Feasibility of concentrating textile wastewater using a hybrid forward osmosis-membrane distillation (FO-MD) process: Performance and economic evaluation

Journal Pre-proof Feasibility of concentrating textile wastewater using a hybrid forward osmosismembrane distillation (FO-MD) process: Performance and economic evaluation Meng Li, Kun Li, Lianjun Wang, Xuan Zhang PII:

S0043-1354(20)30024-5

DOI:

https://doi.org/10.1016/j.watres.2020.115488

Reference:

WR 115488

To appear in:

Water Research

Received Date: 5 December 2019 Revised Date:

2 January 2020

Accepted Date: 6 January 2020

Please cite this article as: Li, M., Li, K., Wang, L., Zhang, X., Feasibility of concentrating textile wastewater using a hybrid forward osmosis-membrane distillation (FO-MD) process: Performance and economic evaluation, Water Research (2020), doi: https://doi.org/10.1016/j.watres.2020.115488. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

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Feasibility of Concentrating Textile Wastewater Using a

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Hybrid Forward Osmosis-Membrane Distillation

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(FO-MD) Process: Performance and Economic

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Evaluation

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Submitted to

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Water Research

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Meng Li, Kun Li, Lianjun Wang, and Xuan Zhang*

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Key Laboratory of New Membrane Materials, Ministry of Industry and information Technology,

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School of Environmental and Biological Engineering, Nanjing University of Science and

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Technology, Nanjing 210094, China

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Corresponding Author:

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X. Zhang.: [email protected]

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Abstract

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The forward osmosis-membrane distillation (FO-MD) hybrid process has shown great promise in

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achieving zero liquid discharge in the textile industry, recovering valuable dye molecules while

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producing large amounts of clean water. However, the progress of this technology seems to have

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stagnated with the direct coupling of commercial asymmetric FO and MD membranes, because water

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management in the system is found to be rather complicated owing to the processing of the different

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membranes. Herein, we propose, for the first time, an FO-MD hybrid process using a custom-made

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self-standing and symmetric membrane and a hydrophobic polytetrafluoroethylene membrane in the

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FO and MD units, respectively. Three types of operation modes were investigated to systematically

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study the process performance in the concentration treatment of model textile wastewater; two

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commercial FO membranes were also tested for comparison. Owing to its low fouling propensity and

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lack of an internal concentration polarization effect, the water transfer rate of our symmetric FO

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membrane quickly reaches equilibrium with that in the MD unit, resulting in continuous and stable

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operation. Consequently, the hybrid process using the symmetric FO membrane was found to

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consume the least energy, as indicated by its lowest total cost in both lab- and large-scale systems.

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Overall, our study provides a new strategy for using a symmetric FO membrane in the FO-MD

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hybrid process and highlights its great potential for use in the treatment of textile wastewater.

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Keywords: FO-MD hybrid process; textile wastewater; symmetric FO membrane; energy/thermal

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efficiency; economic analysis

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1. Introduction

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As the concept of minimal and zero liquid discharge (MLD/ZLD) has been introduced, water

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reclamation, particularly from wastewater, has drawn extensive attention over the past few decades

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(Werber et al., 2016; Wang et al., 2020; Tong et al., 2016). Some valuable resources can be recycled

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or reused for a sustainable purpose, which may significantly reduce the production cost and extent of

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the environmental pollution (Petrinic et al., 2015; Lu et al., 2019; Conidi et al., 2018). Generally

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characterized by both high water consumption and extremely polluted effluents, textile industries

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produce large quantities of saline wastewater containing various dyes during the dyeing process

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(Petrinic et al., 2015; De Vreese et al., 2007). Owing to the complexity of the textile wastewater

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composition, achieving MLD/ZLD has become highly attractive but remains significantly

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challenging.

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Compared with existing treatment processes, e.g., adsorption (Cardoso et al., 2012), biological

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method (Gupta et al., 2014), advanced oxidation (Oller et al., 2011), and coagulation-flocculation

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(Verma et al., 2012), which may produce undesired side products or secondary pollution,

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membrane-based technologies have demonstrated great potential and have been successfully applied

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recently at the lab-scale (Li et al., 2017; Lin et al., 2015; Gillerman et al., 2006; Lau et al., 2009;

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Zhao et al., 2017; Vourch et al., 2008). By using typical pressure-driven membranes [loose

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nanofiltration (Li et al., 2017; Lin et al., 2015) or tight ultrafiltration (Lin et al., 2016)] with

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appropriate pore sizes and surface charges, dye molecules can be selectively retained on the

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concentrated side, whereas most of the inorganic salts penetrate to the permeate. However, the

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subsequent dye recycling step, i.e., the concentration process, seems to be highly risky as the fouling

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of these membranes can easily occur, becoming more severe with an increase in the concentration 3

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factor (CF) (Li et al., 2017).

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By contrast, a high water recovery ratio for the concentration process is theoretically more

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easily achieved through the forward osmosis (FO) process, which is driven by osmotic pressure. Its

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inherently low fouling propensity has also been experimentally proved to be effective, even in

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treatment with a highly concentrated feed solution (Li et al., 2019; Kim et al., 2014; Shaffer et al.,

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2015). Nevertheless, as the water penetrates from the feed to the draw solution (DS), the osmotic

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pressure decreases accordingly owing to the dilution, which inevitably results in a decline in

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performance. Thus, the necessity of DS recovery/reuse has remained as one of the greatest

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drawbacks of FO technology. The integration of FO with a membrane distillation (MD) unit seems

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promising—the DS could be regenerated by a heat-driven process, even if the solution possesses an

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extremely high solute content (osmotic pressure) (An et al., 2019; Wang et al., 2015; Liu et al., 2016;

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Zhang et al., 2014; Xie et al., 2013).

