

Figure 2 shows this plot for experiments performed by

varying the temperature. As can be seen, points lie

satisfactorily around straight lines; this is in accord

with eqn (4) and con®rms the assumed reaction

model. A similar plot was obtained for experiments

carried out by varying pH. After least square regression

analysis, the k'A constants showed in Table 2 were

deduced.

With k'A values, the use of eqn (5) allowed the

Figure 1. Ozonation of OMW. Changes in values of COD, absorbance at254nm (A) and total phenolics (TP) during an experiment. Experiment O-6.

Table 2. Apparent kinetic constants for absorbance (254nm) removal in theozonation process

Experiment CA* x105 (moldmÿ3) k'A (hÿ1)

kA

(dm3molÿ1hÿ1 )

O-1 10.41 0.110 1057

O-2 11.48 0.148 1289

O-3 9.63 0.166 1723

O-4 7.58 0.180 2374

O-5 5.60 0.189 3375

O-6 9.32 0.181 1942

Figure 2. Ozonation of OMW. Determination of apparent kinetic constantsof pseudo-first order in experiments varying the temperature. Experiments:O-2, O-3, O-4 and O-5.

642 J Chem Technol Biotechnol 74:639±646 (1999)

FJ Benitez et al

apparent kinetic constants (kA) for each experiment to

be determined. For this, the ozone equilibrium

concentrations (CA*) were deduced after applying

Henry's law and the Henry's law constants.19 Table 2

also shows the kA values obtained: as they are

in¯uenced by temperature and pH, they can be

correlated by a modi®ed Arrhenius expression in the

form:

kA � k0A expÿEa

RT

� ��OHÿ�p �6�

which illustrates the in¯uence of the operating condi-

tions. According to this expression, multiple regression

analysis of kA constants against temperature and pH

leads to the following values: k0A=4.0�107,

Ea=23.75kJ molÿ1 and p =0.066.

3.2 Aerobic fermentationIn the single aerobic OMW degradation process

experiments were performed by modifying the initial

substrate concentration (COD0) from 20gdmÿ3 to

98gdmÿ3, and the initial biomass X0 between 0.3g of

VSSdmÿ3 and 6g of VSSdmÿ3. Table 3 presents the

values taken for those variables in a set of experiments.

As mentioned previously, the evolution of COD,

biomass and contents of aromatic and phenolic

compounds was followed in every experiment. Thus,

Table 4 depicts the values for experiment B-3 taken as

an example, similar trends being obtained in the other

experiments.

For the substrate concentration COD, a continuous

decrease with time was obtained, as can be observed in

Table 4 for the experiment B-3 taken for illustration.

Table 3 also depicts the CODf ®nal values in each

experiment, and the COD removal as de®ned by eqn

(1). From the XCOD values showed, it can be seen that

for the same initial COD0 (experiments B-1 to B-6,

with COD0 around 20gdmÿ3), the initial biomass X0

hardly affects the COD removal and gives overall

reductions between 81.4% and 87.5%. Simulta-

neously, an inverse effect of the COD0 on the COD

removal is observed (experiments B-6 to B-11, with

similar initial biomass X0, around 0.5gdmÿ3): when

COD0 increases, XCOD diminishes and varies from

84.4% to 58.2%.

Biomass change during the aerobic process is shown

in Fig 3 which gives the biomass concentration with

time in experiments where the initial biomass X0 was

varied (experiments B-1 to B-5). The changes agree

with the typical growth-cycle phases for batch cultiva-

tions:20,21 after a lag phase, a period of rapid growth

takes place where the cell numbers increase exponen-

tially with time: this is the exponential growth phase or

logarithmic phase. When a maximum size of popula-

tion is reached in the stationary phase, a decline in the

cell numbers occurs during the death phase. In the

present system, the lag phase is evident in experiments

B-1 and B-2 with higher X0, while for experiments

B-3, B-4 and B-5, with lower initial biomass already

acclimatized previously, the population of micro-

organisms increases from the ®rst moment of the

culture.

