lable at ScienceDirect

International Biodeterioration & Biodegradation 85 (2013) 237e242

Contents lists avai

International Biodeterioration & Biodegradation

journal homepage: www.elsevier .com/locate/ ibiod

Determination of aerobic biodegradation kinetics of olive oil millwastewater

Ahmet Günay a,*, Mesut Çetin b

a Istanbul Metropolitan Municipality, Environmental Protection Directorate, 34169 Merter-Gungoren, Istanbul, TurkeybMinistry of Health, Gö�güs Hospital, Balikesir, Turkey

a r t i c l e i n f o

Article history:Received 5 July 2013Received in revised form29 July 2013Accepted 30 July 2013Available online 31 August 2013

Keywords:Olive oil mill wastewaterBiological degradationKinetic modelingRefractory

* Corresponding author. Tel.: þ90 536 690 55 16; fE-mail address: ahmetgunay2@gmail.com (A. Gün

0964-8305/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.ibiod.2013.07.012

a b s t r a c t

Aerobic biological treatment was conducted for the treatment of high strength olive oil mill wastewater(OMW). Two different approaches were used for kinetic modeling of OMW biodegradation. TOC removaland CO2eC evolution were monitored in an open and a closed bioreactor systems, respectively. Gom-pertz, Refractory organics plus first-order (RFO) and CheneHashimoto equations were applied to esti-mate the kinetic parameters by using the data from bioreactors. Furthermore, change in oxidation stageof carbon was monitored and temperature dependency of OMW biodegradation was investigated basedon activation energy. At room temperature, 64% of TOC was removed in the open bioreactor while cu-mulative CO2eC evolution was 6.32 g L�1 in closed the bioreactor. Higher biodegradation efficiency andkinetic parameters were obtained at 25 �C rather than 10 �C. Gompertz and RFO equations providedbetter fitting with CO2eC and TOC data, respectively. Experimental and kinetic estimations indicated thatOMW constituted of approximately 30% refractory organics. The comparison of two different modelingapproaches showed that kinetic modeling based on CO2eC provided better correlation with the exper-imental data. Temperature coefficient indicated that biological degradation of OMW is slightly depen-dent on temperature.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Annual olive oil production is about 2.6 million tons worldwideand the majority of the production (>98%) occurs in the Mediter-ranean region, whereas Turkey has 5% contribution to global pro-duction. Olive oil production generates huge amount of wastewaterand effluent characteristics are depended on some factors such asmethod of extraction, type and maturity of olives, region of origin,climatic conditions and associated cultivation/processing methods(Paraskeva and Diamadopoulos, 2006).

Olive oil mill wastewater (OMW) has distinctive dark red toblack color and high concentrations of organics (Adhoum andMonser, 2004). COD of OMW ranges from 35 to 200 g L�1 whileBOD5 concentration is between 15 and 135 g L�1 (Zirehpour et al.,2012). Furthermore, around 10% of the organic matter in OMW isphenolic compounds, which have similar polymer structure tolignin (Tsioulpas et al., 2002). BOD5/COD ratio is used as a keyparameter to evaluate the biological treatability of wastewater and

ax: þ90 212 449 94 94.ay).

All rights reserved.

OMW is widely characterized with high BOD5/COD value. Almostall wastewaters containing biodegradable constituents with aBOD5/COD ratio of 0.5 or greater can be treated easily by biologicalmeans (Chan et al., 2009; Martins et al., 2010).

Treatment of industrial wastewater has been a key aspect ofresearch on wastewater treatment due to the increasing awarenesson environmental issues and strict environmental regulations(Asadi et al., 2012). Discharge of OMW without proper treatmentmay cause serious environmental problems due to high oxygenconsuming capacity of organics, coloring effect and inhibition ef-fects of phenolic compounds to living organism in receiving waterbodies. Treatment of OMW is complicated due to the varyingcharacteristics at different climatic conditions and productionsystems. Various treatment processes such as biological (Eusebioet al., 2007; Karakaya et al., 2012), physico-chemical (Sarika et al.,2005), advanced oxidation (Nieto et al., 2011) and varying combi-nations (Mantzavinos and Kalogerakis, 2005; Khoufi et al., 2006;Azbar et al., 2008) have been employed for the treatment ofOMW. Among them, aerobic treatment has been reported byproviding significant reductions in COD, phytotoxic compoundsand simple phenolics (McNamara et al., 2008).

