Frequently Asked Questions


Here are answers to some of the questions I'm most frequently asked during my seminars. Keep in mind that these answers are my own personal opinions, based on many years of using Katadyn watermakers in a wide variety of situations. In particular, my statements on these subjects do not necessarily reflect the official opinions of Katadyn or their company representatives, unless I've explicitly stated otherwise.



When is it not safe to run my watermaker? There is no unequivocal answer to this question. The potential for damaging the membrane exists to some degree virtually everywhere. It's really a matter of understanding the vulnerabilities of the watermaker and the odds involved in various situations.

There are two things that are well known to cause damage to the watermaker:  exposure to chlorine and petroleum products (e.g., oil, gasoline, petroleum-based lubricants). Either of these can cause extensive and permanent damage to the membrane element. Be aware that chlorine can cause damage even when only present in trace amounts—the membrane is extremely sensitive to chlorine.

The most common source of chlorine damage is inadvertent exposure to chlorine-treated freshwater, such as is typically found in tap water or dockside water supplies. A common scenario for this is the owner who decides to run dockside freshwater through the watermaker as a "temporary flush,"  usually in an attempt to avoid a biocide (pickling) operation. It is true that a freshwater flush can stall the need for a biocide treatment for a few days, but use product water, not dockside water.

Contamination from petroleum products has two common sources:  (1) using a petroleum-based product to lubricate the watermaker or (2) contaminants in the intake water. The latter is the reason owners are advised not to run their watermakers in harbors, marinas, or other populated areas with an enclosed water space. In general, this is a good precaution to observe. However, it's still a matter of the odds. Often the watermaker may be run in a marina with no threat of damage most of the time. It's just that the odds of exposure to contaminants at some time is much higher. If the watermaker is run in such an environment on a regular basis, the odds are quite good that damage will happen sooner or later.

On the other hand, don't be too obsessive about this. I know of several instances where owners have installed new watermakers while in their home port and didn't perform a test run for fear of damage from contamination. In one case I recall, the owner didn't run the watermaker until he was in Mexican waters, which was when he finally learned that there was a problem. If you've installed a new watermaker, you should test it, even if you're in a harbor. Before you run it, look over the side of the boat and check the water quality. If things look good, running the unit for ten minutes or so is not likely to cause a problem.

Sooner or later, you're bound to meet an amateur physicist who will assure you that gas and diesel fuel is lighter than water and, therefore, will float on the water surface and will not get near your watermaker intake thruhull, which is typically far below the waterline. That's true, as far as it goes. But, consider your own observations in a marina. What happens when a nearby boat owner accidentally spills some fuel overboard? Does he do the "politically and morally" correct thing, which would be to immediately call the Coast Guard and report the spill, and then mortgage his home to help pay the fine before he goes to prison? Not likely—I suggest he'll probably grab a bottle of dishwashing detergent and quickly squirt a stream of it onto the spill. The slick immediately disappears like magic. But it doesn't disappear into absolute nothingness. Instead, it's turned into an emulsified cloud that drifts with the currents beneath the water's surface. It could easily drift in the direction of your watermaker intake and—presto!—you're soon digging out your checkbook to pay for a new watermaker membrane. Be wary of such "physicists" and use common sense.

A last word about the odds—although they are very good that you will be safe running your watermaker when at sea, it's still not a 100% sure bet. I'll give you two instances of how things can still go wrong. Near our home port of Santa Barbara, California, there's a place on the coastline called "Coal Oil Point"  Its name is appropriate. There is extensive natural seepage of petroleum from the sea floor in the area. Huge oil slicks extend many miles out to sea and can obviously cause problems. It's so pervasive that I once painted the bootstripe on my boat black, to mask the tarry crud that accumulated on the waterline.

I also recall a cruiser in Mexico who reported in on one of the Ham SSB nets, asking for advice. It seems he had sailed behind a pod of whales, some of whom were indiscreet enough to relieve themselves as he passed. He had sucked up a large quantity of whale excrement into his watermaker. The entire intake system was a gooey mess and he wanted to know what kind of solvent he could use to clean his system. Unfortunately, nobody had an answer for him. Like I said at the beginning, you're always playing the odds.

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Can I use my watermaker in freshwater?  The short answer is—yes. There's nothing about this subject in the Katadyn Owner's Manual, but I've had verbal confirmation from the Katadyn engineers in Switzerland:  processing freshwater will not damage a Katadyn watermaker.

At the present time, I am not aware of any competing brand of watermaker whose manufacturer recommends using their watermakers in freshwater; and the common wisdom among owners of those brands is that freshwater input is not allowed. I suspect that this may be because most other brands of watermakers have pressure regulators which the user can adjust. Watermaker membranes can be damaged by internal pressures that are too high and, because of the complex physics of reverse osmosis, the chances of overpressurizing the membrane are greater when processing freshwater. It is possible for a user of other brands of watermaker to accidentally damage the membrane by adjusting the working pressures, even when using a setting that would be perfectly O.K. when processing seawater. Because of this, the rumor among cruisers is that watermakers should not be used to process freshwater. It may be that Katadyn watermakers, with their system of automatic internal pressure regulation, are the only exception to this general belief. As far as I know that is, indeed, the case.

