Mr. Weber-Shirk,
Hello! My name is Gillian, and I am in 7th grade at Kutztown Area Middle School, Pennsylvania. I am participating in the Future City Competition. Future City is where a team of middle school students creates a metropolis in an area of their choice; the city they build must be set in the year 2150 or beyond. This year my team has chosen Iceland in which to place the city.
In this morning's paper, The Reading Eagle, I read an article that
explained how you created a simple water filter in Honduras. It said that no energy was used, but rather just the force of gravity to move it. I was hoping that you could possibly explain this mechanism in more detail. Also, do you think that this could be mass produced to filter enough water for an entire city of the future? If not, what future technologies would you suggest that we research to help us imagine and describe the sustainable whole-house water system the Future City competition requires us to build?
I thank you for your time, and I hope that you will send a response.
Gillian
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Hi Gillian,
My name is Rachael Moxley. I am a senior at Cornell University and a member of the AguaClara Project. I will happily give you a description of the water treatment process, and some information to allow you to access whether it will be best for your future city. The technology we design to clean water is not a filter. Filters work by passing water through a porous (filled with holes) material. Particles in water are removed in a filter by physical means, for example if the particles are larger than the holes they cannot pass through, and by biological slime layers that form around the filter material and trap (or eat) the smaller particles. The design of an AguaClara water treatment plant incorporates many of the processes that your local water treatment plant uses to treat your water,
that is if you have city water. Personally, my family uses a well, and we don't treat our water because it is assumed to be treated by the slow filtering of surface water through the earth to the water table; for some people even well water needs to be treated.
Back to the story--the basic steps in the water treatment plant process are as follows:
1. Surface water enters the plant via pipes
2. Coagulant, a chemical that makes the particles in the water sticky, is added to the water
3. A device (we use a float control) is used to maintain constant flow through the plant
4. the water flows through the flocculator.
Flocculation is the process by which the particles in the water collide and stick to form large clumps called flocs. In order to get the flocs to collide the water is guided by vertical baffles, or slats: think of a maze where you have to go up, then down, then up, then down. As the water turns the corners the water particles on the outside have to flow faster than those on the inside to cover the same distance. Now think of these flocs travelling at different velocities as cars, now if the cars are wide enough and not in their own lanes, they're going to crash. That's what happens to the flocs. If they hit too fast the bump off of each other, but if they hit just right they'll stick.
5. After flocculation the water flows into the bottom of sedimentation tanks, where the flocs settle, or fall to the bottom of the tank. The water flows up from the bottom of the tank and the clean water is collected from the top of the tank. It is important to get good flocs to form so they sink, but for the smaller particles there are two ways of helping them settle.
1. lamella: these are rows of plates that are tilted so that
the flocs only have to fall the distance between the plates to settle. After they hit the plate below them they can slide down toward the bottom of the tank.
2. a sludge blanket: if you can get the water to flow up
through the tank at the same velocity the particles want to fall down in the tank you can "trap" them in the middle of the tank of water in what is called a sludge blanket. Now when those small flocs that escaped all the collisions in the flocculator flow through the sedimentation tank, they get stuck in the blanket and the water flowing past the blanket is clean.
6. The water collected from the top of the sedimentation tanks is
chlorinated to kill disease causing bugs. There are still a few disease causing bugs that are hard to kill with chlorination, so you can add filters at the end to trap those last guys. Believe it or not, filters and chlorination both work better with cleaner water.
Is this the best solution for your future city?
Well, the reason we use gravity powered plants is to keep costs down and save energy. If you are considering using this technology you might want to consider what your priorities are. Gravity powered plants are good for areas that are hilly or mountainous. You can build a gravity powered plant anywhere, but if the land is not already steep you will have to construct the plant so that one level is higher than another. This makes a more expensive plant because you have to buy more construction materials like cement. It also requires an operator to ensure the right amount of coagulant is added, to empty sludge that settles, and to clean the tanks. A relatively new high-tech solution to water treatment uses only membrane filters. I don't know much about it, but the idea is to send the water through a whole bunch of filters, starting with the ones with the biggest holes, and ending with holes smaller than you can see. With this you can remove smaller contaminents. This technology allows you to get cleaner water and can take up less space, but is less sustainable. You have to clean the filters by forcing the water backwards through the filters using pumps and you have to replace the filters, which (depending on the hole size) can be very expensive to buy or manufacture.
I hope I have provided enough information for you to evaluate our water treatment process as part of your future city. If you have any additional questions, or questions about what I wrote, feel free to email me. I will answer them to the best of my knowledge.
Good luck,
Rachael
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Dear Ms. Moxley,
Hello! This is Gillian, the student from Kutztown Area Middle School, who asked you questions regarding a water purification system. (Since your response, we've been working hard to research future water systems and have just now come to some conclusions.)
Thank you so very much for taking the time to respond to my e-mail. Your explanation regarding the AguaClara system was most helpful. This information will certainly assist us in designing our city water treatment.
Our city is more likely to use the combination of flocculation, chlorine, and filters. It seems as if this system is more efficient and sustainable, that's good! I may contact you again with more questions.
Thanks again!
Gillian