Reverse Osmosis
Introduction to Reverse Osmosis
A technique used in processes requiring high-quality, purified water, such in semiconductor processing or biochemical applications, is reverse
osmosis. It can be used to treat boiler feedwater, industrial
wastewater, or process water. Reverse Osmosis is a water
purification technique that reduces the quantity of dissolved
solids in solution (Kucera, 54). It was first developed in the
1950's by the US government to provide fresh drinking water for
the Navy, and since then, advances have made it much more
feasible for obtaining purified water from wastewaters produced
in many industrial applications. RO uses waterline pressure to
push raw wastewater against a special semipermeable membrane. It
is essentially a molecular squeezing process which causes H20
molecules to separate from the contaminants. The separated water
molecules then pass thru to the inside of the membrane on to a
holding reservoir. The contaminants are washed from the membrane
and disposed of. Recently, RO has been used in treating boiler
feedwater, in addition to industrial and process wastewaters.
Boilers are found throughout the chemical processing industry and
the primary method to treat boiler wastewater is an ion-exchange
based demineralization. However, RO has been demonstrated to be
more cost effective than this demineralization process (Kucera,
54).
Example of RO system
Problems With Reverse Osmosis:
It is necessary to establish feedwater quality guidelines to
optimize system performance and prevent the three main problems
associated with RO: scaling, fouling, and degradation of ROmembranes (Kucera 55) These problems tend to decrease system productivity because they reduce wastewater purity. Scaling occurs on RO membranes when the concentration of scale-forming species exceeds saturation, producing additional solids within
the RO feedwater. Scalants include such chemical species as
calcium carbonate, calcium sulfate, barium sulfate, strontium
sulfate, and reactive silica (Kucera 55). Since these species
have very low solubilities, they are difficult to remove from RO
membranes. Scaling decreases the effectiveness of the membranes
in reducing the solids and causes more frequent cleanings. A
scale on a membrane provides nucleation sites that increase the
rate of formation of additional scale (Kucera 55).
Methods to minimize scaling
In order to minimize scaling, pretreatment methods involving
chemical or ion exchange techniques are used. Ion exchange
methods remove scale-forming species from the RO feedwater, while
chemical techniques change the characteristics of the RO
feedwater so that crystal formation is not favored. An example of
a chemical technique to prevent fouling is lime softening, which
involves chemical processes that reduce the hardness of the
wastewater, essentially preventing material from precipitating
out. (Kucera 56) Lime, soda, ash, and NaOH are used to convert
soluble calcium and magnesium to insoluble calcium carbonate and
magnesium hydroxide. Magnesium hydroxide tends to absorb silica,
another scalant. These solids are then collected as sludge from
the bottom of the "softener". Another softening procedure
involves zeolite in an ion exchange process. A strong acid cation
resin in the sodium is used to remove scale-forming cations, suchas calcium, magnesium, barium, and iron. (Kucera, 56) These cations are exchanged with the sodium to yield "soft water", that is, water of low hardness.
Another pretreatment technique to prevent scaling is
acidification, which specifically reduces the crystallization of
calcium carbonate. Sulfuric acid is most commonly used in this
process, but can often increase the formation of sulfate scales.
Therefore, where sulfuric acid cannot be used, hydrochloric acid
is substituted. (Kucera, 57). Often used with acidification, or
by itself, are antiscalants. Antiscalants are chemicals added to
wastewater to minimize scale carbonate or sulfate based scale
(57). They consist of acrylates and phosphonates which inhibit
the precipitation of carbonate or sulfanates.
Methods to prevent fouling
The second problem with reverse osmosis is with the fouling
of membranes. Fouling occurs when suspended solids, microbes and
organic material deposit on the surface of the membrane. Soluble
heavy metals, such as iron, can be oxidized within membrane
modules and foul the membranes. Another problem is from colloidal
sulfur, which when oxidized from H2S can foul RO membranes.
(Kucera 55). Colloidal sulcar tends to be very sticky and
therefore can attach easily to the surface of RO membranes
Hydrogen sulfide would be found most commonly in well-water. The
primary methods used to combat fouling are mechanical processes
that physically remove the suspended solids or chemical
treatments the deactivate the foulant.
Coagulation is one technique that neutralizes the negative
surface of the suspended solids, allowing the particles to cometogether. (Kucera 57) These large particles are then easy to
remove from the water using filtration. The most common
coagulants used are cationic polymers, inorganic salts, and
aluminum and iron salts. Inorganic solvents tend to form large
particles, while catonionic polymers require much less product
for coagulation. Similar to coagulation is the clarification
method, which destablizes suspended particles through charge
neutralization (58). These particles conglomerate and are removed
using sedimentation or filtration techniques. One particular type
of filtration uses manganese greensand as a filter to remove
soluble iron and manganese from the water source. This is
generally done by oxidizing iron and manganese and physically
removing the precipitates in the manganese greensand bed.
Chlorination is the primary technique to minimize microbiological
foulants, as it is very effective against a wide variety of
microbes and can be easily deactivated using sodium
metabisulfite. (59). After chlorination, activated carbon filters
can be used to remove chlorine and reduce organics. However,
activated carbon tends to foster microbial growth by providing
nutrients for microbes, so it is not a very effective filtration
technique. Finally, to treat H2S containing feedwater, which can
form colloidal sulfur, a combination the above techniques is
used. First, the water is oxidized to precipitate the sulfur,
which is then coagulated and filtered. Any colloidal sulfur that
may have formed is converted to thiosulfates with the addition of
sulfite. Finally, chlorination is done to convert the
thiosulfates to sulfates.(60)
Methods to Minimize Membrane Degradation
The final problem with reverse osmosis is membrane
degradation. It occurs when the membranes are exposed to
conditions that destroy the polymers used to create the
membranes. Some membranes are susceptible to hydrolysis at high
and low pH, while others are degraded by exposure to oxidizers
such as chlorine.(Kocera 56) To prevent membrane degradation by
acidic or alkaline waters, a corrective amount of acid of base
should be added to the feedwater to make the pH approximately
neutral. To prevent oxidation reactions, dechlorination is used.
(57)
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References
"Kucera, Jane". Properly Apply Reverse Osmosis Chemical Engineering
Progress. February 1997. Pgs 54-61.