Treatment and energy utilization of oily water via integrated ultrafiltration-forward osmosis–membrane distillation (UF-FO-MD) system

Author’s Accepted Manuscript Treatment and Energy Utilization of oily water via integrated Ultrafiltration-Forward Osmosis– Membrane Distillation (UF-FO-MD) System Dongwei Lu, Qianliang Liu, Yumeng Zhao, Huiling Liu, Jun Ma www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)32005-7 https://doi.org/10.1016/j.memsci.2017.11.004 MEMSCI15699

To appear in: Journal of Membrane Science Received date: 12 July 2017 Revised date: 27 September 2017 Accepted date: 2 November 2017 Cite this article as: Dongwei Lu, Qianliang Liu, Yumeng Zhao, Huiling Liu and Jun Ma, Treatment and Energy Utilization of oily water via integrated Ultrafiltration-Forward Osmosis–Membrane Distillation (UF-FO-MD) System, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2017.11.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Treatment and Energy Utilization of oily water via integrated Ultrafiltration-Forward Osmosis–Membrane Distillation (UF-FO-MD) System

Submitted by

Dongwei Lua, Qianliang Liub, Yumeng Zhaoa, Huiling Liua and Jun Maa*

a.

State Key Laboratory of Urban Water Resource and Environment, School of Municipal and

Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China b.

Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang

Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China

*Corresponding Author E-mail: [email protected] (J.M.)

Abstract: Oily water of high salinity and temperature causes challenges to eco-environment. Instead of being considered as only pollutants treated by integrated membrane system, oily water was also considered and utilizied as driving-energy resource for the system in this work. This paper proposed and studied integrated UF-FO-MD system for not only treatment of oily water but also utilization of its high salinity and temperature (i.e., osmotic and thermal energies). 50 KDa ceramic membrane was selected for oily water treatment because of high oil recovery rate, low flux decline rate and great reduction of downstream FO-MD fouling, and corresponding membrane fouling mechanism was proposed. After UF, oily water was simultaneously used as FO draw (sewage as FO feed) and MD feed to utilize its osmotic and thermal energies for FO-MD running. Oil content largely influenced FO-MD fouling, while temperature and salt content had little influence. Three scenarios of dynamic mass-transfer process and temperature-salt content equilibrium curve for FO-MD were proposed, which provide guidance for oily water utilization to control mass-transfer process. UF-FO-MD system efficiently treated both oily water and sewage, and recovered high-quality water by utilization of oily water energies at low-energy cost. Oily water after treatment and utilization met reinjection standard. This work helps for oil-field wastewater treatment and utilization to realize water recovery, energy utilization and pollution reduction.

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Graphical abstract

Keywords: Oily water; Water recovery; Energy utilization; Integrated membrane system.

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Introduction Oil-field produced water (denoted as oily water) is the largest waste stream generated in oil and gas industries [1]. The volume of oily water still increases rapidly with the growth of oil consumption and subsequent oil exploitation [2]. It can reach to almost 98% of the total fluid volume when the mature fields are close to the later period of oil production [3, 4]. The physical and chemical properties of oily water from different sources greatly vary and considerably depend on the geological location of oil field, geological formation and reservoir lifetime etc. [5-7]. In general, oily water contains a complex mixture of organic substance (mainly dispersed and dissolved oil compounds) and inorganic substance (high concentrations of mineral salts, total dissoved solids TDS up to ten of thousands ppm) [2, 8, 9]. It normally has high temperature (40-80°C) accompanied with the processes of oil exploitation and crude oil dehydration [10, 11]. Almost 50% of oily water after appropriate treatment can be re-injected for oil production processes, while the others have to be discharged [12]. Oily water without proper treatment can cause severe hazards to the environment and ecology. It could potentially pollute surface water, underground water and soil, and result in bioaccumulation of harmful chemicals [13, 14]. Excessively oily water has become a significant challenge faced by oil and gas industries, which needs efficient treatment to meet the reuse or discharge requirements. Most conventional processes for oily water treatment (e.g., gravity separation, chemical demulsification and air flotation, etc.) are not efficient enough

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to meet the stringent standards. (e.g., oil contents < 10 mg/L and suspended solids (SS) < 10 mg/L) [15-17]. Membrane technology offers a promising solution for oily water treatment to replace traditional treatment due to its advantages of easy operation, high separation efficiency and steady permeate quality, etc. [18-20]. Ultrafiltration (UF) membrane has been widely studied to separate and recover the oil from oily water [21, 22]. Ceramic UF membrane is superior to polymeric membrane for oily water treatment due to its higher stability and longer life [23-25]. Trans-membrane pressure is the driven force for UF process, and oil recovery rate mainly depends on UF membrane pore size [26]. UF process is also used as a pretreatment for downstream membrane technology to reduce membrane fouling [13, 27, 28]. Compared with pressure-driven membrane processes, forward osmosis (FO) and membrane distillation (MD) have received intensive attention for oily water treatment because of their low fouling potential [29-31]. Osmotic pressure difference (i.e., osmotic energy) and vapor pressure difference caused by temperature difference (i.e., thermal energy) forced liquid/gas water molecules from oily water to the draw of FO process and the distillate of MD process, respectively [31-33]. Integrated membrane system combines different individual units with their respective features to achieve comprehensive superiority over single membrane process. Therefore, it has been studied in the fields ranging from water/wastewater treatment to food processing [13, 14, 34]. Various integrated membrane systems such as integrated microfiltration and reverse osmosis (MF-RO), ultrafiltration and reverse

