Treatment+of+Oilfield+Produced+Water+by+Anaerobic+Process+Coupled+With+Micro Electrolysis

7/30/2019 Treatment+of+Oilfield+Produced+Water+by+Anaerobic+Process+Coupled+With+Micro Electrolysis

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Journal of Environmental Sciences 2010, 22(12) 18751882

Treatment of oilfield produced water by anaerobic process coupled with

micro-electrolysis

Gang Li1,2, Shuhai Guo1,, Fengmei Li1

1. Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China. E-mail: [email protected]

2. Graduate School of Chinese Academy of Sciences, Beijing 100039, China

Received 26 January 2010; revised 23 April 2010; accepted 12 June 2010

AbstractTreatment of oilfield produced water was investigated using an anaerobic process coupled with micro-electrolysis (ME), focusing

on changes in chemical oxygen demand (COD) and biodegradability. Results showed that COD exhibited an abnormal change in

the single anaerobic system in which it increased within the first 168 hr, but then decreased to 222 mg/L after 360 hr. The biological

oxygen demand (five-day) (BOD5)/COD ratio of the water increased from 0.05 to 0.15. Hydrocarbons in the wastewater, such as pectin,

degraded to small molecules during the hydrolytic acidification process. Comparatively, the effect of ME was also investigated. The

COD underwent a slight decrease and the BOD5/COD ratio of the water improved from 0.05 to 0.17 after ME. Removal of COD was

38.3% under the idealized ME conditions (pH 6.0), using iron and active carbon (80 and 40 g/L, respectively). Coupling the anaerobic

process with ME accelerated the COD removal ratio (average removal was 53.3%). Gas chromatography/mass spectrometry was used

to analyze organic species conversion. This integrated system appeared to be a useful option for the treatment of water produced in

oilfields.

Key words: heavy oil produced water; anaerobic system; micro-electrolysis; biodegradability

DOI: 10.1016/S1001-0742(09)60333-8

Introduction

Produced water, which is generated in the exploitation

processes of oil and gas industries, is the largest waste

stream source found in oilfields. Nowadays, more-stringent

environmental standards have led to greater efforts being

made to treat produced water. Many technologies have

been developed for the removal of petroleum pollutants

and chemical oxygen demand (COD), including flotation

(Ebrahimi et al., 2010), membrane separation (Qiao et

al., 2008; Cakmakcea et al., 2008), chemical precipitation(Doyle and Brown, 2000; Carvalho et al., 2002; Zhou et

al., 2000), chemical oxidation (Bessa et al., 2001), and

biological treatment (Lu et al., 2009; Li et al., 2005; Zhao

et al., 2006).

Among these technologies, biological treatment is fre-

quently used for the treatment of oilfield produced water

because of its high effectiveness and economical feasibili-

ty. Many laboratory- and pilot-scale experiments have been

performed under aerobic conditions, such as stabilization

ponds (Shpiner et al., 2009), biological aerated filter (Su et

al., 2009), sequencing batch reactor (SBR) systems (Freire

et al., 2001). Nevertheless, because the biological oxygen

demand (five-day) (BOD5)/COD ratio is relatively low in

oilfield produced water, it is difficult to decrease COD

* Corresponding author. E-mail: [email protected]

using a single aerobic biological technology. Therefore,

anaerobic processes have become increasingly popular for

the treatment of produced water with two aims: (1) to

catalyze the hydrolysis of organic compounds into long-

chain fatty acids, and (2) to improve the degradability of

the produced water.

Anaerobic processes are feasible for the treatment of

high-strength wastewater (McHugh et al., 2003), con-

verting organic pollutants to small molecules in various

non-biodegradation wastewaters (Barker et al., 1999).

Heavy oil produced water is, however, difficult to degrade(Guo et al., 2002), because it contains large quanti-

ties of large-molecule non-biodegradation organics. Low

BOD5/COD ratios and long processing time are poten-

tial problems associated with such anaerobic treatments.

Therefore, it is necessary to improve the biodegradability

of produced water and optimize the anaerobic treatment

conditions.

