Treatment of table olive processing wastewaters using electrocoagulation in laboratory and pilot-scale reactors

Process Safety and Environmental Protection 131 (2019) 38–47

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Treatment of table olive processing wastewaters using electrocoagulation in laboratory and pilot-scale reactors Andreas K. Benekos a , Charikleia Zampeta a , Rafailia Argyriou a , Christina N. Economou a , Irene-Eva Triantaphyllidou a , Triantafyllos I. Tatoulis b , Athanasia G. Tekerlekopoulou b , Dimitris V. Vayenas a,c,∗ a

Department of Chemical Engineering, University of Patras, Rio, GR-26504, Patras, Greece Department of Environmental Engineering, University of Patras, 2 G. Seferi Str., GR-30100, Agrinio, Greece c Institute of Chemical Engineering Sciences (ICE-HT), Stadiou Str., Platani, GR-26504, Patras, Greece b

a r t i c l e

i n f o

Article history: Received 14 June 2019 Received in revised form 27 August 2019 Accepted 29 August 2019 Available online 4 September 2019 Keywords: Table olive processing wastewater Electrocoagulation Color removal Operating cost Pilot-scale reactor

a b s t r a c t Electrocoagulation-(EC) is investigated as an alternative, cost-efficient, method for the treatment or post-treatment of table olive processing wastewaters (TOPWs). Experiments were performed in both laboratory and pilot-scale reactors using aluminum and iron electrodes. Different initial chemical oxygen demand (COD) concentrations (3000, 5000 and 9000 mg L−1 ) and current densities (41.7, 83.3 and 166.7 mA cm-2 ) were tested in laboratory-scale experiments to determine maximum COD and color removal from untreated TOPWs. Pilot-scale experiments were also conducted using biologically pretreated TOPW (COD 1000 mg L-1 and current densities of 3.87 and 5.65 mA cm-2 ) to ensure an efficient post-treatment process. Aluminum electrodes were found to be more efficient in reducing COD and color than iron electrodes in both laboratory and pilot-scale experiments. In laboratory-scale experiments the maximum COD and color removal (approximately 50% and 100%, respectively) was recorded for the lowest initial COD concentration of 3000 mg L−1 at 166.7 mA cm-2 . In the pilot-scale reactor the maximum COD and color removal observed was 42.5% and 85.3%, respectively, for the current density of 5.65 mA cm-2 . Lower energy and electrode consumption was recorded when working with aluminum electrodes and optimum results were obtained with the lowest initial COD and current density values tested. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The table olive industry produces considerable amounts of table olive processing wastewaters (TOPWs), approximately 3.9–7.5 and 0.9-1.9 m3 TOPW per ton of green and black olives, respectively (Ayed et al., 2017; Deligiorgis et al., 2008). TOPW contains high levels of inorganic and organic substances, is usually dark brown in color and has an odor similar to that of olive oil (RincónLlorente et al., 2018). The physicochemical composition of TOPWs differs depending on the processing method and variety of olives. For example, black olive wastewaters contain chemical oxygen demand (COD), total phenolic compounds, pH and conductivity values of about 32 g L−1 , 4 g L−1 , pH 4, and 111 mS cm−1 , respectively (Papadaki and Mantzouridou, 2016). Current European legislation sets the maximum permitted COD value as 125 mg L−1 for urban

∗ Corresponding author at: Department of Chemical Engineering, University of Patras, Rio, GR-26504, Patras, Greece. E-mail address: [email protected] (D.V. Vayenas).

wastewaters with 75% organic pollutant removal (Articles 4 and 5 of the Urban Wastewater Directive, 91/271/EEC) (EEC, 1991). Conventional aerobic/anaerobic biological processes (used in single treatment steps) cannot treat these wastewaters efficiently (Aggelis et al., 2002; Zarkadas and Pilidis, 2011). This is especially true for raw TOPWs as the presence of phenolic compounds and high conductivity values inhibit the growth of microorganisms (Ayed et al., 2017; Villegas et al., 2016). In recent years, advanced oxidation processes (AOPs) (Chatzisymeon et al., 2008) and combined (electro-)chemical and biological treatments, have been the most widely accepted methods proposed for TOPW treatment (Ayed et al., 2017; Kotsou et al., 2004; Tatoulis et al., 2016). However, although AOPs are effective at organic matter reduction, their operating cost is high, thus making their use unfavorable especially in small olive processing industries (Ayed et al., 2017). Electrocoagulation (EC) is another alternative technology that could possibly solve the depollution problem (Fajardo et al., 2015; Papadopoulos et al., 2019). EC operates by passing wastewater across metal electrodes, usually aluminum or iron. When con-

https://doi.org/10.1016/j.psep.2019.08.036 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

A.K. Benekos, C. Zampeta, R. Argyriou et al. / Process Safety and Environmental Protection 131 (2019) 38–47

