By Tom Burden
The modern watermaker uses a modest amount of energy to turn undrinkable salt water into pure freshwater. A well designed, properly maintained watermaker can significantly increase safety and comfort while cruising. A watermaker is an appliance that requires a bit of maintenance, on a regular basis, for its continued operation (as is noted below). If you become familiar with the operation of the device, use it every day and keep its components in proper working order, a watermaker is truly a miracle machine.
Reverse osmosis watermakers produce fresh water by pressurizing salt water to around 800psi and forcing it through a semi-permeable membrane. Water molecules can pass through, but practically all contaminants (such as salt, bacteria, viruses, grit, etc.) are left behind. The remaining “brine” (concentrated seawater and contaminants) is discharged overboard, and a new batch of salt water is introduced, in a continuous process that turns 10% of incoming salt water into fresh water, while 90% is discarded.
Katadyn developed an energy-saving method by using the rejected brine to assist in pressurizing the incoming water. This energy recovery principle makes it possible for a manually operated watermaker to produce enough water to hydrate 25 people. The original product, the Survivor 06, led to the introduction of other manual and electric watermakers featuring this technology, which greatly reduces energy use compared to older watermakers.
Beside the desired output, a critical decision concerns how the watermaker will be powered, by 12V or 24V DC, 120V AC or driven by a belt from the engine.
DC watermakers are the easy choice on small boats because of their compact size and manageable current draw (though you may want to run the engine while making water). 120/240V AC and engine-driven systems have higher capacities, and their modular design allows components to be installed separately. But AC systems also have certain drawbacks.
Engine-driven pumps are about the size of a big compressor and may not fit inside the engine compartment of your boat. However, if you can fit an engine-driven pump, you can make water whenever you’re running under engine or at anchor when you’re charging batteries. Because diesel engines don’t like to be underloaded, adding a watermaker pump to the alternator’s load may actually benefit your engine.
120V AC watermakers can be run from either an auxiliary generator or a big inverter with other AC loads to give the genset some additional burden. A better alternative, at least when the generator isn’t required for other tasks, would be to use either a high capacity 12V DC watermaker like the Katadyn PowerSurvivor 80E, or to use a smaller AC watermaker which could be run off of the ship’s inverter. Starting up the generator to make water and adding AC loads to “exercise” it is not ideal and emphasizes the importance of planning and integrating your ship’s systems.
The World Health Organization recommends at least one liter (approx. one quart) of fresh water per person per day to maintain hydration. Obviously, that figure is for survival conditions, not summer vacation. A freshwater consumption of two quarts per person per day is more realistic, assuming that this is augmented with other liquids and that normal water conservation methods are used (saltwater showers, rinsing veggies in salt water, etc.).
Cruiser Linda Dashew stated that her family of four used 5 gallons of fresh water per day initially, but graduated to 12 gallons per day as their children grew and their water storage capacity increased. Your water use also depends on the climate and the kind of activities you perform during the day. The level of perspiration when sitting in the shade of the bimini is quite different from grinding the genoa winch all day long.
These relatively modest amounts are based on necessity, as opposed to desirability. On a luxurious vessel with a large watermaker, a crew of five or six might use 80 gallons per day, taking hot showers, running the washer-dryer and dishwasher. The bottom line is that your water consumption can vary dramatically based on your lifestyle and your ability to produce water.
With increased output comes increased current draw, so plan your electrical system accordingly. Using four gallons per person per day requires about 150 minutes of watermaker operation (PowerSurvivor 40E), or half that much using the PowerSurvivor 80E, equaling roughly 9Ah of battery capacity consumption per day per person. Using these examples, small cruising boats can function comfortably with a PowerSurvivor 40E. When consumption exceeds 12gpd per person, we recommend the PowerSurvivor 80E.
Ideally, you’d run your watermaker when the engine is on for propulsion or battery charging. Anywhere from two to eight hours of operation should cover your needs. This ensures that the voltage is high, and that the energy goes directly into the watermaking load without being stored in the batteries. Watermakers—unlike household water filters—require TLC to perform reliably.
Watermakers are designed for use in clean seawater, not harbor water or tap water. The maintenance intervals given will drop if your watermaker is used in other environments and the following conditions:
When choosing a location for the watermaker drive/pump/membrane assembly:
In addition, you should:
Background: maintaining a healthy watermaker largely involves taking proper care of the pre-filter assembly and seawater intake plumbing. Failure to do so is the most common cause of the two most frequent types of watermaker “failure” we hear about: (1) producing diminished or no freshwater output, or (2) producing “bad-smelling” product freshwater.