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osmosis (UF-RO), UF-MD and FO-MD systems were also reported for oily water treatment. In integrated MF-RO, UF-RO and UF-MD systems, MF and UF were firstly used as pretreatment process to remove oil from oily water to a great extent. After MF and UF treatment, a trace amount of oil was still detected in the permeate. Then, RO and MD worked as enhanced treatment process to further remove the oil from UF/MD permeate. Together, integrated MF-RO, UF-RO and UF-MD processes almost removed all the residual oil from oily water [13, 35, 36]. Integrated FO-MD system was proposed for sustainable water reuse from oily water. Water molecules were transferred from oily water to the draw of FO process and then to the distillate of MD process, where MD process re-generated the draw of FO process to continuously maintain its high osmotic pressure [14]. To sum up, most works so far consider oily water as target pollutants (i.e., only the feed of MF-RO, UF-RO, UF-MD and FO-MD) and mainly focus on treating it to achieve high oil removal and obtain high-quality water through integrated membrane system. However, instead of being considered as only target pollutants treated by integrated membrane system, oily water was also considered and utilized as driving-energy resource for integrated system in this work. As described above, oily water is also characterized with high temperature (i.e., thermal energy) and high salinity (i.e., osmotic energy) besides its water and oil resources [10, 11]. It will be more energy-efficient to treat oily water and deal with the oily water challenges through using oily water simultaneously as draw of FO and feed of MD in FO-MD subsystem and utilizing its osmotic and thermal energies for integrated membrane system

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operation. Therefore, integrated membrane system of ultrafiltration, forward osmosis and membrane distillation (UF-FO-MD) was proposed and studied in this work for the dual purposes of the treatment and utilization of oily water. The schematic diagram of integrated membrane system was shown in Scheme 1. UF ceramic membranes with different pore sizes were firstly used for the treatment of oily water, which can recover oil resource and also reduce the fouling tendency of downstream FO-MD subsystem (Scheme 1A). After UF process, the oily water still had high salinity (i.e., osmotic energy) and high temperature (i.e., thermal energy). FO-MD subsystem was then used for the utilization of oily water (Scheme 1B). Oily water was used as draw solution in FO process for utilizing its osmotic energy (i.e., high salinity property) to draw water molecules from high-pollution and low-salinity wastewater (sanitary sewage in this study) (Section A, Scheme 2) to oily water (Section B, Scheme 2). Meanwhile, it also worked as feed solution in MD process for utilizing its thermal energy (i.e., high temperature property) to drive water gas molecules from oily water (Section B, Scheme 2) to the distillate (Section C, Scheme 2). In this case, water molecules can be recovered and transferred from the high pollution feed of FO to the distillate of MD (i.e., from Section A to Section C, Scheme 2) through energy utilization of oily water at almost no external energy cost. Generally, the support layer of FO membrane and the active layer of MD membrane contacted low-fouling potential salt solution, while they directly contacted the oily water with high-fouling potential in this study (Scheme 2). This may largely influence their performance. Therefore, the influence of oily water properties on FO-MD

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subsystem performance was thus investigated in terms of membrane permeate flux and permeate flux decline rate. The possible scenarios of the dynamic mass transfer process and the temperature-salt content equilibrium curve for FO-MD subsystem were proposed, which can provide guidance for the better utilization of oily water. Finally, oily water after treatment and utilization by the integrated membrane system was analyzed in terms of water quality. This work provided an energy-efficient approach for the treatment of oily water and sewage, and the utilization of oily water energies (i.e., osmotic and thermal energies) to reduce oily water pollution and recover oil/water resources at low-energy cost, which would be of great help for dealing with oily water challenges.

Scheme 1. Schematic diagram of integrated membrane system of UF process (A), FO and MD processes (B).

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Scheme 2. Schematic diagram of dynamic mass transfer process in FO-MD subsystem; section A denotes the feed of FO (sanitary wastewater), section B denotes the draw of FO and the feed of MD (treated oily water by UF), and section C denotes the distillate of MD (produced water).

2. EXPERIMENTAL SECTION 2.1 Materials Ceramic UF membranes, FO membranes and MD membranes were purchased from TAMI Industries (TAMI, France), Hydration Technology Innovations (Albany, US) and Jitian Company (Shanghai, China), respectively. Their characteristics were summarized in Table S1 (Supporting Information, SI). The real oily water and sanitary sewage were obtained from Daqing oilfield wastewater treatment plant (Daqing, China) and the discharge pipe of residential district (Harbin, China), respectively. Synthetic oily water was prepared with different contents of crude oil (Daqing Oilfield Company, China), sodium dodecyl benzene sulfonate (SDBS) (Sigma-Aldrich, US) and sodium chloride (Merck, US) in Table S2 (SI) to investigate the influence of oily water properties on the performance of the integrated membrane system. Preparation process of synthetic oily water was detailed in Text S1 (SI). The oil droplet in real and synthetic oily water was 8

