treatment of secondary effluent from a petrochemical wastewater treatment plant by...

Research ArticleReceived: 6 December 2013 Revised: 27 January 2014 Accepted article published: 5 February 2014 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.4346

Treatment of secondary effluentfrom a petrochemical wastewater treatmentplant by ozonation-biological aerated filterChangyongWu,a,b Zhen Gao,c Yuexi Zhou,a,b* Mingguo Liu,d Jiamei Songc

and Yin Yua,b

Abstract

BACKGROUD: Secondary effluent collected from a petrochemical wastewater treatment plant was treated by ozone–biologicalaerated filter (O3-BAF) to investigate and evaluate the performance, organics and genotoxicity removal characteristics of thiscombined advanced treatment process.

RESULTS: The average dissolved organic carbon (DOC) of the secondary effluent was approximately 23mg L−1. When the ozonedosage was 10 mg L−1 and the contact time was 4 min, the DOC removed by O3-BAF was approximately 9 mg L−1 during which37% was removed by ozonation and 63% by BAF. The ozonation changed the percentage of the small molecular size organics(< 1 k) increasing from 54% to 67%, and the highmolecular size organics (> 100 k) decreasing from 26% to 8%.More than 85%of the genotoxicity of the chemical secondary effluent was removed in the ozonation unit. The genotoxicity of the BAF effluentwas less than 0.71 �g-4-nitroquinoline-N-oxide L−1.

CONCLUSION: Ozonation can change the organics molecular size, increase the biodegradability and obviously reduce thegenotoxicity of petrochemical secondary effluent. The O3–BAF process is suitable for the advanced treatment of petrochemicalsecondary effluent.© 2014 Society of Chemical Industry

Supporting informationmay be found in the online version of this article.

Keywords: petrochemical secondary effluent; ozone; biological aerated filter; organics removal characteristics; genotoxicity

INTRODUCTIONGenerally, industrial wastewater is the major source of toxicorganic compounds to natural waters.1,2 In China, the currentannual discharge of industrial wastewater is more than 2.1× 1010

t, and most of the industrial wastewater is treated through cen-tralized wastewater treatment plants. It is reported that morethan 95% of the biological industrial effluent meets the dis-charge standards, but the implementation standard is relativelyold (GB8978-1996), and the standard lists only COD and a limitednumber of conventional indicators, such as nitrogenandphospho-rus. In many countries, for the protection of natural water bodiesand the improvement of drinking water quality, wastewater dis-charge standards are gradually being addedwith specific organicsand even biological toxicity index.3 In this case, an advancedtreatment technology for biological industrial effluent needsdevelopment.Chemical oxidation technology is one of the common advanced

wastewater treatment technologies used to further improve thewastewater secondary effluentquality or toprovidepotential recy-cled water. The most common oxidants are ozone and hydro-gen peroxide, etc. Ozone has been widely used in water treat-ment. In earlier years, corona discharge was the main source forgenerating ozone. As the cost was high, ozone was used mainly

for the disinfection of drinking water.4,5 With the developmentof ozone generation technology, the costs reduced gradually,and ozone is increasingly being used in advanced wastewatertreatment.6

There are many reports concerning the use of ozone in theadvanced treatment of municipal wastewater treatment planteffluents.7–9 Ozonation can remove part of the residual CODand enhance the biodegradability of the secondary effluent. Many

∗ Correspondence to: Yuexi Zhou, State Key Laboratory of Environmental Criteriaand Risk Assessment, Chinese Research Academy of Environmental Sciences,Beijing 100012, China; Research Center of Water Pollution Control Technology,Chinese Research Academy of Environment Sciences, Beijing 100012, China.E-mail: [email protected]

a State Key Laboratory of Environmental Criteria and Risk Assessment, ChineseResearch Academy of Environmental Sciences, Beijing 100012, China

b Research Center of Water Pollution Control Technology, Chinese ResearchAcademy of Environment Sciences, Beijing 100012, China

c School ofUrbanConstruction, HebeiUniversity of Engineering, Handan056038Hebei, China

d School of Water Resource and Environment, China University of Geosciences,Beijing 100083, China