Here is what happens:
No freshwater output: The most common cause of diminished or no product freshwater output is air entering the seawater intake system at some point. The pump volume is small and the pressure required to press water through the membrane is rather high (about 800 psi). Since air is highly compressible, a very small amount of air can keep the pump from producing enough pressure to produce product freshwater. Periodically inspect and test the entire seawater intake system to assure that all joints and fittings are airtight, especially the connections at the pre-filter assembly.
Note: Be aware that a stable air gap at the top of the pre-filter housing is not uncommon, and doesn’t necessarily mean that air is getting to the pump itself.
Bad-smelling product freshwater: The purpose of the pre-filter assembly is to trap any particulates in the intake seawater that are larger than 30 microns. A coarse strainer (if installed) performs the same chore for contaminants of larger size. In each case, trapped material remains in the pre-filter housing (or strainer bowl) until removed.
Much of the trapped material is organic: plankton, seaweeds and flotsam of all types. After a watermaker has been turned off, this material soon begins to decompose. As it does, it breaks down into a number of chemicals composed of smaller molecules. Some of these molecules are small enough to pass through the watermaker membrane along with the product freshwater. Perhaps the best-known example of such a chemical is hydrogen sulfide, a gas which (in small concentrations) smells like “rotten eggs.”
Two main factors affect the speed with which these products of organic decomposition will contaminate a watermaker system: (1) the ambient temperature and (2) the quantity of trapped material. We realize that many boaters run their watermakers in near-shore situations while anchored. The amount of trapped material is usually high in such locations, and the pre-filter assembly will require more frequent attention. Moreover, the high ambient temperatures in tropical locations greatly accelerate the rate of such decomposition. Users in temperate climate areas or users processing open-ocean seawater during offshore passages are less likely to require the same diligence.
Biocide: a chemical used to inhibit biological growth in the reverse osmosis membrane during storage or extended periods of non-use.
Filter: a device that removes suspended solids from a fluid stream. A filter is not the same as a reverse osmosis membrane.
Micron: a metric unit of measurement equal to one thousandth of a millimeter, or one millionth of a meter.
Osmosis: the process of diffusion between two water sources through a semi-permeable membrane, resulting in both water sources striving to reach equilibrium in the concentrations of dissolved substances.
Pickling: a slang term for the process of flushing the membrane with biocide solution (see Biocide).
Potable: suitable for drinking, especially water. With regard to a desalinator, it is generally defined as water with 1,500ppm or lower of dissolved solids that is relatively free of harmful microorganisms.
PPM: parts per million. Used as a measure of the quantities of substances dissolved in water.
Pre-filter: a filter placed in a desalinator system to remove suspended solids from the feed water before it reaches the high-pressure pump and reverse osmosis membrane.
Pressure relief valve: a valve that relieves pressure in a system to prevent damage to system components. In the PowerSurvivor 40E, the relief valve is set to open at 1,000psi, to prevent damage to the pump and membrane.
Product freshwater: potable water produced by the reverse osmosis process.
PSI: pounds per square inch—a unit for measuring pressure.
Reverse Osmosis (RO): a reversal of the natural osmosis process between two fluids that occurs when water is forced under pressure through a semi-permeable membrane. It produces potable water.
Salinity: a measure of the amount of salts, minerals, and other dissolved solids contained in a water source. Salinity (see TDS) is measured in ppms.
Salt: the common name for the chemical sodium chloride. Open-ocean seawater is about 3% dissolved salt, or about 33,000ppm of sodium chloride.
Seal: a device (such as an o-ring) used to prevent fluid leakage between two system components.
Semi-permeable: the characteristic of some materials (e.g., a reverse osmosis membrane) that makes them relatively porous to some substances while blocking the passage of others. Unlike normal filters, semi-permeable membranes usually operate at the molecular or atomic level, allowing much finer “filtering” than simple mechanical filters.
TDS: Total Dissolved Solids—a type of measure commonly used to indicate the purity of water, usually expressed in ppms. A standard TDS meter measures only the electrical conductivity of the water being tested. Therefore, it is only able to measure dissolved substances that are ionized in solution (e.g., sea salt). In particular, they do not indicate the presence of non-ionized substances, such as bacteria, viruses and many other soluble chemicals.