Guide for Pretreatment to Reverse Osmosis -
Note: Also see
Filtration Section for contaminants removed by various media filters.
Are Lead and Arsenic Removed by Thin Film Reverse Osmosis Membranes?
What are the rejection rates?
There is not
a list of rejection rates for these heavy metals, but we can give you
some general guidelines. However, we recommend that you run the
experiments under your unique set of conditions and ions/counterions.
Rejection of lead by thin film RO membranes has been reported to be
quite high, >99%.
The rejection of arsenic depends on its oxidation state. The +3 acid,
arsenious, is not rejected particularly well in neutral solutions, 70 to
90%, but is well rejected as the pH approaches or exceeds the pKa of
9.2. The +5 arsenic acid is well rejected in neutral solution.
How can I remove the Silica from the water
before passing it through the membranes?
There is no easy method. The
conventional method used is lime soda softening but this is very
expensive. In reverse osmosis no attempt is made to remove silica before
the membranes. Instead the recovery and other operating conditions are
adjusted to prevent silica supersaturation and precipitation.
For a GMP pharmaceutical water pretreatment system to be used for large
scale WFI production, what are the options for removal of chloramines?
the only two options for removal of chloramines are break-point
chlorination, and utilization of catalytic carbon beds.
What water treatment process can be applied for maintaining the feed TDS
but removal of divaent ions such as Ca, Mg & SO4 content?
such treatment is available. The closest thing is Nano-filtration
membranes which will remove less of TDS (about 50%) but most of Ca, Mg
and SO4 (99%).