Project: Watermaker

You don't have to have a watermaker, particularly if you limit yourself to coastal cruising in developed countries.  But if you cruise to some of the more remote and/or less developed areas of the world, you'll either have a watermaker, only go where it rains a lot, or wind up jerry jugging water from some mighty questionable sources.  If the latter, be sure to carry plenty of water purification capability.

All watermakers that we've seen that are intended for boats are of the Reverse Osmosis (RO) type.  They come in a wide variety of outputs and choice of power, from the hand-operated PUR to a variety of 12V powered (e.g. PUR again, Village Marine, Spectra) to AC or direct drive high output units (e.g. HRO Systems and many more).  It's also quite possible to assemble your own from commercially available bits and pieces -- which is what we did and, after all, that's really what most of the manufacturers do too.

In terms of sizing, we find that we use just a bit less than 5 gallons per day per person and so will empty one of our 60 plus gallon water tanks in about a week.  A watermaker that's sized to yield 20 to 30 gallons an hour is about right for us -- we can run the watermaker for around 2 hours once a week and meet our domestic needs quite nicely.  And if we're doing laundry aboard using our washing machine, we'll run the watermaker at the same time and come pretty close to making as much new water as was used to wash a load of dirty clothes.  (If you choose one of the small, low output units, be aware that it may well be running almost all the time.)

On this web page, you'll find some material on:

Ready-made or Do-It-Yourself?

Reverse Osmosis (RO) watermakers are commonly seen as an assembled unit, with controls, pumps, membranes, solenoid valves etc. all in one housing that's installed in a boat and hooked up to the various hoses and wiring it needs to work.  And if that's what you want to do, that's fine.  Although they tend to be fairly expensive, some of these packages are quite clever and functional, even if they are inclined to take up quite a bit of what could otherwise be valuable storage space.

But they're really pretty simple beasts at heart and it's a relatively straightforward task for a person to gather up all the bits and pieces required to assemble one.  If you're uneasy about your ability to track down all the specific bits or just don't want to take the time, there is at least one outfit that has already done the required legwork and will sell you a kit with only a reasonable markup.  (One such source is AquaMarine, Inc.  While they supplied most of the equipment for Daydreamer's watermaker, I have no commercial relationship with them other than being a satisfied customer.)  I have some more specific comments on materials and equipment here.

The do-it-yourself approach has at least three very significant benefits and several minor ones as well.  Most significant are the following:

Minor benefits include better access for maintenance and, potentially at least, a simpler design with minimal automation, hence greater reliability.

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How they work

While I don't want to get into a long workup on how RO works, a bit of simplified explanation might help clarify the installation material that follows.  In essence, salty sea water is pumped up to high pressure and forced through a reverse osmosis membrane.  The membrane acts kind of like a super super fine filter that allows a little bit of fresh water to squeeze through (product water) but keeps the salt from passing through with it. Within limits, the higher the pressure, the more fresh water.  Colder or saltier water will give lower product rates at any given pressure. After passing through the membrane, the slightly saltier (reject) water flows on through a back pressure regulation device and is then thrown away overboard.  More details, up to & beyond info overload, can be easily found on the internet.

As an aside, note that throwing this reject water away by throttling it through a back pressure valve makes an RO watermaker inefficient and quite power-hungry.  After all, we had to put a very significant amount of power into pumping the water up to high pressure.  If we could recover that power instead of just throwing it away, we could use it to help pressure up the water on the membrane feed side.  Doing just that is what allows the highly-efficient watermakers (e.g. Spectra) to operate from relatively small 12V pumps.  It also makes them way more expensive.   It's an elegant idea that really appeals to the engineer in me but I just can't justify the added cost and complexity.  Since Daydreamer already had a generator, power availability wasn't an issue so I'd have to pay out the added cost of a more efficient watermaker on savings in diesel fuel alone.  That is to say, if an efficient watermaker could save me about 1-2 quarts (or liters) of diesel fuel a week for, say, 25 weeks per year, how long would it take to pay out the perhaps $5000 additional that this watermaker would cost?  Probably forever if I consider the time value of money.  If you don't have a generator, the equation might well work out differently.  (Note also, though, that the possibility of saving the incremental cost of such a watermaker may well justify the installation of a low-efficiency watermaker AND a generator instead!  Your engine will be very thankful if you do!)