Many methodologies have been employed previous-

ly to improve the biodegradability of produced water,

including Fenton process, oxidation with ozone and

micro-electrolysis (ME). Among them, ME method is cost-

effective and operationally simple (Wang et al., 2004; Shen

et al., 2001; Nurul Amin et al., 2008). ME is an electro-

chemical process involving an active carbon cathode and

an iron anode. The following half reactions occur at the
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1876 Gang Li et al. Vol. 22

electrodes:

Iron anode (oxidation):

Fe 2e Fe2+, E0 (Fe2+/Fe) = 0.44 V (1)

Active carbon cathode (reduction):

2H+ + 2e H2, E0 (H2+/H2) = 0 V (2)

Most investigations into ME have indicated that

the zero-valent iron can reductively transform electron-

withdrawing constituents, making previously recalcitrant

compounds more amenable to the subsequent biological

oxidation process (Bell et al., 2003; Jin et al., 2003).

Therefore, we suspected that ME methodology might

provide a convenient model system for the improvement

of the biodegradability of produced water.

The objectives for this study were to determine the

effect of a single anaerobic process on the organics speciespresent in produced water and to determine the efficiency

of treating produced water through a combination of

anaerobic processing and ME. Particular attention was paid

to investigating the conversion and removal of organics in

oilfield produced water.

1 Materials and methods

1.1 Raw produced water and materials

Raw produced water was collected from a heavy oil pro-

duced water plant located in the Liaohe Oilfield, Liaoning

Province, northeastern China. The wastewater was treatedwith the intrinsic processes of oil separation and flotation,

providing a mean COD 274 mg/L. Table 1 lists several

chemical parameters of this wastewater sample.

Table 1 Chemical parameters of the produced water sample

Parameter Value Parameter Value

pH 7.4 0.3 TOC (mg/L) 136 5.8

Temp erature ( C) 3 2 4 SS (mg/L) 72 12.9

COD (mg/L) 274 16 TN (mg/L) 12.5 2.4

BOD5 (mg/L) 16 4.7 TP (mg/L) 0.11 0.04

Mineral oil (mg/L) 56.7 4.2

1.2 Performance of reactor system

A semi-hermetic, rectangular (length, 0.4 m; width,

0.3 m; height, 0.4 m) anaerobic bioreactor was con-

structed to treat the produced water continuously on a

laboratory-scale. The sequencing batch reactors were made

of stainless steel consisting of two parts (Fig. 1): one is

ME operation unit and the other is an anaerobic process

unit. The working volumes of the two reactors were 6 and

30 L, respectively. The produced water could be pumped

into the two reactors independently (Path A and Path B,

respectively). A table-flap was set up in the anaerobic

unit. The whole reactor was packed using heat isolating

materials.

In ME unit, industrial cast iron filings and shavings,

that were mainly free of visible rust (Shenyang Institute of

Fig. 1 Schematic representation of the experimental apparatus used

for the treatment of oil produced water. (1) pump; (2) ME operation

reactor; (3) anaerobic reactor; (4) table-flap; (5) heat isolating materials;

(6) overflow weir.

Metal) were added. The iron pieces were cut to a diameter

of ca. 1 mm and a length of 15 mm, with irregular shape.

They were degreased in a 10% hot alkaline solution, thensoaked in a 2% H2SO4 to remove surface rusts; and finally

washed three times with deionized water. The clean iron

was dried naturally. Activated carbon (industrial grade,

Liaoning Province, China) was used as a macroscopic

electrode material having a diameter of ca. 2 mm and

a height of 10 mm, with a columnar shape. In order to

eliminate adsorption effects, the active carbon was satu-

rated with raw wastewater for 24 hr prior to use. NaOH (2

mol/L) and H2SO4 (1 mol/L) solutions were used to adjust

pH. All chemical reagents used in the experiments were

analytical grade and obtained from Shenyang Chemical

Reagent Corporation.

1.3 Experimental procedure and design

The produced water was first treated in the sequencing

batch anaerobic reactor through Path A. The experiment

was conducted to study the removal of organic species in

a single anaerobic reactor. The treatment period was set as

15 days. Next, an experiment was performed to investigate

the COD removal and biodegradability of the produced

water in the anaerobic process coupled with ME, through

Path B.

