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Water filters -- "All you need is terracotta clay, a compliant cow and a match."
Alex Steffen, 17 May 05

God knows we've written our share of water purification posts, but this is pretty cool: An Australian professor has come up with a way of making DIY water purifiers (PDF), suitable for the developing world:

A handful of clay, yesterday’s coffee grounds and some cow manure: the ingredients that could bring clean, safe drinking water to much of the third world. The simple new technology, developed by [Australia National University] materials scientist Mr. Tony Flynn, allows water filters to be made from commonly available materials and fired on the ground using cow manure as the source of heat, without the need for kiln. The filters have been tested and shown to remove common pathogens including E-coli.

“They are very simple to explain and demonstrate and can be made by anyone, anywhere,” says Mr Flynn. “They don’t require any western technology. All you need is terracotta clay, a compliant cow and a match.”

Commercial ceramic filters do exist, Flynn says, but with prices which are often too expensive most people in the developing world. (The PDF also includes a step-by-step guide for making your own -- paging Mark Frauenfelder) Flynn has declined to patent his invention.

It works through microfiltration, as the creator explains in this interview:

BLANCH : So what are the basic principles that allow the filtration process to work so effectively?

FLYNN : Well, in the case of the addition of coffee grounds to the local clay, it does a couple of things. First of all it greatly increases the total volume of the tiny holes or pores within the filter structure and when it’s fired as I’ve just described in the manure mound, the heat burns the coffee out, leaving the holes but which also contain small fractions of silica that aren’t combustible and are a result of the combustion of the combustible fraction of the coffee grounds. Now these small voids or holes in conjunction with their silica content and the network of tiny holes that are joined in three dimensions within the clay particle mass, act as the filter structure and they are small enough to allow the simultaneous passage of water through them, while equally being small enough to remove bacteria that we tested for – in this case E-coli.

Being maybe the least craft-y person in the world, I'm not sure I'd want to trust my life to anything I made by hand, but still, if it holds up under scrutiny, this seems to me to be a great example of the kind of innovation, simple-yet-advanced, that so much of the world needs. It sort of goes well with "Pot-in-Pot" refrigerator designs, I think.


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i love this kind of thing. thanks.

Posted by: csven on 18 May 05

This filter is just for bacteria, not chemical pollutants. You need activated carbon (or charcoal) for that. I've seen ads to buy activated carbon filter made using coconut shells. Anyone know how to make these yourself and how effective they are? Seems like a complete water purification system could be made.

Posted by: EdgeWise on 18 May 05

Ah. A little more research yeilded this:
Charcoal is obtained by burning wood, nutshells, coconut husks, animal bones, and/or other carbonaceous (carbon-containing) materials. Charcoal becomes activated by heating it with steam to approximately 1000 oC in the absence of oxygen (O2). This treatment removes residual non-carbon elements and produces a porous internal microstructure having an extremely high surface area. A single gram of such material can have 400 to 1,200 square meters of surface area, 98% of it internal!
[ ]

However, I'm not sure how to make 1000 degree steam in the absent of oxygen.

Posted by: EdgeWise on 18 May 05

Hmmm... looks like a rotary kiln is the way to go. Lots of different "how to build your own kiln" documents. Haven't found a rotary one, nor how to inject the steam, nor how to remove all oxygen (combust it?).

Posted by: EdgeWise on 18 May 05

perhaps not combusting the oxygen, but rather in the presense of nitrogen or some other element. welding techniques call for an absense of oxygen so the metal doesnt rust, so instead they will use a shielding gas to displace the oxygen with nitrogen and other elements.

Posted by: docpathos on 18 May 05

What is wrong with that pdf? The title and sub-title text show up ok, but the rest of the text is garbled.

Posted by: Burl on 18 May 05

Hey folks, I've been thinking about the potential for a public domain catalog of DIY survival tools for the developing world, also useful for backpackers on-the-cheap. I slapped together a wiki page where instructions can be either collected or actively brainstormed at this address - what do you all think? I love how you folks are actively coming up with a charcoal filter solution and think it would be great to see the design process continue. Please email me with suggestions or comments.

Posted by: Jeffrey Warren on 19 May 05

Working in Cambodia, Engineers Without Borders helped a women's cooperative just outside of Phnom Penh set-up a factory that makes ceramic water filters to sell to rural areas. The filters are high quality with colloidal silver liners to remove viruses and bacteria (sometimes missed by traditional ceramic filters) and a QC department in the factory. This factory employs about 20 people and can ramp up to making about 1000 filters a week. The filters sell for around $5 each, which is affordable even for poor farmers.

The reason I want to point this out is because the filter described in the article you attached is not going to help the 10 million Cambodians who don't have clean water. Training each and every one of them to make their own filter, knowing that they will make something of poor quality that likely won't result in clean water, is not going to solve the problem. What is going to solve the problem is a less-sexy, but highly efficient, series of factories and a reliable supply chain that can get high-quality, yet cheap, ceramic filters to millions of people.

Posted by: paul on 21 May 05

I was wondering whether I could make a water filter using a mixture of maybe sand and crushed organic walnut shells. I read an article some years ago in a science journal/magazine which highlighted the ability of walnut shells to absorb many contaminants. While I cant remember if they were organic (in the chemistry sense of the word) or inorganic I have just been listening to a BBC4 radio programe on wood and how wood is essentially bundles of tiny tubes (I then though walnut shells are wood also). This was the crossed referenced on the show against carbon nano technology which is pretty much a similar thing. Now reading about DIY clay filters here and cross referencing that with activated carbon filters (which I have previousely used while distilling spirits) i wonder if there is any activation of the carbon from the coffee grounds in the clay filter. If the coffee grounds are trapped in the clay where there is little oxygen then maybe when it is fired at a high enough temperature (kiln fired) and the coffee grounds (previousely soaked in water - not dry)will give of some steam while being turned to charcoal within the clay silicate.
I've just come back from Venice where the wood pilons driven into the clay up to and over 500 years ago which were then built upon as a foundation have not actually decomposed at all because of the lack of oxygen in the clay. Interesting that these little strands of information have come to the fore today....not sure if there is any validity to any of this but i'd love to see if the clay filter might be made in some way to absorb organic as well as inorganic and bio contaminants. In terms of the third world im sure it would be no problen to use dung (if not human faeces among other things) to fuel small factories as it gives good heat if burnt in the right way.
(I must say i really hate that term third ...we are all first world we just place a different emphasis on what wealthy is. The use of the term allows a judgement which perpetuates certain ignorance and I feel it is sad that we use the term at all. I'm in london right now and find there is nothing distinctly 1st world comparitively speaking about the air i'm breathing now.
I'd be interested to hear any comments on this. I'm a designer and I just so happen to have a lump of clay which i sometimes use for modelling...i should have taken it to Murano with me on holiday.

Posted by: lothair hamann on 27 Oct 05



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