Treatment of olive oil mill wastewater by combined process electro-Fenton reaction and anaerobic digestion

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Treatment of olive oil mill wastewater by combined process electro-Fenton reaction and anaerobic digestion Sonia Khoufi, Fathi Aloui, Sami Sayadi Laboratoire des Bioproce´de´s, Centre de Biotechnologie de Sfax; B. P. ‘‘K’’, 3038 Sfax, Tunisia

art i cle info

A B S T R A C T

Article history:

In this work, we investigated an integrated technology for the treatment of the recalcitrant

Received 23 October 2005

contaminants of olive mill wastewaters (OMW), allowing water recovery and reuse for

Received in revised form

agricultural purposes. The method involves an electrochemical pre-treatment step of the

19 March 2006

wastewater using the electro-Fenton reaction followed by an anaerobic bio-treatment. The

Accepted 26 March 2006

electro-Fenton process removed 65.8% of the total polyphenolic compounds and subsequently decreased the OMW toxicity from 100% to 66.9%, which resulted in improving the

Keywords:

performance of the anaerobic digestion. A continuous lab-scale methanogenic reactor was

Olive mill wastewater

operated at a loading rate of 10 g chemical oxygen demand (COD) l
1 d
1 without any

electro-Fenton

apparent toxicity. Furthermore, in the combined process, a high overall reduction in COD,

Anaerobic digestion

suspended solids, polyphenols and lipid content was achieved by the two successive

Polyphenols

stages. This result opens promising perspectives since its conception as a fast and cheap

Abbreviations: OMW: olive mill wastewaters COD: chemical oxygen demand

pre-treatment prior to conventional anaerobic post-treatment. The use of electrocoagulation as post-treatment technology completely detoxified the anaerobic effluent and removed its toxic compounds.

BOD5: biological oxygen demand

& 2006 Elsevier Ltd. All rights reserved.

TSS: total suspended solids LMM: low molecular mass AF: anaerobic filter VFA: volatile fatty acids

1.

Introduction

Treatment and disposal of olive mill wastewater (OMW) represents one of the main problems for olive oil producing countries of the Mediterranean area. Tunisia is one of the largest olive oil producers in the world with an average annual production of 450,000 tons. This results in a byproduct of 600,000 m3 OMW. These liquid residues are 100–150 times more heavily loaded with pollutants than ordinary domestic wastewater (Sabbah et al., 2004). The high polluting activity of OMW is linked with their high content of organic molecules, especially polyphenolic mixtures (4–10 g l
1) with different molecular weights (Hamdi, 1992), Corresponding author. Tel./fax: +00 216 74 440 452.

E-mail address: [email protected] (S. Sayadi). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.03.023

as well as their acidity and high concentration of potassium, magnesium and phosphate salts (Arienzo and Capasso, 2000). Besides aromatic compounds, OMW contain other organic molecules including nitrogen compounds, sugars, organic acids, and pectins (Della Greca et al., 2000), that increase their organic load (chemical oxygen demand (COD) ¼ 80–200 g l
1; biological oxygen demand (BOD5) ¼ 50–100 g l
1). Furthermore, the physico-chemical characteristics of OMW are rather variable, depending on climatic conditions, olive cultivars, degree of fruit maturation, storage time, and extraction procedure. Many pollution disposal methods, such as concentration, evaporation, incineration, ultrafiltration/reverse osmosis,

