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Detoxification and discoloration of Moroccan olive mill wastewater by electrocoagulation
Journal of Hazardous Materials 174 (2010) 807–812
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Detoxification and discoloration of Moroccan olive mill wastewater by electrocoagulation F. Hanafi a,b,∗ , O. Assobhei b , M. Mountadar a a Unité de Chimie Analytique et Sciences de l’Environnement du laboratoire de l’Eau et de l’Environnement, département de chimie, Faculté des Sciences, Université Chouaib Doukkali, El Jadida, Morocco b Laboratoire de Biotechnologies marine et de l’environnement, département de biologie, Faculté des Sciences, Université Chouaib Doukkali, El Jadida, Morocco
a r t i c l e
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Article history: Received 29 April 2009 Received in revised form 11 September 2009 Accepted 24 September 2009 Available online 30 September 2009 Keywords: Electrocoagulation Olive mill wastewater Polyphenols Dark color Toxicity
a b s t r a c t The objective of the present study was to assess the electrocoagulation treatment of olive mill wastewater using an aluminum electrode. We have examined the effect of the following parameters on the removal of chemical oxygen demand (COD), polyphenols and dark color removal efficiency: Electrolysis time, Current density, Chloride concentration and Initial pH. The olive mill wastewater (OMW) – diluted 5 times – used in this study had 20.000 mg/L chemical oxygen demand, 3.6 mS/cm conductivity and acidic pH (4.2). It also contains considerable quantities of polyphenols (260 mg/L). The evolution of the physico-chemical parameters during the treatment by electrocoagulation showed that under the following conditions: electrolysis time 15 min, NaCl concentration 2 g/L, initial pH 4.2 and current density 250 A/m2 , the discoloration of the olive mill wastewater, the reduction of the chemical oxygen demand and the reduction of polyphenols exceeded 70%, the electrodes consumption was 0.085 kg Al/kg CODremoved and the specific energy consumed was 2.63 kWh/kg CODremoved . Under these optimal experimental conditions, olive mill wastewater became non-toxic for Bacillus cereus. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Olive oil extraction using the traditional press and three-phase decanter methods results in the production of a highly contaminated olive mill wastewater (OMW). The annual production of olive mill wastewater (OMW) in Morocco exceeds 250.000 m3 [1]. The quantity and the physico-chemical characteristics of OMW, commonly called “vegetation water”, depends on the place, age of growth, harvesting season, yearly changes, olive variety, extraction method, etc. Typically, the composition of OMW is 83–96% water, 3.5–15% organics and 0.5–2% mineral salts. The organic fraction is composed of sugars (1–8%), N-compounds (0.5–2.4%), organic acids (0.5–1.5%), fats (0.02–1%) as well as phenols and pectins (1–1.5%) [2]. The presence of phytotoxic phenolics generally prohibits the use of untreated OMW for irrigation purposes in agricultural production [3]. The complex composition of OMW coupled with the seasonal nature of olive production and the wide geographical dispersion of mills poses considerable technical and economic barriers for efficient effluent treatment and disposal [4].
∗ Corresponding author at: B.P 20, Département de Chimie, Faculté des Sciences, Université Chouaib Doukkali, 24000 El Jadida, Morocco. Tel.: +212 523 373 254/671 974 333; fax: +212 523 342 187. E-mail address: hanafi[email protected] (F. Hanafi). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.09.124
Many processes have been developed to treat this effluent: Simple physical processes such as dilution, evaporation, sedimentation, filtration and centrifugation [5,6]. None of these processes alone is able to reduce the organic load and toxicity of OMW to acceptable limits. Physico-chemical processes such coagulation–flocculation–hydrogen peroxide oxidation has also been employed in OMW treatment [7]. Offer only a partial solution and must be followed by a secondary treatment to comply with legal requirements, and they produce large quantities of sludge that cause other environmental problems. Chemical oxidation using photocatalytic oxidation [8], wet oxidation [9] or advanced oxidation processes (AOP) [10] based on the generation of hydroxyl radicals (i.e., Fenton’s reagent, photocatalysis, a combination of ozone with hydrogen peroxide or UV radiation) destroy completely the organic content of OMW but it involve high operating costs and sophisticated technologies requiring qualified personnel. Biological processes for the treatment of wastewaters have seen worldwide applications. They are considered to be environmentally friendly, reliable and, in most cases, cost-effective. Care needs to be taken in the selection of the microorganisms employed and in their adaptation to treating OMW, as phenolic substances are inhibitory to microorganisms [11,12]. Over the last years, the recovery of polyphenols from OMW has been achieved in the laboratory using membranes [13], for the purpose of application in the pharmaceutical industry. Although the above utilization of olive mill waste is technically feasible, it is too early to achieve large-scale application.