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In light of this, extensive efforts have been devoted to the FO-MD hybrid process in recent

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years (An et al., 2019; Wang et al., 2015; Liu et al., 2016; Zhang et al., 2014; Xie et al., 2013; Ge et

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al., 2012; Kim et al., 2019), including exploration of the polyelectrolyte draw solute (Ge et al., 2012),

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module design (Kim et al., 2019), and development of new membrane materials (Zhang et al., 2014).

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In general, the feasibility of an FO-MD process for continuous operation requires strict water

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management; that is, a water balance between the two separated units. However, FO is typically

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applied with asymmetric membranes (including commercial ones); thus, most studies have focused

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only on their compatibility with the MD process. On the one hand, the existing internal concentration

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polarization (ICP) effect in such asymmetric membrane systems—which is regarded as their Achilles

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heel—exerts great influence on the FO performance (Li et al., 2019). The DS concentration decreases 4

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along with water penetration from the feed to the draw side; however, the decline trend in water flux

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does not have a linear relationship with the salt content owing to the ICP, particularly at a high

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concentration level. In this manner, the water equilibration between the FO and MD components

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becomes rather complicated, resulting in the frequent adjustment of operating temperatures in MD.

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On the other hand, the initial concentrations of the feed solutions in previous studies were generally

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set at a low range (20–300 mg L-1) (Ge et al., 2012; Mahto, et al., 2017; Han et al., 2016; Huang, et

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al., 2017); treatment with these types of model waters may not quite reflect the possible membrane

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characteristics for practical applications, e.g., cake-layer formation on the membrane surface when

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the dye concentration in the feed increases to a certain level.

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The complex combination of the FO-MD hybrid process, choice of FO membranes [i.e.,

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asymmetric or recently reported symmetric structures (Li et al., 2018; Shaffer et al., 2015; Li et al.,

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2019)], and corresponding economic evaluation encouraged us to systematically investigate the

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feasibility of this technology for textile wastewater treatment. Herein, three different types of FO

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membranes (commercial cellulose triacetate (HTI-CTA), commercial thin-film composite FO

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membrane (CSM-TFC), and custom-made symmetric membrane) and one class of MD membrane

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(polytetrafluoroethylene, PTFE) were employed in FO and MD process, respectively. Their

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effectiveness, for concentrating a model dye solution up to a CF of 10, were studied and

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comprehensively compared in three operation modes (i.e., an isolated FO process, separated FO

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integrated with separated MD process, and FO-MD hybrid process). Their associated economic

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attributes, including thermal/energy consumption, gained output ratio (GOR), and system expense,

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were further evaluated or estimated.

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2. Materials and methods

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2.1 Materials and Chemicals

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For the FO unit, a symmetric and self-standing FO membrane, with ultra-low fouling propensity and

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no ICP effect (PTAODH-1.0, here 1.0 represents the molar ratio of 4-aminobenzoic acid to the

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repeating unit of PODH during the synthesis; thickness 8 µm), was fabricated according to our

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previous work (Li et al., 2018), with two commercial FO membranes, i.e., HTI-CTA (Hydration

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Technology Innovations) and CSM-TFC (Toray Chemical Korea, Inc.), for comparison.

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Characterizations including water and salt permeability, structural parameters, and microscale

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morphologies are provided in the earlier work (Li et al., 2018) and partially in Supplementary

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Material (Fig. S1). For the MD process, a commercial flat-sheet PTFE membrane was provided from

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Sterlitech Corporation (PTFE0214225, Washington, USA). Detailed characterizations, including

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pore size, water contact angle, liquid entry pressure, porosity, and morphology, were measured and

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are summarized in Fig. S2 and Table S1. Before operation in the MD unit, the PTFE membrane was

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wetted with ethanol, rinsed using deionized (DI) water, and completely dried in a vacuum oven.

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Congo Red (CR) and sodium sulfate (Na2SO4) were procured from Sinopharm Chemical Reagent

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Co., Ltd. (Shanghai, China). Other reagents and solvents were used as received. DI water with a

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minimum resistance of 18 MΩ-cm (Millipore) was used throughout the study.

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Fig. S1

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Fig. S2

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Table S1

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2.2. Set-up of FO-MD Hybrid System In the current study, the FO-MD hybrid system consists of an individual FO loop and MD loop,

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as graphically shown in Fig. 1A.

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Fig. 1

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In the FO unit, DI water and 1.5 M Na2SO4 solutions were used as the feed solution (FSFO) and

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draw solution (DSFO), respectively. The effective area of the membrane was controlled to 10.0 cm2,

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while the cross-flow rates on both sides (FSFO and DSFO) were fixed at 0.2 L min-1 (10.4 cm s-1), as

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described previously (Li et al., 2018). As the DSFO simultaneously served as the feed solution in the

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MD process (FSMD), it was heated to five different temperatures (25, 35, 45, 55, and 65 °C) to

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optimize the associated MD performance. The temperature of FSFO was kept at 25 °C throughout the

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test.