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lime precipitation, aerobic treatment, lagooning, codigestion, etc were tested on OMW, none of them led to industrial applications. However, anaerobic digestion seems to have some clear advantages that would make it the process of choice (Borja et al., 1992). Indeed, this treatment process produces energy (methane) and a digested effluent with a significant reduction of the organic load (Marques, 2001). However, many problems concerning the high toxicity and inhibition of biodegradation of these effluents were encountered during anaerobic treatments, because some bacteria, such as methanogens, were particularly sensitive to the organic contaminants present (Andreoni et al., 1984). The phenolic compounds severely limit the possibility of using anaerobic digestion (Sayadi et al., 2000). Therefore, the elimination of phenolic compounds from OMW was considered as an important objective in order to reduce its toxicity and to permit the occurrence of microbial fermentation. For this, research turned to a more promising alternative, namely the physico-chemical pre-treatment to remove the toxic compounds of OMW (Beccari et al., 1999). In recent years, there has been increasing interest in the use of electrochemical technologies for the treatment of wastewaters. This technique was found to be successful in removing pollutants in various industrial wastewaters (Lin and Chang, 2000; Ciardelli and Ranieri, 2001; Lai and Lin, 2004). In two recent investigations, Inan et al., (2004) and Adhoum and Moncer (2004), an electrochemical method was used for decreasing the organic matter in OMW. Both investigators found efficient removals of COD, colouration and polyphenols content by electrolysis process using aluminium and iron electrodes. However, a relatively new chemical oxidation method that has not received much attention for OMW or other industrial wastewater treatment is the electroFenton method (Lin and Chen, 1997). This method represents a combination of the electrochemical process and the Fenton oxidation. It is based on the fact that hydrogen peroxide (H2O2) can be used as an oxidant in advanced oxidation processes to decompose refractory or toxic wastewaters (Kusvuran et al., 2004). As indicated in reaction (1), when the ferrous ion reacts with H2O2 it will generate strong oxidant hydroxyl radicals (OH  ). Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
:

(1)

This Fe2+/H2O2 system, often referred to as Fenton’s reagent (Fenton’s), has dual functions of OH radical peroxidation as well as ferrous/ferric coagulation. During this process, the non-biodegradable organics and toxic pollutants present in the wastewaters such as polyphenols are usually destroyed by direct or indirect anodic oxidation via the production of oxidants such as hydroxyl radicals and complex coagulants that promote the flocculation of the matter (Israilides et al., 1997; Panizza et al., 2000; Chen et al., 2002). This paper will attempt to apply the electro-Fenton process to reduce the organic load and the toxicity of OMW in order to improve the anaerobic digestion in terms of biomethane yield. Electro-coagulation was assayed as a post-treatment for complete detoxification and colour removal allowing water recovery and reuse for agricultural purposes.

2.

Materials and methods

2.1.

OMW characterisation

Fresh OMW was obtained from an olive oil continuous processing plant located in Sfax (southern Tunisia). The OMW was characterised by high total suspended solids (TSS) content, COD concentration up to 100 g l
1 and polyphenols up to 12 g l
1. Raw OMW was pre-decanted in a 120 l decanter before being treated by electro-Fenton in order to remove suspended solids (Fig. 1). To confirm the role of electro-Fenton in polymerising and removing the highly polymerised phenolic fraction, experiments were carried out with a low-molecular-mass (LMM) polyphenolic fraction (o2 kDa) obtained by the ultrafiltration of crude OMW using a polysulphone 2 kDa cut-off membrane. The purpose of this ultrafiltration was to study the effect of electro-Fenton reaction on the toxic fraction of OMW which is composed of LMM phenolics such as simple phenolics (hydroxytyrosol, tyrosol, p-OH benzoic acid, p-OH phenyl acetic acid, vannilic acid, caffeic acid, coumaric acid, vanniline, ferilic acid, catechol, methylcatechol), tannins, antocyanins, catechin (Sayadi et al., 2000). The C18-HPLC chromatogram of this OMW phenolic fraction is presented in Fig. 2.

2.2.

Electro-Fenton and electro-coagulation treatment

Preliminary experiments were carried out in a 0.25 l
1 glass reactor for the electro-Fenton of OMW fraction. The aqueous solution of reactants was homogenised by magnetic agitation to avoid concentration gradients. The electro-Fenton reactor was formed by one pair of anodic and cathodic electrodes (cast iron plates) which were positioned approximately 1.5 cm apart from each other and were dipped in the effluent. The total effective surface area of electrodes was 0.2 dm2. The current input was supplied by a convergy power supply. In each run, approximately 0.2 l of OMW fraction was placed in the electrolytic cell. The pH of the solution was adjusted to 4. H2O2 was added to the electrolytic cell before the electrical current was turned on. A batch study was conducted to optimise parameters like H2O2 concentration and current density governing the electro-Fenton process. These parameters were examined in the range of 0–1.5 g l
1 and 1.25–10 A dm
2, respectively. The optimum H2O2 concentration and current density were found to be 1 g l
1 and 7.5 A dm
2, respectively. At these conditions, maximum removal of monomer concentration, COD and colour were attained. For this reason, these conditions were chosen as the optimised parameters and were subsequently used for preparing the pre-treated OMW for the biomethanisation. Experiments of electro-Fenton of crude OMW were conducted in a 5 l glass reactor using iron electrodes having an effective surface area of 150 dm2 (Fig. 1). In each run, 3 l of crude OMW were treated and operated in batch mode. Electro-coagulation of anaerobic effluent was carried out in the same reactor as for the electro-Fenton of crude OMW without stirring. This electrolysis process lasted 2 h at 1.8 A dm
2 and without adjustment of pH.

