air system

Bioresource Technology 102 (2011) 2555–2562

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Treatment of oilfield wastewater containing polymer by the batch activated sludge reactor combined with a zerovalent iron/EDTA/air system Mang Lu a,⇑, Xiaofang Wei b a b

School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, Jiangxi Province, China State Key Laboratory of EOR, Research Institute of Petroleum Exploration & Development, CNPC, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 22 August 2010 Received in revised form 23 November 2010 Accepted 24 November 2010 Available online 30 November 2010 Keywords: Produced water Pretreatment Fenton reaction Hydrolyzed polyacrylamide

a b s t r a c t Laboratory-scale experiments were conducted in order to evaluate the performance of a novel treatment process for oilfield wastewater based on combining chemical oxidation, performed by a zerovalent iron (ZVI), ethylenediamine tetraacetic acid (EDTA) and air process, with biological degradation, carried out in a batch activated sludge reactor. The influence of some operating variables was studied. The results showed that the optimum pretreatment conditions were 150 mg/L EDTA, 20 g/L ZVI, and a 180-min reaction time, respectively. Under these conditions, removal efficiencies for hydrolyzed polyacrylamide (HPAM), total petroleum hydrocarbons (TPH), and chemical oxygen demand (COD) were 66%, 59%, and 45%, respectively. During the subsequent 40 h of bioremediation, the concentrations of HPAM, TPH, and COD were decreased to 10, 2 and 85 mg/L, respectively. At the end of experiments, the total removal efficiencies of HPAM, TPH, and COD were 96%, 97% and 92%, respectively. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Water-soluble polymers injection in petroleum reservoirs has been applied worldwide to enhance recovery of crude oil for decades (Taylor and Nasr-El-Din, 1998). In order to enhance oil recovery, various technologies including polymer (mainly hydrolyzed polyacrylamide, HPAM) flooding have been extensively used in China. Hence, there is a large quantity of HPAM in the produced water during polymer flooding production. Here, ‘produced water’ is designated the wastewater generated after separation from oil during the primary separation process. Presently, discharge into surface-water bodies after treatment and meeting the standard, or re-injection into the reservoir after proper treatment, is the fate of produced water from oilfields (Fakhru’l-Razi et al., 2009). As most of the polymer remains in the produced water, the viscosity of the wastewater is rather high and oil droplets rise very slowly during wastewater treatment. Moreover, the presence of HPAM in produced water can stabilize oil–water emulsions, therefore increasing the difficulty of the demulsification process and the subsequent treatment. Hence the produced water from polymer flooding is more difficult to treat, as compared with that from water flooding (Deng et al., 2002). There is an urgent need for robust ways of decomposing HPAM effectively, therefore improving the treatment efficacy of the ⇑ Corresponding author. Tel./fax: +86 798 8499678. E-mail address: [email protected] (M. Lu). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.11.103

subsequent wastewater remediation, when HPAM is used as a driver for oil exploitation. During the past decade, some researchers have used zerovalent iron (ZVI) as a heterogeneous catalyst to generate Fenton-like reactions for the removal of organic pollutants (Tang and Chen, 1996; Bremner et al., 2006; Kallel et al., 2009). In this process, the corrosion of the iron generates ferrous iron giving rise to a potent Fenton-type reaction. Tang and Chen (1996) demonstrated that H2O2/iron powder system is more effective for decolorization of dyes than Fenton’s reagent due to the continuous dissolution of iron and adsorption of dyes on the iron surface. The particular advantages of this process are cost-saving due to the use of metal iron compared to iron salts and the faster recycling of ferric iron at the iron surface (Bremner et al., 2006). Biotreatment is a cost-effective and efficient method which has been used widely for remediation of oilfield produced water (Campos et al., 2002; Chavan and Mukherji, 2008; Ji et al., 2009; Shpiner et al., 2009). However, a major problem hampering these technologies is the recalcitrance of HPAM to biodegradation. Provision of biotreatment systems able to cope with this problem is a significant technological and environmental challenge. In this study, we proposed a way to partially destroy HPAM contained in oilfield produced water, so as to improve biodegradability of the wastewater, through the application of Fenton-like reactions. In this system, H2O2 was continuously self-produced in the presence of dissolved oxygen (DO) and ethylenediamine tetraacetic acid (EDTA). The evaluation of treatment efficiency was carried out by means of

