This type of filter is a common solution to the problem of obtaining clean water in many parts of the world, especially rural parts of developing nations. It is a fairly simple process that involves only materials that are available in most parts of the world.
A friend of ours is away in Kenya working on various projects having to do with sustainability including filtering water for drinking. Where he is now, "water guard" tablets are used to kill bacteria, but it does nothing to improve the clarity or taste of the water so he has been experimenting with different filtering techniques. Unwanted clothes from all over the world are imported to Kenya so there is a wide variety of fabrics available which makes fabric a good option for filtration.
Note that this filter can only filter particles and color out of the water. It cannot filter bacteria or viruses. Water filtered through this filter is not meant for drinking, and should be boiled before being consumed.
This instructable is the culmination of a project for the Spring 2011 Stuff of History class at the Olin College of Engineering in Needham, MA.
Step 1: Types of Fabrics
We went to the fabric store and got some different remnants, which are cheap pieces of fabric that are the leftover, smaller pieces of fabric that the fabric store sells for half off. We also went to a thrift store and bought a lot of cheap clothes that were made out of all different materials and were constructed in different ways.
Woven fabrics have less give than knitted fabrics which is good for filtration since if fabrics stretch (as knits do), the holes in between the fibers get larger and let more particles through. Another factor that affects the amount that the fabric stretches is the actual fiber that the fabric is made out of. Some fibers have more "give" than others, but we were not sure which ones those would be.
Step 2: Polyester Fabric
Looking at the SEM photographs, the fabric is weaved producing the same pattern on the front-side and back-side of the fabric. Looking at the photo with 85x magnification, the fabric is not very tightly weaved since a lot of the fibers are not lying straight and the pockets produced by the weave is not very visible.
Step 3: Polyester Pajamas
In the pictures from the microscope, the longer weft stitches are clearly visible, which it what give this fabric its smooth, satin-y texture and shiny appearance. Unfortunately, that could mean that the warp threads might separate under pressure of water and might not make the best fabric for filtration.
Step 4: 50/50 Cotton-Polyester
In these microscope pictures, the knit of this fabric is very clear. That indicates that the fabric will stretch, causing the holes in between the threads to become larger and is not ideal for filtration.
Step 5: Nylon Knit Fabric
The pictures from the microscope show that this fabric is knitted and although when testing the fabric by hand is not very stretchy, it still has enough give in it to open holes between the fibers and not filter the water very well.
Step 6: Silk Shirt
This shirt is silk and woven, which is a good combination of a strong, fine, fiber and a non-stretchy weave, however in the microscope pictures, it is clear that the warp and weft threads are not the same width, which could lead to the threads shifting around and creating holes between them therefore, not a perfect filter.
Step 7: Silk Pants
These awful pants were a designer brand and our most successful filter.
In the microscope pictures, you can see that the threads used for the warp and weft are the same size and create a very even, solid woven pattern.
Step 8: Testing
We measured the "cloudiness" of the water using the turbidity tester in our biology lab, though gauging your results by eye (keeping samples from each test to compare) is just as good. We tested our filter fabrics with water mixed with dried clay, which we sieved down to particles of 20 microns (in diameter) or smaller. This was for the purpose of standardization - you can also try using murky water from a nearby pond, or making your own water mixed with dirt or other suspension particles.
We also measured how long it took for all of the water to filter through the fabric (testing a standard quantity of water each time). It is most important that a filter be effective, but it is better if it can do so in a short amount of time.
These were our results for our initial tests (using 30 ml of cloudy water, one layer of fabric, and only gravity as the driving force for the filtration):
See the first table image for our results
These results are similar to what we initially anticipated. The finely woven silk from the awful pants worked to decrease the turbidity the most, taking the water from .832% turbidity to only .011% turbidity. Unfortunately, it took almost an hour to filter a small amount of water. The knitted nylon football jersey did almost nothing, taking the turbidity from .927% to .912%, a difference that is barely visible to the naked eye. This was probably because of the knit of the fabric, which was stretchy, not tightly woven.
For the second set of tests, we took our three most effective fabrics, folded them all twice over, and tested how effectively they filtered the water with a higher applied pressure. Once again, we used a rubber band or two to secure the fabric material (be careful - with this test it's more likely for the fabric to slip off), and also a vacuum flask (an Erlenmeyer flask with a side-arm protruding from the neck) with a pipette bulb attached to the arm to pump air into the flask, forcing the water out.
There are a lot of other ways to force the water through the fabric, though I would suggest using something with at least two openings (we tried with a plain bottle, but it was very troublesome trying to force the air back through the fabric, especially once the clay began to cake the cloth in the interior of the flask).
For this set of tests, we obtained the following results:
See second table image
These results also make sense according to what we have learned about the weaves of the fabrics. The tightly woven pink silk from the pants decreased the turbidity the most, while the polyester fabric, which was less tightly woven, did little to decrease the turbidity.