 | From Ocean Navigator #127 January/February 2003 |
The voyaging sailboat of days gone by: with plastic jugs of water (and diesel fuel!) tied on deck. With a watermaker installed, the need to carry extra jugs of water is eliminated.
Comparatively speaking, even with initial materials and installation as well as maintenance and operating costs figured in, watermakers produce cheap water. In fact, the more they are used, from an amortization standpoint, the cheaper it is to produce each gallon of sweet water.
Additionally, with storage at a premium aboard most far-ranging cruising vessels, a watermaker can be a value added in terms of space. The ability to make one’s own water reduces the need for copious water storage. At the very least, for new designs and retrofits alike, the watermaker installation may occupy the former home of a hitherto necessary water tank. Of course, enough water should be carried to supply the minimum hydration needs of the crew (roughly one quart per crewmember, per day) in the event of equipment failure. Although, in my experience, most watermaker installations simply augment existing tankage, giving the vessel longer water legs, while eliminating the need to get dockside in order to replenish the supply.
Perhaps the most-often overlooked and important advantage to making water aboard is the health benefit. Reverse osmosis (RO) water-making (more on exactly what this is later) doesn’t just produce fresh water, it produces extremely clean water. The process actually has the ability to strain out most bacteria, viruses and other harmful agents. For example, the average bacteria cell is between 1 and 4 microns (a micron is a millionth of a meter) in diameter, and a virus is between 0.4 and 0.02 microns. The size of the “strainer” in the RO process is roughly 0.0001 microns. Therefore, along with the salt, the water that passes through this micro-filtration sieve is relieved of most harmful contaminants. As a result, the water produced by the RO process is often cleaner than domestic dockside water supplies and almost always cleaner than the water found in less developed areas. Furthermore, if only RO water, which is essentially sterile, goes in the vessel’s tanks, the entire water system tends to remain cleaner, without the need for chlorine or other anti-microorganism agents.
Most watermaker manufacturers are quick to caution that microscopic damage to RO membrane systems may allow some harmful contaminants past the gate, so to speak. The user should not, therefore, consider the sterilization process absolute. In practice, however, the product water from most systems, as previously mentioned, is cleaner than the water from the dockside garden hose. Additionally, as good as this filtration process is, it is not capable of filtering out some chemicals, such as formaldehyde and certain solvents. Charcoal and ultraviolet filtration can be added to any onboard domestic water system, whether watermaker-equipped or not, in order to achieve the highest-quality, contaminant-free water.
How it works
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There are essentially two common methods used to make fresh water from seawater. The first and oldest method is through distillation. Very simply, this involves boiling seawater to create steam. Only the fresh water will boil off into steam, leaving behind salt and other minerals. The steam is then cooled, where it condenses into fresh water. Steam-powered ships have made fresh water this way for boiler make-up and domestic uses for ages. Like RO systems, by virtue of the high temperatures involved, the water produced by this process is relatively microbe- and mineral-free.
Distillation has been the salvation of more than a few sailors who have found themselves in extremis. The shipwrecked crew of the U.S. Navy sidewheel steamship Saginaw, in 1870, built a steam-distillation plant while marooned on Ocean Island (now Kure), just west of Midway. Using components salvaged from the ship’s boilers, the chief engineer manufactured a steam-distillation plant that provided enough water for the crew of 93 until a well could be dug.
A derivation of the distillation system involves placing the steam chamber under a partial vacuum, which lowers the boiling point, thus requiring less heat and energy to carry out the process. The drawback to this is the forfeiture of sterilization, because the water is not heated to a high enough temperature to kill all microorganisms. Waste-heat systems, which often use the partial vacuum process, draw byproduct heat from engine-coolant and air-conditioning/refrigeration condensers, work well. However, they require additional treatment to kill any bugs that survive the diminished intensity of the low-pressure/temperature steam chamber.
Solar stills, the simple form of freshwater distillation, also work well, albeit at an agonizingly slow pace. They obviously require sunlight to operate and are all but impractical for seagoing use, with perhaps the exception of the life-raft version; although these have, for the most part, been supplanted by hand-pumped RO units.
The second common method of making fresh water from seawater is by the aforementioned RO process. Before detailing what this is and how it works, a brief description of osmosis is in order. It is the movement of a solvent, water in this case, through a semipermeable membrane into a solution of higher concentration, which then equalizes the concentrations of solution on the two sides of the membrane. In short, the natural process of osmosis will, if one solution is salt water and the other fresh water, cause fresh water to pass through the membrane to the saltwater side. Although this is an interesting process, it is not exactly a useful tool for the voyaging sailor.
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Osmosis also occurs, much to the consternation of many boat owners, within fiberglass hulls. Seawater passes through the semipermeable membrane, gelcoat in this case, and enters the fiberglass laminate. The water-soluble materials within the laminate mix with the seawater, creating a solution that is more concentrated than the seawater on the other side of the gelcoat. The osmotic process goes into high gear at this point, allowing more seawater to enter the more highly concentrated solution. Eventually, considerable pressure builds, leading to the dreaded blister.