I became interested in this subject after reaching the Caribbean side of the Americas and while spending a hurricane season at Mario's Marina in Rio Dulce, Guatemala. The marina is about twenty miles up the river, far from the ocean. I've never taken on dockwater while cruising foreign waters and didn't want to start there. So I ran my watermakers (a PUR Model 35 and a Katadyn Model 40E) while tied up to the dock for almost six months. A couple of years prior to that, I had run my watermakers in the estuary at Bahía de Carácas, Ecuador, for six months. The water there is a dirty mix of fresh river water and seawater.

Although Katadyn watermakers will work well with freshwater input, I have a word of warning. Most freshwater situations should be considered "enclosed water spaces." In such locations, especially in third world countries, rivers are often used—legally or otherwise—as sewage systems. The more populated areas there are upstream from your location, the more likely you will be exposed to various kinds of contaminants—everything from human waste to petroleum products to pesticide runoff. Your chances of experiencing problems from such sources are significantly greater than when in the open ocean. Keep this in mind.

In my own case, to be honest, I had to replace my membranes after six months making water in the Rio Dulce. The membrane elements became slimy and over time the TDS readings rose gradually to the 6-700 ppm range. Bottom line: the problem isn't freshwater, per se, but what other things are likely to be found in the freshwater.

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Is it safe to use product water in my batteries?  I've been using my product water to top off the electrolyte in our batteries for over ten years, with no problems. I've experienced completely acceptable battery life (and then some) during all that time. I use Trojan T-105 6-volt batteries for our house banks.

I should add that, by conventional standards, I could be accused of abusing my batteries. Since we often are at anchor, without access to shore power, the batteries seldom get a full charge. However, I monitor them closely and never discharge them below approximately 25% of their rated full charge. I consider four years to be a good estimate for the average life of the ship's batteries. That estimate works well for quality batteries, but I wouldn't expect that kind of life from the batteries you can buy from West Marine. (O.K., that's admittedly a personal opinion, but it's based on my own experience—call it an anecdote.)

The common wisdom is that you should not use tap water for topping off your batteries. In general, this is a rule you should follow. The problem is the unknown quality of the water. Local water quality varies dramatically from one area to the next. The threat to batteries comes mainly from the possibility of contaminants such as chlorine, metal ions and other substances in the water. These can combine with the sulfuric acid in the electrolyte, thus diminishing its effectiveness. (On the other hand, I would never hesitate to use local tap water in a one-time emergency scenario.)

Since the RO watermaker does a good job, for the most part, of removing metal ions and other larger molecules, it produces product water that shouldn't do any harm to your batteries. If you're hyper-concerned about this matter, you can easily use your watermaker to produce some grade A+++ battery water:  run a couple of lengths of hose from your intake and reject hose barbs into a bucket of product water and run it through the watermaker again. You'll end up with water so pure (less than 5 ppm) that you'll hate to waste it on your batteries.

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Are important trace minerals removed?  Yes, a reverse osmosis watermaker does a pretty good job of that. The implied question is really whether or not this represents a possible health hazard since you won't be getting the minerals and other trace elements you normally do in your drinking water. This is another situation where using a little common sense can be very helpful.

As I've mentioned elsewhere on this page, the quality of local sources of drinking water varies greatly from one area to another. The fact is, in most instances, you're likely to be getting an excess of a few things and little or none of many other trace elements you need. We actually get most of our trace elements from the food in a normal healthy diet. Many people make sure of this by taking mineral supplements. The bottom line is:  you're likely to be treading on thin ice if you're depending on your drinking water to provide all the minerals and trace elements you need to stay healthy.

Nevertheless, this is a complicated topic of interest to all of us. As a result, I did some research on the internet a while back. One of the best documents I found is a draft report issued in 2004 by the World Health Organization on this very subject. For more detailed—and informed—opinions concerning this matter, I strongly suggest you click on the preceding "draft report" link and read it. However, be advised that it is a "draft" document and not officially approved for citation.

If you happen to have a known problem involving mineral (or other dietary) deficiencies, you should seek advice from your physician. In my own experience, I can only recall one case in which product water was suspect. That situation concerned a woman who, for some reason, required an extremely low sodium intake. Even the small amount of salt in a watermaker's product water output was problematic. In that particular case, her cruising partner ran product water through the watermaker a second time, which apparently resulted in such low salt content that the problem was solved. My own experiments with such double runs produced product water with less than 10 ppm of residual salt.

