Xiaoqiang Jia,Dayao Jin,Chen Li,Wenyu Lu,*

1Department of Biochemical Engineering,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2Key Laboratory of Systems Bioengineering,Tianjin University,Tianjin 300072,China

3Synthetic Biology Platform,Collaborative Innovation Center of Chemical Science and Engineering(Tianjin),Tianjin 300072,China

Keywords:Petrochemical wastewater Gradient membrane separation Chemical oxygen demand Biochemical oxygen demand

A B S T R A C T Thecentralizedtreatmentmethodisawidelyusedformofwastewatertreatmentthattendstobelesseffectiveat removing toxic substances.Therefore,a detailed analysis of the composition of wastewater can provide important information for the design of an effective wastewater treatment process.The objective of this paper was to investigate particle size distribution(PSD),biodegradability,and the chemical composition of the petrochemical wastewater discharges.For this purpose,this project selected the petrochemical wastewater and treated wastewater of China National Offshore Oil Corporation Zhongjie Petrochemical Co,Ltd.as the analysis objects.The step-by-step filtration method,along with a molecular weight classification method,was selected to build the chemical oxygen demand(COD)and biochemical oxygen demand(BOD)fingerprints of petrochemical wastewaterandtreatedwastewater.Theresultsshowedthatthemainpollutantsweresettleableparticlesinpetrochemical wastewater,which contributed to over 54.85%of the total COD.The colloidal particles with particle sizes in the range of 450-1000 nm had the highest COD value in the treated wastewater,which contributed 34.17%of the total COD of treated wastewater.The results of the BOD analysis showed that the soluble fractions were the main reason that treated wastewaters did not meetthe treatment standards.Tests on theorganiccompounds in petrochemical wastewater found that there were mainly linear paraffins,branched paraffins,benzene series compounds,and some plasticizers in the influent of the petrochemical wastewater.The most abundant pollutants in treated petrochemical wastewater were the adjacent diisobutyl phthalate and the linear alkanes.Fourier transform infrared(FTIR)transmission spectroscopy analysis showed that the settleable particles of petrochemical wastewater and membrane bioreactor(MBR)-treated wastewater contained multiple types of organic substances.The results also indicated that removing the oil-settleable substances,the colloidal particles(450-1000 nm),and the soluble organics will be necessary for the treatment of petrochemical wastewater.

1.Introduction

The term industrial wastewater refers to the wastewater and liquid produced by industrial activities,which contains industrial production materials,intermediate products,and pollutants produced in the production process.Differentindustries and differentproduction processes will generate a wastewater stream that contains a wide variety of components,some of which can be toxic and possess poor biodegradability.Chemical oxygen demand(COD)is utilized as the index for traditional wastewatertreatmentinadditiontothestandardforpetrochemicaldischarges.However,CODalonecannotbereliedupontoassessthequality of treated waters due to the existence of hazardous substances such as BETX(Benzene,Ethylbenzene,Toluene,Xylene)and PAHs(polycyclic aromatic hydrocarbons)that are independent of COD.The facility discharges wastewater into the municipal pipe network,where it can pollute the environment and threaten human health.So it has become necessary to conduct a qualitative and quantitative analysis of specific pollutants in sewage,especially the prioritized contaminants[1].In addition,ananalysisofconventionalwaterqualityandtheorganiccomponents of oily wastewater can provide insight into the COD compositionof the wastewater and provides the basis for determining a rational wastewater treatment process[2].

In the wastewater treatment process,the identification of complex organic pollutants with different particle size distributions(PSDs)and chemical compositions is a significant step.The type and composition of contaminated water are related to the PSD index of particles in the water[2,3].Sewage composition can be indirectly evaluated by sewage PSD data to save manpower and material resources[4,5].Many studies have focused on the PSD and chemical composition of contaminants in sewage.For example,it was found that the chemical composition and COD was correlated with PSD,and there were significant differences in PSD from different sources of sewage[2,6].The hydrolysis rate of organic matter in water samples is related to particle size.Smaller particles are easier to hydrolyze than larger particles because of their larger specific surface areas[7].The relationship between the number and size of total particles and pathogens in water can be obtained by PSD detection,along with whether the microbial content in sewage is in excess of the standard[8].Continuous research on PSD may promote the formulation of suitable sewage treatment technology,provide insight into the material migration changes in the sewage and evaluate the effectiveness of sewage treatment[4,9-11].

