Advanced oxidation processes for the treatment of olive-oil mills wastewater

Technical Note

Advanced oxidation processes for the treatment of olive-oil

mills wastewater

P.Can ?izares,J.Lobato,R.Paz,M.A.Rodrigo *,C.Sa

′ez Department of Chemical Engineering,Facultad de Ciencias Qu?

′micas,Universidad de Castilla La Mancha,Campus Universitario s/n,13071Ciudad Real,Spain

Received 7April 2006;received in revised form 27October 2006;accepted 30October 2006

Available online 8January 2007

Abstract

In this work,the treatment of an actual industrial waste with three advanced oxidation processes (AOP)has been studied:conductive-diamond electrooxidation (CDEO),ozonation and Fenton oxidation.The wastewater comes from olive-oil mills (OMW)and contains a COD of nearly 3000mg dm à3.CDEO allowed achieving the complete mineralization of the waste with high current e?ciencies.Like-wise,both ozonation and Fenton oxidation were able to treat the wastes,but they obtained very di?erent results in terms of e?ciency and mineralization.The accumulation of oxidation-refractory compounds as ?nal products excludes the use of ozonation and Fenton oxidation as a sole treatment technology.This con?rms that besides the hydroxyl-radical mediated oxidation,CDEO combines other important oxidation processes such as the direct electro-oxidation on the diamond surface and the oxidation mediated by other electro-chemically formed compounds generated on this electrode.ó2006Elsevier Ltd.All rights reserved.

Keywords:Actual wastewater;Diamond;Electrochemical oxidation;Ozonation;Fenton process

1.Introduction

Countries along the Mediterranean coast are among the main manufacturers of olive oil.Although large companies are not excluded in this industrial sector,the manufacture of olive oil is typically carried out by small companies in small facilities.The management of the liquid wastes gener-ated in mills is a subject of the major importance for these companies.

Olive-oil mills generate dark colored,foul-smelling and turbid aqueous wastes.These wastewaters consist of mildly acidic e?uents with high conductivity,especially those coming from traditional mills.Typically,olive-oil mills wastewaters (OMW)are rich in inorganic ions (sodium,chloride and phosphorus),organic matter and suspended solids.According to the literature (El Hadrami et al.,

2004),OMW are characterized by the great variety of pollutants contained,including aromatics (such as cath-ecol,p -coumaric acid,4-methylcathecol,benzeneacetalde-hyde,phenyl ethyl alcohol,benzofurane,and tyrosol)and also aliphatic compounds (such as hexane,octane,nonanol nonanoic acid,decanoic acid,dichloropropene,pentade-cene,and hexadecane).Its organic load is usually around 80–200g dm à3,although it depends on the type of extrac-tion process used.The technologies in the treatment of this kind of aqueous wastes (Beccari et al.,1996;Benitez et al.,1999)are anaerobic digestion and advanced oxidation pro-cesses (AOP).However,it is reported (Capasso et al.,1992)that high amounts of bio-refractory and oxidation-refrac-tory organics remain at the end of both treatments,espe-cially in the case of anaerobic digestion treatment.

Advanced oxidation processes are de?ned as oxidation processes in which hydroxyl radicals are the main oxidants involved.This radical is a very powerful oxidant (E 02.80V vs.SHE)which leads to a very e?ective oxidation process.Among the existing AOPs,three of them are going to be

Corresponding author.Tel.:+34926295300.

E-mail address:manuel.rodrigo@uclm.es (M.A.Rodrigo).

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Chemosphere 67(2007)

832–838

compared in this study:Fenton oxidation,conductive diamond electrochemical oxidation(CDEO)and ozona-tion.In addition to their e?ciency,these three technologies have another important advantage:no dangerous or persis-tent by-products are formed as a consequence of the reduc-tion of the oxidizing agent.

