Membrane and Desalination Technologies - TCE Moodle Website

282 J. Paul Chen et al.
solutions are prevented from diffusing through the membrane, but anions will get through until
equilibrium is reached. For a cation-selective membrane, cations will diffuse through the membrane
while anions will be retained. Potential difference is the driving force behind these
separation processes. It is created by the displacement of the system from the equilibrium ratios,
which can be controlled by adjusting solution concentrations. The application for ion exchange
includes:
l pH control without adding acid or base
l Recovery of acids and bases from salts
l Water softening
4. Membrane bioreactor. Membrane bioreactor (MBR) can be defined as integration of conventional
biological degradation and membrane separation into a single process where microorganisms
responsible for biodegradation and suspended solids are separated from biologically treated water
by a membrane filtration unit. The entire biomass is confined within the system, providing both
perfect control of the sludge retention time for the microorganism in the bioreactor and physical
disinfection of the effluent (12, 29, 30).
As the MBR process does not need the secondary clarifier for the solid–liquid separation,
it can prevent these “inherited” problems that occur in the conventional processes for the
wastewater reclamation (30). The resulting high-quality and completely disinfected effluent
means that the MBR process can be used for many purposes, that is, industrial and municipal
wastewater treatment and reuse. More important, the land requirement of MBR is much less
than that of conventional wastewater reclamation processes.
2.3. Case Studies on Membrane Applications in Water Treatment
2.3.1. Case 1: Desalination of Seawater by RO
Desalination of seawater is one of the important applications of membrane processes.
There are various ways to produce freshwater such as distillation, electrodialysismembrane
distillation, freezing, membrane bioreactor and reverse osmosis. Among them, distillation is
the most used technique, but RO is becoming more popular in the desalination industry. A
flow diagram of a single-stage RO system is shown in Fig. 7.4.
To reduce fouling and protect membrane, pre-treatment must be performed for seawater.
A pre-treatment system removes suspended solids and other potential fouling materials.
Flocculation agents such as iron chloride or polyelectrolytes are added in order to remove
suspended solids. Chlorine is added to remove bacteria and algae.
The water quality is determined by membrane materials and configurations. Cellulose
acetate membranes can be degraded by the formation of biological slimes on the membrane
surface, while polyamide membranes cannot tolerate chlorine. Thus, sodium hydrogen
sulphite is normally needed to remove chlorine in the water. Operation at higher temperatures
and pressures can cause the compaction of the membrane layer and thus reduce output. A
stagnant boundary layer at the membrane surface can cause concentration polarization of
salts, increasing the chance of precipitation and reducing output (26, 27). The operating brine
concentration, pressure and water conversion factor need to be selected to limit both
compaction and concentration polarization effects.