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...

Filtration is worthwhile, as Scafool says, it removes solids and stuff which can make your purification more difficult or less effective. If you can filter, you should, even if it is a little bit. Linen, coffee filters or sand in a plastic bag or water bottle are easy and do not require carrying a lot of supplies.

You really need layers of charcoal and sand to make this work.
Spagum moss if its available.

Precautionary boil afterwards would be advisable.

I am familiar with filtration vs purified.....we didn't expect it to remove everything. Yes we knew it would remove the 'large' chunks....and I am not so naive as to think it was purified......I did however expect there to be some (even slightly) lower numbers of organisms.....I figured that the sand with the coffee filter would do a bit better than it did....and we did start with the gravel > the sand > coffee filter. There was (what I thought was) a decent amount of sand for the water to filter through. We didn't use that much in the way of gravel as we only expected it to get the large stuff....the water did take a decent amount of time to drip through (read that as slowly) so we expected (if nothing else) a "Slightly" better result......were we suprised.......but I agree that it brings up the question....how many people out there have the idea that a similar set up would be good for them? Is there an easier way to accomplish the same goal with less a bulkly set up?

Oh and no the instructor was the first to say..."This is only FILTERED !!!NOT!!! PURIFIED!!! so DON'T drink it." They were surprised though at our results.....

The class was a discussion of things that 'civilizations' need to survive. Water being so precious, the instructor thought that this hands on project would give them an idea of how much you need to do to get 'cleaner' water....the kids realized how hard it is to find clean water........

I think all the bacterial stuff you were looking at were somewhere in the range of .5 to 10 microns, and the coffee filter was in the 15 micron range, so it would be like sifting flour through a wire sieve. All the stuff you were looking at in under the microscope were smaller than the pore size of your filter. All your filter did was hold back the lumps and chunks so you could see the bacteria without other debris in the way.

Sue

Without micro pore pressure filters or semipermeable (osmosis) membranes you will have a lot of bulk.

With slow sand filters you need almost 2 feet of sand and they have to have a biological layer active on the top.

The small slow sand biofilter will clean more than 20 liters of water an hour of water once it is working but it takes about 2 days for the biologically active layer in the top of the sand to get established.

Dr. David Manz at the University of Calgary had managed to get the slow sand filter down to the size of a 5 gallon pail with 99% or better pathogen reduction but that is still a pretty large package.

So I think the answer to the question if there are easier and less bulky expedient filters is no.

For the KISS approach for water filtering, use a Milbank Bag prior to purification. Cheap, small, light weight, and simple to operate. This bag is still used by British military forces.