nected to an external power source, metal cation generation takes place at the anode while hydroxyl anions and hydrogen gas are released from the cathode (Garcia-Segura et al., 2017; Khandegar and Saroha, 2013). These anions and cations interact and combine to form metal oxides and hydroxides that function as coagulants in the EC process. Anodic reactions may also occur during EC, leading to the formation of strong oxidants such as native and free chorine and hypochlorite. Although EC is a common technology in wastewater treatment, relatively little research has been conducted to better understand the fundamental mechanisms of the processes involved (Bocos et al., 2016). The use of EC as a treatment technology has several advantages over standard biological systems and AOPs including: simple apparatus operation, short processing times, no additional chemical requirements, production of colorless and odorless effluents (Fajardo et al., 2015; Garcia-Segura et al., 2017). A literature review shows that EC has been studied extensively in laboratoryscale reactors for the treatment of potable water (Moussa et al., 2017), municipal wastewater (Moreno et al., 2013), and industrial and agro-industrial wastewaters (Khandegar and Saroha, 2013; Papadopoulos et al., 2019; Sahu et al., 2014), while studies in pilot scale reactors are relatively limited and mainly concern potable water and municipal wastewater treatment (McBeath et al., 2018; ´ Smoczynski et al., 2017). The agro-industrial wastewaters to which EC has been applied in laboratory-scale reactors include mainly olive mill wastewaters, dairy wastewaters and wastewaters originating from food and beverage processing (Bazrafshan et al., 2013; Drogui et al., 2008; Sahu et al., 2014; Sharma, 2014). It is worth noting that only one study (García-García et al., 2011) has investigated the treatment of standardized TOPW (20% diluted Spanish-style green olive fermentation brine) using EC in laboratory-scale reactors, and that no research has previously been carried out on this waste in pilot-scale units. García-García et al., (García-García et al., 2011) used TOPW with initial COD and phenol concentrations of about 7000 mg L−1 and 400 mg L-1 , respectively, while different combinations of electrode materials (aluminum and iron) and current densities (25, 50, 75 and 100 mA cm-2 ) were tested to determine the optimum EC conditions for pollutants and color removal (García-García et al., 2011). However, only partial COD and color removal was achieved (maximum removal of about 50% for both parameters). For this reason, hybrid systems are now being investigated. These combine EC with other processes such as anaerobic digestion (Sounni et al., 2018), or EC followed by electro-Fenton (EF) or photoelectron-Fenton (PEF) (Bocos et al., 2016; Flores et al., 2018). In the present work, EC was investigated in laboratory and pilot-scale reactors for complete decolorization and COD reduction of real (i.e., not standardized) TOPW. Initially, the effect of different initial COD concentrations (3000, 5000, and 9000 mg L−1 ), current densities (41.7, 83.3 and 166.7 mA cm-2 ) and electrode material (iron and aluminum) on EC performance was evaluated in laboratory-scale experiments. Pilot-scale experiments were then performed using biologically pre-treated TOPW to study the effectiveness of the EC process as a post-treatment step and examine if a hybrid system (EC followed by biological aerobic treatment) could be successfully scaled-up. Finally, the operating cost of the EC process in relation to color reduction was estimated as a func-

39

tion of electrical energy and electrode consumption per cubic meter of wastewater. 2. Materials and methods 2.1. Main properties of table olive effluents used in laboratory-scale experiments The table olive processing wastewater (TOPW) used in the laboratory-scale experiments was obtained from an olive packaging factory located in Aitoloakarnania (western Greece). This particular plant mostly packs black olives of the Kalamon variety. The characteristic values of the factory’s TOPW vary, as wastewaters are released just before a fresh olive batch is packed. Therefore, organic load is proportional to the duration that the fruit are stored in the factory’s facilities. Experiments were conducted using (a) undiluted TOPW and (b) the same TOPW diluted with tap water, to examine the efficiency of the EC process at different initial COD concentrations (3000, 5000 and 9000 mgL−1 ). 2.2. Main properties of table olive effluents used in pilot-scale experiments The pilot-scale experiments were carried out in a table olive packaging factory located in Messinia (southern Greece). The TOPW, derived from olives of the Kalamon variety, was treated with a biological trickling filter before being discharged into the municipal wastewater treatment plant of Kalamata city. Initial concentrations of COD and phenolic compounds following biological filter treatment were approximately 1000 mg L−1 and 400 mg L−1 , respectively, and the TOPW was dark brown in color. This study investigates the post-treatment of these effluents with EC. 2.3. Experimental design of laboratory-scale experiments Experiments were conducted under batch operation mode. Reactor volume was 600 cm3 and a magnetic stirrer was used to maintain homogenization in the wastewater’s bulk volume. The temperature inside the electrolytic cell was 27–30 ◦ C and kept constant using a cooling jacket. The plate electrodes used (placed in the middle of the reactor) were either both aluminum (Al) or both iron (Fe). The scheme of experimental design is provided by Papadopoulos et al. (Papadopoulos et al., 2019). If they were to be reused, the electrodes were first cleaned with sandpaper to remove any solid particles from their surfaces and then rinsed with (1%) HCl and distilled water to remove any remaining pollutants (Papadopoulos et al., 2019). The active surface of the electrodes was kept constant at 12 cm2 . Inter-electrode distance was also kept constant at 0.3 cm. According to the literature, the minimum distance between the anode and cathode limits the Ohmic drop between the two (Hakizimana et al., 2017; Papadopoulos et al., 2019). Consequently, by keeping that distance as small as possible, electrical energy consumption is minimal. Both electrodes were connected to a power supply unit (model QJ3005C) and a direct current was applied. Unless otherwise indicated, the total treatment time for each experimental run was 90 min. The electric current densities tested were 41.7, 83.3 and 166.7 mA cm−2 . All experiments were

Table 1 Basic physicochemical parameter values recorded in the TOPW prior to and after EC treatment. Parameter

3000 mg COD0 L−1

5000 mg COD0 L−1

9000 mg COD0 L−1

Initial pH before EC Final pH after EC Initial conductivity before EC (mS cm−1 ) Final conductivity after EC (mS cm−1 )

5.5 – 6.0 8.2 – 8.8 10.0 – 10.5 9.5 – 10.0

4.5 – 5.0 6.5 – 7.0 20.0 – 20.5 19.5 – 20.0

4.2 – 4.8 6.2 – 6.8 28.0 – 29.0 27.0 – 28.0

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Fig. 1. Pilot-scale EC reactor for the post-treatment of TOPW.

duplicated (relative standard deviation not exceeding 5%) and samples (6–8 mL) were taken from the bulk volume at regular intervals. Data are presented as mean values calculated from duplicate experiments. Three different initial COD concentrations were studied (3000, 5000 and 9000 mg L-1 ). The basic characteristics of the three effluents tested are presented in Table 1.

2.4. Experimental design of pilot-scale experiments Experiments were performed using a portable EC pilot-unit (Fig. 1) to investigate the percentage removal of COD, phenolics and color from the biologically pre-treated TOPW. The portable unit was an electrolytic cell with 32 (sixteen anode and sixteen cathode) electrode plates (15 cm long and 40 cm wide) with interelectrode gaps of 2 cm. The electrode material used was either Al or Fe. The pilot unit was operated for 50 min at current density 3.87 and 5.65 mA/cm2 using a 2.5 V power supply. The working volume of the reactor was 0.2 m3 and the duplicated experiments (relative standard deviation not exceeding 5%) were conducted at 25 ± 2 ◦ C.

