DIY gravity water filter

[Franco Cuminato]

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.

Franco

[Ken Thompson]

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=8994

[Nicholas Rem]

Thanks for the info, I really like this idea for a filter. The one thing I would like to know, is there anything that you know of that would connect to the coupling to provide a shower? I use the Sea to Summit pocket shower and if I could connect something that would substitute the shower head on the pocket shower, that would be golden! I could potentially kill 2 birds with 1 stone.

[Bob Gross]

You are using the Gravityworks filter with the gray plastic end caps. You can save a little weight by removing those end caps. They don't do much except provide a little bump protection. The way I fold mine up for transport, it doesn't see any bumps anyway.

–B.G.–

[Greg Mihalik]

You will get better flow rates, especially after the filter clogs a bit, if you move the filter unit closer to the ground. Mine is typically just high enough to get the outflow into the receiving container.

[Emily B]

Nice work!
Nicholas, re the shower: Platypus used to make a little black shower head piece that could be stuck into the tubing. (I think sometimes it came in a larger "shower kit".)
Like this:
http://www.outdoorsmagic.com/reviews/accessories/washing-essentials/platypus-shower-kit/19829.html
You'd turn it off/on by using the shutoff clamp on the hose.
It's not on their website now. I found mine in a local store, the type has random stuff sitting and gathering dust on the shelves, so you might have luck with that kind of place, or posting a WTB on gearswap. I've seen other "camp showers" that have nozzles on the end of tubes; maybe one of those nozzles would fit in the platy tube. …or find something that you could poke holes in and attach to the end of the hose?

[Randy Amos]

Very slick design. With the open end design of the dirty bag it would be easy to fill at any water source. I would NEVER go back to a pump filter. I have a 4L Sawyer filter that we take when then the whole family goes but I also have the sawyer squeeze for when I am solo. This design could easily be incorporated to make the squeeze a gravity filter for 2 people.

[David Thomas]

Nice idea. I did something similar in the 80's with a 10-liter water carrier/bladder.

Greg is right, you'll get even faster flow with filter closer to the ground.

Consider black fabric for a little more solar heating for a hot shower on a rest day.

[Nicholas Rem]

Thanks Emily, I will keep an eye out.

I just finished making this filter and can't wait to use it this spring (if it ever gets here). I ordered all the parts as listed and was surprised when McMaster-Carr had it on my doorstep in one day!!

What is the value by using the bag as designed in the thread? Not that I recommend shopping at Walpire (Walmart+Empire), but I remember from another thread that they had dry stuff sacks that I thought could be used. They are polyurethane coated rip-stop nylon. They are the bags that roll over and buckle at the top like an MSR pocket shower. 3 come in a box for $10, and I think for my needs the middle size will do. I doubt it will hold water for long periods of time, but to grab some water from the river/lake and throw it over a branch, I am sure it will hold the liquid long enough to filter, but I could be wrong and it could work out great.

I will weight it all and post some pictures to show my modifications to this awesome idea! Love, love, love the quick release function!

[Frank T]

I just finished a 4-liter gravity filter that uses the 4L Walmart "dry" sack, the Sawyer inline filter cartridge, thin-wall silicon tubing and some plastic fittings from U.S. Plastic. The project is documented in this blog:
http://randomlywalkin.blogspot.com/2013/07/161g-4l-gravity-filter.html

The "damp" weight comes in at 169g or just a hair under 6 oz. I had previously used the same bag & cartridge with thicker tubing and some quick-release parts (also from U.S. Plastic); that system weighed 7.6 oz. By far the biggest contributor to the weight difference iss in the tubing — 1.5 oz. between the two 5-foot lengths.

What I like about the garden hose thread (GHT) couplings is it allows me to use pre-filter discs made out of the diesel filter pouches. The mini stopcock, attached to the bag outlet, provides essentially the same function as the QR connection, so I can fill the bag without the long tube & cartridge attached.

All aspects of this project came in one way or another from BPL, so I thank everyone for their ideas.

[Author]

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