To filter water sink side you have four practical options, and they are not interchangeable. Faucet mounts, countertop units, under sink cartridges and under sink reverse osmosis differ in what they remove, how much cabinet space and plumbing they need, and how often you touch them. Match the option to the contaminant, not to the price.
Faucet mount: least commitment, least capacity
A faucet mount screws onto the aerator threads and diverts water through a small carbon block when you flip a lever. It is the right choice for a renter or anyone who wants better tasting water this afternoon with no tools. The trade off is size: a small cartridge means frequent replacement and a modest flow, and the diverter adds a fitting that eventually drips. It handles chlorine taste and some particulates well and does very little about dissolved contaminants.
Countertop units: capacity without cutting anything
Countertop filters sit beside the sink and connect to the faucet with a diverter hose, or in the gravity fed versions are simply filled by hand. They hold much more media than a faucet mount, so they last longer between changes and can carry more capable media stacks. Vendors sell countertop housings as a distinct product line for exactly this reason. The cost is counter space and a hose that has to be connected and disconnected, which is fine in a rental and irritating in a busy kitchen.
Under sink cartridge: the quiet default
An under sink filter is plumbed into the cold supply and either feeds the existing faucet or a small dedicated one. It is out of sight, holds a serious carbon block, and runs at full flow with no diverter to fiddle with. This is the option most kitchens should default to when the goal is taste, chlorine, and reducing particulates and certain metals. It needs a shutoff, a compression tee and enough clearance under the cabinet to swing the cartridge down when it is time to change it.
Under sink reverse osmosis: when the target is dissolved
If your report shows nitrate, arsenic, fluoride or high total dissolved solids, carbon alone will not move them and reverse osmosis is the answer. A membrane, a storage tank, a dedicated faucet and a drain connection make it the most involved of the four, and it sends some water to the drain by design. Remember that none of these four options treats the shower or the laundry, so if your complaint is scale or staining across the house, the fix belongs on the main line instead.
Questions people ask about filter water sink
Which sink filter removes lead?
Look for a model specifically certified for lead reduction. Some carbon blocks are, most basic taste and odor cartridges are not, and reverse osmosis handles it as part of a broader reduction.
How often do sink filters need changing?
Faucet mounts are the shortest lived and often need replacing every few months, under sink carbon blocks typically run six months to a year, and reverse osmosis membranes last considerably longer with prefilters changed on schedule.
Do I need a separate faucet?
Not for a plain carbon filter, which can feed the existing cold tap. Reverse osmosis normally uses a dedicated faucet because the storage tank supplies it at a lower flow.