Featured Application: High Efficiency Reverse Osmosis Technology
(HERO™)
Save water, money and footprint
while increasing capacity and
performance of your RO.
The beauty of the HERO™ process is its resistance to fouling
that plagues other membrane systems. The first step in the
process is to remove all of the divalent, scale-forming metallic
ions. Calcium, magnesium, strontium, barium and iron are removed
by weak acid cation (WAC) resin.
FIGURE 1. THE HERO SYSTEM
If divalent ions are absent, the resin will accept sodium or
potassium ions to replace hydrogen ions removed from the resin
by the neutralization reaction. However, these monovalent ions
are loosely held and are easily replaced by divalent ions later
in the exchange process. In this way, WAC resin can remove
hardness from saline solutions completely and effectively.
Normal strong acid cation (water softener) resins are
regenerated by high concentrations of sodium and are ineffective
in softening highly saline waters. WAC resin also has an anionic
charge. The resin attracts and removes cationic materials that
would otherwise be attracted to and foul the thin-film composite
membrane.
During the WAC process, alkalinity is converted to carbon
dioxide and the pH of the water is reduced. When treating waters
that contain much more alkalinity than hardness, such as
produced water, additional acid is added to convert the
remaining alkalinity to carbon dioxide. The water is then
decarbonated through a forced-draft air stripper or membrane
degassifier. By removing carbon dioxide and alkalinity, the
water loses its natural buffering capacity and the pH can
subsequently be raised with a small amount of caustic soda.
After carbon dioxide and alkalinity are removed, the pH of
the water is increased to 10 with caustic soda and fed to the
reverse osmosis system. Operating the RO system at this high pH
has three important advantages:
At high pH, the RO operates in a continuous cleaning cycle
for organic materials. Fats and oils are emulsified, and silt is
dispersed and removed from the membrane surface.
Silica is an order of magnitude more soluble in high pH water as
compared to its solubility in neutral pH water. This allows the
RO system to operate at very high recovery rates without silica
scaling.
Weak organic acids are ionized and repelled from the membrane,
so rejection of these organics is greatly improved. Cresylic and
phenolic compounds are ionized and do not foul the polysulfone
layer of the membrane. Rejection of weak inorganic acids such as
silicate and borate is also greatly improved.
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