To study the PSD of wastewater,all particulate matter mixed in water should first be separated according to particle size.Common separation methods include centrifugation,extraction,microfiltration,and ultrafiltration.Gradient membrane separation techniques can shed light on the mechanisms of pollutant migration and transformation[12,13].Next,the parameters of the separated particles of different sizes are detected and analyzed for different parameters,including COD,BOD,UV254,TOC,and TN.Depending upon the different parameters of PSD distribution,the appropriate treatment technology is selected[10,14].However,these parameters alone cannot be used to determine the specific material composition of sewage.When there are more soluble small molecules in sewage,other analytical tools are needed,such as gas chromatography-mass spectrometry(GC-MS),gel permeation chromatography(gpc/gfc),particle technology instruments,infrared spectroscopy,and transmission electron microscopy[15-17].These methods are used to characterize organic pollutants by theirfunctionalgroupsand molecularstructures.Inthepast,qualitative studies on wastewater have accounted for PSD.However,there have been limited studies in which PSD has been analyzed in petrochemical wastewater.

The purpose of this study was to provide additional insight into the characteristics of petrochemical wastewater to establish a suitable treatment system.In this study,we selected the petrochemical wastewater and treated wastewater as the analysis object.The COD and BOD fingerprints of wastewater based on gradient membrane separation were studied along with the biodegradability of the wastewater.To better understand the specific material composition of sewage,the functional groups and molecular structures of organic pollutants were characterized by advanced instrumental analysis methods,such as Fourier transform infrared(FTIR)spectroscopy and gas chromatographymass spectrometry(GC-MS)[18,19].

2.Materials and Methods

2.1.Sample collection

The water samples were taken from the pre-treatment wastewater and the wastewater treated by Membrane Bio-Reactor of CNOOC Zhongjie Petrochemical Co,Ltd.Our study was aimed at the analysis and study of petrochemical wastewater and sewage treated by Membrane biological reaction pool(Membrane bio-reactor).Some basic characteristics of the petrochemical wastewater and treated wastewater are listed in Tables 1 and 2.

2.2.Gradient membrane separation

Thewatersampleswereclassifiedandfilteredbymicrofiltrationand ultrafiltration separation technology.Next,the leachates of different size ranges were tested and analyzed to determine the water quality characteristics of different grades.The process of sewage grading schemes is shown in Fig.1:

Table 1 Quality of influent petrochemical wastewater[20]

Table 2 Quality of MBR-treated wastewater

The circulating water vacuum pump suction filtration device was usedformicrofiltrationexperiments.Thewatersampleswerestreamed through a fast filter paper,a slow filter paper,a 450 nm mixed fiber resin film,and a 200 nm mixed fiber resin film.The filtrate was then directly added to a magnetic stirring agitated ultrafiltration device that possessed a volume of 350 ml.Ultrafiltration experiments were then carried out using high purity N2(99.999%)as a driving force.Thefiltrate was filtered through an intercept with a molecular weight of 100000,30000,10000,3000,and 1000 ultrafiltration membranes,and all levels of the filtrates of different molecular weights were obtained.Finally,20 ml of filtrate at every level was collected for analysis and testing.

To ensure the accuracy of the test results was as high as possible,all exposed laboratory glass equipment was thoroughly cleaned.Glass equipment was not cleaned with substances containing organics,especially detergents.During ultrafiltration experiments,all separation was performed at room temperature(20°C)and pH 7.2-8.8.Restricted by filtration velocity,different membrane materials were selected for different membrane pressures(0.01-0.37 MPa).The smaller the aperture of the membrane,the greater the operating pressure required.

