Reverse osmosis is a filtration method that separates water from what is dissolved in it by pushing it through a membrane. The pores in that membrane are around a millionth of the width of a human hair, small enough to pass a water molecule and hold back most of what water picks up on its way to a tap.
Osmosis, and why this is the reverse of it
Left alone, water moves across a membrane from the less concentrated side to the more concentrated one until both sides match. That is osmosis, and it is how a plant root draws water out of soil. Applying pressure to the concentrated side drives the movement the other way: water crosses, the dissolved load does not. Hence reverse osmosis.
That pressure is the part most descriptions skip. An RO membrane does not work on gravity or on contact time; it works because one side is pushed harder than the other. In a household system that push is usually the mains supply itself, which is why the pressure at the inlet decides whether a system will perform as specified.
Two streams, not one
Everything the membrane holds back has to go somewhere. An RO system therefore produces filtered water on one side and concentrate (the water carrying the rejected load) on the other. A system that produced only filtered water would clog within days. The ratio between the two is one of the numbers worth asking a manufacturer for, because it is what your water bill notices.
What sits around the membrane
A membrane is never the whole system. Sediment and carbon stages run ahead of it to take out the particles and the chlorine that would otherwise shorten its life, and a post-carbon stage often runs after it to finish the taste. The membrane is the stage that does the separating; the others exist so that it can keep doing it.
Pump-free is about the pump, not the pressure
KomiBright's under-sink and commercial systems run without a booster pump and without an electrical supply, on the pressure the mains already provides. That is a statement about the pump, not about pressure: below the minimum inlet pressure a system is rated for, it will not perform. The A95R is the deliberate exception in the range: it carries a 25 W priming pump and is the one model rated for open water sources such as a river or a lake.