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Electrocoagulation (EC) as an electrochemical method was developed to overcome the drawbacks of conventional water and wastewater treatment technologies. In EC, aluminum or iron hydroxide flocs which destabilize and aggregate the suspended particles or precipitates and absorb dissolved contaminants are produced by anodic dissolution followed by hydrolysis. The electrocoagulation has successfully been used for the treatment of wastewaters including dairy wastewater [14], alcohol distillery wastewater [15], textile wastewaters [16], leachate [17], restaurant wastewater [18], oil refinery wastewater [19], chemical mechanical polishing wastewater [20] and also defluoridation of drinking water [21]. The objective of the present study was to assess the electrocoagulation treatment of OMW using aluminum electrode. The effect of four operational parameters namely electrolysis time, current density, chloride concentration and initial pH have been examined on COD, polyphenols and dark color removal efficiency. 1.1. Electrocoagulation process In the electrocoagulation process, the most widely used electrodes are aluminum and iron. The pH, pollutant type and concentration, the bubble size and position, floc stability and agglomerate size all influence the operation of the electrocoagulation unit. Species can interact in solution in several ways: (1) Migration to an oppositely charged electrode (electrophoresis) and aggregation due to charge neutralization. (2) The cation or hydroxyl ion (OH− ) forms a precipitate with the pollutant. (3) The aluminum cation interacts with OH− to form a hydroxide (Eq. (4)), which has high adsorption properties thus bonding to the pollutant (bridge coagulation). (4) The hydroxides form larger lattice-like structures and sweep through the water (sweep coagulation). (5) Oxidation of pollutants to less toxic species. (6) Removal by electroflotation and adhesion to bubbles [22]. The reactions at electrodes are given, for aluminum, as follows: Anode: Al → Al3+ + 3e− +
2H2 O → 4H + O2 + 4e
(1) −
(2)
Cathode (in alkaline solutions): 2H2 O + 2e− → 2OH− + H2
(3)
Table 1 Characteristics of the OMW, diluted 5 times, used in this study. Parameter
Value
pH Conductivity (mS/cm) Chemical oxygen demand (COD) (mg d’O2 /L) Polyphenols (mg/L) Chlorides (Cl− ) (mg/L) Sodium (Na+ ) (mg/L) Potassium (K+ )(mg/L)
4.2 3.6 20.000 260 1160 639 465
chemical oxygen demand after treatment (g/L) and V: volume of the treated wastewater (L). 2. Experimental 2.1. Characteristics of olive mill wastewater Olive mill wastewater was collected from an olive extraction plant which uses a classic process located in south of Morocco (Marrakech). OMW was stored in a closed plastic container at ambient temperature (inhibitory activity of phenolic and fatty acids [23]). The main characteristics of this OMW diluted five times (20% (v/v)) is presented in Table 1. Before electrocoagulation, OMW sample was decanted and filtered by vacuum filter and glass—microfiber filter (Whatman GF/D (porosity 2.7 m)). 2.2. Electrochemical cell The electrochemical cell has two aluminum plates, one serving as a cathode and the other as anode (Fig. 1). The total effective electrode area was 18 cm2 (4.5 cm × 4 cm) and the spacing between electrodes was 2.8 cm. The electrodes were connected to a digital DC power supply (4 A, 30 V). For each run, 100 cm3 of 20% (v/v) of OMW were placed into the electrolytic cell and a gentle stirring rate of about 200 rpm (revolutions per minute) was applied to allow the chemical precipitate to grow large enough for removal (with a stir bar of ø 6 mm × 15 mm long). NaCl was used as electrolyte. Thereafter, the samples were decanted for 24 h before being subjected to vacuum filtration through Millipore membrane filters with a pore size of 0.45 m. In the sample filtrated: COD, polyphenols, dark color intensity and pH were measured.
The metal ions generated are hydrolyzed in the electrochemical cell to produce metal hydroxide ions according to reaction (4) and the solubility of the metal hydroxide complexes formed depends on pH and ionic strength: Al3 + (aq) + 3H2 O → Al(OH)3 (s) + 3H+ (aq)
(4)
The electrode consumption (Celectrode ) having a unit of kg Al/m3 of wastewater treated is calculated from Faraday’s law in the following relation: Celectrode =
(I × t × M) × 10−3 z×P×v
(5)
where I: current intensity (A); t: retention time (s); v: volume of the treated wastewater (m3 ); F: Faraday’s constant (96487 C/mol); M: mass of aluminum (26.98 g/mol) and z: the number of electron transfer (zal = 3). Specific energy consumptions (SEC) can be expressed as: SEC =
U×I×t (COD0 − CODt ) × V
(6)
where SEC: specific energy consumption (kWh/kg of CODremoved ); U: applied voltage (V); I: current intensity (A); t: retention time (h), COD0 : chemical oxygen demand before treatment (g/L), CODt :
Fig. 1. Electrolytic cell.
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2.3. Analysis The polyphenols and the dark color intensity were determined by measuring the sample’s absorbance at 278 nm and 395 nm, respectively, using a CARY 1E VARIAN spectrophotometer, connected to a PC, in 1 cm path-length cells [24]. The equation used to calculate the color removal efficiency in the treatment experiments was: %R =
C0 − C × 100 C0
(7)
Determination of the initial concentration of total polyphenols was carried out with the official spectrophotometric procedure (720 nm) in which the reagent Folin-Ciocalteu was used as a selective reagent for polyphenols [25]. The results were expressed as gram of gallic acid per liter. The chemical oxygen demand (COD) was determined according to the AOAC analytical methods (AOAC, 1990). In order to determine the loss of mass, the electrodes were dried overnight at 100 ◦ C and weighed (balance ANDGF3000) before and after each experience; Yields were expressed as kg of aluminium per m3 of OMW. A digital calibrated pH-meter (jenco 6173) was used to measure the pH of the OMW samples. Sodium (Na+ ) and Potassium (K+ ) were detected by spectrometry ICP-AES. 2.4. Toxicity assays Toxicity assays were performed using Bacillus cereus, strain (6E/2), as described by Kissi et al. [26]. Bacterial cultures were grown in 1.5% tryptone (TY) up to 0.5 OD600 . Each test was carried out in a final volume of 5 ml, containing 0.5 ml of 10× TY broth, 100 L of bacterial suspension and the OMW sample to be tested. All the experimental data were obtained as the means of three determinations. The accuracies of COD, polyphenols and dark color intensity measurements were better than 4%. 3. Results and discussion All experiments were done on the stored OMW filtered and diluted 5 times. 3.1. Effect of operating time on removal efficiency of polyphenols and dark color In this part of study, we have explored the effect of operating time on removal of polyphenols and dark color. The initial pH of the OMW was 4.2. 120 A/m2 of current density has been applied during 3, 6, 10, 15 and 20 min. The treatment of the OMW has been monitored by spectrophotometric measurements, which provided a straightforward way of following the elimination of the polyphenols (absorbance 278 nm) and dark color (absorbance 395 nm) from OMW. The UV spectra, reported in Fig. 2, demonstrated that the percentage of polyphenols and dark color removal depended immediately on the process duration. Therefore, for the 3 min retention time, 10% of polyphenols has been removed whereas the color intensity increased. This has been assigned to the oxidative polymerization of phenols and tannins originally present in the sample that contributed to the increase of dark colored organic compounds [25]. After 15 min of treatment 72% and 80% of polyphenols and dark color have been removed, respectively, OMW samples became visually very clear. Adhoum and Monser [27] reported that to treat 500 cm3 of both fresh and stored OMW, 25 min were sufficient to remove more than 90% and 95% of polyphenols and dark color, respectively. A further treatment time had only a slight influence on polyphenols and dark color elimination. For this reason, 15 min has been chosen
Fig. 2. UV spectra changes during the electrocoagulation of OMW. Evolution of color and polyphenols is shown in the inset. (Current density 120 A/m2 ).
