Attention: That blinking "replace me" light isn't measuring your filter's efficiency. It's measuring time.
I recently purchased and installed an inexpensive (about $150) reverse osmosis water filter that goes under my kitchen sink, freeing me from the tyrrany of bottle water. Aside from the slight gurgling noises it makes as the tank refills, I love being able to refill my stainless steel water bottle any time I like, without adding more plastic to the world's oceans, or putting more money in the pockets of multinational sellers such as Nestle (Poland Springs) of what should be our natural birthright: clean water.
As you can see, I feel strongly that water purification is a much greener alternative than buying bottled water. Just how much "greener," however, has a lot to do with how long the filters last. A three-part reverse osmosis system is complicated, to say the least, because your're talking about three different types of technology that filter your water. And many of the components used in the manufacture of those products-plastics, synthetic membranes and so on, have a high embodied energy. Translation, like most manmade products, they cost the Earth each time we extract, transport, melt and form them.
My point isn't to get deep into the science of water filter cartridges however. Instead, I want to alert you to the fact that the recommended replacement interval on your filter system may be far too conservative. Take my installation, for example. The Whirlpool WHEMBF has a little lithium battery inside the above-sink spout. That battery, activated when you install the filters, is basically on a long slow, slide into the abyss, and just before it dies, a little LED light starts to blink. In other words, it's not a "smart" light, it's a "dumb" light (not unlike what we used to call the "idiot light" on car dashboards).
I can only assume that the technicians at Whirlpool (and these are smart people, trust me, I've known many of them over the years). The specs for the system give it a 350-gallon capacity. But in our two-person household, we're only using about 150 gallons of drinking water annually. All other things being equal, doesn't that mean that my filters should actually last for 2 years between changes, not the measly 6 months determined by the battery life.
I asked David F. Walling, a professional RO installer, to check me on my logic.
"Capacity of any filtration method is a great question to ask," he says. "But one thing to consider when faced with the question to replace or not is the potential for bacteria to grow in the media ... One way to deal with this problem would be of you going to exceed the recommended interval for filter replacement then use UV light as your last line of defense"
I did some research on bacterial contamination of household water filters. Most of the information, unfortunately is anecdotal. A lot of the most negative reports are related to inexpensive Brita filters, which are often basic charcoal filters attached directly to sink faucets. The consensus is that RO filters can last 2 years, and in some cases up to 5 years. That lifespan has much to do with how much crud is in the water, whether it's hard or soft, and so on.
The bottom line. More research is needed before I can recommend that the rest of you ignore the "idiot light," the way I intend to. Perhaps this article will inspire someone with the right tools to test some water out of the tap at monthly intervals from an off-the-shelf RO system. If so, please let us know your findings.