Clean Drinking Water On Board - Part 1, Watermakers For Boats

How do watermakers work?

membrane pump

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.

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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 thru 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.

Seawater supply is achieved via a Thru hull fitting; It is important to have good water flow through the thru hull fitting, by having a forward facing scoop type thru hull as low as possible in the boat. The intake should not be near heads or grey water outlets. Preliminary filtration is achieved by a seawater Strainer; A typical strainer will remove large to medium size debris that comes in with the sea water. The strainer needs to be inspected so it needs to be accessible. Do not install near electrical equipment as water can spill during strainer cleaning. The low-pressure feed pump is to ensure that the high-pressure pump is always supplied with an adequate flow of water. It should also be mounted low in the boat. Particle filtration; The raw water-supply system must remove any particulate matter that could damage the high-pressure pump. Most systems use two filters, which are fitted with progressively finer filter elements. 30 micron and 5 micron filters are typical. The performance of the RO membrane can be degraded by oil, so many systems also include a separate oil-elimination filter. High pressure pump; This pump needs to build up the pressure to around 800 psi to push the water through the membranes. The pump requires a lot of power to produce the 800 plus psi. Two different types of pumps are used in sailboat RO systems: electric-motor-driven plunger pumps similar to those used in pressure washers, but marinised, and hydraulic amplifiers, pumps that amplify relatively low-pressure seawater and obtain the 800 plus psi pressure required for successful RO operation. Membrane; The key element of the RO system, the salt-separation membrane, is semi-permeable, its small pores removing anything other than water molecules. The membranes used in seawater RO systems are usually made of a TFC (thin film composite) membrane consisting of three layers, of spiral wound sheets of PA polyamide. After the membrane the product water is plumbed to the vessels storage water tanks, the by product is brine which is discharged overboard. The brine discharge should be above the waterline so you can monitor the discharge.
Power

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.

AC power supply; Most of the power consumed in an RO system is used to power the high-pressure pump. Large-capacity systems are typically powered with 120- or 220-volt-AC motors and can require generators capable of delivering at least 3 to 4 kilowatts (to handle the motor starting load). On a boat with a genset which is run often or continuously a relatively small watermaker can produce the required water. If the genset is run only occasionally a larger capacity watermaker is needed. DC power supply; Smaller low energy systems can be driven with 12- or 24-volt-DC motors, making them more suitable for typical cruising boats. Selecting the optimum system requires consideration of the vessel's power capability and the number of hours per day the system will have to be operated to provide the desired quantity of water. Running the DC watermaker at the same time as the batteries are being charged will help output of the watermaker. Make sure when you are researching a DC watermaker that the DC system on your boat can handle the loads. Battery capacity may need to be increased
Size Matters

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.

Membrane; Watermakers like to run often even daily for a couple of hours. Running the watermaker like this coupled with automatic back flush, which takes the minerals salt etc off the membrane surface, keeps the membrane clean and in good working order. A membrane not cleaned in a week can get bacterial fouling. If you do not use the watermaker in a week you should flush the system or put it into storage mode which involves pickling. Membranes should not be stored dry. Membrane Flushing; Some systems have an automatic flush system built in, other smaller units require manual flushing. Some systems may have a separate tank for water for flushing. Manual flushing of the membrane requires switching the valves on either side of the membrane, the inlet side and the discharge side, and then turning the unit on to circulate clean water. Many larger systems have an auto flush mode. This can save lots of time and convenience. Flushing water needs to be free of chlorine and so a carbon filter is recommended between the tank containing the flushing water and the membrane. Chlorine can react with some cleaning chemicals. Membrane Storage also called Pickling; The membranes should be permanently immersed in liquid: either sea water before treatment, fresh water provisionally stored or sterilizing liquid if the water maker is not used for extended periods of time Pickling involves special biocides which are mixed with the product water and pumped into the system leaving the membrane saturated. Use the manufacturers recommended biocide. Temperature; The membranes should not be exposed to temperatures freezing. Overpressure due to expansion caused by freezing can rupture the membranes and prevent the salt from being filtered out. The membranes must not be exposed to temperatures above 60°C as high temperatures may also prevent salt from being removed. Brackish water; Watermakers are not recommended for use in muddy or polluted water as the pre filters get clogged and damage the membranes. If you do use the watermaker in these waters only run it for very short periods: as soon as clean sea water becomes available clean the membranes and run the system without pressure for 30 minutes with the pressure regulator open. Spare parts; Carry plenty of spare filters. Filters last 6 months after their first use. Also carry spare parts for pumps etc especially if you are going to be away from land or good stores. Filters; Regularly check the sea water strainer and the two particle filters.

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.

Clean Drinking Water On Board - Part 1, Watermakers For Boats

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