Health Supreme by Sepp Hasslberger

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March 24, 2006

Low-tech Solar Water Purification: It works

TANZANIA - A village is piloting a new way to purify water with simple means. The method is so simple it hurts to think that it has not been put into wide use. Take a transparent plastic water bottle, fill it up with water, lay it on a black roof for several hours. If the sun is beating down good, one hour is enough.


The combination of the sun's ultra violet rays and heat kills any pathogenic germs - the ones that spread much illness in Africa.

The beautiiful simplicity of the solution brings to mind another invention that received the Rolex prize a year ago: a no-electricity refrigerator made of two ceramic pots one inside the other. See this post on boing-boing for a link. There is also a neatly simple ceramic water filter technique linked from the same article.

Compare this with Dean Kamen's latest proposal for a machine that does a very similar job and that will cost between 1000 and 2000 $ when mass produced...

Here is the recent BBC article on solar water sterilization:

- - -

Using the sun to sterilize water

(original on BBC)

Tanzanian villagers have begun using an energy-saving method to sterilize their drinking water - leaving the water under the sun.

The piped water supply to Ndolela village in the central Iringa region is intermittent and even when it does flow, it is not clean enough to drink.

When the pipes run dry, villagers get water from a dirty spring.

Mother of five Rose Longwa says the new process has changed her life.

"We no longer suffer from stomach illness. That's because the water is clean and safe."

Like many other people in rural Africa with no access to safe drinking water, she used to sterilise her water by boiling it.

But she says the smoke from the firewood to heat the water used to irritate her eyes. She is also glad she no longer has to go to fetch wood from the bush.

Ultra-violet rays

About 40 houses in Ndolela are using solar purification.

Mrs Longwa says the process is simple to follow.

"I fill the plastic bottles, put the lids on, then put them on my black-painted roof where they stay for a whole day."

The sun heats the water, helped by the black roof, which helps to absorb the heat.

Solar radiation means a combination of ultra-violet rays and heat destroys the bacteria which cause common water-borne diseases like cholera, typhoid, dysentery and diahorrea.

After eight hours in the sun, it is ready to drink.

If the water reaches more than 50C, it is safe in just one hour.

Pastor Moses Kwanga from the Diocese of Ruaha is behind the project:

"The technology is very easy, but up to now people have not been told about it. We can use old pieces of roofing to put the bottles on. It is also very cheap, so is accessible to everyone."


Up to now, the number of people in Tanzania purifying water using the power of the sun is limited to a few villages like Ndolela, where small-scale education programmes are underway.

Daudi Makamba is a water expert for the aid agency Plan International, which is considering whether to introduce solar purification across the country.

He says it can be difficult to persuade people to use the technology.

"The big resistance from the community is cultural beliefs. People believe the water will be contaminated, or an enemy will put something bad in it, so we need to educate the people."

The technology is working well for at least one community in Tanzania but more work is needed if more people are to taste the benefits.

See also:

OpenWater Project
OpenWater is an educational video about where to find water in a disaster and different methods of purifying water. The five minute video uses a combination of pictures, video footage and music to convey the importance of clean water and that purifying water isn't a complicated process. If you have one, we encourage you to put OpenWater on your portable media player in case of an emergency. If you don't have a portable media player, print out the Water Purification page and store it with your 72-hour kit.

Chinese farmer makes solar-powered water heater from beer bottles


Ma's invention features 66 beer bottles attached to a board. The bottles are connected to each other so that water flows through them. Sunlight heats the water as is passes slowly through the bottles before flowing into the bathroom as hot water, reports China Economy Network. Ma says it provides enough hot water for all three members of his family to have a shower every day.

Here is a video that explains the concept of solar water purification quite well:

Cheap water purification using: SODIS


posted by Sepp Hasslberger on Friday March 24 2006
updated on Saturday July 9 2011

URL of this article:


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Readers' Comments

very, very good and very interesting.This is a MUST for thousands of our rural people. I mean the sun sterilisation of drinking water.

Posted by: ronald kirk on March 25, 2006 03:40 PM


How does the process of solar heating of the drinking water effect the leaching of the toxic chemicals of the plastic bottles into the water?

Granted, ill effects or death from the toxic chemical contamination will take much longer to manifest than the immediate illnesses and deaths produced from drinking water carrying diseases and parasites, but is there any developments looking at eliminating this problem?

