Under Sink Filters

A housing bolted to the cabinet wall and teed into the cold supply line defines this format, and you choose one of two ways to get the water out. A dedicated tap goes through a 1 3/8 inch hole in the sink deck or countertop and serves only drinking water, which makes the cartridge last far longer. An inline connection instead filters everything the main faucet pours cold.

Space is what makes this work. A standard 10 inch cartridge holds many times the carbon of a faucet mount, so water spends several seconds inside dense media and the certifications get serious: NSF/ANSI 53 for lead, cysts, chromium and volatile organic compounds, 401 for pharmaceutical and pesticide residues, and PFOA and PFOS reduction on the better blocks. Capacity lands between 500 and 1500 gallons, six months to a year for most homes, at 0.5 to 1 gallon per minute.

Reverse osmosis shares the same cabinet and is the decision you weigh this against. Carbon under the sink answers chlorine, lead, cysts and organic chemicals cheaply, quickly and with no drain line, while reverse osmosis adds a membrane that removes dissolved solids carbon cannot touch, at the cost of a tank, wastewater and slower delivery. Read your water test: taste and lead point here, nitrate and arsenic point next door. The faucet mount is the same idea shrunk to a fifth of the media.

Homeowners and long term tenants with permission are the natural buyers, especially families passing 120 gallons a month who are tired of cartridge changes. Skip it if you cannot find or turn the cold shutoff under the sink, if the cabinet is already packed with cleaning supplies, or if a slow drip could run for a week before anybody looked.

Favor a standard 10 inch housing, since cartridges in that size come from many sources while a proprietary body ties you to one. Look for shutoff or bypass valves on both sides so a change takes ten minutes without closing down the house, and check that a sump wrench is included. Change on gallons, not on taste, and set a battery leak alarm on the cabinet floor, because a weeping compression fitting destroys the base long before anyone smells it.