Particles in wastewaters have conveniently been grouped into operational size categories:dissolved,colloidal,supracolloidal,and settleable[6].In this study,the microfiltration(MF)and ultrafiltration(UF)procedures could aid in categorizing the petrochemical wastewater as dissolved,colloidal,supracolloidal,or settleable.The particles＞105nm was defined as the settleable fraction and the particles between 103and 105nm were defined as the supracolloidal fraction.The filtrate was categorized as dissolved components after ultrafiltration with 2 nm membranes.Finally,the particles between 2 and 103nm were categorized as the colloidal fraction.

2.3.COD and BOD detection

For COD detection and BOD5detection,a DRB200 chemical oxygen demand(COD)digester and BODTrak?II BOD analyzer manufactured by American Hach Company were used.The analytical method was based on the Hach Company test program.The COD and BOD detection schemes of HACH(HACH)in the United States are fast,convenient,and widely used[21-24].

Fig.1.Sewage grading scheme.

2.4.GC-MS analysis

2.4.1.Liquid-liquid extraction

Approximately 500 ml of the water samples were poured into a 1000 ml funnel,after which 100 ml of hexane extract was added to each sample.After shaken for 5 min,the mixtures were subjected to static stratification,and the organic phase from each sample was collected.The extraction was repeated twice,and the organic phases of each sample were combined.Anhydrous sodium sulfate was added to the organic phases(until a mobile anhydrous sodium sulfate was formed),then allowed to stand 30 min,dehydrated and dried before use.Theorganicphaseswereconcentratedto1mlat50°Cusinganitrogen Sparger.

2.4.2.Solid-phase extraction

To ensure that the adsorption and recovery of polar and non-polar substances were stronger,two solid phase extraction columns were selected for solid phase extraction:a Thermo Hyper Sep C18 solid phase extraction column and a Cleanert PEP-2 solid phase extraction column.10 ml of dichloromethane was added to the C18 column(5 ml each time)and let it flow out naturally after soaked for 5 min.Then 10 ml ofmethanolwasaddedtotheC18column(5mleachtime)andallowed to flow out naturally after being soaked for 10 min to activate the C18 column.Next,10 ml of water was added in the same manner(note thatduringactivation,thecolumnwasnotallowedtorundry).Approximately 500 ml of the pretreated water sample was streamed through the C18 column at approximately 5 ml·min?1.The small column after the enrichment was drained by vacuum.First,3 ml of acetonitrile were added to the C18 column and 2 ml of acetonitrile was added after being soaked for 5 min.Subsequently,the column was eluted and the eluate was collected.The dichloromethane was added to the column to elute in the same manner.The extraction process of the Cleanert PEP-2 solid extractioncolumnwaslike theC18 columnextraction process except for the fact that the Cleanert column was cleaned with dichloromethane and then washed with acetonitrile.The two solid-phase extraction eluents were collected in a 10 ml glass vial,and the eluate was completely volatilized by a nitrogen blowinginstrument at 40 °C.The sample was sealed and placed in a 4 °C freezer to prepare for derivatization.

2.4.3.Sample derivatization method

To prepare the pyridine solution of 20 mg·ml?1methoxy ammonium hydrochloride,200 mg of methoxy ammonium hydrochloride was added to a 10 ml volumetric flask and brought to the desired volume by the pyridine solution.Next,130 μl of pyridine solution of 20 mg·ml?1methoxy ammonium hydrochloride and 20 μl of 0.2 mg·ml?1internal standard D4-succinic acid were added to the solid-phase extraction sample.After full dissolution,the sample was placed in a water bath at 40 °C for 90 min.Then,150 μl of trimethylsilyl trifluoroacetamide(MSTFA)were added to the sample.After sufficient dissolution,the sample was placed in a water bath at 40°C for 30 min.Finally,30 min of derivatization was carried out and the solution was transferred to an autosampler for GC-MS analysis.

2.4.4.GC-MS test

One microliter of each sample was injected into a quadrupole Agilent 7890B GC/MS system(Agilent,USA)equipped with a SE-54 silica capillary column(30 m×0.25 mm×0.25 μm).Instrumental conditions were as follows:

Oven:equilibration time was 0 min,maximum temperature was 300 °C,initial column temperature was 70 °C and hold for 2 min,then increased to 290 °C at 8 °C·min?1,and held for 15 min,run time was 44.5 min.The QQQ(triple quadrupole mass spectrometry)loading chamber from the EPC:He quenching gas 2.25 ml·min?1,N2collision gas 1.5 ml·min?1.Inlet temperature:280 °C.Injection volume 1 μl.Split ratio was 5:1.Scan range:full scan.

