… But we can’t drink it!
We’re surrounded by water. Salty ocean water. And we can’t drink it. Aboard Tangaroa, we need to turn that salty seawater into freshwater.
When we’re docked in a marina we can run a hose from the dock and hook up to public water (just like you would with an RV at a campground). But, if that public water is of questionable origin, we have to be able to purify it to make sure it’s safe to drink.
We also want to cook, clean food, and make ice with water that is free of contaminants. Bathing, washing clothes, dishes, and camera gear is best done without concern for scaling, mildew or lime build-up.
With all that, it was important for us to get the best machine for the job. Because this machine is going to get a lot of use over the coming years! We’ve had watermakers on our other boats that made good freshwater from seawater. But we needed the extra mile of purifying that water, too. After a search of various models out there, we got just the machine to make sure that when we turn on the tap, clean & delicious fresh water comes out!
We chose the XZ Spot Zero watermaker by Dometic. It turns salt water into spot-free water, perfect for drinking or washing down the boat. We could’ve had separate units, one for the reverse osmosis (salt to fresh) system, and another unit for the purification process (spot zero), but this combo unit is really the trick!
At home, we’ve either been on a terrific well that brings water up from an aquifer, or have had city water piped into the house. You almost don’t even think about where your water comes from and take for granted that it is safe to drink. At sea, we have to make all the water we drink and use.
We sure don’t want to be buying bottled water to use on the boat! Not only does that violate our principle about single-use plastic, but the amount of space all those bottles would take up would be crazy! Financially, buying plastic bottles makes no sense, either. Granted, a watermaker is not inexpensive – and our combined, top-of-the-line unit is at the higher end of the scale. For us the upfront investment was worthwhile because this is one of the most critical pieces of gear on the vessel. And it will last as long as we have Tangaroa and far into the future.
We’re still careful about our water use because at sea we have to turn on the generator to make water, and the generator runs on fuel. So we tend to do a few things at once when the generator is running: cook, pump scuba tanks, make water, and charge all of our lithium camera batteries. That way, we aren’t running the generator all the time and wasting fuel.
Tangaroa carries about 130 gallons of water. When we’re diving & filming, we tend to go through more water than usual. We couldn’t do this adventure without our Spot Zero unit! Depending upon the temperature of the ocean water, we can make up to 50 gallons of freshwater per hour right from the sea!
REVERSE OSMOSIS: Freshwater out of Saltwater
Reverse osmosis (RO) is one of the processes that makes desalination (the removal of salt from seawater) possible.
But how does regular osmosis work?
Osmosis is defined by the Encyclopedia Brittanica as “the passage or diffusion of water or other solvents through a semipermeable membrane that blocks the passage of dissolved solutes.”
Imagine a cup of water (the “solvent”). Now, dissolve some salt (the “solute”) in that water . What have you made? Salt water (the “solution”).
Now imagine you have a glass tank that is separated by a semipermeable membrane. (It’s not impermeable, like a piece of plastic, nor is it permeable, like a piece of cotton fabric. It is only partially, or semi, permeable, allowing the water to pass through but not the solutes!
Pour some fresh water (the “solvent”) in one side of your tank. On the other side pour in saltwater (the solution). If you have a device that measure the TDS (total dissolved solids) you will see that the fresh water has a low TDS number while the saltwater has a higher TDS number. Now wait. While you wait, the water will try to reach equilibrium. To do that, the water molecules in the freshwater side will move through the semipermeable membrane to the side with the saltwater, thereby diluting the saltwater. The water is trying to make the concentration on both sides of the membrane more or less equal.
It sounds kinda complicated but basically, osmosis is when a solvent of low concentrated solute solution moves through a membrane to get to the higher concentrated solution, thus diluting it. Osmosis happens naturally in many processes, including throughout the cells in our bodies. Or, imagine putting a raisin in water. That dried shrunken grape will swell up – because osmosis is allowing water to enter the raisin.
In reverse osmosis, we are (literally) just reversing the process, by making our solvent (the water) go from the high concentrate (seawater) into the lower concentrate solution. So instead of creating a more equal balance of solvent (the water) and solute (dissolved material, like salt) in both solutions like we did in our imaginary tank above, RO is separating out solute from solvent.
This reverse osmosis desalination (salt removal) uses pressure to force water through filters, straining out other substances at the molecular level. Besides removing salt, RO removes virtually every mineral and most biological or organic chemical compounds, producing water that is safe to drink.
Since reverse osmosis doesn’t happen naturally, like regular osmosis, we need a powerful pump to make that happen. We’re forcing the water containing all the dissolved material (seawater) through the membrane (filters) in an artificial process.
And that’s where the watermaker comes in: it’s a high-pressure pump that forces the salty seawater through the membranes in the system in such a manner that the larger salt (and other dissolved solids) get left behind (or “concentrated”) and clean water remains to be sent to the storage tank. The salty concentrate goes overboard back into the sea.
So, our Spot Zero first removes 95-99% of the total dissolved solids (TDS) from seawater, flushing that overboard. Roughly 90% of the water supplied to the system is recovered. That water then goes through other filters that further purify it (to remove viruses, cysts, and bacteria) making it deliciously and safely drinkable.
When we are out on the ocean, we make freshwater from seawater using the Spot Zero reverse osmosis machine. When we are tied to a dock in a marina, where the water coming out of the hose is of questionable quality, the Spot Zero doesn’t need to use reverse osmosis. The machine simply runs the water from the hose through it’s system of filters to purify it.
Two systems. One machine. Water, water everywhere – and we have no problem turning it all into fresh clean water for all of our needs!