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time-course physicochemical and instrumental analysis of treated wastewater. 2. Methods 2.1. Materials 2.1.1. Chemicals The analytical grade Na2H2EDTA, HPAM, CdI2, KBr, H2SO4, and NaOH were used as purchased without further purification. The HPAM is a commercial product of Beijing Chemical Co. with a weight-average molecular weight of about 5  l06 Dalton and a hydrolysis degree of about 30%. Iron powder (40–60 mesh, 99%, nitrogen flushed) was purchased from Beijing Yili, China. A stock EDTA solution was prepared by dissolving Na2H2EDTA in water. All solutions were prepared with high purity water. 2.1.2. Produced water The produced water from polymer flooding was obtained from Henan Oilfield, China. Characteristics of the wastewater were as follows: pH 6.5, total suspended solids 135 mg/L, total dissolved solids 4830 mg/L, total petroleum hydrocarbon (TPH) 62 mg/L, total organic carbon (TOC) 371 mg/L, chemical oxygen demand (COD) 1130 mg/L, 5-day biochemical oxygen demand (BOD5) 116 mg/L, Al3+ 3.42 mg/L, Fe2+/Fe3+ 0.11 mg/L, HPAM 237 mg/L, and viscosity 35 mPa s (25 °C). The free oil was skimmed before using and determining. The wastewater was stored at 4 °C until required. 2.1.3. Inoculum Raw aerobic activated sludge was collected from the wastewater treatment plant of a petroleum refinery in China. Acclimation and selective enrichment of microorganisms in the sludge were performed in a 1-L Erlenmeyer flask containing 0.5 L of the produced water. The raw sludge sample was centrifugated at 8000 rpm for 10 min and the pellet was added to the flask. The flask was incubated at 25 °C and stirred at 150 rpm on a rotary shaker for 1 month. Every 2 days, a proper amount of produced water was added to the flask to make up for water loss, and also to serve as a nutrient medium for the microflora. The acclimatized sludge was centrifugated and added to the flask, incubated for another month. The entire procedure was repeated three times. In the end, the resulting pellet was washed three times with sterile high-purify water by centrifugating for 10 min at 8000 rpm, and then stored at 20 °C until use in the biodegradation test. 2.2. Pretreatment in the ZEA system A bench-scale investigation was conducted with the objective of studying the pretreatment behavior of biologically refractory wastewater contents, using the zerovalent iron, EDTA and air (ZEA) process, before discharge to the biological process. The pretreatment was carried out in a 7-L round-bottom Pyrex glass vessel with an adjustable fitted cover at room temperature. There were four holes at the reactor top to allow the introduction of an impeller, an automatic pH controller, a DO probe, and reagent. One sampling port was located along the side-wall of the reactor. A mechanical mixer set at 600 rpm provided mixing. Aeration was via a diffuser stone situated at the bottom of the reactor. In order to initiate treatment, the reactor was filled with 4 L produced water. Then, the compressed air valve was open, and the rate of air flow was adjusted manually to maintain DO level at approximately 3 mg/L. Finally, various dosages of ZVI and EDTA were, respectively added to start the reaction. During reaction, the reactor was shielded with aluminum foil to prevent any light-driven side reactions. The pH was controlled at 6.5 ± 0.2 with the pH controller using 3 M H2SO4 and 1 M NaOH. The samples were taken