HACH (model DR 5000). Decolorization efficiency in each experimental run was determined using the equation (Can et al., 2003): R% =

C0 − C × 100 C

(1)

where, R(%): removal rate; C0 : initial color absorbance, and C: final color absorbance. Concentrations of aluminum and iron were determined using the Optima 8000ICP-OES (Perkin Elmer) instrument that specializes in the determination of metals and trace elements. Argon was used with the ICP-OES system and nitrogen was used as the optical purge gas. The wavelengths used for Al and Fe were 308.215 and 259.933 nm, respectively, while the plasma view for both metals was radial. The experimental conditions applied were: Plasma gas flow: 15 L/min, Auxiliary gas flow: 0.2 L/min, Nebulizer gas flow: 0.55 L/min, RF power: 1300 W, Purge flow: Normal, Peristaltic pump flow rate: 1.5 mL/min, calibration: Linear Calculated Intercept. 3. Results and discussion 3.1. Effect of electrode material and applied current density: laboratory-scale study

2.5. Analytical methods Conductivity and pH measurements were carried out before and after each experimental run using a HANNA HI 5521 multiparameter instrument. Furthermore, anode and cathode electrode masses were also recorded prior to and following the treatment procedure, to evaluate electrode consumption and its impact on treatment cost. Electrode mass loss during the treatment procedure was evaluated using Faraday’s law (Fajardo et al., 2015). Determination of total COD followed the closed reflux dichromate procedure, as described in Standard Methods (APHA/AWWA/WEF, 2012), using a Wastewater Treatment Photometer (HANNA HI 83,214). Total phenolic compounds were determined as syringic acid equivalents using Folin-Ciocalteau’s Phenol reagent according to Singleton et al. (Singleton et al., 1999). The color of the wastewater was evaluated by measuring the absorbance of each sample at the 200–800 nm wavelength range. Since 400 nm is the optimum wavelength for decolorization detection, measurements on that wavelength were isolated. The spectrophotometer used was provided by LANGE

3.1.1. COD removal Experiments on EC treatment of TOPW were performed with different current densities: 41.7, 83.3 and 166.7 mA cm−2 (corresponding to the applied intensities of 0.5, 1.0 and 2.0 A for the specific electrodes used in this study). Process efficiency was investigated using the two electrode materials, aluminum and iron. Fig. 2a shows the recorded effects of each material and applied current intensity for one specific initial COD concentration (COD0 : 3000 mg L-1 ). As shown in Fig. 1, increased electric currents lead to a higher percentage removal of the wastewater’s organic load (expressed as COD) in a shorter time. For example, using the current density of 166.7 mAcm−2 , the maximum COD removal was recorded after 40 min for both electrode materials, while the required treatment time increased to 60 min when currents of 41.7 and 83.3 mAcm−2 were applied. This is consistent with the theoretical background of the EC process according to which higher electric currents lead to increased dissolution of metal cations in the bulk solution and

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Fig. 2. Effect of three different current densities and two electrode materials on COD reduction versus time, for TOPW with initial COD concentrations of: (a) 3000 mg L−1 , (b) 5000 mg L−1 , and (c) 9000 mg L−1 .

therefore higher numbers of coagulant species (metallic hydroxide flocs). As the number of flocs rises, the potential to adsorb organic pollutants in the wastewater also rises (Garcia-Segura et al., 2017). Concerning the effect of the metal electrode material on COD reduction, aluminum is superior to iron as shown in Fig. 2. The increased sedimentation rate of the aluminum species formed during EC of the bulk volume, compared to that of the iron species, is responsible for the enhanced performance of the aluminum electrodes. More specifically, the generated aluminum coagulant species are rather insoluble (with respect to pH) and therefore capable of co-precipitating with organic materials immediately after their formation. On the other hand, iron anodic oxidation follows a more complex mechanism. Since the highly soluble Fe2+ is formed mostly by the aforementioned oxidation process, a further oxidation process in the bulk solution is required for the formation of Fe3+ . The formed Fe3+ can then act as a coagulating agent allowing sedimentation of the pollutant species (Hakizimana et al., 2017). Fig. 3 shows the effectiveness of aluminum at COD removal compared to iron. The effect of current intensity is greater than the electrode material applied. One noteworthy conclusion was drawn by evaluating the current efficiency of each experimental run. The

Electric Current Efficiency (ECE) is defined by the following equation (Equation (2)) (García-García et al., 2011): ECE =

CODinitial − CODfinal minitial − mfinal

(2)

where m denotes the mass of the anode metal (g). Since a significant portion of the anode metal mass that dissolves in the bulk volume precipitates without leading to any pollutant reduction, ECE (g COD removed per liter (L), per g anode electrode dissolved) may express to what extent that metal exploitation occurs in each experimental run (Fig. 3). According to the literature, high initial COD values lead to decreased percentage removal efficiencies of COD (Khandegar and Saroha, 2013). For instance, the maximum removal efficiencies of 5000 mg L−1 COD0 were observed after 60 min reaction time at the highest current density (166.7 mAcm-2 ) and were about 50% and 35% for aluminum and iron electrodes, respectively (Fig. 2b). However, for the high initial COD concentration of 9000 mg L−1 , the maximum COD removal percentages decreased to 35% after 60 min at 166.7 mA cm-2 , for both electrode materials (Fig. 2c). Similar results were reported by Garcia-Garcia et al. (García-García et al.,

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Fig. 3. Electric Current Efficiency (ECE) as a function of different initial COD concentrations and three applied current densities in the EC process, using either aluminum or iron as electrode materials.