characterized in terms of average size distribution and zeta potential in Figure S1 and Table S3 (SI) which were analyzed with a particle size and zeta potential analyzer (Nano ZS, Malvern). 2.2 Analysis methods of water quality Total organic carbon (TOC) and Total Nitrogen (TN) of the real and synthetic oily water, sanitary sewage and the permeates were measured by a TOC/TN analyzer (Analytik Jena, Germany). NH4+-N concentration was examined by means of nessler's reagent colorimetric method. Major cations were determined via ICP-MS (NexION 300 Q, US). Prior to analysis, the sample was first filtered by a 0.22 μm glass fiber membrane and then acidified with 2% HNO3, and the tubes of ICP-MS were rinsed with a 1 mg/L L-cysteine solution in 2% HNO3. Major anions were analyzed by an ion chromatograph (Dionex ICS–3000, US). 30 mM KOH was used as isocratic eluent at a flow rate of 1.0 mL/min, and the suppressor current was set to 75 mA. The oil content was determined by an ultraviolet (UV) spectrometer (Varian Cary 300 UV–vis spectrometer, USA) at UV absorbance of 228 nm. The NaCl content was measured by conductivity meter (DDSJ-308A, China). The pH value of all solutions was measured by pH meter (PHS-3C, China). The fluorescent contents of synthetic oily water before and after ultrafiltration were characterized with three-dimensional fluorescence excitation-emission matrices (FEEM) (Aqualog CDOM Fluorometer, France). The wavelength range of excitation and emission scans were 200-600 nm and 211.44-620.81 nm, respectively. The bacterial counts were measured by Accuri C6 Flow Cytometry (BD, USA). The samples (500 μl)

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were stained with 0.5 μl of SYBR Green I (Sigma-Aldrich, USA). Then, all samples were incubated for 10 min in dark at constant temperature (30 ℃ ) before measurement. 2.3 Membrane processing procedures Integrated membrane system of UF, FO and MD processes was designed for the treatment and utilization of the oily water in Scheme 1. 2.3.1 Ultrafiltration process A constant pressure dead-end filtration setup connected with back flush function was applied to evaluate the performance of ceramic UF membranes in terms of oil recovery rate and permeate flux decline rate (Scheme 1A). The ultrafiltration procedure was detailed in Text S2 (SI). 2.3.2 FO-MD subsystem process A setup of FO-MD subsystem with active layer against feed solution mode (AL-FS) was designed to utilize the osmotic and thermal energies of oily water (Scheme 1B). Individual FO process and MD process were also conducted to investigate the influence of oily water properties on their performances in terms of membrane flux and permeate flux decline rate (Scheme S1, SI). The detailed processes were described in Text S3 (SI). 2.3.3 Characterization of integrated membrane system of UF, FO and MD processes The permeate flux J (L m-2 h-1) of UF, FO and MD membranes were calculated by equation (1). Permeate flux decline rate (FDRn) of all membranes was calculated with equation (2). Oil recovery rate (RO) of UF membrane was calculated with

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equation (3). J = V/AΔt

(1)

V (L) is the permeate volume, A (m2) denotes the membrane area, and Δt (h) represents processing duration.

FDRn= 1-Jn/J0 × 100%

(2)

Jn denotes the initial permeate flux after each back flush and n represents filtration cycles for UF process, while it denotes relatively stable flux (semi-stable flux) at the processed volume of 40 mL for FO and MD processes. J0 is pure water permeate flux for all processes.

RO =1-Cp/Cf ×100%

(3)

RO is oil recovery rate, and Cf and Cp are oil concentrations of the feed and the permeate in UF process, respectively.

3. RESULTS AND DISCUSSION 3.1 Treatment of oily water by UF process 3.1.1 UF membrane filtration Ceramic membranes with different membrane pores (0.14 μm, 150 KDa, 50 KDa and 5 KDa) were used for the treatment of oily water with 100 mg/L oil content, 0.25 mol/L salt content and 50°C temperature. Figure 1A and Figure 1B presented volume-dependent decline of the normalized permeate flux (J/J0) during the treatment of oily water. All the membranes had different extents of flux decline during filtration, which can be ascribed to membrane fouling (i.e., oil foulant adsorption and deposition on membrane surface and pores) and concentration polarization (i.e., oil foulant accumulation on membrane surface) [37, 38]. Figure 1a

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and Figure 1b presented permeate flux decline rate (FDRn, n represents the filtration cycles) of the membranes after each back flush. The FDRn of all the membranes increased with the increase of filtration cycles, indicating that the hydraulic irreversible fouling increased gradually. A remarkable flux decline was observed on 0.14 μm membrane (FDR6 = 83.9%). It can be explained by that the oil droplets more easily penetrate, absorb and stick into large membrane pores, causing serious hydraulic irreversible fouling [23, 24]. 5KDa membrane also had a severe flux decline (FDR6 = 74%). This is because most of oil foulants were not timely removed and closely compacted on membrane surface due to its low backwash frequency compared with 50 KDa and 150 KDa membranes for treating equal volumes of oily water (60 mL). 150 KDa membrane had moderate flux decline (FDR6 = 40.6%). In comparison, 50 KDa membrane showed least flux decline rate (FDR6 = 29.6%) of all membranes (Figure 1a and 1b). More oil droplet penetration into membrane pores may also account for the more flux loss of 150 KDa membrane than that of 50 KDa membrane. The flux decline rates of these membranes followed the order of 0.14 μm membrane ~5 KDa membrane < 150 KDa membrane < 50 KDa membrane. It should be mentioned that the filtration volume of 0.14 μm membrane in the first cycle was above 60 mL because its permeate flux was too high at the beginning (1357 L·m-2·h-1 in Table S1) to be controlled.

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1.0

1.0

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5KDa 50KDa 150KDa

B

0.14 μm

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Nomalized flux J/J0

Nomalized flux J/J0

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3

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Figure 1. Normalized flux declines of ceramic membranes with different pore sizes (A and B) and their corresponding flux decline rates with back flush times (a and b).