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www.soci.org C Wu et al.

studies have shown thatwith ozonation, residual refractory organ-ics with large molecules can be transformed into smaller ones,and this can increase the biodegradability of the wastewater. Thebiodegradable dissolved organic carbon (BDOC) in the ozonationeffluent can be removed by a subsequent biological treatmentprocess, such as biological aerated filter (BAF).5,10–12 Tripathi et al.reported that the aldehydes and carboxylic acid can be formedin the secondary effluent after ozonation, and the formation wassignificantly related with the ozone dosage and contact time.5

It is reported that a municipal secondary effluent by dosage of10 mg L−1 ozone, the main types of organics detected in thewastewater increased from 58 to 73, and BDOC increased from0.8–1.1 to 2.0–2.7mg L−1. The biodegradability of the wastewateris improved obviously.8 However, the application of ozone in theindustrial wastewater treatment is mainly the pretreatment ofsome toxic or refractory industrial wastewaters and there is littleinformation concerning the treatment of industrial secondaryeffluent by ozone.13–15

Ozonation-biological treatment is one of the most promisingprocesses among advanced wastewater treatment processes, andthe core unit of the combined process is the ozonation unit. Thedegree of ozonation has a large impact on the subsequent biolog-ical treatment. Some studies show that many of the intermediatesproduced during the ozonation are even more toxic organicsthan the initial organics.16–18 However, some studies also haveshown that the toxicity of the wastewater decreased obviouslywhen applying an ozonation or ozonation-biological process.19,20

Much research is currently focused on the performance and char-acteristics of the ozonation-biological process on the treatmentof municipal secondary effluent.7–9 Generally, the residual CODconcentration in industrial secondary effluent is higher than thatin municipal secondary effluent, and most of them are refractoryorganics or even toxic organics.21,22 Therefore, there is a needto investigate the ozonation-biological process in the treatmentof industrial secondary effluents. In China, the percentage ofdischarged petrochemical wastewater in industrial wastewatersis about 3–4%. However, the percentage of volatile phenols dis-charge by petrochemical wastewater is over 35%. Although mostpetrochemical secondary effluents meet the discharge standardnowadays when treated by conventional biological treatment

processes, the discharge standards in China do not include spe-cific organics and biological toxicity index. It is expected that theywill be involved with executing increasingly stringent dischargestandards in the future to protect natural water bodies.In this study we investigate the O3 –BAF process treating sec-

ondary effluent collected from a petrochemical wastewater treat-ment plant. The performance, characteristics and genotoxicitychanges of O3 –BAF were investigated.

MATERIALS ANDMETHODSExperimental setup

The experimental setup mainly consisted of one ozonation reac-tor and two parallel BAFs (#1 BAF and #2 BAF). #1 BAF was fedwith ozonation effluent and #2 was fed with petrochemical sec-ondary effluent (Fig. 1). The ozonation reactor consisted of ozonegenerator (YG-5, Beijing Shanmeishuimei Company, China), con-tact reactor, ozone-online detector and off-gas absorption device.The ozone generator uses air as the gas source and the ozoneyield capacity is 5 g h-1 with gas flow rate of 0–300 L h-1. Theozone contacting reactor ismade of plexiglasswith a height of 100mm and a microporous diffuser in the bottom of the reactor. Anozone-onlinedetector (Ideal 2000, Zibo Ideal Company, China)wasused to detect the ozone concentration generated by the ozonegenerator. Off-gas ozoneglass bottleswere fittedwith 20%KI solu-tion. #1 BAF and #2 BAFwere the same, made of plexiglass with aninner diameter of 100 mm and a height of 2000 mm with microp-orous air diffuser fixed in the bottom of the BAF column. Sampleports were set at intervals of 10 cm. Particle ceramsites with diam-eter 4–6mmanddensity 2.26× 103 kgm-3 filled the BAF to a fillingheight of 1200 mm.