The anaerobic reactor was seeded with anaerobic sludge

from a full-scale anaerobic pond at the produced water

plant with volatile suspended solid (VSS) concentration of25 g/L. The reactor was started using produced water at a

desired temperature, and pH 7.2. No nutrients were added

in the anaerobic system.

During the adaptive phase, COD of the influent was

maintained in the range 260300 mg/L at a constant

hydraulic retention time (HRT) of 360 hr in the anaerobic

reactor. When COD removal rate of the integrated system

was stable, the influent concentration was maintained

relatively constant (average COD: 274 mg/L). The system

was operated for three periods, with the pH, COD, BOD5,

and TOC measured routinely.

For ME process, the experiments were firstly performed

in beakers to determine the optimal parameters. Pretreated

iron and active carbon were added to beakers at various

dosage ratios. Wastewater (1 L) was then added into each

beaker and mixed. During the reaction process, an agitation


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No. 12 Treatment of oilfield produced water by anaerobic process coupled with micro-electrolysis 1877

apparatus with six agitators that rotated simultaneously

at the same speed (JJ-4A Jiangsu Province, China), was

used under insufficient oxygen as soon as practically pos-

sible. Each batch experiment lasted for 8 hr. At designed

intervals, the supernatant was removed and kept still for

60 min prior to analysis, and the dosage of active carbonwas 40 g/L, and the dosage of iron was 20, 40, 80 or

100 g/L. The initial pH was adjusted from 4.0 to 9.0 to

determine the optimal reaction condition. The experiments

for determining the parameters were conducted in a water

bath at 32C for determining the parameters.

For anaerobic process coupled with ME (Path B), the

pH of the influent was adjusted to 6.0 to facilitate the

ME reaction. The appropriate HRT was employed in ME.

The wastewater treated through ME was then collected

in the anaerobic reactor; its pH was adjusted to the same

value as that used for anaerobic treatment. The treatment

parameters in Path B were identical to those in Path A. pH,

COD, BOD5, and TOC were measured routinely.

1.4 Analytical methods

Prior to analysis, the supernatant sample was filtrated

through 0.45-m filter paper. All analytical procedures

were performed according to standard methods. COD

was measured using the potassium dichromate oxidation

method. BOD5 were determined using the five-day BOD

test method (APHA, 1999). TOC was analyzed using

a TOC analyzer (TOC-500, Shimadzu). The content of

hydrocarbons in the produced water was measured using

gravimetric methods (Wei et al., 1998; Reddy and Quinn

et al., 1999; Huang et al., 2003). Alkyl, aromatic, pectin,and bitumen samples were separated and measured using

methods described previously (Wang et al., 2009).

Gas chromatography/mass spectrometry (GC/MS) was

used to analyze of organic compounds in pretreated sam-

ples that had been subjected to liquid-liquid extraction

using CCl4 (chromatogram pure grade, Fisher Corporation,

USA). The extraction procedure was conducted under

acidic, neutral, or alkaline conditions (Cao et al., 2007).

Pretreated samples (1 L) were analyzed using a Trace

MS Trace 2000GC/MS system (Finnigan, USA). Highly

pure He (99.999%) was used as a carrier gas (flow rate:

1 mL/min). A DB-5MS capillary column (inner diameter:

0.25 mm; length: 30 m) was adopted in the separation

system. The temperature for the gasification compartment

was maintained at 260C. The temperature control pro-

gram for the column involved maintaining the temperature

at 90C for 1 min and then increasing it to 300C at

an increment of 5C/min. The electron energy and the

electron double voltage were set at 70 eV and 1200 V,

respectively. The molecular weights were scanned from 90

to 900 Da.

2 Results

2.1 Effect of anaerobic process on oilfield producedwater

COD, BOD5, and TOC were measured to evaluate the

removal of organics by the single anaerobic process. In

addition, the composition of hydrocarbons was analyzed to

investigate the changes in the levels of organics after single

anaerobic treatment.

