Mesophilic and Thermophilic Anaerobic Co-digestion of Olive Mill Wastewatersand Abattoir Wastewaters in an Upflow Anaerobic Filter

Hana Gannoun, Nada Ben Othman, Hassib Bouallagui, and Hamdi Moktar*

Laboratory of Microbial Ecology and Technology, Department of Biological and Chemical Engineering,National Institute of Applied Sciences and Technology, BP 676, 1080, Tunisia

The mixture of olive mill wastewater (OMW) with abattoir wastewater (AW) induced the precipitation ofOMW phenolic compounds with AW proteins and the removal of chemical oxygen demand (COD). Anaerobicbatch experiments with different proportions of OMW:AW (v:v) (10:90; 20:80; 40:60; 60:40) showed thatthe 40:60 mixture can be co-digested in a continuous system because it improves the C/N ratio and reducesthe inhibition of methanogenic bacteria by phenolic compounds. The continuous co-digestion of this mixturewas tested under mesophilic (37°C) and thermophilic (55°C) conditions using an upflow anaerobic filter(UAF). The organic loading rates (OLRs) which could be achieved during the co-digestion were higher thanthose obtained during the anaerobic treatment of OMW alone. The change from a mesophilic to a thermophilicenvironment in the UAF was carried out with a short start-up of thermophilic condition. A higher discoloration,higher COD removal (80%), and biogas yield (0.52 L/g of COD removal) were obtained at 55°C and theperformance of the reactor was maintained even at an OLR as high as 12 g of COD/L‚d. The reduction offaecal coliforms achieved after digesting the influent was important in mesophilic (11-21 MPN/mL) andthermophilic conditions (0-11 MPN/mL) and influenced by the combined effect of the antimicrobial activityof OMW and thermophilic operating conditions. The enhanced performance obtained with the UAF treatingOMW:AW (40:60) could be attributed to the dilution of OMW with AW, which reduces the toxicity, and thethermophilic conditions, which improves the biodegradability.

Introduction

Olive oil and abattoir factories produce vast amounts of liquidand solid waste. In Mediterranean countries, treatment of OMWis becoming a serious environmental problem due to its highcontent of phenolic compounds, suspended solids, and itsresistance to biodegradation.1 Several methods have beenproposed for treating OMW such as physical and chemicaltreatment, coagulation, filtration, and evaporation in lagoons.However, the proposed treatments only partially solve theproblem.2-4 Therefore, great interest has been focused onbiological treatment of OMW as an alternative to the conven-tional treatment processes. The aerobic biodegradation of OMWis limited by auto-oxidation of phenolic compounds intorecalcitrant polyphenolic compounds with high molecularweight.4 Anaerobic treatment of OMW is considered to befeasible, but several difficulties were noted, due to the inhibitoryeffects toward methanogenic bacteria of the high concentrationof aromatic compounds and the lack of ammonia needed asnitrogen source for synthesis of bacterial biomass.5 Erguder etal.6 concluded that an anaerobic reactor treating OMW mustbe supplied with NH4Cl and KCl in addition to alkalinity(NaHCO3) in required amounts. Many anaerobic processes suchas anaerobic contact, “UASB reactor”, and anaerobic filters havebeen applied to treat diluted OMW but problems arose due tothe toxicity and biodegradability of this effluent and theacidification of reactors.7 Moreover, the dilution that is requiredin order to reduce OMW toxicity is not compatible with thewater resources scarcity in most Mediterranean countries.

Co-digestion of OMW with animal waste has been studied.5,8,9

It is one way to dilute toxicants and to supply missing nutrients.Several studies demonstrated that co-digestion of different

organic wastes showed a distinct increase in methane yield andthat individual waste streams could be combined as a substratefor more efficient treatment.10 AW contains a high concentrationof biodegradable organics mostly in the form of fats andproteins, sufficient alkalinity, adequate nitrogen, and micro-nutrients for bacterial growth. Then OMW could be mixed withAW in order to decrease the toxicity of phenol compounds andprovide a source of nitrogen needed to achieve a favorableCOD/N ratio. Indeed, preliminary work of mesophilic anaerobicco-digestion of OMW:AW in batch reactors showed that dilutionof OMW with AW reduced the toxicity and improved theanaerobic digestion.11

Performance of anaerobic biodegradation of the mixture ofOMW:AW should be improved in thermophilic conditions.Compared to a mesophilic anaerobic digestion (30-40 °C)thermophilic anaerobic digestion (50-60 °C) usually brings anacceleration of biochemical reactions, higher efficiency in thedegradation of organic matter, and the production of a loweramount and better quality of digested sludge.12 Moreover, mostoften a higher destruction of pathogenic organisms can beachieved at these temperatures.13,14

The aim of the present work was to investigate the anaerobicco-digestion of OMW and AW under mesophilic and thermo-philic conditions in order to evaluate the stability and theperformance of the upflow anaerobic filter (UAF).

