Water Purification, Elegance = Simplicity
by John Laumer, Philadelphia on 09.19.05
The Engineers Without Border's, Madison WI Chapter recently sent a student delegation to a rural Rwanda community to help implement a sustainable drinking water project. They were led by a UW Professor interested in averting the resource intensive "high tech, low impact" solutions engineers typically pursue. The project faced cultural challenges and yet met with success, as exemplifed by the blue water bottle shown. Look closely TreeHuggers. It's not your ordinary water bottle.
Professor Boscher..."took with him a device that represents the kind of elegant design that could change lives in developing nations. He had been thinking about the problem of purifying water. The simplest way to kill bacteria in contaminated water is to heat it, but boiling water consumes precious fuel. Solar energy — one thing Rwanda has in abundance — can also do the trick, but the problem comes in knowing if water left out in the sun has heated enough to eliminate contamination.
To solve that dilemma, Professor Bosscher had located a simple, clear tube with a daub of wax at one end. The tube hangs on a string inside a water jar with the wax end up, and once the water around it becomes hot enough to kill the bacteria — about 150 degrees Fahrenheit — the wax melts, running from the top part of the tube to the lower end. All villagers have to do is look at the tube to be sure the water is safe to drink. To use it again, they simply invert it".
For you borderless non-engineers, wax can be formulated to have a a variety of melting points and to melt either very slowly or relatively quickly (all at once). The trick is to choose a wax that indicates that surrounding water has been exposed to the sun long enough to ensure adequate sterility. Obviously the tube is cheap and lasts through many uses -- an ordinate of systainability. And any old jar or clear bottle will do. [Thanks to Shauns' comment, (below) an error in the original version of this paragraph has been corrected. In essence, sterility is a function of ultraviolet exposure as much or more than heat.]
Water.+ Bottle. + Sun.+ Tube.+ Time. = Water Free of Disease Causing Organisms
Too bad there weren't a whole bunch of these and some basic charcoal filters stored in the New Orleans hurricane shelters. Elegant simplicity might work in developed nations too.
Brilliant. I can see this becoming a prevalent thing in places where clean water is scarce. The magic appears to be in the little tube, rather than the bottle. Everyone in the developing world has bottles, just send them those tubes to suspend in the water, or teach people how to make their own.
Boscher and the team should be given a Nobel or some humanitarian equivalent.
===author's response follows ====
Agree. I've not taken the time to investigate whether this is an adaptive use of an existing product (the melt tube) or a custom built prototype made by the team. Whichever party(s) deserves the credit ... could include several people/organizations...should get further recognition.
Wow, that is world changing.
==== author's response follows =====
It's also engineer changing. Made me wonder if the changing nature of the profession had an influence.
The Water Pasteurization Indicator (WAPI) has been around since 1992... see solarcooking.org/metcalf.htm for details.
It is an amazingly simple system, and I agree it is worthy of global recognition. The link above also gives details on the wax inside the WAPI, in case you want to make your own.
Another useful article: http://aem.asm.org/cgi/content/full/65/2/859. Many more articles on http://www.solarcooking.org also cover water pasteurization.
Cheers,
Dave
Please don't use this method to heat water to 150 Fahrenheit and expect it to be safe to drink! Many very dangerous viruses and bacteria can easily withstand 150 Fahrenheit. The real equation is:
Water + UV Transparent Container + UV rays from the Sun + Time = Water Free of Disease Causing Organisms
Heating water to 150 fahrenheit WILL NOT KILL all of the disease causing bateria or spores or virues. You need to first clear the water of any cloudiness. Then simply lay a clear, water filled container in the direct sun. 15 minutes in bright high-angle sunlight will kill all diesease causing bacteria, viruses, and spores in a small container. Cloudy weather, large containers, and low-angle sunlight will require longer sterilization times.
If sterilizing water was as easy as this post makes it seem, access to clean water would not be an issue!
