I have used hand-pump water filters like the First Need or the Katadyn for years, believing them to be the only “real” option for filtering out cryptosporidium, giardia and other bacteria from the water sources in the Pacific Northwest where I hike. But I’ve been forced to consider other options in an effort to move towards lightweight hiking and backpacking.
Chemical and Ultra-Violet Water Purification
Chlorine dioxide tablets or drops are a good alterative except for the waiting time before you can consume the water. Many people have also found that the tablets did not always dissolve as expected. Ultra-violet water treatment solutions depend on whether the UV lamp is working and whether your backup batteries (which are not standard) are still good. But neither the chemical or UV treatments actually filter the water, leaving unappealing floaters in your bottle. If a water source is stagnant, you are still left with an unpleasant swill of technically drinkable but unenjoyable water. This was a deal-breaker for me.
The one option that I kept coming back to was the Sawyer Mini filter. The Sawyer Mini is a true revolution in lightweight water filtering. Weighing in wet at about 3 oz, it is the lightest true water filter on the market and is highly flexible in how you choose to use it. The advertised method involves squeezing the “dirty” water through the filter. It certainly works but many Sawyer users will tell you how the included bags have failed.
Gravity-fed Water Filters
Sawyer, Platypus and others also sell a gravity-fed system with the filter in between the “dirty” bag and the filter bag. Rather then depend on mechanical squeezing to supply pressure to the filter, gravity can do it with less stress on the physical components if you are willing to wait a bit longer. However both Sawyer and Platypus’s solutions are surprisingly expensive for a 2L set-up.
Pressure can be calculated using the equation P=pgh, where P equals pressure, p equals density, g equals gravity and h equals height. The operative variable is really just height in any gravity-fed water system. The diameter of the tube or the amount of water in theory does not make a difference. Increase height to increase pressure. In reality, the friction of the fluid passing through a tube does act to reduce this. A simple figure to remember is that every five feet of fluid height (excluding friction) results in a pressure of 2.2 pounds per square inch (PSI). This is the force that will push the water through the Sawyer.
The basic components of my MYOG Sawyer-based gravity filter areas follows:
I wanted the dirty squeeze bag to be easily suspended without ripping out the bottom of the bag. To do this I decided to reinforce the holes with metal grommets and then thread nylon string through them so that the weight would be distributed over all four holes. The Sawyer bags were selected because they are very light and inexpensive for the volume of water they can hold.
Start out by finding the bottom of the squeeze bag where the bag part ends and the expansion baffles begin. Reinforce all four corners with some duct tape which will act as reminder that this is the dirty reservoir.
Then drill a hole on each side of the bag through both front and back expansion baffles. Fit one side of a grommet through a hole, fit the other side of the grommet on where it emerges from the bag and give it a couple whacks with the grommet tool to bend the grommet into shape. Repeat three more times.
Lastly, thread the nylon string through the grommets. You should also put grommets on your clean bag, in case your dirty bag becomes compromised and you need another bag to suspend.
Next set up the inflow tube. I chose to use the 5/16” outer diameter tubing rather than the much less expensive 3/8” outer diameter tubing because it’s less than half the weight (1.7 oz) . The thicker tubing is more kink-resistant and less than a quarter the cost ($3 versus $13).
Cut a five foot section of tube, put the tube section through a tube clamp, plug one end to the Sawyer filter and the other end to the blue inline adapter from Sawyer (see parts list). The blue adapter fits the Sawyer bag and also has a neoprene washer.
Use the Sawyer bag cap to draw a circle on the fine-mesh nylon bag and then cut inside the line by 1/16” to 1/32″.
Fit both pieces of mesh inside the threads of the blue screw-top adapter. This mesh acts as a large particulate filter and keeps any floaters from reaching the filter and clogging it. Very fine particles, such as glacier silt, will go through so if you are using water that is cloudy with silt. Allow it to settle in a collapsible bucket before filtering. Cut out a couple of extra mesh circles and keep them in your “extras” kit just in case you lose the ones in the adapter.
The last section is the outflow side. The Sawyer adapter kit comes with a screw-top piece (blue) and a screw-in piece (grey). The grey piece does not fit in any normal bottle so you have two options: buy another Sawyer adapter kit and toss the grey parts, or buy a Tornado Tube.
I chose to go with the Tornado Tube which is actually a kids’ toy but it works great in this application. The grey adapter will fit in one side and the other side will screw onto pretty much anything: Sawyer bags, standard water bottles, Platypus reservoirs, etc. You can see the red Tornado Tube in the assembled picture above. Use a 2” section of tubing to connect the outflow end of the Sawyer filter to the grey Sawyer adapter screwed into the tornado tube.
I cut the top off a standard 1L water bottle and use that as a container for all the filter parts and the Sawyer bags. One advantage to using part of a bottle is that you can use it as a scoop if your available water is in a low-flow seep and you can’t just dip your dirty bag in to gather it. It will also prevent the Sawyer bags from getting punctured.
With the dirty bag suspended over the filtered bag by the full five feet of tubing, I was able to filter 2L in an average 139 seconds in three trials. I also tested with the Mini right under the dirty bag so that pressure from gravity was minimized. Three trial runs in this configuration resulted in an average time of 207 seconds, proving that in this case, gravity is your friend.
Fully assembled, including the 1L bottle container, my filtering system weighs 8.4 oz and cost $75. That cost included many redundant or extra things. I did not need: 10′ of tubing, but that was the length it came in. I also did not need 12 tube clamps, but a single clamp was $5, so for twice the cost, I got extras in case I lost or broke one. The mesh bag also has lots of space to cut out more filter circles. Go in with a friend and make two filter set-ups to split the costs and extra materials.
Other options to add to your MYOG gravity filter:
I ended up adding the quick-connect fittings right above the Sawyer Mini on the inflow tube. They add a convenient way to unhook your filter from the hose when you are setting things up or packing it away.
All finished, my gravity fed water filter system weighs 2 ounces less than a Platypus GravityWorks Water Filter System and about costs about a third less. But making your own lightweight gear, that is a priceless experience.
Nathan Taylor lives in Portland, Oregon and enjoys the great variety of hiking climates available in the Pacific Northwest.