Interestingly, osmosis is responsible for the death of many castaway sailors. Those who are foolish or desperate enough to drink seawater actually perish because of dehydration. Seawater in the stomach and intestines is more concentrated than the fluid in the tissues in these organs. Water is drawn out, through osmotic pressure, to dilute the concentrate, seawater. This is why seawater ingestion actually hastens the demise of these unfortunate souls, as opposed to their comrades who simply remain thirsty.
Reverse osmosis actually puts this process to good use by forcing Mother Nature’s hand. If, in nature, the osmotic process causes fresh water to go through a semipermeable membrane to saltwater, reverse osmosis simply reverses the process. The catch is, accomplishing this requires a considerable amount of energy in the form of high-pressure hydraulic action, approximately 600 to 800 pounds per square inch.
The heart of the RO process is the semipermeable membrane. This component can be thought of as an ultra-fine filter. Water molecules (H20) are comparatively small, considerably smaller than sodium and chloride ions (essentially sea salt). Under pressure, the water molecules slip through the membrane’s pores while virtually everything else – salt, debris, other minerals – are unable to pass.
The amount of engineering required to develop membranes that are able to accomplish the above-described process reliably, while withstanding the considerable working pressure, is appreciable. Add to this the capability for self-cleaning (the blocked salt can’t be allowed to clog the membrane like sediment in a diesel filter) and affordability, and the achievement is all the more impressive. Most membranes used today, which are made from polymer thin film composites, are capable of withstanding 1,000 psi thanks to their spirally wound design.
The key to efficient membrane operation and long life is a formula known in the RO industry as flux ratio. This is the ratio of seawater that is pumped through the RO system to fresh water produced, also known as product. It is a commonly accepted belief within the small-volume RO community that 10 percent – one gallon of fresh water for every 10 that are pressurized – is the maximum output that should be attempted. If this quantity is exceeded, the membranes do not receive the necessary amount of flushing action and can clog. Therefore, the unused water, known as brine, is no more than 10 percent saltier than the seawater from which it came. Even at this rate, a considerable amount of fresh water may be produced from relatively compact RO units whose power needs are modest, even by cruisers’ standards.
It is easy to see why desalination is accomplished almost exclusively by RO where voyaging vessels and motor yachts are concerned. Additionally, commercial and military vessels that are not steam-powered – a growing number – are also turning to RO for their freshwater needs.
The complete system
The membrane is only one component of many in the average RO system. The remainder of the equipment plays a support role in facilitating the passage of seawater through the membrane at high pressure. If the incoming seawater, or feed water, were to be followed from the moment it entered the boat until it ran from a galley or head tap, it would take the following circuitous path. First, it would pass into a through-hull fitting located as far below the waterline as possible. This depth requirement serves two purposes. Deep through-hulls are less likely to pull in floating debris and chemicals. The worst enemy of an RO membrane is oil and fat. This can include petroleum-based oils, such as fuel or crankcase oil, or as unpleasant as it is, human waste. Needless to say, RO intakes should not be located downstream of a head discharge. Additionally, the deep RO intake is less likely to pull air into the system. Because of the violent turbulence and resultant damage it causes, especially under high pressure, it is undesirable to allow air to enter the intake side of the RO system, whether from inside the vessel, through leaks in the vacuum side of the plumbing, or from through-hull aeration.
Incoming seawater, once it passes into the boat, enters a raw-water strainer. This is not unlike the strainer that would be found on your engine’s cooling-water intake. It is relatively coarse and designed to catch visible debris, such as weeds and sea creatures that are large enough to see.
After passing through the primary strainer, the boost or feed pump pressurizes the seawater, depending on the manufacturer, to somewhere between 80 and 120 psi. Because the primary strainer only catches the big stuff, the next stop is further filtration. Again, this varies from manufacturer to manufacturer. Some offer a single 20-micron pre-filter (about the same pore size as your primary diesel filter), while others recommend a 20- or 25-micron pre-filter and a 5-micron secondary filter. As previously mentioned, because oil is so damaging to RO systems, an oil/water separator may be supplied as well.
The ultra-fine filtration is necessary in order to prolong membrane life. Plankton fouling is one of the primary contaminants found in pre-filters. If it were allowed to pass to the membrane, freshwater production would diminish quickly and eventually cease. The self-cleaning nature of the membrane is only designed to cope with salt and other atomic-sized particles, not small plants and animals. Some RO manufacturers go so far as to recommend that their systems not be used in coastal waters because of the presence of abundant micro plant and animal life. Additionally, because pollutants, waste and silt are often found coastally, in crowded harbors and estuaries, this is probably sound advice.
