PARTIAL PURIFICATION OF IRON SOLUTIONS FROM RIPE TABLE OLIVE PROCESSING USING OZONE AND ELECTRO-COAGULATION

Pedro García-García*, Francisco Noé Arroyo-López & Francisco Rodríguez-

Gómez.

Food Biotechnology Department, Instituto de la Grasa (CSIC).

Padre García Tejero 4, 41012 Sevilla (Spain).

*Corresponding author:

Pedro García García

Food Biotechnology Department, Instituto de la Grasa (CSIC)

Avda. Padre García Tejero 4, 41012 Sevilla (Spain).

Tel: +34 954 690 850

Fax: +34 954 691 262

E-mail address: pedrog@cica.es

1

Abstract

This study investigates the application of electro-coagulation and ozonation

technologies for the partial depuration of ferrous solutions deriving from the

color fixation stage of ripe olive processing. Different operational conditions

were investigated along with the possibilities of combining the two techniques

sequentially. In electro-coagulation the best depuration conditions were

obtained using a current density of 25 mA cm-2 for 40 min; COD elimination

reached 30%, the phenols and color of the solutions were also considerably

reduced and the dissolved Fe was completely removed. On the contrary, the

microbial population mainly composed of lactic acid bacteria and yeasts, hardly

decreased. The application of ozonation also reduced the total phenols, color

and COD of solutions, but did not decrease the iron concentration. However, it

considerably affected the microbial population in a higher proportion than

electro-coagulation. In light of the results obtained, the best working conditions

consist of applying 40 min electro-coagulation with aluminum in the anode and

iron in the cathode at 25 mA cm-2 current density, followed by the storage of a

partially purified solution bubbling ozone to reduce the microorganism

population and even eliminate it completely when 6 g O3/L were added. This

would allow for a possible re-use of these solutions for preparing new fixing

color solutions or as cover brine in packaging.

Key-words: Contamination removal; Electro-coagulation; Ferrous solution;

Ozonation; Ripe olive.

2

1. Introduction

Olives darkened by oxidation [1] are commonly known by their original

American name: ripe olives. The production of this preparation was reached

around 630,000 tons in the 2011/2012 season, 30% of the world’s table olive

production [2].

The darkening process consists of successive treatments of the fruits

with a dilute solution of NaOH (lye); during the intervals between lye treatments

the fruits are suspended in water through which air is bubbled [3]. Nowadays, a

preservation solution is added to the tap water in this phase of oxidation [4].

Throughout this operation the fruits darken progressively, but the color formed

is not stable permanent and fades progressively after oxidation; to prevent this

deterioration, the fruits are immersed in ferrous lactate or gluconate for several

hours [5]. The product has a final pH above 4.6 and its preservation is only

achieved by sterilization [3].

Electro-coagulation (EC) is an electrochemical method consisting in

treating polluted water whereby sacrificial anodes dissolve to produce active

coagulant precursors (usually aluminum or iron cations) in the solution. The

generation of coagulants in-situ means that EC process does not require the

addition of any chemicals. The gases produced at the cathode during the

electrolysis of water and metal dissolution allow the resulting flocks to float [6].

EC has been successfully used in the treatment of wastewaters from

food industries such as alcohol distillery [7], dairy [8], olive oil mill [9] and oil

refinery [10]. In green table olive processing, the combined biological and EC

treatments of the debittering solution (lye) and washing water reduce the

chemical oxygen demand (COD) up to 97% [11]. Also, EC application to

3

wastewaters from the packaging industry results in a practically colorless

solution of lactic and acetic salts which could be reused in packing [12]. In

natural black table olives, the EC of wastewaters by boron-doped diamond

electrodes led to 73% COD removal [13]. However, this technology has not yet

been studied for the treatment of effluents from the darkening process of ripe

olive industries.

Ozone has been declared Generally Recognized as Safe (GRAS) and it

was admitted as a direct food additive [14, 15]. The treatment of wastewaters

with ozone from the table olive industry [16] and olive oil extraction process [17]

lightened the final color of the solutions due to destruction of the brownish

polyphenol polymers.

The green olive alkaline solutions (lye and washing waters) treated with

ozone may be re-used as fermentation brines after adding the appropriate NaCl

quantity [18, 19]. Also, Spanish green olives may be packed with diluted (1:1)

ozonated fermentation brines with similar organoleptic characteristics to those

using fresh brine [20]. In ripe olives, preservation solutions can be re-used in a

new preservation cycle after ozone treatment, the resulting quality of the final

product being similar to the ones stored in a fresh acid solution [4].

Color fixation solutions in the darkening process could be reused after

the addition of the required quantity of ferrous gluconate if the organic matter,

mainly phenols, is removed. These compounds can be chemically bonded to

the ferrous ions when adding more ferrous gluconate and preventing the cations

penetrating into the olives, forming the Fe-phenol complexes in the liquid and

the color of olive not being fixed [5].

