Reverse Osmosis vs Under-sink Water Filter - Which is Better?
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The water here in Las Vegas is not great. While it's certainly safe to drink, it has a very high mineral content which plays havoc with water heaters and plumbing fixtures, and which, coupled with the chlorine that gets added to it, makes for an unpleasant taste.
Six or seven years ago, we installed a water softener system to reduce the mineral content. It was a salt-based unit, and it seemed to be effective. We did have less mineral deposit build-up on our faucets, sinks and toilets than previously. The downside was that the water often felt sort of slimy, and it didn’t do much to improve the taste. It recently crapped out, however, and I’ve been researching alternatives over the past few weeks. More on that in a later blog.
When we lived aboard Nine of Cups, we had a reverse osmosis system (RO unit) aboard that we used to convert sea water to fresh. It used a high pressure pump that typically operated around 800-1000 psi, to force water through a membrane, leaving a salty brine on one side and great tasting, fresh water on the other. It was invaluable, especially on long passages - we no longer had to depend on rainwater catchment systems and/or water rationing to make it to the next water source. On the other hand, it operated on battery power, and in our 12v, off-the-grid world, it constituted a significant portion of our daily power requirements.
The small, under sink RO units meant for use in the kitchen use the same principle to improve the taste of tap water, as well as remove minerals and other undesirable content. These devices use a carbon filter to remove chlorine and particulate matter from the water, then force the tap water through a membrane to remove minerals and other undesirable, dissolved content. Since these contaminants are much easier to remove than the high concentrations of salt in sea water, they can operate at a much lower pressure - so low, in fact, that the normal household water pressure of 60 psi is sufficient. Thus, no electricity is required. They typically cost a couple hundred dollars - as compared to the several thousand dollars that a marine RO unit will set you back. And most of the reviews indicated that the units worked well and the water tasted great. Seemed like a no-brainer.
We bought and installed one about six years ago. The unit was easy to install, and the end result was a slow but steady stream of pretty good tasting water.
Advantages
The advantage of an RO unit is that it removes most harmful contaminants like mercury and lead from the drinking water, as well as most chemicals, like chlorine.
Disadvantages
Wasted water.
What we didn’t realize was how much water it wasted. Want to take a guess? If you guessed 3 gallons for every gallon produced you get partial credit. A new unit, operating at an optimal water pressure of 60 psi, does waste about three times the water produced. But the efficiency starts falling off with use or if the pressure drops below 60 psi. By year three, it’s more like 10 gallons wasted for every gallon produced, and if the water pressure drops, this number gets even worse.
Between all our household family members, we average almost two gallons a day through the RO unit, so that translates to about 180 gallons of water poured down the drain each month when the unit was new, and more like 600 gallons a month at year three. Wow! That’s a lot of wasted water, especially since we live in the desert where water is precious and wasting it is a major sin. Replacing the membrane every year or two will return the unit to maximum efficiency again, but they aren't cheap - typically $80.
Operating costs.
The filters are pricey - about $60 a set and need to be replaced twice a year.
To keep the water consumption to as little as possible, the membrane should be replaced each year at $80.
Add to that the cost of all that wasted water, and the operating costs are not "chump-change", not to mention the pangs of guilt from wasting that much water.
Loss of minerals.
An RO unit is quite good at removing all those bad minerals and contaminants in the water, and just as efficient at removing the good minerals like calcium and sodium, making the water less healthful. Some systems include an element that adds these minerals back in, but this increases the operating costs even more.
When we reached year six on our RO unit, it was time to replace the filters and membrane. In addition, the RO water tank would no longer hold pressure and needed replacing. The cost of all these components was more than the cost of a new system, so it was a good time to evaluate whether we should buy a new unit or explore other options.
Under Sink Filter
An alternate possibility was an under sink water filter. This uses a carbon-based filter along with one or two particulate filters, and will remove most contaminants and chemicals, like chlorine, resulting in better tasting water.
A Diagram of an Under Sink Filter
Advantages
Initial cost.
The initial cost of an under sink water filter is less than an RO unit. A two stage filter system, probably adequate for our needs is about $125 versus around $200 for an RO unit.
Operating costs.
The two filters should be replaced twice a year and typically cost around $60 a set. So, while we could expect to pay about $120 per year in filters, this is still considerably less than an RO unit.
Wasted water.
Unlike an RO unit, a simple filter system wastes no water.
Installation.
While neither an RO unit nor an under sink filter is difficult to install, the RO unit is slightly more complicated and requires considerably more space.
Disadvantages
Dissolved Minerals.
Unlike an RO unit, a simple filter system is not effective at removing dissolved minerals like cadmium, lead, and iron.
For our Las Vegas house, this wasn't a real disadvantage. The water here is hard, but it contains well under the EPA established maximum contamination levels of all those undesirable contaminants like lead, cadmium and mercury. To prevent mineral build-up on our plumbing fixtures and pipes, and to extend the life of our hot water heater, we do need a whole-house water softener system, however.
Decision Made
There's little doubt as to which system was best for us - the under sink filter won hands down. We did our research on which one seemed the best for our needs and decided on a 2-stage filtering system from Aquasana. Installation was quick and easy, and the water tastes fine. One definite positive is that the new system produces a higher flow of water than our previous RO unit. Also, after half a gallon or so, as when filling a coffee pot, the flow from the RO unit would taper off to just a trickle, while the filter system maintains the same flow for any length of time.
The Aquasana Filter we decided on
The installed filter
So far, so good.
If you're interested, here's a how-to video on how to install it.
See you next time…