IntroductionMany researches and studies havebeen carried out toward developing practical techniques and approaches todecline the adverse impact of highly toxic generated oily wastewaters, andtherefore to resolve the scarcity of water resources in many regions around theworld. Oily wastewater is wastewaterblended with oily materials under a wide range of concentrations. The soil,water, air and human beings are threatened by oily wastewater because of itsextremely toxic contaminants and also hazardous nature of its oil contentsincluding radionuclides, persistent organic pollutants, particular hazardouscompounds (PHCs), polycyclic aromatic hydrocarbons (PAHs), inorganics anddissolved minerals, hydrocarbons (i.e. oil, fats, grease), etc. (Jamalyet al., 2015). Heavy metals (e.
g. Cr, Cd, Hg, Ag,etc.) in conjunction with particularly hazardous chemicals (PHCs) could beobserved in oily wastewater especially in petroleum refineries andpetrochemical industries (Malakahmad et al.
, 2011).Various approaches have been studiedfor treating oily wastewater including: physical treatment (e.g. membranefiltration (polymeric/ceramic membranes (Salahi etal., 2010)), reverse osmosis (Duong et al.,2014), submerged membrane (Yuliwati et al.
,2012), hollow fiber, PVDF membrane (Zhang etal., 2013), flotation, biological treatment (e.g. up-flow anaerobicsludge bed reactor (Rastegar et al., 2011),membrane bioreactor (Pendashteh et al., 2012),sequence batch reactor (Malakahmad et al.
, 2011;Shariati et al., 2011), biopolymeric flocculation (Ahmad et al., 2004), etc.), electrochemicaltreatment (e.g.
electrocoagulation (Xu and Zhu, 2004), Fenton (Madani et al., 2015 ; Ishak and Malakahmad, 2013),electrochemical oxidation, electroflotation (Ibrahimet al., 2001), chemical coagulation (Madaniet al., 2015), or the like (Körbahti and Artut, 2010; Yan et al., 2011)),chemical treatment (e.g. adsorption, chemical coagulation (Ngamlerdpokin et al., 2011), treatment usingultrasound-dispersed nanoscale zero-valent iron particles, titanium dioxide,etc.
), as well as application of biosurfactants, destabilization of emulsionsthrough the use of zeolites and other natural minerals, treatment by vacuumultraviolet radiation, hybrid technologies, etc. Although oil, fats, grease are ofbiodegradable organics that could be readily be degrade through biologicalmethods and substrates, high attention is required for selecting properbiological treatment due to the presence of toxic pollutants in oilywastewaters. However, the general parameters which are considered as analyticalparameter to evaluate the efficiency of oily wastewater treatment are COD, BOD,TSS, TOC, TDS, TPH, TFAs, VS, turbidity, oil/grease, salinity, and also othersignificant factors based on the target wastewater are assessing PAHs &boron concentration, FFA & FAME, as well as the concentration of nitrogen,sulphate, phosphorus, phenol and phenolic or our target compounds. Also, thereare some other factors for assessment of aerobic processes (such as, MLSS,MLVSS, SVI).Selection of the appropriate treatmentsystem for any discharge greatly depends on its characteristics and dischargingcriteria.
To do so, various oily wastewater treatment technologies arecurrently used; however, the percentage of oil or other contaminants removalvaries among them. The assessment of the most efficient technology depends onseveral factors including (I) the influent quality, (II) the treatment cost,(III) the environmental footprint, (IV) and energy consumption (Jamaly et al., 2015). So, considering most of theprevious approaches are either too expensive to be implemented on a commercialscale or require large environmental footprints, novel sustainable methods foroily wastewater treatment, that will be geared toward economic savings andenvironmental preservation, is needed to be devised.Although, based on thecharacteristics of oily wastewater and target contaminant the design and amountof operational would vary, investigating real oily wastewater under treatingprocess would generate more reliable and precise outcomes, because throughscrutinizing previous researches it could be inferred that there aresignificant variations among results of experiments done on synthetic and realoily wastewater. It has been declared that real oily wastewaters are majorlycontained some other particles such as colloids, solid particles and the like (Pendashteh et al., 2012; Madaeni et al., 2013).