Finally, both the aromatic and total phenolic

contents decreased continuously during each experi-

Table 3. Experimental conditions and COD removals obtained in the aerobicdegradation process

Experiment

X0

(gdmÿ3)

COD0

(gdmÿ3)

CODf

(gdmÿ3) XCOD (%)

B-1 5.63 22.20 4.04 81.8

B-2 6.16 22.53 4.20 81.4

B-3 0.86 23.43 2.93 87.5

B-4 2.05 20.73 2.87 86.2

B-5 0.32 21.33 3.32 84.4

B-6 0.46 21.56 3.36 84.4

B-7 0.59 29.81 7.09 76.2

B-8 0.58 41.50 10.75 74.1

B-9 0.68 65.80 21.10 68.0

B-10 0.57 79.60 32.00 60.0

B-11 0.53 97.75 40.83 58.2

Table 4. Changes in values of biomass, COD, absorbance and totalphenolics during aerobic degradation of OMW (experiment B-3)

Time (day) X (gdmÿ3) COD (gdmÿ3) A TP (gdmÿ3)

0 0.86 23.43 1.553 0.446

1 1.83 13.20 1.484 0.404

2 2.05 9.22 1.395 0.477

3 2.19 7.15 1.331 0.236

4 2.32 4.82 1.228 0.054

5 2.24 3.48 1.158 0.033

6 2.20 3.08 1.110 0.032

7 1.66 2.93 1.152 0.026

Figure 3. Aerobic degradation of OMW. Evolution of biomass inexperiments varying the initial X0. Experiments: B-5, B-3, B-4, B-1 and B-2.

J Chem Technol Biotechnol 74:639±646 (1999) 643

Treatment of olive mill wastewaters

ment (see Table 4 for experiment B-3). Of special

interest is the intense reduction in the total phenolic

content, greater than 90% in all experiments. As was

described in Section 1, these organics are toxic to

methanogenic bacteria, and their elimination reduces

the toxicity for any subsequent anaerobic treatment.

Grau et al16 proposed a model for the degradation of

the organic matter in an aerobic process, the speci®c

substrate decomposition rate being expressed by the

following eqn:

q � K2

S

S0

� �n

�7�

where n is a constant to be evaluated, and S0 is the

initial substrate concentration.

In accordance with this model, the objective of the

kinetic study was to evaluate the kinetic parameters K2

and n. For this purpose, eqn (7) can be linearized in

the form:

ln q � lnK2

S0n

� �� n ln S �8�

Following eqn (8), a plot of ln q vs ln S must lead to

straight lines whose slopes and intercepts are n and

ln(K2/S0n) respectively; and from the intercept, K2 can

be determined.

For this procedure, the speci®c utilization rate of

substrate q, given by the expression:

q � ÿ dS

Xdt�9�

must be evaluated for each time of bioreaction.

In the present study, those values were determined

calculating dS/dt by ®tting the experimental data (S,t)to a polynomic expression by least-square regression

analysis, and dividing by the biomass concentration,

where S represents the biodegradable substrate con-

centration which in this study was determined by

subtracting the non-biodegradable CODnb value from

the COD concentration.

Table 5 shows as an example of this procedure, the

values obtained for ÿdS/dt and q in experiment B-4.

Similar results were obtained in other experiments.

Knowing the speci®c rate, eqn (8) can already be used

as previously described. Figure 4 shows this plot for

experiments where the initial substrate concentration

S0 was varied, and they con®rmed the agreement

between the experimental system studied and the

model of Grau.16 Additionally when S0 increased, the

slope n also increased.

After least square regression analysis, the slopes and

intercepts are evaluated and they are summarized in

Table 6. It can be observed that n and K2 values are

very close in experiments B-1 to B-6, where the initial

biomass X0 was varied and S0 remained almost

constant around 20gdmÿ3: it indicates that the

biomass has no effect on these kinetic parameters.