Experimental Section

OMW and AW Sampling. Fresh OMW used in the presentstudy was obtained from an olive oil production plant locatedin the north of Tunisia, which uses a continuous process forextraction of olive oil. Because of the seasonal production andthe instability of the waste, it was stored at-20 °C. The AWwas collected from an abattoir factory (El Ouardia City, Tunis).Analysis of raw OMW and AW were carried out and the averagecomposition is shown in Table 1.

* Corresponding author. E-mail address: moktar.hamdi@insat.rnu.tn.Tel : 0021671703627. Fax: 0021671704329.

6737Ind. Eng. Chem. Res.2007,46, 6737-6743

10.1021/ie061676r CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 08/07/2007

Experimental Setup. (1) Anaerobic Batch Reactors.Dif-ferent proportions of OMW:AW (v/v) (10:90; 20:80; 40:60; 60:40) were digested in 1 L batch reactors to study their anaerobicbiodegradability and toxicity under mesophilic conditions (37°C). The inoculum used (pH) 6.93; TS) 4.15% w/v) wastaken from an active mesophilic digester treating OMW.

(2) Continuous System.The anaerobic digestion of theOMW:AW 40:60 was carried out in a UAF (Figure 1). Itconsisted of a vertical cylindrical glass column (60 cm lengthand 10 cm in diameter). The active liquid volume was 2 L.The digester was filled with Flocor (Φ3L3, porosity 95%,specific surface 230 m2‚m-3) as media support entities for thegrowth of microorganisms. The sludge issued from the batchtest (40:60) was used to inoculate the UAF. Sodium hydroxide(NaOH) solution was used for pH adjustment of the feed inbatch and continuous reactors to neutral value. Feed wassupplied by a peristaltic pump connected to a programmabletimer. Different OLRs were applied by varying HRT from 13.66to 3.33 days under mesophilic and thermophilic conditions.

Analytical Methods. Chemical oxygen demand (COD) wasmeasured spectrophotometrically.15 Total solids (TS), total

suspended solids (TSS), and total nitrogen were determinedaccording to the procedure listed in Standards Methods for theExamination of Water and Wastewater.16 The biogas producedwas collected daily in plastic bags at room temperature. Thetotal volume was later determined with a wet gas meter andtime to time the methane content was estimated using anORSAT apparatus. In this way the biogas volume productionsof mesophilic and thermophilic reactors were directly compa-rable. Volatile fatty acids (VFA) were measured by HPLC(Waters) equipped with a polypore H column (250 mm× 7.8mm [inside diameter]) connected to a differential refractometer(RI-401 Wates) and a CR-6A Shimadzu integrator. The mobilephase was 0.02 N H2SO4 at a flow rate of 0.6 mL‚min-1. Itwas centrifuged for 15 min at 13000 rpm and filtered througha 0.22µm filter (Millipore) before use. The volume of injectionwas 20µL. Discoloration was assayed by the measurement ofabsorbance at 390 nm (Jenway UV-visible spectrophotometer).Total polyphenol content was determined using the Folin-Ciocalteu method.17 To a 3.6 mL of sample appropriatelydiluted, 200 µL of Folin-Ciocalteu reagent was added andvortexed. After exactly 3 min, 800µL of sodium carbonate (20%w/v) was added, and the mixture was vortexed and allowed tostand at 100°C for 1 min. The absorbance was read at 750 nm,and the total polyphenol concentration was calculated from acalibration curve, using gallic acid as standard. The mostprobable number technique (MPN) was used to determine faecalcoliforms.18 Based on dilutions down to nearly 1 remainingbacterium per test tube (3-fold setups repeated two times), theexit concentration can be estimated statistically.