Once thoroughly filtered, feed water then enters the high-pressure pump. These vary a bit, but the net result is the same – high-pressure water supplied to the membrane. These pumps may operate from a number of voltages or energy sources, including 12, 24 and 110 volts, as well as from a propulsion engine or genset via a belt or coupling.
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Because of the corrosive nature of seawater, and the effects of cavitation and velocity erosion, the pump materials must be of the highest quality, 316 stainless steel, titanium, bronze and high-strength composites.
Once pressurized, the water then enters the now-familiar membrane. From the membrane, two plumbing lines deliver product water (fresh) and brine (concentrated seawater) respectively. The overboard discharge for the brine should be plumbed above the waterline for monitoring purposes and to reduce backpressure and siphoning. Some prefer to plumb this into the cockpit, just above a drain, so that any interruption of production will be noticed quickly, while others connect it to a galley or head sink.
Additional filtration may be employed at this point. Some systems offer charcoal post filters, to remove unpleasant odor or taste, as well as UV sterilization, which will kill the few remaining microbes that may have passed the membrane. A salinity meter and flow meter may also be installed at this point, to monitor salt content and product volume, respectively. Some manufacturers offer automatic diversion in the event an unacceptably high level of salt is detected in the product stream, to prevent contamination of the vessel’s tanks.
A drop in product flow, noticeable on the flow meter, usually graduated in gallons per hour, is often a warning of clogged pre-filters or a pump malfunction, which might require the attention of the crew. Production fall-off can also be caused by a drop in water temperature or an increase in salinity. However, these are usually minute changes; whereas, a filter full of plankton will have a noticeable effect on the product volume.
Product water is usually plumbed into the vessel’s water-tank vent or fill. Because of the possibility of inducing backpressure into the system, product plumbing should not be connected to main freshwater manifolds or into the bottom of a tank. Using a Y-valve to select which tank will receive product water is, however, acceptable.
Not mentioned in the above description of components are the myriad possibilities for flushing, cleaning and product-diversion valves, and plumbing. These all serve a useful and necessary purpose. Each manufacturer offers suggestions for these auxiliary components as to how and where they should be plumbed into the system. They will facilitate freshwater flushing, cleaning and troubleshooting of the system.
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Maintenance
By far, the most common complaint heard about watermakers is their need for regular maintenance. To be fair, that has changed over the years. Systems have become easier to service and maintain while becoming more owner/operator friendly. The best maintenance advice regarding a watermaker is to use it. Watermakers are one of those pieces of shipboard gear that benefit greatly from regular use.
If you have yet to install or use your watermaker, be sure to create a start-up log. That is, when it’s first used, log the variables, feed-water temperature, battery voltage, amp draw, feed pressure and product-water flow rate. This will enable you to determine the health of the RO system down the road, warning of impending failure before it strikes, as well as serving as a troubleshooting tool.
Biofouling, one of the most common failures in RO systems occurs almost invariably because of disuse. This malady occurs when a watermaker is left idle and improperly preserved. Microorganisms present in all seawater – and thus in the feed-water side of your RO system – consume the available food, multiply and eventually die. Their colonies, both alive and decaying carcasses, are often quite noticeable, thanks to the dark color and putrid odor they emit. Alive or dead, it’s not something you want to have inhabiting your drinking-water supply. Additionally, severe biofouling can permanently damage the membrane (even the smallest membranes cost upwards of $500) Depending on the manufacturer and the ambient temperature (as any voyager knows, biological growth is accelerated in tropical climates), the flushing threshold may be as short as three days. Some RO users insist, erring on the side of caution, on flushing their membranes after every use.
The prevention for this disease is simply to fill the entire system, from the intake seacock onwards, with sterile water produced by the watermaker. The problem is you may need to do this if the system breaks down, and if a failure should occur, then you cannot produce the necessary sterile solution. Most RO-savvy cruisers carry a supply of sterile water in the event of just such an emergency. Depending on the size of the RO system, this could be between three and five gallons, or more for very large systems. In an emergency, this cache may be turned into a life-raft drinking-water supply.
A word of caution where flushing is concerned, many membranes have been permanently damaged in the flushing/preserving process thanks to chlorine. Chlorine is just as deadly to membranes as oil. If you fill your tanks dockside, from a safe, municipal water supply, it may be, unbeknownst to you, chlorinated. If you then use water from your tanks for backfilling the RO system, you may actually be doing more harm than good. This is why it’s best to use your own product water, freshly produced or from a clean tank that never sees dockside water, for a storage solution.
Because of the rapidity with which biological growth occurs in warmer climates, some folks opt to freshwater flush their systems before sailing into potentially heavy weather. The logic here is, once you’re in it, the last thing you may feel like doing is working on the watermaker. If an unfavorable weather report materializes, fill the tanks, flush the system and secure it for the duration. When it’s over, all you need to do is open the seacock, and you’re back in the freshwater-production business.