Determination of biodegradation kinetics of organics is usefulto design and scale-up a biological treatment plant as well as

A. Günay, M. Çetin / International Biodeterioration & Biodegradation 85 (2013) 237e242238

prediction of the effluent quality. Due to the large variety ofcompounds present in wastewaters, it is widely accepted thatchemical oxygen demand (COD) and total organic carbon (TOC)are used as global parameters to express the pollution strength ofwastewater and kinetics of wastewater treatment processes(Pocostales et al., 2012; Amor et al., 2012). TOC and COD mea-surements have some inaccuracies and combining those withmean oxidation number of organic carbon (MOC) provides valu-able additional information concerning the reaction developmentand formation of intermediates during wastewater treatmentprocesses, especially when oxidative transformations are involved(Vogel et al., 2000).

Monod-type equations have been widely used to describe theaerobic degradation kinetics. So far, few studies have been per-formed to determine the kinetic values under aerobic treatment ofOWM. Sánchez et al. (2007) investigated the aerobic biodegrada-tion kinetics of anaerobically treated OWM. In this study, twodifferent approaches were used for kinetic modeling of OMWbiodegradation: one was conducted based on CO2eC evolution intwo closed reactors, and the other was TOC removal in an openreactor during the biodegradation experiment. Additionally, valuesof oxidation state of organic carbon and activation energy werecalculated.

2. Materials and methods

2.1. Experimental setup

Three completely-mixed bioreactors were operated in batchmode for 90 days. Two bioreactors were operated in closed systemand one was in open system. CO2eC evolution data from the closedbioreactors and TOC removal data from open reactor were used toestimate the kinetic parameters of OMW. The closed bioreactorswere made of dark brown glass with 1 L volume. To investigate theeffect of temperature on biodegradation kinetics of OMW, one ofthe closed bioreactor was operated at low temperature (10 �C) andthe other at moderate (25 �C) temperature. Schematic illustrationof closed bioreactor system is given in Fig. 1. Five liter of beaker wasused as an open bioreactor and it was operated at room tempera-ture (20 � 2 �C) without temperature control.

Wastewater sample was taken from the process effluent of anolive oil production mill in Balıkesir district, Turkey. Following tothe removal of settleable particles by waiting 30 min, pH ofwastewater was adjusted to 7.2 using NaOH solution. To prevent

Fig. 1. Schematic illustration of closed bioreactor.

the substrate inhibition on microorganisms, OMWwas diluted 50%by distilled water adding before feeding to the bioreactors.Municipal wastewater was used as inoculums following 2-dayaeration and it was adapted to OMW before starting kinetic ex-periments. 250 mL of diluted wastewater was fed to the closedbioreactors with 10 mL of inoculum. Open bioreactor was fed with4 L of diluted wastewater and 50 mL of inoculum.

Atmospheric air was continuously pumped to the bioreactors asoxygen source after removing CO2 andmoisture by passing through5 N KOH and distilled water solution, respectively. CO2eC evolutionfrom the closed reactors during the biodegradation of OMW wascontinuously monitored and absorbed CO2 in 1 N NaOH wasdetermined.

2.2. Chemical analyses

Characterization of raw and treated wastewater was performedby analytical methods as described in Standard Methods (APHA,2005). COD, BOD5, Total Kjeldahl nitrogen and NH3 analyses wereconducted by following the descriptions in the methods of 5220 D,5210, 4500-Norg B and 4500-NH3 B, respectively. TOC wasmeasured by wet combustion method (Nelson and Sommers,1996). Oil and grease measurement was done according to Soxh-let extraction method of 5520B (APHA, 2005). pH was measuredusing a pH meter model WTW 350i.

2.3. Kinetic modeling

The aim of this study was to determine the kinetic parametersduring aerobic biodegradation of OMW. A number of kineticmodels exist to describe biological treatment processes. The formand complexity of substrate biodegradation kinetics depend onmicroorganism growth, decrease in substrate concentration,product formation, substrate or product inhibition; or both,enzyme reactions and reactor type. The kinetic expressions used inthis study depend on assumptions about decrease in substrateconcentration and product formation that based on CO2eC evolu-tion. Refractory organics plus first-order (RFO), Chen andHashimoto (1980) and Gompertz equations were applied toexperimental data. These equations were adapted to experimentaldata in terms of remaining TOC in the open reactor and CO2eCevolution data obtained from closed reactors.

