by William Lidwell
August 08, 2005
Solid analysis and approach. I particularly liked the proposed regeneration process of the carbon-filled tubes. Dissolving iron in the mix is an interesting way to deal with the arsenic, and the sand filters with activated charcoal should take care of the other contaminants. Like several of the other entries, the main challenge would be time–doing this in two days would be tough duty. Since the UN rarely works at that pace anyway, I consider your approach worthy of both the UN and honorable mention. Congratulations and have a great year at college!
Our filtration process involves two main steps: the precipitation of arsenic out of solution via a chemical reaction mediated by dissolved iron, citric acid, and sunlight, and the filtering of the subsequent solution through bamboo tubes filled with layers of sand, gravel, and coconut-derived activated carbon.
The first step in creating a water treatment plant will be the production of activated carbon. Activated carbon is widely used as a filter material to separate organic materials, hydrated ions, and biological material from drinking water. It consists of highly unsaturated porous carbon material. Activated carbon has a large surface area, facilitating adsorption of impurities. Coconut shells are one of the most common starting materials for the production of activated carbon worldwide, so we will readily be able to produce it. The production of activated carbon involves two steps. First, the coconut shells must be heated at low temperature in order to carbonize (char) them. This could be achieved by holding the coconut shells a short distance from a cool brush fire until they are blackened all the way through. Next, this coconut char should be broken up and disintegrated into smaller parts, and then heated to as high a temperature as possible, preferably in the presence of steam. The resulting compound will be our DIY activated carbon, and the most important element of our filtration process, responsible for removing the bacteria, viruses, benzene, and precipitated arsenic.
Given the materials on hand, we suggest dismantling the bicycle and using the wheels as makeshift grilling surfaces. Some kind of semi-permanent bamboo scaffolding should be constructed to position one of the wheel-grills in close proximity to a fire pit, facilitating the initial charring of the coconut shells. The second wheel should be used in conjunction with portions of the barrels (which are both assumed to be metal and capped at both ends) to form a sort of steam oven. A third of one of the barrels should be combined with the very upper portion of the other barrel to create a Dutch oven, which should produce the hottest temperatures. The wheel should be secured inside the oven to provide a platform for the charred coconut, and several inches of water (soon to be steam) should be added to the bottom of the oven. The previous coconut-charring fire should have been constructed in a depression in the ground, and our Dutch oven should be placed on its coals. More coals should be added to the upper lid, with dirt and sand added around the sides to provide insulation. After several hours when all the water has boiled off, we should have our activated carbon, which may then be packed into lengths of the wide-diameter bamboo tube. We suggest covering one end of the tube with cloth from locally available clothing to prevent undue filter leakage, then using smaller-diameter tubes to tamp down the activated carbon, and then finally adding the sand and gravel layers (assumed to be locally available). NOTE: Once the filter has been used a number of times, the activated carbon will become saturated with impurities, and it will no longer be effective. Fortunately, the carbon can be regenerated simply by taking it out of the bamboo and heating it over a fire to drive off organic impurities and kill any microbes. Even simpler, potentially, the tube itself could be carefully combusted and the resulting (re)activated carbon/ash/gravel packed into new tubes. In the absence of practical experience, we suggest initial tube lengths of 5-6 feet, subject to adjustment.
These filter tubes should do a good job of removing all the dirt, silt, parasites, bacteria/protozoa, viruses, and benzene from the water. Arsenic, however, will most likely slip through unfazed due to its ridiculously tiny size. Hence the need for arsenic pre-processing.