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What does a TDS meter tell about product water?  A TDS meter can tell you a lot about the quality of your product water if you fully understand both its capabilities and—probably more important—its limitations. Therefore, I'll begin by describing just what a TDS meter is and how it works.

"TDS" is an acronym for "Total Dissolved Solids."  Stated that way, its name is a serious misnomer. A TDS meter does not give you a measure of the total amount of dissolved materials in a water sample. The TDS meter is nothing more than a conductivity instrument; that is, it provides a measure of how much electrical current a sample of water will conduct at a given applied voltage. Another way of expressing this is to say that it measures the electrical resistance of the water sample. Either way you want to look at it, its output reading is only affected by dissolved materials that affect electrical conductivity; dissolved substances that do not affect conductivity are not represented at all.

At this point, a careful reader will have inferred that not all dissolved substances affect the conductivity of the water. This is true and an important thing to remember. In fact, only dissolved substances that ionize when they dissolve are capable of affecting the conductivity reading. Common examples of such substances include salts, acids and bases. When such substances dissolve in water, their molecules separate into positive and negatively charged particles called ions. Without getting too technical, this is the characteristic that allows them to conduct electricity through the water. Therefore, such substances will be measurable—the more of them that are in the water sample, the greater will be the sample's conductivity, the lower will be its resistance to electric current flow, and the higher the ppm reading on the TDS meter. As you've already learned, this is exactly what a TDS meter is designed to measure.

Unfortunately for us, there are a whole multitude of substances that will dissolve without ionizing in solution. A few examples should serve to emphasize the importance of this fact: substances that do not affect the conductivity of the sample include: viruses, petrochemicals, bacteria, sugar—the list could be extended far beyond what I have mentioned here. For those of you who are from Missouri, or who simply want to confirm this fact for yourself, try the "Dr. Science" experiment I designed, which is described on page 79 of my Watermaker Book.

So, you might wonder, what good is a TDS meter? Well, plenty good, now that you understand what it does and does not do. Since the residual amount of salt in your product water is ionized, the TDS meter is excellent for giving you a pretty accurate reading of how much is there. Katadyn watermakers in normal good working condition will remove a minimum of 96% of the salt contained in the input seawater. Typically the percentage of salt rejection is considerably higher, usually more than 98%. The TDS meter will indicate the amount of salt remaining in ppm (parts per million). A new membrane should produce product water in the range of about 200-300 ppm. I don't contemplate replacing a membrane until the ppm reading exceeds 500-600 ppm, all other factors being equal.

The actual reading you will get—that is, the quality of your product water—depends on several factors that have a fairly wide range of variation. The most important of those factors include:

All of these factors can influence the quality of your product water and, therefore, the TDS meter readings. It's not uncommon to see variations of 100-200 ppm, depending on the circumstances, even with the same input source. You should expect this.

So, now for the good news. If your watermaker is making product water with less than about 500 ppm of residual salt, you can be assured that it is doing a reasonable job of removing the salt. The even better news is the related fact that, if it's removing that much salt, it's also certainly removing potentially harmful viruses and bacteria. Although the actual chemistry is much more complicated, you can think of the membrane as a very fine filter with a pore size of about 0.001 microns! If that's fine enough to be effective at removing the small salt molecules and ions, it's surely also removing the viruses and bacteria, whose molecules are orders of magnitude larger. The analogy I use in my seminars is, trying to get a virus or bacterium through the membrane is like trying to push a cat through a chain-link fence. (I hope I don't offend any animal rights advocates with that metaphor.)

Be aware, however, that our discussion here is only relevant for normal seawater input. If you are using your watermaker to process other sources, like freshwater from a river, you should realize that there are a multitude of other types of harmful substances that could possibly be contained in the input stream and are of small enough molecular size to pass through the membrane and contaminate your product water. In fact, there are so many that I won't attempt to discuss them here. You can find some representative examples of these potential sources of contamination on page 11 and the Appendix of my Watermaker Book.

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Does the chlorine in seawater cause damage?  Not at all. The reason involves a little knowledge of basic chemistry. Chlorine is a chemical element that can exist in a couple of common configurations. As "free" chlorine, it is a very active chemical element. This is the state in which it is not combined with other elements to form a compound and, because of its chemical nature, it is extremely active and corrosive. That's one reason why it's so good at bleaching things. This is basically the form it has in dockwater, where it's great for attacking and killing potentially harmful bacteria. It's also the form of chlorine that is harmful to watermaker membranes.

Since it's such an active chemical, chlorine easily combines with other elements to form compounds. When it does so, it shares electrons with the other element(s). When this happens, the chlorine atoms become "satisfied" and lose much of their aggressiveness. They then exist as "ions" and are very stable. It is true that roughly 3% of seawater is salt (sodium chloride), which equates to about 33,000 ppm salt in the seawater. If you do the math, you'll realize that's a lot of chlorine. But it's all in the ionic form and is no threat to your watermaker membrane. Don't worry, be happy—like the chlorine in seawater.