It may seem like an oxymoron, but there’s a good reason to try it. Forcing water through a filter more slowly could make it last longer. Does lowering pressure increase service life? With a new Sawyer Squeeze, I began my hike along the Pacific Northwest Trail with the intention of finding out.
It’s been a long time since I first became frustrated with my Sawyer Mini on the PCT and I’ve learned a lot since then. This is what I have written previously.
At first glance, gravity filtering seemed to require more elements than I expected.
- Cnoc Vecto 72 ounces (2.1 liters). If you want to speed things up, use the 3L version.
- Trekking pole. Because there isn’t always a place to hang the Vecto.
- Mesh storage bag. The lanyard attaches the Vecto to the cane strap.
- Sawyer Squeeze, outlet plug, homemade inlet plug, backwash syringe.
- 12″ gravity tubes and in-line adapters. These come with the Squeeze.
- Cleaning coupling. This does not come with the Squeeze.
- 32 ounce bag. To collect filtered water.
I soon realized that the only extras needed for gravity filtering are items 5 and 6.
Pre-tour tests
Before leaving for the PNT, I measured the filter flow rate with and without the gravity tube. I also filled a 24 ounce bottle by squeezing water through the filter. Each measurement was repeated three times and the results below are the average of those three values.
I was doubtful of the accuracy of the fluid ounce markings on the Vecto, but I wanted to use them to calculate the water pressure at the filter inlet. Measuring the distance of each mark from the filter was easy, but calculating the amount of water remaining in the Vecto was more complicated.
- I filtered water into a 64 ounce measuring bucket. I assumed their markings were accurate and calculated the time it took for the water to reach each marking.
- I then calculated the time it took for the Vecto’s level to pass each mark.
- By combining the two sets of times and interpolating, I calculated the actual amount of water in the Vecto at each mark.
Results of gravity filtering prior to the tour.
- (A) The reference line shows how much water would still be in the Vecto if its markings were accurate. The green and red lines show how much is still inside.
- (B) The 12″ gravity tube significantly increases the water pressure at the filter inlet.
- (C) Shows how much of the Vecto has leaked into each of its brands.
- (D) Higher inlet pressure causes faster filtration, but it would still take 45 seconds to fill a 24-ounce bottle. In comparison, filling that bottle by squeezing water through the filter took 19 seconds. This implies a compression pressure of 2.06 pounds per square inch, more than twice that of gravity filtration.
without squeezing
On the PNT, I never expelled water through the filter. No squeezing, just gravity. The goal of the experiment was to see if it could extend the life of the filter. Whenever possible, I completely filled the Vecto before filtering. Although I didn’t want that much water, I did want to speed things up. Then I just threw away what I didn’t need.
Several times when it rained I had to sit in my tent to filter the water. Using the gravity tube was not practical, so I connected the filter directly to the Vecto. With my elbows on my knees, I held everything high enough for gravity to do its work; It’s something slow, slow.
During previous hikes, squeezing water through the filter, I often noticed that at first, the water coming out of the filter contained many air bubbles. This was because the filter was stored “dry” after use. Gravity filtering does not create enough pressure to remove this air from the filter, which can further slow the process. To clean the air, I connected the complete Vecto, gravity tube and filter; the outlet cover is still in place. I then shook the filter from side to side, or tapped it against my leg, with enough force to produce a stream of air bubbles inside the Vecto. When the bubbles stopped, I stopped.
No spills
The four 32-ounce (0.95 L) Sawyer Squeeze bags I brought with me on the CDT managed to survive their trip. So, since I like to test things to destruction, I took them with me on the PNT. I used them only to store clean water, but I still found their limits pretty quickly.
- Day 3. Splash! A full bag fell from the bench he was sitting on and hit the ground on its side. It only fell about 18″, but that was enough to burst a seam.
- Day 9. Fatigue. A bag had a leak in one of its many folds.
- Day 21. Fatigue. Another weak point of worn and worn plastic.
Bag number four survived the trail and is now in the trash retirement. Once I was down to my last bag, I primarily used a soda bottle to collect filtered water. The downside is that air must escape as the bottle fills, so it can overflow if you don’t keep an eye on it. The bags can be screwed tightly to the filter outlet and there is no risk of spills.
A full 32-ounce bag will hang from the end of the gravity tube for a few minutes at most. It’s heavy enough to slowly pull the in-line adapters out of the tube, and then the Vecto will empty its contents onto the ground. This can be avoided by shortening the trekking pole.
