## 1 Introduction  
In contemporary production scenarios, the need for increasingly high-performance water purification processes is becoming more and more present. Solute purification, solvent recovery, and solvent separation in organic solvents are more and more widely used in the chemical, pharmaceutical, and food processing industries [1]. For instance, in a large number of industrial processes within the pharmaceutical industry, water needs to be purified before entering the production system, and the success of the water purification process is a crucial condition for the success of all the subsequent processes. In fact, introducing incorrectly purified water into the system can lead to failures and damage to the water processing machinery. Moreover, proper water purification is paramount to ensure the appropriate level of quality for the final product. In fact, the quality level of final products in the pharmaceutical sector are extremely stringent since the regulations are very demanding, and the compliance with such regulations needs to be consistently controlled, both from a quality and a safety point of view [2]. As a consequence, the slightest mismatch in product quality can lead to its complete disposal with economic losses considering the high production costs of many pharmaceutical products. It should be noted that nowadays the implementation of membrane technology at a large scale is a reality at water reclamation and drinking water treatment facilities [3]. Reverse osmosis, which is also known as hyperfiltration, is a process by which solvent molecules are forced from the more concentrated solution into the less concentrated one when a pressure greater than the osmotic pressure is applied. Hence, the reverse osmosis system uses cross-filtration, more commonly called tangential filtration. The greater the concentration, the greater the pressure required. Moreover, the turbulence derived from the induced pressure is strong enough to allow the membrane surface to remain clean. Therefore, another fundamental characteristic of the reverse osmosis process is that it does not alter the characteristics of the product, thus not compromising the subsequent characteristics of the processes that will be implemented in the product. The aim of this process is to obtain a pure solvent by preventing the passage of the solute from one side to the other with a membrane. Figure 1 shows the reverse osmosis process. The above-described technique not only uses the presence of a physical obstacle to filter the water, but it also uses the membrane to separate the different species based on their different chemical affinities. For instance, in this way only hydrophilic molecules can pass through the membrane. Reverse osmosis is used in water treatment mainly for two purposes [3]:  
* Desalination
* Removal of traces of polluting molecules that could compromise the success of the process and the quality of the process output  
For more than a decade now, the reverse osmosis process has been establishing itself as a water desalination process by virtue of its low cost and energy efficiency [4]. Reverse osmosis is also gaining wide acceptance in water desalination plants. In fact, in this field the water purification process allows for the obtaining of drinking water, which is especially useful in geographical areas where drinking water is a scarce resource. Furthermore, as far as energy consumption is concerned, in the last 20 years reverse osmosis ensured an 80% reduction of the energy required for the production of desalinated water compared to other technologies applied for the same purpose [5]. Interesting considerations have been made on the necessity of correct and structured monitoring of the membrane conditions in reverse osmosis processes. The monitoring process is in fact crucial to avoid permanent damage to the membrane itself and to hinder inadequate quality levels and production slowdowns, as previously mentioned [6].