Results and Discussion

Effect of Pretreatment Assay on the Anaerobic BatchReactor Performances.The precipitation of phenolic com-pounds of OMW with the proteins of AW was studied aftermixing both effluents at different proportions of OMW:AW (10:

Table 1. Characteristics of Olive Mill Wastewaters (OMW),Abattoir Wastewaters (AW), and OMW:AW 40:60 Mixture afterSettling

parameterolive mill

wastewaterabattoir

wastewaterOMW:AW

40:60

pH 5.16( 0.09 7.5( 0.05 6.5( 0.3conductivity (ms/cm) 15.33( 3.5 7.26( 2 11.71( 1TS (g/L) 48( 2.5 5.44( 1.5 28( 3TSS (g/L) 6.5( 0.3 0.5( 0.1 1.8( 0.2COD (g/L) 110( 5 6 ( 1.5 41( 2total Kjeldhal nitrogen (g/L) 0.1( 0.02 0.81( 0.04 0.69( 0.01COD/N 50:0.045 50:6.75 50:0.84absorbance at 280µm 234( 9 94absorbance at 390µm 37.3( 2.5 15faecal coliforms (MPN/mL) 45.104 230

Figure 1. Schematic diagram of the pretreatment unit and the upflow anaerobic filter used for mesophilic and thermophilic anaerobic digestion of OMW:AW (40:60) in a continuous system.

6738 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

90; 20:80; 40:60; 60:40). The total suspended solids (TSS)formed by precipitation increased with the increase of OMWproportion (Figure 2). Different mechanisms of interactionbetween phenolic compounds and proteins may be involved suchas hydrogen bonding,19 covalent bonding,20 hydrophobic inter-actions,21 and ionic bonding in aqueous media.22 The removalof TSS by settling improved the water quality for all OMW:AW mixtures and in some cases a favorable COD:N ratio (COD/N, 50:1)23 could be obtained (Table 2).

The results with anaerobic batch tests (Table 3) showed thatfinal pH, biogas yield, and COD removal increased with dilutionof OMW by AW up to 40:60. The mixture of AW and OMWcan improve the anaerobic digestion because of the reductionof toxicity of OMW by dilution24 and the removal of phenolcompounds.4-25 COD removal obtained with the 0:100 and 10:90 mixture was lower than those obtained with 20:80, 40:60,and 60:40 mixtures probably as a result of the inhibitory effectof the initial nitrogen content of AW.26 The maximum CODremoval efficiency of 66% was obtained with the batch reactor

fed with the 40:60 mixture of OMW:AW for which the biogasyield remained stable compared to the 20:80 mixture. Inaddition, TSS removal and phenol reduction obtained aftersettling for this proportion (40:60) indicate that it can alsoenhance the stability process (Figure 2, Table 2).

According to the results obtained with pretreatment assay andbatch reactors, it could be concluded that the co-digestion ofthese two wastes is more advantageous than processing eachone separately. The 40:60 mixture was selected for the experi-ments with continuous reactors because it provides the necessarynutrients and buffer capacity.

Mesophilic Anaerobic Digestion of OMW:AW in UAF.The UAF was fed initially with an organic loading rate (OLR)of 3 g COD/L‚d corresponding to a hydraulic retention time(HRT) of 13.66 days. The OLR was increased gradually byvarying the HRT, from this value to 4.5 days (OLR) 9 g COD/L‚d) (Figure 3). The influent pH remained in a range of 5.8-6.8 during all of the process. At the HRT of 13.66 days, a COD

Figure 2. Total suspended solids (TSS) and soluble COD contents ofOMW:AW mixture after precipitation but without TSS removal (9) andafter precipitation and TSS removal by sedimentation (0).

Table 2. Characteristics of OMW:AW Mixture before and afterPrecipitation Sedimentationa

10:90 20:80 40:60 60:40

Different Mixtures of (OMW:AW)pH 7.02 6.5 6 5.38COD (g/L) 21 34 41 73total phenol (g/L) 0.51 1.02 2.04 3.06COD:N 50:2.5 50:1.27 50:0.55 50:0.28

After Precipitation, SedimentationCOD removal (%) 26.6 20 13.5 9total phenol removal (%) 22 17 12 5.23COD:N 50:1.67 50:0.67 50:0.31

a Mesophilic anaerobic digestion of OMW:AW in UAF.