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Fig. 1 – Schematic representation of OMW treatment process.

Fig. 2 – C18-HPLC chromatogram of phenolic compounds present in OMW fraction. 1: Hydroxytyrosol; 2: 3, 4 dihydroxyphenyl acetic acid, 3: tyrosol; 4: p-OH benzoı¨c acid; 5: p-OH phenylacetic acid; 6: vanillic acid; 7: caffeic acid; 8: coumaric acid; 9: vanillin; 10: ferulic acid.

After electro-Fenton reaction and electro-coagulation, treated effluents were placed in decanter tanks (Fig. 1) to eliminate sludge formed during electrolysis. During the experiments, samples were withdrawn and immediately analysed for water quality measurements.

2.3.

Anaerobic biotreatment and biogas analysis

Two anaerobic filters (AFs) were used in this study. These reactors were made of a glass column having a working volume of 3 l. The inner tubes were enclosed in a jacket

through which hot water was circulated to maintain the temperature of the filter at 37 1C. These AFs were packed with polyurethane foam cubes 2 cm  2 cm  1 cm (Filtren T45, from Recticel, Wetteren, Belgium) as support and inoculated with an 8-year-old digester operated with pre-treated OMW. The influent was fed in six times into the reactor using a pump connected to a programmer. For monitoring the volatile fatty acids (VFA) inside the reactor, three sampling points were made in the AF. Level (A) was at the bottom of the reactor. Level (B) corresponded to the middle and level (C) was at the top of the reactor.

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The gas flow rates of the AFs were measured by liquid displacement. Gas samples were taken with a syringe from the tank of biogas. CH4, CO2 and N2 were measured using a gas chromatograph GC11 (Delsi instruments) equipped with a Haye SepQ 60/80 (SUPELCO) column (maintained at 60 1C), a thermal conductivity detector (current intensity of 160 mA) and a servotrace integrator (SEFRAM). Helium was used as a carrier gas at a pressure of 1.3 bar.

2.4.

0.5 ml of OMW and 0.5 ml luminescent bacterial suspension. After a 15 min exposure at 15 1C, the decrease in light emission was measured. The toxicity of the OMW is expressed as the percent of the inhibition of bioluminescence (%IB) relative to a non-contaminated reference. A positive control (7.5% NaCl) was included for each test. Phytotoxicity test was estimated by the determination of the germination index (GI) according to Wong et al. (2001) using Lycopersicon esculentum (tomato) seeds.

