Over the centuries, alchemists devoted their lives to converting lead into gold. Had they asked any sailor they would have been told that their fortunes would be better assured by finding a way to convert sea water into drinking water. An adequate supply of water is the most critical supply for any voyage lasting more than a few hours. The amount of water available often defines the difference between a pleasure cruise and a survival exercise.
Most pleasure boats depend on periodic replenishment of water storage tanks. This process works well for voyages of short duration or, with stringent control of water use, for longer trips. However, even with the most careful management it will eventually become necessary to refill the tanks, either from a shore-based source or from rainfall. Land-based water sources are often of questionable quality and can be relatively expensive at ports other than those on the North American mainland. Water shortages and high prices are also becoming a problem in some parts of the U.S.
The combination of questionable supply, cost, and a desire for more comfortable sailing (including frequent showers) has created an ever-increasing demand for water on voyaging boats. Fortunately, modern alchemists (engineers) have developed equipment with which even quite small boats can convert sea water into fresh, potable (from the Latin potare, to drink) water. First, let’s define the problem. Sea water contains compounds you don’t want to drink. The dissolved solids in sea water average about 3.5% by weight; 300 pounds of solids per 1,000 gallons, or 35,000 parts per million. Sodium chloride (table salt) accounts for 80% of the dissolved solids. Various other tasty compounds, including calcium sulphate, calcium bicarbonate, magnesium sulphate, and various chlorides make up the rest. The dissolved-solids content of potable water should not exceed 500 parts per million (ppm), 1/70th the dissolved-solid content of sea water. To taste good, the solids content should be less than 200 ppm, with levels between 100 and 150 ppm desirable.
The on-board desalination of sea water, making sea water drinkable, began in the 19th century, when steam engines were installed on ships. The availability of a heat source made it practical to install distillation equipment. From the beginning, distillation desalinators were plagued by the aggregation of scalehard mineral depositswhich are particularly troublesome whenever sea water is heated much above 150° F. Later designs incorporated vacuum pumps that, by reducing the pressure in the evaporation section, allow the water to boil at temperatures low enough the avoid the worst of the scale problem. Improvements in technology led to multi-stage flash evaporators, vapor compression systems, and electrodialysis systems in which an electrical current is used to separate impurities from the water. It is interesting to note that water can be desalinated by chilling the feed water to temperatures at which the pure water component freezes into ice. The resulting fresh water ice is then washed to remove surface contamination and melted. This freezing process can be more energy efficient than most other methods and avoids the problems associated with scale deposits. During the World War II, life rafts were equipped with inflatable, balloon-like solar stills. They worked to a degree, but were only suitable as a last-ditch effort at obtaining drinking water.
During the past 20 to 25 years, reverse-osmosis (R/O) desalination has become practical (see sidebar). R/O systems have found wide application in both large, shoreside plants and on ships of all kinds. Beginning about 20 years ago, R/O systems suitable for use on large yachts became available. Today’s equipment is practical for even quite small boats. System elements
A typical R/O membrane assembly consists of a tubular pressure housing, with the hollow, cylindrical osmotic filtering element, usually made with poylamide plastic deposited on a polysulfone substrate. High-pressure pipe fittings deliver pressurized sea water, carry away the brine, and drain off the fresh water. R/O systems require control valves to permit the initial flow of product water, which contains significant impurities, to be diverted to waste. In addition, valves are usually provided that allow the system to be purged of sea water prior to shutdown. During this process, the pump is supplied from a source of already desalinated water. The water used for this flushing process must be free of chlorine, whose presence will harm the osmotic membrane. For this reason, many installations incorporate a separate storage container whose contents are reserved for the pre-shutdown flush.
A built-in or portable water-quality monitor is a part of most R/O systems. For desalination purposes, the quality of the product water can be determined by measuring its electrical conductivity with an ohmmeter (the more dissolved salt in the water, the more electrically conductive it is). Some systems require manual adjustment of the water pressure developed by the high-pressure pump; others operate with a preset pressure. (The higher the sea water temperature, the less pressure is required to create a sufficient reverse-osmotic flow.) Gauges showing the pressure applied to the membrane are common on many systems. The pressure gauge can also be very useful in evaluating system performance and in diagnosing problems. Flow meters displaying the volume of sea water delivered to the membrane and product water output are included in some larger systems.