2011). They applied EC to treat TOPW with an initial COD concentration of approximately 7000 mg L-1 and also found that Al electrodes were more effective than Fe electrodes at reducing COD. The maximum COD removal recorded in that study was about 50% after 30 min and 30% after 60 min at 100 mA cm-2 , using aluminum and iron as working electrodes, respectively. The negative effect that high initial pollutant concentrations impose on EC performance could be related to the initial pH of the treated solution. Strongly polluted effluents tend to be more acidic (pH values of approximately 4.2–4.8 for initial COD value about 9000 mg L−1 ) while diluted wastewaters with less pollutant load are rather neutral (pH values of around 6 for the initial COD value of 3000 mg L−1 due to dilution with tap water). As known from the theoretical background on the precipitation of electrogenerated metallic species, neutral conditions favor precipitation, therefore the insolubility of aluminum and iron is the critical factor in this process, since both metals form different compounds depending on the solution’s pH value (Garcia-Segura et al., 2017; LinaresHernández et al., 2009). In the present study, a gradual increase of pH was observed during the EC process in all experiments conducted (see Table 1), since OH- is liberated from the cathode and contributes to the neutralization of the bulk volume’s final pH (García-García et al., 2011; Mechelhoff et al., 2013). The increased pH could also be attributed to the removal of acidic polyphenolic compounds (Kraljic´ Rokovic´ et al., 2014). Similar observations were also reported by Salameh et al. (Salameh et al., 2015) and Ghanbari et al. (Ghanbari et al., 2014) who evaluated the efficiency of EC in treating olive mill (OMW) wastewater and wastewaters from the textile industry, respectively. It is worth noting that the main advantage of using EC to treat TOPW is that low current densities can be applied throughout the process, since TOPWs contain high NaCl concentrations (resulting in high conductivity values), therefore resistance to electric current through the bulk solution is rather low. Moreover, the high cost of electrolyte addition is avoided. In addition, the presence of Cl− (due to the presence of NaCl), which leads to the formation of neutral particles, contributes to more effective removal of pollutants by precipitation (Kraljic´ Rokovic´ et al., 2014). In this study, the initial conductivity of TOPW, which is significantly higher than that of other agro-industrial wastewaters (Khansorthong and Hunsom, 2009), differed for each initial COD concentration examined (due to wastewater’s dilution with tap water) and remained constant throughout the treatment process (Table 1). The maximum COD removal of about 50% was observed for the initial COD concentration of 3000 mg L−1 using aluminum electrodes at 166.7 mA cm-2 (Fig. 2a). In all experiments COD reduction was between 40 and 50%, with final COD concentrations above the maximum permitted limit of 125 mg L−1 set by European leg-

islation (EEC, 1991). Therefore, EC could be used as an effective pre-treatment method for partial COD removal. Similar observations were made by Sounni et al. (Sounni et al., 2018) who studied EC to treat olive mill wastewaters (OMW) prior to anaerobic digestion and recorded a 60% reduction in COD values. Additionally, Tatoulis et al. (Tatoulis et al., 2016) showed that TOPWs can be effectively treated using a two-stage hybrid system comprising an aerobic biological filter followed by electrochemical oxidation for complete decolorization and COD removal of the biologically treated effluent. 3.1.2. Color removal Effluent color density is strongly dependent on its organic matter content (Deligiorgis et al., 2008). However, in contrast to COD reduction, color removal was complete when treating TOPW with EC. Experimental results for the effluent with initial COD of 3000 mg L−1 (Fig. 4a) indicated that it is possible to completely remove the effluent’s color, regardless of the current density value or the electrode material used. However, it is apparent that faster decolorization rates can be achieved when higher currents are applied, as significant removal (99%) was possible in the first 10 min of the procedure when the current intensity was increased to 166.7 mA cm-2 . Moreover, aluminum electrodes proved more efficient than iron electrodes at removing color as shown in terms of decolorization kinetics in Fig. 4a. One interesting observation was recorded when using iron electrodes and 41.7 mA cm-2 current density. As shown in Fig. 4a, the color of the bulk solution darkened progressively in the early stages of the EC process, however, complete decolorization was achieved towards the end of the treatment time. It is possible that the high solubility of iron species (mainly ferrous cations) leads to the delayed sedimentation of such polymeric species formed during the treatment process (García-García et al., 2011). As in the case of COD reduction, higher initial COD values tended to limit the decolorization rate (Fig. 4b and c). Additionally, decolorization of TOPW with the initial COD value of 5000 mg L−1 was highly dependent on electric current. It was observed that both aluminum and iron electrodes produced maximum color removal in the first 20 min of the process, while aluminum proved more effective at the highest current density (Fig. 4b). Dark colored effluents were also observed when using aluminum electrodes at 41.7 mA cm-2 and iron electrodes at 83.3 and 166.7 mA cm-2 . The higher initial COD concentration combined with low metal dissolution rates and the weak performance of iron electrodes are considered the main causes for the darker colored effluents observed during the EC process compared to those observed in experiments applying effluents with the lowest initial COD value of 3000 mg L−1 (Fig. 4a). Concerning the decolorization of TOPW with initial COD

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Fig. 4. Effect of current density and electrode material on color reduction of wastewater absorbance at 400 nm versus time, for initial COD concentrations of: (a) 3000 mg L−1 , (b) 5000 mg L−1 , and (c) 9000 mgL−1 .

concentrations of 9000 mg L-1 , dark color was observed in each experiment, even those where the highest current intensity was applied (Fig. 4c). It is worth noting that Garcia-Garcia et al. (GarcíaGarcía et al., 2011) also observed the same trend in color when applying aluminum and iron electrodes to treat TOPW. As mentioned above, the formation of polyphenolic compounds could be responsible for the noticeable colorization of the bulk volume (Ayed et al., 2017; Deligiorgis et al., 2008), while their sedimentation is strongly dependent on the concentration of dissolved metals within the bulk solution (García-García et al., 2011). However, complete decolorization of TOWP effluents is feasible using EC, as in all experimental runs color removal percentages of over 85% were achieved. This may be attributed to the short distance between the electrodes (0.3 cm) applied in this study, whereas in the study of Garcia-Garcia et al. (García-García et al., 2011) where the distance between electrode materials was greater (1 cm), lower color removal percentages were achieved (50% in all experiments). Generally, in this study, the performance of the EC process was significantly more efficient at color removal than COD reduction.

3.1.3. Phenolic compounds removal Phenolic compounds have phytotoxic effects and also inhibit the growth of microorganisms, thus their removal from wastewater is essential prior to discharge (Villegas et al., 2016). Moreover, the removal of phenolic content could enhance the efficiency of biological treatment methods (Sounni et al., 2018). In this study, the removal of phenolic compounds was examined at the lowest initial effluent COD value of 3000 mg L−1 using both aluminum and iron as electrode materials and the current density of 41.7 mA cm-2 (Fig. 5), since with these experimental conditions the maximum COD and color reduction percentages had been observed. In this series of experiments the initial concentration of total phenolic compounds in the effluent was 630 mg L−1 . Following EC application it was observed that the removal percentages of phenolic compounds exceeded 80% for both electrode materials tested. Further experiments on effluents with higher initial COD concentrations produced similar values (c. 80%, data not shown). Fig. 5 presents a typical profile of phenolic compound removal. High removal percentages of phenolic compounds have also been reported when applying EC as a single-stage treatment method (Bazrafshan et al., 2012).

