RO 101

Rich Boren of Cruise RO Water and Power describes for us the reverse osmosis process, pointing out a system similar to that which we installed on Kandu:

RO 101, an end-user’s perspective:

Disclaimer:  The detailed description and observations presented below make the operation sound more complicated than the practice.  Because the system is preconfigured and tested, the hard part is done.  Once properly set up, making water is simple.  AC power supplied, it only takes two to three minutes from start before fresh water is pouring into your tanks.  The freshwater rinse process following water making takes about 5-7 minutes from start to finish.  I’m fascinated by the engineering art that occurs behind the scene and like to share my understanding (albeit, likely flawed).  All in all, based on our current water consumption rate, we can make all the water we use in a week in about three and half hours.  While docked, we’ve been using about 100 gallons a week, but that excludes the showers we take and the laundry we do on land.  Don’t yet know what our consumption rates will be once untethered to land.  So, back to this layman’s behind-the-scenes understanding of water making . . . .

The reverse osmosis (RO) process starts with raw seawater and ends with fresh safe drinking water.  To begin with, the seawater sourced must be oil-free as oil will destroy the membrane, the core of any RO solution.  Fortunately oil floats.  Seawater extracted feet below the waterline (the further the better) will be oil-free.  To get the oil-free seawater up and into the RO system, usually requires a water pump, called a “boost pump.”   Inside the boat’s pump locker, Kandu has a slightly noisy 12-volt low-pressure water pump for the job.  They’re all a little noisy.  It pulls seawater from outside the boat through a through-hull.   Any hole, intentionally, installed in the side of a boat is called a through-hull.  The intake for the RO unit on Kandu is shared with the engine’s raw water intake (raw is another way of describing unfiltered seawater that comes from outside the hull).  Our engine’s raw water through-hull is located three feet below the surface, well below floating oils.  Before it can be pressed into freshwater, the seawater must have all debris and nearly all marine-life removed.  So with help from the boost pump, the seawater passes first through a large bronze screen plumbed just behind and below Kandu’s diesel engine, keeping the big stuff out.  Via hoses, the seawater is then lifted up and through two sediment filters, cylindrical cartridges of folded paper, one finer than the other. The first and more coarse of the two, filters up to 20 microns.  A micron is one-thousandths of a millimeter or 0.000039 of an inch.  To give some perspective, a human hair is around 90 microns.  The diameter of wool fiber used for making garments is less than 25 microns.  So 20 microns is small, keeping out most particles and plankton.  The next and finer of the two paper filters sifts out up to 5 microns, the size of a human blood cell, knocking out most of the remaining sediments and plankton, but not all bacteria. They get filtered out in the next phase.

The components of our Cruise RO SM-30 water maker (photo by Cruise RO Water and Power).
The components of our Cruise RO SM-30 water maker (photo by Cruise RO Water and Power).

After the filters, the boost pump pushes the thrice filtered seawater to the noisier high-pressure (HP) AC electric water pump (Kandu uses AC, many boats use DC electric pumps). Using back-pressure, this power-hungry device, drawing about 8 to 10 amps (equivalent to a toaster*), forces seawater through the membrane at about 800 psi (that’s equivalent to the force of an adult male sea lion doing a one-finger (or flipper) pirouette on a light switch!**).  When making water, the operator must first start the HP pump at its lowest pressure setting, waiting first for the boost pump to provide an adequate volume of seawater.  Once that is achieved, the operator adjusts the back-pressure (preventing the water from leaving the membrane cylinders) created against the force of the HP pump.  This is done by turning a knob, watching the pressure dial meter carefully, and adjusting to keep the pressure at a steady 800 psi.  At such great force, over-pressurizing the system could cause great damage to the components or the locker compartment where it’s installed.  The membrane is housed in a white plastic canister, capable of withstanding pressures of over 1000 psi.  Inside the canister, the membrane itself comes incased in an unpainted cylindrical fiberglass shell.  At its center extends is a dime-sized tube.  Inside the shell, the membrane skin wraps around this center tube. Between the fiberglass shell and the center tube is the space where the filtered water is pumped into the membrane’s container; seawater enters through eight circular openings that encircle both ends.  By restricting the exit flow at the outer edges of the canister, the pressurized water seeks a place to go.  At 800 psi, the osmotic pressure of seawater, about 20% of the seawater finds its way through the membrane filter and into the center channel tube.   Squeezing through the membrane, the filtered seawater is thus converted to bacteria-free “product,” otherwise known as fresh tasting drinking water. The rest of the unprocessed water, called “brine,” gets evacuated and directed back to sea.  On Kandu, the brine is directed from the RO membrane canister to Kandu’s starboard cockpit drain, where we hear it spit and pour as some of the brine sprays out and onto the cockpit sole (floor).   Kandu’s RO system control panel has a flow meter that measure the output rate of the product water in gallons per minute (gpm).  Filled with a yellow liquid, the meter percolates like a chemistry instrument, lifting a small flying-saucer type disk along a vertical wire.  Etched in the glass tube are marked the various flow rates, the smaller at the bottom; the greater, above.  At 0.5 gpm, the rated output of our model, we produce about 30 gal./hr; an enviable quantity of water for most any sailboat.