R/O systems require a substantial flow of sea water. The water must be carefully filtered, usually in a multi-step process, beginning with a sea water strainer. In many ocean and harbor areas, the required flow, about 10 gallons of sea water for every gallon of fresh water produced, can quickly foul the sea water strainer. The system must then be shut down, the strainer opened, and the trapped debris removed from the basket. The fouled strainer problem can be eliminated with installation of a Groco Hydromatic self-cleaning sea water strainer. The Hydromatic is also used as a sea chest, simultaneously supplying filtered sea water to the vessels R/O system, air conditioning system, and generator from a single through-hull and seacock. The sea water delivered from the strainer must be carefully filtered before it is delivered to the high-pressure pump. The close mechanical tolerances required in the pump can be damaged by abrasive particles in the feed water. Two-element prefilter systems are common, composed of a 30-micron element followed by a five-micron element. As with diesel fuel filtration, use of progressively finer filters lengthens filter life. Additionally, special oil-separator filters are available for use in areas where there is risk of petroleum contamination of the sea water. Use of R/O systems should be curtailed in areas where the water contains large amounts of silt, industrial waste or excessive organic materials, all of which can rapidly foul the prefilters.
Many watermaker manufacturers specify that the high-pressure pump be located below the waterline to ensure adequate water flow to the prefilters and pump. When this is not practical, an additional low-pressure boost pump can be installed at the outlet of the sea water strainer, thereby providing a positive flow of water to the prefilters and high-pressure pump. Boost pumps are commonly installed when multi-stage prefilters are used.Sea water corrosive
Sea water, even after careful filtering is highly corrosive. It can destroy high-pressure pumps that are not specifically chosen for R/O service. While early R/O units used off-the-shelf industrial pumps, today’s systems use pumps made with stainless steel, titanium, and ceramics to provide greatly improved service life. The suitability of a specific R/O desalinator for a sailboat often depends on the energy consumption of the high-pressure pump. While one- to three-hp AC motors pose
no power problems when installed on powerboats and larger sailboats, they are generally unsuitable for most sailboats less than about 50 to 60 feet in length. Direct belt drive of the pump from the engine is attractive; however, finding suitable mounting space for the pump may be difficult, and the system will only operate when the engine is in use.
Improved pump and motor efficiency has made 12- or 24-volt DC powered systems practical. Systems that drive conventional high-pressure pumps from low-voltage DC motors typically consume almost 50 watts for each gallon of desalinated water produced. Systems using improved pumps and high-efficiency DC motor drives consume on the order of 36 to 40 watts per gallon.
The pressure required for production of salt-free water will depend to a significant degree on the temperature of the sea water feed, with less pressure required at higher temperatures. For this reason, the power demand will also vary with sea water temperature, decreasing somewhat in warmer waters. This relationship can work to a voyager’s advantage. While water consumption increases in hot climates, this is somewhat compensated by a decreasing energy demand per gallon. For one particular system the power demand decreases from 14 watts per gallon of desalinated water at a sea water temperature of 50° F to 12 watts per gallon when the sea water temperature is 77° F and to 11.6 watts per gallon with 90° F sea water.
The very fine pores of the R/O system’s osmotic membrane allow the passage of pure water but stop all larger molecules, including those of salts and other inorganic materials, including bacteria, which are too large to pass though the filter. It is important to note, however, that viruses can pass through the filter and emerge in the fresh product water. To counter this, R/O systems are frequently fitted with an ozone water purifier capable of eliminating potentially harmful viruses. Product water flowing through the device is exposed to ultraviolet energy, creating ozone, which oxidizes the pathogens that may have passed through the R/O membrane.