out periodically and were analyzed immediately. An abiotic control experiment was also performed under the identical conditions but in the absence of ZVI and EDTA. 2.3. Batch activated sludge treatment Subsequent tests were conducted in the same reactor as above. After the chemical pretreatment, residual oxidant agents in the wastewater were eliminated by the addition of a proper amount of Na2SO3 to avoid oxidant toxicity to microorganisms. The solution was allowed to settle under gravity for 2 h, and then filtered by vacuum through Whatman No. 1 paper. Approximately 5 g of the activated sludge pellets were transferred to the reactor, and K2HPO4 (20 mg/L) was added to provide a phosphorus source. Timing started when the inoculation step was finished. Oxygen was supplied by dissolving compressed air into the water through bubble diffusers. The rate of air flow was adjusted manually to maintain the DO level above 2 mg/L. Meanwhile, a comparison study was performed in another reactor. This treatment was the same as above, except that raw produced water was used instead of the chemically treated water. 2.4. PCR-DGGE analysis Polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) was conducted to analyze microbial community dynamics during acclimation following the methods of Choi et al. (2007). The sample used in this study was raw and acclimatized activated sludge. DNA was extracted with a commercially available kit (Dingguo, China). All PCR amplifications were conducted in a GeneAmp PCR system 9600 (Applied Biosystems, CA). Clones matching bands of interest were sequenced at the ABI Prism 310 genetic analyzer (Vilber Lourmat, France). Sequence inserts were trimmed using Sequencher 4.7 (Gene Codes; Ann Arbor, MI) and close relatives determined by BLASTN searches of GenBank (Altschul et al., 1990). 2.5. Water analysis Hydrogen peroxide concentration was determined by spectrophotometer method proposed by Masschelein et al. (1977). In order to remove potential interferences of hydroxyl radical and H2O2 in the following measurements, pH of the collected effluents was increased above 11 with the addition of 6 M NaOH solution, and an appropriate amount of Na2SO3 was added to destroy residual H2O2 and O2 in the treated solution. The suspensions were centrifugated to separate liquid and solid phases. Water samples were analyzed according to standard methods (State Environmental Protection Administration of China, 2002). Briefly, TOC was measured by a TOC analyzer (Shimadzu TOC-VCPH, Japan). COD was determined by titrimetric method after dichromate closed reflux. BOD5 was measured via the oxygen consumption of bacteria breaking down organic matter in the sample over a 5-day period under standardized conditions. The seeding source of microorganisms used for BOD5 measurement came from a municipal wastewater plant. The concentration of HPAM in solution was measured by starch-CdI2 spectrophotometry (Scoggins et al., 1979). The Cl interference was eliminated by adjusting the pH to 3.5 with dilute H2SO4. The influence of Fe3+ was eliminated by maintaining a constant excess of Al3+ in the buffer solution. 2.6. Precipitation of HPAM An excess of methanol (2:1, v/v) was added dropwise into the wastewater, and the precipitated polymer was filtered off and washed 10 times with methanol. The molecular weight of HPAM

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was determined from the intrinsic viscosity measured in 2 M NaC1 solution at 20 °C using the equation (Ramsden and McKay, 1986):

