In reply to TobyA:
> UKC Dan just checked for me with the UK distributors of the MSR filter I'm trying. It's a pretty top level (expensive!) filter designed for the US military and very impressive in many ways BUT it won't remove chemicals such as fertilizers and the like. I wonder if any chemists here can explain why, but I suspect that it might be that those sort of chemicals once in solution are smaller than the viruses, bacteria etc that filters do get? My GCSE chemistry and biology doesn't help much here I'm afraid
The chemicals in fertilisers are orders of magnitude smaller than viruses - think of it this way viruses (much smaller than bacteria on the whole) are made out of thousands of large carbon chains each with hundreds of carbon atoms in them, whereas most bog standard water soluble chemicals would be in the order of 10s or maybe 100 carbon atoms. So a filter is not going to physically remove chemicals by sieving the water (essentially what most filters do).
What will remove some chemicals is sticking them to a filter, so-called 'activated carbon' can do that to some extent, which is why it is found in many filters but it won't be able to form bonds with all chemicals you might encounter in polluted lowland water supplies, and I imagine it would get saturated before too long. Thus why MSR can't guarantee nor even recommend their filters for that purpose.