2. Different categories of membrane processes

Recently, membrane technology has gained great attention as a powerful separation technique. Figure 1 shows the main categories of the membrane processes. They are categorized mainly based on the size of the contaminants they can exclude from the input stream. Nanofiltration (NF) is one of the fourth classes of pressure-driven membranes appeared after microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO). It was first developed in the late 1970s as a variant of reverse osmosis membrane [ROM] with reduced separation efficiency for smaller and fewer charged ions such as sodium and chloride. As the term, NF was not known in the 1970s, such that membrane was initially categorized as either loose/open RO, intermediate RO/UF, or tight UF membrane. The term NF appears to have been first used commercially by the Film-Tec Corporation (now the Dow Chemical Company) in the mid-1980s to describe a new line of membrane products having properties between UF and RO membranes. Owing to the uniqueness and meaningfulness of the word NF, other membrane scientists have begun using it [9–11].

3. Recent drifts in pH-responsive separation techniques

weight rejected materials [12].

• Adsorption treatment before membrane filtration (pretreatment layout)

• Adsorption treatment after membrane filtration (post-treatment layout)

• Integrated adsorption/membrane processes (IAMPs)

Recently, membrane technology has gained great attention as a powerful separation technique due to prominent advantages over common processes such as high removal efficiency, low energy consumption, fast kinetic, small footprint, and ease of scale up. They are favored for full-scale applications due to normal operating conditions, high productivity, and low energy consumption. They can efficiently eliminate many contaminants including proteins, macromolecules, natural organic matters (NOMs), dyes, dissolved organic matter (DOM), boron, and compounds responsible for odor and color, from aqueous media. However, the recent achievements for pH-responsive membranes require an ion exchange separation in some cases. Figure 2 shows a combination between adsorption and membrane separation. The overall removal efficiency of the hybrid process would be enhanced [17–19]. Generally, three different procedures for hybridization of membrane systems with adsorption processes may be found:

Figure 1. Classification of membrane processes according to separation type, relative size, and approximate molecular

Recent Drifts in pH-Sensitive Reverse Osmosis http://dx.doi.org/10.5772/intechopen.75897 3

Because of late advancements and advances in osmosis innovation, fascinating film operations, including membrane desalination (MD), pressure retarding osmosis (PRO), and reversed electrodialysis (RED), have developed. These operations are equipped for creating spotless and reasonable power from different waste streams, including brackish water and debilitated water, which generally are viewed as natural liabilities. PRO and RED require blending of a high salinity content (e.g., seawater or brackish water and wastewater, separately) with a low salt content to produce power. MD has demonstrated the possibility to produce freshwater and power as an independent process. Reconciliation of MD with PRO or RED upgrades the execution of these procedures and gives a perfect and practical course to create freshwater and vitality [13–16].

Figure 1. Classification of membrane processes according to separation type, relative size, and approximate molecular weight rejected materials [12].
