## 3 Evaluation of New Metrics  
A comparison of different overall desalination processes in terms of various metrics for process intensification has been provided in Figure 6. MSF (multi-stage flash), being the most widely used thermal desalination process, has been considered as the base line and therefore the value of all the metrics for this is one. The figure indicates that various metrics show huge variations for different processes. MI (mass intensity) for RO and MD reduce greatly due to improved recovery factor (considered 50% and 86%, respectively, in current study). RO shows the maximum value of productivity/weight ratio (PW) due to high membrane permeability and elimination of heavy metallic parts, which are essential components of MSF plant. This aspect is particularly important for off-shore or remote installations. It is also evident from the figure that the PW for RO is higher than MD, which can be attributed toward the high flux of RO considered in the current study (Table 1). Larger membrane area requirement in case of MD implies a larger number of modules that will increase the weight, and thus PW will go down. Similarly, PS for MSF is the least, thereby indicating the large foot print of the plant. Overall, the comparison indicates that new membrane operations (MD in the current example) can be optimum candidates to overcome the drawbacks (limited MI) of conventional RO processes.  
Besides the traditional low concentrated solutions, emerging membrane operations have the capability to treat highly concentrated solutions which are beyond the application limit of traditional processes, thus offering the possibility to achieve the values of MI and WI (Waste intensity) which otherwise are not feasible. This aspect has been explained by considering the example of the treatment of produced water by using membrane crystallization. The experimental details of the study have been explained elsewhere [35]. Changes of mass and waste intensities and overall recovery with respect  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Feed solution** & **Searwater** \\ \hline Plant capacity (m\({}^{3}\)/h) & 1250 [33] \\ MD flux assumed (kg/m\({}^{2}\)-h) & 4 \\ Volume of MSF (multi stage flash) unit (m\({}^{3}\)) & 18 \(\times\) 4 \(\times\) 3 [33] \\ Material of construction of MSF unit & Stainless steel \\ Density of steel (kg/m\({}^{3}\)) & 8000 \\ Average permeate flow for RO (reverse osmosis) (m\({}^{3}\)/h) & 24.6 \\ Weight of one element for RO/MD (kg) & 16 \\ Volume of one RO/MD element (m\({}^{3}\)) & 1.016 \(\times\) 0.0286 \(\times\) 0.201 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters and assumption used in calculation of various PI (process intensification) metrics shown in Figure 6.  
Figure 6: PI (process intensification) metrics for MD, multi stage flash (MSF), and RO. MI, mass intensity; PW, productivity/weight ratio; PS, productivity/size ratio.  
to size of membrane (0.2 m\({}^{2}\)--active surface area) and weight (0.467 kg--module) were identified (Figure 7). In the beginning of the experiment, water was only considered the product and in the end of the experiment both water and salt were considered. From the time of saturation, NaCl was being produced at a rate of 0.063 \(\pm\) 0.012 kg/h. MI and WI decreased significantly with increases in water recovery factor from above 35 to below 3 and 2 for MI and WI, respectively. In the carried out experiments a recovery factor of only 37% was obtained and continued treatment could further improve MI and WI.