The freshwater flush is a short-term storage solution. For long-term storage – more than a few weeks – biocide-pickling solutions are available for the membrane itself. The primary filters can be changed and left dry to inhibit growth. Check your RO-system manufacturer guidelines for exact storage procedures and interval thresholds.
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Some manufacturers offer an automatic freshwater flush option, which will automatically flush your RO system with sterilized product water at set daily or weekly intervals (one manufacturer of such a unit is Filtration Concepts Inc., Santa Ana, Calif., 800-850-0123, www.filtrationconcepts.com). If you just don’t want to be troubled with the flushing routine, or if you would like to leave your unit ready to use but unattended for long periods, this might be worth considering. It does, of course, add some complexity and expense to any installation. Spectra (San Rafael, Calif., 415-526-2780, www.spectrawatermakers.com), a well-known RO-watermaker manufacturer, offers the Zetaguard System, which they claim prevents biofouling and scale, eliminating the need for flushing, pickling or cleaning by electrostatically charging the entire RO system’s water content. Again, this feature adds complexity and expense; however, it may be a worthwhile investment for some installations.
Feed-water quality, as previously mentioned, has a dramatic effect on watermaker operation; however, it is also a maintenance issue. It’s worth repeating. Don’t run your RO system in waters that are of questionable cleanliness. Some RO operators only run their units when well offshore. Of course, if you are inshore and need water, go ahead and run your RO system. Be vigilant and watchful of the strainer, pre-filters and oil separator (if equipped) and note any drop in production, which often indicates clogging. If you intend to run inshore regularly, a 5-micron pre-filter and oil separator are must-haves.
If you are of the preventive-maintenance mind, consider carrying out a seal replacement every thousand hours of run time (if your watermaker doesn’t have an hour meter, installing one is easy and will facilitate scheduled maintenance). This is a relatively straightforward task, and if you do it yourself, it will enable you to become more familiar with the unit, should it require emergency repairs at some point.
A complete spares kit is essential for those venturing far from civilization. Even if you’re not traveling to remote parts, having your own spares will enhance self-sufficiency and perhaps save some of the cruising kitty as well. Spares should include several sets of filters (in my experience, polyester filters, while initially more expensive, will last longer than paper filters), biocide-pickling solution, seals, high- and low-pressure hose lengths, and end fittings. Some prefer to carry a spare boost pump as well.
In the event your RO membrane becomes fouled with biomass or calcium deposits, you’ll have to be prepared to perform a full acid or alkaline (or both, respectively) cleaning. Keep in mind, cleaning, particularly with alkaline solution, will shorten the life of the membrane, perhaps by as much as 10 percent with each treatment.
Installation
Where RO system installations are concerned, the old real-estate axiom, “location, location, location,” holds true, thanks to a large volume of high-pressure water contained within every unit. Consider this: If you were to install your membrane above a generator or alternator regulator (because of their sensitivity to heat, membranes should not be installed in engine compartments), and it leaks, the volume of seawater it could pump, perhaps in the form of a fine mist or spray, before you notice it, may be considerable.
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Containerized RO systems offer all or most of the components packaged in a single frame, something akin to a generator. These systems are relatively easy to install. Essentially, you hook up water and power and go. However, they are sometimes simply too large to be accommodated by small- and medium-sized cruising vessels.
Modular systems, on the other hand, provide the freedom to pick the best or most available locations for each item. When choosing real estate for pressurized items (everything downstream of the boost pump), keep in mind the aforementioned potential for collateral damage in the event of a leak. Additionally, plan on spilling some seawater when replacing pre-filters and cleaning the strainer. Finally, the location of the flushing and cleaning outfall must be considered a potential splash hazard to any nearby equipment.
As mentioned previously, RO systems are available in several different power configurations. Smaller, 12- and 24-volt DC units are ideal for voyagers, while 120- and 240-volt AC models are more appropriate for larger genset-equipped yachts. A variation on this theme is to power the high-pressure pump from the vessel’s auxiliary engine (or genset) crankshaft, through a belt, much like a high-output alternator or refrigeration compressor. The logic for this setup is quite sound. If you must run your engine once a day to charge batteries, why not put some of that horsepower (even large alternators rarely use more than three or four horsepower) to use? Running a diesel lightly loaded – a state of affairs that leads to cylinder-wall glazing, excess oil consumption and overcooling – borders on sacrilege. An RO pump can help rectify this scenario by adding some load to the diesel’s workout schedule.
If, however, you opt for the 12- or 24-volt version, as most voyagers do, it’s best to run it while the engine is running during the battery-charge regimen. This will provide the watermaker with the highest possible voltage, which will enable it to produce more product in less time. Inductive loads, such as boost and high-pressure pumps, will also last longer if run at the higher limit of a nominal 12-volt system, 13.8 rather than 12.6 volts.