Table 3. Performance Data of Mesophilic Anaerobic Batch Reactorsof Different Mixtures of OMW:AW after 10 days of Digestion

OMW:AW 0:100 10:90 20:80 40:60 60:40 100:0

final pH 7.2 7.5 7.3 7.01 6.4 4.6conductivity (mS/cm) 4.8 5.18 9.78 9.01COD removal (%) 30 28 43 66 58 12discoloration (%) 34.6 55.3 29.5 41.8total phenol removal (%) 52.5 35.4 29 30.5Y (L biogas/g of COD

removal)0.112 0.125 0.201 0.189 0.04

Figure 3. Effect of loading rate by varying HRT on pH variation of theinfluent (9) and effluent (0); the CODinlet (O), the CODoutlet (b), and thebiogas production rate (2) during anaerobic digestion of the mixture ofOMW:AW in UAF at mesophilic temperature.

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076739

removal of 75% was obtained. COD removal remained stablebetween 70 and 75% when HRT was reduced to 10 days (OLR) 4.1 g COD/L‚d). This indicates that the mesophilic digestershowed initially stable conditions, although it presented a VFAconcentration of 3000 mg/L (Table 4). Despite this VFA amount,no pH drop was observed thanks to the ammonium coming fromAW. Evidence of process upset due to an organic overloadingwas observed when the VFA concentration rose to 5000 mg/Lat an OLR of 9 g COD/L‚d (Table 4) and biogas productiondecreased (Figure 3). The anaerobic process was overloadedand the VFAs were built up due to the faster growth rate of themesophilic VFA producers compared to the mesophilic VFAconsumers. In addition, the acetogenic and methanogenicbacteria may be more inhibited by phenolic compounds thanthe acidogenic bacteria.24 The OLR reached with anaerobic co-digestion of the mixture OMW:AW was higher than thatobtained with OMW alone.4 This is probably due to the toxiceffect of undiluted OMW.27 It has been well-established thatphenolic compounds are major contributors to the toxicity andthat their antibacterial activity limits the anaerobic biodegrad-ability.24,28Biogas production rate was improved by the increaseof the OLR up to 6 g COD/L‚d; it averaged 1.5 (70% of CH4)to 4.8 L/d (67% of CH4) at an HRT of 13.66 and 6.7 days,respectively. The increase in biogas production was in someway compensated by a lower methane percentage.

Thermophilic Anaerobic Digestion of OMW:AW in UAF.The temperature was increased in one step from 37 to 55°Cwith a simultaneous decrease of the OLR from 9 to 4.1 g COD/L‚d in order to have a fast start-up of the process. The digesterstability was reached after a week, and the OLR was thenincreased gradually from 4.1 g COD/L‚d to 12 g COD/L‚d(Figure 4). The reactor effluent pH remained between 7.2 and8.4 during all of the study. The increase of pH from the influentto the effluent can be due to the degradation of protidiccompounds. In fact, changing from mesophilic to thermophilicconditions usually results in an increase in protein hydrolysiswhich in turns leads to an increase in the buffer capacity of thesystem. This result is in agreement with previous studies.29-31

For all OLR applied, the COD removal efficiencies were quitehigh. Indeed, a COD removal of 80% was obtained at organicloads of 8.2 g COD/L‚d and remained constant at both OLRsof 10 and 12 g COD/L‚d. The one-step increment of the reactortemperature was followed by an increase of the biogas produc-tion rate which averaged 2.75, 3, 4.15, and 3.9 L biogas/L‚d atan OLR of 6, 8.2, 10, and 12 g COD/L‚d, respectively. Theanaerobic filter showed a little sensitivity to the increase of theOLR under thermophilic conditions. These results are inaccordance with those reported by van Lier32 and Kim et al.33

The biogas production rate obtained in thermophilic condi-tions was satisfactory and the performance of the reactor wasshown to be stable, but the methane content decreased from 75to 70% with the increase of the OLR from 4.1 g COD/L‚d to12 g COD/L‚d, respectively. This is thought to be due to the

increase of inhibitory effect of the recalcitrant phenolic com-pounds on methanogenic bacteria.34