½g ¼ 6:31  105 M0:8

where½gis in dl=g

2.7. Ecotoxicity tests Microtoxicity of effluents from the treatment process was measured by an SDI M500 analyzer (SDI, USA). Results were expressed as EC50 (15 min, 15 °C), which was defined as the effective concentration of pollutant for a 50% reduction of the luminescence of the bacterium Photobacterium phosphoreum. 2.8. Instrumental analysis 2.8.1. GC–MS analysis Analysis of water samples for saturates and aromatics were carried out using a simplified sample clean-up and a gas chromatography–mass spectrometric (GC–MS) system. Briefly, wastewater samples were extracted by liquid–liquid technique with dichloromethane three times for pH 2, 7, and 12. The three extract layers were combined and condensed to 1 mL in a rotary evaporator, and then fractionated by silica-gel column chromatography to separate saturate and aromatic fractions, following the method given by Bastow et al. (2007). The fractions were analyzed by an Agilent7890-5975c gas chromatograph–mass spectrometry equipped with an Agilent HP-5MS fused silica capillary column (60 m  0.25 mm  0.25 lm). The injection volume was 1 lL. The carrier gas was helium at 37 kPa. Flow velocity was 1 mL/min. The analytical conditions were: initial temperature of 50 °C, with isothermal operation for 1 min; heating to 120 °C at a constant rate of 20 °C/min; heating to 250 °C at a constant rate of 4 °C/min; and heating to a final temperature of 310 °C at a constant rate of 3 °C/ min, with a 30 min isothermal. Mass spectrometer conditions were: electron impact, electron energy 70 eV; filament current 100 lA; multiplier voltage, 1047 V; full scan. 2.8.2. ESI FT-ICR MS analysis An aliquot of the above extract without fractionation was examined by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). For FT-ICR MS analysis, the extract was dissolved in dichloromethane at a concentration of 0.1 mg/mL. A 1-mL aliquot of this solution was diluted with 1 mL methanol, and then concentrated with 10 lL of 35% (v/v) conc. NH4OH to facilitate deprotonation of acidic compounds by negative ion electrospray. The analysis was performed on an Apex-ultra FT-ICR MS (Bruker Daltonics, USA) equipped with a 9.4 T actively shielded magnet. Ions were generated by negative ion electrospray equipped with a 50 lm id fused silica ESI needle. Samples were infused at a flow rate of 250 lL/h. The operating software was XMASS version 6.0 (Bruker Daltonics, USA). Each spectrum was composed of 64 scans.

the activated sludge was significantly different after acclimation, as illustrated by a number of new DGGE bands (Fig. 1). Bands b, d, e, and w were consistently intense throughout operation, indicating that the populations represented by these bands were present in high levels throughout acclimation. Bands h, o, p, q and r were not prominent although the bands were consistently found. Bands g and m disappeared from the band patterns after acclimation. In addition, bands k and s appeared after acclimation and band s became prominent, suggesting that certain microorganisms were enriched through HPAM utilization and biodegradation. The newly emerging bands, k and s, were identified. Nucleotide sequences were compared to the GenBank database using BLASTN. Two sequenced clones, representing bands k and s, were most closely related to Enterobacter agglomerans and Paracoccus denitrification sequences, respectively. Nakamiya and Kinoshita (1995) isolated two strains of bacteria from soil, identified as E. agglomerans and Azomonas macrocytogenes, which can use HPAM as the sole source of carbon, nitrogen and energy. 3.2. Effect of initial EDTA concentration on ZVI-induced oxidation The effect of the initial EDTA concentration and ZVI amount was first investigated separately at 25 °C with an initial COD of 1130 mg/L. ZVI dose was set to 10 g/L, and the initial concentration of EDTA was varied from 0 to 150 mg/L. The change of COD removal efficiency with different dosage of EDTA is shown in Fig. 2a. The plots clearly demonstrated that an increase in the EDTA concentration enhanced the COD removal process. With the dosage of EDTA at 0, 50, 100, and 150 mg/L, COD removal efficiencies were 6.6%, 20.4%, 24.8% and 26.3%, respectively, after 300 min of reaction time. It has been demonstrated that ZVI can react with O2 to form ferrous ion and H2O2 (Eq. (1)), which leads to the Fenton reaction (Eqs. (2) and (3)) (Kang and Choi, 2009).

Fe0 þ O2 þ 2Hþ ! Fe2þ þ H2 O2 2þ

Fe

3þ

þ H2 O2 ! Fe



ð1Þ 

þ OH þ OH

ð2Þ

Fe0 þ H2 O2 ! Fe2þ þ OH þ OH

ð3Þ

In this system, the Fenton reagent is generated in situ, which makes the ZVI technology more versatile. The low COD removal in the case without EDTA addition may be attributed to the fast precipitation of Fe2+/Fe3+ at near neutral pH. EDTA is the most effective chelating agent in the enhancement of the degradation of hydrophobic organic contaminants under Fenton’s chemistry (Rastogi et al., 2009; Xue et al., 2009). In the ZEA system, the role of EDTA is two-fold, a chelating agent for maintaining Fe2+/Fe3+ under dissolved condition, and an essential activator of H2O2 generation (Eqs. (4)–(8)) (Seibig and van Eldik, 1997).