Finally, choose your watermaker’s capacity carefully. Unlike anchors, bigger is not always better where watermakers are concerned. Because watermakers suffer when sitting idle for even a short period of time, it’s best to size your system so that it has to run, under normal cruising conditions, once every one to three days. If your watermaker has so much capacity that you only need to run it every week to 10 days, then it is more likely to suffer from biofouling and scale buildup. You may even have to run it just to prevent this from happening, even if there’s no need for fresh water, which is economically unsound and wasteful.
Of course, this equation must also take into account tank capacity. However, if you design your system to work in such a way that the RO system runs regularly, it will be more reliable and long-lived. Most crews find that once the water supply is augmented by a watermaker, water usage increases (more showers, freshwater dishwashing and even decadent deck washdowns) two- or threefold. So consider this when making your selection.
Considering the size, simplicity and relative expense, RO watermakers are a truly remarkable development that has changed the lifestyle of extended voyaging immeasurably.
Contributing Editor Steve C. D’Antonio is a freelance writer and the boatyard manager of Zimmerman Marine in Mathews, Va.
How it works
There are essentially two common methods used to make fresh water from seawater. The first and oldest method is through distillation. Very simply, this involves boiling seawater to create steam. Only the fresh water will boil off into steam, leaving behind salt and other minerals. The steam is then cooled, where it condenses into fresh water. Steam-powered ships have made fresh water this way for boiler make-up and domestic uses for ages. Like RO systems, by virtue of the high temperatures involved, the water produced by this process is relatively microbe- and mineral-free.
Distillation has been the salvation of more than a few sailors who have found themselves in extremis. The shipwrecked crew of the U.S. Navy sidewheel steamship Saginaw, in 1870, built a steam-distillation plant while marooned on Ocean Island (now Kure), just west of Midway. Using components salvaged from the ship’s boilers, the chief engineer manufactured a steam-distillation plant that provided enough water for the crew of 93 until a well could be dug.
A derivation of the distillation system involves placing the steam chamber under a partial vacuum, which lowers the boiling point, thus requiring less heat and energy to carry out the process. The drawback to this is the forfeiture of sterilization, because the water is not heated to a high enough temperature to kill all microorganisms. Waste-heat systems, which often use the partial vacuum process, draw byproduct heat from engine-coolant and air-conditioning/refrigeration condensers, work well. However, they require additional treatment to kill any bugs that survive the diminished intensity of the low-pressure/temperature steam chamber.
Solar stills, the simple form of freshwater distillation, also work well, albeit at an agonizingly slow pace. They obviously require sunlight to operate and are all but impractical for seagoing use, with perhaps the exception of the life-raft version; although these have, for the most part, been supplanted by hand-pumped RO units.
The second common method of making fresh water from seawater is by the aforementioned RO process. Before detailing what this is and how it works, a brief description of osmosis is in order. It is the movement of a solvent, water in this case, through a semipermeable membrane into a solution of higher concentration, which then equalizes the concentrations of solution on the two sides of the membrane. In short, the natural process of osmosis will, if one solution is salt water and the other fresh water, cause fresh water to pass through the membrane to the saltwater side. Although this is an interesting process, it is not exactly a useful tool for the voyaging sailor.
Osmosis also occurs, much to the consternation of many boat owners, within fiberglass hulls. Seawater passes through the semipermeable membrane, gelcoat in this case, and enters the fiberglass laminate. The water-soluble materials within the laminate mix with the seawater, creating a solution that is more concentrated than the seawater on the other side of the gelcoat. The osmotic process goes into high gear at this point, allowing more seawater to enter the more highly concentrated solution. Eventually, considerable pressure builds, leading to the dreaded blister.
Instrumentation, like the gauges on this watermaker unit from Sea Recovery, eases the task of monitoring operation.Interestingly, osmosis is responsible for the death of many castaway sailors. Those who are foolish or desperate enough to drink seawater actually perish because of dehydration. Seawater in the stomach and intestines is more concentrated than the fluid in the tissues in these organs. Water is drawn out, through osmotic pressure, to dilute the concentrate, seawater. This is why seawater ingestion actually hastens the demise of these unfortunate souls, as opposed to their comrades who simply remain thirsty.
Reverse osmosis actually puts this process to good use by forcing Mother Nature’s hand. If, in nature, the osmotic process causes fresh water to go through a semipermeable membrane to saltwater, reverse osmosis simply reverses the process. The catch is, accomplishing this requires a considerable amount of energy in the form of high-pressure hydraulic action, approximately 600 to 800 pounds per square inch.
The heart of the RO process is the semipermeable membrane. This component can be thought of as an ultra-fine filter. Water molecules (H20) are comparatively small, considerably smaller than sodium and chloride ions (essentially sea salt). Under pressure, the water molecules slip through the membrane’s pores while virtually everything else &mdash salt, debris, other minerals &mdash are unable to pass.