Average mean values of 1.22 for n, and 6.37g Sg

VSSÿ1 dayÿ1 for K2 can be proposed in this group of

experiments. However, in experiments B-6 to B-11

(where X0 was almost constant and the substrate

concentration S0 was modi®ed), increases of n and K2

when S0 increases can be observed, as already been

illustrated by Fig 4. So, linear relationships can be

Table 5. Determination of specific substrate degradation rate during aerobicdegradation of OMW (experiment B-4)

Time

(day)

X

(gdmÿ3)

S

(gdmÿ3)

ÿdS/dt

(gSdmÿ3 dayÿ1)

q (gSg VSSÿ1

dayÿ1)

0 2.05 19.07 5.48 2.67

1 2.60 9.74 4.55 1.75

2 3.32 7.42 3.62 1.09

3 3.69 6.12 2.68 0.73

4 3.99 3.24 1.75 0.44

5 3.59 1.90 0.82 0.23

Table 6. Values of the Grau model kinetic parameters in the aerobicdegradation of OMW

Experiment S0 (gdmÿ3) ln (K2/S0n) n

K2 (g S g VSSÿ1

dayÿ1)

B-1 20.42 ÿ2.46 0.81 6.34

B-2 20.73 ÿ2.92 0.96 4.09

B-3 21.55 ÿ1.69 1.23 7.80

B-4 19.07 ÿ2.15 1.10 7.81

B-5 19.62 ÿ2.56 1.69 5.42

B-6 19.84 ÿ2.35 1.53 6.79

B-7 27.12 ÿ4.10 1.96 10.65

B-8 38.18 ÿ6.55 2.50 12.99

B-9 60.53 ÿ8.87 2.90 20.57

B-10 73.23 ÿ13.76 3.93 22.44

B-11 89.92 ÿ15.62 4.26 34.28

Figure 4. Aerobic degradation of OMW. Determination of Grau modelkinetic parameters in experiments varying the initial substrateconcentration S0. Experiments: B-6, B-7, B-8, B-9, B-10 and B-11.

644 J Chem Technol Biotechnol 74:639±646 (1999)

FJ Benitez et al

proposed for both parameters in the form:

n � k0nS0 � k00n �10�

K2 � k0GS0 � k00G �11�After regression analysis of n vs S0, and K2 vs S0, the

following ®nal values are obtained: k'n=0.0405dm3g

Sÿ1; k@n=0.742; k'G=0.346dm3g VSSÿ1 dayÿ1 and

k@G=0.271g Sg VSSÿ1 dayÿ1.

3.3 Combinations of ozonation and aerobicdegradation stagesThe combined OMW degradation processes were

studied with the aim of evaluating the in¯uence of each

respective pretreatment on the second stage. The ®rst

combined process C-1 comprised ozone oxidation

pretreatment followed by aerobic biodegradation.

Table 7 shows the operating conditions, the initial

and ®nal COD concentrations, and the conversion

values obtained (XCOD) in each stage individually

considered, as well as the conversion achieved by the

overall process.

The total conversion obtained by the successive

stages was 84.6%, a higher value than achieved by

either single process under the same operating condi-

tions. With aerobic biological degradation of the

ozone-pretreatred OMW (C-1-B stage), COD con-

version of 82.5% was higher than that (76.2%)

obtained in the single aerobic experiment with a

similar initial concentration of substrate and operating

conditions (experiment B-7 of Table 3). This suggests

that ozone pretreatment enhances the subsequent

aerobic process, probably by removing some phenolic

compounds capable of inhibiting biological oxidation,

although it could be expected that the ®nal COD

removal by aerobic treatment following the ozonation

could reach higher values. In this case it can be

attributed to either the fact that the ozone treatment

was not suf®cient to remove all the recalcitrant

compounds, or that acclimatization of the microbial

consortia with the OMW in¯uent was not completely

achieved and the microbial population was not able to

degrade completely the loaded COD within the time

of the experiment.

This enhancement in the organic matter removal of

the second stage was con®rmed by the kinetic study

performed, by applying the Grau model to experi-

mental data as with single stage aerobic experiments.

Using eqn (8), q values were plotted against the

concentration of biodegradable material (S) in loga-

rithmic coordinates, and after regression analysis the

following kinetic parameters were determined:

n =2.58 and K2=17.85g Sg VSSÿ1 dayÿ1. These

values are higher than that obtained in the equivalent

experiment (B-7 of Table 6) with untreated OWM

(1.96 and 10.65g S g VSSÿ1 dayÿ1 for n and K2 re-

spectively). Thus, the rate of substrate removal was

increased by the initial chemical ozonation.

