Challenges and Opportunities for the Future There are some opportunities for achieving benefits from oilywastewater such as:1- the reuse of oily wastewater in steam boilers,2- recycling in injection wells for the enhancement of crude oilexploitation,3- opportunities for the sale of the oil concentrate from oily watertreatment to oil recycling companies,4- And also, opportunities for the recovery of precious metals fromoily wastewater, especially from petrochemical industries.While, turning these opportunitiesinto reality has remained a complex undertaking (Jamalyet al., 2015).A great amount of oily wastewater isdischarged into the environment by industries. As a result of the diversequantities of the fluctuating compositions of different components in oilywastewater under real operating conditions, there is no one-size-fits-allapproach for the removal and/or recovery of these components (Jamaly et al., 2015).
Even though, the presence of heavymetals (i.e. Cr, Cd, Hg, Ag, etc.) in oily wastewater is one of the majorproblems militating against the recycling or reuse of oily wastewater becauseof their highly hazardous and toxic nature, limited studies concerning therecovery or removal of heavy metals from oily wastewater has been done.Furthermore, the presence of heavy metals along with particularly hazardouschemicals (PHCs) in oily wastewater from petroleum refineries and petrochemicalplants could even result in more deleterious effects when discharged into the environment(Malakahmad et al., 2011; Rocha et al.
, 2012). 1.1.
Heavy Metals Removals and Recovery Depend on what a petrochemical orother industrial factories manufacture, the type and concentration of heavymetals (i.e. Cr, Cd, Hg, Ag, Cu, Ni, Pb, Zn, Al, Fe, Ba, Mn, Sn) and othercontaminants such as PHCs (particular hazardous compounds/chemicals) and PAHs(polycyclic aromatic hydrocarbons) vary from one factory to another. To bespecific, various technologies have been applied for removing heavy metals fromoily wastewaters and the like.
Usually physicochemical treatmentssuch as precipitation, filtration, ion exchange, electron-deposition, andreverse osmosis have been used as conventional methods of heavy metals removalfrom aqueous solutions (Ong et al., 2005).There are some common problems associated with these methods e.
g. they are morecostly compared to biological treatment methods and can themselves produceother waste problems; which has limited their industrial applications (dos Santos et al., 2014).
3.1.1 Electrocoagulation Amongelectrochemical oxidation processes, electrocoagulation has been found to bemore effective than many other treatment technologies for heavy metal removalfrom oily wastewater, because it requires no addition of chemical compound, lowcapital cost, and enhances the settling of the oily sludge produced (dos Santos et al., 2014).
However, electrocoagulation requires high operating cost because of theelectrical energy requirement. Apart from this, there is a release of high quantitiesof metals into the oily sludge produced, thereby making the sludge morehazardous and creating another environmental concern. 3.1.2 Biological Treatment of Metals The application of biologicalprocesses, for treating water and wastewater, is gradually attracting interestsbecause of reasons including reduced chemicals additives, low operating costs,eco-friendly and cost-effective alternative of conventional techniques and,higher efficiency at lower levels of contamination (Ahluwaliaand Goyal, 2007; Gonçalves et al., 2007; Srivastava and Majumder, 2008; Appelset al., 2010; Malakahmad et al.
, 2011; Kieu et al., 2011).22.214.171.124 SBRSequencing Batch Reactors (SBRs) arean attractive alternative to conventional biological treatment methods, due totheir simplicity and flexibility in operation. In addition, in several casesSBR was used for removing heavy metals, for instance Malakahmadet al. (2011) removed Hg2+ and Cd2+, Sirianuntapiboon and Hongsrisuwan (2007) studiedZn2+ and Cu2+ removal, alsoSirianuntapiboon and Ungkaprasatcha (2007) testified removal of Pb2+and Ni2+ through a hybrid Granular Activated Carbon and SequencingBach Reactor.
An SBR is a periodically fill-and-draw reactor, so that fivediscrete periods in each cycle are filling, reaction, settling, drawing, andidle time (Ong et al., 2005). Reactionsstart during filling stage and continue during reaction step. After reaction,the mixed liquor suspended solids (MLSS) are allowed to separate bysedimentation during settle in a defined time period; the treated effluent iswithdrawn during draw. The time period between the end of the draw and thebeginning of the new fill is termed idle.The high performance of SequencingBatch Reactor (SBR) which was based on aerobic process (like activated sludge)was shown for treating synthetic petrochemical factory wastewater containing Hg2+and Cd2+ (Malakahmad et al.
