Recently I have been studying and writing about various methods for disinfection of water . Since I first learned about solar stoves, I have made a number of them in different designs and have also given several workshops about how to make them. Solar stoves have the capacity to heat water to temperatures adequate for Pasteurization, and small temperature indicators can verify that. Of all the methods available this one of the few that has a secure endpoint, positive proof that each batch of water has been disinfected. Other methods can only assume that the water is safe, based on previous experience. Dr. Metcalf who developed the testing methods I advocate calls this “evidence based versus faith based methodologies”.
Sand filtration systems are in this category. When working correctly they are effective and produce a fair volume of water. They have many possible defects however and require proper setup and maintenance to work safely.
One organization, Aqua Clara, has designed a small sand filtration system that seems simple enough and inexpensive enough to be used by families . It is designed to be constructed in a plastic garbage can or similar sized container, and uses PVC pipe for the internal piping. At the bottom of the container is a layer of gravel or small stones, larger than the hole sizes in the pickup tube running across the bottom. Above that layer is one of finer gravel. On top of that second layer is a thin layer of a metallic biocide, a material that kills bacteria. Finally, above that is a layer of fine sand. Thus there are at least 4 layers from bottom to top of increasingly fine sand material. In practice a layer of active bacteria and yeasts slowly forms on the surface of the sand, called a “schmutdecke” in German. This bioactive layer also helps to kill or inactivate pathogenic organisms.
The principal of the sand filter is that contaminated material slowly passes down through the schmutdecke and the very fine sand and the contaminants get trapped in the pores and interstitial spaces between the sand grains. There is also some biological activity killing pathogens in the sand layer. Any pathogenic materials that make it through the sand layers are killed when they pass through the biocide layer. Finally water pressure from above forces the clean, decontaminated water up and out the outlet pipe.
Aqua Clara suggests that the sand layers be put down very evenly and carefully so that no small channels develop through the sand, thus allowing the water to pass though without being filtered adequately. Operating properly they say it should take up to 24 hours for water to filter from the top to the bottom. Also, to be effective it make take several weeks for the bioactive schmutdecke to develop and the water may not be completely decontaminated or safe until that time.
After studying the informational material provided by Aqua Clara, I decided to try and make a small sand filter model that would demonstrate the principal of the system. I wanted something that I could take with me as an example for people to examine. I also wanted to test the methods of construction that Aqua Clara use to make their filters. After learning and practicing their procedures by making a model, I planned then to make a full size filter similar to theirs and test it for effectiveness.
I had some coarse sand from previous projects so I made two sifting screens similar to Aqua Clara's instructions and sifted the sand into three sizes: small gravel, course and fine sand. After sieving the sand into the three sizes I washed the sand to remove dirt and impurities. The procedure is to take small quantities of sand and swirl them in water for a few seconds then pour off the dirty water while the sand settles back to the bottom. Usually several washes are required to get the sand reasonably clean. The sand is then spread out to dry on clean plastic tarps.
Because of all the sand and water, these filter systems are very heavy and difficult to move, so mine was made in a small waste basket, but when completed it still weighed over 30 pounds. I cut the front panel out of the waste basket and added a
piece of Plexiglas so people could
see the different layers inside. I
then cut and fitted ½ inch PVC pipe
to make the pickup tube and outlet
pipe.
It's important to place the layers carefully in the filter so that no air is trapped and the layers are even and well packed. Starting with the gravel layer on the bottom I placed the material around the pickup tube so that the openings were not blocked. After several inches of gravel came the layer of coarse sand. In practice these layers are put in with lots of water and stirred to settle so that no air is trapped and no channels develop through the sand. I didn't have any metallic biocide material, so I put in a thin layer of white sand to indicate that material. The final fine sand layer extends up to within an inch or two of the outlet tube so that the water level remains above the sand and the sand cannot dry out.
Due to weather and other commitments this whole project took me several weeks to complete, but turned out pretty well when done. My goal was to have it ready by the time of the World Water Day activities in Portland, I was scheduled to give a presentation and I wanted the filter for a display. I made up an exhibit that included the water filter and a solar reflector oven and some other things with explanations and I thought it went pretty well.
One small problem with the filter was that the different layers tended to sift together, especially when I moved the filter. In normal operation that would not be an issue, especially if the layers were well settled, but in this case where the layers are visible they don't look as clean and distinct. If I redo this filter or make another for display, perhaps I can put something like plastic wrap in between the layers to keep them separate.
Another issue is its weight of about 40 pounds. Maybe I could make a smaller filter that isn't so heavy or substitute some other lighter material for the sand, or I could just have a nice picture of the layers and have the filter itself empty.
My next goal is to make a full size filter in a large garbage can similar to the ones made by Aqua Clara. They are quite specific about every detail of the construction so I'll try to follow their instructions closely. I have a power-point of one of their training sessions so I can try to watch it as I work. I want to build it in my back yard and use rainwater from my roof to charge it. Originally I had planned to make it a simple system that just passed the rainwater directly through the filter, but Aqua Clara says the water has to be added slowly every twelve hours, so I guess I'll have to have a rainwater storage tank that I can then pipe to the filter at intervals. I'll try to build it this summer during the dry season so that it will be ready by next fall.
Aqua Clara International has been very helpful to me, sharing their methodologies and giving me the biocide material to test and use. There are other manufacturers of water filter systems, but Aqua Clara seems committed to designing systems that local communities can build and maintain for themselves and to transferring the technology to them. I appreciate their concept and I hope that I can support them in their work. This should be a really interesting project to complete for me. I'm anxious to see how it works and to test it for a while here at home. It's important to use things yourself that you advocate for others; to practice what you preach. Just as I've used and tried to improve the latrines I've built and cooked and drank the water from the stoves I've made, I want to build and use water filters before I promote them to others.
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