Membrane and Desalination Technologies - TCE Moodle Website

Membrane Separation: Basics and Applications 281
In the last 6–8 years, the technology has gained industry acceptance as a viable water
treatment option for many different fluid separation applications. Low operating costs and the
ability to remove organic contaminants and 95–99% of inorganic salts with minimal chemical
requirements make RO an attractive technology for many industrial applications.
A study by Subhi and Anne (24) shows that RO was capable of separating oil from oily
wastewater with a rejection of 99% when oil content was up to 30%. For industrial applications,
RO was generally used to treat groundwater for production of pharmaceutical-grade
water; the retention reported by Belkacem and coauthors was more than 95% for the totality
of ions and conductivity was decreased successfully (25). RO also has extensive applications
in the following water treatments:
l Boiler feed water and cooling tower blow down recycle for utilities and power generation
l Cleaning of contaminated surface water and groundwater
l Potable water from sea or brackish water
l Ultra-pure water for food processing and electronic industries
l Water for chemical, pulp and paper industry
2.2.5. Other Membrane Technologies
Apart from the above four major separation processes, there are other membrane technology
applications in water and wastewater treatment:
1. Dialysis. Dialysis is a process where solutes travel from one side of the membrane to the other side
according to their concentration gradients (13, 26, 27). It uses a semi-permeable membrane
capable of passing small solute molecules, such as salts and small organic species, while retaining
colloids and solutes of higher molecular weight. The transfer through the membrane is by
diffusion, rather than by the hydrodynamic flow that would occur in a porous medium. The
current applications of dialysis include:
l Hemodialysis
l Purification techniques in pharmaceutical and biochemical laboratories
l Separating nickel sulphate in the electrolytic copper refining industry
2. Electrodialysis. Electrodialysis (ED) is a process in which ions are transported through ionpermeable
membranes from one solution to another under the influence of a potential gradient
(26, 27). The electrical charges on the ions allow them to be driven through the membranes
fabricated from ion-exchange polymers. Applying a voltage between two end electrodes generates
the potential field required for this. As the membranes used in ED have the ability to selectively
transport ions having positive or negative charge and reject ions of the opposite charge, useful
concentration, removal or separation of electrolytes can be achieved by ED. Sadrzadeh and
Mohammadi (28) reported that ED was also very efficient for desalination of seawater especially
at lower concentrations. The current applications of ED also include:
l Electroplating rinse water
l Etch bath rinse water
l Removal of organic acids from wine and fruit juices
l Radioactive wastewater treatment
l Regeneration of ion-exchange resins
l Ultra-pure water production
3. Donnan dialysis. Donnan dialysis uses an anion- or cation-selective membrane, which functions
similarly to ion-exchange resins (15, 26). For an anion-exchange membrane, cations in both