2.5.FTIR analysis

Themain functional groups in suspended particleswere detected by FTIR spectroscopy.Specific methods were as follows:100 ml untreated sewage and 500 ml membrane bio reactor(MBR)treated water werefiltered using a 0.45 mm MF membrane,the samples remaining on the membranewereusedforanalysis.Nextthesamplesweredriedinadryingoven at60°Cfor 12 h.Approximately 6 mgof thedried samplesand 300 mg of KBr were finely grinded in an agate mortar.The mixture was pressed into a pellet under 10 tons of force for 1 min.The FTIR analysis was conducted using a Nicolet 6700 spectrometer with resolution of 4 cm?1with a scanning range of 4000-400 cm?1.

3.Results and Discussion

3.1.Distributions of COD based on gradient membrane separation

Gradient membrane separation is an effective method to utilize when investigating the distribution of pollutants by particle size[25].For this reason,continuous research of particle size distributions is of may greatly aidineffortstowardstheimprovementofwastewatertreatmenttechnology.Inthis studyCOD was selectedas representativeindex becauseit isa reliable metric for evaluating the degree of pollution.

After the physical classification,the COD value of each particle size range was obtained by detecting and analyzing the petrochemical wastewater and treated wastewater.This was accomplished by comparing the COD value of each particle size range to the total COD value of the wastewater or treated water to obtain the percentage of COD values that fell within each particle size range.

Fig.2.COD particle size distribution of petrochemical wastewater(A)and MBR treated wastewater(B).

The total measured COD of the petrochemical wastewater was 745.87 mg·L?1.As shown in Fig.2,the highest COD value originated from settleable particles(＞1.2×105nm),which accounted for 54.85%of the total COD.The settleable particles in sewage mainly included inorganic particles of sediment,organic particles of animal,plant corruption,and biological particles such as plankton or microorganism.The petrochemical wastewater of this experiment was from petrochemical industrialpark.Accordingtothedataprovidedbythesewagetreatment plantof China seaoilZhongJiePetrochemical Co.LTD,concentrationsof petroleum in sewage can reach 500 mg·L?1.The main components of petroleum are straight paraffins,which are non-polar and almost insolubleinwater,butcaneasilyabsorbbothinorganicandorganicparticles.At thesame time,straightparaffinscan adsorb onto some non-polar organic substances that may be contained in the sewage.Therefore,it can beinferredfromtheexperimentalresultsthathigherreadingsofCODin the wastewater were due to greater concentrations of oil.The presence of particles less than 2 nm in size may have originated from the byproducts of microbial metabolism or small molecules in petrochemicalwastewater.TheCODoftheparticlesizethatrangedbetween1knm and 1.2 × 105nm was 25.3 mg·L?1,which accounted for only 3.39%of total COD possibly because there were more settleable components in the water than non-settleable components.When the sewage passed through the filter paper,the pore diameter of the filter paper was blocked,and the substances of the particle size at 1000 nm 1.2×105nm were trapped in the fast filter paper.The COD value of particles between 450 nm and 1000 nm is the highest in colloidal components.Other types of wastewater,such as waste leachate[26],pulping and papermaking wastewater[27],polymer industrial wastewater[28],and so on,did not exhibit this feature,which may have been due to the high oil content of sewage.

After biochemicaltreatmentreactionsand filtration,thewastewater would flow into the MBR.The total measured COD of MBR treated wastewater was 186.69 mg·L?1.As a result of the membrane filtration process,the COD of substances with particle size of over 1000 nm was lower.As shown inFig.2,thecolloidalparticles made thelargest contribution to COD value,especially the substances with particle sizes between 450 nm-1000 nm(34.17%),which may have been due to the presence of microorganism-produced or microbially-cleaved products and macromolecular extracellular polymers.The COD of the particles less than 2 nm and molecular weight of less than 1000 was 55.1 mg·L?1,and accounted for 29.51%of the total COD of MBR treated wastewater;the particles substances are were mainly derived from small molecular organics that were the metabolites of microorganisms and not fully degraded in the MBR reactor and other small organic molecules already present in wastewater are also difficult to biodegrade.