Gompertz equation is widely used for modeling of logistic-growth of microbial cultures and it can be written according toTOC removal as:

TOCt ¼TOC0 � ðTOC0 � TOCRÞ

� exp�� exp

�Rm � e

TOC0 � TOCRðl� tÞ þ 1

�� (1)

Gompertz equation can be written based on CO2eC evolutionas:

ðCO2�CÞt ¼ ðCO2�CÞu�exp��exp

�Rm�e

ðCO2�CÞuðl� tÞþ1

��

(2)

Chen and Hashimoto equation is related to substrate utilizationand it can be written based on TOC removal as:

TOCt ¼ TOC0 ��Rþ ð1� RÞ � K

mmt � 1þ K

�(3)

Chen and Hashimoto equation can be written based on CO2eCevolution as:

A. Günay, M. Çetin / International Biodeterioration & Biodegradation 85 (2013) 237e242 239

ðCO2 � CÞt ¼ TOC0 � TOC0 ��Rþ ð1� RÞ � K

mmt � 1þ K

�(4)

Table 1Characteristics of olive oil mill wastewater.

Parameter Value

pH 5.56COD (g/L) 63BOD5 (g/L) 38TOC (g/L) 26.8Oil and Grease (g/L) 5.0TS (g/L) 41.4TSS (g/L) 16.8NH3eN (mg/L) 68.8Acidity (mg/L CaCO3) 6636Cl� (mg/L) 1550

The minimum hydraulic retention time (or critical time) in-dicates the washout time of microorganism and it is equal to thereciprocal of the maximum growth rate:

tc ¼ 1mm

(5)

Biodegradation rate in refractory organics plus first-order (RFO)equation is first-order with respect to substrate utilization orproduct formation. All organics in olive wastewater can not beutilized during biological treatment since some of organics haverefractory property. So, RFO equation can be justified by assumingthat microbial growth is negligible and that no nutrient, substrateor electron acceptor limitations exist as fallow:

TOCt ¼ TOCR þ ðTOC0 � TOCRÞ � expð � kc � tÞ (6)

and for CO2eC evolution as:

ðCO2 � CÞt ¼ ðCO2 � CÞu � ð1� expð � k� tÞÞ (7)

In these equations,

TOCt is the amount of TOC at time t, g L-1

TOC0 is initial TOC, g L�1

TOCR is the amount of refractory TOC value, g L�1

t is hydraulic retention time, daytc is minimum hydraulic retention time, d(CO2eC)u is ultimate CO2eC evolution(CO2eC)t is CO2eC evaluation at any timeRm is the maximum daily TOC removal amount, g L�1 day�1

l is lag-phase time, dayK is kinetic constant, day�1

R is refractory organic ratiokch is kinetic constant, day�1

mm is maximum specific growth rate, day�1

Kinetic parameters were estimated by nonlinear regressionanalysis. Nonlinear regression was performed using trial and errorprocedure with the help of solver add-in functions of MicrosoftExcel software. In trial error procedure, kinetic parameters wereestimated by maximizing the coefficient of determination (R2) andminimizing average relative error (ARE). R2 and ARE have beenwidely used for kinetic modeling of data from various environ-mental processes (Gunay, 2007; Bhunia and Ghangrekar, 2008;Ghaniyari-Benis et al., 2012).

Coefficient of determination was calculated as;

r2 ¼P�

Cmod � Cexp2

P�Cmod � Cexp

2 þ �Cmod � Cexp

2 (8)

On the other hand, ARE can be calculated as;

ARE ¼ 1N

XNi¼1

�Cexp � Cmod

Cexp

�� 100 (9)

where Cexp (mg/l) is the amount of remaining TOC value in openreactor or CO2eC evolution data obtained from closed reactors,Cmod is calculated TOC values or CO2eC evolution by the kineticmodels and N is the number of experimental data.