Experimental procedure#1 BAF and #2 BAF were incubated with secondary effluent col-lected from a petrochemical wastewater plant for 20 days. Theparameters of the two BAFs during the incubation time were thesame: gas–water ratio 5:1, hydraulic retention time (HRT) 5 h. TheDOC removal efficiencyof the two reactors increasedgradually dayafter day and eventually stabilized at 8 to 10%. Then the influent

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Figure 1. Schematic diagram of the O3-BAF process (1) ozone generator; (2) feed tank; (3) ozone concentration detector; (4) ozone reaction column;(5) peristaltic pump; (6) residual ozone absorption device; (7) ozone stripping tank; (8) air compressor; (9) # 1 BAF; (10) #2 BAF; (11) pH, DOmeter.

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of #1 BAF changed from secondary effluent to ozonation effluentfrom day 21. The operating parameters of ozonation reactor andBAFwere ozone dosage 10mg L−1, contact time 4min, gas–waterratio 3:1, HRT 3 h, which were determined by further experiments,as shown in the Supplementary data. The ozonation effluent wasfirst transferred into anozone stripping tank to remove the residualdissolved ozone and then pumped into #1 BAF. Samples were col-lected once a day. Immediately after collection, the samples weretransported to the laboratories for analysis.

Wastewater collectionThe wastewater used in this study was the secondary effluent of apetrochemical wastewater treatment plant with anoxic/oxic (A/O)as the core process. The wastewater treated by this petrochemicalwastewater treatment plant is the mixed wastewater dischargedfrom a petrochemical industrial park containingmore than 50 setsof petrochemical production plant, including typical petroleumrefining plants, basic petrochemical raw materials (intermediates)production plants and petrochemical synthetic material plants.Themain qualities of the secondary effluent were (inmg L−1): COD70–120, DOC 16–30, BOD5 1–5, NH4

+–N 0–0.2, TN 10–15, TP0.5–2.0 and pH 6–8.

Analytical methodsMolecular size distributionThe molecular size distribution (MSD) measurement of watersamples was carried out by ultrafiltration (driven by 0.1 MPahigh-purity N2) using different pore size membranes, 1, 3, 5, 10, 30and 100 k, respectively. The water samples were filtered by a 0.45μm cellulose acetate membrane and the measured DOC is thetotal DOC. Then each water sample was filtered by different poresize membranes with stirred ultrafiltration cell (model 8400, Milli-pore, USA). The DOC of each fraction of the sample wasmeasured.TheMSD is calculated based on the ratio of each fractional DOC tothe total DOC.8 A DOC mass balance was made in this study withtotal DOC equal to the sum of every partial DOC in the differentMSD ranges within the allowed error factor.

GC-MS analysisGas chromatography–mass spectrometry (GC–MS) was used fororganics analysis. The preparation of the sample was the same asin a previous study.23 After preparation, a 1�L samplewas injectedinto a 7890/5975 GC–MS system (Agilent, USA) equipped witha HP-5MS capillary column. The temperature program of the GCwas: initial oven temperature 40∘C for 2 min, raised at 5∘C min−1

to 100∘C and held for 2 min, raised at 10∘C min−1 to 220∘C andheld for 1 min, raised to 300∘C at 20∘C min−1 and held for 4 min.The operating parameters of the MS were ion source temperature230∘C, quadrupole temperature 150∘C, electron energy 70 eV, sol-vent delay time 6min, interface temperature 280∘C. The identifica-tion and analysis of the organics were based on the NIST 05 massspectral library database.

