Membrane filtration is one principle at four scales, and the scale decides everything: what is stopped, how much pressure is needed, whether there is a drain line, and what the treated water tastes like. Placing the four on a ladder from coarse to fine makes the choice straightforward, because you are really choosing how far down the ladder your problem sits.
Microfiltration and ultrafiltration
Microfiltration has the largest pores of the four and stops suspended solids and larger organisms, functioning much like a very fine mechanical filter. Ultrafiltration goes finer, excluding bacteria and protozoan cysts reliably while letting dissolved minerals pass. Both work at low pressure, need no drain connection and no storage tank, and leave the water's taste and mineral content unchanged. Viruses are smaller than most ultrafiltration pores, so neither is a complete microbiological barrier on a source suspected of viral contamination.
Nanofiltration
Nanofiltration is selective rather than thorough. It rejects multivalent ions, meaning calcium and magnesium, along with larger organic molecules, while passing much of the sodium and chloride. That makes it a membrane route to softening with no salt, no brine tank and no regeneration discharge, and a good tool for removing the organic colour behind tannin stained water. It needs pressure and produces a reject stream, though less of both than reverse osmosis.
Reverse osmosis
The finest of the four rejects almost everything dissolved, which is why it addresses the widest range of contaminants and why its product water is flat tasting and mildly aggressive to plumbing until remineralized. It needs the most pressure, a drain connection for the concentrate, and normally a storage tank because production is slow relative to demand. For a household with a specific dissolved contaminant to remove, it is usually the answer, fitted at the point of use rather than across the house.
What every one of them needs
Pretreatment, and the same pretreatment. Sediment filtration first, because particles foul any membrane surface. Dechlorination by carbon on a chlorinated supply, because chlorine degrades thin film composite material permanently and no cleaning reverses it. Scale control where hardness is significant, since the minerals being rejected concentrate exactly at the membrane face. And iron removal on well water. Skipping any of these turns a long lived component into a consumable. Our whole house filter guide covers the train that protects it.
Questions people ask about membrane water filter
Which membrane type removes bacteria?
Ultrafiltration and finer exclude bacteria and cysts by pore size. Viruses are smaller than most ultrafiltration pores.
Which one softens water?
Nanofiltration rejects calcium and magnesium, and reverse osmosis removes them along with everything else. Microfiltration and ultrafiltration do not.
Which needs a drain line?
Nanofiltration and reverse osmosis, because both reject a concentrate stream. Microfiltration and ultrafiltration do not.
Does chlorine damage all of them?
It degrades thin film composite membranes permanently, which is why a carbon stage ahead of nanofiltration and reverse osmosis is mandatory.