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In the MD unit, the performances were evaluated in direct contact membrane distillation

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(DCMD) mode with an effective membrane area of 10.0 cm2. The cross-flow rates of solutions were

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fixed at 0.2 L min-1 (10.4 cm s-1) and 0.1 L min-1 (5.2 cm s-1) for the feed FSMD (same as DSFO) and

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distillate streams, respectively. As the most optimal temperature for FSMD was found to be 55 °C (as

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discussed later), the temperatures for the distillate side were set at 5, 15, and 25 °C, respectively,

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corresponding to temperature differences of 30, 40, and 50 °C, respectively. For each test, the system

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would be run for at least 30 min for equilibration before the reliable performance data were collected. 7

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The reverse salt flux (Js, FO) was measured with an electric conductivity (DDS-307 conductivity

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meter, China), while the water fluxes (Jw, FO or Jw, MD) were obtained by measuring the weight change

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in the DSFO and distillate in MD, respectively. Notably, when the CR solution was employed as FSFO,

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the reverse salt contents in the FO unit were measured by ion chromatography (Dionex ICS-2100,

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U.S.A.) in terms of SO42-, and the CR concentrations were measured by UV/VIS spectrometry

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(Lambda 25, PerkinElmer, U.S.A.). Detailed calculations methods are provided in the

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Supplementary Material.

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2.3 Model Textile Wastewater Tests

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In this study, CR was used as a typical dye to prepare the model textile wastewater (Li et al.,

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2017; Li et al., 2019), with an initial concentration of 1.0 g L-1. The experiments proceeded by three

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types of operation modes, i.e., Mode A (isolated FO process), Mode B (separated FO integrated with

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separated MD process), and Mode C (FO-MD hybrid process), as mentioned earlier (Fig. 1B). In the

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FO unit, an initial 300 mL of Na2SO4 solution (1.5 M) and 500 mL of CR solution (1 g L-1) were

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used as DSFO and FSFO, respectively. All membranes in the FO unit were tested in FO mode (active

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layer facing the feed solution), while the active layer of the PTFE membrane faced the hot side

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(FSMD or DSFO) in the MD unit. The concentration process of the CR solution continuously

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progressed until the water recovery ratio reached 90%, corresponding to a CF of 10.

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Mode A: In this mode, the concentration of DSFO was maintained at 1.5 M by manually adding the

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salt (Na2SO4) with a conductivity monitor over a certain time period. The FO process ended once the

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CF reached 10. As the concentration of DSFO was fixed over the entire test, no further treatment was

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performed for DSFO regeneration. However, the DSFO volume would notably increase accordingly as

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the water permeated from the feed solution. 8

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Mode B: In this mode, the FO process operated in a continuous manner without the supplement of

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salt, also resulting in a CF of 10. Similar to Mode A, the volume of DSFO would also increase but

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with a much lower concentration. To be specific, the concentration of DSFO decreased to 0.6 mol L-1

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at the end of the FO process; such an “extra” amount of water (450 mL from FSFO) needed to be

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distillated until the concentration reached 1.5 M again, corresponding to a water recovery ratio of 60%

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for the MD process.

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Mode C: Different than Modes A and B, the concentrating of the dye solution and the regeneration

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of DSFO (or FSMD) proceeded simultaneously. In this mode, DSFO was diluted by accepting the

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penetrated water from FSFO, whereas it would be concentrated in the MD process at the same time

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from the internal circulation, serving as fresh DSFO again. Specifically, as the water fluxes for all

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three membranes (PTAODH-1.0, HTI-CTA, and CSM-TFC) in the FO process were different, the

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temperature differences applied in their MD processes were optimized separately (will discuss later),

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fully taking the water transfer rate (WTR) in this hybrid process into account.

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2.4 Evaluation of Energy/thermal Consumption

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In this study, the total energy consumption was calculated by the summation of energy

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consumption in the FO loop and MD loop, respectively. It should be mentioned that energy

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consumption in the FO process only comes from the pumping operation, whereas it originates from

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both the pumping operation and heater/cooler for solution heating/cooling in the MD process.

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Detailed calculation methods, including the thermal efficiency (TE), GOR, specific thermal energy

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consumption (STEC), and specific energy consumption (SEC), of the hybrid process were provided

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in Supplementary Material.

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3. Results and discussion

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3.1 Optimization of FO and MD Processes

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Fig. 2

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As DSFO also serves as the feed solution in MD unit, its operating temperature was initially

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studied and optimized separately for both units. As shown in Fig. 2A–C, the water fluxes (Jw, FO) for

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all three FO membranes (PTAODH-1.0, HTI-CTA, and CSM-TFC) increase with an increase in the

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DSFO temperature, which is attributed to the elevated osmotic driving force and draw solute diffusion

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at higher temperatures (Kim et al., 2019; Phuntsho et al., 2012). All the salt reverse fluxes (Js, FO)

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increase accordingly, suggesting a greater salt loss. Nonetheless, the reverse flux selectivity, Jw/Js, a

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parameter that reflects the reverse flux selectivity for an FO membrane (Li et al., 2018; Shaffer et al.,

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2015; Kim et al., 2019), was found to be less significant for the temperature variations (Fig. 2D) of

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all three membranes, indicating the minor effect on temperatures that may vary their physical

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properties. Considering the energy consumption that is closely associated with the operating

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temperature, the temperature of DSFO was finally determined to be 55 °C, in which Jw/Js for all three

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FO membranes were at their maximum levels, i.e., 432.1, 375.6, and 502.0 L mol-1 for PTAODH-1.0,

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HTI-CTA, and CSM-TFC membranes, respectively.