Three-dimensional fluorescence scanThe pH of the water samples was adjusted to 7.0 and then filteredusing a 0.45 μm cellulose acetate membrane. The water sampleswere diluted 10-fold before analysis. A fluorescence spectropho-tometer (F-7000, HITACHI, Japan) was used for the measurement.The operating parameters were: excitation and emission slit width5 nm, range of excitation and emission wavelength 200–500 nm,intervals 10 nm.

Genotoxicity measurementWater samples (100 mL) were acidified to pH 2.0 with 2 molL−1 H2SO4, and then passed through resin cartridges contain-ing 1 g of CHP20P resin (Mitsubishi Chemical, Japan), whichconsisted of 75–150 μm poly(styrene-divinylbenzene), as pre-viously described.24 Organics retained on the cartridge wereeluted with acetone and completely dried by nitrogen flow. Thedry residues were then dissolved in 100 �L dimethylsulfoxide(DMSO) to obtain 1000-fold concentration (volume of water sam-ple to volume of extract). The genotoxicity of the concentratedwater samples was evaluated with the SOS/umu test based onSalmonella typhimurium TA1535/pSK1002 without S9 activationaccording to ISO 13829.25 In this assay, the �-galactosidase wasapplied to monitor overall genotoxicity. The DMSO solution of4-nitroquinoline-N-oxide (4-NQO) was used as positive control.The genotoxicity and its calculation were as described by Wuet al.24 The genotoxicity of the sample was standardized as theequivalent 4-NQO concentration. Each sample was measuredthree times in parallel.

Other analytical methodsThe ozone concentration in the wastewater was determined byan iodometric titration method.8 Prior to analysis, samples werefiltered through a 0.45 μm cellulose acetate membrane, adjust-ing the pH to 7.0. The DOC was measured with a TOC ana-lyzer (TOC-VCPH/CPN, Shimadzu, Japan) and UV254 was measured bya UV-Vis spectrophotometer (UV-1700, Shimadzu, Japan). Otherwater qualities, such as COD and BOD5 were measured accordingto the Chinese NEPA standard methods.26

RESULTS ANDDISCUSSIONThe optimization of ozonation and BAF unitsIt is clear that ozonation can degrade organics into simpler com-pounds which can be biodegraded in a bioprocess. The parame-ters of the ozonation unit, such as contact time and dosage, arevery important for BAF. The contact time and ozone dosage of theozonation unit were determined as 4 min and 10 mg L−1 in thestudy, as shown in the Supplementary data. The ozone utilizationratewas higher than 79%and BOD5/CODof the ozonation effluenthad the highest value of 0.15 under this condition (less than 0.06in untreated secondary effluent). The gas–water ratio and HRT ofthe BAF unit were 3:1 and 3 h as determined by experiment (Sup-plementary data).

Performance and characteristics of the O3–BAF processDOC and UV254 removal characteristicsThe average DOC value of the secondary effluent collected fromthe petrochemical wastewater treatment plant was 23± 3 mg L−1

during the operation. Figure 2(a) shows the DOC removal charac-teristics of the experiment. The DOC of the ozonation effluent wasapproximately 20 mg L−1, and when the ozonation effluent wassubsequently treated by #1 BAF, the DOC further reduced to 14mg L−1. It was less than 50 mg L−1 as measured by COD. The DOCremoved by O3 –BAF was approximately 9 mg L−1 during which37% was removed by ozonation and 63% was removed by BAF.BAF removed a greater amount of DOC, which is similar to someprevious studies.8,20 The ozonation can enhance the treatability ofthe secondary effluent. When the secondary effluent was treateddirectly by #2 BAF, the DOC of the effluent was approximately 21mg L−1, and the DOC removal efficiency was approximately 10%,

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Figure 2. DOC removal (a) and UV254 variation (b) characteristics in theO3 –BAFprocess (#1 BAF feedwith ozonation effluent and #2 BAF feedwithsecondary effluent).

which indicated the low biodegradability of the industrial sec-ondary effluent used in this study.The variations of UV254 of influent, ozonation effluent and #1