as the optimum time to treat 100 cm3 of stored OMW in our case. 3.2. Effect of the current density on the removal efficiency of COD, polyphenols and dark color The current not only determines the coagulant dosage rate but also the bubble production rate and fluid regime (mixing) within the reactor. Hence the collision between particles, the floc growth and the potential for material removal, both pollutant and coagulant, by flotation are determined by the current. Therefore, the effect of current density on the COD, polyphenols and dark color removal efficiency should be investigated, as depicted in Fig. 3. When the current density was raised to 250 A/m2 the removal efficiency of COD, polyphenols and dark color rose to 80%, 77% and 88%, respectively. The amount of sludge formed was 6.8–7 kg/m3 and the electrodes (anode + cathode) consumption was 0.6–0.7 kg Al/m3 of OMW. This value is high compared to the theoretical value 0.55 kg Al/m3 of wastewater as shown in Fig. 4. Any further increase in current density (380 A/m2 ) will induce a relatively high increase of electrode consumption (experimental value 1.4 kg Al/m3 and theoretical value 0.9 kg Al/m3 ) but no more COD and polyphenosl removal has occurred regardless of the current used. A similar observation was previously observed by Holt et al. [28] and was explained by the fact that, at higher currents, the supply of aluminum ions is generated rapidly compared to the coagulation process, resulting in a decrease of removal efficiency calculated on an equivalent aluminum basis. In addition, the rapid removal of aluminum hydroxide from solution by flotation leads to a reduction in the probability of collision between the pollutant and coagulant.
Fig. 3. Effect of current density on removal efficiency of COD, polyphenols and dark color. (Electrolysis time 15 min).
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Fig. 6. Effect of initial pH on removal efficiency of COD, polyphenols and dark color. (Current density 250 A/m2 , electrolysis time 15 min).
Fig. 4. Effect of current density on electrode consumption. (Electrolysis time 15 min).
3.3. Influence of NaCl concentration on the removal efficiency of COD, polyphenols and dark color When the concentration of NaCl in the OMW increases, OMW conductivity increases. The voltage drops from 30 to 15 V and the current increases from 0.65 to 1.05–1.12 A. Furthermore, the addition of Cl− to OMW solution was reported to increase the anodic dissolution rate of Al, either by the incorporation of Cl− to the oxide film or by the participation of Cl− in the metal dissolution reaction [29]. Fig. 5 shows the effect of addition of Cl− ions into the solution on the removal efficiency of COD, polyphenols and dark color. The removal efficiency was greatly enhanced by adding 2 g/l NaCl to the solution, obtaining removal efficiency of 84% of COD, 87% of polyphenols and 92% of dark color. Hence, adding NaCl to the wastewater is probably a better choice for increasing the performance of the electrocoagulation technology [30]. In addition to the coagulation process, when anode potential is sufficiently high, secondary reactions may also occur, such as an indirect oxidation if the solution contains Cl− . The Cl− is discharged at the anode to generate Cl2 , which immediately dissolves in the solution, chemically converted to ClO− . The ClO− oxidizes the pollutants effectively. As a result, the removal efficiency of polyphenols and dark color increases [31]: 2Cl− Cl2 + 2e
(8)
Cl2 + H2 O HClO + H+ + Cl− −
HClO ClO + H
+
(9) (10)
However, the removal efficiency decreases when more NaCl is added to the solution. This demonstrates that an excess amount
of Cl− in the solution is detrimental to the coagulation of the pollutants. The only explanation is that the Cl− ions in the solution containing Al(OH)3 forms some transitory compounds, such as Al(OH)2 Cl, Al(OH)Cl2 , and AlCl3 . The transitory compounds finally dissolve in the solution with excess Cl− , as a form of AlCl4 − [32]. Thus, the amount of Al(OH)3 coagulants decreases, resulting in the decrease of the removal efficiency. 3.4. Effect of initial pH on the removal efficiency of COD, polyphenols and dark color The pH has a considerable effect on the efficiency of the electrocoagulation treatment [33]. In this study, the pH was varied in the range of 2–10 by using sodium hydroxide or hydrochloric acid. Removal efficiencies of COD, polyphenols and dark color as a function of initial pH are presented in Fig. 6. The maximum efficiency of COD, polyphenols and dark color removal was observed at pH in the range of 4–6. The possible explanation of this phenomenon was given from the observation of the solubility diagram of aluminum hydroxide [28]. At pH 4–6 the solid precipitate of aluminium hydroxide is formed. The solubility of aluminium hydroxide increases when the solution becomes either more acidic or alkaline. The typical pH of OMW is between 4 and 5.5, which allows it to be directly treated by electrocoagulation without further pH adjustment. Furthermore, the pH of OMW changes during the electrocoagulation process as observed by other investigators [34]. The final pH increase when the initial pH is low (<7) due to the OH− ion accumulated in aqueous solution during the process. In alkaline medium (pH > 8), the final pH does not vary very much and a slight drop is recorded due to the consumption of the OH− ion and the formation of Al (OH)4 − . This result is in accordance with previously published works [18]. 3.5. Energy consumption
Fig. 5. Effect of the amount of NaCl salt on removal efficiency of COD, polyphenols and dark color. (Electrolysis time 15 min).
