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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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