i j k

s t

2.9. Data analysis All experiments in this study were performed in triplicate to get reliable data, and results represented the average of three parallel experiments. 3. Results and discussion 3.1. Changes in microbial community structure during sludge domestication The bacterial community before and after domestication was compared by PCR-DGGE. The microbial community structure of

B

A

a bcd

efg h

l m n opqr

uvw

Fig. 1. DGGE profiles of PCR-amplified 16S rRNA genes from raw (a) and acclimatized (b) activated sludge samples.

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a

40 0 mg/L EDTA 50 mg/L EDTA 100 mg/L EDTA 150 mg/L EDTA

COD removal (%)

30

20

10

0 0

60

120

180

240

300

Time (min)

b

50

COD removal (%)

40

30

20

10 g/L ZVI 20 g/L ZVI 30 g/L ZVI 50 g/L ZVI

10

0 0

60

120

180

240

300

Time (min) Fig. 2. Efficiency of COD removal versus reaction time for (a) different initial EDTA concentrations, [COD]0 = 1130 mg/L, mFe0 = 10 g/L, pH 6.5, and T = 25 °C; (b) different initial dosage of Fe, [COD]0 = 1130 mg/L, [EDTA]0 = 150 mg/L, pH 6.5, and T = 25 °C.

Fe2þ þ EDTA þ H2 O ! ½Fe2þ ðEDTAÞðH2 OÞ2 2þ

½Fe ðEDTAÞðH2 OÞ

2

þ O2 $ ½Fe ðEDTAÞðO2 Þ

½Fe ðEDTAÞðO2 Þ ! ½Fe ðEDTAÞðO2 Þ2 ½Fe3þ ðEDTAÞðO2 Þ2 þ ½Fe2þ ðEDTAÞðH2 OÞ2 2þ

2

2þ

3þ

4 3þ ! ½ðEDTAÞFe3þ ðO2 þ H2 O 2 ÞFe ðEDTAÞ

ð4Þ 2

þ H2 O

ð5Þ ð6Þ

3.3. Effect of initial iron dosage on ZVI-induced oxidation

ð7Þ

4 3þ ½ðEDTAÞFe3þ ðO2 2 ÞFe ðEDTAÞ

! 2½Fe3þ ðEDTAÞðH2 OÞ þ H2 O2

and oxidative reactions are initiated by the electron transfer on Fe0 surface, the dissolution of iron particle into solution (reactions 1 and 3) becomes a crucial step.

ð8Þ

Hence, higher initial EDTA dosage could bring greater COD removal due to the more rapid H2O2 production. In this process, ferrous is produced and dissolved to solution through the oxidization on Fe0 surface exerted by O2 and/or H2O2 (Eqs. (1)–(3)). Moreover, higher EDTA concentrations increase the rate of iron corrosion and dissolution, because ligands that form bidentate-mononuclear complexes can enhance dissolution of metal surface oxides (Noradoun and Cheng, 2005; Keenan and Sedlak, 2008). Since both the reductive

The role of Fe0 in the ZEA system is also two-fold. On one hand, ZVI is a source for the reduction of Fe3+ to Fe2+. On the other hand, ZVI serves as a source of Fe2+ (Eq. (1)), which would participate in H2O2 production and Fenton reaction (Eqs. (1)–(8)). It was predicted that the removal rate of COD should be highly correlated with ZVI dosage, since the chemical destruction of organics by ZVI involves reaction at the iron surface. ZVI concentration was varied from 10 to 50 g/L, while initial concentration of EDTA was kept at a constant value (150 mg/L). The results are shown in Fig. 2b, where it can be seen that an increase in ZVI mass (surface area) within the system facilitated the COD removal process. Under initial 10 g/L iron loading, 26.3%