Electrochemical treatment is undoubtedly an energy-intense process and its efficiency is usually assessed in terms of specific energy consumption (SEC). This is defined as the amount of energy consumed per unit mass of organic removed. Fig. 7 shows that at 25–250 A/m2 , the SEC were ranged between 0.12 and 4.04 kWh/kg CODremoved while COD, polyphenols and dark color removals were ranged between 42–83.5%, 33–84% and 55–90%, respectively (electrolysis time was 15 min). An increase in the current density causes a proportional increase of the ohmic voltage losses in the cell and also increases SEC. Similarly, a sharp increase in the energy consumption with COD removals was observed by UN [30] who had found, at 250–1350 A/m2 and after 7 h electrolysis, the SEC were ranged between 5.35
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and electrode material costs during an electrocoagulation treatment are taken into account as major cost items. The total cost was calculated and expressed in D per kg CODremoved : Operating cos t = a(SEC) + bCelectrode
Fig. 7. Specific energy consumptions dependence of current intensity, NaCl concentration and initial pH (3.6 mS/cm initial conductivity). ((a) pHi (4.2); (b) CD = 250 A/m2 , pHi (4.2); (c) CD = 250 A/m2 , [NaCL] = 2 g/L).
and 27.02 kWh/kg CODremoved while COD removals were ranged between 63.4% and 99.2%, respectively. The effect of NaCl concentration on SEC is shown in Fig. 7. As the conductivity increased, the SEC was considerably reduced. The SEC of 3.41 kWh/kg CODremoved at 0.5 g/L NaCl decreased to 2.63 kWh/kg CODremoved , with increasing NaCl concentration up to 2 g/L. The sufficiently high chloride concentration resulted in a decrease of the anode potential. As seen in Fig. 7, the SEC increased for highly acidic and basic mediums. This happens because a decrease in COD removal with pH < 4 and pH > 8 is accompanied by a proportionately greater increase in energy consumption. Between pH 5 and 8, the SEC is almost constant at 2.54–2.63 kWh/kg CODremoved . 3.6. Toxicity Toxicity tests performed on B. cereus reveal that the electrocoagulation treatment for 15 min decreases the toxicity of OMW by 70%. As shown in Fig. 8, B. cereus is unable to grow in the presence of crude OMW diluted 5 times (20% (v/v)). Whereas, in the presence of treated OMW (20%) the growth of bacteria is nearly similar to the standard medium. The electrocoagulation of OMW removes a large amount of polyphenols and consequently decreases its toxicity [25]. The obtained pre-treated OMW may be treated by biological purification that reduces wastewater-polluting load. 3.7. Operating cost One of the most important parameters that must be determined to evaluate a method of wastewater treatment is the cost. Energy
(11)
where SEC: specific energy consumption (kWh/kg CODremoved ) and Celectrode : electrode consumption (kg Al/kg CODremoved ), which are obtained experimentally. Unit prices, a and b, given for the Moroccan Market, August 2009, are as follows: (a) electrical energy price: 0.0751 D /kWh [35], (b) electrode materiel price: 0.8843 D /kg of aluminum. The electrode and energy consumption increase with increasing of the current density. The operating cost of electrocoagulation of OMW filtered and diluted 5 times, applied in this study was determined to be in the range of 0.03–0.38 D /kg CODremoved at SEC ranged between 0.12 and 4.04 kWh/kg CODremoved and electrode consumption ranged between 0.020 and 0.085 kg Al/kg CODremoved . When the discoloration of the OMW, the reduction of the COD and the reduction of polyphenols exceeded 70%, operating cost was calculated as 0.27 D /kg CODremoved (the SEC 2.63 kWh/kg CODremoved and electrodes consumption was 0.085 kg Al/kg CODremoved ). UN [30] showed that the cost for electrooxidation of OMW (without any pre-treatment and dilution) were ranged between 0.22 and 1.12 D /kg CODremoved and the complete treatment were achieved with the running cost of 0.88 D /kg CODremoved after 7 h electrolysis at the conditions of 1350 A/m2 , 2 M NaCl, 7.9 cm3 /s and 40 ◦ C. Tzagaroulakis et al. [36] have summarized data regarding running costs for various physical, chemical and biological olive mill effluents treatments showing that electrochemical oxidation would cost about 0.6–0.75 D /kg CODremoved . 4. Conclusion This work involved the electrocoagulation of olive mill wastewater filtered and diluted 5 times, using aluminum electrodes. The experimental results showed that: • Electrocoagulation can remove more than 70% of COD, polyphenols and dark color present in olive mill wastewater due to the in situ electrogeneration of aluminum hydroxide, electrochemical oxidation and reaction with soluble aluminum species. • The COD, polyphenols and dark color removal was affected by the operating conditions. Optimum removals were obtained after 15 min of treatment by the addition of 2 g/L NaCl to the wastewater and by applying 250 A/m2 as current density. • The SEC (2.63 kWh/kg CODremoved ) and electrodes consumption (0.085 kg Al/kg CODremoved ) were significantly low. • The operating costs was 0.27 D /kg CODremoved . • The naturally occurring pH of OMW is appropriate to achieve an effective treatment. • The final pH of treated OMW is nearly neutral which allows it to be directly treated by biological methods. Consequently, electrocoagulation can be considered as a suitable alternative to existing methods or applied as pre-treatment step of biological process used for the treatment of OMW. Indeed, the reported results show that electrocoagulation is faster and more effective process as compared to biological methods alone. Nevertheless, further studies should be carried out to confirm the practical feasibility of downstream biological treatment. References
Fig. 8. Effect of untreated and treated OMW on growth of Bacillus cereus.
