maintained using a lower pressure in the downstream compartment and condensing
the molecules onto a cold surface. The transfer mechanism is called “solutiondiffusion” as the first step is the selective solubilization of compounds at the feed
compartment/membrane interface that obey to a partition coefficient and the second
step is the diffusion-controlled transport within the membrane up to the downstream
compartment. The proper choice of the membrane material is essential since it must
have a high affinity for the targeted molecules and possibly their mobility within the
material must be higher than those of the others to achieve a good selectivity for their
transfer. Thus, materials like polypropylene are suitable for hydrophobic contaminants in aqueous leachates. Concentration factors of 1000 are not uncommon.
Because of the low solubility of hydrophobic contaminants, an NAPL in contact
to a contaminant-saturated water phase is often observed when the permeate vapors
are condensed (Jyoti et al. 2015).
Reverse osmosis (RO) is so called as it reverses the osmosis, that is the spontaneous diffusion of solvent that takes place when two solutions of different solute
concentrations are separated by a semipermeable membrane. Like pervaporation,
RO uses dense membranes and follows the same transfer mechanism. The osmosis
may be reversed when the pressure applied to the more concentrated solution is
higher than the osmotic pressure. The osmotic pressure, π (Pa), is usually estimated
assuming diluted solution conditions using the ideal gas law:
π ¼ RT
X
i
ΔC i
ð1:28Þ
where ΔC i refers to the concentrations difference between the feed and the permeate
compartments for each solute i, R (8.31 J K
À1 mol
À1 ) and T(K) are the gas constant
and the temperature, respectively. Osmotic pressures larger than 1 MPa are not
uncommon in RO. The pressure imposed upon the feed solution in RO has two
components: the one required to overcome the osmotic pressure and the other to flow
water through the membrane. As for all the pressure-driven membrane processes,
there is a linear relationship between the driving pressure and the volumetric flow to
permeate as long as the membrane structure and characteristics remains unchanged.
Because of this, in absence of concentration polarization, it is observed that for all
the driven membrane processes, the rejection increases with the driving pressure.
Therefore, for desalination technologies (RO, nanofiltration (NF)) driving pressures
are commonly larger than 1 MPa at least. RO has limited interests in additive/
contaminant separation because the rejection rates are usually larger than 95%.
Nevertheless, it is a useful technology for concentration, implemented for zero liquid
discharge, since high-quality water recovered in permeates can be reused in many
processes (Tong and Elimelech 2016).
Filtration Using Porous Membranes
Nanofiltration(NF), ultrafiltration (UF) and microfiltration (MF) implement porous
membranes of increasing pore size in order to make solute/solute, solute/colloid and
solute, or colloid/particulate separation.
34
N. Fatin-Rouge
the molecules onto a cold surface. The transfer mechanism is called “solutiondiffusion” as the first step is the selective solubilization of compounds at the feed
compartment/membrane interface that obey to a partition coefficient and the second
step is the diffusion-controlled transport within the membrane up to the downstream
compartment. The proper choice of the membrane material is essential since it must
have a high affinity for the targeted molecules and possibly their mobility within the
material must be higher than those of the others to achieve a good selectivity for their
transfer. Thus, materials like polypropylene are suitable for hydrophobic contaminants in aqueous leachates. Concentration factors of 1000 are not uncommon.
Because of the low solubility of hydrophobic contaminants, an NAPL in contact
to a contaminant-saturated water phase is often observed when the permeate vapors
are condensed (Jyoti et al. 2015).
Reverse osmosis (RO) is so called as it reverses the osmosis, that is the spontaneous diffusion of solvent that takes place when two solutions of different solute
concentrations are separated by a semipermeable membrane. Like pervaporation,
RO uses dense membranes and follows the same transfer mechanism. The osmosis
may be reversed when the pressure applied to the more concentrated solution is
higher than the osmotic pressure. The osmotic pressure, π (Pa), is usually estimated
assuming diluted solution conditions using the ideal gas law:
π ¼ RT
X
i
ΔC i
ð1:28Þ
where ΔC i refers to the concentrations difference between the feed and the permeate
compartments for each solute i, R (8.31 J K
À1 mol
À1 ) and T(K) are the gas constant
and the temperature, respectively. Osmotic pressures larger than 1 MPa are not
uncommon in RO. The pressure imposed upon the feed solution in RO has two
components: the one required to overcome the osmotic pressure and the other to flow
water through the membrane. As for all the pressure-driven membrane processes,
there is a linear relationship between the driving pressure and the volumetric flow to
permeate as long as the membrane structure and characteristics remains unchanged.
Because of this, in absence of concentration polarization, it is observed that for all
the driven membrane processes, the rejection increases with the driving pressure.
Therefore, for desalination technologies (RO, nanofiltration (NF)) driving pressures
are commonly larger than 1 MPa at least. RO has limited interests in additive/
contaminant separation because the rejection rates are usually larger than 95%.
Nevertheless, it is a useful technology for concentration, implemented for zero liquid
discharge, since high-quality water recovered in permeates can be reused in many
processes (Tong and Elimelech 2016).
Filtration Using Porous Membranes
Nanofiltration(NF), ultrafiltration (UF) and microfiltration (MF) implement porous
membranes of increasing pore size in order to make solute/solute, solute/colloid and
solute, or colloid/particulate separation.
34
N. Fatin-Rouge