anaerobic digestion of aegean olive mill effluents with and without pretreatment

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Research ArticleReceived: 23 December 2009 Revised: 19 February 2010 Accepted: 21 February 2010 Published online in Wiley Interscience: 26 April 2010

(www.interscience.wiley.com) DOI 10.1002/jctb.2390

Anaerobic digestion of Aegean olive milleffluents with and without pretreatmentGulseren Pekin, Senem Haskok, Sayit Sargın, Yuksel Gezgin, Rengin Eltem,∗Erdinc Ikizoglu, Nuri Azbar and Fazilet Vardar Sukan

Abstract

BACKGROUND: Olive oil production is an important economical activity in the Aegean region of Turkey. However, the effluentsof the olive oil producing mills with their high organic loads and toxic compounds are causing serious environmental problems.The anaerobic biological treatment of olive mill wastewater (OMWW) using the treatment plants of the regional industriescould be a method of choice and within the scope of this study floccular and granular sludges were investigated in batch modefor their success in the treatment of OMWW while producing biogas.

The major limitation of this treatment is the inhibition of methanogenic bacteria by the phenolic compounds in OMWW. Thusan integrated solution was suggested in which a pre-treatment step (dephenolization) was also introduced before biologicalstep.

RESULTS: The effluents of 27 olive mills out of 47 were found to have total phenolics (TP) less than 3 g L−1 and could betreated anaerobically after simple dilution. The biogas production for the untreated OMWW was higher for floccular sludgethan for the granular sludge (68.5 mL and 45.7 mL respectively). Combined pre-treatment experiments, first coagulation withpolyaluminum chloride, followed by flocculation with cationic polyelectrolyte and finally Fenton’s oxidation, could remove80% of TP and 95% of the total suspended solids.

CONCLUSION: OMWW having TP values less than 3 g L−1 can be treated anaerobically using floccular sludge after simple dilutionand biogas can be produced. For OMWW samples having higher TP values pre-treatment is necessary and the pre-treatmentgiven in this study may be used effectively.c© 2010 Society of Chemical Industry

Keywords: olive mill wastewater; treatment; polyelectrolyte; anaerobic digestion; biogas

INTRODUCTIONTurkey is one of the important olive oil producers in Europe andolive oil is produced seasonally by a large number of small olivemills scattered mainly in the Aegean region of Turkey. The oliveoil production mills are small agro-industrial units producing notonly olive oil but also agro-waste, namely olive mill wastewater(OMWW), which is a problematic effluent having a serious negativeimpact on the environment by polluting soil and water on theAegean coast.

The problems arising from OMWW derive from its high organicload (biochemical oxygen demand (BOD) 35–110 g L−1, chemicaloxygen demand (COD) 45–170 g L−1, total suspended solids(TSS) 1–9 g L−1) and dark colour caused by the organic fractioncontaining recalcitrant compounds (lignins and tannins) and thepresence of phenolic compounds. These phenolic compounds areeither originally synthesized by the olive plants as a defence againstpathogens,1 or formed during the olive oil extraction process,2

and pass into the OMWW due to their high water solubility.3 Theconcentration of phenolic compounds in OMWW varies greatlyfrom 0.002 to 80 g L−1.4

Olive oil phenols show a range of antioxidant, functional,nutritional and anticancer properties,5 while at the same time,ironically, olive oil phenols in the OMWW are reported to be toxicto microorganisms because they inhibit microbial growth6 and

are toxic to plants due to their growth/germination inhibitorycharacteristics.7 Thus dephenolization of OMWW prior to itsdisposal is necessary to eliminate the polluting effects of phenolson soil and water.8

Among the different options, biological treatments, bothaerobic and anaerobic, are considered the most environmentallycompatible and the least expensive.16