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Detoxification and discoloration of Moroccan olive mill wastewater by electrocoagulation F. Hanafi a,b,∗ , O. Assobhei b , M. Mountadar a a Unité de Chimie Analytique et Sciences de l’Environnement du laboratoire de l’Eau et de l’Environnement, département de chimie, Faculté des Sciences, Université Chouaib Doukkali, El Jadida, Morocco b Laboratoire de Biotechnologies marine et de l’environnement, département de biologie, Faculté des Sciences, Université Chouaib Doukkali, El Jadida, Morocco
a r t i c l e
i n f o
Article history: Received 29 April 2009 Received in revised form 11 September 2009 Accepted 24 September 2009 Available online 30 September 2009 Keywords: Electrocoagulation Olive mill wastewater Polyphenols Dark color Toxicity
a b s t r a c t The objective of the present study was to assess the electrocoagulation treatment of olive mill wastewater using an aluminum electrode. We have examined the effect of the following parameters on the removal of chemical oxygen demand (COD), polyphenols and dark color removal efficiency: Electrolysis time, Current density, Chloride concentration and Initial pH. The olive mill wastewater (OMW) – diluted 5 times – used in this study had 20.000 mg/L chemical oxygen demand, 3.6 mS/cm conductivity and acidic pH (4.2). It also contains considerable quantities of polyphenols (260 mg/L). The evolution of the physico-chemical parameters during the treatment by electrocoagulation showed that under the following conditions: electrolysis time 15 min, NaCl concentration 2 g/L, initial pH 4.2 and current density 250 A/m2 , the discoloration of the olive mill wastewater, the reduction of the chemical oxygen demand and the reduction of polyphenols exceeded 70%, the electrodes consumption was 0.085 kg Al/kg CODremoved and the specific energy consumed was 2.63 kWh/kg CODremoved . Under these optimal experimental conditions, olive mill wastewater became non-toxic for Bacillus cereus. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Olive oil extraction using the traditional press and three-phase decanter methods results in the production of a highly contaminated olive mill wastewater (OMW). The annual production of olive mill wastewater (OMW) in Morocco exceeds 250.000 m3 [1]. The quantity and the physico-chemical characteristics of OMW, commonly called “vegetation water”, depends on the place, age of growth, harvesting season, yearly changes, olive variety, extraction method, etc. Typically, the composition of OMW is 83–96% water, 3.5–15% organics and 0.5–2% mineral salts. The organic fraction is composed of sugars (1–8%), N-compounds (0.5–2.4%), organic acids (0.5–1.5%), fats (0.02–1%) as well as phenols and pectins (1–1.5%) [2]. The presence of phytotoxic phenolics generally prohibits the use of untreated OMW for irrigation purposes in agricultural production [3]. The complex composition of OMW coupled with the seasonal nature of olive production and the wide geographical dispersion of mills poses considerable technical and economic barriers for efficient effluent treatment and disposal [4].
∗ Corresponding author at: B.P 20, Département de Chimie, Faculté des Sciences, Université Chouaib Doukkali, 24000 El Jadida, Morocco. Tel.: +212 523 373 254/671 974 333; fax: +212 523 342 187. E-mail address: hanafi[email protected] (F. Hanafi). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.09.124
Many processes have been developed to treat this effluent: Simple physical processes such as dilution, evaporation, sedimentation, filtration and centrifugation [5,6]. None of these processes alone is able to reduce the organic load and toxicity of OMW to acceptable limits. Physico-chemical processes such coagulation–flocculation–hydrogen peroxide oxidation has also been employed in OMW treatment [7]. Offer only a partial solution and must be followed by a secondary treatment to comply with legal requirements, and they produce large quantities of sludge that cause other environmental problems. Chemical oxidation using photocatalytic oxidation [8], wet oxidation [9] or advanced oxidation processes (AOP) [10] based on the generation of hydroxyl radicals (i.e., Fenton’s reagent, photocatalysis, a combination of ozone with hydrogen peroxide or UV radiation) destroy completely the organic content of OMW but it involve high operating costs and sophisticated technologies requiring qualified personnel. Biological processes for the treatment of wastewaters have seen worldwide applications. They are considered to be environmentally friendly, reliable and, in most cases, cost-effective. Care needs to be taken in the selection of the microorganisms employed and in their adaptation to treating OMW, as phenolic substances are inhibitory to microorganisms [11,12]. Over the last years, the recovery of polyphenols from OMW has been achieved in the laboratory using membranes [13], for the purpose of application in the pharmaceutical industry. Although the above utilization of olive mill waste is technically feasible, it is too early to achieve large-scale application.
