There are many different
types of water filter
systems on the market,
appealing to different
budgets and performance
expectations. Also, the
quality of the water
coming to your home can
vary greatly, with some
systems offering very
clean water, but others
including rust, dirt,
bacteria and other
unsavory substances. At
the top of the line in terms
of cleanliness is the
reverse osmosis water
filter system.
Reverse osmosis, or RO, was
originally quite an
expensive process used
mainly in military
vessels back after World
War II to desalinate
seawater for drinking.
Over the next couple of
decades private
manufacturers began to
produce systems for home
and commercial use,
although they were rather
expensive. In more recent
years, the technology has
been refined further, and
costs have come down to
more-affordable levels
(although it is still
noticeably more expensive
than conventionally
filtered water in most
cases).
How does it work? The basic
concept is not unlike using
a filter, although there
are some very important
differences. The water
passes through the
membrane material,
allowing the water
molecules and a few other
small molecules to pass,
but leaving behind the
vast majority of other
substances. A filter has
small holes in it of course;
a Reverse Osmosis water
filter membrane has
vastly smaller holes-pores
really that are thousands
of times smaller than
what is in a typical
filter-and the water
won't really go through it
unless it is forced under
pressure. So there is higher
water pressure on one side
of the membrane and
lower pressure on the other
side, thus pushing the
water past. For this reason
the much of the important
stuff happens inside a
pressurized container, so
you don't want to be
buying cheap materials.
It is not uncommon to
include one or more
cartridge filters upstream
of the reverse osmosis
water filter system to
remove the larger
contaminants; this helps
the more-expensive
membrane to last longer.
Some RO systems have the
ability to clean the
membrane off, thus
extending its life without
maintenance effort.
However, eventually the
membrane and filter
elements must be removed,
so part of the research
process should include
understanding how long
these will last; what is
the capacity in terms of
gallons or months?
You might spend 20 cents
per gallon for water from
a reverse osmosis water
filter, so it does not make
sense to use it for things
other than drinking and
cooking. However, RO will
remove over 99% of most
substances including dirt
and rust of course, but also
stuff like copper sulfate
and the pesticide DDT.
There are other substances
like lead, mercury,
chlorine, bacteria and
viruses that are cleaned to
a good 95% removal level or
so.