Just getting a rainwater harvesting system in place on my boat - very exciting! For purification, as people will know, you have 3 main choices: Chlorine (which our mains water system uses but which is toxic in higher quantities and has a growing number of detractors), U.V treatment (great, but requires a constant source of power which is problematic on a narrowboat) and then there is the ceramic filter - a filter which has colloidal silver content which destroys bacteria such as E-coli, typhus etc. I hadn't heard about this until I discovered a brilliant website called Potters for Peace (http://www.pottersforpeace.org/).
Searching for something similar in this country, I bought a British Berkfield water purifier , made in the UK and using the same principle (though it looks like a tea-urn at a village fete - very English!).
Actually collecting the rainwater can be a big challenge as most boats have their water tank at the front and the boat slopes down at the back, so water flows in the opposite direction. I hope I've found a solution and will get my system up and running shortly, so I'll post back if it all works!

we just got a fairey ceramic from bob bagley at fountainfilters.co.uk
it works the same as a seagul which is advrtised in canal world.
you could technically use canal water but it would clog filter up really fast but wll rotect you from the stuff that grows in the tank.
it cost us £125 and a cartridge replacement is about £40 every 2 years for a two person household although if using canal water it wld be every 3 months!
it goes under sink and has its own tap to plumb in and the flow rate is a 3-4 liters a minute . bob bagley is a dear! call him on 01538750387