Hello, this is my first post on this forum. This summer I went on a multi-day hike with a store-bought pump water filter and came back unsatisfied with the product. Not that it didn't work, but 1) I didn't enjoy squatting uncomfortably over a creek while juggling a pump, bottle, etc., and more importantly 2) I felt the thing had too many moving parts and one day it would break and leave me in trouble. So I set out to build my own gravity water filter. My goals were something that would be lighter, simpler, less expensive, and easier to use. I started off with what Ray Jardine calls "friend" in his book. I cut a 30-inch circle of silicone-nylon and sewed (actually my wife did the sewing as I am useless with that) eight short straps along the edge. Below is a picture of the result:
The straps are 3/8" wide polypropylene (nylon would also work), 3.5" long. You fold the strap in half and you straddle the fabric with it so that it is sandwiched. Then you sew it. Here is a closeup of the strap:
Below is the rest of the hardware needed. The Platypus filter is very light (2 oz.) and not too expensive. I believe MSRP is $59 but I found it for $40 on Amazon. The fittings and tubing I purchased at McMaster-Carr (mcmaster.com), which is an awesome business. The tubing is silicone soft rubber tubing, 1/4" ID, 3/8" OD, 10 feet long, part number 51135K28. One of my goals in my attempt to improve upon the Ray Jardine "friend" was that I didn't want to be dragging the bag along with the filter while collecting water from a creek, so I decided to use quick-disconnect couplings. The female coupling attaches to the bag and also has a great feature which is a valve that closes when the coupling is disconnected. The female coupling is part number 5012K75. The other side of the female coupling goes on the inside of the bag. You could leave that as it is, but I decided to install a small and light strainer so as to hopefully extend the life of the filter. Also, the strainer and the coupling create a better seal. The strainer is part number 98755K11. It uses a mesh size 40. If I had to do it again, I would chose 60 (finer). The last fitting is the male coupling, which is part number 5012K71. The tubing, strainer, and couplings cost $20 all together.
To attach the coupling at the bottom of the bag, first you cut a round hole 0.5" in diameter in the middle of the bag. Then you apply a bed of silicone sealant (for sinks, windows, etc.) to both sides of the bag, just around the hole. Pass the coupling through the hole, and screw the strainer in. The female thread of the strainer is made of rubbery PVC, so don't screw it too hard. The water pressure is very low anyway.
To hang up the bag, you run a length of cord through the straps. 2-3 mm cord will do. Here is a picture of the bag with water inside:
Notice that there is no water pouring out of the coupling thanks to the automatic valve. The idea is to collect the water with the bag alone, hang it up, and then attach the coupling to start filtering. Another benefit is that you could have a showerhead that you can also attach to the coupling.
The last picture is the bag with the filter attached to it. I cut a 6-inch length of tube on one side, and six-feet on the other. You want your tubing to be long enough so that it bends down before going into the bottle. That way, if there were any leaks, the dirty water wouldn't flow into the bottle.
When you attach the coupling, no siphoning is needed. Water will start flowing on its own, and it fills up a one-quart bottle rather quickly (I didn't time it but it took less than one minute). The bag is big enough to easily filter half a gallon. I weighed the system after using it, so things were wet but the water had been drained. This would be the "field" weight. It weighed 7.5 ounces. About one third of the weight is in the bag with fittings, one third in the filter with short length of tube and coupling, and one third is in the six feet of tubing. I was a little disappointed in the weight, but it still beats any commercial pump of gravity filter.
Let me know if you have any questions. Thanks.