All kinds of water filter questions

I’ve done that before – Platypus, squeeze, gravity mode – worked, but I switched to 1 liter soda bottle, squeeze, squeeze mode – easier, but that’s subjective

The thread pitch on the Platypus is different than the squeeze, but it’s plastic so it conforms, but it’s a little tricky getting it to not leak.  If it leaks, that water can make it’s way down to the treated water.  If you watch for this its possible to do it.

I don’t know how you could damage the gravityworks filter by backflushing it with the Sawyer syringe?  Have you asked Cascade Designs customer support?  You might have a whole colony of microorganisms growing inside the filter impeding flow.

Bruce,

I just read this on Sawyer FAQS –

https://sawyer.com/water-filtration/faqs/

Can I backwash a BeFree Filter the same way Sawyer Filters are backwashed?
No.  Sawyer Hollow Fibers are much stronger than those used by Katadyn so backwashing a BeFree with a Sawyer backwashing plunger will more than likely damage the fibers.

Which made me think, maybe I shouldn’t have done this with the Gravityworks.

I did email Platypus and haven’t heard back.  Flow isn’t a problem, it did 2 Liters in a little over a minute, I just want to make sure its actually filtering after I blasted it.

I emailed Sawyer too asking about which filter they recommended for a gravity setup and they responded on a Sunday evening – I was pretty impressed with that.

” Flow isn’t a problem, it did 2 Liters in a little over a minute, I just want to make sure its actually filtering after I blasted it.”

That is a problem with hollow fiber filters, impossible to know if they’re still working

You can try blowing through it.  If any significant air flows through, it’s broken.

But that’s subjective.  It would detect a big hole, but maybe not a small hole

If you squeeze water through it, the flow should be somewhat slow, like a normal filter.  If the flow is high it’s probably broken.

Maybe if you really rely on the filter, if the water is less clean, just get a new filter if you’re unsure

Maybe this is a serious problem with hollow fiber filters.  If they freeze they won’t work anymore but it’s difficult to know.

Feel like we’ve been here before.

The gravity method Platypus describe in their instructions just doesn’t get the gunk out. Squeezing the bag does remove the crud, the syringe should be fine. Here is a screenshot of the integrity test (also in the instructions  from Platypus)

The BeFree and Platypus Meta bottle filters both use a different kind of filter structure, looks more open and not enclosed by a solid casing. Both seem to require shaking in clean water. I suspect this may not be as effective as a pressure flush though.

And to answer some of the other questions. We use a Sawyer Mini in a gravity system. It works fine. The trick is rather than screw direct to the bottle include a section of silicon tube between raw water side and filter, just like the gravity works. The extra head differential pushes the water along fine. We usually give a suck on the outlet side to get things moving but after that it runs well.

If the Sawyer mini was totally dry then freezing would not have harmed it.

ChrisR thanks for the instructions, I must have missed that in my searching.

Probably could test a Sawyer the same way?

The mini was probably dry, but I picked up a sawyer squeeze anyways just because I want to try one out.

Thanks all

My gravity setup. Cnoc 2L or 3L depending on trip>Sawyer Squeeze>Sawyer In-Line Adapter>Osprey Hydraulics Reservoir Hose>Tube Clamp.

Works fantastic.

I’m pretty sure the same integrity test works for the sawyer filters. I’m guessing that neither of these filters are made by an in-house manufacturing facility, more likely a company that makes filters for the chemical/pharmaceutical industry, so pretty generic technology. Katadyn describe the BeFre as using a hollow fiber membrane so maybe it works in a similar but different way.

when you say “if the mini is totally dry then freezing would not have harmed it”

do you mean when you first buy it before first use?

That’s a good question! Maybe someone would care to try it? Since the problem would be water inside the hollow tubes expanding as it freezes and cracking the tubes, how dry would it need to be before it was safe to store? Would warming it push out any residual moisture? If they are air freighted around the world what temperature does the hold get down to?