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´ pilot-scale applications (McBeath et al., 2018; Smoczynski et al., 2017; Thiam et al., 2014; Timmes et al., 2010), there are no studies concerning the (pre- or post-) treatment of TOPW at pilot-scale. This study demonstrates that a pilot-scale EC reactor can be used as a post-treatment step for decolorization and partial removal of COD from TOPW even when operated at low current densities (up to 5.65 mA cm−2 ) to reduce operating cost. Aluminum electrodes proved more suitable than iron for the depollution of TOPW as they achieved color and COD removal up to 85% and 42.5%, respectively, after 50 min of EC treatment. 3.3. Metal concentration in the final bulk volume

Fig. 5. Removal of total phenolic compounds versus time from TOPW with an initial COD value of 3000 mg L−1 using different electrode materials and 41.7 mAcm-2 current density.

Garcia-Garcia et al. (García-García et al., 2011) also studied the efficiency of EC in the treatment of TOPW and referred that phenol degradation occurs in three stages. Initially, the phenol oxidizes to quinonic compounds. Subsequent oxidation of these compounds, following the opening of the aromatic ring, leads to the formation of organic acids that are then mineralized to carbon dioxide. GarciaGarcia et al. (García-García et al., 2011), achieved 75% removal of phenolic compounds when using aluminum electrodes. Such percentages are similar to those achieved in the present study, as the high valence of aluminum (oxidized to Al3+ ) is a critical factor for this behavior (García-García et al., 2011). Sounni et al. (Sounni et al., 2018) also reported significant phenolic compound removal (up to 78%) when applying EC to pre-treat OMW, aiming to reduce its toxic compounds for enhanced biogas production. 3.2. EC as a post-treatment method for TOPW: pilot-scale application In this series of experiments, EC was examined as a posttreatment method for biologically treated TOPW. Experiments were conducted using two different electrode materials (Al, Fe) at current densities of 3.87 and 5.65 mA cm−2 (corresponding to applied intensities of 65 and 95 A). As shown in Fig. 6, results of COD, phenolic compounds and color removal for the pilot-scale study were similar to those observed in the laboratory-scale experiments. Specifically, using an Al electrode and a current density of 5.65 mA cm-2 (Fig. 6b), the maximum COD, phenolics and color removal values were 42.5%, 83.9% and 85.3%, respectively, for a treatment time of 50 min. The current density of 3.85 mA cm-2 led to lower removal percentages of COD and phenolic compounds (Fig. 6a), however high color removal was also achieved (88.7%). Regarding the use of iron electrodes, lower COD, phenolics and color removal were recorded when applying both current densities. For example, using the current density of 5.65 mA cm-2 , the maximum removal values of COD, phenolics and color were 29.3%, 77.9% and 71.1%, respectively; while the removal percentages were significantly lower at 3.85 mA cm−2 (1.2%, 50.3% and 39.8% for COD, phenolics and color, respectively). Although EC has been investigated for the treatment of municipal wastewater, dye wastewater, drinking water and seawater in

The toxic effects of iron and aluminum on aquatic ecosystems are well documented (Fezeu et al., 2009), hence the reduction of residual concentrations of metal species from the bulk volume of treated effluents is essential. In this study, measurements of dissolved aluminum and iron concentrations were taken from the bulk volume at the end of each laboratory and pilot-scale experiment. In all experiments the maximum concentrations of dissolved residual aluminum in the treated TOPW ranged between 0.3 and <0.5 mg L−1 , and between 0.8 and <1.0 mg L−1 for dissolved residual iron. These values do not exceed the permissible limits for discharge of treated wastewaters into aquatic systems (0.5 mg L-1 for Al and 1.0 mg L−1 for iron) (Fezeu et al., 2009; Gardner et al., 2012). The values recorded here are similar to those recorded in previous studies that produced reduced amounts of dissolved metal ions in treated wastewaters due to pollutant precipitation during EC (Khansorthong and Hunsom, 2009; Mouedhen et al., 2008). 3.4. Process cost analysis 3.4.1. Energy consumption Based on the experimental results, a cost analysis of effluent treatment by EC was conducted to evaluate the feasibility of applying the process at industrial scale. Although COD reduction was limited to 40–50% the decolorization results were far more conclusive (almost complete decolorization recorded in all experiments). As the EC process resulted in only limited COD removal from TOPW, the treatment cost analysis was conducted using the single specific criterion that final effluent decolorization exceeds 85%. Consequently, the energy and electrode consumptions for a treatment time corresponding to that specific removal percentage (color removal 85%) were estimated. During the treatment process, electric energy consumption is mostly dependent on the effluent’s conductivity and the treatment time and therefore may be defined as (Papadopoulos et al., 2019):

t

P=

[

t=0

U I dt ]1000 60 V

(3)

where, P is the electric energy consumption (kWh m−3 ); U is the cell’s voltage (V); I is the electric current intensity (A), V is the wastewater’s volume (0.5 L) and t is the treatment time (min). Energy consumption results are presented in Fig. 7a. The energy required for 85% effluent decolorization in experiments using aluminum electrodes is significantly less than that required when using iron electrodes. It can be seen that less energy is required to treat effluents with lower initial COD values as less time is needed for complete decolorization (within the first 10 min of the process). Concerning the effect of applied current density, Fig. 7a shows that the use of lower currents leads to lower energy consumption. Garcia-Garcia et al. (García-García et al., 2011) also estimated total electricity consumption. They found that 0.76 kW h per cubic meter of wastewater was required for the following operating conditions: two different materials for anode and cathode (aluminum

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Fig. 6. Removal of COD, total phenolic compounds and color versus time from TOPW in the pilot scale unit using different electrode materials and current densities of a) 3.87 mA cm−2 and b) 5.65 mA cm-2 .

