Backpacker’s Water Filter

I’m getting into the upcycling lark. Here’s how I made myself a camping water filter from a Brita (TM) filter cartridge and a pop bottle.

Gravity-fed Lightweight Water Filter

Now, it’s not as high-tech as the backpacking filters where you pump the water through a ceramic element, but it’s got some serious advantages, not least that it’s a fraction of the weight and costs virtually nothing.

It weighs about 50 g (1 3/4 oz), and could be a little less. I made it a generous size to allow nearly a litre of water to filter unattended, and put some rather overkill heavy-duty cord on it. Many of the pump varieties are getting on for 10 times that weight, they’re expensive, clog up and require maintenance or more expense replacing the filter elements, and you have to pump a handle to force water at high pressure, sometimes hundreds of times per litre.

My filter uses gravity while I’m doing something else, and is pretty quick. I’ve slowed it down with a thin layer of cotton wool pad to allow the active ingredients to work more effectively. It doesn’t remove all the stuff the ceramic ones would, but has its uses. For instance, you might boil your water to sterilise it, or use drops or tablets, but sometimes it looks unappetising, and chemical treatments are less reliable if the water is cloudy or has other physical crud in it, so a simple gravity filter is a good idea. However, this does a lot more, according to the Brita site. It removes a good portion of chemical contaminants that might be there, and should go some way toward removing micro-organisms and inhibiting their growth, at least.

If you’re out on the trail on a hot day and run out of drinking water, and there’s a clear stream to drink from, you could be fooled into thinking it’s good quality drinking water, when there happens to be contamination from agricultural run-off or biological decay (like a dead animal upstream). Boiling uses a lot of time and fuel and leaves you with warm water to drink; using drops or tablets to kill the bugs works, but if you use them all the time for several litres a day, the weight becomes significant to the ultralight backpacker, and they’re adding more chemicals and sometimes alter the taste. You can use UV sterilisers, but then there’s the battery to carry, and the bulbs only last so long before they have to be replaced too. With a lightweight, gravity-fed filter, you can refill your water bottle from the stream and have a cold drink by the time you’ve rested a moment and drunk in the view, and enjoy natural stream water but with reduced risk of an upset stomach if you elect to skip the sterilisation chemicals or use less of them.

Here’s how I did it. The main parts are a pop bottle (I used a 1.5 litre one), cut off at whatever size is convenient, and the Brita filter cartridge:

The bottom of the filter cartridge has two circular areas of holes to keep the filter material from coming through:

I cut round one with a scalpel and glued it inside the neck of the bottle, like so:

This plastic mesh circle will perform two functions, as a first filter, straining out any particles bigger than about half a millimetre, and to keep the filter medium in place. A portion of Brita filter material will be held between it and a similar mesh fitted in the “bottle top” (now the bottom as the whole thing is upside down), here:

At first I used a solvent-free glue that professed to glue most types of plastic, but it was still slightly tacky after about 3 days and the water actually dissolved it (the photo shows it going white and soft), so I did it again with a solvent-based one, which seems to work fine and dried in minutes.

Next, I took the top off one of those water bottles that has a drinking valve. It looks a bit different because I cut off the white plastic outer tube. As it happens, I didn’t need to do this once I altered the way the filter attaches to the collecting bottle: my first idea was to slot this inner valve into the collecting bottle with a connector, and it only fitted the connector at this smaller diameter. I’ll probably replace it with a new one sometime, so water can be held in the filter reservoir by closing it.

An ordinary bottle top would have done, but not as well – it would pour badly – especially as I wanted it to pour straight into a similar bottle below.

The second mesh circle was cut and gradually reduced in diameter until it fit snugly into the inside, and I cut a ring of foam plastic to act as a seal:

A perfect seal isn’t really necessary, as any water leaking at this point has already been through the filter – it would only be a tiny gap probably smaller than the mesh itself. The mesh is there to stop the filter material washing through into your bottle, that’s all (although it is completely non-toxic anyway).

Actually, the plastic on the inside of the top was moulded with a flange to locate on the inside of the bottle neck, and three prongs of plastic extended a little further below this – why, I have no idea. In order to seat the mesh it was necessary to cut all that off flush, which was a little trouble. An alternative, probably easier in the long run, would be to use an ordinary top, cut a moderately sized hole in it and control the outpouring by inserting a tube of some sort, glued in place. However, in this version there remains a second volume below the mesh, which I thought might prove useful in some way. To get to it I would just need to pull out the O-ring and mesh.

The main filter chamber is reached, to replace the contents, simply by unscrewing the whole top in one. I expect to change the filter material regularly. I may pack a small amount to take on a camping trip, just in case, and it would be best to change it in between trips. Here’s what the filter looks like when you’ve hacked it:

And here’s the carbon and ceramic mixture being loaded into the filter:

Brita filter replacement cartridges are costly enough not to just hack a brand new one for the contents and plastic mesh circles, although you can if you want. All I did was replace the cartridge we use in the kitchen a week or so early, so I know that it’s got some life left in it. There is, of course, a great deal more material in one than you need in this scaled down version, so you can store the rest and replace it however often you like.