2.4. Evaluation of oxidation state of carbon and activation energy

Value of oxidation state of organic carbon (MOC) is useful toevaluate the oxidation of carbon during treatment. Carbon hasoxidation state of 0 as in the form of organic matter and it increasedto þ4 when organic matter is converted to CO2. MOC can becalculated as (Vogel et al., 2000):

MOC ¼ 4� 1:5� CODTOC

(10)

Activation energy (Ea) is a barrier energy to be overcome tocontinue the biochemical reaction. It can be calculated according toArrhenius equations as:

Ea ¼ RT1T2T2 � T1

lnk2k1

(11)

where,

Ea is activation energy, cal mol�1

R is gas constant, 1.986 cal mol�1K�1

T1 and T2 are temperature, Kk1 and k2 values from RFO equation are used to calculate Ea.

3. Results and discussions

3.1. Characterization of olive mill wastewater

Mean characteristics of raw OMW are given in Table 1. OMWwas slightly acidic and pollutant content was mainly constituted oforganics (as in COD and BOD5), solid matter and oil and grease.Biodegradability ratio (BOD5/COD) of OMW was relatively high as0.58, while a wastewater is considered to be very biodegradablewhen the ratio is above 0.50 (Esplugas et al., 2004; Martins et al.,2010). OMW wastewater had also high total solid content and41% was comprised of suspended solid. Other researchers have alsoreported comparable high pollutant concentrations and biode-gradability ratios for OMW from different countries (Beltran et al.,2008; Justino et al., 2012). To prevent the substrate inhibition onmicroorganisms, raw OMW was diluted about 50% by addingdistilled water. Diluted wastewater had COD of 33.5 g L�1, TOC of13.5 g L�1 and BOD5 of 19.5 g L�1.

3.2. Kinetic modeling of data from closed bioreactors

Daily CO2eC evolution in closed bioreactor at 10 �C and 25 �Cwas as shown in Fig. 2. CO2eC evolution at both temperatures hadsimilar pattern during the experimental period. First 2 days, smallamount of CO2eC productionwas observed at both bioreactors dueto the adaption of microorganism to the wastewater. At day 3, CO2e

C amounts were 56.9 mg d�1 and 39.7 mg d�1 at 10 and 25 �C,

Fig. 2. Fluctuations in daily CO2eC production in closed bioreactors.

Fig. 3. Comparing kinetic models at 10 �C and 25 �C.

Table 2Kinetic parameters from closed bioreactors.

Model parameters 10 �C 25 �C

ExperimentalCum. (CO2eC)exp, g/L 5.55 6.32

RFO kinetic model(CO2eC)u, g/L 8.58 8.44TOKR, g/L 4.97 5.11k, d�1 0.014 0.020ARE, % 6.08 5.29r2 (non-linear) 0.987 0.985

Chen and Hashimoto modelR, % 33.79 29.03K 2.687 3.701mm, d�1 0.020 0.049tc(1/mm), d 51.09 20.50ARE, % 4.19 4.52r2 (non-linear) 0.991 0.992

Gompertz model(CO2eC)u, g/L 5.17 5.36Rm 0.131 0.182ʎ, d 5.22 4.12ARE, % 3.52 3.92r2 (non-linear) 0.994 0.982

A. Günay, M. Çetin / International Biodeterioration & Biodegradation 85 (2013) 237e242240

respectively. Daily CO2eC evolution continuously increased and thehighest values were 134.4 mg L�1 d�1 at 10 �C and 213.2 mg L�1 d�1

at 25 �C on day 12. In both reactors, CO2eC evolution steadilydecreased until the end of the experiment. The decrease in CO2eCevolutionwas attributed to the decrease in the amount of substratethat remained in the bioreactors. The comparison of two reactorsrevealed that CO2eC evolutionwas higher at 25 �C than 10 �C due tothe higher biodegradation of organics in the bioreactor. Increase inorganic removal efficiency with temperature is due to theenhancement of microbial activity and the solubility of substrate(Cirja et al., 2008). Positive effect of temperature on biodegradationwas reported during the biological treatment of different waste-waters (Ferrer et al., 2009).