and iron), initial COD concentration of about 7000 mg L−1 , current density of 25 mA cm-2 , and 0.6 cm distance between the electrodes. This energy consumption value is lower than that reported in the present study, however, different operating conditions were examined in both cases and, as such, the values are not directly comparable. Considering the current Greek electric power price of 0.12 D per KWh, the cost of electric energy consumption (D m−3 ) was also evaluated for all the laboratory-scale experiments performed. Costs ranged between 0.17 and 0.90 D m-3 for aluminum electrodes, and 0.29 and 4.56 D m−3 for iron electrode use. Energy consumption was also evaluated for the pilot-scale experiments using the same equation (Eq. 3). For both metals used (Al and Fe) and the same treatment time (50 min), the energy requirement was calculated as 0.68 kW h m−3 , when 3.87 mA cm-2 current density was applied, and 0.99 kW h m−3 with 5.65 mA cm-2

Table 2 Operating parameters of pilot-scale reactor, anode loss (kg) and electrode loss (D m−3 ) recorded at the end of the EC post-treatment on TOPW. Treated Volume (m3 ) Treatment time (min) Voltage (V) Current Density (mA cm−2 ) Al anode loss (kg) Fe anode loss (kg) Al electrode cost (D m−3 ) Fe electrode cost (D m−3 )

0.2 50 2.5 3.85 0.018 0.045 0.45 0.22

5.65 0.027 0.066 0.68 0.33

current density. Using the same electric energy price (0.12 D per KWh), the respective estimated energy costs were 0.08 and 0.12 D m−3 . The data used for these cost estimates are presented in Table 2. An increase in electric current leads to a proportionate rise in energy consumption.

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Fig. 7. a) Electric energy consumption (kWh m−3 of wastewater) and (b) Electrode consumption (kg m-3 of wastewater) recorded during EC using different electrode materials (aluminum, iron), initial COD concentrations (mg L-1 ), and current densities (mA cm-2 ).

3.4.2. Electrode consumption Electrode consumption is the second factor required to evaluate total EC treatment cost. It should be noted that as no experimental details were available during the procedure (the electrodes were weighed only prior to and after EC treatment), Faraday’s law was used to evaluate their consumption (Fajardo et al., 2015) at different time intervals. The criterion used to evaluate anode consumption was that of sufficient color removal (over 85%). Faraday’s law was applied to determine the mass of the anode electrode that was dissolved at the experimental time when 85% decolorization was observed. Fig. 7b shows the results of anode consumption for each run of laboratory-scale experiments. For each electrode material used, the respective consumption was proportionate to the applied current density. For example, when TOPW with initial COD concentration of 9000 mg L−1 was treated with iron electrodes, the electrode consumption recorded was 6.6, 5.4 and 2.9 kg per cubic meter of wastewater for the current densities of 166.7, 83.3 and 41.7 mA cm-2 , respectively. Aluminum electrode consumption for the same initial COD concentration was significantly lower (0.87, 0.85 and 0.83 kg m-3 for 166.7, 83.3 and 41.7 mA cm-2 , respectively). Furthermore, since effluents with higher initial COD values required longer treatment times for complete decolorization (see Fig. 4), they also resulted in higher anode losses. Generally, less electrode consumption was observed when applying aluminum electrodes to treat TOPW with the lowest initial COD value of 3000 mg L-1 and the lowest current density of 41.7 mA cm-2 (Fig. 5b). The cost of the treatment process due to electrode consumption was also estimated taking into account the current Greek market price of Al and Fe (5.00 D and 1.00 D Kg-1 ). Costs for the laboratory-scale experiments varied between 1.00 and 4.35 D m-3 for aluminum electrode use, and between 1.90 and 6.60 D m-3 for iron electrode use. Electrode consumption was also evaluated for the pilot-scale experiments. However, in this case electrode consumption was much lower due to the lower organic loads and current densities applied. Concerning the aluminum electrodes, 0.09 kg m−3 and 0.135 kg m−3 were the calculated consumptions when 3.87 mA cm-2 (65 A) and 5.65 mA cm-2 (95 A) were the respective applied current densities. The respective iron electrode consumption values were 0.22 kg m−3 and 0.33 kg m−3 . Using the current Greek market prices for Al and Fe materials, the indicative metal consumption costs for all the pilot-scale experiments conducted were

0.45 D m-3 and 0.68 D m-3 for aluminum electrodes, and 0.22 D m-3 and 0.33 D m−3 for the iron electrodes. 4. Conclusions Experiments were performed in laboratory and pilot-scale reactors to determine the efficiency of EC to treat TOPW as a single treatment process or a post-treatment step, respectively. From the experimental results it can be deduced that: • Aluminum appears to be the more suitable electrode material for COD reduction and decolorization of untreated or biologically pretreated TOMW. • Higher initial COD concentration resulted in lower process performance for both Al and Fe electrodes used. Specifically, in the laboratory-scale experiments an increase of initial COD concentration from 3000 to 9000 mg L−1 , for all current densities (166.7 - 41.7 mA cm-2 ), resulted in 23–46% reduction of COD removal when applying Al electrodes and 15–31% reduction when applying Fe electrodes. • Aluminum electrodes consumed less energy than iron electrodes (1.41–7.5 KWh m−3 and 2.4–38.0 KWh m−3 , respectively in laboratory-scale experiments. • Lower metal consumption was recorded in experiments using aluminum electrodes (Laboratory-scale experiments: 0.20-0.87 Kgm−3 for Al and 1.9–6.6 K g·m−3 for Fe, Pilot-scale experiments: 0.09-0.135 Kgm−3 for Al and 0.22-0.33 Kg·m−3 for Fe). • EC could be combined with biological treatment methods in a hybrid system to treat TOPW and increase the system’s total pollutant removal efficiency.