Finally, it should be added that the BOD of oily wastewater like refinerywastewater is mainly lower than municipal wastewater due to the existence ofsome materials which could not be easily biodegraded (AlZarooni and Elshorbagy, 2006). 1. Some of Applied Approaches for Treating Oily Wastewater 1.1. Coagulation 1.1.1 Electrochemical Treatment (I) Electrochemical Oxidation (direct/indirect)Investigated parameters: currentdensity and reaction temperature in electrochemical reactor (Körbahti and Artut, 2010),initial pH and cell voltage (Yan et al., 2011).
(II) Electro-Fenton In theElectro-Fenton process, Fe(II) is oxidized by H2O2 to form Fe(III).This lead to forming a hydroxyl radical (HO•) and a hydroxide ion (OH?) in the process aswell. In the next step Fe(III) is then reduced back to Fe(II) by anothermolecule of H2O2, forming a radical of (HOO•)and a proton (H+). The main effect of adding H2O2 isto create two different oxygen-radical species, with water (H+ + OH?)as a byproduct (Ishakand Malakahmad, 2013). Fe2+ + H2O2 ? Fe3+ + HO• + OH? (1) Fe3+ + H2O2 ? Fe2+ + HOO• + H+ (2) In thesecond reaction free radicals of HOO are produced. Hydroxyl radical (HO•)is an authoritative, strong, and non-selective oxidant which can start the newreactions rapidly. Oxidation of anorganic compound by Fenton’s reagent can be done very quickly but it involvedwith exothermic reactions that results in increasing the temperature of thesolutions. The main purpose of this process is to oxidation of pollutants toprimarily carbon dioxide and water (Kavitha, V.
, & Palanivelu, K., 2005). Generally, Fe(II) sulfate (FeSO4)is used as catalyst in the reactions. In case of electro-Fenton process,hydrogen peroxide is produced in situ from the electrochemical reduction ofoxygen. Also, Fenton’s reagent during the radical substitution reaction is usedin organic synthesis for the hydroxylation of aromatic hydrocarbon (Casado et al., 2005).
For instance, classical conversionof benzene (C6H6) into phenol (C6H5OH)can be expressed as: (Casadoet al., 2005) C6H6 + FeSO4 + H2O2 ? C6H5OH (3) Meinero and Zerbinati(2006) investigatedthe oxidative and energetic efficiency of various electrochemical oxidationprocesses. The electro-Fenton process was verified to have the best degradationefficiency in terms of energy consumption: for that case the specific energyconsumption was 0.3 kWh/g of COD, corresponding to 41.8 kWh/m3.Manyworks classified electro-Fenton or the very Fenton process as advancedoxidation process (AOP).
Some of AOPs are, electro-Fenton process, TiO2/H2O2,photocatalysis reactions, etc., that are chemical oxidation processes mainlyused as an attractive pretreatment method to improve the biodegradability ofvarious industrial discharges, that is able to generate and use hydroxyl freeradicals (•OH) as strong oxidant (Klamerthaet al., 2010; Sin et al., 2011). The application of AOPs not onlyreduces the COD load and contaminants levels in wastewater, but also generatesfewer toxic effluents. Besides, AOPs augment the biodegradability of wastewaterthrough forming intermediates similar to the metabolic pathway substances (Ollis, 2000).
Advanced oxidation process (AOP)which employ strong oxidant agents (ozone, hydrogen peroxide and UV, Fenton,etc.), are able to remove organic and phenolic pollutants of the Olive MillWastewater (OMW) (Madaniet al. 2015).
