What is an Under-Sink Water Filter?

An under-sink water filter is a common water purification device that works to remove many different kinds of contaminants from drinking water. 
Purification device means that the system is installed where you need access to clean water, such as at your kitchen or bathroom faucet, office, RV, or anywhere you have an appliance that requires clean, filtered water. Also, under-sink water filters are in-line filters that connect directly to your waterline. 
The water passes through the filter before coming through your tap or other outlets. However, some under-sink filters dispense water through their own specially-installed faucet.The water delivery process of a conventional under-sink filter is pretty simple and straightforward. The filter gets its water via a flexible plastic tube that’s connected directly to your cold-water line under the sink. Essentially, the tube diverts some of the water to the filter.
The water then leaves the filter through a separate flexible plastic tube and is delivered to you through the filter’s own dedicated faucet, mounted on top of the sink. That means there’s no mixing with unfiltered water.
Because of the constant pressure on the system, the water flows from the filter and out the tap when you turn on the dedicated faucet. Only drinking water delivered via the special faucet is filtered. The sink’s regular cold and hot water functions remain independent.
In contrast, simple under-sink filters have a slightly different water delivery system. In this arrangement, all of the water is diverted through the filter, and the filtered water is dispensed only through the regular cold-water sink faucet.