2.1.1 Removal and conversion of organic matter during

single anaerobic process

Figure 2 displays the dynamic changes in COD and

BOD5 after treatment in the single anaerobic system. The

COD of the influent was 274 mg/L, a typical value for

heavy oil produced water. In the steady state, the mean

COD of the sample underwent a continuous increase with-

in the first 168 hr of the experiment, reaching 341 mg/L

(a 24.5% increase). After 168 hr, the COD of the produced

water decreased gradually. After 360 hr of operation, the

CODof the effluent was less than 222 mg/L. Almost 18.9%

of the COD was removed during the entire process. BOD5exhibited a similar trend. Within the first 144 hr of the

anaerobic process, the BOD5 increased from 16 mg/L to

a maximum concentration of 52.3 mg/L. After 360 hr, theBOD5 transmission was 22.6 mg/L. Although we observed

similar trends for COD and BOD5, the range of COD value

exceeded that of BOD5. In addition, the time required to

reach the maximum COD lagged behind that of BOD5. The

maximum BOD5/COD ratio of 0.15 was achieved after 144

hr.

Under anaerobic conditions, increased COD value was

observed for heavy oil produced water, which contrasts

with other refractory organic wastewater treatment sys-

tems. The present results can be explained by considering

that some contaminants in the raw oil water might not

be oxidized by the COD method used in this study.

Whereas, after anaerobic processing, they were degrad-

ed into simpler components that could be measured

using the potassium dichromate oxidation process. (1)

Straight-chain carboxylic acids may have been completely

oxidized, but the aromatics and polynuclear aromatics are

not readily oxidized, even in the presence of a catalyst

(Ag2SO4); (2) pectin in the raw produced water may

have been oxidized only partially, whereas fatty acids

derived from pectin through biotransformation during the

anaerobic process might be oxidized during the potassium

dichromate oxidation process. The relationship between

COD and theoretical oxygen demand (ThOD) has been

investigated for several specific classes of organic chem-icals (Baker et al., 1999). In this study, however, the

Fig. 2 Dynamic change of COD and BOD5 during the single anaerobic

process.


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COD obtained reflects the presence of only some organic

compounds; that is, the measured COD was lower than the

real quantity of organic compounds.

In the anaerobic processing of the heavy oil pro-

duced water, two reactions occurred simultaneously: (1)

biodegradable organic species were degraded, causingthe COD to decrease, and (2) refractory large-molecule

organic species were hydrolyzed into simpler compounds,

causing the COD to increase. The final COD was deter-

mined from the sum of these two processes. We found

that the second reaction was dominant during the first 168

hr. After then, the first reaction became dominant and the

COD decreased gradually.

As shown in Fig. 2, the increase of BOD5 was significant

during the period of anaerobic treatment from 96 to 192 hr;

that is, the period of the increase in BOD5 lagged behind

that of the COD. This situation arose presumably because

the organic species were hydrolyzed slowly within the

first 96 hr, causing BOD5 to increase slowly. The majorityof large-molecule organic species were hydrolyzed after

96 hr. The anaerobic process appeared to improve the

biodegradability of the wastewater organic species; this

hypothesis could be explained by the partial conversion of

COD to BOD5.

TOC is believed to be a better indicator for the organic

content of wastewater (El-Rehaili, 1995; Visco et al.,

2005). As shown in Fig. 3, the TOC content decreased

slowly from 136 to 124 mg/L during the first 168 hr of

the experiment. After 360 hr, the final TOC of the effluent

stream was 82 mg/L.

These findings suggest that the decrease in TOC canmore accurately reflect the degradation of the organic

contaminants. Within the first 168 hr of the anaerobic

process, since the hydrolysis of pectin predominated in

this system, the rate of decrease of the TOC was slow.

After 168 hr, the degradation of the organic species was

performed predominantly by the anaerobic microorgan-

isms, thus the decrease rate of TOC was accelerated. As

a result, the observed changes in COD, BOD5, and TOC

were consistent during the anaerobic process.

2.1.2 Analysis of hydrocarbon group content in anaer-

obic processes

Figure 4 displays the effect of anaerobic treatment on thecontent of hydrocarbons. The system exhibited an overall

Fig. 3 Dynamic change in the TOC during single anaerobic process.

hydrocarbon waste removal efficiency of 29.5% (calculat-

ed by weight), similar to the total COD removal efficiency.

