## 2 New Membrane Operations Aligned with PIS
### Membrane Distillation/Crystallization
Effectively, in a Mcr the membrane matrix acts as a selective gate for solvent evaporation, modulating the final degree and the rate for the generation of the supersaturation. Hence, the possibility to act on the trans-membrane flow rate, by changing the driving force of the process, allows for the potential to modulate the final properties of the crystals produced both in terms of structure (polymorphism) and morphology (habit, shape, size, and size distribution). Moreover, the possibility to separate the solvent evaporation (occurring inside the membrane module where the flowing solution is below the supersaturation condition) and the crystallization stage (performed in a separate tank on the retentate line operating in the metastable regime of supersaturation) allows for the proper control of the crystallization temperature, thus addressing the formation of a particular form (hydrous or anhydrous). The European funded project: _Membrane Based Desalination: An Integrated Approach (MEDINA)_ was launched in 2006. This project focused on integrated membrane systems for improving the efficiency of desalination. Integrated systems consisting of microfiltration (MF), NF, and RO with a membrane crystallizer coupled to NF and RO can achieve a water recovery factor as high as 92.4%, thus approaching zero liquid discharge and recovery of valuable raw materials [21]. Integration can be achieved in several configurations, as mentioned in [22; 23; 24]. Integrated membrane systems are also very interesting from an economical point of view. The specific water cost apparently increases when MD or Mcr are introduced (Figure 3), mainly due to the requirement of steam when operating the thermal processes with respect to electrical energy demand in pressure driven membrane operations [24]. Nevertheless, the water recovery factor increases significantly with the introduction of MD and Mcr from only 40% in RO (configuration 1) to above 90% for integrated operations (configuration 7). However, if the sale of CaCO\({}_{3}\), MgSO\({}_{4}\).7H\({}_{2}\)O, and NaCl is considered the water production cost can be negative (solid lines shown in Figure 3) [24]. In this case, the desalinated fresh water can just be a by-product of mineral production. In addition to RO brine, other industrial saline water streams can also be considered for recovery of contained minerals. Several developments are due for widespread applications of membrane distillation/crystallization process. The biggest issue is the unavailability of specific membranes with the required hydrophobicity, porosity, pore size distribution, and antifouling/wetting character. From a phenomenological point of view, a better understanding and control of temperature polarization is required. Research efforts are also required to improve the module design and to develop better logics for energy recovery. There is  
Figure 2: A comparative analysis of potentially recoverable minerals from brine in percentage with respect to their quantity obtained through traditional mining. a need to develop large scale prototypes to assess the true techno-economic potential of the process and to comprehend the unforeseeable problems.