Analytical methods

As the presence of residual H2O2 introduces a positive error in COD determination (Kang et al., 1999), in electrolysis experiments with H2O2, the pH of the samples was raised to above 10 with NaOH 6 N prior to analysis. The value of COD was estimated using the method described by Knechtel (1978) and fading colour was monitored by measuring the absorbance at 395 nm, the length of the maximum absorbance, using a spectrophotometer (ANTHLIE ADVANCED 5 SECOMAM). Samples were centrifuged for 20 min at 4000 t/min and diluted appropriately before each COD determination. BOD5 was determined by the manometric method with a respirometer (BSB-Controller Model 620T (WTW)). Concentration of ortho-diphenols was determined by the colorimetric reaction with Folin–Ciocalteau reagent. An aliquot of the OMW aqueous methanol extract was mixed with 2 ml of Folin–Ciocalteau reagent (Fluka, Switzerland). A sodium hydroxide solution (6% v/v) was added, and the mixture was shaken. The blue colour formed was measured at 727 nm. The ortho-diphenol concentration of OMW samples, as determined by the Folin–Ciocalteau method, (Folin and Ciocalteau, 1927) were reported as caffeic acid equivalents by reference to a standard curve. As about total polyphenols, they were quantified using the method described by Sayadi et al. (2000). Concentration of aromatic compounds was determined by high-performance liquid chromatography (HPLC) using a Shimadzu 10AVP chromatograph equipped with a Shimadzu 10AVP UV detector. Separation was made by a column (Shimpack CLC-ODS (M) 250 mm  4.6 mm) washed with acetonitrile/water (70/30) before and after analysis. A mixture of 50% acetonitrile in 50% water was chosen as optimal mobile phase. Data were analysed by class VP Shimadzu software. A Progel TSK-G 2000-SW Supelco column (300 mm  7.8 mm) was used with the same Shimadzu apparatus to analyse molecular-mass distribution of the OMW polyphenols. The elution was carried out using a phosphate buffer of pH 6.8 and 0.6 ml min
1 flow rate. The wavelength of the detector was adjusted to 280 nm. The standard method of Soxhlet solid/liquid (organic solids of OMW/hexane) was utilised for the dosage of lipids. VFAs (acetate, propionate, butyrate, isobutyrate and valerate) were measured by HPLC using the method described by Mechichi and Sayadi (2005). The microtoxicity test consists of the inhibition of the bioluminescence of Vibrio fischeri LCK480 using the LUMIStox system (Dr Lange GmbH, Du¨sseldorf, Germany) and was carried out according to ISO 11348-2 (1998). Percentage inhibition of the bioluminescence was achieved by mixing

3.

Results and discussion

3.1.

Electro-Fenton treatment

3.1.1.

Electro-Fenton treatment of OMW fraction

Preliminary tests were conducted to study the effect of electro-Fenton reaction on the pollutant characteristics of the LMM phenolic fraction. Experiments were realised with 1 g l
1 H2O2 added and a current density of 7.5 A dm
2 (see materials and methods section). The initial pH of the OMW fraction (4.8) was decreased to pH 4 in order to allow the Fenton’s peroxidation. Fig. 3 shows the evolution of pH, COD removal, colouration, monomers removal, hydroxytyrosol concentration and toxicity using V. fischeri based on LUMISTox system during the electro-Fenton treatment. The pH increased from 4 to 9 (Fig. 3a). The final pH differed in function of the quantity of the H2O2 added (data not shown) and the duration of the treatment. For 1 g l
1 of H2O2 and a 6 h treatment, the final pH obtained (pH 10.5) was not favourable for anaerobic post-treatment. The COD removal was 26% (Fig. 3b) at the steady state (after 4 h) but could be higher for a prolonged treatment period. The colour intensity of the effluent fraction monitored by measuring absorbance at 395 nm (Fig. 3c) doubled after 30 min of reaction, probably due to the polymerisation of monoaromatic compounds. Then it decreased to 75% of the initial colour after 4 h. This can be explained by the coagulation of the highly polymerised polyphenolic compounds. Chromatography techniques confirmed the removal of the most phenolics of LMM (Fig. 3d) which resulted in decreasing the toxicity from 100% to 67% after 30 min (TF1) and to 28% after 4 h of incubation (TF2) (Fig. 3f). Indeed, approximately 90% of the mono-aromatic compounds were removed after 4 h of incubation. As an example, the concentration of hydroxytyrosol, the major ortho-diphenol, decreased rapidly in the OMW fraction (Fig. 3e); a removal of 98% was achieved at the end of treatment. To test whether the dark colour was caused by polymerisation of the OMW fraction, the molecular-mass distribution of the reaction mixtures was measured by fast SE-HPLC and compared with the control. A polyphenol fraction with high hydrodynamic volumes was formed after OMW oxidation by electro-Fenton oxidation, suggesting that polymerisation had taken place (Fig. 4). After this polymerisation step which was related to the colour increase, a coagulation of the highly condensed polymers followed by rapid sedimentation occurred. This resulted in the decolourisation of the OMW fraction.

