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Chuck Husick: Techno-Talk,
January 2006 from BoatUS Magazine - Still Current as of August 2009
Reverse
Osmosis Watermakers
Water, water, everwhere, but unless you are sailing on an exceptionally
clean freshwater lake or have a reverse osmosis watermaker on board,
you can’t drink it. The value of an unlimited supply of fresh,
good tasting water is well understood by anyone who sails in a hot
climate.
Reverse osmosis (R/O) watermakers suitable for installation on even
modest size boats have been available for quite some time, however
a number of constraints limited their use. Unless the watermaker
was used frequently a time consuming cleaning procedure was necessary
to protect the costly R/O filter membrane. Many units required a
substantial amount of AC power, particularly to operate the high-pressure
pump. Advances in system design have made it practical to use an
R/O system on small and infrequently used boats.
Reverse osmosis systems deliver fresh water by supplying seawater
to a filter whose pores are so fine that only fresh water can pass
through. All salts, other minerals and bacteria are rejected and
disposed of overboard. Normal operation of the system can cause
contaminants, including bacteria and marine organisms, to accumulate
on the seawater side of the membrane. When the system is in operation
the flushing action created by the large volume of feed water flowing
through an operating system (about 10 gallons for every gallon of
fresh water produced) keeps the contamination under control. However,
an idle system provides an ideal environment for nasty things to
grow in the membrane.
The traditional way to deal with this problem is to introduce cleaning
chemicals, usually an acid, followed by a neutralizing base to preserve
the membrane when it is not in use. The process is time consuming
and frequent use of the chemicals reduces membrane life. Today’s
systems eliminate the need to chemically clean and preserve the
system during non-operating times extending for weeks or even a
few months by periodically backwashing the R/O filter with the pure,
chlorine-free water previously produced by the system. Chemical
treatment is required only when the rate of production of fresh
water diminishes after a long period of service or if the system
is unused for months on end.
Manufacturers have reduced operating power required by the system’s
high-pressure pump. Seawater delivered to the filter membrane that
separates the fresh water from the dissolved solids (including salt)
must be pressurized to about 800 pounds per square inch. This can
be done with brute force, as it is in pressure washers. An alternative
is to use the highly pressurized salty water (brine) that can’t
pass through the R/O membrane to aid the pumping action. This process,
first used in hand operated R/O equipment is now employed in motor
powered units to reduce the drain on the boat’s electrical
system.
An alternative approach employs one or more low-pressure pumps to
deliver a large volume of water at modest pressure (usually less
than 100 psi) to a hydraulic amplifier. The amplifier consists of
a pair of directly connected pistons whose areas are in a ratio
of about 10:1. Water at 80 psi supplied to the large piston will
allow the small piston to deliver water at 800 psi. The difference
in efficiency between the low-pressure pumps and the conventional
high-pressure piston pumps, plus the recovery of energy from pressurized
water that does not pass through the R/O membrane, can significantly
reduce the amount of power needed. Many of today’s systems
will consume only 21 watt-hours of electrical energy to deliver
a gallon of fresh water (a 12-volt system consuming 13 amperes can
produce about 8 gallons per hour).
Having an R/O system on your boat brings with it an additional advantage:
the assurance that the water you drink and cook with is free of
both bacterial and viral contamination. Bacteria are too large to
pass through the R/O filter membrane. Viruses can be small enough
to pass through the R/O membrane, however, they are readily killed
by the ultraviolet ozone generating sterilizer that is a common
system accessory. The result— water quality better than the
expensive bottled water. When you next have guests on board, offer
them that exclusive drink, water named after your boat, Eau de Poulet
Vagabond, or the like.
By Chuck Husick
Chuck Husick is a pilot, engineer, sailor and former president of
Chris Craft Boats.
© Copyright BoatUS Magazine 2006
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