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Fig. 3 10

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Subsequently, the MD unit was optimized by adjusting the temperature of the distillate stream,

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resulting in different trans-membrane temperature differences. Owing to the increased water vapor

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pressure (Alsaadi et al., 2014), Jw,MD of the PTFE membrane increases with an increase in the

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temperature difference, as described in Fig. 3A. Although the concentration of FSMD (or DSFO)

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increases as the experiment proceeds, no apparent changes were observed in either the water flux or

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the membrane permeability coefficient (Km, Fig. 3B, see in Supplementary Material), suggesting a

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negligible influence on the solution concentration (Chen et al., 2020). Further calculation of both the

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temperature polarization coefficient (TPC) and TE revealed that the most optimal MD performance

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occurs at a temperature difference of 50 °C, which indicated the lowest temperature polarization and

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highest utilization of heat of the MD system (Liu et al., 2019; Swaminathan et al., 2018).

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Combining the above results, the temperatures for FSFO, DSFO (or FSMD), and DSMD were

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eventually determined and fixed at 25, 55, and 5 °C, respectively, for the subsequent treatment of the

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textile wastewaters.

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3.2 Model Textile Wastewater Tests

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3.2.1 Operation in Mode A and Mode B

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Fig. 4

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Fig. S3

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Fig. S4

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To verify the feasibility of dye concentration in the FO process and water reclamation in MD

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process, two typical modes (A and B) were conducted, and the results are displayed in Fig. 4 and Fig.

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S3. Through the manual supplement of the draw solute (Na2SO4) to maintain a constant osmotic

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pressure (Mode A), membrane PTAODH-1.0 maintains a stable Jw,FO of 18.6±0.4 L m-2 h-1 over the

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entire concentration process until CF reaches ~10 (Fig. 4A). This could be regarded as the primary

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evidence of minimal fouling at the feed side as well as a negligible ICP effect (Li et al., 2019). By

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contrast, the asymmetric HTI-CTA and CSM-TFC membranes both exhibit apparent flux declines,

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suggesting severe membrane fouling and the associated external concentration polarization (ECP)

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effect. Although membrane CSM-TFC possesses the greatest initial Jw,FO, it decreased by 59.6%

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from 25.5 to 10.3 L m-2 h-1 at the end of the experiment (Fig. S3A), indicating its most inferior FO

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performance.

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Different than Mode A, the water fluxes for all three membranes decrease significantly in Mode

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B, as the driving forces continuously decrease with water penetration. More specifically, the Jw,FO of

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membrane PTAODH-1.0 decreases by 56.8% from 18.4 to 7.9 L m-2 h-1 (Fig. S3B), which matches

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the DS dilution ratio of ~60%. Unsurprisingly, the two commercial membranes both exhibit a faster

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flux decline, which is probably attributed to the severe fouling and ECP effect, as also mentioned

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above. The calculated ECP here was found to reduce the driving force by 16.4% and 17.0% for the

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HTI-CTA and CSM-TFC membranes, respectively. It is worth mentioning that the salt and dye loss

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were both maintained at low levels over the entire FO process, regardless of the membrane type or

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operation mode (Fig. 4B and 4D); the slightly higher concentrations of salt in FSFO and dye in DSFO

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were only ascribed to the longer operation durations of Mode B. Symmetric membrane 12

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PTAODH-1.0, possessing the highest running fluxes, lowest solute loss, and the least time

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consumption in both modes (Fig. S3A and S3B), displays the most superior effectiveness for the dye

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concentration process among the three FO membranes. One possible explanation would be its

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ultra-smooth surface roughness, which was demonstrated to effectively prevent the dye molecule

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deposition under the hydraulic shear force (Li et al., 2019; Zhao et al., 2015; Mazlan et al., 2016).

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After the experiments, the dye solution could be successfully concentrated at a CF of 10 for

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both operation modes. As DSFO was still maintained at 1.5 M for Mode A, no further regeneration is

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needed. However, an extra 95.9 g of Na2SO4 was consumed, along with a much enlarged solution

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volume (750 mL). As such, the practical wastewater treatment by Mode A may only be suitable for

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some small-scale and off-grid applications, considering the limited room for infrastructure and

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expenditure on the draw solutes. Although no additional draw solute was required in Mode B, the

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continuously reduced osmotic driving force considerably prolonged the entire concentration process,

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suggesting excessive energy consumption. Consequently, a diluted 750 mL of DSFO (~0.6 M) was

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collected, which required regeneration by a separate MD process. As graphically described in Fig. S4,

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the experiment proceeded for another 28 h until reaching a water recovery ratio of ~60% (1.5 M).

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However, the water flux slightly decreased from 17.0 to 14.7 L m-2 h-1 at the end of the test, possibly

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owing to membrane fouling or scaling. In terms of the FO and MD performances in Mode B, it is

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suggested that practical application of this mode may not be possible, as DSFO underwent

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overdilution, resulting in a much lower time efficiency and higher energy consumption.

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3.2.2 Operation in Mode C

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Fig. S5

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Fig. 5

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Although Mode A and Mode B both eventually completed the concentration process, many

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drawbacks were involved, as mentioned above. Therefore, an FO-MD hybrid process (Mode C) is

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highly attractive because DSFO (or FSMD) is diluted and expected to be regenerated simultaneously,

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maintaining its pristine concentration (osmotic pressure). However, a critical issue remaining in this

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technology is the WTR balance for the FO and MD components (Liu et al., 2016; Kim et al., 2019).

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In brief, if the WTR in FO is higher than that in the MD process, DSFO (or FSMD) would be diluted

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with time, until a water balance is established. Note that the concentration of DSFO (or FSMD) would

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be lower or much lower than that in the initial state, resulting in a longer treatment duration. On the

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contrary, if the WTR in FO is lower than that of MD, DSFO (or FSMD) would still be steadily

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concentrated with time until reaching a stable state as well. However, such an increase in DSFO

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concentration may inevitably exacerbate the ICP effect if asymmetric FO membranes are employed,

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leading to an undesired and fairly slow equilibration process. In addition, precipitation may also

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occur once DSFO (or FSMD) reaches a saturated of oversaturated state, especially when Na2SO4 is

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used as the draw solute, owing to solubility concerns. Moreover, it is important to mention that

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membrane fouling (either FO or MD membrane) is not considered in either of the above two

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situations; water management should be much more complicated in view of the practical

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applications.

