• **Hydraulic resistance (Rm)**

This is the resistance of the membrane to the flow of the fluid to be filtered; the resistance is related to the permeability by Eq. (3).

$$R\_m = \frac{1}{\mu L\_p} \tag{3}$$

μ is the viscosity of the permeate (water).

The resistance can be calculated from the permeate flow through the membrane and the transmembrane pressure.

#### • **Cut-off threshold**

The SC of a membrane is the molecular weight above which the membrane retains at least 90% of the molecules. It is expressed in g/mol or in Dalton [40].

The cutoff is used in the characterization of membranes, but from a scientific point of view it is not rigorous, as it depends on the operating conditions and the characteristics of the solute.

#### • **Lifespan**

This is a characteristic of the membrane, because beyond which the membrane will no longer be effective [41].

#### • **Conversion rate**

The conversion rate is the flow rate fraction of the liquid passing through the membrane [41]:

μ is the viscosity of the permeate.

The resistance can be calculated from the permeate flow through the membrane and the transmembrane pressure.

$$Y = \frac{Q\_p}{Q\_f} \tag{4}$$

#### **Chemical characteristics**

Depending on the chemical nature of the membrane, there are interactions between the membrane and the solutes to be filtered, in particular at the level of clogging [20].

• Hydrophobicity and hydrophilicity:

Hydrophilicity depends on the ionized or polar groups of the polymers used, by nature organic membranes are hydrophobic [20].

Due to their hydrophilic properties, regenerated cellulosic membranes are widely used in ultrafiltration [42, 43].

• Surface electrical charge:

Organic membranes have two functional groups one is amine (basic) and the other is carboxylic (acid) having positive or negative charges respectively.

Due to the partial hydrolysis of the amide functions, NF membranes are negatively charged, inorganic membranes have amphoteric surfaces.