BAF effluent are shown in Fig. 2(b). As is known, UV254 is mainlyabsorbed by aromatic rings and conjugated double bonds, whichrepresents the organic matter with C=C, C=O structures, suchas phenolic, poly aromatic hydrocarbons, aromatic ketone, andaromatic aldehyde, etc.8,27,28 The specific UV absorbance (SUVA)is defined as the UV absorbance per milligram of DOC (L mg-1

m-1), which represents the UV-absorbing aromatic structures anddouble bonds of dissolved organic matters (DOM).29 The SUVA254

of the secondary effluent used in this study was 3.27± 0.31 L mg-1

m-1. GC-MS result shows that approximately 169 types of organicscan be detected in the secondary effluent, most of which (morethan 80%) are unsaturated bond or aromatic organics, such as ani-line, acetophenone, nitrobenzene, nitrochlorobenzene, styreneetc., as summarized in Table S1 (Supplementary data). The UV254

removed by O3 –BAF was approximately 0.275 cm-1 during which75% was removed by ozonation and 25% was removed by BAF.The situation is exactly the opposite of the DOC removal. Ozona-tion removed a greater amount of UV254, which mainly changedthe structure of the DOM in the secondary effluent, and is similarto some previous studies.8,20 However, GC-MS results show thatthe number of organics in the ozonation effluent decreased sig-nificantly, especially the aromatic organics. This is different fromsome previous studies treating municipal secondary effluent.5

Figure 3. Percentage variations of MSD of DOM in influent (a), ozonationeffluent (b) and #1 BAF effluent (c).

MSD variations during the treatmentSize distribution of wastewater COD fractions is an index forbiodegradability. Generally, most of the organics that can be uti-lized by the microorganisms are small molecule organics.30 Ozonehas strong oxidizing power. However, ozonation of secondaryeffluent does not lead to complete degradation of these chem-icals. Ozone can destroy the unsaturated molecules bonds, con-verting macromolecular organics into small molecular ones andenhancing the biodegradability of the wastewater.19 Figure 3 isthe percentage variations of MSD of DOM in the influent, ozona-tion effluent and #1 BAF effluent. More than 54% of the DOM inthe secondary effluent is smaller molecular organics. The organ-ics with molecular size higher than 30 k is approximately 35%,



which is obviously higher than that in the municipal secondaryeffluent.8 After ozonation, the organics with large molecular size(> 30 k) decreased to 12%, while the percentage of small molecu-lar size (< 1 k) organics increased to 67%. This is similar to previousstudies.7–9 When theozonation effluentwas subsequently treatedby BAF, the percentage of organics with molecular size less than1 k decreased to 40%. This is mainly because the small moleculestransformed by ozone can be partly biodegraded by microorgan-isms fixed in the BAF.

Three-dimensional fluorescence characteristicsThree-dimensional fluorescence spectroscopy is a useful technol-ogy to differentiate the changes and transformations of organ-ics in wastewaters.31–33 It is often used for interpreting the DOMfluorescence properties due to its high sensitivity, good selec-tivity, and non-destruction of samples.32 Figure 4 shows thethree-dimensional fluorescence spectra of DOM in influent, ozona-tion effluent and #1 BAF effluent. As shown in Fig. 4, there aretwo fluorescence peaks at �Ex/Em = 235/345 nm (Peak 1) and �Ex/Em=280/345 nm (Peak 2), which represents tryptophan-like aro-matic protein and soluble microbial byproduct-like chemicals.31

Table 1 shows the position and intensity of fluorescence peaksin wastewater samples. The position of the fluorescence peaksof the industrial secondary effluent used in this study is simi-lar to that of the municipal secondary effluent.34 However, thefluorescence fingerprint characteristics are completely different.The main fluorescence peak is the peak 1, and peak 2 is the sec-ondary peak. The FI ratio of peak 2 to peak 1 is 0.57, which isobviously lower than the typical municipal secondary effluent.34