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Surprisingly, CSM-TFC membrane shows a dramatic decline by 65.3% in WTR in the FO

305

process (Fig. S5A), although the entire experiment was completed within 30 h (Fig. S5B). The WTR 14

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in the FO process decreases from 25.4 to 17.0 mL h-1, taking approximately 10 h to reach the same

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WTR level as that in the MD process; however, the decrease continues over the rest of the test (final

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value of 8.8 mL h-1), both of which suggesting that the severe fouling is derived from the deposition

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of the dye molecules on the membrane surface. Considering the initial water flux of the HTI-CTA

310

membrane, the temperature difference for the MD component was adjusted to 40 °C to avoid the fast

311

evaporation of FSMD (or DSFO), forming Na2SO4 precipitates. Consequently, its TE may have

312

inevitably decreased, as discussed earlier (Fig. 3C). In this manner, the WTR decline for the

313

HTI-CTA membrane is found to be more gentle but still at a high level of 31.9% (from 11.6 to 7.9

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mL h-1, Fig. S5C and S5D), indicating membrane fouling as well.

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Compared with the two commercial FO membranes, the FO-MD hybrid process by using our

316

symmetric membrane (PTAODH-1.0) proceeded quite well. Owing to a higher water flux of the FO

317

part, its initial WTR was found to be slightly higher than that of the MD part. However, the dynamic

318

water equilibration was established only after a few hours, as evidenced by the identical WTR for

319

both FO and MD components (Fig. 5A). The conductivity in the distillate was also found to be stable

320

at a low level (< 5 µS cm-1) over the entire test, indicating the excellent stability of the system.

321

Although it was reported in many previous studies that dynamic equilibrium is not prone to being

322

established in the FO-MD hybrid process (An et al., 2019; Lu et al., 2018; Xie et al., 2014), such an

323

ideal operation state was eventually obtained by using our symmetric membrane in the FO part,

324

together with a common hydrophobic membrane (PTFE) in the MD part. Some plausible

325

explanations are provided as follows. On the one hand, as the concentration of DSFO (or FSMD)

326

continues to vary, particularly in the early stages, the water flux in the FO part varies accordingly.

327

However, commercial asymmetric membranes suffer from a large ICP effect, as reported elsewhere 15

328

(Li et al., 2019; Gray et al., 2006). Thus, the water flux of the FO membranes does not display a

329

linear relation with the DSFO concentration. Consequently, the variations in WTR may be difficult to

330

match between the FO and MD parts, without ending with the dynamic equilibration even after a

331

long-term procedure. Owing to the lack of ICP effect, this phenomenon could be well inhibited in the

332

symmetric membrane system, as revealed recently by both mathematical models and experimental

333

observations (Li et al., 2018). On the other hand, the membrane fouling is non-negligible and is

334

perhaps a more critical issue. Although FO membranes were regarded as low-fouling media (Kim et

335

al., 2014; Shaffer et al., 2015), a cake layer would still form when the CF is at a high level. Such

336

molecular deposition was found more easily on the surface of the commercial membranes [roughness

337

of 3.8–46.0 nm and 30.3–65.0 nm for the CTA and TFC FO membranes, respectively (Li et al., 2019;

338

Mazlan et al., 2016; Stillman et al., 2014)], resulting in the inhibition of the water transfer (Fig. S5A

339

and S5C). By contrast, the symmetric PTAODH-1.0 membrane, with an ultra-smooth surface nature

340

(roughness of 0.66±0.08 nm) (Li et al., 2019), was demonstrated to be effective in avoiding the dye

341

adhesion under a crossflow hydraulic scouring, and thus maintaining the WTR in an almost constant

342

level over the entire membrane process.

343 344

3.3 Evaluation of Energy/Thermal Consumption

345 346

Fig. 6

347

Fig. 7

348 349

With the experimental results obtained, the energy and thermal consumptions for different 16

350

operation modes using different membranes are further studied. Fig. 6 shows the changes in the

351

specific energy consumption in the FO process (SECFO), where the consumed energy only accounts

352

for the water pump. The calculated values of SECFO seems relatively high, which is largely attributed

353

to the low WTR caused by low water flux and limited membrane area (only 10 cm2) (Eqs. S(13) and

354

S(25)). However, the SECFO could be significantly reduced if FO membranes with larger effective

355

areas were employed in practical use. Regardless which membrane was employed, Mode B always

356

displayed the largest SECFO among the three operation modes, indicating that it consumes the most

357

energy. A similar trend was also found for the HTI-CTA membrane, which continuously exhibited the

358

largest SECFO among the three FO membranes in the three operation modes. As the water fluxes of

359

both commercial FO membranes decreased considerably at the ending stage of the concentration

360

process, these results clearly reveal that the higher energy consumption is mostly derived from the

361

reduced water fluxes and resulting longer operation durations. In this manner, membrane

362

PTAODH-1.0, with the shortest treatment time, undoubtedly shows the lowest energy consumption

363

in the FO process.