Conductive diamond is an emergent material with very good properties for the electrochemical treatment of waste-waters polluted with organic compounds,including great chemical and electrochemical stability and generation of large amounts of hydroxyl radicals(Marselli et al.,2003). During the recent years,CDEO has been widely studied with both actual and synthetic industrial wastes in lab and bench-scale plants(Gherardini et al.,2001;Rodrigo et al.,2001;Polcaro et al.,2003;Can?izares et al., 2004b,c,d;Can?izares et al.,2005a,2006b).The main con-clusion of these studies is that this technology allows obtaining the almost complete mineralization of the organ-ics contained in the wastes with very high current e?cien-cies.Unfortunately,in the recent years only few papers have engaged on the electrochemical oxidation of olive-oil mills and they have mainly focused in the use of others anodic materials(Longhi et al.,2001;Gotsi et al.,2005; Panizza and Cerisola,2006)(Ti/TiRuO2,dimensional sta-ble anodes(DSA),PbO2,...).The main results of these studies are that this technology allows obtaining the almost complete removal of the organic load.Likewise,low-elec-trical charges and low-energy consumptions are required.

Fenton process is an advanced oxidation technology in which a mixture of hydrogen peroxide and iron(II)salts is added directly to the wastewater.Besides the oxida-tion carried out by the hydroxyl radicals generated by catalytic decomposition of hydrogen peroxide,the iron(III) ions generated during the oxidation stage promote the removal of other pollutants by coagulation and sedimenta-tion.This AOP has been largely studied and great e?cien-cies are reported for the treatment of wastewaters polluted with di?erent organics(Rivas et al.,2001;Vlys-sides et al.,2004).The results show that only60–80% chemical oxygen demand(COD)removal percentages are obtained.

During the recent years,ozone has been used for the non-persistent disinfection of water and treated wastewa-ters,and also for industrial wastewater treatment.Ozone is itself a very powerful oxidant(E02.07V vs.SHE)but in certain conditions it can decompose and lead to the for-mation of hydroxyl radicals.In this later case,the process e?ciencies are strongly increased.During the last years, many works have been published concerning ozonation of OMW(Beltran-Heredia et al.,2001;De Heredia and Garcia,2005).The results obtained show that large amounts of refractory compounds are obtained at the end of the process.For this reason,several authors propose a combined treatment(ozonation-biological treatment)to improve the removal of organic matter.

Thus,the goal of this work is to study the treatment of this actual industrial waste with CDEO,to give more insights about the role of the main oxidative mechanisms in the electrochemical oxidation of organics with conduc-tive diamond electrodes and to compare the results obtained with those obtained using other two AOP tech-nologies:Fenton process and ozonation.

2.Experimental

2.1.Analytical procedure

The carbon concentration was monitored using a Shi-madzu TOC-5050analyzer.The chemical oxygen demand was determined using a HACH DR200analyzer.The mea-surements of pH and conductivity were carried out with an InoLab WTW pH-meter and a GLP31Crison conducti-meter,respectively.

2.2.Determination of the oxygen-equivalent chemical-oxidation capacity(OCC)

To compare the performance of di?erent AOPs,it is desirable one parameter which informs about the chemical e?ciency of the oxidants used in each oxidation process and quanti?es the oxidants added to the waste with the same arbitrary units(kg O2per m3of wastewater).In the literature(Can?izares et al.,2006c;Faouzi et al.,2006)it is proposed to use the oxygen-equivalent chemical-oxida-tion capacity(OCC).This parameter is related to the di?er-ent oxidants used in the three AOPs studied in this work according to Eqs.(1)–(3),where Q is the speci?c electrical charge passed(kA h mà3):

OCCekg O2mà3T?0:298áQekA h mà3Te1TOCCekg O2mà3T?1:000á?O3 ekg O3mà3Te2TOCCekg O2mà3T?0:471á?H2O2 ekg H2O2mà3Te3TThese equations are obtained from stoichiometrical cal-culations,taking into account the number of electrons exchanged in the reduction of the di?erent oxidants(for the case of ozone and hydrogen peroxide)and also the faraday number in the case of CDEO.