Ready to roll:

So here’s how Mk I was meant to work: my drinking bottle, a 1 litre plastic pop bottle, would be held somewhere convenient, between stones or in the side pocket of my pack, and something would hold the filter above it. I toyed with different ideas and poked about in my collection of junk and found this, part of the transit clips from a washing machine (sorry about the focus):

Part of it was just the right shape to act as the connector, if I cut a slot in it. It was light, strong and easily modifiable into a clip that would squeeze into the drinking bottle while holding the filter tight, like so:

You can see the slot in the bottom that I cut, which perfectly located between the gaps in the screw thread.

All that was required was a little support – here using part of my stove setup:

This was useable, but would require the drinking bottle to be held pretty steady to support the heavier top when first adding water. In the field this could be awkward and lead to spillage. So Mk II hangs by a cord threaded through the top, from a tree branch, walking pole, stick in the ground or the tent porch, with the drinking bottle hanging below. The best way to do this, I reckon, is to permanently attach another bottle top, with its end cut off to form a cylinder, by cord or wire to the filter. I used wire at first, which was easier to get the right length, but it snapped from being bent up when packed away, so I replaced it with some strong cotton, which was more fiddly to get right. A loop goes round the neck somewhere that won’t slip off (a good place is just behind the little ring of plastic that remains after first opening your pop, which is integral with the top until you break the connections), and then three cords are attached to that, spaced round the neck (I’m figuring that three is better than four), and tied to little holes melted through the bottle-top-with-no-top (these parts need naming!) – you’ll see in the video that follows how it ends up.

An alternative would be to hang it with some other kind of connector. I almost made it that way using two of those bits of white plastic, fitting them round the bottle necks and bolting them together, but really, cord is simpler and lighter. In the video, even though it’s quite windy and the empty bottle is swinging, it catches all the water. You could make the cord shorter, but you need to be able to move it to one side to remove the filter end to refresh the contents.

Now all I need to do when I want to filter water in the wild is take the normal top off my bottle, screw it onto the filter and hang it up. The only other thing needed is some sort of container to collect the water from the source, which can be a billy can or plastic bag.

Incidentally, I find that those plastic bags that are moderately thick with a reinforced top and press closure particularly good for getting water from even the smallest trickle of a stream. Many types of food come in them.

I deliberately made the filter chamber rather small as I’m on an ultra-lightweight kick this year. Different shaped bottles (like my drinking bottle shown here) have longer necks, tapering more gradually. Using one of those would give a bigger volume for the filter material and make the water pass more of it vertically.

While the Brita material itself is very light indeed, it naturally stays wet for some time in use, so you carry proportionally more water for more material in the chamber. This is at the cost of its functioning, of course – the more material the water passes through the better – but it also speeds up flow. If anything, the flow was rather “too fast” (in my guestimation), so I have since replaced the o-ring with a disc of cotton wool (cut from one of those make-up removers, but using only part of the thickness). The filter isn’t just stopping physical particles, it’s having various chemical effects on the water molecules, and if it isn’t in contact with them very long that effect is reduced. Just physically slowing it down should help remove more nasties for the same amount of material.

I was pretty chuffed with this project, but the best was yet to come, when I found purely by accident that my new lightweight stove-and-kettle combo fits right inside the filter reservoir. The whole thing packs together and goes in a rucksack pocket.

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3 Responses to Backpacker’s Water Filter

  1. John Brand says:

    Awesome! Still crazy after all these years. 🙂 I can’t believe you did all this for a few brief camping trips. You’re betting on the four horsemen to win, place, and show aintcha?

    Always a good show John. Very creative. Lazy slug that I am I’d just boil the water and add some wine to it. But I admire your enterprise. “Zen and the Art of Motorcycle/Hydration Maintenance”??

  2. Andrew says:

    Hope this is a joke. brita filters do not remove the bacteria and viruses that will make you sick, they remove chemicals and flavours/odors. It would be useful to use this after iodine or CLO2 treatment, to remove those chemicals, but so would passing it through ANY activated charcoal filter.

  3. lettersquash says:

    Thanks for you comment, Andrew, but I think I give a fair idea of those risks and advantages. As I said, Brita claim that their filter medium does reduce chemical contaminants, including heavy metals, and that’s why it’s a little better than any activated charcoal filter. They also stated – regarding the older kind of filter jug, where the filter used to sit in the filtered water – that this inhibited microorganism growth (I’ve never asked them why later ones tend to be high and dry).

    As I said, chemical treatments are often reduced in effect by physical crud in the water, so it makes sense to remove it first, or, as I think I explained reasonably well, you could boil filthy water to make it perfectly sterile, but it might not be very appealing, and might still have chemical contaminants, which filtering should reduce.

    It’s all checks and balances at the end of the day. I don’t filter my tap water to sterilize it. It’s pretty well sterile thanks to the chlorine in it. I filter it to remove some of the chlorine, metal compounds, fluorine and god knows what else might be in it.

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