4

In the present work, the use of EC and ozonation to treat ferrous

gluconate solutions from ripe olive processing was studied. Particularly, the

effect of various operating conditions (current intensity and contact time of

solutions in EC or amount of O3 bubbled) on the elimination of COD, iron,

phenols, color and microorganism survival was investigated. This information is

essential both to assess the possibility of reusing these solutions by the industry

and also for scaling up the treatment at the industrial level.

2. Experimental

2.1. Ferrous solutions

Ferrous gluconate solution samples were collected for the darkening

processes of ripe olives carried out at the Instituto de la Grasa pilot plant (CSIC,

Spain) similarly to what industrial processes are [3]: relationship olives/liquid,

1/1; lye treatment (2.5% NaOH, 5 h to reach stone); first washing only tap water

(20 h); second washing (tap water/preservation solution, 1/1, 24 h), ferrous

gluconate solution (0.1%, 20 h).

The initial physico-chemical and microbiological characteristics of the

solutions are shown in Table 1. The high population of microorganisms,

especially lactic acid bacteria (LAB) and yeasts were found in these solutions

due to the preservative liquid reuse in washing during oxidation [4]. This is the

first time that this fact is mentioned in a publication.

2.2. Electro-coagulation procedure

5

The EC was carried out in a plexiglas parallelepiped reactor (2.0 L). Two

pairs of Al/Fe (anode/cathode) electrodes connected in a bipolar mode were

placed in the electrochemical reactor, each one with dimensions of 13 cm × 6.5

cm × 0.2 cm; the distance between the lowest part of the electrode and the cell

was 0.8 cm and between electrodes was 1 cm; the total effective surface area

of the electrodes was 91 cm2. Prior to each experiment; the electrodes were

immersed in 1% HCl for 12 h and rinsed with distilled water.

The volume of liquid treated each time was 1.6 L. The liquid was

magnetically stirred by two bars, allowing a correct homogenization of the

wastewater. A direct current was imposed by a stabilized power supply (Quasar

500, CRS Industrial Power Equipment, Calco, Lecco, Italy) for 1 h and the

range of current density variations was 10–75 mA cm−2. The temperature was

monitored with a thermometer and maintained stable (20–25 °C) during the

experiments by cooling the solution with a cooling jacket. All experiments were

performed in duplicate and samples (10 mL) were withdrawn at 5 min intervals.

Before their analysis, the samples were centrifuged to remove any suspended

solid.

The electrical energy consumption per unit mass of organic load

removed (SEEC) was calculated using the following equation:

(1)

where: V, voltage in volts; I, current in amperes; t, time in hours; Q, volume of

wastewater in liters; and (C0-C), pollutant load removed in g O2/l.

2.3. Treatment with ozone

6

The ozone was produced by TODOZONO1, Mod TD ZN equipment

(Colmenar Viejo, Madrid, Spain). Aliquots of 2.0 L of solution were put in a

cylindrical test tube (52 cm x 8.5 cm diam.). The solutions were then treated by

bubbling ozonated air with an ozone concentration of 3.63 mg/L at a flow rate of

200 L/h (3.63 g O3/h/L of solution) through a synthetic glass diffuser introduced

at the bottom of the tube. The pH of the solution was allowed to evolve freely.

All experiments were performed in duplicate and samples (10 mL) were

withdrawn at 1 h intervals.

The electrical energy consumption was estimated from the power

consumption indicated by the manufacturer of the ozone-producing equipment

(50 w) and the time (h) to produce a given amount of ozone in g (Q) per unit

volume of solution (L).

(2)

2.4. Experimental design

The effect of various EC operating conditions (current intensity and

contact time) and bubbled O3 quantity during ozonation treatment on the

elimination of COD, iron, phenols and color, was investigated in a first step, in

order to know the best individual treatment conditions. Then, using the best

conditions obtained for both treatments (ozonation and EC), additional

experiments were performed alternating the order of operations, first EC and

then ozonation and vice-versa.

2.5. Physico-chemical analysis

7

The COD analysis was carried out according to the procedure described

in the Standard Methods of APHA-AWWA-WPCF [21]. The appropriate amount

of sample was placed with potassium dichromate and sulphuric acid with

mercuric sulphate solutions in a sealed test tube and incubated for 2 h at 150

ºC in a COD reactor (Selecta, Barcelona, Spain). COD was measured by

titration with a ferrous sulphate solution.

Total polyphenols were determined by the Folin-Ciocalteau method [22]

and the iron concentration in liquid was determined by flame atomic absorption

spectrometry [23].

The color of solutions was calculated as the difference in absorbance at

440 and 700 nm (A440–A700), using a fiber optics probe in a Varian Cary 60 UV-

Vis Spectrophotometer (Agilent Technologies, Malgrave, Vi, Australia).

Previously, the liquids were centrifuged at 12,000 g for 5 min [24].

All physico-chemical analyses were carried out at least in duplicate.