Posted by: Donna Russell on March 27, 2006 10:22 PM



I have not heard anything about concerns over the chemicals that at times leak from the plastic bottles. Obviously, for these villagers the immediate gain of absence of infective water-born diseases is of greater concern than a largely theoretical and long-term contamination of the water with phtalates and other chemical goodies.

They could use clear glass bottles, if available, but in a poor rural environment, you use what you have - and that's plastic soft-drink or water bottles.

Posted by: Sepp on March 28, 2006 10:33 AM


This also brings up a related question of inert materials for water containers.

You've pointed out glass as a prime example. I was wondering if there is a list of metals/metallic compounds which provide for the "inert" property? And will these metals be effective for the above mentioned solar heating?

I asked, because glass can be fragile and a suitable replacement would be best for backpackers or campers in the wilderness.

Posted by: vish on May 30, 2006 11:12 AM


This question was looked at in a well designed study, taking plastic bottles from third world countries. The results may be found at: (link no longer active - Sepp)

Basically, they found that the level of "plasticizers" were no higher than water not exposed to plastic.

Visit the home page

for more information.

Posted by: Robert on September 4, 2006 04:40 AM


I've just returned from Tanzania and was investigating using discarded water bottles as building materials. I got very excited when I ready your article. I'm wondering if you have more information about the possibility of toxic ingredients leaching from the plastic when the bottles are heated by the sun or subjected to the UV rays. Please contact me if there is a program already in place because I want to see that one does get started in as many African communities as I can. THanks you, Jacuqie Hale

Posted by: Jacquie Hale on March 1, 2008 11:49 AM


Hi Jacquie,

thank you for your comment on the article regarding solar water purification.

About the possibility of toxins leaching, I saw an article two months ago that said no. Couldn't find that exact one again, but searching today I find:

Almost all soft drink and water bottles are made out of polyethylene terephthalate (PET), which is a polyester that, unlike vinyl, doesn't require the use of orthophthalates for flexibilty. PET has not been found to be toxic (it's biologically inert if ingested) and is not an endocrine disruptor.

(from: )

Whether there is a program in place, I wouldn't know. But whatever you find already exists, or whatever you manage to get going yourself, if you could post some information as a comment here, it would help others in the future...

Posted by: Sepp on March 1, 2008 01:19 PM


How does the recent evidence of plastic bottle "out-gassing" affect how the solar purification can be implemented?

Out-gassing, in this case, refers to the leaching of harmful (carcinogenic) substances from a plastic water bottle into its contents. It occurs when the bottle is exposed to heat, such as that of a car in a parking lot, or a tar roof in Africa. The affects of drinking such water are long-term.

I'm no expert, and this article was published before the more recent evidence of out-gassing of which I am aware. Of course, having safe drinking water trumps most concerns, especially those of the long-term.

Perhaps the use of glass, or any non-plastic bottles, as with the Chinese farmer above, would be a simple solution.

Posted by: Eric on July 29, 2008 04:13 PM


Great article. Did a search to advise someone on Youtube about the use of these bottles and clicked on the first link (Auto Pilot) Was delighted to find I was viewing Your Pages again Sepp. :)

This use of Solar Disinfection has great potential throught areas that have little in the way of safe drinking water. I first read about this being used in Africa many years ago, adopted by people have no option but to drink dirty water if there is no safe method of decontaminating it. These bottles are ingenious and we thrown them away in the West without giving them a second thought.

Andrew K Fletcher

Posted by: Andrew K Fletcher on August 10, 2008 05:41 AM


hi - i can see that working in countries close to the equator but how would it translate to higher or lower latitudes where more UV is blocked by passing through a thicker atmosphere?

Posted by: threenorns on August 27, 2008 08:35 AM


hey this is very good and quite authenticccc but i need some more information so please help!!!!!

Posted by: kanza on September 19, 2008 12:00 PM


I believe that the plastic used for water and soft drink bottles (which is what's being used here) is not made with bisphenol A or BPA, which is supposed to be the toxic compound. Unless you have different information, this should not be a great worry.

Posted by: Sepp on September 24, 2008 04:57 PM


1. How would this Solar Disinfection method work in a place such as India that has monsoon seasons?

2. Would you recommend an alternative and easily maintained water purification system that could supplement the use of plastic bottles in the Solar Disinfection method?

3. I'm looking for a system that would purify water for a community of a few hundred people. I've found a link to the PotPaz site lists commencial vendors. Any recommendation for a vendor?

Posted by: Joann on April 29, 2009 12:12 PM


I would think that during monsoon, it is possible to catch rainwater and drink it. If you have a few hundred people, you should probably look around for something more sturdy than disinfection with the use of plastic bottles. That PotPaz ceramic filter looks good. I have no experience with this however to recommend a vendor.

