Hi, quite a few of us have filters of one form or another, Rangeing from the basic millbank bag upto the all singing and dancing MSR waterworks 2 with all of its attachments.
I would be a little wary of filtering water from canals especially if they are near industrialised areas and filters cannot remove industrial poisons from your water supply, but water filters are pretty good for streams, rivers, lakes in the UK and around the world.
Be aware that filters tend to fall into a couple of categories;
The first being a straight filter examples being the likes of the millbank bag and heading upto the more expensive (and probably effective) Katadyn systems. With these you may have to add a chemical afterwards depending on your water source.
The second is a filter combined with a chemical purifier that sterilises the water post filtration by using an active ingredient like iodine or chlorine these include the some of the PUR systems. With these you have no option about chemicals they will be in the water whether you like it or not.
The katadyn systems use a ceramic disc that is impregnated with silver and has miniscule holes in it (about 0.2 of a micron) and effectively you use a pump to force the water through the semi - porous disc of ceramic. The silver impregnation is also supposed to create a small electrical field that will hold onto small biological contaminents thus limiting the size of just about anything that can get through. Katadyns were (and probably still are) on issue to the world health organisation.
The PUR type systems use a filter and chemical mix normally in two stages (filter then purify with chemicals), with these you will also be forcing water through a a fine filter, but at the end they then run it through chemicals (anything that is nasty and biological gets zapped by the chemicals should they get through the filter), the down side can be aftertaste.
Availability, most good outdoor shops sell them, Blacks, Feild and Trek, Taunton Leisure and quite a few others sell them, cost on the other hand can be excessive depending on what you buy, the Katadyn Pocket Filter is probably the most expensive, followed by the MSR with all of its attachments, PUR and then the Katadyn Mini filter so on and so forth.
It is worth looking at the overall cost of the filter, how many litres will it filter before needing to buy replacement cartridges, how much do they cost, what sort of flow rate are you going to get from them are all things to be considered and of course how much is it going to get used.
The British Army have got by using Millbank bags for years they are cheap, easy to use and about as robust as you can get as they have no working parts to break. I carry one as a backup and they are in the end probably one of the most tried and tested methods of filtration going. In the end there is only one thing to remember about them and that is don't forget to sterilise the water either by boiling or by adding tablets or drops, in the case of iodine, to the water post filtration. If you after one of these look at strike force supplies or on e-bay as they appear on there every now and again for less than £20.
I hope this will be of some use.