2.6. Microbiological analysis

Ferrous gluconate solutions or their decimal dilutions were plated using a

Spiral System model dwScientific (Don Whitley Scientific Limited, England) on

the appropriate medium described below.

Lactic acid bacteria (LAB) were plated on de Man, Rogosa and Sharpe

(MRS) agar media (Merck KGaA, Darmstadt, Germany) supplemented with

0.02% (w/v) sodium azide (Sigma, St. Louis, MO, USA), and yeasts on a yeast–

malt–peptone–glucose medium (YM agar, DifcoTM, Becton and Dickinson

Company, Sparks, MD, USA) supplemented with 0.005% (w/v) gentamicin

8

sulphate and oxytetracycline antibiotics (Oxoid LTD, Basingstoke, Hampshire,

England). The plates were incubated at 30 ºC for 48 h, counted using a

CounterMat v.3.10 (IUL, Barcelona, Spain) image analysis system and

expressed as log10 CFU/mL.

2.7. Statistics

Statistica version 6.0 (StatSoft, Tulsa, USA) for windows was used for

data analysis. Comparisons of the average values were performed by one-way

analysis of variance (ANOVA) followed by the multiple Duncan’s multiple range

test (p< 0.05).

3. Results and discussion

3.1. Effects of EC on ferrous solutions

3.1.1. Influence on physico-chemical parameters and chemistry of the process

Figure 1 shows the influence of different current densities on the

evolution of COD, polyphenols and iron dissolved in the ferrous gluconate

solutions during the application of EC. A decrease in the COD content occurred

in all EC processes at the different current densities assayed (Fig 1A).

Increasing current density led to an increase in the COD reduction rate and less

final COD values after 1h of treatment were obtained at the highest current

density values.

9

The concentration of total phenols in the solutions followed a similar

trend to that observed for the COD. When current density increased the phenol

contents in the solutions decreased. The final concentrations obtained in the

gluconate solutions ranged from 62 to 88 mg L-1 (Fig 1B).

The iron concentration also decreased with time and increasing current

density led to an increase in the iron reduction rate and less time was required

to achieve dissolved metal disappearance. With the lower tested current density

(10 mA cm-2) all dissolved iron disappeared in 40 min (Fig 1C).

Thus, removal efficiency increased significantly as the current density

increased. The highest current densities produced the fastest treatment with the

lowest values of COD, total polyphenol concentration and dissolved iron. This

behavior is ascribed to the fact that, at a high current density, the amount of

oxidized metal increases, resulting in a greater amount of precipitate for the

removal of more pollutants [10]. In addition, bubble size decreases with

increasing current densities and bubble density increases [25], resulting in a

greater upwards flux and a faster removal of pollutants and sludge flotation.

During the EC treatment, a gradual increase in the pH of the solution was

observed (Fig 2A). The values of pH after 1 h ranged from 8.0 to 8.9, with

higher final values with increasing current density.

A complete examination of what happens in the EC process shows the

following electrochemical and chemical reactions at anodes (3)-(4), both in the

bulk solution (5) and in the cathode (4) [8,10,26]:

Al-anode:

(3)

10

Al - 3e = Al3+ (4)

Solution:

Al3+ + 3H2O = Al(OH)3 + 3H+ (5)

Cathode:

3 H2O + 3e- = 3/2 H2 (g) + 3OH- (6)

The anodic oxidation of organic compounds (3) is the most important

reaction that should be produced in the EC of the ferrous gluconate solution

[28]. The indirect oxidation of organic matter can hardly occur, mainly phenols

by the action of the hypochlorite eventually formed from the NaCl dissolved in

the wastewater [27,28] because today in the industrial process NaCl is not

used, the preservation of the olives is carried out with organic acid solutions [3,

4] .

The reactions in the cathode (6) produce OH- and other anions such as

Cl-, HCO3-, etc. which are dissolved in the wastewater can exchange partly with

OH- in Al(OH)3, to liberate free OH-, which causes an increase in pH.

At the beginning of the EC treatment, the liquid became progressively

darker, as evidenced by the increase in the (A440–A700) parameter (Fig 2B). This

fact has also been observed in the wastewater EC of green olive industries [12].

This effect may be due to the initial formation of more colored polymers, as was

also noticed during the oxidation phase of ripe olives [29] and in the ozonation

of green olive brines [20]. Later on, the polymers and phenols were apparently

oxidized and their disappearance (Fig 1B) produced the subsequent brine

discoloration, which led to a decrease in the difference in absorbance (A440–A700)

(Fig 2B).

11

3.1.2. Electrode consumption

Information on electrode consumption is essential for estimating the cost

of the EC process because the dissolution rate affects the life of the electrodes.

Theoretically, according to Faraday’s law, whenever one Faraday of charge

passes through the circuit, 9.0 g of aluminum is dissolved at the anode.

However, the effective electrode consumption may be reduced or increased

from this theoretical value depending on the wastewater characteristics and

operational conditions.

