With membranes it is about filtration of ‘particles’. For filtration special (semipermeable) membranes are developed with small to very small ‘pores’. In other words, membrane filtration is a selection principle on particle size. Where water (and some other small particles) can go through the membrane, the larger particles are retained on the membrane. To get a water flow through the membrane a pressure difference over the membrane will be introduced.
See some simplified (and generalised) information of the main system types below:
microscreen filtration MSF
pores: 10 µm
removal of: suspended solids, algae
lowest pressure (below 0.1 bar)
micro filtration MF
pores: 1 µm or 1000 nm
removal of: suspended solids, algae, bacteria
low pressure (below 2 bar)
ultra filtration UF
pores: 0.1 µm or 100 nm
removal of: suspended solids, algae, bacteria, viruses
less low pressure (below 3 bar)
nano filtration NF
pores: 0.01 µm or 10 nm
removal of: suspended solids, algae, bacteria, viruses, micro contaminants, (‘larger’) polyvalent ions
higher pressure (above 5 bar)
reversed osmosis RO
pores: 0.001 µm or 1 nm
removal of: suspended solids, algae, bacteria, viruses, micro contaminants, ions.
high pressure (above 10 bar or even much higher in case of sea water)
For the different types of membranes, different types of technical solutions are developed to get a high surface membrane area on a small foot print. Depending on the particle size to be separated, more or less solids will be captured on the membrane surface. Such ‘deposit’ must be removed frequently to maintain a good flux (= water flow through the membrane) at reasonable low pressure (= energy) input. For removal, a so called backwash (BW) is used. Sometimes chemically enhanced backwash (CEB) is needed as well. To remain good performance over long time, also cleaning in place (CIP) is used.
Alternatively, membrane systems can be designed for continuous mode with a concentrate (or brine = captured ‘particles’) and permeate (= purified water) flow. Basically such systems don’t need a BW or CEB. A periodically CIP remains, depending of the incoming water quality and type of membranes applied. To keep the membrane surface ‘clean’ such systems often rely on a crossflow setup. That a (turbulent) water flow prevents particles from depositing on the membrane, to keep it in suspension.
RO 
NF 
UF 
MSF