, 2011).To detect this, COD, TSS, MLSS (mixed liquor suspended solid), MLVSS (mixedliquor volatile suspended solid), and SVI (sludge volume index) were consideredfor assessment of reactor performance. At maximum concentrations of the heavymetals, the applied lab-scale SBR removed about 76–90% and 96–98% of Hg2+and Cd2+, respectively. Whereas, increase of heavy metalsconcentration (Hg2+ and Cd2+) led to decrease of CODremoval, as well as decrease of MLSS and MLVSS, the percentages of Hg2+and Cd2 removal were increased not only because of microbialadaptation, but also due to biosorption of the sludge.
In other word,percentage reduction of COD removal reveals the poor performance of microorganisms in the SBR which it was confirmedthrough MLSS and MLVSS values during addition of Hg2+ and Cd2+ (Malakahmad et al., 2011). Besides, increasingrate of SVI indicates decrease in settleability of the sludge (Malakahmad et al., 2011). In several work, thepopulation of microorganism in SBR system have been detected with which thesignificant role of those microorganisms (including Rhodospirilium-likebacteria (Srivastava and Majumder, 2008; Malakahmad et al.
,2011), Gomphonema-like algae (Ivorra et al., 2002; Malakahmad et al., 2011),and Sulfate reducing-like bacteria (McGregor etal., 1999; Malakahmad et al., 2011)) in removing heavy metals from oilywastewater were proven. The Rhodospirilium cells, thesulfate reducing bacteria, and the diatom Gomphonema algae are originally ormostly anaerobic, and they are able to be adapted to aerobic condition.
While achieved outcomes revealedthat in the SBR removal efficiency of cadmium was higher than mercury, when theconcentrations of mercury and cadmium were increased by magnitude 5 as asignificant shock for the SBR system, for mercury removal efficiency of SBRreduced sharply to 40%, and for cadmium reduction was decreased slightly to76%, but due to adaptation of microorganisms with new condition, they couldrespectively reach more than 80% and 95% for Hg2+ and Cd2+after passing several days (Malakahmad et al.,2011).However, 95% mercury removal and 99%cadmium removal have been gained by means of the SBR throughout the processeven with considerable shortage of microorganisms. It verifies that in additionto activities of microorganisms, some portion of heavy metals concentration wasremoved from wastewater as a result of biosorption which plays a significantrole in biological treatment of metals compounds confirmed formerly by otherresearches (Al-Qodah, 2006; Anayurt et al., 2009;Sar? and Tuzen, 2009; Tuzen et al.
, 2009). Some important notes on Malakahmad etal. (2011) 1- Testifying a synthetic petroleum wastewater (COD 110 mg/L; urea 33mg/L; Hg2+ 0.1-9 mg/L; Cd2+ 0.1-15 mg/L etc.). In fact,analyzing real wastewater would reveal more logical and authentic results to beconsidered in industrial sites.
2- Only two types of heavy metal were studied – mercury and cadmium – which the reason hasbeen due to the intended petroleum refinery wastewater: 3- As it has been ascertained that SBR act as both biological processand biosorption of sludge (Malakahmad et al., 2011),hence it could be implied that heavy metals bear the potential ability to beadsorbed, so devising of a novel biopolymer flocculants system as pretreatmentor the like might be helpful in enhancing the performance and performance ofthe reactor, even though there sre many researches about the performance ofbioadsoption processes in removing heavy metals (Yanand Viraraghavan, 2001; Alluri et al., 2007). 4- Despite high performance of SBR in removal of Hg2+ andCd2+ presented by Malakahmad et al.(2011), none of contaminant removal reached discharge criteria: based onUS EPA (1995) the amount of mercury andcadmium concentrations of effluent discharge are 0.013 mg Hg2+/L and0.73 mg Cd2+/L, respectively. To do so, presenting a cutting-edgetechnology is required by which not only discharge standards should be met, butalso shocks originated from variation of contaminant concentration should befaced.
5- Impact of operational factors (such as COD concentration, eachheavy metal concentration, temperature, aeration speed, etc.), as well as theinteraction among factors have been ignored. Finally, the amount of optimalcondition could be evaluated by means of DX 7 (Design Expert Software – version7), by which the correlation between factors could be achieved too.Todo so, the impact of each contaminant concentration on each other, and theirinteraction with each other as well as COD removal should be scrutinizedprecisely. As it has been found out that by increase of heavy metal to oilywastewater the percentage of COD removal decrease, and the toxicity andinhibitory impact of heavy metal on COD removal during biological treatment (Malakahmad et al., 2011),it would be a good opportunity to discover a better arrangement for removingheavy metal through a selective pretreatment methods (like biopolymerflocculants, AOP (electro-fenton reactor)) or removing COD by an anaerobicreactor such as EGSB, UASB, and so forth, if it is possible to divide thetreatment process.Furthermore,as a significant shock for the SBR system, when the concentrations of mercuryand cadmium were increased by magnitude 5, for mercury removal efficiency ofSBR reduced sharply to 40%, and for cadmium reduction was decreased slightly to76%, but due to adaptation of microorganisms with new condition, they couldrespectively reach more than 80% and 95% for Hg2+ and Cd2+ after passingseveral days (Malakahmad et al., 2011).