The membrane fouling mechanism of UF ceramic membrane for oily water treatment was hypothesized in Scheme 3, which presumptively included three stages: i. At the beginning of the filtration, part of small oil droplets directly penetrated and stuck inside or absorbed on membrane pores, which partially blocked the membrane pore (i.e., pore blockage fouling). Meanwhile, some oil droplets absorbed on the membrane surface. In this stage, the membrane permeate flux suffered a relatively slow decline as the effective membrane pore size gradually decreased (i.e., the first stage); ii. As the processing time increased, some oil droplets were rejected and piled up on membrane surface and membrane pore mouth, which subsequently promoted

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their collision and coalescence [39, 40]. Consequently, the coalesced oil droplets largely covered the membrane pore mouth (i.e., pore mouth coverage fouling), thus causing significant reduction of the effective pore size and a rapid permeate flux decline (i.e., the second stage); iii. At the end of the filtration, the oil droplets continuously merged with each other and eventually grew into a compact oil layer spreading out on membrane surface (i.e., cake layer fouling). This uniform oil layer fully covered the membrane pore mouth as a “second membrane” and dominated membrane permeate flux, ultimately leading to a semi-stable permeate flux (i.e., the third stage) [25, 41, 42].

Scheme 3. Possible fouling mechanism of UF ceramic membrane for oily water treatment.

For 0.14 μm membrane, the first stage happened in short time since oil droplets can easily enter into its large membrane pore and quickly stick inside or absorb on membrane pores. Subsequently, the second stage dominated the main filtration process. In the late period, the filtration process approached to the third stage. For 50 KDa and 150 KDa membranes, the first stage was clearly observed in the initial

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period as oil droplets gradually penetrated into their moderate membrane pores. Afterwards, the second stage showed up. The third stage was not observed in this case probably due to the timely backwash, which was then confirmed by a long run without backwash (Figure S2, SI). In contrast, 5 KDa membrane had no obvious “the first stage” possibly because of the low penetration of oil droplets into its small membrane pore. It soon entered into the second stage. Overall, the first and second stages took place in a relatively short period due to the quick coverage of small pore mouth by oil droplets. Then the third stage occurred and occupied the predominant position in the whole filtration process. Overall, the first stage of all membranes gradually shortened and even disappeared while their second and third stages remained as the backwash times increased. It indicated that membrane fouling in the first stage was mainly hydraulically irreversible and hardly recovered by backwash, but the membrane fouling in the second and third stages was mainly hydraulically reversible and easily recovered. This phenomenon was well consistent with our proposed membrane fouling mechanism in Scheme 3 that the pore blockage fouling (mostly hydraulic irreversible fouling) mainly occurred in the first stage and the pore mouth coverage and cake layer fouling (mostly hydraulic reversible fouling) mainly occurred in the second and third stages [23]. 3.1.2 Oil recovery rate and UF permeate quality Table 1 showed the oil recovery rates (RO) of all membranes. 5 KDa membrane had almost 100% of oil recovery rate, indicating nearly all the oil was recovered. High oil recovery rate (RO = 95.6%) was also observed on 50KDa membrane. 150

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KDa membrane and 0.14 μm membrane had oil recovery rate of 83.9% and 63.5%, respectively. The oil recovery rates of all membranes decreased in the order of 5 KDa membrane > 50 KDa membrane > 150 KDa membrane > 0.14 μm membrane. It is reasonable that oil recovery rate increased as the membrane pore size decreased. The permeate oil contents of 5 KDa membrane and 50 KDa membrane were less than 5 mg/L, which meet the injection standard of oilfield water (oil content < 5 mg/L) (SY/T5329-1994). As expected, the salt contents of the permeates were still 0.25 mol/L indicating that all the ceramic UF membranes had no salt rejection in Table 1. Table 1. Oil recovery rate, permeate quality and initial permeate flux*

Membrane

*

Permeate oil

Permeate salt

Permeate flux

content (mg/L)

content (mol/L)

(L m-2 h-1)

Oil recovery rate (%)

5 KDa

>99.0

-

0.25

10.6

50 KDa

95.6

4.4

0.25

77.3

150 KDa

83.9

16.1

0.25

180.3

0.14 μm

63.5

36.5

0.25

218.1

Synthetic oily water had 100 mg/L oil content, 0.25 mol/L salt content and 50°C temperature; initial

permeate flux was the initial flux of the membranes at the fifth cycle.

In addition, the fluorescent organic contents of oily water before and after filtration by the membranes were compared using FEEM (Figure 2). Two major peaks adjacent to each other (centered at Ex/Em of 215/297 and 225/337 nm) presented in feed, which can be ascribed to benzene derivatives [43, 44]. Another intense peak (centered at Ex/Em of 250/450 nm) can be assigned to fulvic acid-like compounds [44]. 5 KDa membrane had the highest fluorescence rejected rate, while 0.14 μm membrane had the lowest removal rate in comparison of the feed with all 16

the permeates (Figure 2). The fluorescence rejected rates of all membranes were consistent with the oil recovery rates in the order of 5 KDa membrane > 50 KDa membrane > 150 KDa membrane > 0.14 μm membrane.

Figure 2. FEEM spectra of the feed and the permeates treated by different sizes of membranes.

