Article by Capt. Steve Katz
“Water, water, everywhere, and not a drop to drink.” This quote by Samuel Taylor Coleridge 200 years ago still holds true today. We still venture out on the ocean with no easy way to make drinking water. A boat trip involves a checklist that often includes, bait, fishing gear, food, fuel and water. Whether a short trip or an offshore adventure, having fresh drinking water on-board is a necessity.
While many boaters fill their water tanks before they leave the marina, some require more water than they can carry for the voyage. Whether it’s showering, using the toilets or washing the boat, fresh water can be consumed very quickly. Back on land, the average American uses about 100 gallons of water a day. Even without this level of consumption, unless you are really frugal, your boats freshwater tank can run dry faster than you expect.
With all that seawater around, how can we convert it to freshwater? We can desalinate it. The process of desalination is any process that removes salts and minerals from saltwater, with distillation and reverse osmosis the most common processes.
Distilling water is the oldest and most common method used to remove salt from water. In simple terms, distillation involves heating and evaporating the water, and then condensing the steam back into a liquid. The salt will stay behind when the water boils and the clean water is collected in a separate container. As an extreme example, the US Navy ship, U.S.S. Carl Vinson, makes 400,000 gallons of freshwater per day by distillation.
Another method, reverse osmosis (RO), uses pressure to force water through a semi-permeable membrane, straining out other substances at the molecular level. Developed in the 1960’s, the process became feasible on a commercial scale in the 1970’s, ultimately replacing distillation as the method used in most new desalination facilities, in part because it requires less energy than distillation. Besides removing salt, both methods remove virtually every mineral and most biological and organic chemical compounds, producing water that is safe to drink, far exceeding federal and state drinking water standards.
Desalination of saltwater for pleasure boats is usually accomplished by reverse osmosis (RO) equipment. A RO system or “watermaker” will consist of a booster pump to supply water to the high pressure pump. The high pressure pump then pressurizes the seawater to 600 to 1,000 psi and forces the water through the RO membrane. The RO membrane is like a filter, only allowing water molecules to pass through, leaving behind a brine mixture of salt and minerals. About 90% of the water pumped through a RO system is discharged back to the sea as brine and the remaining 10% becomes clean pure freshwater.
Sounds simple? The process is basic, but the equipment can be complex. Pleasure boat systems come in many styles and capacities. They are rated by the daily water making throughput capacity, meaning how much water can be made when running the system for a 24 hour period. If you have ever been on a boat with a watermaker, you know when it’s running. It’s often loud and vibrates through the boat, almost like a main generator. It is the high pressure pump that consumes the most energy and makes the most noise. These pumps are similar to a large commercial power washer pump and require a lot of electricity.
While there are some RO watermakers that have the pump operated by the engine, most systems are run with electricity, usually requiring 240 volts, though some small system run using 12 or 24 volts of DC power.
A RO watermaker also requires routine maintenance. The RO membrane is very sensitive and expensive and works best when used daily since this inhibits biological growth within the membrane. It is best to flush the system with non-chlorinated freshwater (not city water) after each use, as chlorine will damage the membrane. If the watermaker will be sitting unused for a week or longer, it should be "pickled" with a biocide to keep bacteria from growing in membrane.
Watermakers have other filters that clean the water before it gets to the RO membrane. These are called pre-filters. Most systems contain at least two pre-filters, but often more depending on the environment. Many units use a 30 micron and 5 micron pre-filter to capture the silt and organic matter. If operating in oily waters, special oil removing pre-filters are important since oil will damage the membrane. Prefilters need to be cleaned and replaced on a regular basis, lasting about 6 months, depending on usage.
Most watermakers have some sort of automated system to monitor the water quality of the freshwater or product water, only allowing high quality water to enter your tanks while low quality water is discharged back to the sea. This is most often done by measuring the TDS or Total Dissolved Solids in mg/liter, otherwise known as parts per million (ppm) in the product water. The EPA indicates that water above 500 (TDS) should not be consumed. Many health specialists believe that ideal drinking water should be under 50 TDS and end user tests on popular bottled water showed most to be below 100 TDS.
Watermakers are available in many styles and capacities, with the smallest manually operated survival unit making about 6 gallons a day and sells for $1,000. An 80 gallon/day, 12 volt electric unit costs about $4,000 and a 1,200 gallon per day unit runs about $12,000.
Before you purchase a watermaker be sure to consider the amount of water you will need on your trips, the ability to store clean water and the ability to generate the electricity to run the watermaker. Popular watermaker manufacturers are: FCI, Watermakers Inc, Village Marine Tec, Dometic, Spectra, Sea Recovery and others. While a watermaker may be a significant purchase, getting access to clean and safe water is not always easy or cheap and making your own is often the most reliable and safest method of keeping your water tanks full.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers.