The changes in the hydrocarbon group content in anaerobic

processes are due to the synergetic actions of anaerobic

microorganisms. The contents of alkanes, aromatics, and

pectin in the raw wastewater were 31.6, 8.5, and 13.2mg/L, respectively. The levels of the alkanes and aromatics

in the effluent after 360 hr were decreased by 18.7%

and 14.1%, respectively. The removal of pectin was high

(60.6%) during the first 168 hr, and low (11.4%) thereafter.

The content of bitumen barely changed during the entire

anaerobic process. The results in Fig. 4 indicate that the

transformation of pectin to medium-length and long-chain

fatty acids improved the BOD5 and biodegradability to

some extent.

2.2 Additional ME process

2.2.1 Effect of dosages of iron and active carbon

Figure 5 displays the removal efficiencies of COD as

a function of reaction time in the presence of various

dosages of iron and active carbon at pH 6.0. Overall, the

removal efficiencies of COD in the wastewater exhibit

similar trends. With active carbon of 40 g/L, the removal

efficiencies of COD at iron dosages of 20, 40, and 60

g/L were 21.6%, 31.4%, and 32.5%, respectively, within

a reaction time of 8 hr. When the dosage of iron exceeded

80 g/L, the removal rate of COD increased dramatically.

Notably, we observed no obvious additive effect when

employing a higher dosage. At 100 g/L, the removal

efficiency of COD remained at 38.5% after 8 hr. It can be

concluded that a transition exists in the ME process at aniron dosage between 80 and 100 g/L. Because an excess of

iron would affect the mass transfer efficiency, a dosage of

80 g/L would be preferable. Therefore, the optimal ratio of

iron to active carbon was 2:1 (W/W).

Increasing the retention time caused the removal effi-

ciency of the COD to increase as a result of the formation

of macroscopic electrodes. We observed little enhance-

ment upon increasing retention time (Fig. 5). When the

reaction was 4, 6, and 8 hr, the COD removal efficiencies

with optimum iron and active carbon dosages were 33.2%,

34.2%, and 37.6%, respectively. The greater retention

time did not improve the effi

ciency dramatically. Taken

Fig. 4 Composition of hydrocarbons in the heavy oil produced water

during the single anaerobic process.


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Fig. 5 COD removal as a function of reaction time in the presence of

various amounts of iron, active carbon with 1 L wasterwater.

together, we considered the optimal retention time as 4 hr.2.2.2 Effect of initial pH of produced water

Figure 6 reveals that the ME process was more effective

under acidic conditions. When the initial pH decreased, the

removal efficiency of COD increased dramatically. After 8

hr, the COD removal efficiencies at pH 4.0, 5.0, 6.0, and

7.0 were 40.3%, 37.2%, 36%, and 29.3%, respectively.

Higher efficiency might be expected for the ME process

performed under acidic conditions, partially because of

the accompanying coagulation process. On the other hand,

the ME technique differs from coagulation because the

former involves redox processes. The electrical potential

energy between the anode and cathode was enhanced whenpH decreased in the reaction system. Extremely low pH,

however, is not suitable for anaerobic system. Therefore,

the optimal pH for ME process was in the range 5.06.0.

2.2.3 Enhanced biodegradability of produced water in

ME process

Table 2 lists data reflecting the effects of ME on heavy

oil produced water. For HRT of 4 hr and pH of 6.0, the

COD and TOC decreased by 38.3% and 23.5%, respec-

tively. As expected, the COD of the produced water was

effectively eliminated through ME treatment. The behavior

of the COD here is dissimilar to that observed for the

Fig. 6 COD removal at various initial pH values of raw produced water

during the ME process. Iron: 80 g/L; active carbon 40 g/L; wastewater: 1

L; temperature: 32C.

biological treatment. The greater removal of COD in this

case was due to both chemical reactions and adsorption

effects being involved. In addition, the decrease in TOC

during the 4 hr ME process was also significant, but not

as great as that of COD. The BOD5/COD ratio increased

from 0.05 to 0.17. Thus, ME has the ability to alter themolecular structures of dissolved compounds, resulting

in the increase in wastewater biodegradability. Particular,

the ME process produced compounds of lower molecular

weight that are more biodegradable.