====== author's response follows =====
Thank you for adding a comment which takes our understanding to the next step. While some plastic bottles may contain UV interrupt additives for the purpose of preventing sunlight-based degradation of the plastic, and would thus slow the desired UV sterilizing effect of the contents, the property is incomplete and not identical with all plastics; plus most glass jars do not have this added property. In other words, UV exposure is part of the equation as you say. My presumption, when putting this post together was that heat, or in this case thermal history, of the water is taken to be a surrogate indicator of the amount of time that UV exposure has occured. I also assumed that the wax tube was easy to explain, in spite of any language barriers or lack of technical education, and therefore is relativley fool proof for teaching. The larger the bottle, the longer the exposure time needed, proportional to the drop in surface area to volume ratio. So, tis good that people don't need to think about what the processes are occuring and are taught not to just wait for a single time period. Just watch the wax.
Light penetration will be greatest at the internal surfaces most perpendicular to the sun's rays, which would lead to a non uniform heat flux, and hence a continuing turbulence that would bring all water "parcels" in close contact with incident sunlight. Should someone put the wax tube inside a dark-walled containe,r it may still be possible to achieve the melt eventually; but the tube itself would not be observable unless removed, presenting a clue that this type container is incompatible!
As for the turbidity issue, that is complex as the ability to absorb UV light is not directly proportional to particle size or particle density alone. Very large particles are resistant even to chemical sterilants, so this problem is common to all water treatment methods.. One would hope that relatively clear water is available to fill the bottle with. On the other hand, if only cloudy water sources are available, then people are already drinking out of muddied streams and puddles and using the new method will not add disease risk. It is possible, depending on what kind of particles are causing turbidity, that the device may still perform some exposure reduction and offer an health benefit.
Your post make the following ascertation:
"The trick is to choose a wax that indicates that surrounding water has been hot enough, long enough to ensure adequate heat-induced sterility."
This statement leads people to believe that it is the heat being generated that sterilizes the water. I simply wanted people to understand that while heating water to 150F does help destroy bacteria, UV sterilization is the important component of this process.
===author's response ====
Right you are Shaun. The original post was not written thoughfully and I will make a correction. Thanks for pointing this out so gracefully.
Being the person that was referenced, I would also like to set the record straight. I want to first say that we were not the originators of that concept. We merely borrowed a good idea for our project. Dale Andreatta, at that time a graduate engineering student at the University of California, Berkeley, was the originator. The device is called a WAPI (water purification indicator). The WAPI itself can be ordered on http://solarcooking.org/sci_orderform.htm. Dale did the original research to determine the physical conditions needed to kill the pathogens.
We make them because we need many for this project but you can buy them and other solar stoves at http://solarcooking.org/. This is a wonderful organization. You'll see a number of useful solar methods on that website. A good description of what is currently being used in water purification/testing is at http://solarcooking.org/metcalf.htm as mentioned above.
Unfortunately, they don't really have anything about it up on their web page yet, but the D-Lab at MIT is also working on this sort of stuff (along with other neat engineering tricks for solving problems in the developing world). http://web.mit.edu/d-lab/portfolio/solardis.htm
==== author's response ====
Great feedback. Makes TreeHugger a happening place for intelligence gathering / distribution.
It is such a great idea. I think it would be a very interesting topic for a science fair. I would love to involve or focus my topic on this. I wonder if you could help me refine this idea into something I can test. Maybe something like, the bacteria that survive 150 degrees fahrenheit. Something like that would be perfect for my year long biotechnology II project. I would aprreciate it so much if there was someone that would like to help me out a bit with that.
I am one of the students on the EWB-UW team that implemented solar water pasteurizers and WAPIs in Rwanda this summer. In preparing for this project, we talked to various experts in the field including Bob Metcalf (one of the co-founders of Solar Cooking International) and they all agreed that heating water to 65*C (~150*C) would kill the bacteria of primary concern for unclean drinking water in developing countries (e.g. E. coli and coliform bacteria). Professor Metcalf, a microbiologist, acknowledged that UV radiation can help to kill bacteria but that it is a far more uncertain method (consider all of the qualifiers used above--direct sunlight, a non-reflecting container, etc.) and in the time one would need to maybe pasteurize with the UV effect, the water could be sufficiently heated in a solar pasteurizer with a WAPI to ensure it is clean. Field testing of several designs of solar pasteurizers in Rwanda confirmed that heating alone eliminated all presence of E. coli and coliform bacteria. UV was not a factor in these tests as they were performed in opaque metal pots. From talking with researchers informally at UW, it appears that UV pasteurization of water is not as promising as once hoped.
==== author's response follows =====
Thanks...great feedback. This comment has IMPORTANT information for anyone reading down-thread.