Whether you are looking for an ultra-light and simple gravity water filter for backpacking, or a filter to have around the house in case of a natural disaster, this affordable setup uses off the shelf parts, is easy to make, even easier to use, and should serve you well. They are great for backpacking, weighing in at only 6.84 oz for the one with the 1.1oz silnylon bag and CamelBak HydroLock Filter Adapter, and 7.61 oz for the one with the 1.9 oz silnylon bag.

Updated March 2016 – added links to make it easy to purchase the supplies for this project or get more details.

The directions below look longer and more complicated than they really are. This is an easy-moderate skill level project. If you prefer to purchase a Gravity filter or pump water filter, we recommend the following respected retailers:

Here are two videos of the finished DIY gravity water filter in action:

Materials:

Untreated water reservoir: About 1 sq yard of silicone impregnated nylon cloth. I’ve had success with either 1.1 or 1.9 oz silnylon Alternately, a light-weight silnylon waterproof “dry bag” or Platypus bag that will hold about 4 liters

Tools: Sharp knife or xacto knife Lighter

1/4″ nylon barb to MIP adapter. I used Watts part A-193B (UPC 048643123595), which is a 1/4″ to 1/2″ adapter. If I could have found a 1/4″ Barb to 1/4″ MIP adapter, that would have been better (lighter).

5 feet of 3/8″ x 1/4″ clear food grade vinyl tubing. I used Watts part SVGE10, which is 10′ of tubing, and made 2 gravity filters.

A 1/2″ female threaded nut that fits on the 1/2″ end of the barb to MIP adapter. I couldn’t find one at the local big box home improvement store, so I got a 1/2″ threaded T-fitting from the PVC pipe section and sawed off 2 off the 3 ends to make 2 small, thin threaded nuts. There is likely a better solution, but I was in a hurry and this is what I could find quickly. 🙂

Water filter: I chose a Sawyer inline water filter because it was readily available, inexpensive, and had decent reviews. The first 2 filters I made used the filter from a Sawyer model SP140 kit that I found on close-out at WallyWorld. The SP140 kit contains the SP121 filter, which is fantastic for bacteria and protazoa, but does not filter out virus. I recommend the SP135 filter, which is similar to the SP121, but does filter out virus as well. The SP135 can be bought separately or in the SP149 kit. I recently acquired the SP128 “Mini water filtration system” for backpacking, which uses a smaller filter similar to the SP121, but is reported to have about 1/2 the filter fibers… so it will need to be cleaned more often. Still no Virus filtration, but I’m told that in North America the risk of picking up a virus from water is statistically insignificant. I recommend that before you choose a filter check out all of the options. Campsaver has a good selection HERE. And here is a 3-way Sawyer filter from REI. You can also get a Sawyer water purifier filter with a bucket adapter kit, if you think you may need to purify larger quantities.

Assembly directions:

1) Decide which type of unfiltered water bag you will make. A rectangular bag is a more efficient use of material, likely resulting in a lighter bag. However, you will have to seal the seams with a very thin layer of silicone caulk. A circular bag has no seams to seal, but is not the most efficient use of the fabric, and will be heavier for the same capacity. Either will work.

Alternately, I’ve heard of folks buying silnylon waterproof “dry bag” or Platypus bag and using them as their unfiltered water bags. If you purchase a dry bag, skip to step 3a. If you purchase a Platypus bag, skip to step 4.

2) Make the unfiltered water bag.

2a) Rectangular bag:

-Determine the dimensions based on your desired amount of water to be filtered in each use. I started with a piece of 1.1oz silnylon about 15″ x 20″.-Roll hem one of the long edges of the silnylon. This will be the top of the finished bag.

-Fold the silnylon in half so that you have a piece 15″ tall (hem at the top) and 10″ wide.-Holding both of the loose edges on the 15″ side together, fold twice and sew to create a seam. This will be the side of the bag.

-Fold both layers of the 10″ side together and sew to create a seam. This will be the bottom of the bag.

-Sew 4 tie points of 1/2″ grosgrain ribbon along the rolled hem of the top of the bag. I sewed mine in pairs across from each other.

-Cut 2 one foot lengths of line. Tie one end of each line to a grosgrain tie point so that you have 2 loops of line to hang the bag from.