It’s about time some college kids did something useful and found us the answers to the things we really need to know.

I just put mine in a ziplock and keep it in my sleeping bag at night.

yeah, exactly

I don’t think it’s possible to get the water out of the hollow tubes.  Maybe blow air

But, to backflush the filter, you pump water through backwards which expands the hollow tubes.  This also expands the pores which releases any trapped particles

But, if the tubes can expand a little to dislodge particles, they should be able to withstand expansion by freezing water.  That’s my logic for theorizing it should withstand freezing.  Maybe expansion from freezing is larger than expansion from backflushing.

If the filter was filled with water inside and outside the tubes, and it froze and expanded, that would be a problem, the housing is not designed to expand, it would crack.

Maybe, it you shake all the water out (which is the water outside the hollow tubes) the filter would withstand freezing.

But, you’d really have to test that if you were depending on the filter for health.  Take a bunch of filters.  Use the filters, shake out water, freeze, then take water with parasites, treat water, count the number of parasites and see if they’re reduced by 99.99…%.  Impractical.

I’ve had my filter for years and it might have frozen.  I can’t blow through it.

I just read this from sawyer.com:

“While we have no proof that freezing will harm the filter, we do not have enough proof to say it will not harm the filter, therefore we must say that if you suspect the filter has been frozen, to replace it — this is especially true with a hard freeze.”

Did they change this?  I think they used to say just replace if you suspect it froze.  Added the part about proof.

(previous threads about this)

I think the way that hollow fibre filters work is water flows into the tubes which have pores engineered to be a specific size. Anything smaller than this can escape the tubes and flow out the other end of the filter. Back flushing is simply reversing that process; water is going in through the out door. This pushes the gunk back out the tubes.
I presume that as the tubes are glass they are brittle though when we used to pull glass tubes by heating with a Bunsen burner the tubes very flexible though of course the wall on these is incredibly thin.
oh the joy of confidence levels. Even if 99 times out of 100 it was fine after freezing would you want to take the chance. Of course if it accidentally freezes and that is all you’ve got it still likely better than nothing until you get home.

Are there no options as convenient as the hollow tube filters, besides say the UV light, that you wouldn’t have to worry about freezing?

I think you can freeze ceramic filters, like that MSR filter.  Or Katadyn pleated filter.

They’re heavier though

https://patents.google.com/patent/EP1463578B1/en

hollow fibers are polymer plastic

to filter, the water flows from outside tube to inside.  This contracts the tube a little.

to backflush, the water flows from inside the tube to outside.  This expands the tube a little.

this is part of why they work so good

“Even if 99 times out of 100 it was fine after freezing would you want to take the chance. Of course if it accidentally freezes and that is all you’ve got it still likely better than nothing until you get home.”

Yeah, I agree, base your decisions based on what the manufacturer recommends, based on testing.

It’s too bad though, if the filter actually does tolerate freezing.  Like they said, they can’t prove one way or another whether freezing damages the filter.

I bet there are people that have left the filter full of water, accidentally frozen, and the housing broke.

Freezing and hollow fibers. I have always assumed from the warnings that the fibers are rigid and therefore do not expand, Same with the warning about dropping the hollow fiber filter.

Backflushing: The assumed failure is congestion from micro flora and micro fauna and perhaps very small vegetable or mineral participles whose greatest width is about the same as the hollow fiber. I would argue it is the force of the water that dislodges them, not expansion but don’t really know.

Chad,

Not sure how quick the gravityworks is, but I rigged up the sawyer mini with a gravity system and it worked fine. To put a time on it, I’d say it put out about 0.5L/min. I don’t use a clean water bag, but I was using a 2L dirty water bag and 1L bottle, so I just had to watch it to avoid spilling over.

So, I don’t think it’s that slow. Maybe relative to the gravity works (idk), but I would think you mostly are filtering at camp anyway. Even so, on the trail, you can should be able to fill a 3L bladder in 6 min+/-.