Flow Diagram

Take a look at the diagram below.  Seawater comes in through a strainer that takes out the big chunks of material, then goes through a small boost pump.  The two pre-filters upstream of the high pressure pump take out sediment, algae, and other suspended material so the membranes sees only clean water.  Salt water from the high pressure pump then goes on to the RO membrane.  Reject water goes from the membrane through a needle valve that is used to control the operating pressure in the RO membrane and then goes overboard.

ERRATA:  The HP pump sizing shown on the diagram is incorrect; it should read "1.8 GPM"

Fresh, product water comes from the membrane at low pressure and goes through a two-way valve.  When first starting the system, the product water is routed to a sink and the watermaker run for a while.  A simple taste test of the product water will tell if the RO membrane is working properly.  When the product water tastes good, the valve is then used to switch product water to the tanks. 

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Customization on Daydreamer

The do-it-yourself approach taken for Daydreamer's installation has allowed us to make a number of modifications that significantly improve the flexibility and reliability of our system when compared to standard RO watermakers.

Alternative Drive Mechanism

Typically, the electric motor that is used to drive the HP pump is a single-shaft 3450 RPM motor (for 110V 60 Hz service) which will fit compactly in a watermaker skid housing.  Since we were using a separately-mounted motor and pump, we were able to select an electric motor that has a shaft on each end.  One end had the pump mounted directly to the motor via a bell housing, as is standard.  The other end of the motor's shaft has an electric clutch & sheave, similar to those used on automobile air conditioners; this allows the pump/motor combo to also be driven by a V-belt from the main engine.

The motor/pump is mounted next to Daydreamer's engine on a modified engine mount.  When the generator is running, the HP pump can be driven directly by the electric motor, as per normal practice.  Alternatively, a V-belt can be run from the engine to the HP pump/motor clutch.  If we're motoring or if 110V electrical problems were to crop up, we can drive the pump from the engine and the electric motor simply acts as a ball bearing mounted lay shaft.  We were able to specify a motor/pump combo that would run at 1725 RPM, which is much easier to match to the engine speed allowing for a reasonable choice of drive and driven sheave diameters.

Dual RO Membranes

The "standard" RO membrane used on yachts is about 4 feet long.  Some of the pre-assembled watermakers use half-length membranes to cut down on the size of the skid and double up on the number of membranes to yield the same product water rate.  Daydreamer's system has two full-sized membranes that will yield almost twice the product volume at any given operating pressure.  Alternatively, the membranes can be operated at a lower pressure and still have the same yield as a single membrane operating at higher pressure, which is the approach we take.  Operating membranes at lower pressure & output will extend their service life.  Also, having two membranes in service provides a bit of redundancy in that a failure of one membrane still allows the system to function on on the other membrane only.

Observable Reject Water Flow

The diagram above shows the reject water going out through a thru-hull below the water line.  And that'll work OK.  But on Daydreamer, we took a bit of a different tack.  I installed the two RO membranes vertically in the hanging locker in the aft head with the control panel inside the hanging locker door.  The aft head is equipped with foot pumps and spigots for both fresh water and salt water.  So I teed the "test" output into the fresh water pump discharge which then flows directly into the sink.  I did the same thing with the reject water by teeing it into the salt water foot pump discharge.  We do get a pretty high rate of salt water flow into the sink but that can be very valuable, because it allows direct observation of the reject water flow.  If the HP pump is loosing suction because the filters are plugging up or some check valves in the pump head are acting up, we can not only hear a problem but we can see it as well.

Tank Selector Valve

The custom design allowed us to include a tank selector valve in the control panel.  It's a three-way valve plumbed in downstream of the test/tank selection valve and allows a selection of the tank to be filled at the control panel.