Several dephenolization methods have been proposedfor OMWW such as coagulation with lime and alum,9

electrocoagulation,10 flocculation using polyelectrolytes,11 chemi-cal oxidation with Fenton’s reagent,12 their various combinations13

and biological treatments.14,15

Anaerobic biological processes are particularly advisable be-cause of their well-known advantages related to energy andchemicals saving17 and to the low production of sludge, especiallywhen it comes to treatment of high COD wastewaters. More-over, the seasonal nature of the operation of olive mills (typicallyNovember to February) is not a disadvantage for anaerobic pro-cesses because the observed decay rates for methanogens are very

∗ Correspondence to: Rengin Eltem, Ege University, Department of Bioengineer-ing, 35100 Bornova-Izmir, Turkey. E-mail: [email protected]

Ege University, Department of Bioengineering, 35100 Bornova-Izmir, Turkey

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Anaerobic digestion of Aegean olive mill effluents www.soci.org

low and a digester can easily be restarted after several monthsof shut-down period.18 However, OMWW is a problematic sub-strate for anaerobic digestion and requires pre-treatment becauseof its high organic load and the presence of inhibitory com-pounds. The various pre-treatment methods together with theirdephenolization efficiencies have recently been reviewed.4,19,20

The aim of this study was first, the dephenolization of theAegean region OMWWs and second, the production of biogas.Effluents from 47 Aegean olive mills were monitored, collectedand analysed for their total phenols, thus aiming to prepare usefulreference data for the Aegean coast OMWW with respect to wastesand their phenolic content. The commercial polyelectrolytes usedin the OMWW pre-treatment experiments have been tested for thefirst time for their flocculation efficiencies. During studies aimed atthe production of biogas, the efficiencies of floccular and granularsludges were compared, to the best of our knowledge for the firsttime in OMWW studies.

MATERIALS AND METHODSMaterialsFeSO4.7H2O was supplied from Merck (Germany), H2O2 wassupplied as 35% from Fluka (Germany). PAC was supplied from SNFFloerger (France). pH was adjusted manually using 10% Ca(OH)2

and 0.1N H2SO4 and these chemicals were supplied from localsuppliers and Merck respectively.

Five cationic (CP1, CP2, CP3, CP4 and CP5); five anionic (AP1,AP2, AP3, AP4 and AP5) and one nonionic (NP) polyelectrolyteswere obtained from SNF Floerger. In all cases, the appropriateamount of polymer was dissolved in de-ionized water to give 0.1%w/v stock solutions. This was done since addition of polymersdirectly into the effluent proved difficult owing to their moderatesolubility in water.

Wastewater (OMWW)OMWW samples were collected from 47 olive oil mills in the Aegeanregion of Turkey. The map given in Fig. 1 shows the location ofthese mills together with their processing systems. As is seen fromFig. 1, the olive mills have high geographic scattering. The majorityof them (29) use three-phase production technology; 18 use batchproduction technology, while only two use a two-phase system.

The OMWW samples from the monitored mills were firstanalyzed for their TP contents and then stored at −18 ◦C.

Pre-treatment experimentsOMWW samples were pre-treated with a combined treatment(first coagulation; followed by flocculation, and finally Fenton’soxidation) and the samples obtained at the end of thesepretreatment experiments were further degraded biologicallyby anaerobic microorganisms. OMWW sample W1 having initialTP and initial TSS 2.904 ± 0.18 g L−1 and 8800 ± 960 mg L−1,respectively, was used. All experiments were run in triplicate andmean values are reported.

Coagulation experimentsFor coagulation experiments, an appropriate amount of coagulantwas added to a 400 mL OMWW sample having a pre-determinedpH value of 8.5 (data not shown) and the sample was left to settlefor 2 h, and at the end of the sedimentation period the supernatantwas analyzed for its TSS and TP contents.

Experiments coupling coagulation with flocculationFor experiments in which coagulation was followed by flocculation,50 mL of 0.1% polyelectrolyte solution (11 different polyelec-trolytes were used; one nonionic, five anionic and five cationic)was added to the 400 mL PAC coagulated OMWW sample, the

Figure 1. The geographical distribution of olive oil production plants which have provided OMWW samples. (�) three -phase, (�) two-phase, (�) batch.