The comparison of kinetic equations based on CO2eC evolutionis presented in Fig. 3 while the estimated kinetic parameters aregiven in Table 2. All kinetic equations had well description withexperimental data. At the end of experiment, the highest experi-mental CO2eC values were 5.55 and 6.32 g L�1 at 10 and 25 �C,respectively, however, refractory TOC values decreased with theincreasing of temperature. At both temperatures, Gompertz equa-tion had the highest correlation coefficient and the lowest AREvalues compared to other equations while Refractory organics plusfirst-order (RFO) had the lowest correlation with experimentaldata. In Gompertz equation, refractory TOC was estimated as 5.17and 5.32 g L�1 at 10 and 25 �C, respectively. When temperatureincreased from 10 to 25 �C, maximum daily CO2eC productionenhanced from 0.13 to 0.19 mg day�1 while the lag-phase timedecreased from 5.22 to 4.52 days. Similarly, other researchers re-ported lower-lag phase time at higher temperatures for thebiodegradation of varying wastes (Kumar and Lin, 2013). Temper-ature also had positive effect on the reaction rate constants of RFand CheneHashimoto. At the end of experiment, microbial specificgrowth rate (mm) in Chen andHashimotomodel increased from0.03to 0.07 day�1 with the increasing of temperature. Gompertz andRFO equations resulted in closer refractory organic amounts at10 �C while RFO had lower amount at 25 �C.

3.3. Kinetic modeling of data from open bioreactor

Initial TOC in open reactor was 13.50 g L�1 and it decreasedsharply during the first 40 days while degradation of organicsstabilized after day 60 (Fig. 4). At the end of the experiment, TOCwas measured as 4.9 g L�1 with 64% treatment efficiency. Threekinetic models, namely Chen and Hashimoto, RFO and Gompertzmodels were applied to the TOC data.

Comparable fitting degrees of kinetic models with experimentaldata are shown in Fig. 4 while estimated kinetic parameters are

given in Table 3. All equations had a higher correlation (R2 > 0.90)with experimental datawhile RFOmodel had the lowest ARE value.Refractory TOC ratios were estimated as 28% and 16.5% with RFOand Chen and Hashimoto kinetic models which are quite lowerthan experimental ratio of 36.3%. Kinetic rate constant (k) of RFOwas 0.06 day�1 and ratio of refractory was estimated as 37.58%.Maximum microbial specific growth rate (mm) of CheneHashimotoequation was estimated as 0.23 day�1 and lag-phase time ofGompertz model was 2.78 day. All three equations resulted

Fig. 4. Kinetic comparisons with for TOC removal in open bioreactor.

Fig. 5. Changes in oxidation stages of carbon.

A. Günay, M. Çetin / International Biodeterioration & Biodegradation 85 (2013) 237e242 241

different amounts of refractory TOC and the average was one thirdof initial TOC.

3.4. Oxidation state of carbon and activation energy

At the beginning of experiment, oxidation state of carbon wascalculated as 0.27 (Fig. 5). This relatively low value indicated thatOMW was mostly comprised of hardly degradable organics. Likelyto sugars, oxidation stage of raw OMW is close to zero due to thehigh content of cellulosic materials. Oxidation state steadilyincreased with time and reached to 1.26 at the end of the experi-ment. In aerobic treatment, organic carbon is finally converted toCO2 having oxidation state of þ4. Oxidation state of 1.26 indicatesthe oxidation state of organics remaining in the liquid phase.Oxidized organic carbon leaves the reactor in the form of CO2 whilevalue of 1.26 indicates the oxidation state of organic carbon thatremained in the liquid phase.

Activation energy (Ea) of biological degradation of OMW wascalculated using the degradation rate constants from RFO model at10 and 25 �C. Activation energy value was calculated as1.75 kcal mol�1 while temperature coefficient was 1.17. Researchers

Table 3Comparison of kinetic parameters.

Experimental data Value

TOC0, g/L 13.55TOCR, g/L 4.90R, % 36.16

RFO modelTOCR, mg/L 5.09k, d�1 0.060R, % 37.60ARE, % 4.24r2 (non-linear) 0.969

Chen and Hashimoto modelR, % 25.66K 2.55m, d�1 0.23tc, d 4.39ARE, % 5.66r2 (non-linear) 0.925

Gompertz modelTOKR, g/L 5.34Rm, 0.380ʎ, d 0.92R, % 39.42ARE, % 5.09r2 0.956

reported that microbial activity increases two fold when temper-ature increases 10 �C with the activation energy of 11.94 kcal mol�1.Lower temperature coefficient indicates that aerobic biologicaldegradation of OMW is slightly dependent on temperature.