Table 2 Effects of ME on heavy oil produced water

COD BOD5 BOD5/COD TOC

(mg/L) (mg/L) ratio (mg/L)

Influent 274 16 0.05 136

Effluent 169 28.6 0.17 104

2.3 Effect of anaerobic process coupled with ME

2.3.1 Removal and conversion of organic matter by

anaerobic process coupled with ME

In the system of anaerobic process coupled with ME,

the HRT of ME treatment and anaerobic treatment were 4

and 360 hr, respectively. Figure 7 displays the changes in

COD and BOD5 in the reactor. The mean COD increased

slowly from 169 to 184 mg/L during the first 96 hr, then

decreased slowly. After 360 hr, the COD in the effluent was

138 mg/L. Therefore, ca. 18.3% of the COD was removed

during the anaerobic process. As a result, the total removal

of COD in the system combining the anaerobic processwith ME was 49.6%. The BOD5 exhibited a similar trend.

The initial BOD5 of the effluent after ME processing was

28.6 mg/L. After 120 hr of anaerobic treatment, the BOD5increased to a maximum concentration of 41.3 mg/L. The

BOD5/COD ratio in the effluent varied from 0.17 to 0.22.

After 360 hr, the decrease in the content of biodegradable

organics resulted in a final BOD5/COD ratio of 0.14. Thus,

most of the organic compounds were removed when using

the integrated system; that is, the biodegradability of the

produced water was improved through pretreatment with

the ME system.

Although many of the organic components in pro-

duced water are non-biodegradable, the redox processes

Fig. 7 Dynamic changes in COD and BOD5 during the anaerobic pro-

cess after ME treatment. Iron: 80 g/L; active carbon: 40 g/L; temperature:

32C; HRT for ME treatment: 4 hr.


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Table 3 Identification of peaks using GC/MS

Number Identified compound Number Identified compound

Peak 1 C14H22O3 Peak 10 C29H48O2Peak 2 C15H24O Peak 11 C18H24O2Peak 3 C16H30O4 Peak 12 C17H36

Peak 4 C16H30O4 Peak 13 C16H22O4Peak 5 C15H22O2 Peak 14 C35H72Peak 6 C16H22O4 Peak 15 C30H50Peak 7 C20H30O4 Peak 16 C28H42O4Peak 8 C18H24O2 Peak 17 C29H50O

Peak 9 C23H38O2

occurring during the ME stage caused some refracto-

ry organic compounds to be transformed or removed

and other organic compounds to become water-soluble.

Furthermore, the biodegradability of effluent was also

enhanced, increasing the BOD5 and the BOD5/COD ratio.

Therefore, ME is an appropriate method for enhancing the

biodegradability of heavy oil produced water.

2.3.2 GC/MS analysis

We used GC/MS to analyze the raw influent and the

effluent from the anaerobic process coupled with ME

(Tellez et al., 2005; Middleditch, 1984). The GC/MS

chromatogram of the organic species extracted from the

heavy oil produced water was extremely complex. In

Fig. 8a, the 17 major peaks having the greatest areas are

marked with the largest as peak 10. Figure 8b displays

the chromatogram obtained for effluent from the anaerobic

process coupled with ME. The content of large-molecular

substances (e.g., peaks 12, 14, and 15) had been reduced,

consistent with the trend for BOD5. These large moleculeshad been transformed into smaller ones through hydrolysis

in the anaerobic system. A mass of the small-molecule

substances were biodegraded (e.g., peaks 1, 2, and 5),

consistent with the decrease in TOC. Furthermore, we

performed mass spectrometric analyses to identify the 17

main petroleum-based compounds (Table 3). The contents

of high molecular weight compounds, such as those rep-

resented by peaks 5, 10, 11, 12, and 14, decreased by an

average of 37% after anaerobic treatment.