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This Z Guard module from Spectra is designed to reduce biofouling via the use of electrostatic charges. The amount of engineering required to develop membranes that are able to accomplish the above-described process reliably, while withstanding the considerable working pressure, is appreciable. Add to this the capability for self-cleaning (the blocked salt can’t be allowed to clog the membrane like sediment in a diesel filter) and affordability, and the achievement is all the more impressive. Most membranes used today, which are made from polymer thin film composites, are capable of withstanding 1,000 psi thanks to their spirally wound design.
The key to efficient membrane operation and long life is a formula known in the RO industry as flux ratio. This is the ratio of seawater that is pumped through the RO system to fresh water produced, also known as product. It is a commonly accepted belief within the small-volume RO community that 10 percent &mdash one gallon of fresh water for every 10 that are pressurized &mdash is the maximum output that should be attempted. If this quantity is exceeded, the membranes do not receive the necessary amount of flushing action and can clog. Therefore, the unused water, known as brine, is no more than 10 percent saltier than the seawater from which it came. Even at this rate, a considerable amount of fresh water may be produced from relatively compact RO units whose power needs are modest, even by cruisers’ standards.
It is easy to see why desalination is accomplished almost exclusively by RO where voyaging vessels and motor yachts are concerned. Additionally, commercial and military vessels that are not steam-powered &mdash a growing number &mdash are also turning to RO for their freshwater needs.
The complete system
The MaxQ unit from Filtration Concepts can be set to flush with product water at timed intervals.The membrane is only one component of many in the average RO system. The remainder of the equipment plays a support role in facilitating the passage of seawater through the membrane at high pressure. If the incoming seawater, or feed water, were to be followed from the moment it entered the boat until it ran from a galley or head tap, it would take the following circuitous path. First, it would pass into a through-hull fitting located as far below the waterline as possible. This depth requirement serves two purposes. Deep through-hulls are less likely to pull in floating debris and chemicals. The worst enemy of an RO membrane is oil and fat. This can include petroleum-based oils, such as fuel or crankcase oil, or as unpleasant as it is, human waste. Needless to say, RO intakes should not be located downstream of a head discharge. Additionally, the deep RO intake is less likely to pull air into the system. Because of the violent turbulence and resultant damage it causes, especially under high pressure, it is undesirable to allow air to enter the intake side of the RO system, whether from inside the vessel, through leaks in the vacuum side of the plumbing, or from through-hull aeration.
Incoming seawater, once it passes into the boat, enters a raw-water strainer. This is not unlike the strainer that would be found on your engine’s cooling-water intake. It is relatively coarse and designed to catch visible debris, such as weeds and sea creatures that are large enough to see.
After passing through the primary strainer, the boost or feed pump pressurizes the seawater, depending on the manufacturer, to somewhere between 80 and 120 psi. Because the primary strainer only catches the big stuff, the next stop is further filtration. Again, this varies from manufacturer to manufacturer. Some offer a single 20-micron pre-filter (about the same pore size as your primary diesel filter), while others recommend a 20- or 25-micron pre-filter and a 5-micron secondary filter. As previously mentioned, because oil is so damaging to RO systems, an oil/water separator may be supplied as well.
The ultra-fine filtration is necessary in order to prolong membrane life. Plankton fouling is one of the primary contaminants found in pre-filters. If it were allowed to pass to the membrane, freshwater production would diminish quickly and eventually cease. The self-cleaning nature of the membrane is only designed to cope with salt and other atomic-sized particles, not small plants and animals. Some RO manufacturers go so far as to recommend that their systems not be used in coastal waters because of the presence of abundant micro plant and animal life. Additionally, because pollutants, waste and silt are often found coastally, in crowded harbors and estuaries, this is probably sound advice.
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The system shares a strainer with a raw-water washdown pump. Normally, this is not a problem as long as the intake seacock is located well below the waterline. Once thoroughly filtered, feed water then enters the high-pressure pump. These vary a bit, but the net result is the same &mdash high-pressure water supplied to the membrane. These pumps may operate from a number of voltages or energy sources, including 12, 24 and 110 volts, as well as from a propulsion engine or genset via a belt or coupling.
Because of the corrosive nature of seawater, and the effects of cavitation and velocity erosion, the pump materials must be of the highest quality, 316 stainless steel, titanium, bronze and high-strength composites.
Once pressurized, the water then enters the now-familiar membrane. From the membrane, two plumbing lines deliver product water (fresh) and brine (concentrated seawater) respectively. The overboard discharge for the brine should be plumbed above the waterline for monitoring purposes and to reduce backpressure and siphoning. Some prefer to plumb this into the cockpit, just above a drain, so that any interruption of production will be noticed quickly, while others connect it to a galley or head sink.
Additional filtration may be employed at this point. Some systems offer charcoal post filters, to remove unpleasant odor or taste, as well as UV sterilization, which will kill the few remaining microbes that may have passed the membrane. A salinity meter and flow meter may also be installed at this point, to monitor salt content and product volume, respectively. Some manufacturers offer automatic diversion in the event an unacceptably high level of salt is detected in the product stream, to prevent contamination of the vessel’s tanks.