As Malakahmad et al. (2011) hasillustrated whenever there was a shock during increase of heavy metalconcentration, the performance of reactor for Hg2+ and Cd2+decrease significantly till the microorganism would be adapted to the situationafter several days. For industrial purposes, that circumstance should be solvedto avoid unwilling discharge of pollutants into environment.
6- Malakahmad et al. (2011) have considered some variations for operational parameters andbased on those random data the operational condition have been designated. I amwondering why the authors have not used other useful techniques and softwaresuch as “Design of Experiments” to have more extensive views of experiment andbetter outcome analyzing. 7- Hydraulic Retention Time (HRT 15 day) = Reactor volume (24L)/Feeding (1.
6 L/day) has been mentioned that HRT was 15 days Malakahmad et al. (2011) pp-120. But what does itmean? I mean how the runs are done? 8- In this study, the impact of aeration variations in aerobic tankhas been ignored. How the aeration speed has been changed?? From X vvm to Y vvm?A compressor with capacity of 150 L is vague! 3.1.2.
2Biosorption Technology Al-Qodah (2006) removed heavy metal ions from aqueous solutions by activatedsludge. Moreover, Anayurt et al. (2009) and, Sar? and Tuzen (2009) investigated the removal ofPb(II) and Cd(II) from aqueous solution by green alga (Ulva lactuca) andmacrofungus (Lactarius scrobiculatus) biomasses. And also, lichen(Xanthoparmelia conspersa) biomass was applied for the removal of Hg2+from aqueous solution (Tuzen et al., 2009). 3.1.2.
3 Hybrid Reactor for Heavy metals TreatmentSirianuntapiboonand Ungkaprasatcha (2007) testifiedremoval of Pb2+ and Ni2+ through a hybrid GranularActivated Carbon and Sequencing Bach Reactor. In anotherresearch, a field pilot plant consisting UASB-constructed wetland systems wasinvestigated for long-term removal of heavy metals from municipal wastewater (De la Varga et al., 2013).
They analyzed theevolution of heavy metals removal from the water stream over time (over aperiod of 4.7 year of operation) and the accumulation of heavy metals inUASB sludge and constructed wetland sediments at two horizons of 2.7 and4.0 year of operation. High removal efficiencies were achieved for somemetals in the following order:Sn > Cr > Cu > Pb > Zn > Fe(63–94%). Also, medium removal efficiencies were found for Ni (49%), Hg (42%),and Ag (40%), and finally Mn and As showed negative percentage removals.
According to obtained outcomes, removal efficiencies of total Heavy metals werehigher in UASB (De la Varga et al., 2013).3.1.3 Removal or Recovery of Heavy Metal from Anaerobic and AerobicSludge by Bioleaching and BiocharBioleaching hasbeen illustrated to be a feasible method for removing heavy metals from sludge (Xiang et al., 2000; Yoshizaki and Tomida, 2000; Wong etal., 2002). According to Wong et al.
(2002),the effect of pH requirement for isolated indigenous Thiobacillusferrooxidans for bioleaching heavy metals from wastewater sludgehas been studied, because based on fundamental concepts the leaching mediumneeds to be pre-acidified to less than 4. They used isolated sludge-indigenousiron-oxidizing bacteria for the bioleaching experiments to find out thedissolution behavior of heavy metals, including Zn, Cu, Ni and Cr, from sludgeset at an initial pH (3–7) with the purpose to decline the consumption ofacid. In another paper, bioleaching ofheavy metals from from anaerobically digested sludge using isolated indigenousiron-oxidizing bacteria has been testified by Wonget al. (2004), in which the impact of using FeS2 as an energysource, on the bioleaching of Zn, Cr, Cu, Pb, Ni, as well as nutrients such asnitrogen and phosphorus. Observations revealed that addition of FeS2accelerated the acidification of sludge and raised the oxidation–reductionpotential of sludge medium, thus resulting in an overall increase in metaldissolution efficiency. The removal ofheavy metals from anaerobically digested sewage sludge was studied by usingferric sulfate and it was compared with the using sulfuric acid at pH 3 inorder to clarify the effect of ferric iron as an oxidation reagent on elutionof heavy metals (Ito et al., 2000). Thisresearch showed that the addition of ferric sulfate to the sludge led to theacidification of the sludge and the elution of heavy metals from the sludge.