364

Analogously, relevant performance parameters, including TE, STEC, GOR, and SECMD of all

365

MD-involved processes, were also calculated and analyzed in this study (Fig. 7). In general, all the

366

obtained SECMD were found to be only 7.2%–17.5% higher than STEC through the experiment,

367

indicating that the heat effect dominates the overall energy consumption of the MD process. Just as

368

the findings above, the HTI-CTA membrane showed the worst performance for all processes, that is,

369

the highest STEC (1859.1–2008.8 kWh m-3) and SECMD (2083.3–2396.0 kWh m-3) and the

370

corresponding lowest GOR (0.30–0.26). Therefore, such a result further indicates the unreliability of

371

this type of FO membrane for use in any MD-coupled hybrid process. One possible reason is its 17

372

inherently lower water flux, which requires a lower temperature difference in MD (40 °C, whereas

373

50 °C for the CSM-TFC and PTAODH-1.0 membranes) to balance the WTR, resulting in the lowest

374

TE (60%–70%). Again, owing to the fact that it exhibits the most stable WTR and shortest operation

375

duration among all three FO membranes, symmetric membrane PTAODH-1.0 is expected to display

376

the most superior performance in the hybrid process.

377

More specifically, a significant decline in TE and simultaneous increase in STEC were observed

378

for both commercial FO membranes during the second half of the experiments (Fig. 7). Note that for

379

a long period of time, WTR in MD was greater than that in the FO process (Fig. S5); thus, FSMD (or

380

DSFO) may undergo excessive evaporation, generating a saturated or oversaturated solution, as

381

discussed earlier. Inorganic scaling was likely to form and accumulate on the PTFE membrane

382

surface, resulting in a reduction in water productivity and an increase in the thermal conductivity

383

coefficient, consequently reducing the membrane desalination and thermal/energy efficiency

384

(Deshmukh et al., 2018). In terms of the overall results of the energy evaluations, commercial FO

385

membranes, especially the CSM-TFC membrane, may still be applicable for the FO-MD hybrid

386

process for a dye concentration treatment. However, a stable water recovery ratio of less than 30% is

387

expected, corresponding to a CF of ~3.3. By contrast, a water recovery ratio as high as 90% (CF of

388

~10) could be steadily achieved for the symmetric membrane PTAODH-1.0 without much variation

389

in energy consumption, which is highly beneficial for the subsequent dye recovery process (Lin et al.,

390

2015; Chen et al., 2015; Cong et al., 2007). A further increase in the CF may be possible; however,

391

the practical application should also consider the dye solubility and viscosity increase of the solution.

392

18

393

3.4 Economic Analysis

394 395

Table 1

396

Table 2

397

Table S2

398 399

Economic feasibility of the proposed FO-MD process in this study is another concern for

400

practical application. Therefore, we summarized the related data and calculated some key cost

401

parameters by comparing the implementation of the three types of FO membranes and their use in

402

the three operation modes (Table 1). Specifically, the cost of the fabricated PTAODH-1.0 membrane

403

was estimated to be 56 USD/m2 by considering the total expense of chemicals, solvents, purification

404

processes, etc., which was further multiplied by 1.5 to consider other potential expenditures.

405

Meanwhile, the costs for the two commercial FO membranes were assessed by their purchase prices

406

in the module, followed by a simple calculation of the area of the membranes. Furthermore, the

407

electric costs only comprise the cost of pump operation, solution heating/cooling, and heat exchange.

408

Without an MD unit, operation in Mode A shows a reasonably low electricity cost; however,

409

additional chemicals (draw solute) significantly contributes to the total cost (TC), resulting in the

410

highest TC among the three operation modes. Storage or treatment of the DS, with the continuously

411

increased salt amount and solution volume, would be another critical issue in the consideration of the

412

environmental impact. For comparison, the implementation of the MD unit (MD membrane and

413

MD-related electricity cost) rendered the largest TC for both Modes B and C. By using the

414

symmetric PTAODH-1.0 membrane in the FO-MD hybrid process, the lowest TC was achieved 19

415

(approximately 0.17 USD) for treating 500 mL textile wastewater to a CF of 10. This simultaneously

416

produced 450 mL of pure water that could be reused in the dyeing process. Nevertheless, it is worth

417

mentioning that MD can benefit from using low-grade or waste heat from industrial sources and

418

power plants, as reported elsewhere (Deshmukh et al., 2018). Therefore, by employing these heats to

419

offer the heat energy in MD process, considerable reduction in the electricity cost is highly possible,

420

which may further facilitate the economic viability.

421

Expanding the wastewater capacity results in a significant increase in TC, largely due to the cost

422

of FO membranes, accounting for 60.8%–86.2% (Table 2). Another issue is the FO membrane

423

fouling, which greatly affects the effectiveness of the system process. While the performance

424

recovery of FO membranes was mostly ignored in the lab-scale experiments, recovery was critically

425

needed on an industrial scale. Assuming a backwashing process using pure water, together with the

426

consideration of the water flux recovery ratios of the three FO membranes (in our previous study, Li

427

et al., 2019, Table 2), the membrane cleaning cost was estimated to be 0.030, 0.025, and 0.024

428

USD/m3, corresponding to TC values of ca. 14.15, 48.34, and 47.54 USD/m3 for the PTAODH-1.0,

429

HTI-CTA, and CSM-TFC membranes, respectively.