This is because the tryptophan-like aromatic proteins are themainfluorescence organics in the municipal secondary effluent. How-ever, most of the DOM (Table S1) in the industrial effluent usedin this study has �–� conjugated double bond and the �Ex and�Em of these organics are often less than 250 nm and 350 nm,respectively.35 The FI of the two peaks reduced greatly when thesecondary effluent was treated by ozone. However, the FI has acertain degree of increase when the ozonation effluent was sub-sequently treated by BAF, as shown in Table 1. This is becausesome soluble microbial byproduct-like chemicals such as polysac-charides, protein and humic, can be produced when the ozona-tion effluent is treated by BAF, and these organics have a certainfluorescence.

Genotoxicity removalFigure 5 shows the genotoxicity changes in secondary effluent,ozonation effluent, #1 BAF effluent and #2 BAF effluent. The geno-toxicity of the secondary effluent is approximately 23.50± 1.23μg-4-NQOL−1. It is reported thatmost of thegenotoxicitymayexistin the hydrophobic acids (HOA) andhydrophobic bases (HOB) frac-tions of DOM in petrochemical wastewater.36 In this study, HOAand HOB occupy a total of 41% of the DOM in the secondary efflu-ent. Further study is needed to interpret the relationship betweenDOM fractions and genotoxicity in this study. Some studies indi-cate that ozonation of treatedwastewater can increase the toxicityand affect the development and reproduction of fish.17,18 How-ever, in this study, ozonation can greatly reduce the genotoxicityof the industrial secondary effluent, and it decreased to 3.52± 0.45μg-4-NQO L−1 after ozonation. More than 85% of the genotoxicityof the secondary effluent can be removed. The genotoxicity wasfurther reduced to 0.71± 0.24 μg-4-NQO L−1 when subsequentlytreated by BAF. However, the genotoxicity reduced slightly when

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Figure 4. Three-dimensional fluorescence spectra of DOM in influent (a)ozonation effluent (b) and #1 BAF effluent (c).

directly treating the secondary effluent by BAF, and only 8% ofgenotoxicity can be removed during this process. More study isneeded to further reveal the relationship and response of removedorganics and genotoxicity during the process.

CONCLUSIONSIn this study, O3 –BAF was used to treat petrochemical secondaryeffluent, and the following conclusions are noted:

(1) when the ozone dosage was 10 mg L−1 and the contact timewas 4 min, more than 37% of the DOC was removed duringozonation. BAF removed more DOC than ozonation and 63%of it was removed by BAF;



Table 1. Position and intensity of fluorescence peaks in wastewater samples

Influent Ozonation effluent #1 BAF effluent

Peaks Ex/Em FI (nm) Ex/Em FI (nm) Ex/Em FI (nm)

Peak 1 235/345 6746 235/345 1219 235/345 2157Peak 2 280/345 3824 280/345 512.4 280/345 892.4

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#1 BAF #2 BAF

Figure 5. Changes of genotoxicity in secondary effluent, ozonation efflu-ent, #1 BAF effluent and #2 BAF effluent.

(2) the percentage of the small molecular size organics (< 1 k)increased from54%to67%, and thehighmolecular size organ-ics (> 100 k) decreased from 26% to 8% during ozonation;

(3) the fluorescence intensity (FI) reduced greatlywhen the chem-ical secondary effluent was treated by ozone. However, the FIhas a certain degree of increase when the ozonation effluentwas subsequently treated by BAF;

(4) the genotoxicity of the secondary effluent was higher than 20μg-4-NQO L−1 and it was reduced greatly by O3 –BAF whilemore than 85% of it was removed by the ozonation unit.

ACKNOWLEDGEMENTThe work is financially supported by the China special S&T projecton treatment and control of water pollution (2012ZX07201-005)and the National Natural Science Foundation of China (51208484).

Supporting InformationSupporting informationmay be found in the online version of thisarticle.

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