TheFenton process could be enumerated as one of the promising alternativeoxidation methods because of its cost efficiency, operation simplicity, lack ofresidue, and ability to treat a spectrum of substances. Fenton process, whichis in fact a synthesis of oxidation and coagulation reaction, reduces toxicityand COD concentration using hydrogen peroxide and ferrous sulfate (Madani et al. 2015). To be specific, the oxidationmechanism by the Fenton process is due to the generation of hydroxyl radical inan acidic solution by the catalytic decomposition of hydrogen peroxide and inpresence of ferrous (II) ions (Ledakowiczet al., 2001). Fenton’sreagent (a solution of hydrogen peroxide (H2O2) and an iron catalyst (like FeSO4,iron electrode, FeSO4.
7H2O (ferrous sulfateheptahydrate), etc.)) is used to oxidize contaminants or organic compounds inwastewaters such as trichloroethylene (TCE), tetrachloroethylene (perchloroethylene, PCE), andrefinery wastewater to augment biodegradability. The Fenton reaction is shownin Eqs. (4) to (13). At acidic pH it leads to the production of ferric ion andhydroxyl radical (Ishakand Malakahmad, 2013): H2O2 + Fe2+ ? Fe3++ •OH + OH- (4) Fe3+ + H2O2 ? Fe-OOH2+ + H+ ? •H2O + H+ (5) Hydroxyl radicals may be scavengedby reaction with another Fe2+ or with H2O2: •OH + Fe2+ ? OH? + Fe3+ (6) •OH + H2O2 ? HO2 • + H2O (7) Hydroxylradicals may react with organic and starting a chain reaction: •OH + RH ? H2O + R• (RH=organic substrate) (8) R• + O2 ? ROO• ? products of degradation (9) Ferrous ion and radicals are produced during the reactions: H2O2 + Fe3+ ? H+ + FeOOH2+ (10) FeOOH2+ ? HO2• + Fe2+ (11) HO2• + Fe2+ ? HO2? + Fe3+ (12) HO2• + Fe3+ ? O2 + Fe2++ H+ (13) Ishak and Malakahmad(2013) showedthat Fenton process is able to augment the biodegradability of refinerywastewater as a pretreatment for recalcitrant contaminants. Studied operationalparameters were reaction time (20 – 120 min), H2O2/COD (2 – 12) andH2O2/Fe2+ (5 – 30) molar ratios.
Theydetermined that BOD5/COD as an index of biodegradability ofwastewater increased from 0.27 to 0.43 under optimum conditions of operationalparameters, including reaction time (71 min), H2O2/COD (2.8) and H2O2/Fe2+(4) molar ratios: the process was optimized using response surface methodologybased on a five-level central composite design. In addition to low biodegradability of petroleum refinerywastewater, the higher concentration of COD in characterized refinerywastewater is because of presence of some compounds such as phenols andsulfide. So, such wastewater with low BOD and high COD is consider as lowbiodegradability wastewater (Metcalf and Eddy,2003). Moreover, considering high concentration of some contaminantsincluding oil and grease; Benzene and Toluene as PHCs; Ethylbenzene and Xyleneas aromatic hydrocarbons, it could be implied that the petroleum wastewater orother oily wastewaters containing biorecalcitrant contamination or heavy metalsrequires pretreatment before application of any biological decontamination (Ishak and Malakahmad, 2013).According to Ishak and Malakahmad(2013), although the range of time factor in Fenton process was from 20to 120 min, the results revealed that in the first 20 minutes of the Fentonreaction, more than 90% of COD and BOD removal was achieved.
Also, BOD5/CODratio of 0.40 was attained within 20 minutes. This finding, shows very shortperiod of time required for a significant biodegradability improvement andpollution reduction in a Fenton process which is of special interest in theindustrial application of Fenton’s reagent: hydroxyl free radicalsbear a the short half-life, so the extension of reaction timedoes not improve degradation. Even though by increasing of H2O2concentration better organic degradation will be attained due to moregeneration of more hydroxyl radicals (Kang andHwang, 2000), at a certain limit, the complete organic removal could notbe obtained even with higher than stoichiometric quantity of H2O2/CODand this eventually led to reducing the removal efficiency.