430

According to some statistics, approximately 2.1 billion ton of textile wastewater (assuming an

431

average dye content of 0.1%) was discharged in China in 2015 (Zheng et al., 2019). By using the

432

same process presented in this study, approximately 1.9 billion liters of fresh water and 2.1 million

433

ton of dyes could be reclaimed and reused, which would complete the MLD/ZLD mission while

434

largely relieving the stress of the industrial water supply (~1 USD/m3 in 2015). To realize the

435

FO-MD hybrid process in the textile wastewater treatment plant, reducing the FO membrane cost is

436

the paramount issue in reducing TC. Applying the PTAODH-1.0 membrane in the FO-MD hybrid 20

437

process has been demonstrated to be highly promising; however, its performance regeneration after

438

each treatment cycle may still need further optimization to extend the membrane lifespan. Moreover,

439

as DCMD was regarded as convenient but the most energy-consuming process compared with the

440

other MD configurations (sweeping gas MD and vacuum MD, Karanikola et al., 2019), integrating

441

FO with an appropriate MD unit may be another compelling option to further strengthen the

442

competitiveness of this technology. Additionally, the MD process may also be substituted by some

443

novel desalination processes, such as low-salt-rejection RO (LSRRO) technology revealed in a very

444

recent research by Wang et al. (Wang et al., 2020). It can be used to desalinate or concentrate

445

hyper-saline brines using moderate hydraulic pressure with relatively low energy consumption.

446

Therefore, we’re curious but highly anticipate a substantial reduction of the total energy consumption

447

for the hybrid system, if LSRRO would be coupled with the symmetric FO membrane, and achieve a

448

facile water equilibration as well.

449

450

4. Conclusions

451

In this work, we systematically evaluated the feasibility of the concentration process of a model

452

textile wastewater by using three different types of FO membranes under three types of operation

453

modes. Although all processes could achieve a high CF of ~10 for the feed solution, operation in

454

Mode A requires extra draw solute consumption to maintain the osmotic driving force, producing a

455

larger DS volume. Without the supplement of the draw solute, the water fluxes were found to be

456

continuously decreasing over the entire process for Mode B, which greatly extended the treatment

457

duration. The most promising approach was determined for the FO-MD hybrid process, where the

21

458

draw solute was diluted in the FO part and was simultaneously regenerated in the MD part. However,

459

owing to the existence of the ICP effect and membrane fouling, the two commercial FO membranes

460

did not show satisfactory performance, owing to the difficulty in equilibrating WTR. By contrast,

461

symmetric FO membrane showed superior performance in the FO-MD process, as its WTRs were

462

found to be almost identical between the FO and MD process, resulting in much lower energy

463

consumption. Economic analysis revealed that the lowest TC of ca. 0.17 USD for treating 500 mL

464

textile wastewater to a CF of 10 was achieved by using the symmetric PTAODH-1.0 membrane in

465

the hybrid process. Possible improvements, including the feasibility of various dyes and the possible

466

selection of other draw solutes, need to be examined to highlight the great potential of symmetric FO

467

membranes for use in the dye concentration and recovery of textile wastewaters.

468 469

Declaration of competing interest

470

The authors declare no competing financial interest.

471

Acknowledgement

472

This work was supported by the National Natural Science Foundation of China (21774058,

473

51778292), the Natural Science Foundation of Jiangsu Province (BK20180072). We also

474

acknowledge the support from the China Scholarship Council (CSC) for Meng Li (Grant No.

475

201906840105).

22

476

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26

594

595 596 597 598 599 600 601 602 603 604

Fig. 1. (A) Schematic diagram of FO-MD hybrid system containing two individual processes (FO loop and MD loop). (B) Three types of operation modes applied in this study. Mode A: an isolated FO process. The concentration of DS (Na2SO4) was maintained at 1.5 M by manually adding the salt with the monitor. Mode B: a separated FO integrated with a separated MD process. The DSFO concentration was diluted continuously without the supplement of salt, and the solution was further diluted in the MD process. Mode C: an FO-MD hybrid process. The DSFO was diluted in the FO part and concentrated in the MD process simultaneously.

27

605 606 607 608 609 610 611 612 613 614

Fig. 2. FO performances, in terms of Jw and Js, of (A) PTAODH-1.0 membrane (8 µm), (B) HTI-CTA membrane, and (C) CSM-TFC membrane as a function of DSFO temperature. (D) Jw/Js of three membranes as a function of DSFO temperature. All FO experiments were carried out at a flow-rate of 0.2 L min-1 with an effective membrane area of 10.0 cm2. DI water and a 1.5 M Na2SO4 solution were used as the FSFO and DSFO, respectively. The temperature of the FSFO was kept at 25 °C, while the temperatures of DSFO were set at 25, 35, 45, 55, and 65 °C, respectively. At least two parallel tests were conducted, presented as average values with error bars.

28

615

616 617 618 619 620 621 622 623 624 625 626

Fig. 3. (A) Water flux of commercial PTFE membrane with time under different temperature differences. (B) Water flux and mass transfer coefficient (Km) as a function of temperature difference, by using DI water or 1.5 M Na2SO4 as the FSMD, separately. The MD performance of commercial PTFE membrane was evaluated by a lab-scale DCMD unit with the effective membrane area of 10.0 cm2. The flow-rates were kept at 0.2 L min-1 (10.4 cm s-1) and 0.1 L min-1 (5.2 cm s-1) for the FSMD and distillate, respectively. As the temperature of the FSMD was fixed at 55 °C, the temperature differences in MD process were adjusted to 30, 40, 50, and 60 °C, by varying the temperatures of the distillate stream.

29

627 628 629 630 631 632 633 634 635 636

Fig. 4. (A) Variation of the normalized water flux and calculated/actual CR concentrations in FSFO with the permeate volumes in Mode A. (B) The detected mass transfer of Na2SO4 in FSFO and CR in DSFO, respectively, in Mode A. (C) Variation of the normalized water flux and calculated/actual CR concentrations in FSFO with the permeate volumes in Mode B. (D) The detected mass transfer of Na2SO4 in FSFO and CR in DSFO, respectively, in Mode B. An initial 1 g L-1 of Congo Red (CR) solution was used as the FSFO at 25 °C, and a 1.5 M of Na2SO4 solution was used as the DSFO/FSMD at 55 °C.