Fig.3.BOD particle size distribution of petrochemical wastewater(A)and MBR treated wastewater(B).

According to the COD particle size distribution characteristics of the petrochemical and MBR treated wastewaters,it was observed that the settleable components were completely removed after the following steps were carried out:oil removal,vortex gas float,dissolved gas airfloat,acidizing pool,aerobic pool,and MBR pool treatment;and the COD in treated wastewater was mainly contributed by colloidal and soluble particles.Particularly,the COD of the colloidal components with a particle size between 2 and 3 nm did not decrease but increased.The COD of petrochemical wastewater in this particle size range was 21 mg·L?1and the COD of MBR treated wastewater was 24.7 mg·L?1.This may have been because some of the microbiological pyrolysis byproducts or the substances contained in the water were difficult to biodegrade in MBR treated wastewater.In the other soluble components,the COD value of petrochemical wastewater was approximately 145.5 mg·L?1,and treated wastewater's COD was approximately 55.1 mg·L?1,which was the main reason that treated wastewater is substandard,so the soluble components were the focus of sewage treatment[29,30].

3.2.Distributions of BOD based on gradient membrane separation

DuetotheCODparticlesizedistributioncharacteristicsandtheneed to analyze the BOD5particle size distribution characteristics,the petrochemical wastewater and the MBR treated wastewater passed through the following membranes:a slow filter paper,a 200 nm film,a 100000film,a 10000 film and a 1000 film.The experimental results are shown as follows:

As shown in Fig.3,the total BOD of petrochemical wastewater is 156.54 mg·L?1.The BOD of the substances greater than 1000 nm and were trapped by the rapid filter paper is about 73.4 mg·L?1,accounting for 46.89%of the total BOD of petrochemical wastewater and it is the highest BOD value for all particle size ranges.The substances with particle sizes＜2 nm have the second highest BOD whose molecular weight are less than 1000 and the BOD is approximately 48.1 mg·L?1;accounting for 30.72%of the total BOD.The substances with particle sizes between 200 nm and 1000 nm have the third highest BOD and the BOD is approximately 20 mg·L?1.The proportion of BOD in other particle size ranges do not exceed 10%of the total BOD.

The total BOD of the treated wastewater was 11.2 mg·L?1.Approximately 73.21.%of the total BOD of the MBR treated wastewater primarily originated from particles less than 5 nm in size,especially the soluble particles less than 2 nm,the ratio reached 59.82%.The BOD of particles in the range of 2 nm-5 nm contributed 13.39%of the total BOD.These indicate that substances in this range had the capacity for biodegradability.

3.3.GC-MS analysis of components in MBR treated wastewater and petrochemical wastewater

Fig.4.GC-MS graph of liquid-liquid extraction(A)and solid phase extraction(B)of MBR treated wastewater.

The GC-MS graphs are shown in Figs.4 and 5.To better understand the main contributions of the filtrate COD,the material which contributedthemostCODwasselected.Themainsubstancesareshown in Tables 3 and 4.As shown in Table 3,the most prevalent pollutants in the treated wastewater include:1,2-benzenedicarboxylic acid;bis(2-methylpropyl)ester;1,2-benzenedicarboxylic acid;and bis(2-methylpropyl)ester which is mainly used as a plasticizer of PVC.It has the same plasticizing effect as dibutyl phthalate,but the volatility and water extraction loss are relatively large.It can also be used a substitute for dibutyl phthalate as well as a plasticizer for cellulose resins,vinyls,nitrile rubbers,and neoprene.In addition,the contribution of straight-chain alkanes is still high,especially for long-chain and longchain paraffins,indicating that after several processes,petroleum hydrocarbonshavenotcompletelydegraded.Forthesereasons,thesesubstances are responsible for the COD of MBR treated wastewater not up to standard.

