## 2 New Membrane Operations Aligned with PIS  
### Membrane Distillation/Crystallization  
Besides the widely accepted RO in desalination which has already outclassed traditional thermal processes, new and relatively less explored membrane operations with more promises are emerging. In the desalination industry, membrane distillation and membrane crystallization, in particular, can add a positive effect on the process by increasing the overall water production and recovering valuable salts from the brine, thus approaching zero-liquid-discharge and the goals of process intensification strategy. The recovery of minerals from brine through the use of membrane distillation/crystallization might have a significant role in determining the future outlook of the mining industry. While the mining industry is under extreme stress due to excessive exploitation of minerals, the volume of brine (and therefore minerals contained in it) is increasing across the globe due to increases in the net volume desalination capacity. Seawater brine contains a number of essential as well strategic elements including sodium, magnesium, barium, lithium, etc. and can thus serve as a form of open sky mine for the recovery of these components. Dramatic increases in the consumption of some of these materials have been observed recently. For example, the use of strontium has increased in the oil and gas industry as weighing material in mud. Similarly, traditional sources of lithium might not be enough in the near future to fulfil its requirements in the electronic industry (especially considering hybrid vehicles). Sea-mining offers a potential solution to the problem of mineral depletion. A comparison of potentially recoverable minerals from the brine of existing desalination plants and their current exploitation through conventional mining has been illustrated in Figure 2[16]. It can be noted from the figure that the amount of Na and Mg in brine from current desalination capacities is more than that obtained through conventional mining. Similarly, strontium and lithium are both found considerable quantities in brine, indicating an attractive opportunity for the recovery of these minerals from the brine. The situation will further improve on the completion of contracted and planned desalination capacities across the globe. In 2002, [PERSON] and co-workers [17] suggested for the first time membrane crystallizers for seawater desalination in an integrated approach with RO. In this study real seawater from the Tirrenian coast was first treated by nanofiltration (NF) and RO followed by membrane crystallization (MCr) treatment of the RO concentrate with the production of NaCl [17]. The prospect of introducing MCr to RO brine has the potential to increase the fresh water recovery factor from around 50% to above 90% in combination with salts recovery. In fact, one of the main advantages of MCr is that it does not suffer from osmotic phenomena like RO because the driving force is a temperature gradient instead of a pressure gradient. Therefore, MCr can treat highly concentrated and even saturated solutions without any particular reduction in driving force and, as a consequence, in trans-membrane flux. The hydrophobic character of the membrane provides a complete rejection of non-volatiles, thus producing water at very high qualities. Moreover, the low used temperatures (normally in the range of 40-80 \({}^{\circ}\)C) permit the use of waste grade heat or other renewable energy sources. In the subsequent years of this first study, several research activities have focused on integrated membrane systems in desalination. In 2004, a MCr unit was applied on a synthetic NF retenate. That resulted in the recovery of NaCl and magnesium sulfate in the form of Epsomite (MgSO\({}_{4}\).7H\({}_{2}\)O) [18], a hydrated form of magnesium sulphate of great economic value. Another great advantage of membrane crystallizer is, in fact, the possibility to obtain different polymorphs and forms (hydrous or anhydrous) by simply tuning the operative conditions [19; 20].