M. Lu, X. Wei / Bioresource Technology 102 (2011) 2555–2562

of COD was removed, at the end of reaction. A significant increase in COD removal was observed when increasing iron loading to 20 g/L, and 45.4% of COD was destroyed after 300 min of reaction. However, COD removal increased little when the dosage of iron was elevated from 20 to 50 g/L. This may be due to a limited mass transfer of oxygen and the reaction factors in the presence of high concentrations of solid particles in the solution. During treatment of coking wastewater by the ZVI method, Lai et al. (2007) also observed a mass transfer barrier occurring at iron loading higher than 30 g/L. One can observe from Fig. 2 that after a certain period of time, COD removal increased slowly, and the prolonged treatment time did not significantly increase COD removal. As the reaction progressed, EDTA was also gradually oxidized. Although complexation of Fe3+ by ligands can enhance Fe3+ solubility, in this study, the concentrations of Fe0 (P10 g/L, 0.18 M) were far more than that of EDTA (6150 mg/L, 5.1  10–4 M). Therefore the precipitation of Fe2+/Fe3+ would be inevitable, which may lead to passivation of the ZVI surface and decrease reaction activity (Keenan and Sedlak, 2008). Certainly this surface passivation can be removed by the addition of an appropriate amount of EDTA. 3.4. TOC, BOD5, and microtoxicity analysis during the ZEA process Based on the above results, the optimum concentrations of EDTA and ZVI for the development of the ZEA process were 150 mg/L EDTA and 20 g/L ZVI, respectively. These quantities were chosen for further experimental series. COD is an important parameter that was followed in order to know the degree of substances oxidation changes. TOC has been also measured in order to know the amount of organics that were mineralized to CO2 and H2O during Fenton oxidation. Fig. 3a shows COD, TOC, BOD5, and microtoxicity of produced wastewater with time as treated in the ZEA system. At the beginning of reaction (within 10 min), those small molecule and easily oxidized substances were mineralized quickly, therefore TOC and BOD5 decreased. Thereafter, the increase in BOD5 production indicated that the compounds in the wastewater were degraded into smaller molecules with greater biodegradability by Fenton oxidation. After 60 min of reaction time, BOD5 was reduced again, indicating that some biodegradable intermediates were gradually consumed. Fig. 3b presents the results regarding the variations of COD/TOC and BOD5/COD ratios during the development of the process. The COD/TOC ratio was increased from 3.059 to 3.207 after the reaction for 10 min, and then it was decreased gradually until the end of the experiment. The initial increase in the COD/TOC ratio was mainly a result of the greater rate of TOC reduction than that of COD due to the complete mineralization of organics. Basically, lower COD/TOC ratio means higher degree of oxidation. For example, the theoretical value of this parameter is 4.667 for C2H6, 2.667 for C2H4O2, and 0.667 for C2H2O4. As shown in Fig. 3b, the decrease in COD/TOC of the wastewater after 10 min of reaction suggested that the contaminants became more oxidized and more polar. The BOD5/COD ratio decreased from 0.103 to 0.081 after the reaction for 10 min. One possible explanation for this was that some toxic intermediates with low biodegradability were generated during the reaction. Afterwards, the BOD5/COD ratio went up continuously with the reaction except for a decrease occurring at 60 min, and reached 0.438 at the end of the experiment. As shown in Fig. 3a, the reduction of microtoxicity (increase in EC50) was obvious during the first 30 min of reaction and then declined, indicating production of some more toxic intermediates. After this, microtoxicity decreased again. The BOD5/COD ratio is the most extensively used biochemical index for quantifying biodegradability, and wastewater is considered with high biodegradability as the ratio is higher than 0.4 (Torres et al., 2003). On the