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Fig. 3 – Variations of pH (a), COD removal (b), residual colour (c), monomer removal (d) (1: hydroxytyrosol; 2: 3, 4 di-OH phenylacetic acid, 3: tyrosol; 4: p-OH benzoı¨c acid; 5: p-OH phenylacetic acid; 6: vanillic acid; 7: caffeic acid; 8: coumaric acid; 9: vanillin; 10: ferulic acid), hydroxytyrosol concentration (e) and the relative toxicity of OMW fraction (F) and the reactants collected during the Fenton process at 30 min (TF1) and at 4 h (TF2) of treatment (f).

3.1.2.

Electro-Fenton treatment of crude OMW

Table 1 shows the characteristics of OMW before and after electro-Fenton treatment. After decantation, the TSS of raw OMW decreased from 59 to 12 g l
1. The residual TSS in crude OMW was unsettlable suspended matter, which presents a major difficulty in the treatment and handling of OMW. During electro-Fenton treatment, pH increased from 4 to 7.6, which may be attributed to the smaller production of H+ than OH
as was explained by Israilides et al. (1997) and the reduction in phenol concentration. Indeed, phenols are acids in liquids, and their removal from a solution reduces its acidity. The pH value of electro-Fenton-treated OMW can be considered favourable for anaerobic bio-treatment.

Biodegradability is determined by measuring the ratio between COD and BOD5, whose value must be in the range of 2–2.5. After electro-Fenton, the COD of crude OMW drops to approximately 68% of the initial value. This result points out the ability of the electrolysis process to eliminate soluble compounds present in OMW. BOD5 values decreased from 19.25 to 15.5 g l
1 before and after treatment, respectively. Thus, COD/BOD5 ratio decreased from 5.84 before to 2.26 after. It appears that a significant proportion of the non-biodegradable matter present in OMW was removed by electro-Fenton. Degradation and mineralisation of phenolic compounds can occur during Fenton reaction. Kavitha and Palanivelu (2004) reported that in Fenton process, biodegradable aliphatic

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Fig. 4 – Molecular-mass distribution of phenolics from the untreated ultra-filtrated OMW phenolic fractions (___) and treated fraction ( ). Arrows indicate standard elution times: from left to right: 1: blue dextran (MW ¼ 2000 kDa), 2: lysozym (MW ¼ 15 kDa), 3: syringic acid (MW ¼ 198 Da).

Table 1 – Compositions of olive mill wastewaters before and after the treatment with the electro-Fenton process (S1, see Fig. 1) then with anaerobic bio-treatment (S2, see Fig. 1) and finally by electro-coagulation (S3, see Fig. 1) Parameter pH Color (absorbance 395 nm) UV absorbance 280 nm BOD5 (g l
1) COD (g l
1) COD/BOD5 Total solids (%) Total volatiles (%) Total suspended solids (g l
1) Volatiles suspended solids (g l
1) Ortho-diphenols (mg l
1) Total polyphenols (g l
1) Residual oils (g l
1)

Crude OMW

S1

S2

S3

5.44 73.00 167.30 19.25 112.50 5.84 12.20 10.90 59.00 55.17 6025.50 11.75 12.00

7.60 16.10 54.20 15.50 36.00 2.26 3.90 2.10 2.70 2.30 1536.70 4.20 1.30

7.80 13.16 49.60 — 8.30 — 1.95 1.50 1.80 1.65 861.80 1.20 Not detected

9.20 1.19 4.60 — 2.50 — 0.97 0.65 0.53 0.32 28.57 Not detected Not detected

compounds such as acetic acid and oxalic acid were identified as the major products during the degradation of synthetic phenol. However, transformation of phenolic polymers to simple phenolic compounds was not demonstrated. Chromatography analysis (data not shown) confirmed the removal of most LMM phenolics. Besides, the concentration of ortho-diphenols, monitored by Folin–Ciocalteau method, was significantly reduced during the electro-Fenton process. Removal efficiency was about 65.8% for total polyphenols and 74.5% for ortho-diphenols. Crude OMW was highly coloured due to its high content of polyaromatic compounds. In the beginning of the electrolysis treatment, the colour intensity of the effluent increased (data not shown) as a result of phenolic compounds polymerisation. However, colour intensity decreased to 78% of the initial colour at the end of treatment. During the electrolysis treatment, a part of the solute and particle matter present in OMW turned out to be a suspended solid that could reach 40 g l
1 at the end of the electrolysis reaction. These TSS were rapidly eliminated by simple sedimentation. After decantation, the obtained effluent has a