A drop in product flow, noticeable on the flow meter, usually graduated in gallons per hour, is often a warning of clogged pre-filters or a pump malfunction, which might require the attention of the crew. Production fall-off can also be caused by a drop in water temperature or an increase in salinity. However, these are usually minute changes; whereas, a filter full of plankton will have a noticeable effect on the product volume.
Product water is usually plumbed into the vessel’s water-tank vent or fill. Because of the possibility of inducing backpressure into the system, product plumbing should not be connected to main freshwater manifolds or into the bottom of a tank. Using a Y-valve to select which tank will receive product water is, however, acceptable.
Not mentioned in the above description of components are the myriad possibilities for flushing, cleaning and product-diversion valves, and plumbing. These all serve a useful and necessary purpose. Each manufacturer offers suggestions for these auxiliary components as to how and where they should be plumbed into the system. They will facilitate freshwater flushing, cleaning and troubleshooting of the system.
Maintenance
By far, the most common complaint heard about watermakers is their need for regular maintenance. To be fair, that has changed over the years. Systems have become easier to service and maintain while becoming more owner/operator friendly. The best maintenance advice regarding a watermaker is to use it. Watermakers are one of those pieces of shipboard gear that benefit greatly from regular use.
If you have yet to install or use your watermaker, be sure to create a start-up log. That is, when it’s first used, log the variables, feed-water temperature, battery voltage, amp draw, feed pressure and product-water flow rate. This will enable you to determine the health of the RO system down the road, warning of impending failure before it strikes, as well as serving as a troubleshooting tool.
Biofouling, one of the most common failures in RO systems occurs almost invariably because of disuse. This malady occurs when a watermaker is left idle and improperly preserved. Microorganisms present in all seawater &mdash and thus in the feed-water side of your RO system &mdash consume the available food, multiply and eventually die. Their colonies, both alive and decaying carcasses, are often quite noticeable, thanks to the dark color and putrid odor they emit. Alive or dead, it’s not something you want to have inhabiting your drinking-water supply. Additionally, severe biofouling can permanently damage the membrane (even the smallest membranes cost upwards of $500) Depending on the manufacturer and the ambient temperature (as any voyager knows, biological growth is accelerated in tropical climates), the flushing threshold may be as short as three days. Some RO users insist, erring on the side of caution, on flushing their membranes after every use.
The prevention for this disease is simply to fill the entire system, from the intake seacock onwards, with sterile water produced by the watermaker. The problem is you may need to do this if the system breaks down, and if a failure should occur, then you cannot produce the necessary sterile solution. Most RO-savvy cruisers carry a supply of sterile water in the event of just such an emergency. Depending on the size of the RO system, this could be between three and five gallons, or more for very large systems. In an emergency, this cache may be turned into a life-raft drinking-water supply.
A word of caution where flushing is concerned, many membranes have been permanently damaged in the flushing/preserving process thanks to chlorine. Chlorine is just as deadly to membranes as oil. If you fill your tanks dockside, from a safe, municipal water supply, it may be, unbeknownst to you, chlorinated. If you then use water from your tanks for backfilling the RO system, you may actually be doing more harm than good. This is why it’s best to use your own product water, freshly produced or from a clean tank that never sees dockside water, for a storage solution.
Because of the rapidity with which biological growth occurs in warmer climates, some folks opt to freshwater flush their systems before sailing into potentially heavy weather. The logic here is, once you’re in it, the last thing you may feel like doing is working on the watermaker. If an unfavorable weather report materializes, fill the tanks, flush the system and secure it for the duration. When it’s over, all you need to do is open the seacock, and you’re back in the freshwater-production business.
The freshwater flush is a short-term storage solution. For long-term storage &mdash more than a few weeks &mdash biocide-pickling solutions are available for the membrane itself. The primary filters can be changed and left dry to inhibit growth. Check your RO-system manufacturer guidelines for exact storage procedures and interval thresholds.
Some manufacturers offer an automatic freshwater flush option, which will automatically flush your RO system with sterilized product water at set daily or weekly intervals (one manufacturer of such a unit is Filtration Concepts Inc., Santa Ana, Calif., 800-850-0123, www.filtrationconcepts.com). If you just don’t want to be troubled with the flushing routine, or if you would like to leave your unit ready to use but unattended for long periods, this might be worth considering. It does, of course, add some complexity and expense to any installation. Spectra (San Rafael, Calif., 415-526-2780, www.spectrawatermakers.com), a well-known RO-watermaker manufacturer, offers the Zetaguard System, which they claim prevents biofouling and scale, eliminating the need for flushing, pickling or cleaning by electrostatically charging the entire RO system’s water content. Again, this feature adds complexity and expense; however, it may be a worthwhile investment for some installations.
