Membrane filtration covers four categories separated by how small the things they retain are, and they are often discussed as though reverse osmosis were the only one. Each has a distinct place, and choosing the tightest available is not automatically right, because tighter membranes need more pressure and produce more waste.
Microfiltration and ultrafiltration
Microfiltration is the most open, retaining suspended solids and bacteria while passing everything dissolved. Ultrafiltration is tighter, retaining bacteria, viruses and larger organic molecules, and still passing dissolved salts and minerals entirely. Both work at low pressure and, importantly, most ultrafiltration designs run with little or no waste stream, which makes them attractive where the concern is microbiological rather than chemical and where the water's mineral content is wanted.
Nanofiltration
Nanofiltration sits between ultrafiltration and reverse osmosis, retaining most divalent ions, which includes calcium and magnesium, while passing a good proportion of monovalent ones such as sodium and chloride. That selectivity is why it is sometimes called a softening membrane: it reduces hardness and larger organic molecules substantially while leaving the water less stripped than reverse osmosis does, at lower pressure and with less waste. It is common in municipal treatment and less so domestically.
Reverse osmosis
The tightest of the four, retaining dissolved salts and minerals broadly, which brings nitrate, arsenic, fluoride, lead, many heavy metals, PFAS compounds and total dissolved solids into range. It needs the most pressure and produces a reject stream that goes to drain, which is the cost of keeping the membrane surface swept clean. It is the correct answer when the concern is dissolved and the wrong one when the concern is only microbiological or only particulate.
What every membrane needs
Protection. Chlorine degrades thin film composite material, which is why carbon stages sit ahead of reverse osmosis membranes and a spent carbon cartridge costs a membrane rather than a cartridge. Sediment fouls the surface irrecoverably, which is why a sediment stage comes first. And hard feed water scales the membrane, which is why softening ahead of reverse osmosis extends life considerably on hard supplies. Our softener guide covers protecting a membrane's feed.
Questions people ask about water membranes
What are the four membrane types?
Microfiltration, ultrafiltration, nanofiltration and reverse osmosis, in order from most open to tightest.
Which one removes bacteria without wasting water?
Ultrafiltration retains bacteria and viruses at low pressure, and most designs run with little or no waste stream.
What is nanofiltration for?
It retains most divalent ions including calcium and magnesium while passing many monovalent ones, so it softens at lower pressure than reverse osmosis.
What damages membranes fastest?
Chlorine reaching a thin film composite membrane through a spent carbon stage, then sediment fouling, then scale from hard feed water.