Some obstructions
From time to time, the water he collected was not as clear as it seemed. The Vecto contained a cloud of tiny particles, and after hanging it from the trekking pole, the particles began to sink to the bottom. I connected the gravity tube and then sat with my thumb over the outlet for about 10 minutes. Once the debris settled, I moved it closer to the gravity tube by gently moving it a few times. I took my thumb off the outlet for a split second, washed away the debris, and finally attached the filter. It would have been quicker to let the debris clog the filter and then backwash it. But like I said, the goal of the experiment was to extend the life of the filter.
I backflushed an average of once every two days, which is about half the frequency I needed during previous hikes. It’s not that the filter clogged more slowly on the PNT, it just caused me to waste less time by being slow. While gravity pushed the water through the filter, I was busy eating a snack, emptying the remains of my shoes, exploring the next landmarks in FarOut, or setting up my tent.
Usually, while I was busy with some other task, the filtering finished successfully in the background. The result was an almost full collection bottle, an empty Vecto and a “dry” filter. If I later wanted to backwash the filter, I first had to make sure to clean the air, as described above. Then, with the filter full of water, I banged it against a tree trunk (or the edge of my boot sole) for 30 seconds. Finally, the countercurrent.
Filter Preparation
After the PNT, the gravity filtering portion of the experiment ended. All that was left was to restore the filter as much as possible. I started by giving him 30 seconds of Knock-on-Wood (KW) treatment. Basically the same backwash procedure I used on the trail, except this time I slammed the filter against a block of wood.
Next, Sawyer Squeeze received the vinegar (VG) treatment. I pushed air through the filter, screwed it into a vinegar bottle, squeezed out some of the contents and left it for 30 minutes. I then squeezed in some more vinegar and left it for another 30 minutes.
Finally, I subjected the filter to three rounds of HK treatment.
Hot water treatment plus wood blow (HK)
- Put 24 ounces. water in a Pyrex container
- Drop the filter into the container without the inlet plug and outlet cap.
- Heat the bowl in the microwave on high power until it reaches 140℉. It takes about three minutes; use a thermometer to check it.
- Remove the hot water filter, insert the inlet plug and cover the outlet.
- Give the filter 30 seconds of wood tapping treatment.
- Backwash the filter with a syringe of 140℉ water.
- Fill the container with a syringe with cold water and replace the filter.
- Heat the bowl in the microwave on high power until it reaches 140℉. It takes less than 30 seconds; use a thermometer to check it.
- Repeat steps 4 through 8 as necessary. (I did it 18 times).
- Squeeze 24 oz. of cold water through the filter.
Results
The table below shows how long it took (min:sec) to fill a 24 oz container. (0.71 L) by squeezing water through the filter as quickly as possible. I used the average of three measurements and converted the time to gallons per minute (1 US gallon = 3.79 L). The graphs show how the flow rate decreases, assuming I drink two gallons of water per day while on the trail.
Comparing my latest Sawyer Squeeze to previous filters.
- Row 1. This Sawyer Squeeze is faster than the last one.
- Row 2. After PNT, this Sawyer Squeeze retained 42% of its initial flow.
- Row 3. Vinegar treatment had no benefit.
- Rows 4 to 6. The first round of HK treatment achieved significant improvement.
- (A) Shows the reduction in flow with use. This Sawyer Squeeze has filtered no more than 130 gallons, which is about half as much as the previous two filters.
- (B) Shows a relative reduction in flow with use.
Conclusions
I spent 65 days in the PNT and gravity leaked two gallons a day at most. In that time, the flow rate of my Sawyer Squeeze dropped to 42% of its original value. The HK treatment described above is the most effective way I know of to unclog a filter. A single treatment, approximately 40 minutes long, restored the filter to 70% of its initial flow. Two more treatments provided diminishing returns and I stopped once the filter reached 76% of its original value.
The Sawyer Squeeze I used on the CDT was significantly slower at the same point in its life. It retained less than 65% of its original flow after filtering the same amount of water. One likely explanation for this difference is gravity filtering, although there are two possibilities that I can’t rule out: I drank less per day in the PNT and their water sources were cleaner.
The only times I ran water through this filter were before the tour, when the filter was clean, and after the tour, when I wanted to compare it to previous filters. Gravity filtration results in a lower inlet pressure and should reduce the likelihood of debris becoming permanently trapped. In other words, it should improve backwash efficiency. For every 10,000 particles trapped on the filter membrane, the PNT backwash consistently dislodged 25 more particles than the CDT backwash. That small increase resulted in a significant difference over time.
In the future, I will use gravity when I can and tighten when necessary. My Vecto and filter will last longer, I’ll have more free time, and last but not least, I’ll have fewer opportunities to complain about frozen fingers.