2.3.3 Efficiencies of the integrated system

Figure 9 displays the changes in COD in the effluent

of the ME and integrated systems during continuousprocessing for 45 day. The CODs of the two systems re-

mained steady at ca. 185 and 133 mg/L, respectively, with

average COD removal efficiencies of 35.4% and 53.3%,

respectively, during the operation period. The anaerobic

reactors coupled with ME were efficient at removing

organic compounds, suggesting that an integrated system

featuring anaerobic process coupled with ME might be

suitable for the purification of oilfield produced water.

3 Discussion

Earlier publications have reported that the oil-in-wateremulsified heavy oil is difficult to be biodegraded (Wu

et al., 2003; Zhao et al., 2006). It is difficult to treat the

oil wastewater through conventional biological treatment

Fig. 8 GC/MS analyses of organic contaminants in the raw influent (a)

and effluent (b) of the anaerobic process coupled with ME.

Fig. 9 COD variation in the raw influent and the effluent from the single

ME and integrated system.

system mainly due to slow biodegradability. Surface flow

constructed wetland and anaerobic baffled reactor (ABR)

systems have been used to treat heavy oil produced wa-

ter with high concentrations of salts and low levels of

nutrients (Ji et al., 2007, 2009). In contrast, there are

few reports regarding the changes in COD during the

anaerobic processing of heavy oil produced water. In

this study, we observed an unexpected initial increase in

COD during anaerobic treatment, due to refractory organicspecies being transformed into small degradable organic

compounds, primarily through hydrolysis. Lu et al. (2009)

demonstrated that, for a strictly anaerobic process, the


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influent COD content is generally transformed into volatile

fatty acids (VFA), alcohol, hydrogen, and biomass; the

overall COD content, however, should remain unchanged.

Although anaerobic processing is usually designed as

the pre-treatment system for aerobic processing (Chan et

al., 2009; Tellez et al., 2002), heavy oil produced wateris not suited for aerobic treatment because of its low

BOD5/COD ratio. Therefore, anaerobic processes have

been designed with longer HRT and lower COD loading

(Guo et al., 2002), with the main purpose of enhancing the

BOD5/COD ratio.

Anaerobic process has a shortage of long HRT. In

general, additive methods are used to enhance the anaer-

obic effect. ME processing is being used increasingly for

COD removal, particularly for refractory organics reme-

diation (Wei et al., 2001). Zero-valent iron is a strong

reducing agent that can reductively transform relatively

oxidized pollutants, including chlorinated solvents, metals,

nitrates, and explosives. We were interested in exam-ining pretreatment with iron as a means to reductively

transform electron-withdrawing moieties and render recal-

citrant compounds more amenable to subsequent oxidation

processes. In this study, we found that 80 g/L of ironand 40

g/L of active carbon were the optimal dosages at a retention

time of 4 hr. The optimal pH range for ME process was

5.06.0. We suspect that the parameters are different from

others wastewater due to the hydrocarbon content in heavy

oil. Meanwhile, it should be noted that ME process was in

favor of enhancing biodegradability of produced water.

In addition, the effluent COD concentrations and the

corresponding removal efficiencies in the reactor weregratifying when coupling the anaerobic and ME processes.

The combined method led to a significant decrease in the

content of refractory organic species. This phenomenon

suggests that the integrated system is suitable for the

treatment of produced water in oilfields.

4 Conclusions

We have experimentally investigated the removal of

organic species from heavy oil produced water through

single anaerobic treatment and the coupling of anaerobic

processing with ME. In the single anaerobic system, the

COD and BOD5 both increased initially over time and then

decreased gradually. The BOD5/COD ratio of the produced

water increased upon treatment. The organic composition

changed during anaerobic processing. Coupling anaero-

bic treatment with ME accelerated the conversion and

biodegradation processes. GC/MS analysis revealed that

combined treatment was particularly effective for the

conversion of large-molecule organics to smaller ones

that were biodegraded only partially. Thus, the combined

process was very effective at transforming most of the

organic pollutants found in the heavy oilfield produced

water.

Acknowledgments

This work was supported by the Water Pollution Control

and Management Project, China (No. 2009ZX07208). The

authors wish to thank the Ministry of Science and Technol-

ogy, China, for partially funding this study.

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