The results of Table 1 and Figure 2 indicated that ceramic UF membranes of 50 KDa and 5 KDa were highly efficient to recover oil from oily water. Although 50 KDa membrane had slightly lower oil recovery rate than 5 KDa membrane, it had nearly 8 times the fifth initial permeate flux of the latter (77.3 vs. 10.6 L m-2 h-1 in Table 1). Therefore, 50 KDa membrane was selected for oily water treatment due to

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its low flux decline rate, high permeate flux, high oil recovery rate and permeate quality (Figure 1, Table1 and Figure 2). 3.2 Utilization of oily water by FO-MD subsystem After UF process, the oily water still had high salinity (i.e., osmotic energy) and high temperature (i.e., thermal energy). It was simultaneously used as the draw solution of FO process and the feed solution of MD process in FO-MD subsystem to utilize its osmotic energy and thermal energy, respectively (Scheme 2). In this case, water molecules can be recovered and transferred from sanitary sewage (i.e., FO feed in section A of Scheme 2) to the distillate of MD (i.e., section C of Scheme 2) by utilization of oily water energies at almost no external energy cost. 3.2.1 Influence of oily water properties on FO-MD subsystem performance The influences of main oily water properties (i.e., oil contents, salt contents and the temperature) on FO-MD subsystem performance were investigated in terms of membrane permeate flux and permeate flux decline rate (i.e., membrane fouling). Oil content influence: Oily water with different oil concentrations from 5 to 50 mg/L, constant salt content of 0.25 mol/L and temperature of 50oC was used as the draw of FO and the feed of MD to investigate the oil content influence on membrane performance. Figure 3A and 3B showed the permeate flux-volume curves of FO and MD membranes, respectively. The permeate flux of all membranes declined as the permeate volume increased. This can be attributed to that oil foulants adsorbed and deposited on membrane surface during the FO and MD processes, which hindered the mass transfer of liquid/gas water molecules. Figure 3a and 3b illustrated the

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permeate flux decline rates of FO and MD as a function of oil contents, respectively. Oil contents of oily water largely influenced membrane fouling. Higher oil content resulted in higher flux decline rate, thus higher membrane fouling of FO and MD membranes (Figure 3a and 3b), which was consistent with the optical images of fouled MD and FO membranes (Figure 4B and 4C).

4 3

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Figure 3. Permeate flux-volume curves of FO (A) and MD (B); permeate flux decline rates of FO (a) and MD (b) at different oil contents; filtration condition: oily water had constant salt content of 0.25 mol/L and temperature of 50°C.

Figure 4 showed the optical images of the support layer and active layer of fouled FO and MD membranes, which can provide direct insights into the exact fouling status. Increasing membrane fouling was clearly observed on the support layer of FO

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membrane (Figure 4B) and the active layer of MD membrane (Figure 4C) directly contacted with oily water, as oil content increased from 5 to 50 mg/L. In contrast, the active layer of FO membrane (Figure 4A) and the support layer of MD membrane (Figure 4D) respectively faced to sanitary sewage and the distillate suffered less fouling. This phenomenon was probably because crude oil contamination of oily water (aromatic-like compounds and fulvic acid-like compounds) more easily adsorbed on membrane interface and caused serious membrane fouling compared to sanitary sewage [23]. Furthermore, MD membranes (Figure 4C) overall suffered more membrane fouling than FO membranes (Figure 3 and Figure 4B). This may be ascribed to two reasons: i. organic oil foulants were inclined to adsorb on the hydrophobic MD membrane (Figure 4C) compared with the hydrophilic FO membrane (Figure 4B); ii. the actual flow of oily water was backward to FO support layer but forward to MD active layer (i.e., from FO membrane to MD membrane), where the oil accumulated more easily on MD active layer than FO support layer. FO and MD membranes had lowest flux decline rate (FDR < 20%) and least membrane fouling when the oil content of oily water was less than 5 mg/L, which was consistent with the optical images of the fouled membranes (Figure 4).

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Figure 4. Optical images of the active layer (A,C) and support layer (B,D) of FO membranes and MD membranes for the utilization of oily water with different oil contents.

According to UF results, 50 KDa membrane removed most oil from the oily water (oil content of the permeate < 5 mg/L). Therefore, UF treatment of oily water by 50 KDa membrane is an efficient pretreatment for reducing the membrane fouling tendency of downstream FO-MD subsystem. Considering that oily water after UF treatment had residual oil contents of no more than 5 mg/L, oily water with constant oil content of 5 mg/L was then used to invetigate the influence of its salt content and temperature on FO-MD subsystem performance in the following experiments. Salt content influence: Oily water with different salt contents from 0.1 to 0.75 mol/L, constant oil content of 5 mg/L and temperature of 50oC was used as the draw of FO and the feed of MD to investigate the salt content influence on membrane performance. The permeate flux variation of FO and MD with permeate volume was 21

separately shown in Figure 5A and 5B. As expected, the permeate flux of all membranes for FO and MD slowly decreased with increasing permeate volume. The absorption and accumulation of oil foulants on membrane surface may account for this phenomenon. As the salt content increased, the permeate flux of FO membrane increased (Figure 5A) while the permeate flux of MD membrane decreased (Figure 5B). This can be explained by as follows: higher salt solution has higher osmotic pressure, and thus stronger driving force across FO membrane [32, 33]; in contrast, high salt content decreases the water vapor pressure (i.e., colligative property of salt solution), which consequently weakens the driving force across MD membrane [13, 45]. Moreover, the increase of the permeate flux was disproportional to that of the salt content for FO process (Figure 5A). This was probably because of the concentrative and dilutive concentration polarization on both sides of FO membrane surface [29]. In contrast to the permeate flux of FO and MD membranes (Figure 5A and 5B), their permeate flux decline rates (Figure 5a and 5b) hardly changed as salt content increased from 0.1 to 0.75 mol/L indicating that salt contents of oily water in low range had little influence on subsystem membrane fouling.

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0.75

Figure 5. Permeate flux-volume curves of FO (A) and MD (B); permeate flux decline rates of FO (a) and MD (b) at different salt contents; filtration condition: oily water had constant oil content of 5mg/L and temperature of 50°C.

