treatment of green table olive waste-waters by...
TRANSCRIPT
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: [email protected]
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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.
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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
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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].
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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
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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
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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
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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
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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.
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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)
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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).
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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.
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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.
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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
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(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.
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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
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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
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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).
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References
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International Olive Oil Council.
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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