**3.1 MD module design**

DCMD is one of the most used MD configurations due to its simplicity. Different MD modules have been developed to address a variety of process requirements. Based on membrane type and design principle, MD modules have been developed into three different designs:


#### **Figure 11.**

*(A): Flat sheet membrane module, showing a rectangular membrane between two channels. (B): A schematic of Hollow Fiber module set up, comprising hollow fiber membrane cased in a housing. (C): A spiral wound DCMD module commercialized by Solar Springs, adapted from [42].*

permeate flux of the hollow fiber module is lower to that of the flat sheet. Additionally, the cleaning process and the membrane replacement is difficult in the hollow fiber module [4, 41]. **Figure 11(B)** shows a typical set up of hollow fiber DCMD.

3. Spiral wound: Flat sheets are spirally coiled in a cylindrically wound shape. The membrane is rolled with the spacer between the feed and coolant fluids, creating evaporator and condenser channels. Such type of system was first commercialized by Solar Springs as Oryx unit as shown in **Figure 11(C)** [42]. The condensation process first preheats the cold feed as a condenser stream in the spiral wound design. Then, preheated feed is heated to a required temperature before entering from its center. The feed solution flows axially in the evaporator channel, and the permeate which passes through the membrane flows spirally. The condenser channel is placed toward the shell side while feed towards the core side to get the advantage of counter-current circulation [6].