The aluminum consumption (y) was linearly related (R2>0.95) to the

increase in current density (x) and it was higher (y=0.079 x) than theoretically

estimated values (y=0.061 x) and slightly greater than in green table olive

wastewaters EC (y= 0.074 x) [12].

The higher than expected dissolution of aluminum on the anode is due to

the electrochemical process; a reduction in water can modify the pH on the

anode with respect to the pH in the rest of the solution [9]. In the cathode, a

decrease in iron weight was not observed.

3.1.3. Efficiency and cost of process

The COD removal per theoretical unit of electrode consumption

decreases with treatment time (Fig 3A). The best COD removal versus

electrode consumption was obtained at the lowest current density (10 mA cm-2).

Using higher current densities, efficiency values were lower.

12

On the contrary, the electrical energy consumption per unit mass of

organic load removed (SEEC) increased, as the current density was higher (Fig

3B), due to a proportional increase in ohmic voltage loss in the cell. The energy

efficiency was higher (lower SEEC) when the current density was 10 mA cm-2.

Figure 3B shows that the specific power consumption is very similar when

working at 10 and 25 mA cm-2. However, the final value of COD was

significantly lower when operating at 25 mA cm-2 (Fig 1A). Thus, it can be

concluded that after the 40 min treatment, the COD and the color of the solution

(Figure 2B) remained unchanged and the dissolved Fe was completely

removed (Fig 1C).

Then, with a 25 mA cm-2 current density and 40 min treatment, the power

consumption expected would be around 5 kWh/kg COD (Fig 3B), which means

that for one cubic meter of wastewater and a COD reduction of around 1,300

mg O2 L-1 (Fig 1A), the total consumed electricity should be 6.5 kWh and the

theoretical sacrificial electrode loss 0.89 g of aluminum.

3.1.4. Influence on microbial population

The LAB and yeast population slightly decreased during EC at 25 mA

cm-2 current density (Fig 4A). After the 40 min treatment, the LAB population

only decreased about a third (≈0.25 log10 cycles). There is a slightly greater

influence on yeasts and the population was reduced by almost 60% in the same

period of time (≈0.5 log10 cycles). Continuous EC involves the reduction of the

microbial population.

13

The inactivation of LAB and yeast cells by electrochemical means has

been well documented [30]. The cells were probably inactivated during the EC

process by the chemical oxidizing products and the short-lived germicidal

agents produced, such as free radicals [31], but the performance of the process

strongly depends on the characteristics of the initial effluent (microorganism

population, organic load, etc.) and the type of electrodes used [32]. The low

inactivation of bacteria and yeast observed in this study may be related to the

high organic loading presented by the ferrous solution and the fact that EC is

more effective at inactivating microbes at lower population densities than at

higher ones [33], as occurred in this work.

3.2. Effects of ozonation on ferrous solutions

3.2.1. Influence on physico-chemical parameters and treatment cost

Ozone treatment decreased the concentration of phenols and the color of

the solutions (Fig 5A, 5B). After bubbling 6 g O3 L-1, both parameters had lower

values than under the best conditions in EC (Fig 1B, 2B).

The ozonation of phenols implies diverse breakdown reactions that give

rise to short chain carboxylic acids [34] producing a gradual decrease in pH

values (Fig 5C). A faster pH decrease which dropped the pH to about 5.3 was

observed after the added ozone reached 4.0 g L -1, to then decrease slowly to

reach a value of 5.2 when 10 g O3 L-1 were added.

Bubbling ozone produced a decrease in COD until 5 g O3 L-1 were added.

Adding more ozone does not involve a significant decrease in the pollutant load

14

(Fig 5D). Finally, a decrease in the iron concentration due to the effect of

bubbling ozonated air was not observed (data not shown).

The cost of the process is only due to the electrical energy consumption.

Figs 5A, 5B and 5D show that after adding 5 g O3 L-1, the total phenols, color of

the solution and COD were practically unchanged. Therefore, for one cubic

meter of solution, the total consumed electricity should be 68.5 kWh.

3.2.2. Effect on microbial population

The ozone also produced a decrease in the microbiological population,

as reported by other authors using different matrices [15,20]. After adding 6 g

O3/L. the population was reduced by about 1.0-1.5 log10 cycles for both LAB and

yeasts (Fig 6A).

The residual free ozone not used in phenol oxidation could have reacted

earlier with the cellular membrane of the microorganisms and cause their initial

death [15]. This was also observed when ozone is bubbled into the fermentation

brines of green olives [20].

3.3. Ozonation of ferrous solutions followed by EC.

The previous experiments allowed us to determine the best conditions for

the individual application of EC and ozonation to the ferrous solutions.

Consequently, the following step consisted in assessing the influence of the

sequential application of both techniques. Thus, solutions previously treated

with ozone (5 g O3 L-1) were then submitted to an EC procedure with 25 mA cm-2

current density. The main results obtained from EC are shown below.

