A fantastic bit of kit to have in your bag, used since the second (or first?) world war and still going strong.Clearly only a filtration device but in its own element for highly silty/dirty water that will then allow your choice of purification to act properly. Nothing to break and works by gravity. available here on t'internet.

http://cgi.ebay.co.uk/New-Survival-Water-purification-bag-(MILLBANK-Bag)-SAS-_W0QQitemZ110401600892QQcmdZViewItem

http://www.greenmanbushcraft.co.uk/water/millbank-bag-water-purification-kit-.htm