The difference between membrane microfilters and membrane ultrafilters is the pore size. 

For example, LifeStraw's membrane ultrafilter pores are 0.02 micron —10 times smaller than the pores of the membrane microfilter. Both technologies work via the same mechanism, but ultrafilters also block viruses due to the smaller pore size. When dirty water enters the ultrafilter, the microscopic pores block viruses, bacteria, parasites, microplastics, dirt, and other turbidity forms. Clean water passes through the pores in the membranes making the water microbiologically safe to drink. Because of the smaller pore size, ultrafilters often have a slower flow rate or require more pressure to purify water than microfilters. LifeStraw Membrane Ultrafilters are capable of removing 99.999% (log 5) of viruses (including Rotavirus), 99.999999% (log 8) of bacteria (including E. coli and Salmonella), 99.999% (log 5) of parasites (Giardia and Cryptosporidium), and 99.999% (log 5) of microplastics. We classify our products that use membrane ultrafiltration technology as purifiers because they remove virus.