2.3.Conductive diamond electrochemical oxidation

CDEO assays were carried out in a single-compartment electrochemical?ow-cell working under a batch operation mode(Can?izares et al.,2005b).Diamond-based material (Adamant Technologies,Switzerland)was used as anode and stainless steel(AISI304)as cathode.Both electrodes were circular(100mm diameter)with a geometric area of 78cm2and an electrode gap of9mm.The wastewater was stored in a glass tank(0.6dm3)and circulated through the electrolytic cell by means of a centrifugal pump(?ow-rate2.5dm3minà1).Electrolyses were carried out in galva-nostatic mode.During the electrolyses no control of pH was carried out.

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2.4.Ozonation

Ozonation experiments were carried out by continu-ously feeding an ozone–oxygen gas stream in a mixed semi-batch bubble reactor(continuous for gas and batch for liquid).The experimental set-up consists of an ozone generator(Ambizon,Model GMF-10,Sistemas y Equipos de Ozonizacio′n S.L.,Madrid,Spain)and a jacketed cylindrical reactor(2.5dm3)equipped with a porous gas distribution plate and ba?es to increase the capacity of absorption of ozone.The ozone–oxygen mixture gas stream was sparged with a constant?ow rate of0.5dm3 minà1(?ow controller Cole Parmer,model#32907-39) and the average production of ozone was around1g hà1. The volume of wastewater treated in each assay was 2dm3.The pH was maintained in a set point close to 12±0.1.A jacket reactor coupled with a controlled thermostatic bath(ectron,model3473200Selecta,Madrid, Spain)was used to maintain the temperature at the desired set point(25°C).

2.5.Fenton process

Fenton oxidation assays were carried out in lab-scale thermostated mixed batch reactors.The experimental setup consists of a multistirrer device(Ikamag RO5power,IKA-WERKE GmbH&Co.KG,Staufen,Germany)with15 mixing sites coupled with a controlled thermostatic bath (Digiterm100,JP Selecta,Barcelona,Spain).Pyrex?asks (250cm3)hermetically sealed and equipped with magnetic

stirrers were used as reactors.In every assay,the reactors were?lled with100cm3of wastewater.Then,the iron dose was added(as acidic aqueous solution of FeSO4?7H2O) and the pH was adjusted to3±0.1with sodium hydroxide or sulphuric acid.Hydrogen peroxide was measured according with Eisenberg(1943).

2.6.Wastewater characterization

The wastewater used in this work is the e?uent of an actual olive-oil mill(Unaproliva,Jae′n,Spain).It is mainly composed of a great variety of aromatic and aliphatic compounds,which characteristics and composition depend on the type of extraction process used.Its organic load is around3000mg dmà3of COD and840mg dmà3of TOC.Its conductivity is 2.29mS cmà1and the pH is around6.

3.Results and discussion

3.1.Advanced oxidation processes

Fig.1shows the results obtained in the batch electrooxi-dation with conductive-diamond anode of the OMW stud-ied in this work.As it can be observed,the complete removal of the COD and TOC concentration is obtained. Likewise,COD and TOC pro?les are overlapped.This fact can be indicative of the small amount of intermediates generated and of the fast oxidation of organic compounds contained in the e?uent to other more simple organics and to carbon dioxide.Part b of the?gure shows the changes in pH and potential during the same electrolysis.As it can be observed,the pH of the waste slightly increases at the beginning of the oxidation process.These changes can be explained in terms of the anodic and cathodic reactions that take place in the cell.The generation of hydroxyl anions on the cathode(from water reduction)can be com-pensated by the protons generated from water oxidation in the anode.However,this last process coexists with the ano-dic oxidation of organics(and/or inorganic compounds) and,as a consequence,the proton generation rate on the anode is lower than that of hydroxyl anions.Thus,the anions generated in the cathode are only partially compen-sated and,consequently,the pH increases.It can be also seen that the cell potential is maintained almost constant during the electrolysis,indicating that the formation of non-conductive layers on the surface of the electrodes and the electrodes corrosion does not occur.