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sample was left to settle for 1.5 h and then the supernatant wasanalyzed for TP and TSS contents.

Experiments coupling coagulation-flocculation with Fenton’soxidationFor the experiments in which coagulation–flocculation wasfollowed by Fenton’s oxidation, the pH of the supernatant ofthe coagulated and flocculated sample was adjusted to 3.0 withsulphuric acid. A pre-determined optimum amount (2.8 g L−1) ofFeSO4.7H2O and a pre-determined optimum volume of 50% H2O2

solution (10 mL) were added to the sample and the sample wasleft for 20 min and then TP was determined.

Analytical methodsThe TP content was determined according to a modification of theFolin–Coicalteau method detailed elsewhere.21 Total suspendedsolids (TSS) and COD were measured in accordance with StandardMethods.22 Gas produced in each serum bottle was measuredusing a gas displacement device.

Biological treatmentsThe effluents of pre-treated OMWW samples were further de-phenolized biologically. Biological treatment was done anaero-bically where biochemical methane potential (BMP) assay wascarried out and cumulative biogas production was monitored.

InoculaAnaerobic mixed cultures obtained from industrial granular andfloccular sludges were used as inocula for the BMP test. Granularsludge was taken from a beer factory (Anadolu Efes Biracılık ve MaltSan. A.S. Izmir) from the upflow anaerobic sludge blanket (UASB)reactor of its wastewater treatment plant, and floccular sludge wastaken from the UASB reactor of the wastewater treatment plant ofBeaker’s Yeast Plant (Pakmaya-Pak Gıda Uretim ve Pazarlama A.S.Izmir). Initial activity of the inocula were controlled by using sugarsolution and both sludges were found to be initially active withgranular sludge having higher COD removals (83.3% for granularsludge; 69% for floccular sludge) (data not shown).

Basal mediumA defined basal medium that contained all the necessary micro-and macro-nutrients for optimum anaerobic microbial growth,composed of 1200 mg L−1 NH4Cl, 400 mg L−1 MgSO4· 7H2O,400 mg L−1 KCl, 300 mg L−1 Na2S· 9H2O, 50 mg L−1 CaCl2 2H2O,80 mg L−1 (NH4)2HPO4, 40 mg L−1 FeCl2· 4H2O, 10 mg L−1 CoCl2·6H2O, 10 mg L−1 KI, 0.5 mg L−1 MnCl2· 4H2O, 0.5 mg L−1 CuCl2·2H2O, 0.5 mg L−1 ZnCl2, 0.5 mg L−1 AlCl3· 6H2O, 0.5 mg L−1

NaMoO4· 2H2O, 0.5 mg L−1 H3BO3, 0.5 mg L−1 NiCl2· 6H2O,0.5 mg L−1 NaWO4· 2H2O, 0.5 mg L−1 Na2SeO3, 10 mg L−1 Cys-teine, 6000 mg L−1 NaHCO3.23

Biochemical methane potential assay (BMP)Biochemical methane potential assay was used23 to monitor theanaerobic biodegradability. 100 mL serum bottles were seededeither with 10 mL of granular (TSS: 69600 mg L−1) or 10 mL offloccular (TSS: 15400 mg L−1) industrial sludges. OMWW sampleswere added (up to 20 mL) depending on the pre-determinedinitial COD concentration (10000 mg L−1) into serum bottles andthe total working volume of the reactor was made up 40 mL with

Table 1. Ranges of total phenol (TP) content obtained from olive millwaste water (OMWW) samples used in various studies