Although the water delivered to a boat’s storage tanks from the R/O system is pure, airborne contaminants can enter the storage tanks. Water added to tanks from other sources may also introduce contaminants. For these reasons, it is common to add a small amount of chlorine to the water storage tanks. An activated charcoal filter can be connected at the outlet of the potable water pump to eliminate the taste and odor of residual chlorine. Some boats equip the ozone sterilizer with valves that allow the sterilizer to be put in line to purify water drawn from the storage tanks when the R/O system is not in operation.R/O systems need careful installation.
Maintenance access, especially for inspection and change of prefilter elements, is an important consideration. Unless the system is equipped with a remote control panel and power-operated valves it will also be necessary to have direct access during system operation. The length of R/O membranes can complicate the task of finding a home for the system. R/O systems can be on the noisy side. Sound levels in excess of 75 dbA are common for systems using conventional high-pressure pumps. While it may be tempting to use the underutilized space beneath a bunk, the sleepy occupant may not thank you if you do. Adequate electrical connections
The electrical connections to the vessel’s battery bank must be done with due regard for the power demand of the system’s motors, which can require as much as 40 amps of current. The circuit supplying the power should be designed to limit voltage drop to no more than 3%. Once an R/O system is on board a boat, water use can increase dramatically as crewmembers neglect previous water conservation habits. One result of this is that the R/O system may be operated for many more hours than were contemplated before this source of potable water was available. Although it may be practical to power some low-drain R/O systems from the vessel’s battery bank when the engine is not running, few battery banks will support the long hours of operation most users require. Some vessels not equipped with a genset fit a small engine-driven high-output alternator. Small engine packages incorporating direct drive high pressure pumps in addition to a high current output alternator are also available. Other installations rely upon the ship’s genset or use solar panels and wind generators to supply some or all of the R/O system’s power appetite.R/O systems work best when used frequently.
Daily use is the best operating condition for the system. Such use tends to prevent fouling of the R/O membrane and inhibits the growth of bacteria on the surface of the membrane. Whenever a system is to be left unused for more than a few days, most manufacturers recommend that fresh water be used to flush all salt water and brine from the membrane. The water used for flushing the system should be pure product water and free of chlorine or any other disinfectant. Even small amounts of chlorine can damage the osmotic membrane. Using water from the main storage tanks is inadvisable. Water in these tanks may contain chlorine from a shoreside source or chlorine added on board. A supply of about five gallons of pure water may be needed for system flushing depending on model. If the system is to remain idle for more than a few days to a week it will be necessary to clean and preserve the membrane in accordance with the manufacturer’s instructions. Each manufacturer offers a particular chemical kit for the purpose of “pickling” the membrane. These chemicals can be caustic and must be handled with care. While these chemicals aren’t cheap, they’re much less costly than membrane replacement. One set of chemicals is usually acid based and is used to remove mineral scale and kill live microbes. This chemical mix is often allowed to remain in the R/O membrane assembly during long storage periods. The second chemical cleaner is alkaline and is used to remove biological byproducts, oil, and dirt that may have gotten past the prefilters. The system manufacturer will provide specific instructions for cleaning, pickling, and returning to service. It is important to follow the instructions of the system manufacturer with regard to both the specific chemicals used and the sequence in which they are used.
Reverse-osmosis desalinators are by no means cheap. However, they can bring about very significant and desirable changes in the way a voyaging boat operates, from a crew comfort standpoint and by eliminating the necessity to plan voyages based on all-too-frequent stops to replenish limited water stores. However, and there always is a however, an adequate supply of stored potable water must always be maintained against the possibility that the R/O system may not operate properly. The stored quantity need not match the daily consumption rate allowed when the R/O system is operating but must be sufficient to provide a reasonable amount of water for all on board during the time it may take to reach a port where potable water is available.
You’ll need to know how the system is supposed to work and how to troubleshoot it when it is not working properly. A full spares kit should be on board, along with a supply of prefilters, and the chemicals recommended by the manufacturer. Don’t assume that the chemicals recommended by one maker will be satisfactory for another brand of equipment.