Posted by: Sepp on April 29, 2009 04:32 PM


its really wierd what people have to do to get clean drinking water. and i wish this website was a bit more helpful i cant find what i need anywhere.

- - -

Reply by Sepp @viny,

and what is it you are looking for?

Posted by: viny on June 9, 2009 02:51 AM


Hello, thank you very much for this article. It is very interesting as we are always looking for ways to improve the drinking water of rural communities in Africa.

The quality of water is directly related to the health of local populations and most of the water sources are contaminated so this method if efficient could be absolutely revolutionary.

We will test it and go from there.


Posted by: hadji on November 5, 2009 01:03 PM


GREAT article...I am on my way to Nepal to work in a small orphanage right out of kathmandu..the water is a BIG issue with the children getting sick all the time. Can I take BLACK plastic and use this instead of black painted glass?
It would be awesome to build this. Would one of those thermometers made of wax be good to know when the temperature is good?

thank you

Posted by: Pam Smith on November 8, 2009 03:55 PM


go ahead and experiment, Pam.

I would think black plastic is fine instead of painted glass. Try it out.

Wax thermometers? never heard of them, but I suppose you'll be able to find a normal thermometer there as well. Just don't break it ;)

Posted by: Sepp on November 8, 2009 04:51 PM


hi...its a great idea to purify water with saving energy, sir i m manjesh the student of diploma chemical engg. from india (m.p.) and i want to do this experiment as my colleg project, may i do it? and please give me advise as soon as that how can i do it. i hope u help me about this. thanks

Posted by: manjesh gurubele on March 21, 2010 11:56 AM


hi ... no need to ask. Just do your experiment and write up what you find. I would be happy if you could tell us about your results, when you are finished.

Posted by: Sepp on March 22, 2010 09:30 AM


I think it is important to note that it is the ULTRAVIOLET RAYS not THERMAL HEAT destroying the pathogens. Using painted glass or anything which blocks the light rays from penetrating the water would nearly negate the sterilzation technique.

Posted by: Jonathan Lowe on May 24, 2010 10:42 PM


good and very interesting

Posted by: suriya on February 5, 2011 05:18 AM


Wonderful. We can use this SODIS in the community we are helping in Masbate here in the Philippines. How I wish there is also a similar way of converting salt water to tap water.

Posted by: Justino B. Cabarles, Jr. on October 18, 2011 09:41 PM


it is nice
let i want know total working principle

Sepp's answer:
The principle is simple. The Sun has ultraviolet rays, which kill germs. When the water stays in the sun for a day, the germs are killed and the water is safer to drink.

Posted by: chinna on June 8, 2012 03:41 AM


thank you for the data

Posted by: honey on September 20, 2012 09:35 PM


Subhas Mohapatra comments by email: Dear Sepp the solar water purification method described by you is innovative. I have been using it in rural India for nearly a decade. However, in rural India people have thatched houses and can not get black roof. I have solved this problem through "mini-greenhouse" technology. All one needs to do is: a) Place 4 rocks, one in each corner. b) place the water bottles on a piece of black plastic film which is laid on the ground within the 4 rocks making sure that the rocks are at least 6" tall. If rocks are not available, earth can be piled high to represent the rock. c) A clear plastic film is placed above the bottles such that there will be at least 6" clearance between the plastic film and the bottles underneath. d) Stretch the plastic film in all directions over the rocks/earth mounds and then lay the edges of the clear plastic film such that it is buried under the ground and is "taut". That is all needed. Leave the bottles for several hours and use. The temperature under the clear plastic will be 10 C-40C higher depending on the intensity of the sun. In other words this becomes like a black car under hot sun (in the USA many children die from excessive heat inside these cars). Let me know if you have any questions.

Posted by: Sepp for subhas mohapatra on March 4, 2013 11:00 AM


is there any chance to see the scientific research/sources on this technique? this is something i would like to implement in a project i am working on but i need to be sure the science adds up before i do.

Posted by: Thomas on March 25, 2014 12:33 AM


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The Individual Is Supreme And Finds Its Way Through Intuition


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This work is licensed under a Creative Commons License.

These articles are brought to you strictly for educational and informational purposes. Be sure to consult your health practitioner of choice before utilizing any of the information to cure or mitigate disease. Any copyrighted material cited is used strictly in a non commercial way and in accordance with the "fair use" doctrine.



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