another concentrate stream) in alternate compartments. The diluent is the desalinated
solution.
Electro-deionization This process is the modified version of electrodialysis (Nagel
2005) wherein ion exchange resins are filled in each compartment. The process is
applicable to very low-salinity water (much less than 50 ppm). Upon application of
the electrical potential, ions migrate and saturate the resins of alternate compartments. As the ionic concentration reduces, water splitting occurs leading to the
migration of hydrogen and hydroxyl ions which regenerate the resins, thus avoiding
the need for acid and alkali. This process is useful only at very low concentrations.
Hence, it is used as a polishing step for the preparation of ultrapure water from
reverse osmosis product.
Electrodialysis with Bipolar Membrane This is another modified version of
electrodialysis where a bipolar membrane (anion and cation membrane joined
together) is inserted between two pairs (each pair having one anion exchange and
one cation exchange) of membranes, and as a result, the dissolved salt is converted to
parent acid and base from the solution (Oztekin and Yazicigil 2007). Further studies
have indicated the possibility of fractionation of the ionic species (Reig et al. 2016).
These processes are still in the development stage particularly with reference to the
membranes whose stability under the process conditions is a challenge.
8.3.4 Other Membrane Processes
Concentration-Driven Processes
Forward Osmosis All the concentration-driven processes are passive in nature. In
forward osmosis, water flows through the membrane from the feed solution toward
the draw solute motivated by the osmotic pressure difference through the semipermeable membrane. The draw solution gets diluted, but the ultimate osmotic pressure
of the draw solution is always greater than the feed solution. A second step
separation is required to get pure water as shown in Fig. 8.7.
Selection of draw solute, which can be separated by a simple process, is critical to the
performance.
The challenges are the development of membranes with a good flux and a suitable
draw solute from which water could be recovered. Recent development of aquaporin
(Ma et al. 2012) membrane with a water channel to transport water is expected to
trigger practical applications in many areas including wastewater treatment.
Diffusion Dialysis Diffusion dialysis describes the movement of ionic species
through a charged membrane. The membrane can be anionic or cationic. Unlike
electrodialysis, where both cationic and anionic membranes are used, diffusion
dialysis requires only one of the two membranes. When an anion membrane is
8 Role of Membranes in Wastewater Treatment
259
solution.
Electro-deionization This process is the modified version of electrodialysis (Nagel
2005) wherein ion exchange resins are filled in each compartment. The process is
applicable to very low-salinity water (much less than 50 ppm). Upon application of
the electrical potential, ions migrate and saturate the resins of alternate compartments. As the ionic concentration reduces, water splitting occurs leading to the
migration of hydrogen and hydroxyl ions which regenerate the resins, thus avoiding
the need for acid and alkali. This process is useful only at very low concentrations.
Hence, it is used as a polishing step for the preparation of ultrapure water from
reverse osmosis product.
Electrodialysis with Bipolar Membrane This is another modified version of
electrodialysis where a bipolar membrane (anion and cation membrane joined
together) is inserted between two pairs (each pair having one anion exchange and
one cation exchange) of membranes, and as a result, the dissolved salt is converted to
parent acid and base from the solution (Oztekin and Yazicigil 2007). Further studies
have indicated the possibility of fractionation of the ionic species (Reig et al. 2016).
These processes are still in the development stage particularly with reference to the
membranes whose stability under the process conditions is a challenge.
8.3.4 Other Membrane Processes
Concentration-Driven Processes
Forward Osmosis All the concentration-driven processes are passive in nature. In
forward osmosis, water flows through the membrane from the feed solution toward
the draw solute motivated by the osmotic pressure difference through the semipermeable membrane. The draw solution gets diluted, but the ultimate osmotic pressure
of the draw solution is always greater than the feed solution. A second step
separation is required to get pure water as shown in Fig. 8.7.
Selection of draw solute, which can be separated by a simple process, is critical to the
performance.
The challenges are the development of membranes with a good flux and a suitable
draw solute from which water could be recovered. Recent development of aquaporin
(Ma et al. 2012) membrane with a water channel to transport water is expected to
trigger practical applications in many areas including wastewater treatment.
Diffusion Dialysis Diffusion dialysis describes the movement of ionic species
through a charged membrane. The membrane can be anionic or cationic. Unlike
electrodialysis, where both cationic and anionic membranes are used, diffusion
dialysis requires only one of the two membranes. When an anion membrane is
8 Role of Membranes in Wastewater Treatment
259