TP values of OMWW(g L−1) References

3.176-3.256 2002 samples of this study

2.328- 3.568 2003 samples of this study

2.848- 4.872 2004 samples of this study

5.0–80 Hamdi (1996)14

0.98- 10.7 Roig et al. (2006)19

17.7–21.1 Obied et al. (2007)25

basal medium. All bottles were purged with a gas mixture of 75%of N2 and 25% of CO2 for 4 min to provide anaerobic conditionsand were then capped with rubber stoppers and sealed withaluminum covers. The serum bottles were then incubated at 35 ◦Cin an incubator. The total gas production was recorded eachday using a hypodermic needle connected to a calibrated fluidreservoir through the serum cap. The methane content of biogaswas determined as described in detail elsewhere.24 All experimentswere run in triplicate and the mean values of net biogas productionare reported. The untreated (raw) OMWW sample used in BMPassay (W2) had 2.848 g L−1 TP and 103.500 g L−1 COD initially.

BMP assay was applied to the following OMWW samples: GW2(raw W2 treated using granular sludge); FW2 (raw W2 treatedusing floccular sludge); GC (PAC coagulated + CP3 flocculatedW2, treated using granular sludge); FC (PAC coagulated + CP3flocculated W2, treated using floccular sludge); GF (PAC coagulated+CP3 flocculated and Fenton’s oxidized W2, treated using granularsludge); FF (PAC coagulated + CP3 flocculated and Fenton’soxidized W2, treated using floccular sludge).

RESULTS AND DISCUSSIONCharacterization of OMWW samplesFor the characterization studies, first, TP values of OMWW sampleswere determined. The data given in Table 1 compares the TPrange obtained in this study with some other TP values from theliterature.14,19,25

TP values from batch and three-phase systems were comparedto determine the effect of processing technique on TP values.The average TP values of three-phase systems were found to beconsiderably higher than those of batch systems (4.03±1.99 gL−1

and 1.86 ± 0.80 gL−1, respectively). All the olive mills using batchproduction systems (18) had effluents with TP values below 3 g L−1,while only nine of the olive mills using three-phase productiontechnology had effluents with TP values below 3 g L−1.

TP values were also monitored within the same plant (Bornovaolive oil plant) for 3 years.

As is seen in Table 1, although the olives were from the sameregion and the processing technique was the same, TP valueswithin the same plant were not constant and varied from 2.84 g L−1

to 4.872 g L−1, implying that seasonal conditions and cultivationmethods also affected the TP values.

Pre-treatment experimentsThe main objective of this research was to minimize the negativeenvironmental impacts of OMWW in the Aegean region of Turkeyby developing a suitable OMWW treatment method. Anaerobictreatment of OMWW was seen as the method of choice, since

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Figure 2. The effect of coagulant concentration of PAC on TP and TSSremoval. ( ) TP; (�) TSS.

the treated effluent could be re-used in irrigation, and the biogasproduced could supply a substantial portion of the energy needs ofthe treatment plant.26 The major limitation of anaerobic treatmentis the inhibition of methanogenic bacteria by phenolic compoundsand organic acids present in OMWW. In previous studies, a highlevel of phenolic compounds was identified as a limiting factorfor the anaerobic digestion of olive mill waste.27 To eliminatethis inhibition, a combined pre-treatment was introduced prior tothe anaerobic treatment. Pre-treatment studies were conductedstarting with coagulation and flocculation. These two treatmentstages aimed at removal of the suspended and colloidal fractions(pectins, proteins, oils and tannins). The final step of the pre-treatment was Fenton’s oxidation, this step being aimed at thedephenolization of the coagulated and flocculated OMWW priorto the anaerobic treatment stage.

Experiments with coagulantsIn the coagulation experiments PAC was used. To determine theoptimum coagulant dosage, W1 was treated with various dosagesof PAC and the dephenolization and coagulation of supernatantswere compared. Figure 2 shows the effect of coagulant dosageon TP and TSS removal. Increasing the amount of PAC addedfrom 1.25 g L−1 to 5.00 g L−1 increased TP removal from 26% to40%. Increasing PAC concentrations also affected TSS removal,decreasing it from 86% to 50%. At the end of the coagulationstudies, TP and TSS removals at the optimum PAC concentration(1.25 g L−1) were determined as 26% and 86%, respectively. Thehigh value of TSS removal (86%) may be due to the pH of thesolution (8.5) since PAC is a coagulant particularly effective for pHvalues above neutral.28 This result is in agreement with others inthe literature. Ginos et al.13 studied the coagulation capacity ofPAC in OMWW and found 20% TP and 60% TSS removals at a lowersolution pH (4.7).