-Spread a thin layer of clear 100% silicone caulk on each side of the sewn side and bottom seams to seal the needle holes.-Set the bag aside to dry for 24 hours.

3a) Install the 1/4″ barb fitting:

-Cut a small hole in the silnylon bag about 1″ from the bottom corner on the side without a seam. If you are using a 1/4″ barb to 1/2″ MIP fitting like I did, then make your hole about 1/4″. You want the hole to be smaller than the inside diameter of the barb fitting that you will screw in place shortly.

–Optional: Only if you think you can do this safely, use the lighter to VERY CAREFULLY melt the edge of the hole so it won’t frey. You just want to singe it to melt the nylon fibers together. Don’t let it catch on fire! Silnylon is really flamable.

-With the 1/4″ barb side of the fitting on the outside of the bag, and the threaded nut on the inside of the bag, align the hole over the rim of the threaded side of the fitting and screw on the threaded nut, capturing the silnylon between and sealing the hole. Dry fit it at first. You can always add some silicone caulk or teflon tape later if you find that it leaks.

2b) Circular bag:

-Determine the dimensions based on your desired amount of water to be filtered in each use. I started with a piece of 1.9oz silnylon and traced a circle with a 30″ diameter. You can do larger or smaller, based on your needs. Mark the center of the circle with a dot.

-Cut along the traced circle line.

-Roll hem the edge of the silnylon circle. This will be the top of the finished bag.

-Sew 4 tie points of 1/2″ grosgrain ribbon with a zig zag stitch at equally spaced points along the rolled hem of the top of the bag. I used 10″ sections of grosgrain, folded in a V shape, with arms of the V on the silnylon, and the point of the V hanging about 1/2″ past the rolled hem and off of the silnylon.

-Mid way between each inverted grosgrain V sew a group of 3 one inch” pleats into the rolled hem with a straight stitch. The pleats will reduce the diameter of the top of the unfiltered water bag, turning it from a cone to a bag shape.

-Cut a one foot length of line and feed it through all 4 grosgrain loops (the point of each V makes a loop) and tie the ends together so that you have a loop of line connecting all 4 grosgrain loops. This line is what you will hang the untreated water bag from.

3b) Install the 1/4″ barb fitting:

-Cut a small hole in the silnylon bag at the center dot that you marked in step 2b. If you are using a 1/4″ barb to 1/2″ MIP fitting like I did, then make your hole about 1/4″. You want the hole to be smaller than the inside diameter of the barb fitting that you will screw in place shortly.

-Optional: -Only if you think you can do this safely, use the lighter to VERY CAREFULLY melt the edge of the hole so it won’t frey. You just want to singe it to melt the nylon fibers together. Don’t let it catch on fire.

-With the 1/4″ barb side of the fitting on the outside of the bag, and the threaded nut on the inside of the bag, align the hole over the rim of the threaded side of the fitting and screw on the threaded nut, capturing the silnylon between and sealing the hole. Dry fit it at first. You can always add some silicone caulk or teflon tape later if you find that it leaks.

(all remaining directions are the same for either bag type)

4) Slide one end of the vinyl tube on to the 1/4″ barb fitting on the unfiltered water bag.

5) Slide the other end of the vinyl tube on the “IN” side of the water filter.

6) Optional: My filter came with a 6″ length of silicon tubing already attached to the output side of the filter.

-If your filter did not, then you can attach an output tube of clear food grade vinyl tubing, if you like.

-You can also install the CamelBak HydroLock Filter Adapter on/off switch to the end of the output hose, if you plan to use your filter to connect to a hydration pack.

How to use a gravity water filter:

Take care not to let unfiltered water run down the outside of your water tube and contaminate your filtered water. A loop in the unfiltered water tube can be effective at stopping any drips from running down the tube and contaminating the filtered water.

Fill your unfiltered water bag with unfiltered water.

Hold the unfiltered water bag as high as you can above the container. You can hold it or hang it from a branch, fence post, etc.

Put the filter output tube in the container that will catch the filtered water.

Reflect on your great decision to make a gravity filter for a few minutes while the water runs down through the filter and into your container.

Try not to taunt your friends who are sweating as they pump water through their manual filters, probably slower than your gravity filter.

Please post any questions or comments and tell us if you’ve made a gravity filter. 🙂