weak quantity of TSS (2 g l
1) in comparison with the decanted crude OMW (12 g l
1). The formation of suspended particles was caused presumably by electro-coagulation process. The polymers were precipitated with iron which was continuously dissolved into the wastewater from the cast iron anodes, as governed by the Faraday’s law (Pletcher and Walsh, 1990). This result confirms the hypothesis that the electro-Fenton reaction would have a strong ability to eliminate polyphenols from OMW. Furthermore, as can be seen in the Table 1, the concentration of lipids was decreased by 89.2%. The pH, COD, colouration, polyphenols and lipids removal were consistently very good. Indeed, the effluent quality of the pre-treated OMW by electro-Fenton process (S1, see Fig. 1) was rather excellent (Table 1). It could be directly fed as influent to anaerobic reactor.

3.2.

Anaerobic bio-treatments

3.2.1.

Anaerobic digestion of non pre-treated OMW

The anaerobic treatment of non pre-treated OMW was performed in a 3-l AF reactor. The yield of methanisation of

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this untreated diluted OMW was higher than 0.3 l CH4 g
1 COD introduced at low loading rates. However, since the 26th day, when the loading rate reached a mean of 4 g l
1 d
1 of COD, a decrease in the biogas production and yield was observed (Fig. 5A). This toxicity was accompanied by a pH decrease in the three levels of the reactor and an accumulation of the VFA (Fig. 6). This test of the anaerobic digestion of untreated OMW by an 8-year OMW-acclimated consortium will serve as a control for comparing the efficiency of the electro-Fenton pretreatment in the detoxification of this effluent.

Anaerobic digestion of electro-Fenton pre-treated OMW

The AF was loaded with undiluted pre-treated OMW at a starting loading rate of 2 g COD l
1 d
1. The reactor was operated at influent OMW concentration of 35.5 g COD l
1

10

10 Loading rate (g COD l-1 d-1)

(mean value). The hydraulic retention time (HRT) varied between 17.7 and 3.5 days. In general, the percentage of COD removal decreased with increased loading rate during the fermentation of OMW in the AF. The percentage of COD removal decreased from 88.8% to 68% when the organic loading rate increased from 2 to 10 g COD l
1 d
1 (Fig. 7). The evolutions of the loading rate, biogas productivity and methane yield are presented in Fig. 5B. At the higher loading rates (9–10 g COD l
1 d
1), the yields obtained were approximately 0.3 l CH4 g
1 COD introduced. The volume of biogas reached 12 l d
1 (4-fold of the volume of the digester). The higher values of yields (0.32–0.34 l CH4 g
1 COD introduced) were obtained for loading rates lower than 8 g COD l
1 d
1. In addition, Fig. 5Ba and Bc show that the methane yield increased with the increase of the loading rate

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1) (a), biogas production (l l
1) (b) and methane yield (l CH4 g
1 COD introduced) (c) during anaerobic digestion of crude OMW (A) and electro-Fenton pre-treated OMW (B).

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up to the theoretical steady yield value, reported to be around 0.35 l CH4 g
1 COD introduced. This observation can be considered a solid proof for the ability of the anaerobic biomass to degrade most organic matter present in the electro-Fenton-pre-treated OMW. Besides, it may confirm the gradual increase of the methanogenic activity. The biomethanisation process was found to be stable during 3 months of operation. No toxicity phenomenon was observed. VFAs have long been recognised as the most important intermediates in the anaerobic process and were proposed as a control parameter (Ahring et al., 1995; Mechichi and Sayadi, 2005). Therefore, VFA and pH were analysed in

20

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Fig. 8 – Evolution of pH (a) and concentration of VFA (b) in the anaerobic filter during the methanisation of electro-Fentonpre-treated OMW (A: bottom of the reactor, B: middle of the reactor, C: top of the reactor).

