Inline reverse osmosis is used for two quite different things and the confusion is worth clearing up before buying. Sometimes it means a tankless reverse osmosis unit that produces water on demand with no storage vessel. Sometimes it means the sealed inline cartridges that make up the pre and post stages of an otherwise conventional system.
Tankless reverse osmosis
A tankless unit uses a booster pump and a higher capacity membrane to produce water fast enough to serve a tap directly, so there is no storage tank taking up half the cabinet and no stored water sitting for days. The benefits are space, no tank to sanitize or to lose its air charge, and no stale first draw. The costs are a power supply at the sink, more moving parts, a pump that can be audible, and no water at all during an outage since nothing is stored.
Inline cartridges on a conventional system
The other meaning is the sealed cylindrical cartridges with push fit connections at each end that make up the sediment, carbon and post filter stages on many modern systems, in place of cartridges sitting inside a screw off housing. They are quicker to change, since the whole cartridge is swapped rather than opening a sump, and there is no O ring to reseat and no water to spill. They cost more per change, and they are usually proprietary to the system rather than a standard size.
Which arrangement suits which kitchen
A conventional tank system is cheaper, needs no power, delivers a burst of water quickly and keeps working through an outage, at the cost of cabinet space and periodic tank maintenance. A tankless unit fits where space is genuinely tight and where nobody wants to think about a tank, provided a socket is available. Inline cartridges suit anyone who wants servicing to be quick and clean. None of these choices changes what the membrane removes, which is the same in every case.
What to check before buying either
Feed pressure, because conventional systems need reasonable line pressure to work at all and tankless units are pumped but still specify a minimum. Drain access, since every reverse osmosis system produces a reject stream that has to go somewhere. Cartridge availability and cost over several years, particularly for proprietary inline cartridges, which is where the running cost hides. And whether a remineralizing stage is wanted, since RO water tastes flat. Our reverse osmosis guide compares the formats in detail.
Questions people ask about inline reverse osmosis water filter
Does inline mean tankless?
Sometimes. It also describes the sealed push fit cartridges used for the pre and post stages of a conventional tank system.
What are the drawbacks of tankless RO?
It needs a power supply at the sink, has a pump that can be audible, and stores nothing, so an outage means no filtered water.
Are inline cartridges better?
Quicker and cleaner to change, with no sump to open or O ring to reseat, but usually proprietary and more expensive per change.
Does either remove more contaminants?
No. What is removed is set by the membrane, and that is the same regardless of whether the system is tanked or tankless.