Acknowledgements ¨ ¨ This study was financially supported by the CLIENTDR project funded by the Region of Western Greece under the Promoting Transnational Research Projects for Small and Medium Enterprises Framework - Operational Program Western Greece 2014-2020, grant agreement No. 5021383/INCOMERA project that received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 618103. The

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pilot unit was designed and constructed by ECOMEX Company ¨ ¨ under the framework of the CLIENTDR project. References Aggelis, G., Ehaliotis, C., Nerud, F., Stoychev, I., Lyberatos, G., Zervakis, G., 2002. Evaluation of white-rot fungi for detoxification and decolorization of effluents from the green olive debittering process. Appl. Microbiol. Biotechnol. 59, 353–360, http://dx.doi.org/10.1007/s00253-002-1005-9. APHA/AWWA/WEF, 2012. Standard methods for the examination of Water and wastewater. Stand. Methods 541, https://doi.org/ISBN9780875532356. Ayed, L., Asses, N., Chammem, N., Ben Othman, N., Hamdi, M., 2017. Advanced oxidation process and biological treatments for table olive processing wastewaters: constraints and a novel approach to integrated recycling process: a review. Biodegradation 28, 125–138, http://dx.doi.org/10.1007/s10532-017-9782-0. Bazrafshan, E., Biglari, H., Mahvi, A.H., 2012. Phenol removal by electrocoagulation process from aqueous solutions. Fresenius Environ. Bull. 21, 364–371. Bazrafshan, E., Moein, H., Kord Mostafapour, F., Nakhaie, S., 2013. Application of electrocoagulation process for dairy wastewater treatment. J. Chem. 2013, 7–10, http://dx.doi.org/10.1155/2013/640139. Bocos, E., Brillas, E., Sanromán, M.Á., Sirés, I., 2016. Electrocoagulation: simply a phase separation technology? The case of bronopol compared to its treatment by EAOPs. Environ. Sci. Technol. 50, 7679–7686, http://dx.doi.org/10.1021/acs. est.6b02057. Can, O.T., Bayramoglu, M., Kobya, M., 2003. Decolorization of reactive dye solutions by electrocoagulation using aluminum electrodes. Ind. Eng. Chem. Res. 42, 3391–3396, http://dx.doi.org/10.1021/ie020951g. Chatzisymeon, E., Stypas, E., Bousios, S., Xekoukoulotakis, N.P., Mantzavinos, D., 2008. Photocatalytic treatment of black table olive processing wastewater. J. Hazard. Mater. 154, 1090–1097, http://dx.doi.org/10.1016/j.jhazmat.2007.11. 014. Deligiorgis, A., Xekoukoulotakis, N.P., Diamadopoulos, E., Mantzavinos, D., 2008. Electrochemical oxidation of table olive processing wastewater over borondoped diamond electrodes: treatment optimization by factorial design. Water Res. 42, 1229–1237, http://dx.doi.org/10.1016/j.watres.2007.09.014. Drogui, P., Asselin, M., Brar, S.K., Benmoussa, H., Blais, J.F., 2008. Electrochemical removal of pollutants from agro-industry wastewaters. Sep. Purif. Technol. 61, 301–310, http://dx.doi.org/10.1016/j.seppur.2007.10.013. EEC, 1991. Council directive concerning urban waste-water treatment. Off. J. Eur. Communities 10 http://eur-lex.europa.eu/legal-content/en/ALL/ ?uri=CELEX:31991L0271. Fajardo, A.S., Rodrigues, R.F., Martins, R.C., Castro, L.M., Quinta-Ferreira, R.M., 2015. Phenolic wastewaters treatment by electrocoagulation process using Zn anode. Chem. Eng. J. 275, 331–341, http://dx.doi.org/10.1016/j.cej.2015.03.116. Fezeu, W.M.L., Ngassoum, M.B., Montarges-Pelletier, E., Echevarria, G., Mbofung, C.M.F., 2009. Physicochemical characteristics of Lake IRAD, an artificial lake in Wakwa region, Cameroon. Lakes Reserv. Res. Manag. 14, 259–268, http://dx.doi. org/10.1111/j.1440-1770.2009.00402.x. Flores, N., Brillas, E., Centellas, F., Rodríguez, R.M., Cabot, P.L., Garrido, J.A., Sirés, I., 2018. Treatment of olive oil mill wastewater by single electrocoagulation with different electrodes and sequential electrocoagulation/electrochemical fenton-based processes. J. Hazard. Mater. 347, 58–66, http://dx.doi.org/10.1016/ j.jhazmat.2017.12.059. García-García, P., López-López, A., Moreno-Baquero, J.M., Garrido-Fernández, A., 2011. Treatment of wastewaters from the green table olive packaging industry using electro-coagulation. Chem. Eng. J. 170, 59–66, http://dx.doi.org/10.1016/ j.cej.2011.03.028. Garcia-Segura, S., Eiband, M.M.S.G., de Melo, J.V., Martínez-Huitle, C.A., 2017. Electrocoagulation and advanced electrocoagulation processes: A general review about the fundamentals, emerging applications and its association with other technologies. J. Electroanal. Chem. 801, 267–299, http://dx.doi.org/10.1016/j. jelechem.2017.07.047. Gardner, M., Comber, S., Scrimshaw, M.D., Cartmell, E., Lester, J., Ellor, B., 2012. The significance of hazardous chemicals in wastewater treatment works effluents. Sci. Total. Environ. 437, 363–372, http://dx.doi.org/10.1016/j.scitotenv.2012.07. 086. Ghanbari, F., Moradi, M., Eslami, A., Emamjomeh, M.M., 2014. Electrocoagulation/Flotation of textile wastewater with simultaneous application of aluminum and iron as anode. Environ. Process. 1, 447–457, http://dx.doi.org/10.1007/ s40710-014-0029-3. Hakizimana, J.N., Gourich, B., Chafi, M., Stiriba, Y., Vial, C., Drogui, P., Naja, J., 2017. Electrocoagulation process in water treatment: A review of electrocoagulation modeling approaches. Desalination 404, 1–21, http://dx.doi.org/10.1016/j.desal. 2016.10.011. Khandegar, V., Saroha, A.K., 2013. Electrocoagulation for the treatment of textile industry effluent - A review. J. Environ. Manage. 128, 949–963, http://dx.doi. org/10.1016/j.jenvman.2013.06.043.

