Membrane and Desalination Technologies - TCE Moodle Website

102 L. Song and K.Guan Tay
Key Words Fouling characterization reverse osmosis full-scale processes fouling
potential thermodynamic equilibrium.
1. INTRODUCTION
Industrial applications using (RO) membranes as separation medium began in 1960s when
Loeb and Sourirajan (1) successfully developed high flux cellulose acetate RO membrane.
Today, RO has been widely used for separation and concentration of solutes in many fields,
such as chemical and biomedical industry, food and beverage processing, and water and
wastewater treatment. The most common uses of RO membrane are in the seawater desalination
and water reclamation industries. There are estimated 12,500 desalination plants
around the world, supplying 20 million cubic meters per day or 1% of world’s production
of drinking water. With the continuous decrease in cost of RO desalination, the number of RO
desalination plants and its share in world drinking water supply is expected to rapidly increase
in the near future (2, 3).
The increasing acceptance of RO technology in seawater desalination and water reclamation
is no coincidence. Shortage of potable water around the world and more stringent water
quality requirements by the regulating bodies (4, 5) are two major driving factors that make
RO an attractive option for separation process in water treatment and reclamation. In
addition, RO has many advantages over conventional separation processes such as distillation
and other physical operations. Energy consumption in terms of electricity use by RO is only
about one third to that of distillation (6). RO does not require clarification tanks and
disinfection units; hence, it usually requires smaller space and is easy to operate. Compared
with conventional separation processes, RO produces less sludge as it does not or seldom
involves the use of chemicals such as coagulants or polymers (6).
Despite the attractive attributes of RO, the separation process is unfortunately plagued by
the single most critical problem – membrane fouling (7, 8). Fouling is the accumulation of
contaminants or foulants on the membrane surface, which results in the reduction of water
production, product water quality, and membrane lifespan (9). Membrane fouling can be
significantly reduced by appropriate pretreatment of the feed water to the RO process. The
fouled membranes can be cleaned to restore its permeability. As a guide, cleaning is usually
done when there is a 10% decrease in water production (10) at constant operating conditions,
or a 10% increase in the driving pressure to maintain the same production at constant
temperature, or an increase of 15–20% in the pressure differential between feed and reject
flows. However, the costs that associate with fouling control and membrane cleaning
represent a significant proportion in the total operating cost. Studies indicated that scale
inhibition and cleaning chemicals account for about 5–20% of the total cost for a typical
seawater RO system (11, 12) and the pretreatment cost in RO systems in Middle East to range
between 10 and 25% of the total cost (13). If membrane fouling is not effectively controlled
and the extent of fouling is so severe that membrane cleaning fails to reverse the decline in
water production, as in the case of Yuma Desalting Plant in Arizona (14), plant has to be shut
down and membrane replacement has to be carried out.