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basis of data shown in Fig. 3, the optimum pretreating reaction time for the produced water by ZEA oxidation was 180 min because at this moment, BOD5/COD reached 0.421, and microtoxicity was decreased by 84%. Moreover, the average molecular weight of HPAM has been reduced from 2.4  l06 to 0.6  106 after 180 min of treatment (data not shown). Ramsden and McKay (1986) suggested that hydroxyl radicals formed in the presence of DO can cause chain scission of the HPAM and a reduction in the molecular weight of the polymer, but chain scission did not occur in the absence of oxygen. 3.5. Biological process Based on the above results, the reaction under the optimum dosage of EDTA and ZVI (150 mg/L and 20 g/L, respectively) was quenched at a reaction time of 180 min by the addition of Na2SO3 to avoid oxidant toxicity. Iron powder and ferric hydroxide were removed through precipitation separation. The biodegradation experiment was started by inoculation with the sludge pellets. Removal efficacies for HPAM, TPH, and COD were 66%, 59% and 45%, respectively during the chemical pretreatment. As shown in Fig. 4, after an 8-h adaptation period, microorganisms started to quickly degrade pollutants. After 40 h of biodegradation, the concentrations of HPAM, TPH, and COD were decreased from 81, 25 and 617 mg/L, to 10, 2 and 85 mg/L, respectively. Percent HPAM, TPH, and COD removal efficiency obtained after the combined chemical and biological treatment was 96%, 97% and 92%, respectively. The TPH and COD of effluent can satisfy the professional emission standard (grade one) (TPH < 5 mg/L, CODCr < 100 mg/L) of petrochemical industry of PR China (GB4287-92). During the biological treatment of raw wastewater, however, COD was only decreased to 728 mg/L from the initial value of 1130 mg/L after 72 h of biodegradation (data not shown). Moreover, the prolonged treatment time did not significantly enhance COD removal. From the results obtained via combined chemical and biological oxidation of oilfield produced water, it can be safely said that advanced oxidation is reasonably suited and feasible for pretreatment of biorefractory and extremely polluted wastewater streams. The process is aimed to simplify the chemical structure of recalcitrant organic substances for enhancing their biodegradability. The effectiveness of biological and advanced oxidation processes for treating recalcitrant wastewaters has already been extensively explored by many authors (Scott and Ollis, 2006). 3.6. Removal of hydrocarbons A gas chromatographic investigation was made into oil oxidation on the extracts obtained from the raw stream water, after 180 min of ZEA oxidation, and after biodegradation, respectively. The chromatograms clearly demonstrate the effects of chemical oxidation and biodegradation on the chemical composition of saturates and aromatics (see Supplementary Electronic Annex, Fig. S1). From the gravimetric analysis of the saturate and aromatic fractions it was estimated that a degradation of 96% saturates and 85% aromatics had occurred at the end of the experiment. After the ZEA process, the fraction present in the highest percentage shifted from the C25 fraction to C27 one. Most peaks of n-alkanes have been depleted, and an unresolved complex mixture (UCM) occurred. The UCM includes branched and cyclic compounds that cannot be resolved by capillary columns, and has generally been attributed to degraded petroleum contamination. The most abundant peak in Fig. S1c corresponded to squalane (2,6,10,15,19,23-hexamethyltetracosane,C30H62), which is a multiply branched saturated hydrocarbon and much less susceptible to spontaneous chemical oxidation. The increase in the relative abundance of squalane could be attributed to concentration resulting from the greater

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a

COD BOD5

1000

TOC EC50

60

50 800 40

30

(%)

600

EC50

COD, TOC, and BOD 5 (mg/L)

1200

400 20 200

10

0

0 0

60

120

180

240

300

Time (min)

b

0.5

4

0.3

COD/TOC 3

BOD5/COD 0.2

BOD5/COD

COD/TOC

0.4

0.1

2

0.0 0

60

120

180

240

300

Time (min) Fig. 3. Changes in (a) COD, TOC, BOD5, and microtoxicity; (b) COD/TOC and BOD5/ COD ratios, during the chemical oxidation process using the optimum dosage of ligand and iron, 150 mg/L EDTA and 20 g/L ZVI, respectively.