30

637

638 639 640 641 642 643 644 645 646 647

Fig. 5. Performance of FO-MD hybrid process (Mode C) for concentration of CR solution. Membrane (PTAODH-1.0) was used in the FO process, while PTFE was used in the MD process. (A) The WTR of the FO loop and MD loop with time over the entire concentration process. (B) Calculated/actual CR concentrations in FSFO with the permeate volumes in Mode C. The calculated CR concentration was obtained by the initial CR concentration and the CF, assuming no adsorption of dye on the membrane surface. Here, the temperatures of FSFO, DSFO (FSMD), and the distillate (in MD) were set at 25, 55, and 5 °C, respectively.

31

648 649 650 651 652

Fig. 6. Variation on specific energy consumption in the FO process (SECFO) with the permeate volume for different FO membranes.

32

653 654 655 656 657

Fig. 7. Variation on MD-relevant parameters. (A) TE, (B) STEC, and (C) GOR and SECMD in the FO-MD hybrid process (Mode C) by using different FO membranes. Parameters in Mode B were also investigated here for comparison.

33

658

Table 1. Comparison of the assumptions, key cost parameters, and time consumption for the three FO membranes in the three operation modes (small-scale). Items Mode A Mode B Mode C Assumptions

(isolated FO process)

(separated FO and MD process)

(FO-MD hybrid process)

Wastewater volume (mL)

500

500

500

CF/Water recovery

10/-

10/90%

10/90%

Fresh water price (USD/m )

~1

~1

~1

Fresh water production (mL)

NONE

~450

~450

Power cost (USD/kWh)

~0.1

~0.1

~0.1

70%

70%

70%

~2.85

-

-

3

Pump efficiency (%)

a

Draw solute price (USD/kg) Membranes price

PTAODH-1.0

HTI-CTA

CSM-TFC

PTAODH-1.0

HTI-CTA

CSM-TFC

PTFE

PTAODH-1.0

HTI-CTA

CSM-TFC

PTFE

56

100

82

56

100

82

40

56

100

82

40

10

10

10

10

10

10

10

10

10

10

10

Operation process b

FO1

FO2

FO3

FO1+MD

FO2+MD

FO3+MD

FO1-MD

FO2-MD

FO3-MD

Draw solute supplement (g)

~95.9

~95.9

~95.9

-

-

-

-

-

-

Pump energy consumption

0.019

0.039

0.020

0.044

0.074

0.043

0.035

0.063

0.041

Heat and cooling (kWh)

0.028

0.028

0.028

0.046

0.046

0.046

0.030

0.028

0.030

Heat exchange

-

2

(USD/m )

Required membrane area (cm2)

3

(kWh/m )

-

Only consider MD process

Only consider MD process

(kWh) Time consumption (h)

Estimated costs

659

0.7059

0.7059

0.7059

0.6658

0.9395

0.7895

24

51

27

64

103

64

26

46.5

30.3

~0.273

~0.273

~0.273

-

-

-

-

-

-

Membrane cost (USD)

0.056

0.100

0.082

0.096

0.140

0.122

0.096

0.140

0.122

Electricity cost (USD)

~0.003

~0.003

~0.003

0.080

0.083

0.080

0.073

0.100

0.090

Total cost (USD)

0.332

0.376

0.358

0.176

0.223

0.202

0.169

0.240

0.212

Draw solute cost (USD)

c

1

2

3

a) Pump efficiency is determined by actual condition; b) FO : PTAODH-1.0 membrane; FO : HTI-CTA membrane; FO : CSM-TFC membrane; c) the cost of draw solute supplement. 34

660 661 662

Table 2. Comparison of the assumptions and cost parameters for the three FO membranes in a large-scale hybrid system. FO1-MD

FO2-MD

FO3-MD

56

100

82

FO Membrane area (m )

7.9

7.9

7.9

MD membrane cost (USD/m2) c

40

40

40

MD membrane area (m2)

7.9

7.9

7.9

MD membrane lifetime (years)

5

5

5

Wastewater capacity per entry (m3) d

3.95

3.95

3.95

FO membrane regeneration efficiency (%) e

94.5

86.0

83.2

Available cycles of FO membrane cleaning f

12

4

3

Total FO membrane operation cycles g

13

5

4

Total wastewater treatment capacity (m3) h

51.35

19.75

15.80

FO membrane replacement (USD/m3) i

8.6

40

41

MD membrane replacement (USD/m3) j

0.04

0.09

0.06

FO membrane cleaning cost (USD/m3) k

0.030

0.025

0.024

Total electricity cost (USD/m3) l

5.48

8.22

6.46

Total cost (USD/m3) m

14.15

48.34

47.54

Items Assumptions

FO Membrane cost (USD/m2) a 2 b

Estimated costs

663 664

* FO1: PTAODH-1.0 membrane; FO2: HTI-CTA membrane; FO3: CSM-TFC membrane. Detailed information for denotations (a-l) were provided in Table S2 in Supplementary Material.

35

Highlights 1. A novel FO-MD hybrid process using symmetric FO membrane was developed. 2. A model textile wastewater was concentrated by 10 times in the hybrid process. 3. Water equilibration was readily established in novel hybrid process. 4. Novel hybrid process consumes lower energy than that using commercial FO membranes.

Declaration of competing interest The authors declare no competing financial interest.