Under sink filtration treats the water at one tap, usually the kitchen, by splicing a filter into the cold line inside the cabinet. The options run in three tiers: a single inline carbon cartridge, a multi-stage cartridge system, and full reverse osmosis. The right tier depends on whether your problem is taste, specific contaminants, or dissolved solids only a membrane can remove.
Tier one: the inline carbon cartridge
The simplest setup is one carbon cartridge plumbed straight into the cold line feeding your existing faucet. Every drop from that tap passes through it, installation is quick, and nothing new appears on the counter. Carbon at this scale improves chlorine taste and odor convincingly, which is the majority complaint on city water. Its limits are equally clear: modest capacity, no effect on dissolved solids or hardness, and the cartridge treats even the water you use for rinsing dishes, so it exhausts faster than a dedicated-faucet setup.
Tier two: multi-stage cartridge systems
Multi-stage systems chain purpose-built cartridges, typically sediment then carbon block, sometimes a stage targeting lead or other specific contaminants, and usually dispense through a small dedicated faucet so the filtered water is reserved for drinking and cooking. The dedicated faucet stretches cartridge life dramatically. This tier is the sweet spot when your water report names a concern that a certified carbon-block stage addresses and you want more thoroughness than one inline cartridge without committing to a membrane and drain line.
Tier three: reverse osmosis
RO is under sink filtration's ceiling: prefilters, a membrane that rejects most dissolved solids, a storage tank or on-demand pump, a postfilter, a drain connection and a dedicated faucet. It is the tier that changes what is dissolved in the water rather than just its taste, and the one to choose for high dissolved solids, salty or mineral-heavy supplies, or the broadest contaminant coverage at a single tap. It asks the most in return: cabinet space, a drain saddle, and a multi-part filter schedule. Our best under sink reverse osmosis roundup compares the current systems.
Renters and the no-drill alternative
If you cannot drill a faucet hole or modify plumbing, the countertop route delivers most of tier one and two's benefit with zero permanent changes: units that connect to the faucet aerator or stand alone on the counter, like the countertop filters Aquasana publishes specifications for. They are also a cheap way to trial filtered water before committing to an under-sink install in a home you own. The tradeoff is counter clutter and slower dispensing, which is exactly what under sink filtration exists to hide away.
Questions people ask about under sink filtration
Does under sink filtration reduce water pressure?
A well-sized inline or multi-stage system costs little noticeable pressure at the tap. RO is different by design: its dedicated faucet pours from a tank or pump at gentler flow than the main faucet, which is normal, not a fault.
Which under sink tier do I actually need?
Match the tier to the complaint: chlorine taste alone points to an inline carbon cartridge, a named contaminant on your water report points to a certified multi-stage system, and dissolved solids, salinity or maximum coverage point to reverse osmosis.
Can under sink filtration soften hard water?
No cartridge system softens water; hardness removal at a single tap is only meaningful with reverse osmosis, and whole-home scale problems need a point-of-entry softener rather than anything under the sink.
Do I need a plumber for under sink filtration?
Inline and multi-stage systems with push-fit connectors are within reach of a careful DIYer, especially reusing an existing sprayer hole for the faucet. Drilling stone counters or adding an RO drain saddle is where hiring out starts to make sense.