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Electrocoagulation (EC) as an electrochemical method was developed to overcome the drawbacks of conventional water and wastewater treatment technologies. In EC, aluminum or iron hydroxide flocs which destabilize and aggregate the suspended particles or precipitates and absorb dissolved contaminants are produced by anodic dissolution followed by hydrolysis. The electrocoagulation has successfully been used for the treatment of wastewaters including dairy wastewater [14], alcohol distillery wastewater [15], textile wastewaters [16], leachate [17], restaurant wastewater [18], oil refinery wastewater [19], chemical mechanical polishing wastewater [20] and also defluoridation of drinking water [21]. The objective of the present study was to assess the electrocoagulation treatment of OMW using aluminum electrode. The effect of four operational parameters namely electrolysis time, current density, chloride concentration and initial pH have been examined on COD, polyphenols and dark color removal efficiency. 1.1. Electrocoagulation process In the electrocoagulation process, the most widely used electrodes are aluminum and iron. The pH, pollutant type and concentration, the bubble size and position, floc stability and agglomerate size all influence the operation of the electrocoagulation unit. Species can interact in solution in several ways: (1) Migration to an oppositely charged electrode (electrophoresis) and aggregation due to charge neutralization. (2) The cation or hydroxyl ion (OH− ) forms a precipitate with the pollutant. (3) The aluminum cation interacts with OH− to form a hydroxide (Eq. (4)), which has high adsorption properties thus bonding to the pollutant (bridge coagulation). (4) The hydroxides form larger lattice-like structures and sweep through the water (sweep coagulation). (5) Oxidation of pollutants to less toxic species. (6) Removal by electroflotation and adhesion to bubbles [22]. The reactions at electrodes are given, for aluminum, as follows: Anode: Al → Al3+ + 3e− +
2H2 O → 4H + O2 + 4e
(1) −
(2)
Cathode (in alkaline solutions): 2H2 O + 2e− → 2OH− + H2
(3)
Table 1 Characteristics of the OMW, diluted 5 times, used in this study. Parameter
Value
pH Conductivity (mS/cm) Chemical oxygen demand (COD) (mg d’O2 /L) Polyphenols (mg/L) Chlorides (Cl− ) (mg/L) Sodium (Na+ ) (mg/L) Potassium (K+ )(mg/L)
4.2 3.6 20.000 260 1160 639 465
chemical oxygen demand after treatment (g/L) and V: volume of the treated wastewater (L). 2. Experimental 2.1. Characteristics of olive mill wastewater Olive mill wastewater was collected from an olive extraction plant which uses a classic process located in south of Morocco (Marrakech). OMW was stored in a closed plastic container at ambient temperature (inhibitory activity of phenolic and fatty acids [23]). The main characteristics of this OMW diluted five times (20% (v/v)) is presented in Table 1. Before electrocoagulation, OMW sample was decanted and filtered by vacuum filter and glass—microfiber filter (Whatman GF/D (porosity 2.7 m)). 2.2. Electrochemical cell The electrochemical cell has two aluminum plates, one serving as a cathode and the other as anode (Fig. 1). The total effective electrode area was 18 cm2 (4.5 cm × 4 cm) and the spacing between electrodes was 2.8 cm. The electrodes were connected to a digital DC power supply (4 A, 30 V). For each run, 100 cm3 of 20% (v/v) of OMW were placed into the electrolytic cell and a gentle stirring rate of about 200 rpm (revolutions per minute) was applied to allow the chemical precipitate to grow large enough for removal (with a stir bar of ø 6 mm × 15 mm long). NaCl was used as electrolyte. Thereafter, the samples were decanted for 24 h before being subjected to vacuum filtration through Millipore membrane filters with a pore size of 0.45 m. In the sample filtrated: COD, polyphenols, dark color intensity and pH were measured.
The metal ions generated are hydrolyzed in the electrochemical cell to produce metal hydroxide ions according to reaction (4) and the solubility of the metal hydroxide complexes formed depends on pH and ionic strength: Al3 + (aq) + 3H2 O → Al(OH)3 (s) + 3H+ (aq)
(4)
The electrode consumption (Celectrode ) having a unit of kg Al/m3 of wastewater treated is calculated from Faraday’s law in the following relation: Celectrode =
(I × t × M) × 10−3 z×P×v
(5)
where I: current intensity (A); t: retention time (s); v: volume of the treated wastewater (m3 ); F: Faraday’s constant (96487 C/mol); M: mass of aluminum (26.98 g/mol) and z: the number of electron transfer (zal = 3). Specific energy consumptions (SEC) can be expressed as: SEC =
U×I×t (COD0 − CODt ) × V
(6)
where SEC: specific energy consumption (kWh/kg of CODremoved ); U: applied voltage (V); I: current intensity (A); t: retention time (h), COD0 : chemical oxygen demand before treatment (g/L), CODt :
Fig. 1. Electrolytic cell.
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2.3. Analysis The polyphenols and the dark color intensity were determined by measuring the sample’s absorbance at 278 nm and 395 nm, respectively, using a CARY 1E VARIAN spectrophotometer, connected to a PC, in 1 cm path-length cells [24]. The equation used to calculate the color removal efficiency in the treatment experiments was: %R =
C0 − C × 100 C0
(7)
Determination of the initial concentration of total polyphenols was carried out with the official spectrophotometric procedure (720 nm) in which the reagent Folin-Ciocalteu was used as a selective reagent for polyphenols [25]. The results were expressed as gram of gallic acid per liter. The chemical oxygen demand (COD) was determined according to the AOAC analytical methods (AOAC, 1990). In order to determine the loss of mass, the electrodes were dried overnight at 100 ◦ C and weighed (balance ANDGF3000) before and after each experience; Yields were expressed as kg of aluminium per m3 of OMW. A digital calibrated pH-meter (jenco 6173) was used to measure the pH of the OMW samples. Sodium (Na+ ) and Potassium (K+ ) were detected by spectrometry ICP-AES. 2.4. Toxicity assays Toxicity assays were performed using Bacillus cereus, strain (6E/2), as described by Kissi et al. [26]. Bacterial cultures were grown in 1.5% tryptone (TY) up to 0.5 OD600 . Each test was carried out in a final volume of 5 ml, containing 0.5 ml of 10× TY broth, 100 L of bacterial suspension and the OMW sample to be tested. All the experimental data were obtained as the means of three determinations. The accuracies of COD, polyphenols and dark color intensity measurements were better than 4%. 3. Results and discussion All experiments were done on the stored OMW filtered and diluted 5 times. 3.1. Effect of operating time on removal efficiency of polyphenols and dark color In this part of study, we have explored the effect of operating time on removal of polyphenols and dark color. The initial pH of the OMW was 4.2. 120 A/m2 of current density has been applied during 3, 6, 10, 15 and 20 min. The treatment of the OMW has been monitored by spectrophotometric measurements, which provided a straightforward way of following the elimination of the polyphenols (absorbance 278 nm) and dark color (absorbance 395 nm) from OMW. The UV spectra, reported in Fig. 2, demonstrated that the percentage of polyphenols and dark color removal depended immediately on the process duration. Therefore, for the 3 min retention time, 10% of polyphenols has been removed whereas the color intensity increased. This has been assigned to the oxidative polymerization of phenols and tannins originally present in the sample that contributed to the increase of dark colored organic compounds [25]. After 15 min of treatment 72% and 80% of polyphenols and dark color have been removed, respectively, OMW samples became visually very clear. Adhoum and Monser [27] reported that to treat 500 cm3 of both fresh and stored OMW, 25 min were sufficient to remove more than 90% and 95% of polyphenols and dark color, respectively. A further treatment time had only a slight influence on polyphenols and dark color elimination. For this reason, 15 min has been chosen
Fig. 2. UV spectra changes during the electrocoagulation of OMW. Evolution of color and polyphenols is shown in the inset. (Current density 120 A/m2 ).