Combined use of polyelectrolytes and PACThe combined use of polyelectrolytes and PAC is compared inFig. 3. The cationic polyelectrolyte CP3 gave the highest TSSremoval (95%) together with the highest TP removal (40%). Recentreports on polyelectrolytes parallel the TSS removal capacity (95%)found in this study. Gomec et al.29 observed 95% TSS removalin OMWW when combined with acid cracking and Sarika et al.11

showed that polyelectrolytes were generally capable of completely(100%) removing the TSS in OMWW.

Figure 3. TP and TSS removal efficiencies of various polyelectrolytes.( ) TP; (�) TSS.

As is depicted in Fig. 3 cationic, anionic and nonionic poly-electrolytes showed varying TSS and TP removals, with amaximum TP removal of 40%, which was lower than re-ported in other studies. This may be due to the working pH(8.5) of this study and the presence of coagulant (PAC). Gi-nos et al.13 studied the TP-removing capacity of anionic andcationic polyelectrolytes, showing that type and concentrationof the coupled coagulant and the pH of the solution haddetrimental effects on separation and gave varying TP removalvalues.

In the next step, the optimum concentration of polyelectrolyte(CP3) to be used was determined in the presence of theinorganic coagulant (PAC). Increasing amounts of CP3 wereadded to PAC- treated W1 samples. The best treatment of W1was achieved with a dosage of 100 mg L−1 CP3 (64% TP and95%TSS). The result of polyelectrolyte flocculation experimentsindicated that CP3 was a promising polyelectrolyte and couldbe used successfully in OMWW treatment experiments. Tothe best of our knowledge, the polyelectrolytes tested inthis study were used for the first time in OMWW treatmentstudies.

Fenton’s oxidation experiments combined with coagulationand flocculationFenton’s reaction is very efficient as a pre-treatment stepfor degradation of polyphenolic compounds in OMWW. Thedegradation of polyphenolic compounds by Fenton’s reac-tion ranges from 80 to 95%.30 In this study, combinedcoagulation and flocculation studies were observed to re-move only 64% TP, thus Fenton’s oxidation was added toachieve higher TP removal. Pre-treatment using Fenton’s oxi-dation were conducted to decrease the TP content of OMWWand also to improve the anaerobic biological degraation ofOMWW.

The optimum Fe2+ concentration and the optimum H2O2

concentration for Fenton’s oxidation were determined as 2.8 gL−1

and 10 mL at 50%, respectively (data not shown).The experiments coupling the separation with Fenton’s oxida-

tion gave 80% TP removal. This result was in accordance with theliterature.13

Biological degradation experimentsThe Aegean region olive mill plants are small–scale enterprisesand cannot afford to build their own anaerobic OMWW treatment


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plants. One alternative is constructing a central biologicaltreatment plant for the area and then forcing the OMWW producersby some legal means to convey their effluents to the treatmentplant. However, such a plant will only work for 3–4 months a yearand may not be feasible. In this study the active sludges were takenfrom the anaerobic wastewater treatment plants of the regionalindustries to investigate the possibility of utilizing these treatmentunits during the OMWW production months. The granular andfloccular sludges were obtained from the regional industries andtheir efficiencies for OMWW treatment were compared.

