## 1 Introduction  
Membrane distillation (MD) is a thermal-driven separation process that uses hydrophobic membranes as an excellent way to treat water and wastewater. Unlike pressure-driven membrane process, the driving force to produce membrane permeate is the vapor pressure difference between the two sides of the membrane pores, allowing for the mass transfer of the volatile solution components [1; 2; 3]. In addition, hydrostatic pressure encountered in MD is lower than that used in pressure-driven membrane processes such as reverse osmosis (RO) and nanofiltration (NF) [4]. One of the great advantages of the MD technology in water and wastewater treatment is that the industrial sources and renewable energies such as waste heat or solar energy can be used as the heat energy to operate MD system [5; 6]. Interests in MD technology have been growing rapidly in the areas of seawater desalination due to relatively low fouling potentials and energy requirements compared to high-pressure driven membrane process [7; 8]. Nevertheless, the MD process has disadvantages such as low permeate flux compared to NF or RO and high susceptibility permeate flux to the concentration and temperature of the feed conditions due to the concentration and temperature polarization phenomena [4]. Furthermore, few studies have been done to better understand the performances of MD processes in wastewater treatment in comparison to seawater desalination [9; 10]. Direct contact membrane distillation (DCMD) is the most commonly used MD module configuration. In DCMD, both feed wastewater and permeate are in direct contact with a hydrophobic membrane. The water vapor transferred across the membrane is directly condensed in the permeate produced by the membrane module from which temperature is lower than feed one [4]. The DCMD is attractive particularly for wastewater treatment because it does not require external equipment such as vacuum pump or condensation plate, which may be necessary for other types of MD configuration such as air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD) or vacuum membrane distillation (VMD) [11; 12; 13]. For DCMD, a submerged configuration using a bundle of hydrophobic hollow-fiber membranes is preferentially used [14; 15; 16]. Furthermore, the DCMD configuration is especially suitable for hybrid membrane technologies such as membrane distillation bioreactor (MDBR) or integrated forward osmosis-membrane distillation (FO-MD) [17; 18; 19]. Hollow-fiber membrane is considered more favorable than flat sheet membrane in submerged direct contact membrane distillation (SDCMD) process due to its high packing density as well as relative easiness in membrane module design. Nevertheless, thermal efficiency can often be lower as membrane packing density is higher because the permeate can be heated up through the geometry of hollow-fiber membrane [20; 21; 22]. Most of all, hydrodynamic conditions should affect trans-membrane temperature and permeate flux because they can influence temperature polarization. Recently, the interest in MD processes such as submerged configuration have grown rapidly for resource recovery from brine solution [23; 24; 25]. In a submerged MD configuration, temperature polarization across the membrane should occur more easily due to the lack of turbulence on the membrane surface compared to the cross-flow MD system [26]. However, the submerged MD system requires less energy and a smaller foot-print than that needed to generate a high cross-flow velocity from a side-stream of MD process. In addition, the submerged MD can be integrated with a bioreactor to obtain energy more easily, for example, anaerobic MD bioreactor process [19]. In spite of many advantages, there is still lack of effort applying the SDCMD process to understand the effect of operational conditions on MD performances in treatment of low-strength wastewater.