Just as with the back-pressure of the filtered seawater, the unrestricted flow of product water is equally important to configure.  With all that pressurized water looking for a place to go, the product water flow must never be blocked, or risk over-pressuring the system.  To direct its flow unfettered, only constant flow valves are installed along the product water’s exit path. With these valves, no matter which way you set the valve handle, you can’t shut it off: the water goes one way, the other, or both, but never stopped.

Once pressurized past 600 psi, the boosted, filtered seawater begins to push readily through the system’s RO membrane and out the “sample” spout, located on Kandu at the galley sink where it drains to the sea.  The system is easily brought up to the model’s preferred 800 psi.  The initial water coming from the membrane is a little salty, but quickly freshens up.  Before directing the product water into the ship’s tanks, the water is sampled, or tested, with a digital total dissolved solids (TDS) meter, mostly salt (NaCl).  Power it up, drop the bottom end of the meter about 1/2″ into a small sample of the water and the meter provides an instant reading. Once the produced water measures less than 500 ppm, the minimum allowed by the EPA (other countries accept higher salt levels), the water is sent to the tanks by turning two valve sets: one re-directs the water from the sample waterspout to the second valve, which in turn is set to direct the flow to whichever tank the operator wishes to receive the freshly made water, either port or starboard.  As product water continues to flow, the TDS reading quickly drops to below 150 ppm, the soft water range.

The system runs for whatever time the operator feels necessary.  Between the generator and the pumps, the operation is noisy enough that it should only be performed when neighbors would typically be awake.

Shutting down the system occurs in two phases: the saltwater stop and the freshwater rinse.  Before shutting down the system while seawater is still being processed through it, the product water is redirected to the sink and the HP pump dialed down to the lowest pressure possible.  Then the high-pressure pump is shut off, followed by shutting off the low-pressure boost pump.  The seawater intake valves are turned off and re-configured for the freshwater rinse.  At this point, the two paper saltwater sediment filters and RO membrane are saturated in seawater, with all its lovely marine life.  Saltwater doesn’t harm the filters or the membrane.  It’s the marine life that does.  Left un-rinsed, the microscopic organisms will eventually die and be consumed by anaerobic marine bacteria that will off hydrogen-sulfide (H2S) gas, a corrosive environment for RO elements, producing water that tastes like rotten eggs.  To prevent this unpleasant biological reaction, freshwater from the ship’s tanks is redirected into the system to flush its components.  Because chlorine is destructive to the RO membrane, an active carbon filter is installed in-between the freshwater tanks and the RO’s boost pump, ensuring that no residual chlorine that may have been added to the tanks by the cruiser to kill algae or bacteria growth (which occur naturally in stored freshwater) comes in contact with the membrane.  So as part of the RO systems maintenance, it’s important to remember to change the carbon filter at least every 6 months.  Just as with the pulling/pushing of seawater into the RO system, so too does the operator engage the pumps to pull/push freshwater.  The RO system’s internal intake valve is turned away from the seawater intake and toward the freshwater intake.  With the HP pump off and at its lowest pressure setting, with the product water directed to pour into the sink through the sample spout, the boost pump is again engaged.   Again, once it’s flowing to where the operator hears a steady flow of waste/brine water, the HP pump is activated with the pressure setting still at its lowest setting.  Freshwater pushed through the membrane at high pressure would damage the membrane so the HP remains at its lowest setting throughout the freshwater rinse.  After pulling/pushing freshwater for about 3-5 minutes, the saltwater around the paper filters and RO membrane has been replaced by fresh and the system can be shut down: first turning off the HP pump, then the low pressure boost pump. The intake valve is shut off. The rinse stage complete.