the three levels of the AF (Fig. 8). Level A is at the bottom of the reactor, level B corresponds to the middle and level C is in the top of the reactor. The pH at these three levels of the reactor was higher than 7.0 for all the loading rates applied. The VFA concentrations were low even at the higher loading rates. Knowing that untreated OMW causes inhibition of methanisation at a loading rate of 2–4 g COD l
1 d
1 (Kang and Chang, 1997; Hamdi, 1991; Rozzi et al., 1989, this work), it can be concluded that electro-Fenton of OMW resulted in decreasing the toxic effect of this wastewater on anaerobic digestion. Moreover, this experiment was stopped at a loading rate of 10 g COD l
1 d
1 while the biological process did not show any apparent toxicity. These results also suggest that anaerobic digestion can be a practical alternative for the treatment of OMW.

3.2.3.

Characterisation of the anaerobic effluent

The anaerobic effluent (S2) was characterised with common parameters (pH, COD, colouration, TSS, ortho-diphenols). Main results were plotted in Table 1. Results showed that the colouration and the residual COD (hardly biodegradable compounds) of S2 remained relatively high. The phytotoxicity test of OMW samples were carried out using the germination index (GI) of L. esculentum (tomato). Results showed that electro-Fenton treatment increased the GI percentage of L. esculentum from 4.4% (for crude OMW) to 30% while the anaerobic effluent led to an increase of the GI to

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Table 2 – Inhibition of Vibrio fisheri luminescence (IB) after exposure with different OMW samples during 15 min and the GI percentage of Lycopersicon esculentum OMW sample Untreated OMW Electro-Fenton-treated OMW (S1) Electro-Fenton anaerobic-treated OMW (S2) Electro-coagulated S2 (S3)

IB (%)

GI (%)

100 66.9 45.2 —

4.4 30 121 140

2015

The anaerobic process applied as post-treatment reached a loading rate of 10 g COD l
1 d
1 without any apparent toxicity. Finally, electro-coagulation of the anaerobic digestion effluent could be used as polishing step for improving the quality of the treated water for potential reuse.

Acknowledgements This research was supported by C.I.U.F. (Belgium), EEC Contract no. ICA3-CT-2002-00034 and Contract Programmes (MRSTDC, Tunisia).

121% compared to 100% for the control (Table 2). Indeed, approximately 43.9% of the ortho-diphenols were removed after anaerobic bio-treatment. As shown in Table 2, untreated OMW exercised 100% inhibition on V. fischeri. It was reduced to 66.9% after pretreatment by electro-Fenton and to 45.2% in the anaerobic effluent. Microtoxicity of S2 remained high due to the residual VFA. Yet, the characteristics of anaerobic effluent do not comply with legal requirements. To overcome this problem, a tertiary treatment step was necessary if we want to reach the Tunisian standard requirements. For this purpose, experiments of electro-coagulation of S2 were carried out using the same electro-Fenton reactor, in order to remove the residual polyphenols, COD and colour.

3.3. Improvement of the quality of the effluent using electro-coagulation The purpose of this part of study was directed to treat the anaerobic effluent (S2) by electro-coagulation process. During this process, when direct current passed though the Fe anodes, Fe2+ and Fe3+ correspondingly dissolved and combined with hydroxyl ions in the water. They formed metal hydroxyls ions, which are partly soluble in water under definite pH values and play the role of coagulant. The electro-coagulation step was performed at a current density of 1.8 A dm
2 and without adjustment of pH. The determination of the physico-chemical parameters of the electro-coagulated anaerobic effluent (S3) showed that the electrolysis process was able to remove 70.55% of TSS, 91% of the colour and 70% of the residual COD (Table 1). Moreover, the analysis of ortho-diphenols showed a removal efficiency of 97% while polyphenolic compounds were not detected. Hence, the final effluent (S3) was free of toxic compounds as can be seen in Table 1. Furthermore, the phytotoxicity of S3 was significantly reduced by the application of electrocoagulation, reaching 140% germination index (Table 2). As conclusion, the proposed process of OMW treatment reduces significantly its biotoxicity. For this, OMW can be used as fertiliser.

4.

Conclusion

The electro-Fenton method applied on raw OMW as pretreatment resulted in removing a large amount of recalcitrant polyphenolic compounds as well as in decreasing toxicity.

R E F E R E N C E S

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