Temperature influence: Oily water with different temperature from 40 to 55oC, constant oil content of 5 mg/L and salt content of 0.25 mol/L was used as the draw of FO and the feed of MD to investigate the temperature influence on membrane performance. The permeate flux of FO and MD was shown in Figure 6A and 6B, respectively. FO and MD membranes all had slow permeate flux decrease with increasing permeate volume due to oil foulants attachment and accumulation on membrane surface. In addition, the higher the oily water temperature was, the higher the permeate flux was in FO process (Figure 6A) and MD process (Figure 6B).

23

High temperature contributed to promote the mass transfer of liquid/gas water molecules for FO and MD processes. Two reasons may contribute to the promotion for FO process: 1) the osmotic pressure increases with increasing temperature based on the van’t Hoff equation; 2) the increased diffusion coefficient of solute molecules (i.e., molecular thermal motion) lowers the solute resistivity in the support layer and hence reduces internal concentration polarization [29, 46, 47]. For MD process, high temperature leads to high pressure difference of water vapor across membrane, thus promoting its mass transfer [48, 49]. 7

12

6

10

Permeate flux J(Lm-2h-1)

Permeate flux J(Lm-2h-1)

B

A

5 4 3 2 1

55℃ 50℃ 45℃ 40℃

0 0

1

10

20

Volume (mL)

30

00

40

a

55℃ 50℃ 45℃ 40℃ 10

20

30

Volume (mL)

40

0.20 0.15 0.10

b Permeate flux decline rate %

Permeate flux decline rate %

4

0.30

0.25

0.25 0.20 0.15 0.10 0.05

0.05 0.00

6

2

0.30

2

8

40

45

50

Temperature (℃)

0.00

55

40

45

50

Temperature (℃)

55

Figure 6. Permeate flux-volume curves of FO (A) and MD (B); permeate flux decline rates of FO (a) and MD (b) at different temperatures; filtration condition: oily water had constant oil content of 5mg/L and salt content of 0.25 mol/L.

24

Figure 6a and 6b demonstrated that the permeate flux decline rates of FO and MD membranes slightly increased as oily water temperature increased. The following reason may account for this phenomenon: high temperature accelerated the motion and unstability of oil droplets which tended to attach on the interface between aqueous phase and membrane phase and hindered the mass transfer process. It should be mentioned that all the permeate-flux decline of FO and MD membranes consisted of two stages during the process: i. a rapid permeate-flux decline stage and ii. a quasi-steady permeate-flux decline stage (Figure 3, Figure 5 and Figure 6). The development of the membrane fouling during the process was hypothesized as followed. In the beginning, small oil droplets promptly penetrate and stuck inside membrane pores, and relatively large oil droplets absorbed on membrane surface of FO support layer; in contrast, oil droplets absorbed on membrane pore entrance and membrane surface of MD active layer. Subsequently, the follow-up oil droplets deposited on the membrane surface or piled up on the absorbed ones for both MD and FO processes, and some oil foulants may gradually spread into MD membrane pores because of membrane wetting and hydrophobic interaction [50, 51]. Meanwhile, some adsorbed and deposited oil droplets were removed from membrane surface by shear force under cross flow condition. As a whole, the adsorbed and deposited oil droplets on membrane were more than the removed droplets, which reduced effective transfer area of FO and MD and caused a quick permeate flux decline (i.e., the first stage). As the process proceeded, the adsorption-dissociation process of oil droplets-membrane and adsorbed oil

25

droplet-deposited oil droplet tended to be equilibrium. Eventually, the total amount of the adsorbed and deposited oil droplets approached to that of the removed droplets. In such case, the permeate flux of FO and MD was closed to be relatively stable (i.e., the second stage). 3.2.2 Produced water quality of FO-MD subsystem Produced water (i.e., the distillate of MD process in section C of Scheme 2) was compared with sanitary sewage (i.e., section A of Scheme 2) and oily water (i.e., section B of Scheme 2) in terms of salt contents and TOC. Table 2 demonstrated that the temperature (40-55oC) of oily water with constant salt content of 0.25 mol/L hardly influenced the salt contents of the produced water (around 0.13 mmol/L). TOC of produced water slightly increased from 0.77 to 1.02 mg/L as oily water temperature increased. This is probably because more volatile components of oily water (i.e., section B of Scheme 2) passed through MD membrane into the produced water (i.e., section C of Scheme 2) at high temperature. In contrast, the salt contents (0.1-0.75 mol/L) of oily water with constant temperature of 50oC had almost no influence on salt contents and TOC of the produced water in Table 2. As a whole, all the produced water had low salt contents (≤ 0.15 mmol/L) and TOC (≤ 1.02 mg/L) compared with sanitary sewage (salt content = 2.79 mmol/L and TOC = 32.2 mg/L) and synthetic oily water (salt content = 0.1~0.75 mol/L and TOC = 7.46 mg/L). It should be noticed that trace amount of salt was observed in produced water. The reason may be that MD membrane fouling resulted in partial membrane wetting, thus leading to the passage of some salt and liquid water through membrane into the

26

produced water [52, 53]. The comparison results of water quality indicated that FO-MD subsystem can recover high-quality water from oily water and sanitary sewage through utilization of the osmotic and thermal energies of oily water. Table 2. Comparison of water quality indexes in FO-MD subsystem* Sanitary sewage Salt contents (mmol/L)

2.79

TOC (mg/L)

32.2

Synthetic oily water Temp. (°C)

Salt contents (mol/L)

40

Produced water TOC (mg/L)

Salt contents (mmol/L)

TOC (mg/L)

0.25

0.12

0.77

45

0.25

0.15

0.79

55

0.25

0.13

1.02

50

0.25

0.14

0.82

50

0.10

0.13

0.80

50

0.50

0.15

0.85

50

0.75

0.15

0.81

7.46

*Salt content of sanitary sewage was shown in Cl- concentration; the temperature of produced water and sanitary sewage was 25°C; oil contents of oily water and sanitary sewage were 5 mg/L and less than 1 mg/L, respectively.