In the above state, the system can be left idle for 3-7 days (the greater the marine life, the earlier the interval) before it must either be put into service to make water and rinsed again, or can just simply be rinsed again with freshwater, without making water.  If for whatever reason a cruiser will not be engaging the RO system for an extended period of time, she can saturate the components in a food-grade antiseptic by employing a process called “pickling.”  Pickled, the unit can be held in stasis for six months or more before it should be re-picked or brought back into service.  A properly maintained membrane can last as long as 10 years before the product water begins to taste not-so fresh.  The paper sediment filters are changed regularly too.  In addition to the footprint an RO system occupies, supporting a water maker also requires stowage space for the three types of water filters, pickling power, and crank case oil. Even though RO water is free of bacteria, without a disinfectant in the water storage tanks, algae and bacteria can grow.  For this reason, it’s prudent to introduce a product like bleach (1 tsp per 10 gallons) into the tanks, and then filter out the chlorine taste by passing the sanitized water through a carbon filter before drinking. Ultra-violet therapies are also available.  On Kandu, our carbon filter has also a KDF*** element to remove heavy metals and kill bacteria.  Between the financial costs, the 3-7 day interval, the stowage burden, and the noise factor, watermaking is not every cruiser’s cup of tea.

For those willing to accept the burdens that come with the freedom of converting engine time into safe drinking water, several makes and models of RO watermakers exist, in both DC or AC versions.  Whether DC or AC, electric motors that pressurize seawater consume a lot of electricity  To protect the boat’s house batteries from being drawn down too quickly, when making water, sailors support the electrical demand by either run the boat’s engine, an electric generator, or are connected to shore power. In essence, desalinators convert a petroleum fuel (gasoline or diesel) into freshwater. For the freshwater rinse, a cruiser with large batteries supported by passive charging sources such as solar panels and wind generators, may be able to get by without having to start an engine.  The units typically found on a cruising boat Kandu’s size use DC electricity. They employ specially designed, expensive proprietary components, and produce around 6-8 gallons of water an hour.  They cost about $5k-$6k. Some units are more automated and quieter than others.  For Kandu, we went with the SM-30 by Cruise RO Water and Power, a non-propertary dual membrane AC model that efficiently outputs an impressive 30 gallons an hour.  As I understand it, just idling or running free (not under the load of a spinning propeller) a diesel engine, it doesn’t get hot enough to burn off the carbon that otherwise builds up inside its cylinders and injectors, gumming them it up. So, to prevent possible problems with our boat’s auxiliary (another name for a boat’s internal engine), we felt it wise to buy a Honda generator to support the electrical needs of the HP pump’s AC motor when making water.   For the freshwater rinse, we’re hoping our large battery banks and passive energy generation will support the occasional rinse cycle. The cost including the generator was just north of $6k.  We’re excited and pleased to have it.  Leslie set our smart phones to remind us to run the unit every three days (it just went off as I wrote this) The freshwater rinse process takes less than 10 minutes to complete.  Overall, so far, and it’s early yet, the watermarking and rinsing processes have been painless, and in an odd way, rewarding and liberating.


* 10 amps at 120VAC is equivalent to 100 amps at 12VDC.  Batteries should not be drawn down below half their capacity. At 900 amp/hrs. total, Kandu has an available draw of 450 amp/hrs.  Were we to run the water maker strictly off our batteries, we would in 4.5 hours deplete our “house” batteries (the term used to describe a boat’s main battery bank, supporting all but the engine’s starter) to a level that would normally take 5 days, a rate faster than the house batteries are designed to support.

** While researching 800 lbs animals, I learned that the testicle of a right whale weighs 1100lbs!

*** “Kinetic Degradation Fluxion (KDF) is a high-purity copper-zinc formulation that uses a basic chemical process known as redox (oxidation/reduction) to remove chlorine, lead, mercury, iron, and hydrogen sulfide from water supplies. The process also has a mild anti-bacterial, algaecidic, and fungicidic effect and may reduce the accumulation of lime scale.” –-Home Plus Water

A box of "pet food" awaits Kandu.
A box of “pet food” awaits Kandu’s water maker.

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