Feed-water quality, as previously mentioned, has a dramatic effect on watermaker operation; however, it is also a maintenance issue. It’s worth repeating. Don’t run your RO system in waters that are of questionable cleanliness. Some RO operators only run their units when well offshore. Of course, if you are inshore and need water, go ahead and run your RO system. Be vigilant and watchful of the strainer, pre-filters and oil separator (if equipped) and note any drop in production, which often indicates clogging. If you intend to run inshore regularly, a 5-micron pre-filter and oil separator are must-haves.
If you are of the preventive-maintenance mind, consider carrying out a seal replacement every thousand hours of run time (if your watermaker doesn’t have an hour meter, installing one is easy and will facilitate scheduled maintenance). This is a relatively straightforward task, and if you do it yourself, it will enable you to become more familiar with the unit, should it require emergency repairs at some point.
A complete spares kit is essential for those venturing far from civilization. Even if you’re not traveling to remote parts, having your own spares will enhance self-sufficiency and perhaps save some of the cruising kitty as well. Spares should include several sets of filters (in my experience, polyester filters, while initially more expensive, will last longer than paper filters), biocide-pickling solution, seals, high- and low-pressure hose lengths, and end fittings. Some prefer to carry a spare boost pump as well.
In the event your RO membrane becomes fouled with biomass or calcium deposits, you’ll have to be prepared to perform a full acid or alkaline (or both, respectively) cleaning. Keep in mind, cleaning, particularly with alkaline solution, will shorten the life of the membrane, perhaps by as much as 10 percent with each treatment.
Installation
Where RO system installations are concerned, the old real-estate axiom, “location, location, location,” holds true, thanks to a large volume of high-pressure water contained within every unit. Consider this: If you were to install your membrane above a generator or alternator regulator (because of their sensitivity to heat, membranes should not be installed in engine compartments), and it leaks, the volume of seawater it could pump, perhaps in the form of a fine mist or spray, before you notice it, may be considerable.
Containerized RO systems offer all or most of the components packaged in a single frame, something akin to a generator. These systems are relatively easy to install. Essentially, you hook up water and power and go. However, they are sometimes simply too large to be accommodated by small- and medium-sized cruising vessels.
Modular systems, on the other hand, provide the freedom to pick the best or most available locations for each item. When choosing real estate for pressurized items (everything downstream of the boost pump), keep in mind the aforementioned potential for collateral damage in the event of a leak. Additionally, plan on spilling some seawater when replacing pre-filters and cleaning the strainer. Finally, the location of the flushing and cleaning outfall must be considered a potential splash hazard to any nearby equipment.
As mentioned previously, RO systems are available in several different power configurations. Smaller, 12- and 24-volt DC units are ideal for voyagers, while 120- and 240-volt AC models are more appropriate for larger genset-equipped yachts. A variation on this theme is to power the high-pressure pump from the vessel’s auxiliary engine (or genset) crankshaft, through a belt, much like a high-output alternator or refrigeration compressor. The logic for this setup is quite sound. If you must run your engine once a day to charge batteries, why not put some of that horsepower (even large alternators rarely use more than three or four horsepower) to use? Running a diesel lightly loaded &mdash a state of affairs that leads to cylinder-wall glazing, excess oil consumption and overcooling &mdash borders on sacrilege. An RO pump can help rectify this scenario by adding some load to the diesel’s workout schedule.
If, however, you opt for the 12- or 24-volt version, as most voyagers do, it’s best to run it while the engine is running during the battery-charge regimen. This will provide the watermaker with the highest possible voltage, which will enable it to produce more product in less time. Inductive loads, such as boost and high-pressure pumps, will also last longer if run at the higher limit of a nominal 12-volt system, 13.8 rather than 12.6 volts.
Finally, choose your watermaker’s capacity carefully. Unlike anchors, bigger is not always better where watermakers are concerned. Because watermakers suffer when sitting idle for even a short period of time, it’s best to size your system so that it has to run, under normal cruising conditions, once every one to three days. If your watermaker has so much capacity that you only need to run it every week to 10 days, then it is more likely to suffer from biofouling and scale buildup. You may even have to run it just to prevent this from happening, even if there’s no need for fresh water, which is economically unsound and wasteful.
Of course, this equation must also take into account tank capacity. However, if you design your system to work in such a way that the RO system runs regularly, it will be more reliable and long-lived. Most crews find that once the water supply is augmented by a watermaker, water usage increases (more showers, freshwater dishwashing and even decadent deck washdowns) two- or threefold. So consider this when making your selection.
Considering the size, simplicity and relative expense, RO watermakers are a truly remarkable development that has changed the lifestyle of extended voyaging immeasurably.
Contributing Editor Steve C. D’Antonio is a freelance writer and the boatyard manager of Zimmerman Marine in Mathews, Va.