3.2.3 Dynamic mass transfer process of FO-MD subsystem Knowledge on dynamic mass transfer process of FO-MD subsystem would help for its better utilization of oily water. As described above, high salinity (i.e., producing osmotic pressure difference) and high temperature (i.e., producing vapor pressure difference) of oily water were the driving forces of FO process and MD process, respectively. They mainly determined the dynamic mass transfer process of FO-MD subsystem. Figure 7A summarized the semi-stable permeate flux of FO and MD at different salt contents and temperatures but constant oil content of 5 mg/L considering that oil contents were less than 5 mg/L after ultrafiltration. The cross

27

points of FO and MD in Figure 7A (i.e., equal permeate fluxes at certain temperature and salt content) were drawn as an equilibrium curve line in Figure 7B.

Figure 7. Semi-stable permeate flux of FO and MD at different salt contents and temperatures (A); temperature-salt content equilibrium curve (B) obtained from cross points of Figure 7A; filtration condition: produced water and sanitary sewage were at 25°C; oil content was 5 mg/L.

For the FO-MD subsystem, the dynamic mass transfer process included three scenarios in Scheme 2: I. on the line of Figure 7B where the corresponding salt 28

content was equal to the equilibrium content at specific temperature, the water transfer mass of MD (abbreviated as MMD) was equal to the water transfer mass of FO (abbreviated as MFO), i.e., MMD = MFO; II. below the line of Figure 7B where the corresponding salt content was higher than the equilibrium content at specific temperature, the water transfer mass of MD was less than that of FO, i.e., MMD < MFO; III. above the line of Figure 7B where the corresponding salt content was lower than the equilibrium content at specific temperature, the water transfer mass of MD was higher than that of FO, i.e., MMD > MFO (Scheme 2). In the scenario I (i.e., MMD = MFO), the end produced water (section C, Scheme 2) was equal to the outlet water of sanitary sewage (section A, Scheme 2) indicating that the outlet water in section A was totally recovered. In the scenario II (i.e., MMD < MFO), the end produced water in section C was less than the outlet water in section A. In such case, oily water volume (section B, Scheme 2) would increase as processing time increases, which ultimately increases the follow-up processing load on oily water discharge. In the scenario III (i.e., MMD > MFO), the amount of end produced water in section C was greater than that of the outlet water in section A. It indicates that additional amount of oily water in section B (i.e., MMD-MFO) was also recovered besides the recovery of the outlet water in section A (i.e., MFO). The scenarios I and III (MMD ≥ MFO) will be more favorable than the scenario II for oily water utilization from the perspective of industrial application and efficiency. Our results can provide knowleges for regulating the temperature or salt content of oily water to guide the dynamic mass transfer process of FO-MD subsystem occurring in

29

the scenarios I and III. 3.3 Treatment and utilization of real oily water Real oily water was treated and utilized in the integrated membrane system of UF, FO and MD (Scheme 1). It was firstly treated by ceramic ultrafiltration membrane of 50 KDa (Scheme 1A). Main water quality indexes of all solutions were listed in Table 3. Table 3. Main water quality indexes of all solutions* Main indexes

Real oily water

Treated oily water

Sewage

Produced water

Temp. (oC)

48

45.5

25

25

Oil contents (mg/L)

118

4.6

<1

<1

Suspended solids (mg/L)

34.5

-

203

-

Bacterial counts (CFU/mL)

1.38×104

121

2.2×107

36

TOC (mg/L)

299.2

12.57

32.2

1.1

TN (mg/L)

43

39

65.12

0.12

NH4+-N (mg/L)

11

10

46.03

0.05

Na+ (mg/L)

4336

4309

7.145

2.92

K+ (mg/L)

256

262

2.001

0.12

Ca2+ (mg/L)

844

831

4.129

0.05

Mg2+ (mg/L)

161

157

0.936

0.01

Fe3+ (mg/L)

0.7

0.7

0.08

Trace

Cl- (mg/L)

10136

10109

99.17

5.81

SO42-(mg/L)

35

32

5.94

Trace

Br- (mg/L)

60

62

2.85

Trace

*Treated oily water was the permeate of real oily water treated by 50 KDa UF membrane; total bacterial counts of DI water were 35 ± 10 detected by Flow cytometer.

The treated oily water had 4.6 mg/L oil content, indicating that most oil was recovered (Ro = 96.4%) by 50 KDa UF. Relatively high TOC of the treated oily 30