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Notes on Materials, Installation, & Operation

(Click to go to notes on Installation, Operation, "Pickling", and Miscellaneous)

Material/Equipment

1)    Strainer:  Nothing special here, just a standard marine raw water strainer.  If it's large enough, it may be acceptable to just tee off of the main engine water strainer.

2)    Boost Pump:  Use a centrifugal pump installed below the water line so as to maintain a flooded suction to the HP pump.  I use my refrigeration condenser pump for this service by just teeing off its discharge and using diodes in the power line so either refrigerator or watermaker controls can turn it on.  I originally used a Jabsco Centri-puppy for this service but found out that I could only get about one year’s service, at best, before the lip-type shaft seals started to leak.  I eventually changed over to a Shurflo Magnetic Drive Bait Sentry pump (MDBS-1100) in 2005.  Being a magnetic drive, it has no seals.  It also has a water-cooled motor housing and looks like it’ll be a more-or-less permanent solution.  (Update:  I found that the Shurflo pump's non-replaceable motor brushes lasted a bit over 3 years in this service.  That's not too bad and ours, at least, started to get pretty noisy about 2-4 weeks before the pump stopped running, so we had plenty of warning.)

3)    Pre-filters:  standard, plastic domestic water filter housings’ll do the trick, you can find ‘em at Home Depot/Bunnings.  (See-thru is best to allow you to visibly check out the filter element in service.)

4)    HP pump:  Pick an AC(mains)-powered 1 horse power electric motor and a high pressure (HP) pump that’ll put out 1.8 gallons per minute at the RPM the motor runs at.  Tie them together with a bell housing and shaft coupling … easy as.  The HP pump needs to be compatible with salt water service and have an operating pressure in excess of 1000 psi.  Many RO system builders use triplex pumps (i.e. a piston pump with three pistons) made by CAT but certainly other manufacturers might work just as well or even better. 

5)    High Pressure hose:  You need high pressure hose to and from the membrane housing.  Parker(US) or Enzed (Aus/NZ) will be glad to make them up for you or sell you the parts you need to do it yourself.

6)    Membrane:  Dow Filmtec 5W30-2540 will do the trick, the supplier will undoubtedly have fiberglass housings as well.  You can almost double your fresh water production with no additional power consumption by having two of these membranes in series.

7)    Pressure Gauge:  A liquid-filled gauge is best as there will be a bit of pulsation.

8)    Flow Meter: a standard rotameter works fine here.

9)    It’s convenient to make up a control panel with the on/off switch, needle valve, pressure gauge, and test/taste valve.  Place the control panel near your test/taste station for maximum convenience.  If you do go with a panel, be sure to get a gauge, rotameter, and needle valve that are designed to be panel mounted.

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Installation:

1)    The boost pump is important to maintaining a flooded suction to the high pressure pump.  It's not likely but you MAY be able to avoid installing one if the HP pump is down in bilge WAY below the water line. 

2)    Don’t put any valves in the fresh water product line that might block flow and allow backpressure to build up on the membrane.  In fact, if you use hoses and hose barbs to hook up the fresh water product piping, you might consider just leaving off the hose clamps.  That way, any backpressure that might somehow buildup will cause a hose to leak at or blow off its hose barb.

3)    Be sure to install the pre-filters in a location that allows easy access; you’ll probably be changing at least the 20 micron element frequently.  We often only get a few of weeks run before we have to change it out.  The problem is not that it gets plugged up with particulates but rather it filters out algae, which then die and begin to smell really bad!

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Operation:

1)    Starting conditions: 2 way flush valve in operating position, 2 way test/tank valve in test position, and needle valve full open.

2)    Turn on boost and High Pressure pumps and let run for a short period to stabilize operation and flush out any fresh water that’s residual from previous flushing.  If the membrane has been “pickled” (see below), let pumps run at least a half hour without backpressure to flush out any pickling solution.