Fig.2shows the changes in the Average and the Instan-taneous Current E?ciencies during the electrolysis of OMW.It can be observed that high e?ciencies are obtained for the initial stages of the process(high COD concentration),and that from a given COD the e?ciencies decrease continuously down to0.This behaviour is charac-

834P.Can?izares et al./Chemosphere67(2007)832–838

teristic of discontinuous CDEO of wastewaters and it is usually explained in terms of mass transfer limitations assuming that the electrochemical oxidation on boron doped diamond(BDD)anodes is a direct or a hydroxyl radical mediated electrochemical process(Panizza et al., 2001;Rodrigo et al.,2001).

Fig.3a shows the COD and TOC pro?les during the discontinuous ozonation(at alkaline pH)of the OMW studied in this work.As it can be observed,the ozonation process can be used to treat this kind of actual wastewater. However,it is not able to achieve the complete removal of organic load and a signi?cant concentration of organic carbon(around30%)remains at the end of the process. According to the literature,this fact can be explained in terms of the generation of high concentration of intermedi-ates(mainly carboxylic acid)that cannot be further oxidized and,consequently,they are accumulated in the system.In the ozonation process at alkaline pH several oxidation mechanisms coexist,being the oxidation with hydroxyl radicals one of the more important in the conditions maintained.Hence,it can be assumed that the primary oxidation mechanism is same as in CDEO.However,the di?erences observed in both oxidation technologies are indicative of the lower oxidizability of the pollutants con-tained in the actual waste towards ozonation and may be indicative of the weaker activity of hydroxyl radicals over these pollutants and of the existence of other important oxidation mechanism in CDEO and in ozonation(such as persulphate oxidation and direct-oxidation in CDEO and molecular ozone oxidation in ozonation).

Fig.3b shows the changes in the global and instanta-neous e?ciency during the discontinuous ozonation pro-cess.This parameter can be estimated taking into account the ratio ozone used/ozone supplied.The amount of oxi-dant used is calculated taking into account the COD removal at each interval of time.As it can be observed,a maximum of60%of the ozone supplied is employed in the oxidation process,and this percentage decreases with the initial COD concentration.

In Fig.4a,the variations of COD and TOC with the amount of hydrogen peroxide supplied in the Fenton oxi-dation treatment of OMW are shown.As it can be observed,this technique is not able to achieve the complete mineralization of the waste,and a TOC concentration around200mg dmà3remains at the end of the treatment. These high concentrations of refractory carbon also appear in the Fenton oxidation of other kind of aqueous wastes (Can?izares et al.,2006a,c)and seem to be characteristic of this oxidation process.The di?erences observed between this AOP and the previous ones can be explained in terms of the mechanisms involved in the oxidation process.In this case,hydroxyl radical oxidation is the sole oxidation reactions.Fig.4b shows the e?ciency pro?le as a function of the amount of oxidant supplied.As it can be observed, at the beginning of the process the e?ciency is maximum and from a given ratio hydrogen peroxide used/hydrogen peroxide supplied the e?ciency decreases.This fact indi-cates a fast oxidation of raw compounds to intermediate compounds(mainly carboxylic acids)that cannot be fur-ther oxidized by hydroxyl radicals.

https://www.360docs.net/doc/c511699297.html,parison of three advanced oxidation processes

Fig.5shows the changes in the COD removal with the quantity of oxidant added in the three AOPs studied.

