Kitchen Water Filter Varieties: Under-Sink, RO, and More

To ensure the purity of your home's water supply, there are three primary categories of water filters specifically designed for kitchen use: under-sink filters, reverse osmosis systems, and multistage filters. While these filters effectively improve water quality, they might not eliminate all contaminants.

Under-Sink Filters

The under-sink water filter, as the name implies, is installed beneath your kitchen sink. The mechanism of under-sink filters relies on water pressure, not electricity, to drive water through the filter. Contaminants in the water are captured as it passes through the filter medium.

Budget-friendly under-sink filter systems often come with a dedicated faucet and filters that are effective for a limited period, usually around 12 months or 8,000 gallons. Different manufacturers have varying recommendations for filter replacement. Some suggest changing filters every six months if the water tastes or smells unpleasant, while others propose setting reminders on your phone or calendar. More advanced models feature filter replacement indicators to help you stay on track.

Reverse Osmosis Systems

The reverse osmosis (RO) system employs water pressure to push tap water through a semipermeable membrane, resulting in two streams: permeate (clean water) and concentrate or brine (rejected water). Commonly referred to as a point-of-use reverse osmosis system, this filter is typically installed at a single fixture, often the kitchen sink. A properly functioning reverse osmosis system has the potential to filter out contaminants such as lead, bacteria, viruses, volatile organic compounds (VOCs), and per- and poly-fluoroalkyl substances (PFAS).

Most reverse osmosis filters incorporate multiple filtration stages. Tankless models are designed to save space under the sink. The Centers for Disease Control and Prevention (CDC) recommends filters with a 1 micron or less pore size to effectively target Cryptosporidium and remove microbes larger than 1 micron. This is sometimes indicated as an "absolute 1 micron."

Multi-Stage Filters

Multistage filters, also referred to by the number of stages they offer, are designed to address different contaminants as water passes through each stage. Each layer serves a distinct purpose: one might reduce larger particles, sand, rust, and nematodes; another might focus on carbon and odors; another targets water scale; another is dedicated to reverse osmosis; and one might be an alkaline filter. Positioned somewhere between, a filter layer effectively removes tiny particles such as lead, mercury, zinc, iron, copper, cadmium, aluminum, and chlorine.

Ultrafine-membrane filters can remove volatile organic compounds (VOCs) and microorganisms like E. coli and Cryptosporidium. The greater the number of stages in a filter, the higher its cost. Multistage filters are a versatile choice, catering to various needs and preferences for improved water quality in the kitchen.