as the optimum time to treat 100 cm3 of stored OMW in our case. 3.2. Effect of the current density on the removal efficiency of COD, polyphenols and dark color The current not only determines the coagulant dosage rate but also the bubble production rate and fluid regime (mixing) within the reactor. Hence the collision between particles, the floc growth and the potential for material removal, both pollutant and coagulant, by flotation are determined by the current. Therefore, the effect of current density on the COD, polyphenols and dark color removal efficiency should be investigated, as depicted in Fig. 3. When the current density was raised to 250 A/m2 the removal efficiency of COD, polyphenols and dark color rose to 80%, 77% and 88%, respectively. The amount of sludge formed was 6.8–7 kg/m3 and the electrodes (anode + cathode) consumption was 0.6–0.7 kg Al/m3 of OMW. This value is high compared to the theoretical value 0.55 kg Al/m3 of wastewater as shown in Fig. 4. Any further increase in current density (380 A/m2 ) will induce a relatively high increase of electrode consumption (experimental value 1.4 kg Al/m3 and theoretical value 0.9 kg Al/m3 ) but no more COD and polyphenosl removal has occurred regardless of the current used. A similar observation was previously observed by Holt et al. [28] and was explained by the fact that, at higher currents, the supply of aluminum ions is generated rapidly compared to the coagulation process, resulting in a decrease of removal efficiency calculated on an equivalent aluminum basis. In addition, the rapid removal of aluminum hydroxide from solution by flotation leads to a reduction in the probability of collision between the pollutant and coagulant.
Fig. 3. Effect of current density on removal efficiency of COD, polyphenols and dark color. (Electrolysis time 15 min).
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Fig. 6. Effect of initial pH on removal efficiency of COD, polyphenols and dark color. (Current density 250 A/m2 , electrolysis time 15 min).
Fig. 4. Effect of current density on electrode consumption. (Electrolysis time 15 min).
3.3. Influence of NaCl concentration on the removal efficiency of COD, polyphenols and dark color When the concentration of NaCl in the OMW increases, OMW conductivity increases. The voltage drops from 30 to 15 V and the current increases from 0.65 to 1.05–1.12 A. Furthermore, the addition of Cl− to OMW solution was reported to increase the anodic dissolution rate of Al, either by the incorporation of Cl− to the oxide film or by the participation of Cl− in the metal dissolution reaction [29]. Fig. 5 shows the effect of addition of Cl− ions into the solution on the removal efficiency of COD, polyphenols and dark color. The removal efficiency was greatly enhanced by adding 2 g/l NaCl to the solution, obtaining removal efficiency of 84% of COD, 87% of polyphenols and 92% of dark color. Hence, adding NaCl to the wastewater is probably a better choice for increasing the performance of the electrocoagulation technology [30]. In addition to the coagulation process, when anode potential is sufficiently high, secondary reactions may also occur, such as an indirect oxidation if the solution contains Cl− . The Cl− is discharged at the anode to generate Cl2 , which immediately dissolves in the solution, chemically converted to ClO− . The ClO− oxidizes the pollutants effectively. As a result, the removal efficiency of polyphenols and dark color increases [31]: 2Cl− Cl2 + 2e
(8)
Cl2 + H2 O HClO + H+ + Cl− −
HClO ClO + H
+
(9) (10)
However, the removal efficiency decreases when more NaCl is added to the solution. This demonstrates that an excess amount
of Cl− in the solution is detrimental to the coagulation of the pollutants. The only explanation is that the Cl− ions in the solution containing Al(OH)3 forms some transitory compounds, such as Al(OH)2 Cl, Al(OH)Cl2 , and AlCl3 . The transitory compounds finally dissolve in the solution with excess Cl− , as a form of AlCl4 − [32]. Thus, the amount of Al(OH)3 coagulants decreases, resulting in the decrease of the removal efficiency. 3.4. Effect of initial pH on the removal efficiency of COD, polyphenols and dark color The pH has a considerable effect on the efficiency of the electrocoagulation treatment [33]. In this study, the pH was varied in the range of 2–10 by using sodium hydroxide or hydrochloric acid. Removal efficiencies of COD, polyphenols and dark color as a function of initial pH are presented in Fig. 6. The maximum efficiency of COD, polyphenols and dark color removal was observed at pH in the range of 4–6. The possible explanation of this phenomenon was given from the observation of the solubility diagram of aluminum hydroxide [28]. At pH 4–6 the solid precipitate of aluminium hydroxide is formed. The solubility of aluminium hydroxide increases when the solution becomes either more acidic or alkaline. The typical pH of OMW is between 4 and 5.5, which allows it to be directly treated by electrocoagulation without further pH adjustment. Furthermore, the pH of OMW changes during the electrocoagulation process as observed by other investigators [34]. The final pH increase when the initial pH is low (<7) due to the OH− ion accumulated in aqueous solution during the process. In alkaline medium (pH > 8), the final pH does not vary very much and a slight drop is recorded due to the consumption of the OH− ion and the formation of Al (OH)4 − . This result is in accordance with previously published works [18]. 3.5. Energy consumption
Fig. 5. Effect of the amount of NaCl salt on removal efficiency of COD, polyphenols and dark color. (Electrolysis time 15 min).
