balance of hydrophilic and hydrophobic character so that water alone can be
preferentially sorbed onto its surface, and under the application of external pressure
greater than the osmotic pressure. The sorbed water flows through the capillaries,
thus effecting the separation of water from solution. Nanofiltration membranes are
also similar to reverse osmosis membranes except that the pore sizes are slightly
larger. These membranes may have slight positive or negative charge depending on
the type of chemical treatment (Teixeira et al. 2005). In both the processes, hydrostatic pressure in excess of osmotic pressure of the boundary layer has to be applied
for the removal of water from the solution. Since nanofiltration allows the permeation of monovalent solutes, the product water will have significant osmotic pressure
compared to reverse osmosis permeate which is nearly pure. In view of this, for the
same values of total dissolved solids (containing a mixture of ionic solutes),
nanofiltration would require less applied pressure compared to reverse osmosis.
When macromolecules like dyes are to be separated from a given solution,
nanofiltration would be a better choice, as it would allow the solutes to permeate
through the membrane retaining macromolecules. Both in reverse osmosis and
nanofiltration, as the process proceeds, the feed solution becomes more and more
concentrated resulting in rise of osmotic pressure and becomes susceptible for
scaling, consequently limiting the percent separation (percent recovery). The inability to achieve total separation is one of the major limitations of nanofiltration and
reverse osmosis.
Ultrafiltration whose pores are larger than nanofiltration operates based on size
exclusion mechanism. The attractive features of the process are the flexibility to
operate both in dead-end and cross flow mode and the backwash possibility. Unlike
reverse osmosis and nanofiltration, osmotic pressure has no significant impact on the
performance of ultrafiltration membranes affording the operation at very low pressures in the range of 1–5 bar. Table 8.2 presents the comparative aspects of pressuredriven membrane processes.
8.3.3 Electro-membrane Processes
Electro-membrane processes are performed under electrical potential gradient,
where ions migrate from the solution toward the corresponding electrodes.
Depending on the arrangement of the membranes, desired separation can be
achieved. The processes include electrodialysis, electrodialysis reversal, electrodeionization, and electrodialysis with bipolar membrane.
Electrodialysis In electrodialysis (Campione et al. 2018), a number of pairs of
cation and anion exchange membranes (cell pair) are arranged alternately in between
two electrodes. Feed solution containing ionic solutes are fed parallelly between
each pair of membranes (Fig. 8.6). When connected to power source, the ions
(cations and anions) start moving in opposite direction toward their respective
electrodes. During the migration, the cations pass through cationic membranes and
8 Role of Membranes in Wastewater Treatment
257
preferentially sorbed onto its surface, and under the application of external pressure
greater than the osmotic pressure. The sorbed water flows through the capillaries,
thus effecting the separation of water from solution. Nanofiltration membranes are
also similar to reverse osmosis membranes except that the pore sizes are slightly
larger. These membranes may have slight positive or negative charge depending on
the type of chemical treatment (Teixeira et al. 2005). In both the processes, hydrostatic pressure in excess of osmotic pressure of the boundary layer has to be applied
for the removal of water from the solution. Since nanofiltration allows the permeation of monovalent solutes, the product water will have significant osmotic pressure
compared to reverse osmosis permeate which is nearly pure. In view of this, for the
same values of total dissolved solids (containing a mixture of ionic solutes),
nanofiltration would require less applied pressure compared to reverse osmosis.
When macromolecules like dyes are to be separated from a given solution,
nanofiltration would be a better choice, as it would allow the solutes to permeate
through the membrane retaining macromolecules. Both in reverse osmosis and
nanofiltration, as the process proceeds, the feed solution becomes more and more
concentrated resulting in rise of osmotic pressure and becomes susceptible for
scaling, consequently limiting the percent separation (percent recovery). The inability to achieve total separation is one of the major limitations of nanofiltration and
reverse osmosis.
Ultrafiltration whose pores are larger than nanofiltration operates based on size
exclusion mechanism. The attractive features of the process are the flexibility to
operate both in dead-end and cross flow mode and the backwash possibility. Unlike
reverse osmosis and nanofiltration, osmotic pressure has no significant impact on the
performance of ultrafiltration membranes affording the operation at very low pressures in the range of 1–5 bar. Table 8.2 presents the comparative aspects of pressuredriven membrane processes.
8.3.3 Electro-membrane Processes
Electro-membrane processes are performed under electrical potential gradient,
where ions migrate from the solution toward the corresponding electrodes.
Depending on the arrangement of the membranes, desired separation can be
achieved. The processes include electrodialysis, electrodialysis reversal, electrodeionization, and electrodialysis with bipolar membrane.
Electrodialysis In electrodialysis (Campione et al. 2018), a number of pairs of
cation and anion exchange membranes (cell pair) are arranged alternately in between
two electrodes. Feed solution containing ionic solutes are fed parallelly between
each pair of membranes (Fig. 8.6). When connected to power source, the ions
(cations and anions) start moving in opposite direction toward their respective
electrodes. During the migration, the cations pass through cationic membranes and
8 Role of Membranes in Wastewater Treatment
257