Membrane and Desalination Technologies - TCE Moodle Website

280 J. Paul Chen et al.
The application of high pressure to the concentrated side will cause this process to reverse.
This results in solvent flow away from the concentrated solution, leaving a higher concentration
of solute. In application, the waste stream flows past the membrane; while the solvent
(e.g. water) is driven through the membrane, the remaining solutes (e.g. organic or inorganic
components) do not pass through, and become increasingly concentrated on the feed side of
the membrane.
Almost all RO membranes are made of polymers, cellulosic acetate and matic polyamide
types, and are rated at 96–99% NaCl rejection. Generally, there are two types of RO
membranes: asymmetric or skinned membranes and thin-film composite membranes. The
support material is commonly polysulphones, while the thin film is made from various types
of polyamines and polyureas.
RO membranes have the smallest pore structure with pore diameter ranging from approximately
5 to 15 A ˚ (0.5–1.5 nm). The extremely small size of RO pores allows only the smallest
organic molecules and unchanged solutes to pass through the semi-permeable membrane
along with the water. Greater than 95–99% of inorganic salts and charged organics will also
be rejected by the membrane due to charge repulsion established at the membrane surface.
The major advantage of RO for handling process effluents is its ability to concentrate dilute
solutions for recovery of salts and chemicals with low power requirements. No latent heat of
vaporization or fusion is required for effecting separations; the main energy requirement is
for a high-pressure pump. It also requires relatively limited floor space for compact, high
capacity units, and it exhibits good recovery and rejection rates for a number of typical
process solutions (15).
The major problem of RO is its higher potential of fouling. This is caused by particulate
and colloidal matters that become concentrated at the feed side of the membrane surface.
Fouling of membranes by slightly soluble components in solution or colloids has caused
failures, and fouling of membranes by feed water with high levels of suspended solids can be
a problem. Pre-treatment is used to remove particulate matters from the feed water. A system
designed to operate at a high permeate flux is likely to experience high fouling rates and
will require frequently chemical cleaning. Limited operational temperature range is another
limitation for RO process. For cellulose acetate membranes, the preferred range is 18–30 C,
lower temperatures will cause decreased fluxes and higher temperatures will increase the rate
of membrane hydrolysis, hence reduce system life. These membranes are also chemically
sensitive. Strongly acidic or basic solutions, strong oxidizing agents, solvents and other
organic compounds can cause dissolution of the membrane. Poor rejection of some compounds
such as borates and low molecular weight organics is another problem. Finally, there
are some concentrated solutions having initial osmotic pressures, which are so high that they
may exceed the available operating pressures and make it uneconomical (15).
The product water from an RO unit will have a low pH and most probably a high concentration
of carbon dioxide (15). The carbon dioxide can be removed and the pH of the product
increased by use of a decarbonator. A decarbonator is a packed column in which product water is
introduced at the top while either forced or induced air is introduced at the bottom. The air and
water flow counter-currently over and around the column packing. The carbon dioxide is
stripped from the water and exits from the decarbonator at the top in the air stream.