3)    Increase the operating pressure of the membrane by slowly closing the needle valve until the membrane "breaks" osmotic pressure and begins to produce some fresh product water, typically around 700-750 psi for normal sea water.  (This pressure will be lower if water is brackish and higher if the feed water is relatively cold.)  Continue to close the needle valve until the fresh water product rate is around 30 gals/hour*.  Do not go to or above 1000 psi, do not exceed the 30 gals/hr*/membrane product rate.

4)    Monitor product water quality at the taste/test station.  You can either use your sense of taste or a hand-held conductivity meter.  When the water quality is good, turn the test/tank 2 way valve to divert the fresh product water to your tanks.

5)    When you have made enough water, turn the test/tank 2 way valve back to the test position and open the needle valve.  Turn off the pumps.

6)    Turn the 2 way flush valve about half way to the flush position and back-flush the prefilters for about 15-20 seconds.  Turn the flush valve to the full flush position and flush through the HP pump and the RO Membrane.  The flushing discharge will come out the test spigot.  Monitor this discharge until it is free of any saltiness, which indicates that the membrane has been adequately flushed.

7)    Close the needle valve and return the flush valve back to the operating position.

*30 gph is sort of a nominal value.  The particular membrane I’ve suggested is rated to produce 700 gal per day of potable water from seawater, but your actual mileage may vary by ± 10 to 20% because of manufacturing variables and water temperature.  The thing to do is to find out how much fresh water your membrane likes to produce on normal seawater salinity at around 900 psi or so, then don’t significantly exceed this product rate even if the feed water is only brackish.  I’ve heard that some membranes will actually see a slight increase in production over time, a sort of “break-in” effect.  And productivity will suffer if the membrane gets fouled with scale/solids or when operating on cold water.  If not damaged by chlorine or oil, you may well be able to clean the membrane and restore production.

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“Pickling”:

If you aren’t going to be using your watermaker for an extended period, say 3-4 weeks, it’s best to pickle the membrane, i.e. fill the membrane housing with a preservative solution.  Essentially, I just mix up a batch of preservative solution and pump it through the filters, HP Pump and membrane housing and out the reject line.

The recipe for the preservative solution is:  2 tablespoons of Sodium Metabisulphite per gallon of water (available wherever fine watermakers are sold and might also be available at your your friendly neighborhood pharmacy/chemist…I think this is the same stuff that wineries use to stop fermentation per the “contains sulphites” found on most wine labels).  Beer brewing kits are sold in Australia in the supermarkets (Woolworths, Coles); I saw jars of sodium metabisulphite on the shelves right next to the Cooper Ale kit at my local Woolies today, presumably to sterilize bottles, vats etc.  Elsewhere, you might have luck at your local or internet home brew supplier.  For example, I bought my last batch of Sodium Metabisulphite at home brew supply store in Phoenix for $3.19/lb.

I usually mix up a jerry can’s worth (5 gallons) and use a spare water pressure pump to circulate the solution.  By putting a hose on the reject water spigot, I can return the solution to the jerry can and continue circulating without worrying about running out of solution.

I’ve not shown any pickling valve(s) on the schematic.  It’s the builder's choice as to if and where you put them.  One consideration is the solution can sometimes cause the check valves in the HP pump to stick together.  It’s happened to me.  So it’s probably best to tee a valve into the high pressure line between the pump and membrane and just circulate through the membrane housing only.

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Miscellaneous:

Chlorinated water will ruin or at least damage RO membranes.  Don’t flush the membrane with chlorinated water or try to pickle using chlorinated water.  Essentially, this means that you won’t be able to flush the membrane or make up pickling solution with tap water from the dock.  (You might be able to get around this problem if you have an inline charcoal filter that'll remove chlorine.)

Don’t operate the watermaker in waters that have oily discharge floating around.  I also don’t operate in waters with a lot of other boats around, particularly where the anchorage is a bit confined.  In theory, viruses and bacteria are too large to get through an RO membrane, but why test that theory?

It is possible to more-or-less clean membranes that have gotten fouled with organic matter or scale/particulates.  You can do it yourself with alkaline or acid cleaning solutions made up for that purpose or you can take the membrane into a service bureau that’ll do it for you (my choice.)