Arsenic may be present in water in two forms: it may be incorporated in organic molecules or it may be present in inorganic forms. Naturally occurring organic molecules that contain arsenic are relatively nontoxic to humans. Therefore, the primary concern is with removing inorganic forms of arsenic. Inorganic arsenic may be present in water in two oxidative states: arsonite (As(III), primarily in the form of H3AsO3), and arsonate (As(V), primarily in the form of HAsO4). It is easier to remove arsonate from aqueous solution, so the first step will be the oxidation of arsonite to arsonate. Industrial processes for arsonite oxidation generally involve oxidants such as ozone or hypochlorite (bleach), but arsonite can be oxidized by atmospheric oxygen. This is a slow process, but the rate can be increased by photocatalysis. In the presence of iron (III), the process can also be mediated by the iron. It has been found that citrate, which forms coordination complexes with iron (III), can mediate the reduction of iron (III), leading to the production of reactive oxidants such as oxygen and hydroxyl radicals, which can quickly oxidize arsenite. Therefore, the first step in the water treatment should be to add a few drops of lemon or lime juice or some local analog (anything containing citrate in the form of citric acid) to the water and to allow it to sit for a couple of hours in bright sunlight. It may also be possible to increase the rate of oxidation by aerating the solution.
If aqueous solutions containing iron are allowed to sit still, iron (III) complexes will precipitate out of solution. Arsenate compounds will co-precipitate with iron (III), removing it from solution. With any luck, the iron content of the local water will be sufficiently high to allow this to happen. If no precipitate forms, then the iron content of the water may not be great enough and it will be necessary to dissolve some iron in the solution. Steel wool is made primarily of iron and can be dissolved in water by treating it with acid (possibly in the form of citrus juice — treat the steel wool with citrus juice and then wash the juice into the solution). The following precipitate could then be removed with the rest of the unhealthy additives during passage through the bamboo filter tube.
Thusly, we present the following design:
The top 1/3 of one of the barrels is cut off with the saw, and the very top lid of the other barrel is also removed.
The 1/3 barrel + lid + 1 bicycle wheel are combined into a dutch-oven.
The 2/3 barrel is placed on an elevated (~4ft) bamboo platform in a sunny area, and several bamboo- diameter holes are bored into its lower side. This will be the cistern, the starting point of dirty water.
The other barrel is placed on the ground near the platform.
5-10x 5ft length, 3in wide coconut-derived-activated-carbon bamboo filtration tubes are constructed
A portion of the bicycle tube is wrapped around the outside of the gravel end of several of the filtration tubes, and a cloth cover is attached to the other, carbon-filled end. The rubber-wrapped ends are tightly fit into the holes of the cistern on the platform and positioned to drain into the other larger barrel.
Villagers fill the cistern via any means they please, perhaps shuttling river water via the 6 1-liter bottles.
The $10 in American coins is used to purchase as much citric fruit and steel wool as possible. Juice from one of the fruits is run through the steel wool and into the cistern. Some of the fruits are planted (hopefully providing a replenishable citrus source), and the rest are stored.
The villagers are instructed to never just take clean water from the purification system, but to always trade dirty water for clean. In this way, the job of keeping the dirty cistern full is distributed fairly across the community.
Additionally, one or two specific people should be given the daily responsibility of adding the citric acid/iron to the dirty cistern, and of maintaining the integrity and (re)construction of filter tubes.
Hope you enjoyed our solution! We just got the latest issue earlier this week, and have been playing around with this design since. We really dig your magazine! In fact, one of us (Nick) just yesterday completed the construction of a iSight teleprompter. Asked what in the world he could practically do with it, he immediately responded that it would come in handy for presenting our water purifier design when the United Nations calls. (Hah. Hah. Right.) He later went on to admit that by present, he meant inundate, and by United Nations, he meant the next hapless relative that called.
Anyway, it was fun, and we definitely learned a lot. We’re particularly interested to hear the other solutions, and sincerely hope that some brilliant reader submits one that has real potential for implementation and really making an impact. Furthermore, we give you our greatest encouragement to continue “throwing down the gauntlet of innovation” with the MakeShift section of the publication. It seems like an incredibly powerful way to encourage a lot of really intelligent, innovative, creative people to develop novel solutions to pressing social problems. You are doing the world a service, and it is a blast!