The second combined process (C-2) involved

aerobic pretreatment followed by ozonation. Table 7

gives the operating conditions, the initial and ®nal

COD concentrations, and the conversion XCOD

obtained for each stage, in addition to the total

conversion achieved in the overall process. Similarly

to combination C-1, the overall process achieved

81.8% degradation, greater than was obtained by the

individual chemical or biological processes under the

same operating conditions.

In combined process C-2, the ozone oxidation of the

biologically pretreated OMW (C-2-B stage) yielded a

conversion of 30.3%, signi®cantly greater than that

(19.9%) attained in the equivalent single ozonation

process (O-3 of Table 1) with a similar initial substrate

concentration of 10gdmÿ3. This suggests that aerobic

pretreatment enhanced the subsequent ozone oxida-

tion by removing most of the biodegradable organic

matter. The ozonation step then degraded some of the

non-biodegradable organic matter and much of the

residual phenolic compounds.

This was reinforced by a kinetic study of this stage,

applying eqns (4) and (5) to the experimental data.

Thus, according to eqn (4), a plot of ln (A0/A) vs t in

logarithmic coordinates gave a straight line of slope

k'A. After least square regression analysis, the slope

obtained was 0.221hÿ1. As the C*A of this experiment

was known (8.4�10ÿ5moldmÿ3), with eqn (5) the

apparent kinetic constant kA was determined to be

2512dm3molÿ1hÿ1. The results obtained were com-

parable with those from the equivalent experiment in

the single ozonation process without aerobic pretreat-

ment (experiment O-3 of Table 2): the values for k'Aand kA were 0.166hÿ1 and 1723dm3molÿ1hÿ1

respectively. Again, the enhancement of substrate

removal in the chemical ozonation of OMW by aerobic

pretreatment of the ef¯uent was demonstrated.

Table 7. Combined processes

C-1. Ozonation followed by aerobic degradation

C-1-A ± Ozonation stage:

Operating conditions: T=20°C; PO3=1.73kPa; pH=7;

COD0=34.05gdmÿ3

Substrate removal obtained: CODf=29.9gdmÿ3;

XCOD=12.2%

C-1-B ± Aerobic degradation stage:

Operating conditions: X =0.59gdmÿ3; COD0=29.85gdmÿ3

Substrate removal obtained: CODf=5.22gdmÿ3;

XCOD=82.5%

Total removal in process C-1: XCOD=84.6%

C-2. Aerobic degradation followed by ozonation

C-2-A ± Aerobic degradation stage:

Operating conditions: X =0.53gdmÿ3; COD0=41.95gdmÿ3

Substrate removal obtained: CODf=11.07gdmÿ3;

XCOD=73.6%

C-2-B ± Ozonation state:

Operating conditions: T=20°C; PO3=1.69kPa; pH=7;

COD0=10.95gdmÿ3

Substrate removal obtained: CODf=7.63gdmÿ3;

XCOD=30.3%

Total removal in process C-2: XCOD=81.8%

J Chem Technol Biotechnol 74:639±646 (1999) 645

Treatment of olive mill wastewaters

4 CONCLUSIONSThe study shows that over the range of variables

tested, ozonation of OMW achieved a moderate

reduction in the COD, and signi®cant removal of

aromatics and total phenolic compounds. The micro-

bial aerobic treatment achieved signi®cant removal of

COD and phenolics but less elimination of aromatic

substances. Kinetic studies of both processes identi-

®ed parameters useful for the design of treatment plant

reactors. For ozonation, the apparent kinetic constants

for the reduction of aromatic compounds were

evaluated and correlated as a function of the tempera-

ture and pH. For microbial aerobic degradation the

kinetic study was carried out using the Grau model;

constants K2 and n were calculated and correlated as a

function of initial substrate concentration.

The two combined processes studied both achieved

higher COD removal ef®ciencies than either single

stage treatment under similar operating conditions.

They can treat OMW to meet discharge norms and

reach treatment ef®ciencies required by national

regulations of the Mediterranean countries.

ACKNOWLEDGEMENTSThis research has been supported by the `Comision

Interministerial de Ciencia y Tecnologia' (CICYT) of

Spain, under Project AMB97-339, and partially by

Junta de Extremadura under Project IPR98A014.

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