Clean Drinking Water on Board - Part 1, Watermakers for Boats
How do watermakers work?Watermakers use Reverse Osmosis to make water. This is the opposite process of osmosis that plants use. Wiki definition of Reverse Osmosis and description; Reverse osmosis (RO) is a separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. The membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2 to17 bar 30 to 250 psi, for fresh and brackish water, and 40 to 70 bar 600 to 1000 psi, for seawater, which has around 24 bar 350 psi, natural osmotic pressure which must be overcome.Types of watermaker Modular, Self Contained or Portable units are made for use on boats. For very small boats or even kayaks portable units are the way to go. For most boats however modular systems are best. Modular systems can be mounted in individual parts and do not require one large space, but several much smaller spaces. Most manufactures produce self contained units that house all the parts of the watermaker between the through hull and the water tanks. These setups are easily controlled but require a large amount of space.Components of a typical modular system A typical installation involves a seawater supply, preliminary filtration to remove weeds and large contaminants, a low pressure pump to push the water through the particle filter which remove particles down to 5 microns, a high pressure pump to supply the RO membrane. Fresh water from the membrane is then sent to the water tank. For every gallon of fresh water produced roughly 10 gallons is required to pass through the membrane, the remainder goes overboard in the form of brine. Hose runs should be minimal especially in low energy systems.Power to the pumps, can be supplied by battery, generator or engine driven pumps. Large capacity watermakers generally need an AC power source. Smaller capacity and low energy watermakers can use DC AC or engine driven pumps.How much water are you going to need, what size watermaker do you need? Calculate how much water you need by estimating your daily usage/person/day. Older estimates allowed for a minimum of 1/2 to 1 gallon/person/day. This rate does not include showers, dishes and other extras. Nowadays with modern conveniences its more like 2 1/2 to 3 galls/person/day. Work out how much you want to run the watermaker. You don't want to run it all day, but you may want to run it while the generator is on, while the engine is on and your charging your batteries. So maybe 2-3 hour per day run time is reasonable. Tank size is also an issue. Its no good making 400 gallons if the tanks are not big enough. Normally watermakers give their estimated outputs based on 70 degree F. Colder water temps may produce less water.Maintenance The membrane needs to be flushed regularly to remove the particles and salt that have been built up on the supply side of the membrane during the RO process.Conclusion You now have fresh water, what no, what do you mean no. Water from the membrane goes into the water tank however there maybe contaminants in the tanks and pipes. Read part 2 of the making fresh water series to find out how to keep water fresh in the water tank and water lines before use.