Also, the pH of the sludge decreased through increase of the added iron anddecrease of the sludge concentration. Comparative results revealed that ferriciron eluted cadmium, copper and zinc more effectively than sulfuric acid. Thiseffective elution of heavy metals was caused by the oxidation of the sludgesolid by ferric iron added. Therefore, it could be inferred that ferric ironplayed a role to acidify the sludge and to oxidize metallic compounds in thesludge and this new chemical method was useful for removing heavy metals fromanaerobically digested sewage sludge (Ito et al., 2000).The ability oftwo biochars converted from anaerobically digested biomass to sorb heavy metalsusing a range of laboratory sorption and characterization experiments wasexamined by Inyang et al. (2012). Initialevaluation of digested dairy waste biochar and digested whole sugar beetbiochar revealed that both biochars were effective in removing a mixture offour heavy metals (Pb2 +, Cu2+, Ni2+, and Cd2+)from aqueous solutions.
Further investigations of lead sorption by the twobiochars indicated that the removal was chiefly through a surface precipitationmechanism, which was proved by batch sorption experiments, mathematicalmodeling, and examinations of lead-laden biochars samples using SEM–EDS, XRD,and FTIR (Inyang et al., 2012). Madani et al. (2015) Case study: Process: Treatment ofolive mill wastewater using physico-chemical and Fenton processesWaste water source: effluentsof an olive oil production plant located in Lushan Industrial, Gilan city, IranCapacity: Oil processingcapacity of this factory is 60 tons at an average waste effluent of 430,000L/day when it works.
Type of experiments: Twotypes of experiments were performed: physico-chemical treatment and advancedoxidation studies using the Fenton process. (Detail of this experiments areexplained in (Madani et al. 2015). Chemicaloxygen demand (COD), total phenols, color, and aromaticity were examined tocheck the efficiency of the methods. Tabel-1:efficiency of different process in treating OMW. (c) (b) (a) Figure 1: a)effect of Fe concentration; T0 = 298 K, pH = 3, H2O2concentration of 0.5 M, and reaction time of 4 h at a stirring rate of 90 rpm.b) T0 = 298 K, ferrous iron initial concentration 0.
02 M, H2O2concentration 0.5 M, and reaction time of 4 h at a stirring rate of 90 rpm. c)T0 = 298 K, ferrous iron initial concentration of 0.02 M and areaction time of 4 h at a stirring rate of 90 rpm.Results: In Acidcracking experiment 1.5 ml per one liter of wastewater sulfuric acid was required. pHof 2.
5 was the optimum number and lower pH values reduced the experimental timebut did not shows a significant difference in the removal efficiency. However,at a low pH, more NaOH is required to adjust the pH in other steps of theexperiment to complete treatment, which is not an economic solution. After 60minutes 47.1, 97, 29.
6, 57.4, and 63.2% removal efficiency was obtained in COD,Turbidity, Total phenol, Aromaticity, and Color respectively.Table 1 shows the physico-chemical analysis of OMW beforeprocessing and after acid cracking, the Fenton test, and the coagulation test. Acidcracking and Fenton process showed high efficiency of COD (83%), total phenols(98.6%), color (77%), and aromaticity (67%) removal from the OMW. As a resultof this study, acid cracking and Fenton process have a significant effect inreducing the COD and total phenols from OMW.
Figure 1a shows the effect of Fe concentration in COD, totalphenol, aromaticity, and color removal efficiency. It reveals that optimum loadof ferrous iron is 0.02 M which leads to 53, 96, 30, and 16 percent removalefficiency for COD, total phenol, aromaticity, and color respectively.
Infigure 1b the effect of H2O2 concentration in Fe load of0.02M is illustrated. As can be seen higher efficiency was obtained in 0.05M ofH2O2. In the same type of experiment, the effect of pHwas tested and results are shown in figure 1c. The optimum pH reveals to be atpH=3. ConclusionThe conclusions can be drawn from this study in applying acidcracking, chemical coagulation, and Fenton processes on Olive Mill Wastewater(OMW).
(1) The acid cracking at the optimum pH 2.5 removed 97, 47, 30, 63,and 57% of the turbidity, COD, total phenol, color, and aromaticity,respectively. Acid cracking has dual effect; sulfuric acid is a powerfuloxidant agent that results in oxidizing the OMW component also it can lower thepH and improves the coagulation process as well. A set of experiments showsthat only 10% of the acid cracking efficiency was due to its oxidation effect,while the other 90% was related to the coagulation effect(2) However increasing temperature from 298 to 308 K can increasethe efficiency of phenol removal from 96 to 98%, but does not have aconsiderable effect on the efficiency process and only slightly increases the rateof the reactions. (3) From the set of experiments it reveals that more than 83, 98.
6,77, and 67% removal of COD, total phenol, color, and aromaticity can beobtained by applying the combination of acid cracking, chemical coagulation,and Fenton processes. Consequently, it can be concluded that this combinationof processes can be used as a suitable way to treat OMW.