Evolution of Performances of a UAF Treating a Mixtureof OMW:AW (40:60) from Mesophilic to ThermophilicConditions. The mesophilic and thermophilic anaerobic diges-tion are characterized by an optimal temperature range andoverpassing the upper limit would cause an immediate deathof the considered group of bacteria.35 Since the thermophilicmicroorganisms are the main interest in the thermophilicanaerobic digestion, the transition from mesophilic to thermo-philic temperature may therefore require a long acclimationperiod. Different strategies of adapting mesophilic reactors tothermophilic temperature have been described in the literatureat constant HRT and OLR: one-step36,37or a stepwise temper-ature increase.38 Nevertheless, there is a lack of studies wherethe adaptation of stable mesophilic reactor to thermophilictemperature is applied in a one-step increase of temperature atdifferent OLR and HRT in order to establish the best digestingcapacity with the shortest adaptation time to thermophilicoperation. According to Bousˇkovaet al.,35 a one-step increasingtemperature from mesophilic to thermophilic is the best strategy

Table 4. Decolorization Removal, Faecal Coliform, and Total VFAObtained with Mesophilic and Thermophilic Anaerobic Co-digestionof OMW:AW at Different OLRs

Different Organic Loading Rates (OLRs: g/L‚d)

Mesophilic Thermophilic

3 4.1 6 9 4.1 6 8.2 10 12

faecal coliform(MPN/mL)

21 20 16 11 10 8 5 0 0

discoloration (%) 40 43 42 43 45 49 50 66 62total VFA (mg/L) 3000 3200 3260 5000 1500 1800 2040 2300 3700

Figure 4. Effect of loading rate by varying the HRT on pH variation ofthe influent (9) and effluent (0); the CODinlet (O), the CODoutlet (b), andthe biogas production rate (2) during anaerobic digestion of the mixture ofOMW:AW in UAF at thermophilic temperature.

6740 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

in changing operational temperature in anaerobic digestion. Thefast adaptation of the mesophilic sludge to the thermophilicconditions indicates the presence of thermophilic microorgan-isms in the mesophilic inoculum. In a study on characterizationof the microbial community during the start-up of a thermophilicanaerobic digester, Chachkhiani et al.39 concluded that thedominant species taking part in the anaerobic thermophilicdigestion were not adapted mesophiles but the thermophilesalready present in the inoculum at a subdominant level whichquickly became dominant under thermophilic anaerobic condi-tions. As expected from the previous reports and also confirmingthem, the one-step increase of temperature coupled to a reductionof OLR was very efficient in our case since it allowed us toreach good and stable performances in only 1 week after theshifts.

COD removal, biogas production, and biogas yield obtainedwith anaerobic co-digestion of OMW:AW under mesophilic andthermophilic conditions in UAF are used to evaluate the processefficiencies (Figure 5). The COD removal efficiency of thethermophilic anaerobic filter was greater than the mesophilicanaerobic filter treating the mixture of OMW:AW for the HRTvarying from 4.5 to 6.8 days. Maximum COD removal wasobtained at an HRT of 10 days for the mesophilic anaerobicfilter while the thermophilic anaerobic filter presented only slight

changes for the different HRT studied. The COD efficienciesobtained with mesophilic and thermophilic anaerobic co-digestion showed that at high HRT (13.66, 10, and 6.8 days),COD removal was limited to 80%. As could be expected, thisis probably the result of the presence of recalcitrant compoundslike pseudolignin and condensed tannins in OMW.24

The highest biogas production rate (4 L/L‚d) was obtainedwith thermophilic temperature at an HRT of 4 days (Figures 4and 5). It has been shown that biogas production rates are higherunder thermophilic conditions than under mesophilic conditions.This is mainly due to a higher maximum specific growth rate(2-3 times) of thermophilic microorganisms compared tomesophilic analogues.40 The biogas yield presented stablebehavior at mesophilic temperature at a level varying from 0.3to 0.35 L/g COD removed following the perturbation of HRTfrom 13.66 to 4.5 days (Figure 5). The thermophilic anaerobicdigestion induced a little increase of the biogas yield from 0.5to 0.52 L/g COD removed at an HRT of 5 and 4 days,respectively. The thermophilic digester remained stable at anHRT of 3.33 days (OLR) 12 g COD/L‚d) and still presenteda biogas with a methane content of 70%. In contrast, themesophilic digester was already overloaded at an HRT of 4.5days. At this HRT, the biogas production had a lag phase, andthe methane content was less than 65%. This is in agreementwith several works in the literature which have shown thatthermophilic systems are capable of achieving higher organicloadings compared to their mesophilic counterparts.41,42