Characterization of the OMWW samples used in biological treatmentstudiesThe choice of the OMWW sample to be treated for biogasproduction was critical, since according to findings in the literature,an OMWW sample having TP values above 3 g L−1 cannot be usedfor methane gas production due to toxic effects.31 Taking this factinto consideration and also the fact that 27 out of 47 olive millswere producing OMWW with TP values less than 3 g L−1, an OMWWsample (W2) having initial TP value just below 3 g L1(2.848 g L−1)was chosen as the sample for biogas production assays. Theexperiment also aimed to determine whether a pre-treatment wasneeded for OMWW samples having TP values below 3 g L−1or not.

Biogas productionThe BMP assay was carried out using both granular and floccularsludges and cumulative biogas production was monitored dailyfor 81 days. Figure 4 and Fig. 5 show the change in cumulative gasproduction with respect to time for granular and floccular inocula,respectively.

Biogas production was lowest for the GW2 bioreactor (45.7 mL)indicating possible inhibition (Fig. 4) due to the presence ofpolyphenols. Although the untreated sample was chosen to haveTP less than the critical value (3 g L−1) the composition of thesingle phenolic compounds might be posing a problem as well asthe composition of the bacterial consortium within the granularsludge.32

As is shown in Fig. 4 the biogas production of the pretreatedsamples, in the GC reactor and the GF reactor were higher than theGW2, which suggests that pre-treatment of OMWW as carried outin this study increases the anaerobic biodegradability of OMWW.Other researchers who carried out physico-chemical pre-treatmentstudies confirm this interpretation.33

Figure 4. Cumulative biogas production in granular sludge inoculumbased (�) GW2; (�) GC; (�) GF; (�) GControl; (�) GControl-basal medium;bioreactors.

Figure 5. Cumulative biogas production in floccular sludge inoculumbased (�) FW2; (�) FC; (�) FF; (�) FControl; (�) FControl-basal medium;bioreactors.

Figure 6. Cumulative biogas production (mL) versus time (days) of (�)granular and (♦) floccular sludge.

Since the amount of biogas produced using the pretreatedsamples in the GC and GF reactors were not significantly different(67.1 mL and 65.5 mL, respectively), it was concluded that Fenton’soxidation may not be necessary subsequent to PAC coagulationand polyelectrolyte flocculation. This was an important findingsince Fenton’s oxidation is an expensive pre-treatment, with theassociated safety hazards of using H2O2 and pH adjustment.34

Figure 5 compares the average cumulative net gas readingsof the floccular-sludge-containing reactors where the BMP assayswere monitored for FW2, FC, and FF samples and for 81 days.As Fig. 5 shows, biogas production was lowest for the FW2reactor for the first 14 days, suggesting inhibition. However,biogas production increased rapidly and on the day 81 the FW2bioreactor yielded the highest biogas production (68.5 mL). Thisfinding suggests that W2 was not toxic to the floccular sludgeafter day 14. When the biogas production of pretreated samples inthe FC and the FF bioreactors were compared (Fig. 5), the resultswere not significantly different (53.8 mL and 55.5 mL, respectively)which suggests that for biogas production, additional Fenton’spre-treatment might not be needed and should only be usedwhen the TP content of the samples is higher than 3 g L−1.

When the biogas production of W2 in different activated sludgeswas compared (Fig. 6), the different behavior of the same crudeolive oil mill effluent in granular and floccular sludges suggeststhat both the bacterial composition and physical composition ofthe activated sludges might be of importance. These findingsare in accordance with other studies since Borja et al.35 observed


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Table 2. Changes in chemical oxygen demand (COD) removal levelsin bioreactors

Bioreactor

InitialCOD

(mg L−1)

Initialreactor

COD(mg L−1)

Final COD(mg L−1)

COD removal(%)

