Simple paper water filter under development

There are a lot of good ideas in play there – the very inexpensive nature of such a filter, including the water-safety tips on different pages and the feel-good aspects of rich Westerners donating to get "books" into the third world that directly help avoid disease.

But the form factor is all about it being "a book". The "funnel" and receiving vessels nest around the book of filter pages, but provide very little volume. For someone to use this a lot, it needs to be as easy to use as possible and treating a liter at a time isn't very easy.

My hackles are raised by the claims here. To the best of my knowledge, silver does not 'kill' bacteria. It prevents bacteria from dividing and multiplying. That's why it is used in the tanks of yachts and so on: to stop bacteria (which are always there) from spreading. But, (again, to the best of my knowledge), what bacteria are there remain alive.

Twoo other things to watch: viruses and protozoa are easily as much of a risk to us, but this filter will do little or nothing to handle those, and – where's the legal statement of compliance with EPA regulations? Not seen!

Will anything come of this one? I suspect not. It does not provide the protection people need – like against viruses and protozoa.

Cheers

Roger, are you sure you're not mixing up silver with UV? Every source I can find indicates silver and certain other metals are bactericidal. In terms of MoA, it's thought that silver ions damage bacteria by altering protein structures, deactivating key enzymes, and possibly denaturing the DNA.

Hi Jeremy

To The Best Of My (limited) Knowledge:

UV actually hacks some of the internal bonds and kills the bacteria or virus or protozoa. That is hardly surprising, considering the energy in the UV-C photons. They are quite capable of disrupting DNA bonds.

My understanding is that silver does 'damage' a bacterial cell so it cannot fission, but the cell remains 'alive' – as far as a single bacterial cell can be said to be alive. If not 'alive' at least intact. Good question though: if the cell cannot fission, is it alive? Standard tests for bacteria mostly assume that the bacteria will mulitply (like in a Petrie dish on agar gel), so if they don't many simply call them dead.

Exactly how the silver ion really works is still not fully understood, so I may be wrong.

A caution is needed about the water to be treated: if there are other impurities in the water they may preferentially absorb the silver. Other metal ions like iron will interfere – and we often see iron bacteria in the water. They are harmless to us.

Now, what the marketing guys make of all this is another matter. The general public might have a problem with the fine distinctions here, leading marketing to just say silver 'kills' bacteria. I can understand the problem: marketing wants to sell their silver-based chemicals against chlorine dioxide chemicals (for instance), and that can be a hard sell.

Regardless, neither viruses nor protozoa are handled by this. Both will likely slip through the paper filter. And other contaminants may render the filter paper worthless. I suggest waiting to see if they ever get any EPA certification.

Cheers

Hi Roger,

Yes, I sent them a query asking about non-bacterial agents. I'll post up if there's any response.

The reason I asked about UV was that it tends to neuter bacteria by forming thymine dimers that block transcription/replication rather than killing them. (Thus one worry people had regarding UV disinfection is photoreactivation, where leaving the water exposed to sunlight might permit photolyase enzymes to repair the damange.)

Regarding the silver ions, at least one thing they do is interfere with cellular respiration, which is a little more vital to the cell then reproducing. Also, here's a nice image of their effect on cell membranes:
http://aem.asm.org/content/71/11/7589/F2.expansion.html

CDC says “Silver ion has bactericidal effects in low doses and some attractive features, including absence of color, taste, and odor. The use of silver as a drinking water disinfectant is popular in Europe, but it is not approved for this purpose in the United States, because silver concentration in water is strongly affected by adsorption onto the surface of the container, and there has been limited testing on viruses and cysts. In the United States, silver is approved for maintaining microbiologic quality of stored water.”

They also point to ceramic filters impregnated with silver that do well on bacteria but not viruses. My hunch would be that these are roughly equivalent to things like the Sawyer mini, with the added benefit that the silver content keeps bacterial growth in the filter down so you don’t have to clean with bleach – probably a big benefit in more remote/undeveloped areas.