Electrochemical treatment is undoubtedly an energy-intense process and its efficiency is usually assessed in terms of specific energy consumption (SEC). This is defined as the amount of energy consumed per unit mass of organic removed. Fig. 7 shows that at 25–250 A/m2 , the SEC were ranged between 0.12 and 4.04 kWh/kg CODremoved while COD, polyphenols and dark color removals were ranged between 42–83.5%, 33–84% and 55–90%, respectively (electrolysis time was 15 min). An increase in the current density causes a proportional increase of the ohmic voltage losses in the cell and also increases SEC. Similarly, a sharp increase in the energy consumption with COD removals was observed by UN [30] who had found, at 250–1350 A/m2 and after 7 h electrolysis, the SEC were ranged between 5.35
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811
and electrode material costs during an electrocoagulation treatment are taken into account as major cost items. The total cost was calculated and expressed in D per kg CODremoved : Operating cos t = a(SEC) + bCelectrode
Fig. 7. Specific energy consumptions dependence of current intensity, NaCl concentration and initial pH (3.6 mS/cm initial conductivity). ((a) pHi (4.2); (b) CD = 250 A/m2 , pHi (4.2); (c) CD = 250 A/m2 , [NaCL] = 2 g/L).
and 27.02 kWh/kg CODremoved while COD removals were ranged between 63.4% and 99.2%, respectively. The effect of NaCl concentration on SEC is shown in Fig. 7. As the conductivity increased, the SEC was considerably reduced. The SEC of 3.41 kWh/kg CODremoved at 0.5 g/L NaCl decreased to 2.63 kWh/kg CODremoved , with increasing NaCl concentration up to 2 g/L. The sufficiently high chloride concentration resulted in a decrease of the anode potential. As seen in Fig. 7, the SEC increased for highly acidic and basic mediums. This happens because a decrease in COD removal with pH < 4 and pH > 8 is accompanied by a proportionately greater increase in energy consumption. Between pH 5 and 8, the SEC is almost constant at 2.54–2.63 kWh/kg CODremoved . 3.6. Toxicity Toxicity tests performed on B. cereus reveal that the electrocoagulation treatment for 15 min decreases the toxicity of OMW by 70%. As shown in Fig. 8, B. cereus is unable to grow in the presence of crude OMW diluted 5 times (20% (v/v)). Whereas, in the presence of treated OMW (20%) the growth of bacteria is nearly similar to the standard medium. The electrocoagulation of OMW removes a large amount of polyphenols and consequently decreases its toxicity [25]. The obtained pre-treated OMW may be treated by biological purification that reduces wastewater-polluting load. 3.7. Operating cost One of the most important parameters that must be determined to evaluate a method of wastewater treatment is the cost. Energy
(11)
where SEC: specific energy consumption (kWh/kg CODremoved ) and Celectrode : electrode consumption (kg Al/kg CODremoved ), which are obtained experimentally. Unit prices, a and b, given for the Moroccan Market, August 2009, are as follows: (a) electrical energy price: 0.0751 D /kWh [35], (b) electrode materiel price: 0.8843 D /kg of aluminum. The electrode and energy consumption increase with increasing of the current density. The operating cost of electrocoagulation of OMW filtered and diluted 5 times, applied in this study was determined to be in the range of 0.03–0.38 D /kg CODremoved at SEC ranged between 0.12 and 4.04 kWh/kg CODremoved and electrode consumption ranged between 0.020 and 0.085 kg Al/kg CODremoved . When the discoloration of the OMW, the reduction of the COD and the reduction of polyphenols exceeded 70%, operating cost was calculated as 0.27 D /kg CODremoved (the SEC 2.63 kWh/kg CODremoved and electrodes consumption was 0.085 kg Al/kg CODremoved ). UN [30] showed that the cost for electrooxidation of OMW (without any pre-treatment and dilution) were ranged between 0.22 and 1.12 D /kg CODremoved and the complete treatment were achieved with the running cost of 0.88 D /kg CODremoved after 7 h electrolysis at the conditions of 1350 A/m2 , 2 M NaCl, 7.9 cm3 /s and 40 ◦ C. Tzagaroulakis et al. [36] have summarized data regarding running costs for various physical, chemical and biological olive mill effluents treatments showing that electrochemical oxidation would cost about 0.6–0.75 D /kg CODremoved . 4. Conclusion This work involved the electrocoagulation of olive mill wastewater filtered and diluted 5 times, using aluminum electrodes. The experimental results showed that: • Electrocoagulation can remove more than 70% of COD, polyphenols and dark color present in olive mill wastewater due to the in situ electrogeneration of aluminum hydroxide, electrochemical oxidation and reaction with soluble aluminum species. • The COD, polyphenols and dark color removal was affected by the operating conditions. Optimum removals were obtained after 15 min of treatment by the addition of 2 g/L NaCl to the wastewater and by applying 250 A/m2 as current density. • The SEC (2.63 kWh/kg CODremoved ) and electrodes consumption (0.085 kg Al/kg CODremoved ) were significantly low. • The operating costs was 0.27 D /kg CODremoved . • The naturally occurring pH of OMW is appropriate to achieve an effective treatment. • The final pH of treated OMW is nearly neutral which allows it to be directly treated by biological methods. Consequently, electrocoagulation can be considered as a suitable alternative to existing methods or applied as pre-treatment step of biological process used for the treatment of OMW. Indeed, the reported results show that electrocoagulation is faster and more effective process as compared to biological methods alone. Nevertheless, further studies should be carried out to confirm the practical feasibility of downstream biological treatment. References
Fig. 8. Effect of untreated and treated OMW on growth of Bacillus cereus.
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