In terms of home water treatment, things rarely get more powerful than reverse osmosis filters. These devices are capable of removing contaminants down to the size of individual water molecules—and when combined in a multi-stage system with other types of filter, can produce water that’s completely clean, safe, and great tasting.

Based on our review process, here are the best whole house reverse osmosis systems available to buy online:

How does Reverse Osmosis work, exactly?

Reverse osmosis (RO) is the opposite of osmosis. This is the natural flow of water from solutions of a lower concentration to a higher concentration. Left alone, water divided by a semipermeable membrane will travel in the direction of the most solutes, until the entire water system is equally concentrated.

By applying pressure to a water source—either through a pressure tank or simply the wider system pressure—reverse osmosis systems push water through a membrane in the opposite direction. That is, from an area of more to less concentration. The membranes used in a RO filter can have pores as small as 0.0005 microns, which is basically the minimum size through which water itself can pass.

As a result, any contaminant of equal or larger size than water molecules should be caught by the RO membrane. This includes sediments, bacteria, asbestos, lead, and other heavy metals. Some common drinking water contaminants do have a smaller molecule size than water, and can therefore make it through the membrane. Chlorine, for example, as well as some other organic chemicals, are not filterable by RO.

For this reason, it’s common for reverse osmosis systems to include a carbon filtering stage, to take care of any leftover materials. A pre-filtering screen can also extend the lifespan of a filter system, by removing large particles before they reach the more delicate RO membrane.

Point of use (POU) Vs Point of entry (POE)

Whole-house reverse osmosis systems are installed at the earliest point in a building’s plumbing, either where the mains water supply enters a house, or after the pressure tank of a private well.

This makes them a point-of-entry or POE system. These filters provide the ultimate peace of mind for home water treatment, ensuring that anything drawing water inside your home, be it sinks, showers, dishwashers, or laundry machines, is accessing pre-filtered water.

Despite this benefit, some find that smaller, point-of-use or POU systems better fit their needs. Only filtering water at specific outlets—the kitchen faucet, for example—reduces the cost of filter components, as well as lowering the possibility of reduced water flow, which can come with some POE filter systems.