Generally, it meansbiodegradability declined after increasing H2O2/COD molarratio to more than 2 (Ishak and Malakahmad, 2013).Regarding the third studied influential factor, i.e. H2O2/Fe2+,it has been verified that both peroxide dose and iron concentration (Fe2+)are influential factors in the Fenton reaction for better degradationefficiency and reaction kinetics,respectively (Kavitha and Palanivelu, 2005;Siedlecka and Stepnowski, 2005).
In that experiment, decrease of H2O2/Fe2+molar ratio (i.e. higher concentration of Fe2+) caused morebiodegradability and higher removal of the target compound and formation ofearly intermediates, i.e. generating more hydroxyl radicals for the degradationprocess (Catalkaya and Kargi, 2007; Ishak andMalakahmad, 2013). Excessive amount of Fe2+ competes with theorganic carbon for hydroxyl radicals when high Fe3+ concentration isused.(III) ElectrocoagulationSomeelectrode materials like iron, aluminum, boron-doped diamond, platinum-iridium,and titanium-rubidium have been testified for treating varied types of oilywastewater so far. In electrocoagulation processes more current density (mA/cm2),electrolysis time, salinity cause more removal of turbidity, COD, TSS,contaminants (such as sulphate, phenol, etc.
). However, several investigationhave tried to evaluate the the optimum amount of operational parametersincluding current density, electrodematerials, the distance and arrangement of electrodes, reaction temperature,initial pH, voltage, effluent concentration, salinity (NaCl) (Xuand Zhu, 2004). Moreover, Santos et al. (2006) applied full-scaleelectrocoagulation reactor for organic components removal from oily wastewater,consequently about 57% COD removal was achieved after 70 hr reaction. Fouad (2014) did a study to separate cottonseed oil from oil–water emulsionsusing an electrocoagulation method, where the power consumption was calculated.
He reported that the power consumption increased from 0 to 0.9 kW/kg-oilremoved when the current density increased from 0.0009 to 0.02 A/cm2.Moreover, he showed that the oil removal percent was higher with lower sodiumchloride concentration.
Removal was 90% with fresh water containing 85 mg/LNaCl, whereas, with seawater containing 3.5% NaCl, the oil removal was 80%.However, higher concentrations of NaCl were preferred as the power consumptionwas lower for seawater with 3.5% NaCl, 0.
017 kW/kg-oil removed compared withfresh water containing 85 mg/L NaCl, 0.022 kW/kg-oil removed (Fouad, 2014).Among electrochemical oxidationprocesses, electrocoagulation has been found to be more effective than manyother treatment technologies for heavy metal removal from oily wastewater,because it requires no addition of chemical compound, low capital cost, andenhances the settling of the oily sludge produced (dos Santos et al., 2014). However, electrocoagulation requires high operating cost becauseof the electrical energy requirement. Apart from this, there is a release ofhigh quantities of metals into the oily sludge produced, thereby making thesludge more hazardous and creating another environmental concern.
(IV) ElectroflotationThe removalof finely dispersed oil from oil-water emulsions of different Egyptian oilcrudes by either batch wise or continuous processes was analyzed to identifythe effect of various operating and design parameters under experimentalcondition in which electroflotation cells had been equipped with a set ofelectrodes mounted in the cell bottoms (Ibrahim et al., 2001). The recommended conditions foroperating batch runs were current density from 5 to 20 mA/cm2,temperature from 30 to 40°C, and pH =6.
Achieved data from continuous runsdemonstrated that, at almost complete separation of oil, the minimum powerconsumption was 0.08 kWh/m3 of a 200 mg/L emulsion flowed at 300mL/min (Ibrahim et al., 2001).