## 1 Introduction  
Anodizing is an electrochemical process applied at the metallic surfaces to enhance their decorative, durable, appearance enhancement, and corrosion resistance properties. Anodizing treatment can be broadly divided into two main steps: (i) surface cleaning and preparation (degreasing, pickling etc.) and (ii) metallic coating and surface finishing (electrolating, anodizing, and immersion). The process generates various wastewater streams, which are generally mixed into a single stream before disposal [1]. Due to the presence of several metals into the final stream, the anodizing industry is moving toward zero liquid discharge to meet stringent environmental regulations [2]. Depending upon the composition of effluent and final requirements, various technologies are applied in practice for the treatment of wastewater from the anodizing industry. However, each of the applied technologies addresses a very specific problem within the overall wastewater treatment line. For instance, combined chemical precipitation, coagulation, and flocculation followed by filtration is used to partly remove the dissolved metals from wastewater. However, this approach is time consuming, uses excessive toxic chemicals, generally requires a polishing step to meet the discharge requirements [3], and has large footprints due to the involvement of multiple unit operation. Reverse osmosis (RO) or ion exchange are applied to remove dissolved salts and thus to produce water with low conductivity. However, RO requires extensive pretreatment, and generates a concentrated stream that needs proper disposal, while ion exchange technology requires the regeneration of the resins. Zero liquid discharge units, being applied in the anodizing industry today, are a complex combination of several unit operations such as chemical-physical purification, mechanical separation, membrane bases purification, and evaporation [4]. Various chemicals are used to adjust pH levels at different stages during these processes. The energy consumption of such methods is high, and footprints are large. Furthermore, this approach does not consider the recovery of valuable components from the wastewater streams. Membrane distillation/crystallization (MD/MCr) has emerged as an attractive process to concentrate the solutions to their saturation level [5]. MD is a temperature-driven process in which the vapors of volatile components pass through a hydrophobic membrane, due to a vapor pressure difference across the membrane. Due to the hydrophobic nature of the membrane, it has theoretically a 100% rejection of non-volatile substances present in wastewater streams. The process is operated at lower temperatures (~40-60 \({}^{\circ}\)C [6]) compared to the conventional distillation process; accordingly, cheap low-grade heat can be used to heat the feed solution [7]. Compared to conventional thermal desalination processes, the process has smaller footprints, and is lightweight [8; 9]. The process can simultaneously produce freshwater and crystals of the dissolved salts, thus approaching zero liquid discharge without any additional post-treatment [10]. The technical potential of MD for various applications, such as desalination, environmental/waste clean-up, water reuse, and in the food and medical industry has been well-demonstrated in the literature [11; 12; 13]. MCr has been particularly investigated for the simultaneous recovery of salts and freshwater from various types of brine solution [14; 15; 16]. [PERSON] et al. [17] applied MCr to recover high-purity NaCl from the reverse osmosis concentrate of real seawater. In another study [10], MCr was applied to recover NaCl crystals from microfiltered produced water. [PERSON] et al. [18] integrated a reverse osmosis unit with MCr to recover Na\({}_{2}\)SO\({}_{4}\) from a single salt solution. [PERSON] et al. [19] coupled MCr with nanofiltration to recover Na\({}_{2}\)SO\({}_{4}\) from its single salt aqueous solution.