Table 4 summarizes the color removal, the faecal coliformreduction, and the VFAs of the mesophilic and thermophilicanaerobic co-digestion of OMW:AW. The coliform removalobtained under mesophilic conditions did not present significantdifferences at the different OLRs tested. The thermophilicprocess was apparently more efficient in the destruction ofcoliforms at the high OLR (10 and 12 g COD/L‚d) (MPN/mL) 0). The important faecal coliform reduction achieved couldbe attributed to the combined effect of the anaerobic environ-ment, temperature,43 and the antimicrobial activity of OMW.44

The digester VFA levels were higher at 37°C than at 55°C.This seems to suggest that methanogenic bacteria with higheraffinity for VFA were selected and dominated in the thermo-philic digester after the temperature increase. The high bufferingcapacity in the thermophilic digester was attributable to theenhanced degradation of nitrogenous compounds. This issupported by previously reported comparative studies of meso-philic and thermophilic co-phase anaerobic digestion.31

The discoloration correlated to the removal of phenoliccompounds was improved at the higher OLR in thermophilicconditions (Table 4). Converti et al.45 suggest that the thermo-philic microflora have the capacity to use more carbon sourcesthan the mesophilic and psychrophilic microflora. Moreover,with higher OLR, biodegradation of several compounds can beimproved thanks to cosubstrate assimilation.46

The dark effluent from the anaerobic co-digestion of OMW:AW still contained a significant concentration of COD, due tothe non-degraded residual phenolic compounds as mentionedby Hamdi.4 Previous works on the anaerobic treatment of OMWhave shown that a chemical47 or a biological48 post-treatmentis capable of removing if not degrading these residual aromatics.

Conclusions and Practical Strategy for the Managementof Olive Mill and Abattoir Wastewaters. This investigationhas shown that the anaerobic co-digestion of OMW and AW isa technically viable solution for these wastes since their mixingreduces the main problems encountered during their separate

Figure 5. COD removal, biogas production rate, and biogas yield obtainedwith the upflow anaerobic filter treating AW:OMW in mesophilic (9) andthermophilic (0) conditions at various HRTs.

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076741

anaerobic treatment: polyphenol toxicity for OMW and am-monium toxicity resulting from protein hydrolysis for AW.Moreover, co-digestion gives more energy thanks to the increaseof the COD:N ratio which depends on the OMW:AW ratio.The OMW:AW ratio, which allows an optimal precipitation ofphenolic compounds and proteins, must be adjusted as a functionof the amount of phenolic compounds and proteins present inOMW and AW, respectively.

This OMW and AW treatment solution will be possible atthe condition to have a strategy of constructing olive mills andslaughterhouses in the same industrial complex in order toreduce transport costs. Presently, OMW, which is produced inlarge quantities over a short period of the year (3-5 months),is stored in evaporation basins, a practice with a lot of negativeimpacts for the environment. The mixing of AW produced allover the year with stored OMW will allow detoxification ofOMW and implementation of an efficient and stable anaerobicco-digestion. Thanks to the organic carbon provided by OMWthe co-digestion of these two wastes should produce enoughenergy to operate the installation and this both under mesophilicand thermophilic conditions.

From a sanitary point of view, in addition to the effect of theprecipitation step, anaerobic co-digestion will contribute to thedisinfection of AW particularly when performed under ther-mophilic conditions.

The solids generated during the precipitation and recoveredby settling represent an organic waste which can be dischargedin sanitary landfills with a minimum of biological risks due tothe fact that the precipitated bacteria are killed by the OMWphenols. The quality of this solid could be improved by aerobiccomposting or anaerobic digestion after mixing it or not withother solid wastes.

Acknowledgment

The authors thank Prof. Sami Sayadi for his help in HPLCanalysis and the anonymous reviewers of this paper for theirconstructive criticism which allowed them to improve theoriginal manuscript and lead to the proposal of some advicefor the management of OMW and AW.

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ReceiVed for reView December 27, 2006ReVised manuscript receiVed June 9, 2007

AcceptedJune 12, 2007

IE061676R

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