15

3.3.1. Physico-chemical changes

A decrease in pollutant load was observed during the first 30 min of EC

treatment (Fig 7A); later, unchanged COD and the final value was slightly lower

than when no pretreatment with ozone was carried out (Fig 1A). The iron

concentration decrease rate (Fig 7B) was higher than when no pretreatment

with ozone was carried out (Fig 1C) and the dissolved metal disappeared

completely in 30 min. An increase in the pH of the solutions was also observed

(Fig. 7C); after 30 min of EC, the pH reached about 7.0 units.

The total phenol concentration was reduced from 50-60 mg L -1 (Fig 5A)

to about 30 mg L-1 in 30 min; also, a nearly complete discoloration of the

solution occurred (data not shown).

In accordance with the above mentioned results, it can be concluded that

the purification process finished after 30 min EC in previously ozonated iron

solutions with 5 g O3 L-1.

3.3.2. Effect on microbial population

EC at 25 mA cm-2 current density after ozonation of solutions also

produced a decrease in the microbial population. After 30 min the LAB

population was reduced by about 0.5 log10 cycles and the yeast population

hardly decreased (Fig 6B).

When EC extended for over 30 min, a greater inactivation rate of

microorganisms was produced than in both the first 30 min (Fig 6B) and also

when EC was applied to the solution without any pretreatment (Fig 4A). This

16

may be related to both lower organic load and lower population density, which

makes EC more effective in killing the microorganisms [32,33].

3.3.3. Efficiency and cost of process

After ozonation, the initial COD removal per theoretical unit of electrode

consumption (Fig 8A) was higher (about three times) than when no

pretreatment was performed (Fig 3A) with the same current density (25 mA cm-

2). After 30 min of EC treatment COD removal per g of consumed aluminum

was also higher (around 2.5 kg DQO g-1 Al consumed) than when no

pretreatment was carried out (1.7 kg DQO g-1 Al consumed).

Also, at the same current density (25 mA cm-2), the electrical energy

consumption per unit mass of organic load removed (SEEC) was higher in

solutions with a previous ozonated treatment (Fig 8B) than when no

pretreatment was performed (Fig 3B).

Therefore, the power consumption expected after 30 min of EC would be

around 7 kWh/kg COD (Fig 8B), which means that, for one cubic meter of

wastewater and a COD reduction of around 1,000 mg O2 L-1 (Fig 7A), the total

consumed electricity should be 7 kWh. The expected sacrificial electrode loss is

around 2.5 g of aluminum m-3.

The previous cost of the electricity needed for producing 5 g O3 L-1, 68.5

kWh/m3 of solution, must be added.

3.4. EC of ferrous solutions followed by ozonation.

17

Solutions previously treated with EC (40 min at 25 mA cm -2 current

density) were filtered and then submitted to ozonation.

3.4.1. Effect on physico-chemical parameters

The application of ozone (2 g L-1) produced a phenol concentration

reduction from about 80 mg/L to one fourth of the amount (data not shown); it is

necessary to bubble 7 g O3 L-1 to reach values below 10 mg L-1. At the same

time, a nearly complete discoloration of the solution occurs in addition to a slight

decrease in the pH with a final value near neutrality (7.0). The COD of the

solution decreases only 100 mg L-1 after bubbling 10 g O3 L-1 (data not shown).

Therefore, the ozonation of the gluconate solutions after EC does not

significantly decrease the pollution load and only the removal phenols and

discoloration occurred.

3.4.2. Effect on microbial population

Ozonated air bubbling after EC treatment reduced the microorganism

population and a complete destruction of the LAB was achieved after the total

bubbled ozone reached 4.0 g O3 L-1 (Fig 4B). Yeasts were more resistant to the

applied ozone than LAB and at least 5.8 g O3 L-1 must be applied to achieve

total elimination. This different effect of ozone on microbial inactivation was also

observed in the ozonation of green table olive brine [20].

The reduction in the microorganism population, and even its complete

elimination, may prevent secondary fermentations that could lead to the

18

formation of both strange smells and tastes that would make the re-use of these

liquids unviable in ripe table olive processing.

3.4.3. Efficiency and cost of process

Previously it has been shown that the power consumption of the EC for

40 min was 6.5 kWh m-3 of solution and the theoretical sacrificial electrode loss

was 0.89 g of aluminum. To this previous cost, the electricity need for bubbling

6 g O3 L-1 of ferrous solution to kill all microorganisms must be added, 82.2 kWh

m-3.

Alternatively, the solution could be used directly without ozone, reducing

this way the expenditure. When the solution is stored temporarily for later reuse,

bubbling ozone would be convenient in order to limit the possible development

of microorganisms and even their complete elimination.

3.5. Choice of the purification procedure

When the solutions were treated with ozone alone, the worst results in

removing pollution were obtained: highest values of COD and color, without

removing any ferrous ion and maintaining a high population of microorganisms.

On top of this, the energy cost was very high (Table 1).