4. Conclusions

Olive mill wastewater (OMW) was aerobically treated both inopen and closed bioreactors. Non-linear regression was applied forkinetic modeling of TOC removal and CO2eC evolution data fromthe bioreactors. Initial TOC of OMW was 13.50 g L�1 and biologicaltreatment provided 64% treatment efficiency. Closed bioreactorwas operated at 10 and 25 �C and higher biodegradation was ob-tained at elevated temperature. Gompertz and RFO equationsprovided better fitting with CO2eC evolution and TOC removaldata, respectively. According to modeling results, microbial kineticvalues increase at high temperature whereas lag-phase time de-creases with temperature. Experimental data and kinetic estima-tions indicated that approximately one third of organic content ofOMW was refractory characteristics. Evaluation of two approachesfor modeling the OMW biodegradation revealed that kineticmodeling based on CO2eC production provides better fitting withexperimental data. Oxidation stage of carbon continuouslyincreased during the experiment and biodegradation of OWM wasfound as slightly dependent on the temperature.

Acknowledgment

This research was funded by Balıkesir University ScientificResearch Projects Department (Project No: 2008/37). The authorsthank Dr. Dogan Karadag from Yildiz Technical University forhelpful suggestions on manuscript writing.

References

Adhoum, N., Monser, L., 2004. Decolourization and removal of phenolic compoundsfrom olive mill wastewater by electrocoagulation. Chemical Engineering andProcessing: Process Intensification 43, 1281e1287.

Amor, C., Lucas, M.S., Pirra, A.J., Peres, J.A., 2012. Treatment of concentrated fruitjuice wastewater by the combination of biological and chemical processes.Journal of Environmental Science and Health, Part A: Toxic/Hazardous Sub-stances and Environmental Engineering 47, 1809e1817.

APHA, 2005. Standard Methods for the Examination of Water and Wastewater,twenty-first ed. APHA, Washington, DC, USA.

Asadi, A., Zinatizadeh, A.A.L., Sumathi, S., 2012. Simultaneous removal of carbon andnutrients from an industrial estate wastewater in a single up-flow aerobic/anoxic sludge bed (UAASB) bioreactor. Water Research 46, 4587e4598.

Azbar, N., Keskin, T., Catalkaya, E.C., 2008. Improvement in anaerobic degradation ofolive mill effluent (OME) by chemical pretreatment using batch systems.Biochemical Engineering Journal 38, 379e383.

A. Günay, M. Çetin / International Biodeterioration & Biodegradation 85 (2013) 237e242242

Beltran, J., Gonzalez, T., Garcia, J., 2008. Kinetics of the biodegradation of green tableolive wastewaters by aerobic and anaerobic treatments. Journal of HazardousMaterials 154, 839e845.

Bhunia, P., Ghangrekar, M.M., 2008. Analysis, evaluation, and optimization of kineticparameters for performance appraisal and design of UASB reactors. BioresourceTechnology 99, 2132e2140.

Chan, Y.J., Chong, M.F., Law, C.L., Hassell, D.G., 2009. A review on anaerobiceaerobictreatment of industrial and municipal wastewater. Chemical EngineeringJournal 155, 1e18.

Chen, Y.R., Hashimoto, A.G., 1980. Substrate utilization kinetic model for biologicaltreatment process. Biotechnology and Bioengineering 22, 2081e2095.

Cirja, M., Ivashechkin, P., Schaffer, A., Corvini, P.F.X., 2008. Factors affecting theremoval of organic micropollutants fromwastewater in conventional treatmentplants (CTP) and membrane bioreactors (MBR). Reviews in EnvironmentalScience and Biotechnology 7, 61e78.

Esplugas, S., Contreras, S., Ollis, D., 2004. Engineering aspects of the integration ofchemical and biological oxidation:simple mechanistic models for the oxidationtreatment. Journal of Environmental Engineering 130, 967e974.

Eusebio, A., Mateus, M., Baeta-Hall, L., Sagua, M.C., Tenreiro, R., Almeida-Vara, E.,Duarte, J.C., 2007. Characterization of the microbial communities in jet-loop(JACTO) reactors during aerobic olive oil wastewater treatment. InternationalBiodeterioration & Biodegradation 59, 226e233.

Ferrer, I., Gamiz, M., Almeida, M., Ruiz, A., 2009. Pilot project of biogas productionfrom pig manure and urine mixture at ambient temperature in Ventanilla(Lima, Peru). Waste Management 29, 168e173.

