What’s Wrong With Under-Sink Water Filters (An Engineer’s Honest Take)

Under-sink water filters are the most popular home water treatment, and for good reason. They remove contaminants at the point where you actually drink, they don’t require whole-house plumbing work, and the technology behind them (carbon block, reverse osmosis membranes) is proven. After 9 years building filtration systems, I’d still recommend an under-sink filter over no filter at all if your water test says you need treatment. That being said, under-sink water filters have some serious structural problems that the industry has zero incentive to fix. Here are five that I’ve spent years thinking about. Key Takeaways — What’s actually wrong with under-sink filters? Under-sink filters are the best point-of-use option available — and I’d still recommend one over nothing if your water test says you need treatment. But the category has real problems the industry ignores. RO systems waste 2–4 gallons for every gallon they produce, then add minerals back into the water they just stripped. Carbon filters expire invisibly — no affordable system tells you when it stops working. Installation means a second faucet on your sink that most people forget to use. The deeper issue: the filtration in...