Message-ID: <363775AF.2CC8@AfricaOnline.Co.Ci> Date: Wed, 28 Oct 1998 20:51:11 +0100 From: Reid Harvey <mailto:ceramics@AfricaOnline.Co.Ci> Subject: An A.T. Water Filter To: mailto:DEVEL-L@AMERICAN.EDU
Thanks to those who have been helpful in response concerning the silver-impregnated water filter. I am now getting the strong feeling that the filter can actually be very effective. (I've been away from the internet for two years, and the kind of helpful dialogue these lists make possible.) It seems that part of the impression the filter is not viable may be coming from a few failures by well meaning researchers.And thanks to the people in Guatamala who actually made the whole thing work!
I cannot recall the name of the CDC expert who told me you couldn't get enough silver into the filter to make it effective, but I do recall the detail he went into about activated charcoal, and some other processes that seemed amazingly complicated. Where did he get these ideas? If I may be simplistic for a moment, I suspect that some of the people who profit from these filters are trying to make the whole thing sound complicated.
Sorry for that bit of paranoia. I usually refrain. I now think that in order to go forward with this filter we need real names in the various organisations that are involved with water. And since continued research on the filter is much too big a job for me alone we'll need to contact others who can do this as well. One invaluable response came from a man who's working on the same kind of filter for another organisation. Perhaps there are others I do not know about. I'd be happy to turn the whole idea over to someone else since I have plenty of work in other areas.
Obviously, the need for the filter is now more urgent then ever. Where I was working in Guinee Forestiere, in 1995 there were 500 cases of cholera. Perhaps just ten years ago this situation could not have existed. Sorry if mine seems like righteous indignation.
Thanks again. Reid Harvey
P.S. In brief, here are the approximate steps needed to make a silver impregnated, ceramic water filter. The basic filter can be in two parts (excluding lid, spigot, etc.): a catchment container and a filter that fits half way down inside the catchment container. This technology is well within the means of village women potters.
1. Form the filter, by hand or mold, etc. Use a lowfire, earthenware clay, without rocks or sand. Add in fine sawdust to give enough porosity for good flow. 2. Fire the filter for strength, somewhat less than 900C, depending on the clay material used. Some sacrifice in strength is necessary for good flow. 3. Apply 4% silver nitrate solution to the outside of the filter, either by dipping or painting. 4. The filter is now ready for firing. (It is possible to use a 'primitive' cylindrical kiln, with a dome that is built on top for each new firing. The dome consists of brick or once-fired clay scraps, mortared together with a mud/sand mix. This kiln is fired with wood.) Fire to about 475C, the first visible red. It is easy to judge this temperature at night, or when outside lighting is subdued. 5. It is now necessarry to put the filters in reduction, that is starve the fire of oxygen. This can be done either by filling the kiln with sawdust or a similar combustible, or by removing the filter, then covering it with the combustible. Seeking oxygen, the continued combustion pulls the nitrate from the silver, thus leaving silver metal behind in the ceramic. 6. Run water through the filter several times, in order to remove any extraneous nitrates or silver. 7. The filter is now ready to use. Micro-organisms are killed when they make contact with the silver metal. END