Table 3 Major compounds of MBR treated wastewater

Table 4 shows that the pollutants in petrochemical wastewater include:direct alkane,branched alkane,benzene,ammonia and some plasticizers(1,3-benzenedicarboxylic acid bis(2-ethylhexyl)ester).It can be seen from Table 2 that the content of major substances in the water were relatively average,while the relative content of major substances was under 10%.

3.4.FTIR analysis of the settleable or supracolloidal components

Fig.6 shows the IR spectra of the retentate on the 0.45 mm MF membrane.Spectrum A shows that the petrochemical wastewater has a broad absorption peak at the 3426.8 cm?1mark,which is a mainly free or associated-OH absorption peak,and may be an amide group absorption peak.The absorption peak at 1042.65 cm?1could indicate the presence of an ether or Ar--O--R groups.The absorption peak at 2524.45 cm?1indicates the presence of the carboxylic acid.The peak at 2924.93 cm?1is assigned to the stretching vibration of--CH2--groups.The peaks at 1436.19 cm?1(CH3)and 2924.93 cm?1(--CH2--),showed that alkane was present in the wastewater.The peak at 1635.189 cm?1is the most common absorption peak of C＝C groups on the benzene ring.The peak at 880.38 cm?1indicated the presence of R2C＝CH2or Ar-NO2groups.The peak at 728.67 cm?1may be the absorption peak of an m-disubstituted benzene or 1,3,5-trisubstituted benzene.According to these groups,there is an abundance of oil pollutants in the settleable or supracolloidal components,as well as prove the speculation that the main COD content of petrochemical wastewater is due to oil.According to the FTIR transmission spectrum analysis of petrochemical wastewater,the organic substances that may be contained in the settleable particles of petrochemicalwastewater include alcohols,acids,ethers,phenols,alkanes,proteins,and aromatic compounds.

Table 4 Major compounds of petrochemical wastewater

According to spectrum B,the MBR treated wastewater had more absorption peaks.The absorption peak at 3425.29 cm?1indicated the presence of--OH groups or amide group.The absorption peak at 2924.18 cm?1and 2854.07 cm?1indicated the presence of--CH2--group.The absorption peak at 1644.34 cm?1showed that C＝C groups were present in MBR and are on the benzene ring or on the straight chain.The group represented by the peak at 1536.70 cm?1and 1455.69 cm?1are most likely a C＝C group of thebenzenering.The absorption peak at 1227.76 cm?1showed that phenol or ether is present in the sewage.The absorption peak at 771.63 cm?1is not obvious whichmayindicate thepresenceof monosubstituted benzeneor disubstituted benzene.It also prove that oil exist in the settleable or supracolloidal components of treated wastewater.The peak at 1036.03 cm?1is the absorption peak of the Ar--O--R group.According to the FTIR transmission spectrum analysis of MBR treated wastewater,the organic substances that may be contained in settleable particles of treated wastewater include alcohols,phenols,ethers,alkanes,aromatic hydrocarbons,and other compounds.

4.Conclusions

This study provided valuable details regarding the characteristics of petrochemical wastewater.In the petrochemical wastewater,the particlesthatcontributedto54.85%ofCOD,themostofanyparticlecategory,were the settleable particles＞1.2×105nm.In the MBR treated wastewater,the particles that contributed to 34.17%of the total COD were the colloidal particles(450-1000 nm).Additionally,the BOD analysis results showed that the soluble fractions were primarily responsible for the MBR treated wastewater's failure to meet treatment standards.The FTIR and GC-MS results showed that there were mainly hydrocarbons and hydrocarbon derivatives in the petrochemical wastewater and treated petrochemical wastewater.It was observed that removing theoil settleable substances,thecolloidal particles(450-1000nm),and the soluble organics was crucial for the petrochemical wastewater to meet treatment standards.Judging from these results the suspended componentsandcolloidalcomponentsshouldbetreatedbycoagulation and sedimentation processes for targeted deep processing and oxidationoradsorptionforsoluble organic components,suchas chainhydrocarbons,esters and alcohols,should be adopted.

Fig.6.FTIR transmission spectrum of petrochemical wastewater(A)and MBR treated wastewater(B).