47

Khansorthong, S., Hunsom, M., 2009. Remediation of wastewater from pulp and paper mill industry by the electrochemical technique. Chem. Eng. J. 151, 228–234, http://dx.doi.org/10.1016/j.cej.2009.02.038. Kotsou, M., Kyriacou, A., Lasaridi, K., Pilidis, G., 2004. Integrated aerobic biological treatment and chemical oxidation with Fenton’s reagent for the processing of green table olive wastewater. Process. Biochem. 39, 1653–1660, http://dx.doi. org/10.1016/S0032-9592(03)00308-X. ˇ ´ M., Cubri ´ M., Wittine, O., 2014. Phenolic compounds removal from Kraljic´ Rokovic, c, mimosa tannin model water and olive mill wastewater by energy-efficient electrocoagulation process. J. Electrochem. Sci. Eng. 4, 215–225, http://dx.doi.org/10. 5599/jese.2014.0066. ˜ ˜ Linares-Hernández, I., Barrera-Díaz, C., Roa-Morales, G., Bilyeu, B., Urena-Nú nez, F., 2009. Influence of the anodic material on electrocoagulation performance. Chem. Eng. J. 148, 97–105, http://dx.doi.org/10.1016/j.cej.2008.08.007. McBeath, S.T., Mohseni, M., Wilkinson, D.P., 2018. Pilot-scale iron electrocoagulation treatment for natural organic matter removal. Environ. Technol. 0, 1–9, http:// dx.doi.org/10.1080/09593330.2018.1505965. Mechelhoff, M., Kelsall, G.H., Graham, N.J.D., 2013. Electrochemical behaviour of aluminium in electrocoagulation processes. Chem. Eng. Sci. 95, 301–312, http:// dx.doi.org/10.1016/j.ces.2013.03.010. Moreno, H., Parga, J.R., Gomes, A.J., Rodríguez, M., 2013. Electrocoagulation treatment of municipal wastewater in Torreon Mexico. Desalin. Water Treat. 51, 2710–2717, http://dx.doi.org/10.1080/19443994.2012.749366. Mouedhen, G., Feki, M., Wery, M.D.P., Ayedi, H.F., 2008. Behavior of aluminum electrodes in electrocoagulation process. J. Hazard. Mater. 150, 124–135, http://dx. doi.org/10.1016/j.jhazmat.2007.04.090. Moussa, D.T., El-Naas, M.H., Nasser, M., Al-Marri, M.J., 2017. A comprehensive review of electrocoagulation for water treatment: potentials and challenges. J. Environ. Manage. 186, 24–41, http://dx.doi.org/10.1016/j.jenvman.2016.10.032. Papadaki, E., Mantzouridou, F.T., 2016. Current status and future challenges of table olive processing wastewater valorization. Biochem. Eng. J. 112, 103–113, http:// dx.doi.org/10.1016/j.bej.2016.04.008. Papadopoulos, K.P., Argyriou, R., Economou, C.N., Charalampous, N., Dailianis, S., Tatoulis, T.I., Tekerlekopoulou, A.G., Vayenas, D.V., 2019. Treatment of printing ink wastewater using electrocoagulation. J. Environ. Manage. 237, 442–448, http://dx.doi.org/10.1016/j.jenvman.2019.02.080. Rincón-Llorente, B., De la Lama-Calvente, D., Fernández-Rodríguez, M.J., BorjaPadilla, R., 2018. Table olive wastewater: problem, treatments and future strategy. A review. Front. Microbiol. 9, http://dx.doi.org/10.3389/fmicb.2018. 01641. Sahu, O., Mazumdar, B., Chaudhari, P.K., 2014. Treatment of wastewater by electrocoagulation: A review. Environ. Sci. Pollut. Res. 21, 2397–2413, http://dx.doi. org/10.1007/s11356-013-2208-6. Salameh, W.K.M.B., Ahmad, H., Al-Shannaq, M., 2015. Treatment of olive mill wastewater by ozonation and electrocoagulation processes and water resources management. Int. J. Environ. Chem. Ecol. Geol. Geophys. Eng. 9, 296–300. Sharma, D., 2014. Treatment of dairy waste water by electro coagulation using aluminum electrodes and settling, filtration studies. Int. J. ChemTech Res. 6, 591–599. Singleton, V.L., Orthofer, R., Lamuela-Raventos, R.M., 1999. [14] analysis of total phenols and other oxidation substrates and antioxidants by means of folin-ciocalteu reagent. Polyphenols Flavonoids 299, 152–178. ´ Smoczynski, L., Kalinowski, S., Ratnaweera, H., Kosobucka, M., Trifescu, M., ´ K., 2017. Electrocoagulation of municipal wastewater Pieczulis-Smoczynska, a pilot-scale test. Desalin. Water Treat. 72, 162–168, http://dx.doi.org/10.5004/ dwt.2017.20654. Sounni, F., Aissam, H., Ghomari, O., Merzouki, M., Benlemlih, M., 2018. Electrocoagulation of olive mill wastewaters to enhance biogas production. Biotechnol. Lett. 40, 297–301, http://dx.doi.org/10.1007/s10529-017-2464-5. Tatoulis, T.I., Zapantiotis, S., Frontistis, Z., Akratos, C.S., Tekerlekopoulou, A.G., Pavlou, S., Mantzavinos, D., Vayenas, D.V., 2016. A hybrid system comprising an aerobic biological process and electrochemical oxidation for the treatment of black table olive processing wastewaters. Int. Biodeterior. Biodegrad. 109, 104–112, http:// dx.doi.org/10.1016/j.ibiod.2016.01.013. Thiam, A., Zhou, M., Brillas, E., Sirés, I., 2014. A first pre-pilot system for the combined treatment of dye pollutants by electrocoagulation/EAOPs. J. Chem. Technol. Biotechnol. 89, 1136–1144, http://dx.doi.org/10.1002/jctb.4358. Timmes, T.C., Kim, H.C., Dempsey, B.A., 2010. Electrocoagulation pretreatment of seawater prior to ultrafiltration: pilot-scale applications for military water purification systems. Desalination 250, 6–13, http://dx.doi.org/10.1016/j.desal. 2009.03.021. Villegas, L.G.C., Mashhadi, N., Chen, M., Mukherjee, D., Taylor, K.E., Biswas, N., 2016. A short review of techniques for phenol removal from wastewater. Curr. Pollut. Reports 2, 157–167, http://dx.doi.org/10.1007/s40726-016-0035-3. Zarkadas, I.S., Pilidis, G.A., 2011. Anaerobic Co-digestion of table olive debittering & washing effluent, cattle manure and pig manure in batch and high volume laboratory anaerobic digesters: effect of temperature. Bioresour. Technol. 102, 4995–5003, http://dx.doi.org/10.1016/j.biortech.2011.01.065.