Ghaniyari-Benis, S., Martín, A., Borja, R., Martín, M.A., Hedayat, N., 2012. Compari-son of two mathematical models for correlating the organic matter removalefficiency with hydraulic retention time in a hybrid anaerobic baffled reactortreating molasses. Bioprocess and Biosystems Engineering 35, 389e397.

Gunay, A., 2007. Application of nonlinear regression analysis for ammonium ex-change by natural (Bigadiç) clinoptilolite. Journal of Hazardous Materials 148,708e710.

Justino, C.L., Pereira, R., Freitas, A.C., Rocha-Santos, T.A.P., Panteleitchouk, T.S.L.,Duarte, A.C., 2012. Olive oil mill wastewaters before and after treatment: a criticalreview from the ecotoxicological point of view. Ecotoxicology 21, 615e629.

Karakaya, A., Laleli, Y., Takoc, S., 2012. Development of process conditions forbiodegradation of raw olive mill wastewater by Rhodotorula glutinis. Interna-tional Biodeterioration & Biodegradation 75, 75e82.

Khoufi, S., Aloui, F., Sayadi, S., 2006. Treatment of olive oil mill wastewater bycombined process electro-Fenton reaction and anaerobic digestion. WaterResearch 40, 2007e2016.

Kumar, G., Lin, C.Y., 2013. Bioconversion of de-oiled Jatropha Waste (DJW) tohydrogen and methane gas by anaerobic fermentation: influence of substrateconcentration, temperature and pH. International Journal of Hydrogen Energy38, 63e72.

Mantzavinos, D., Kalogerakis, N., 2005. Treatment of olive mill effluents Part I.Organic matter degradation by chemical and biological processesdan over-view. Environment International 31, 289e295.

Martins, R.C., Rossi, A.F., Quinta-Ferreira, R.M., 2010. Fenton’s oxidation process forphenolic wastewater remediation and biodegradability enhancement. Journalof Hazardous Materials 180, 716e721.

McNamara, C.J., Anastasiou, C.C., O’Flahertyd, V., Mitchell, R., 2008. Bioremediationof olive mill wastewater. International Biodeterioration & Biodegradation 61,127e134.

Nelson, D.W., Sommers, L.E., 1996. Total carbon, organic carbon, and organic matter.In: Sparks, D.L., Bartel, J.M. (Eds.), Methods of Soil Analysis: Chemical MethodsPart 3. Soil Science Society of America Inc., Wisconsin, USA.

Nieto, L.M., Hodaifa, G., Rodríguez, S., Giménez, J.A., Ochando, J., 2011. Degradationof organic matter in olive-oil mill wastewater through homogeneous Fenton-like reaction. Chemical Engineering Journal 173, 503e510.

Paraskeva, P., Diamadopoulos, E., 2006. Technologies for olive mill wastewater(OMW) treatment: a review. Journal of Chemical Technology and Biotechnology81, 1475e1485.

Pocostales, J.P., Álvarez, P., Beltrán, F.J., 2012. Kinetic modeling of granular activatedcarbon promoted ozonation of a food-processing secondary effluent. ChemicalEngineering Journal 183, 395e401.

Sánchez, E., Rincón, B., Borja, R., Travieso, L., Raposo, F., Colmenarejo, M.F., 2007.Aerobic degradation kinetic of the effluent derived from the anaerobic digestionof two-phase olive mill solid residue. International Biodeterioration & Biodeg-radation 60, 60e67.

Sarika, R., Kalogerakis, N., Mantzavinos, D., 2005. Treatment of olive mill effluents,Part II: complete removal of solids by direct flocculation with polyelectrolytes.Environment International 31, 297e304.

Tsioulpas, A., Dimou, D., Iconomou, D., Aggelis, G., 2002. Phenolic removal in oliveoil mill wastewater by strains of Pleurotus spp. in respect to their phenol oxi-dase (laccase) activity. Bioresource Technology 84, 251e257.

Vogel, F., Harf, J., Hug, A., Von Rohr, P.R., 2000. The mean oxidation number ofcarbon (MOC) e a useful concept for describing oxidation processes. WaterResearch 34, 2689e2702.

Zirehpour, A., Jahanshahi, M., Rahimpour, A., 2012. Unique membrane processintegration for olive oil mill wastewater purification. Separation and Purifica-tion Technology 96, 124e131.