GW2 103500 10000 2490 75

GC 39150 10000 4860 51

GF 29400 10000 4170 58

FW2 103500 10000 3105 69

FC 39150 10000 4210 57.9

FF 29400 10000 5365 46.4

that some bacteria could degrade tyrosol and p-hydroxybenzoicacid and thus stimulate methane production. Various wastewatertreatment studies also suggest that granular sludge and floccularsludge have different performances. Hussain et al.36 studiedmethane generation from phenol and found that the methanegas production rate and the total yield were lower in floccularsludge than in granular sludge using the same substrate (phenol)and inoculum concentrations. In this study biogas productionobserved in the floccular sludge bioreactor was higher than inthe granular sludge bioreactor (68.5 mL and 45.7 mL, respectively)while TP removal was lower in the floccular sludge bioreactorthan in the granular sludge (88.5% and 91.3%, respectively). Thissuggests that some degradation components other than phenolspresent in the OMWW might be affecting biogas production. Thesubstrate for gas production in this study was not only phenolbut the un-treated OMWW containing suspended and colloidalfractions such as pectins, tannins, proteins and oils, which could beinterfering. Koster and Cramer37 studied the inhibition effects ofsaturated long-chain acids on methane gas formation in granularsludge and also showed that methanogenic activity changed withcomposition of the oils and could even be reduced by 50% withlauric acid.

Total organic matter removalAs is seen from Table 2, 75% COD removal was achieved in theGW2 bioreactor while the FW2 bioreactor gave 69% COD removal,which suggested that anaerobic treatment alone would not besufficient for COD removal from OMWW samples. Comparison ofthe data from this study with other anaerobic studies using OMWWshowed that COD removal efficiencies vary considerably (55% and87%) according to the type of digester and the microorganismsused in the pre-treatment, and also the available total organicmatter. In this study total organic matter available was 10 g CODL−1 in the reactors. Hamdi and Garcia38 found COD removal of55% with a batch loading of 10 g COD L−1.

DephenolizationThe amount of biogas produced and the simultaneous TP removalare presented in Table 3. When both dephenolization and biogasproduction are taken into consideration, the combined data givenin Table 3 shows that the final step in pre-treatment, namelyFenton’s oxidation, could be excluded since both the GF and FFreactors gave lower TP removals than the GC and FC bioreactors,while their biogas productions were not significantly different.Also the data given in Table 3 shows that an OMWW samplehaving initial TP concentration below 3 g L−1 could be treatedsuccessfully using a floccular sludge without any pre-treatment.

Table 3. Changes in total phenol (TP) and total biogas production inbioreactors

BioreactorInitial TP(g L−1)

Final TP(g L−1)

TPremoval

(%)

Totalbiogas

(mL)

GW2 2.848 0.248 91.3 45.7

GC 1.720 0.112 93.5 59.3

GF 1.352 0.464 65.7 56.8

FW2 2.848 0.328 88.5 68.5

FC 1.720 0.200 88.4 53.8

FF 1.352 0.712 47.3 55.5

CONCLUSIONA considerable number of olive mills on the Aegean coastof Turkey have total phenolics of less than 3 g L−1 in theireffluents. The results of this study indicate that sludges fromindustrial wastewater treatment plants in the region could be usedefficiently to biodegrade these effluents after simple dilution, whileproducing biogas. Further investigations and feasibility studiesmust be performed to evaluate the possible use of such regionaltreatment plants as potential central OMWW treatment and biogasproduction facilities. Constructing olive mills in the same industrialcomplexes with suitable treatment facilities could also be astrategy to reduce transport costs. Environmental awareness mayeven legally force olive oil mill owners to pay these industries forthe biodegradation of their OMWW.

For OMWW having TP content equal to or greater than 3 g L−1,simple dilution before the anaerobic treatment will not be enoughand some type of pre-treatment process needs to be developedto avoid toxicity of the phenolics on methanogens. A combinedpre-treatment as suggested in this research can be carried out andthe cationic polyelectrolyte CP 1 introduced in this study with its95% TSS removal capacity can be used successfully to flocculatethe OMWW samples.

ACKNOWLEDGEMENTThe authors thank Izmir Environmental Protection Foundationfor financial support and also Izmir Environment and ForestDirectorate for providing the OMWW samples.

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anaerobic digestion of aegean olive mill effluents with and without pretreatment

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