Statistically (p<0.05) same final COD values (around 3,200-3,300 mg O2

L-1) were obtained when performing only EC with Al(anode)/Fe(cathode)

electrodes at 25 mA cm-2 current density for 40 min and combined with

ozonation regardless the order in which the treatments were performed (Table

19

1). However, the use of ozone results in removing color and also the lowest final

concentration of phenols.

The order of application of treatments influences the final population of

microorganisms and, also, in aluminum and energetic consumptions. Ozonation

performed first and then the EC (30 min at 25 mA cm -2) reduced the population

of LAB in 2 log10 cycles while the yeast in 1.1 log10 cycles (Fig 6, Table 1);

however, in the reverse order, it is possible to eliminate all microorganisms

(Table 1, Fig 4). In the latter case the aluminum consumed at the anode was

lower (0.89 g/m3) due to the increased effectiveness of the EC when the liquid

to be treated had a higher organic load, although the energy consumption was

greater (88.7 kWh m-3) than when ozone was first bubbled and the EC was

performed after (75.5 kWh m-3).

The first application of the EC (40 min at 25 mA cm-2) eliminated the

dissolved iron. Additionally, organic load, phenol concentration and color of the

solution, decreased. A priori, this partially purified solution could be reused after

the addition of the required quantity of ferrous gluconate in new fixation

treatments, since the concentration of phenols and organic load have

decreased, thereby partly preventing ions ferrous bonded to these compounds

from penetrating into the olives, not properly fixing the color [5].

In the industries, the partially purified solutions must be stored for a

range time (2-24 h) until possible reuse. During storage after EC, because of

the population of microorganisms is present in these solutions, fermentations

can occur producing unusual smells and tastes [3]. To prevent this, the bubbling

of ozone can control the population of microorganisms [15] and even eliminate it

20

completely (Fig 4B), same as in the ozonation of Spanish green olive

fermentation brines [20].

According to the results presented here, the best method for partially

purifying color fixing solutions in ripe olive processing consists in performing an

EC with Al(anode)/Fe(cathode) electrodes set at 25 mA cm-2 current density for

40 min and storing the obtained partially purified solution bubbling ozone until

further reuse.

4. Conclusions

In this study, the application of EC and ozonation for the regeneration of

the iron solutions from the ripe table olive processing industry was assessed

and the following results were obtained:

1) The EC with aluminum in the anode and iron in the cathode removed all

dissolved iron, most of the initial phenols in the solution and about 30%

of the initial pollutant load. The LAB and yeast population slightly

decreased during EC. The chemical depuration efficiency increased

significantly as the current density increased.

2) The bubbling of ozone produces a decrease in total phenols, a

discoloration of the solution but only an initial limited reduction of COD

and without removing any iron ions. The ozone bubbling also produced a

decrease in the microbiological population. The energy cost of ozonation

is much higher than EC.

3) Combined EC and ozonation, regardless of the order in which treatments

are performed, produces a colorless solution, with lower phenol

21

concentration and pollutant load than when only EC or ozonation are

applied. Ozonation reduces the population of LAB and yeast faster than

EC. When it is applied after EC, it is possible to eliminate all

microorganisms.

4) The best working conditions consist in applying 40 min EC with aluminum

in anode and iron in the cathode at 25 mA cm-2 current density, followed

by storage of the partially purified solution after bubling ozone to reduce

the microorganism population and even eliminate it completely. Thus the

formation of possible unusual odors and tastes is eliminated. Total cost is

due to EC (6.5 kWh m-3) plus ozone (approximately 13.7 kWh g-1 O3 m-3);

therefore the aluminum consumed at the anode is 0.89 g m-3.

Acknowledgments

This work was supported by the Spanish Government (project AGL2010-

15494 partially financed from European regional development funds, ERDF)

and Junta de Andalucía (through financial support to group AGR-125).

22

References

[1] IOOC. (2004). Trade standard applying to table olives. Madrid, Spain:

International Olive Oil Council.

[2] IOOC. (2012) On line reference included in World table olives figures:

production, URL http://www.internationaloliveoil.org/estaticos/view/132-

world-table-olive-figures, Accessed 2/2013.

[3] A.H. Sánchez, P. García, L. Rejano, Elaboration of table olives, Grasas

Aceites 57 (2006) 86–94.

[4] E. Medina, P. García, C. Romero, M. Brenes, Recycling preservation

solutions in black ripe olive processing, Int. J. Food Sci. Techn. 46

(2011) 1685–1690.

[5] P. García, M. Brenes, C, Romero, A. Garrido, Colour fixation in ripe olives.

Effects of type of iron salt and other processing factors, J. Sci. Food

Agric. 81(2001) 1364-1370.

[6] N. Daneshvar, A. Oladegaragoze, N. Djafarzadeh, Decolorization of basic

dye solutions by electrocoagulation: an investigation of the effect of

operational parameters, J. Hazard Mater. 129 (2006)116-122.

[7] Y. Yavuz, EC and EF processes for the treatment of alcohol distillery

wastewater, Sep. Purif. Techn, 53 (2007) 135–140.