water (12.57 mg/L) may be due to the existence of low molecular weight-polyacrylamide used for polymer recovery oil-extraction [54]. Almost all the suspended solids and bacteria in oily water were rejected (>99%) by 50 KDa ultrafiltration, while the ionic concentrations (i.e., salt contents) had almost no change (Table 3). 50 KDa UF membrane for real oily water treatment had higher permeate flux decline rate than that for synthetic oily water treatment (FDR6 = 37 % in Figure 8 vs. FDR6 = 29.6% in Figure 1). This can be explained by that real oily water had not only oil but also suspended solids and bacteria, etc. which also led to membrane flux decline. It should be noticed that advanced treatment may be needed for further separation of the suspended solid and bacteria from oil. After ultrafiltration treatment, the real oily water still had temperature of 45.5oC and salt content of 0.28 mol/L in Cl- concentration (Table 3). It was then utilized in FO-MD subsystem (Scheme 1B). Based on the equilibrium curve of Figure 7, the salt content of the real oily water was lower than the corresponding equilibrium content (0.43 mol/L) at the temperature of 45.5oC. Therefore, mass transfer process of FO-MD subsystem would occur in the scenario III of Scheme 2, which helps for the energy-efficient utilization of oily water. FO-MD subsystem for real oily water utilization had membrane flux decline rates of 17.2% for FO membrane and 19.5 % for MD membrane (Figure 8). Real oily water caused more membrane fouling of FO-MD subsystem than synthetic oily water probably due to its more complex components. The main indexes of the produced water (Table 3) in FO-MD subsystem reached the drinking water standard of China (GB5749-2006). This

31

further confirmed that high- quality water was recovered from sanitary sewage and oily water by utilization of osmotic and thermal energies of oily water at low-energy cost. The oily water after treatment and utilization by integrated membrane system had low contents of oil, SS and bacteria (Table 3), which met the injected water requirements and can be re-injected for oil production [55, 56]. The results of Table 3 and Figure 8 proved that the integrated membrane system of UF, FO and MD was feasible, efficient and low energy-consuming for oily water treatment and utilization to realize oil recovery and high-quality water regeneration. 1.0

Permeate flux decline rate %

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

UF

MD

FO

Figure 8. Permeate flux decline rates of UF, MD and FO of integrated membrane system for the treatment and utilization of real oily water.

4. Conclusion This study provided a feasible and highly efficient integrated membrane system for not only the treatment of oily water and sewage but also the utilization of oily water to realize water resource recovery and pollution reduction at low-energy cost.

32

Main conclusions were summarized as follows: (1). 50 KDa UF ceramic membrane not only efficiently treated oily water but also worked as pretreatment to mitigate membrane fouling of the downstream FO-MD subsystem, and corresponding UF membrane fouling mechanism was proposed. (2). Sanitary sewage was used as the feed of FO process, and treated oily water was simultaneously used as the draw of FO process and the feed of MD process to utilize its osmotic and thermal energies for FO-MD subsystem running. (3). Oil contents of oily water greatly influenced membrane fouling of FO-MD subsystem, while its temperature and salt content had slight influence. (4). Three possible scenarios for dynamic mass transfer process and temperature-salt content equilibrium curve of FO-MD subsystem were proposed, which can provide guidance to control mass-transfer process for better utilization of oily water. (5). Integrated UF-FO-MD system efficiently treated both oily water and sewage, and recovered high-quality water from them by utilization of oily water energies at low-energy cost. (6). Oily water after treatment and utilization by integrated UF-FO-MD system meets the injection water standard and can be re-injected for oil production. Influence of oily water composition on integrated membrane system should be further investigated in detail considering its complex components. In addition, study on membrane antifouling modification and antifouling material development is also needed to improve membrane antifouling ability and the performance of the

33

integrated membrane system. Acknowledgment This work was supported by National Natural Science Foundation of China (51378141), National Key Technology Support Program of China (2016DX04) and Heilongjiang Province Science Foundation for General Program (E201427). Notes The authors declare no competing financial interest. Appendix A. Supplementary data Three texts, three tables, two figures and one scheme were included in the supporting information. References [1] K.S. Ashaghi, M. Ebrahimi, P. Czermak, Ceramic Ultra- and Nanofiltration Membranes for Oilfield Produced Water Treatment: A Mini Review, Open Environ. J. 1 (2007) 1-8. [2] U. Daiminger, W. Nitsch, P. Plucinski, S. Hoffmann, Novel techniques for oil/water separation, J. Membr. Sci. 99 (1995) 197-203. [3] E.T. Igunnu, G.Z. Chen, Produced water treatment technologies, Int. J. Low-Carbon Technol. 9 (2014) 157-177. [4] S. Alzahrani, A.W. Mohammad, N. Hilal, P. Abdullah, O. Jaafar, Comparative study of NF and RO membranes in the treatment of produced water—Part I: Assessing water quality, Desalination 315 (2013) 18-26. [5] J.L. Weeber, R.S. Bowman, L.E. Katz, E.J. Sullivan, J.M. Ranck, BTEX Removal from Produced Water Using Surfactant-Modified Zeolite, J. Environ. Eng. 131 (2005) 434-442. [6] G.T. Tellez, N. Nirmalakhandan, J.L. Gardea-Torresdey, Performance evaluation of an activated sludge system for removing petroleum hydrocarbons from oilfield produced water, Adv. Environ. Res. 6 (2002) 455-470. [7] J.A. Veil, Produced Water Management Options and Technologies, 2011. [8] P. Jain, M. Sharma, P. Dureja, P.M. Sarma, B. Lal, Bioelectrochemical approaches for removal of sulfate, hydrocarbon and salinity from produced water, Chemosphere 166 (2017) 96-108. [9] E.V. dos Santos, J.H. Bezerra Rocha, D.M. de Araujo, D.C. de Moura, C.A. Martinez-Huitle, 34

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Highlights

1.

Integrated membrane system of UF, FO and MD treated and utilized oily water

2.

FO-MD subsystem simultaneously utilized osmosis and thermal energies of oily water

3.

Temperature-salt content equilibrium curve of FO-MD guided for oily water usage

4.

Three scenarios of dynamic mass transfer process of FO-MD were proposed

5.

Integrated system recover water from oily water and sewage by using oily water energies

38