degradation of other compounds, when more easily degradable compounds were degraded and squalane accumulated. Squalane often is recalcitrant to biodegradation. However, Berekaa and Steinbüchel (2000) demonstrated that squalane was susceptible to microbial degradation by actinomycetes Mycobacterium fortuitum and Mycobacterium ratisbonense. Furthermore, they found that squalane degradation would be inhibited in the presence of acrylic acid. As to the aromatics, the major feature was a series of resolved alkyl aromatic hydrocarbons comprising mainly naphthalenes and phenanthrene, besides sterane and bisbenzothiazole (Fig. S1d). Methylated naphthalenes and C21-triaryl-sterane seemed to be resistant to radical induced oxidation (Fig. S1e). Interestingly, however, they were highly susceptible to bacterial attack (Fig. S1f). Although Fenton’ reagent is believed to be an indiscriminate oxidizing agent, some compounds show resistance to oxidation by Fenton reaction (Bigda, 1995). Note that 4-methyl-dibenzothiophene was more resistant than other compounds to both chemical and micro-

bial oxidation (Fig. S1d–f). It is interesting to note that UCM was reduced greatly by Fenton oxidation; however it became larger than initially, and dominated the peak area, after undergoing bioremediation. Booth et al. (2007) found that UCM of aromatic hydrocarbons contained thousands of previously unidentified branched alkyl homologues of known aromatic hydrocarbons, and these compounds showed persistence, bioaccumulativeness, and toxicity. 3.7. Analysis of polar compounds by FT-ICR MS A major problem in the study of petroleum degradation is the complexity of crude oil because it comprises an enormous number of components (Dutta and Harayama, 2000). A large number of polar species in crude oil cannot be directly analyzed by GC–MS, because of the highly complexity in chemical structure, the thermal lability, excessive low volatility, or strong interaction between the carbonyl groups and the stationary phase in the GC column (Letzel et al., 2001). Some polar components can be analyzed by

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700

PAM TPH COD

600

500

60 400 40

300

COD (mg/L)

PAM and TPH concentration (mg/L)

80

200 20 100

0

0 0

8

16

24

32

40

Time (h) Fig. 4. Time course of HPAM, TPH concentrations, and COD of the produced water treated in batch activated sludge systems.

GC–MS by derivatizing the analytes to less polar, but this method is tedious and time consuming and commercial standard substances are sometimes hard to obtain. In order to check the formation of partially oxidized compounds due to both Fenton oxidation and biological activity, the oil and grease extracted from wastewater was characterized by ESI FT-ICR MS. The molecular weight distribution of polar compounds was significantly altered by chemical oxidation and biodegradation (figure not shown). The weight-average molecular weight (Mw) for the three oils (raw, after chemical oxidation, and after biodegradation) was 485, 426, and 213, respectively. The number of highintensity peaks was increased significantly after Fenton oxidation. This was mainly due to the oxidation of hydrocarbons by hydroxyl radical, because a series of oxygen containing compounds can be produced during Fenton oxidation of hydrocarbons (Tapper, 1992). Some degraded products of HPAM were also identified. The substrate C16H33N1 was n-cetamide, a possible degraded product of polymer after CAC bond breakage. Compounds C15H21O10 and C28H52N2O2, C8H18O1 and C13H24O4 were also degraded products from the polymer. The degraded products consisted of fractured HPAM with duplet bonds, epoxy and carbonyl groups and derivatives of acrylamide. It should be pointed out that HPAM is not amenable to FT-ICR MS, because the molecular weight of this polymer far exceeds the detection threshold of the instrument, and HPAM cannot be ionized by ESI. 4. Conclusions The degradation of pollutants present in oilfield produced water was carried out by using a combination of advanced oxidation and aerobic biological degradation process to investigate the synergism between the two modes of treatment. The results demonstrated that over the range of operating conditions tested, single-advanced oxidation or biological degradation achieved moderate reductions of COD. However, significant removal levels were obtained using the combination of the advanced oxidation by ZEA process followed by biodegradation process. The main advantage of ZEA oxidation over other hydroxyl radical generating systems is its simplicity. The chemicals are commonly available and inexpensive, and there is no need for special equipment like ultraviolet radiation lamps, complex reaction vessels, expensive TiO2 particles, or

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