[8] I.A. Şengil, M. Özacar, Treatment of dairy wastewaters by

electrocoagulation using mild steel electrodes, J. Hazard Mater. 137

(2006) 1197-1205.

23

[9] S. Khoufi, F. Feki, S, Sayadi, Detoxification of olive mill wastewater by

electrocoagulation and sedimentation processes, J. Hazard. Mater. 142

(2007) 58-67.

[10] U. Tezcan, A. Savas, U. Bakir, Electrocoagulation of vegetable oil refinery

wastewater using aluminium electrodes, J. Envir. Manag. 90 (2009) 428-

433.

[11] A. Kyriacou, K.E. Lasaridi, M. Kotsou, C. Balis, G.Pilidis, Combined

bioremediation and advanced oxidation of green table olive processing

wastewater, Process Biochem. 40 (2005) 1401-1408.

[12] P. García-García, A. López-López, J. M. Moreno-Baquero, A. Garrido-

Fernández, Treatment of wastewaters from the green table olive

packaging industry using electro-coagulation, Chem. Eng. J. 170 (2011)

59–66.

[13] A. Deligiorgis, N. P. Xekoukoulotakis, E. Diamodopoulos, D. Mantzavinos,

Electrochemical oxidation of table olive processing wastewater over

boron-doped diamond electrodes: Treatment optimization by factorial

design, Water Res. 42 (2008) 1229-1237.

[14] United States Enviromental Protection Agency Office of Water, Alternative

disinfectants and oxidants guidance manual EPA 815-R-99–014.,

http://www.epa.gov/OGWDW/mdbp/alternative_disinfectants_guidance.p

df

[15] M. A. Khadre, A. E. Yuoseft, J. Kim, Microbiological aspects of ozone

applications in food: a review, J Food Sci 6 (2001) 1242–1252.

24

[16] P. García, M. Brenes, J. de Vicente, A. Garrido, (1990) Depuración de las

aguas residuales de las plantas envasadoras de aceitunas verdes

mediante tratamientos físico-químicos, Grasas y Aceites 41(1990) 263–

269.

[17] N. Zouari, Decolorization of olive oil mill effluent by physical and

chemical treatment prior to anaerobic digestion, J Chem Tech

Biotechnol 73 (1998) 297–303.

[18] K. A. Segovia, F.N. Arroyo, P. García P, M-C. Durán, A. Garrido,

Treatment of green table olive solutions with ozone. Effect in their

polyphenol content and on Lactobacillus pentosus and Saccharomyces

cerevisiae growth, Int J Food Microbiol 114 (2006) 60–68.

[19] K. A. Segovia-Bravo, F. N. Arroyo-López, P. García-García, M. C. Durán-

Quintana, A. Garrido-Fernández, reuse of ozonated alkaline solutions as

fermentation brines in Spanish green table olives, J Food Sci 72 (2007)

126–133.

[20] K. A. Segovia-Bravo, P. Garcia-Garcia, F. N. Arroyo-López, A. López-

López, A. Garrido-Fernández, Ozonation process for the regeneration

and recycling of Spanish green table olive fermentation brines, Eur Food

Res Technol 227 (2008) 463–472.

[21] APHA-AWWA-WPCF, Standards methods for the examination of water and

wastewater American Public Health Association. Washington DC, 1985.

[22] M. Brenes, P. García, A. Garrido, Regeneration of Spanish style green

table olive brine by ultrafiltration, J. Food Sci. 53 (1988) 1733-1736.

25

[23] P. García, C. Romero, M. Brenes, A. Garrido, Validation of a method for

the analysis of iron and manganese in table olives by flame atomic

absorption spectrometry, J. Agric. Food Chem. 50 (2002) 3654–3659.

[24] A. Montaño, A.H. Sánchez, L. Rejano, Método para la determinación del

color en salmueras de aceitunas verdes de mesa, Alimentaria 193 (1988)

137-139.

[25] N.K. Khosla, S. Venkachalan, P. Sonrasundaram, Pulsed electrogeneration

ob bubbles for electroflotation, J. Appl. Electrochem. 21 (1991) 986–990.

[26] T. Picart, G. Cathalifaud-Feillade, M. Mazet. C. Vandensteendam, Cathodic

dissolution in the electrocoagulation process using aluminium electrodes,

J Envirom. Monit. 2 (2000) 77-80.

[27] C.A. Martinez-Huitle, S. Ferro, Electrochemical oxidation of organic

pollutants for the wastewater treatment: direct and indirect processes,

Chem. Soc. Rev.35 (2006) 1324–1340.

[28] Ch Comnillelis, A. Nerini, Anodic oxidation of phenol in the presence of

NaCl for wastewater treatment, J. Appl. Electrochem. 25 (1995) 23–28.

[29] P. García, M. Brenes, T. Vattan, A. Garrido, Kinetic study at different pH

values of the oxidation process to produce ripe olives, J. Sci. Food Agric.

