A few weeks ago, my boys (9 and 14) attended a class. Part of the 'hands on' was making a water filter with a 2 litre coke bottle. Imagine you cut the top off the bottle, invert the top, place a coffee filter over the opening. Now fill the (inverted) top with sand and then 'gravel'. Place the sand filled top into the 'bottom' of the bottle that you cut it from. Now pour the water into the sand and allow it to filter through. They provided us with the water. It came from an outdoor tank that houses reptilian animals. We took samples home and continued the filtering 'test'. We put the water under the microscope both before and after. I wouldn't put my faith in coffee filters people.....while the filter and sand 'removed' the chunks from the water....they let through just about everything else....I don't remember my microbiology all that much, but...rods, cocci, flagellates.....others I couldn't identify......No gram staining just visualized under a scope.....But the 'after' filtration looked just as bad a the 'before'. I would have thought that the set up would have caught SOMETHING...but no. Evaluate you set up if needed....

What was the class for? Water sanitation/conservation, or...?

I would imagine that water from a reptile tank to have a much higher amount of micro fauna than, say, a mountain stream. (Salmonella and other nasties comes to mind. Blegh!) That said, even a cold, clear alpine stream can harbour giardia, which can only be filtered out with a 1 micron pore size. A coffee filter only sieves out to about 20 microns, so yeah, I'm not surprised that some critters got through.


As for me, I'm a big fan of filtering then boiling. (Potable Aqua works in a short-term pinch, but doesn't kill cryptosporidium and a few other bugs.)

The filtration method described is not intended to purify the water. It is intended to do just what it did, remove the chunks. It is still necessary to purify, either chemically, thermally, or some other method which does kill the bacteria. Purifying without filtration will kill the bad things, but leaves the water very unappetizing.

See if the "instructor" is willing to drink the "filtered" water.

Montanero has it right

Filtering the crud out lets the chemicals (Chlorine, Iodine?)work better too.
Large lumps of matter can protect the pathogens from the disinfectants because they react with the chemicals or they have the pathogens inside where the chemicals can not reach.

The school demonstration likely was not a very effective filter either but your city water supply is likely using a sand filter too.
When they are properly set up they remove almost all the solids and then the city chlorinates to kill any bacteria and viruses that might get through.
Slow sand filters are even more effective.

+1 to Montanero and Scafool. Also, it seems the filter wasn't designed properly because there are several stages missing. The first (top) layer should be gravel, then progressively finer sand. Next a layer of very fine mesh fabric or cotton wool, then a layer of charcoal (extremely important!) and another thin layer of fabric or cotton wool.

That's the kind of filter we were taught to build and it works fairly well for removing the major gunk though you'll probably still need to boil or chemically purify the water. Anyway, it's a pretty slow process. If there are enough stages and all the material is compacted down correctly, the water will drip out slowly, measured in drops per second! If it goes any faster than that, the filter is probably not working right.

We once build a waterfilter sas survivalbook style, with sand, moss and charcoal. One of use worked for a water-company was able to test the water for bacteria. We took 3 samples: untreated, filter through the filtered and boiled.

The boiled water was the best with no living bacteria. Surprisingly the filtered water had more bacteria than the untreated water....

I'm sure this is a stupid question,and Montero and scafool's comments cause me to think of it. Particularly the former's. Given that filtration in and of itself isn't enough and purification still must follow, could it be argued that this filtration process was TOO cumbersome? My thinking is, and obviously I've never had to do it, is that if in fact a coffee filter works to 20 microns perhaps a filter of sand and a coffee filter might have accomplished the same filtering of larger stuff since all the purification still had to follow anyway? That probably ain't too smart, or someone else would have said so. Just curious.

"Surprisingly the filtered water had more bacteria than the untreated water...."

Is it possible that the bacteria came from the moss that was part of your filter?

Well, it makes some difference inasmuch as the filter removes any major particles, dead insects and that kind of crap. Filtering may not be necessary if you get water from a reasonably clean source. If your only source of water is a stagnant pool though, you'd better make sure to filter first. That is, unless you don't mind drinking stuff that tastes like boiled mud, cattle [censored] and diarrhoea with a bunch of drowned flies and ants to spice it up...