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As it can be observed,for low OCC concentration the e?-ciencies of both CDEO and Fenton oxidation are similar. However,Fenton process is not able to achieve percentage of COD elimination higher than70%,whereas CDEO allows obtaining the almost complete COD removal.Like-wise,it can be observed that ozonation is signi?cantly less e?cient and a maximum removal percentage of70%is obtained.

These results can be explained taking into account the oxidation mechanisms involved in the three AOPs.It is known that besides hydroxyl radicals formation(Marselli et al.,2003)CDEO combines other kind of oxidation mechanism(Can?izares et al.,2005b):direct electro-oxida-tion on the BDD surface and mediated-oxidation by other electrochemically formed compounds such as persulphate (Michaud et al.,2001),perphosphate(Can?izares et al., 2005c)or hypochlorite(Can?izares et al.,2004a)depending on the electrolyte.Conversely,in Fenton process and ozon-ation at alkaline pH the hydroxyl radical and molecular ozone oxidation(for the ozonation)or hydrogen peroxide (for Fenton process)are the only oxidant agents involved in the treatment.Likewise,these two former technologies seem to be strongly in?uenced for the composition of the waste.Thus,it can be concluded that CDEO competes favourably with ozonation and Fenton oxidation due to the higher number of oxidants involved in the electrochem-ical process.

To verify the role of the di?erent mechanisms involved in the electro-oxidation of organic compounds with BDD electrodes,a last combined oxidation essay was carried out.Fig.6shows the COD pro?le with the amount of oxi-dant supplied(in terms of kg O2mà3)during the combined Fenton-CDEO of OMW.As it can be observed,Fenton oxidation is not able to achieve the complete removal of the waste and a residual e?uent containing Fenton-refrac-tory organics is obtained(COD around700mg dmà3). From this point,the CDEO of this refractory e?uent was carried out.The results show that the complete miner-alization of the waste is obtained and none refractory com-pounds remain at the end of the process.

On the other hand,the OCC parameter only provides information about the chemical e?ciency of the oxidants

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involved in the treatment with these AOPs,but it does not give any additional information about the real cost of the treatment,as the price of the oxidants used can vary.How-ever,to compare operating cost of ozonation and electro-chemical technology is not di?cult,as both oxidizing agents can be generated in situ and the cost can be com-pared in terms of energy consumption.Nevertheless,to include in this comparison the operating cost of the Fenton oxidation is very di?cult because the cost of the raw mate-rials(hydrogen peroxide and iron),sludge treatment and reagents used to change the pH can vary a lot.Moreover, they depend on the country,the year and many other parameters that should not be included in a scienti?c work. Fig.7shows the energy consumption requirements in the treatment of an OMW by CDEO and ozonation as a func-tion of the COD removal.It can be observed that the energy requirements are lower for the CDEO.This obser-vation was previously reported in the literature for other type of wastes(De Lucas et al.,2002;Can?izares et al., 2005a,2006a,c).It was very interesting to compare these results with those obtained in the Fenton process but unfortunately nowadays,the lack of the literature about the comparison of di?erent AOP technologies leads to the lack of parameters for comparison of the technologies.

4.Conclusions

The three AOPs studied in this work(CDEO,ozonation and Fenton oxidation)are able to treat OMW.However, only CDEO allows achieving the complete mineralization of the waste and with high e?ciencies.Both ozonation at alkaline pH and Fenton oxidation lead to the generation of high concentration of intermediates(mainly carboxylic acids)that cannot be further oxidized by hydroxyl radicals. As a consequence,TOC around30%remains at the end of the process.The di?erent behaviour observed can be explained taking into account that besides hydroxyl radi-cals generation,CDEO combines other mechanisms such as direct-oxidation and mediated oxidation by electro-reagent.The energy requirements of CDEO are lower than those of ozonation.Acknowledgements

This work was supported by the JCCM(Junta de Com-unidades de Castilla La Mancha,Spain)through the Pro-ject PBI-05-043and by the Spanish government through the Project CONSOLIDER-INGENIO2010(CSD2006-0044).

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