60 (1992) 327–331

[30] T. Grahl, H. Markl, Killing of microorganisms by pulsed electric fields. Appl.

Microbiol. Biotechnol. 45 (1996) 148–57.

[31] H.F. Diao, X.Y. Li, J.D. Guc, H.C. Shi, Z.M. Xie, Electron microscopic

investigation of the bactericidal action of electrochemical disinfection in

26

comparison with chlorination, ozonation and Fenton reaction Process

Biochem. 39 (2004) 1421–1426.

[32] S. Cotillas, J. Llanos, P. Cañizares, S. Mateo, M. A. Rodrigo, Optimization

of an integrated electrodisinfection/electrocoagulation process with Al

bipolar electrodes for urban wastewater reclamation, Water Res. 47

(2013) 1741-750.

[33] K. P. Dreesa, M. Abbaszadeganb, R. M. Maiera, Comparative

electrochemical inactivation of bacteria and bacteriophage, Water Res.

37 (2003) 2291–2300.

[34] J. Beltran, Ozone reaction kinetics for water and wastewaters systems,

Lewis Publisher, Washington D.C., 2004.

27

Fig 1. COD, total phenols and iron removal in the ferrous gluconate solution

from ripe olive processing during EC treatment, using different current densities

with an electrode sets of Al (anode)/Fe (cathode).

28

Fig 2. Changes in pH and color in the ferrous gluconate solution from ripe olive

processing during EC treatment, using different current densities with an

electrode set of Al (anode)/Fe (cathode).

29

Fig 3. Changes in COD removal per unit of aluminium electrode consumption

(anode) (Fig 3A) and specific electrical energy consumption (SEEC) (Fig 3B)

during the EC treatment of ferrous gluconate solution from ripe olive processing,

using different current densities with electrode sets of Al (anode)/Fe (cathode).

30

Fig 4. Lactic acid bacteria (LAB) and yeast population counts during the EC

treatment of ferrous gluconate solutions using 25 mA cm-2 current density and

Al (anode)/Fe (cathode) electrode sets (Fig 4A), and after the subsequent

treatment of solutions with ozone (Fig 4B).

31

Fig 5. Total phenols, color and COD removal as pH changes in the ferrous

gluconate solution from ripe olive processing during ozonation.

32

Fig 6. Lactic acid bacteria (LAB) and yeast population counts during the

treatment with ozone of ferrous gluconate solutions (Fig 6A) and after the

subsequent EC treatment of solutions using 25 mA cm -2 current density and Al

(anode)/Fe (cathode) electrode sets (Fig 6B).

33

Fig 7. COD and iron removal as pH changes in the ferrous gluconate solution

from ripe olive processing during EC treatment after previous treatment with 5 g

ozone per litre. Electrode sets of Al (anode)/Fe (cathode) and current density 25

mA cm-2.

34

Fig 8. Changes in COD removal per unit of aluminium electrode consumption

(anode) (Fig 8A) and Specific electrical energy consumption (SEEC) (Fig 8B)

during the EC treatment of the ferrous gluconate solution from ripe olive

processing after previous treatment with 5 g ozone per liter. Electrode sets of Al

(anode)/Fe (cathode) and current density 25 mA cm-2.

35

36

Table 1. Initial characteristics of ferrous gluconate solutions and after tested treatments to remove pollution. Electric power and aluminum consumptions.

TREATMENTSCOD

(mg O2 L-1)

TOTAL PHENOLS

(mg L-1)

COLOR(A440-A700)

IRON(mg L-1)

MICROORGANISM POPULATION(log10CFU mL-1)

ELEC-TRIC

CONSU-MPTION

(kWh m-3)

CONSU-MED

ALU-MINUM

(g m-3)

LAB Yeast

INITIALa 4700(88) e af 171 (9) a 1.05 (0.08) a 51 (5)a 6.5 (0.3) a 5.5 (0.3) a - -

ECbc (40 min) 3300 (94) c 80 (7) b 0.27 (0.05 )c 0 b 6.3 (0.2) a 5.3 (0.1) ab 6.5 0.89

OZONEb (5gO3 L-1) 4200 (97) b 55 (8) c 0.46 (0.04) b 51 (4)a 5.2 (0.3) b 4.9 (0.1) b 68.5 -

OZONE + ECbc

(5gO3 L-1)+(30 min)d3190 (99) c 30 (12) d ≈0 d 0 b 4.5 (0.2) c 4.4 (0.2) c 68.5 + 7 2.5

ECc + OZONEbd

(40 min)+(6g O3L-1)d3200 (95) c 15 (5) d ≈0 d 0 b 0 d 0 d 6.5+ 82.2 0.89

a Averages of samples collected from 6 different darkening processb Averages of two experimentsc EC at 25 mA cm-2 current densityd Time (min) by EC and bubbled O3 (g L-1) by ozonatione Standard deviation in parenthesisf Column values followed by the same letter do not differ at the 5% level of significance according to Duncan’s multiple-range test

37