Two different processes.  The ceramic Katadyn filter mechanically removes
particulates much like a sieve or strainer removes tomato seeds from the
tomato pulp.  This is filtration.  Can't handle viri as the partical size is
so small that the flow characteristics would be abysmal.  Can handle most
bacteria and cysts.

The iodine mesh systems like the PUR purifiers expose the liquid to iodine
as it is pumped through the mesh.  Will handle viri, bacteria.  Cysts are an
issue because of the exposure time.  Some cysts are tough enough to not be
destroyed in the short time they are exposed to iodine.  It is not as fine a
filter as the ceramics and is susceptible to clogging as its surface is not
easily cleaned.

Filtration or purification of muddy water can be aided by a settling process
or using a paper coffee filter to eliminate many of the larger particles.

Neither system will remove dissolved contaminants.

A solar 'still will get all the evil out, including dissolved contaminants.

A surface filter like finely divided charcoal will remove some contaminants
but not bacteria or viri.  It will make the water taste better.

The gravity fed Katadyn ceramic system is quite good and lets gravity do the
work.  It is of course bulkier than the "pocket" filters, but hey, that's
why you own the vanagon, so you have a home for all the niftyneato
accoutrements.

pensioner