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Sub 3oz gravity filter setup
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Colin Krusor
(ckrusor) - M

Locale: Northwest US
Sub 3oz gravity filter setup on 05/24/2012 21:25:40 MDT Print View

I’ve fantasized about a gravity filter with a long hose (a good flow rate) that is as light and durable as a Sawyer Squeeze. Even the lightest gravity setups (including cleverly designed MYOG systems using cuben reservoirs, etc.) are five ounces or more. So, I decided to try making a lighter one.

The filter itself is a Barnstead 0.2 micron hollow-fiber filter. Like the Sawyer filters, it should have a functionally limitless lifetime if it gets an occasional backflushing. It is smaller (and lighter) than the Sawyer filters, but has a comparable flow rate due to the larger pore size. In my opinion, there is no difference between 0.1 and 0.2 micron filters with respect to backcountry infection risk. None of the relevant waterborne pathogens in my area are between 0.1 and 0.2 microns in size. I’m open to any other opinions about that.

The dirty water reservoir is just a 2L Sawyer Squeeze bag. The prefilter (in the cap) is a wafer of polyester felt, just like the Sawyer Squeeze prefilter. The hose is five feet of thin walled polypropylene tubing. This setup, all parts included (reservoir, hose, clamp, fittings, filter, etc.), weighs 2.75 ounces dry and 2.95 ounces when wet (the lightest I've seen, I think). Any feedback is welcome.









Edited by ckrusor on 05/25/2012 01:32:38 MDT.

Steve C

Locale: sierra nevada
filter on 05/24/2012 21:47:55 MDT Print View

Where did you source the various parts and what did it cost to build?

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Parts on 05/24/2012 22:02:02 MDT Print View

The Barnstead filter is lab surplus (a departing researcher left a pile of unused consumables in my lab), and the tubing, the fittings, and the clamp are from ebay. I've been using the Sawyer Squeeze for some time, so I already had the Sawyer bag. So, total investment for me was about thirty dollars. I would guess that the filters probably sell for about $30-50 each from lab supplies distributors.

Edit: a BPL member let me know that she recently saw the same filters for sale on ebay for about $9 each. They weren't there when I checked just now, though.

Edited by ckrusor on 05/24/2012 23:06:13 MDT.

Chad "Stick" Poindexter
(Stick) - F

Locale: Wet & Humid Southeast....
Re: Parts on 05/25/2012 07:30:38 MDT Print View

Nice setup. Just curious if someone could post a link to those filters or a model number? I am having a difficult time tracking those down using my phone... or I can look later when I get home. Thanks.

Paul Wozniak

Locale: Midwest
filter price on 05/25/2012 07:41:44 MDT Print View

A quick look at two sites show prices for the filter at $96.00 and $116.00 respectively.

Someone may be able to find these cheaper.

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Filter on 05/25/2012 08:15:53 MDT Print View

I just found the filters on Amazon for $25 each:

Edit: As of 5/29/12, it seems that they are no longer available from Amazon.

Edited by ckrusor on 05/29/2012 19:17:23 MDT.

adam blanton
(adamallstar) - MLife

Locale: Central Texas
Re: Filter on 05/25/2012 08:46:41 MDT Print View

Very cool, I wonder if an open top feed bag would work well. You could use another bag to pour water into it as needed without having to take it down.

Brad Walker

Locale: SoCal
Re: Sub 3oz gravity filter setup on 05/25/2012 10:37:49 MDT Print View

Nice system and ingenious part-sourcing. I've spent the past 4 years of work using the same exact filter and never have put 2 and 2 together. At $30 shipped it sure is tempting to buy one for myself.

I wouldn't worry about the 0.2 um pore size. The Katadyn filters are what, 0.3 um, and they've been "standard" forever. I'm no expert and I know there's smaller bugs out there, but seems good enough to me.

I know that filter can take some pretty high forward pressures (rated 50psi) and gets great flow. I'm not sure about it's back-flush-ability (new word!) though, so it'd be great to hear if someone's tried that out.

What wall thickness tubing did you use? The thin-walled Tygon kinks way too much IME. Or did you go for silicone?

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Filter on 05/25/2012 11:51:07 MDT Print View

Brad, I agree that the 0.2 micron pore size should be fine. It will remove all protozoans (ie, Giardia and Crypto) and all pathogenic bacteria.

I've backflushed this little filter several times and it seems to work fine. To backflush, I just squeeze my platypus bottle to force some of the clean water back through the filter. The flow rate of these little filters is impressive, as you said, and it doesn't take much squeezing to get a good backflush.

The thin-walled tubing is 1/4" diameter heat-shrinkable polypropylene. Some might object that "it isn't food grade" but I think that concern is not warranted in this case. It is cleaner (undyed and no phthalates or BPA), more inert, and more environmentally friendly than the vinyl tubing used in most gravity setups. It kinks and flattens very easily. For this application it works fine, because the filter is at the bottom so hydrostatic pressure opens up the tubing, but it wouldn't work with the filter at the top (the tubing would flatten out).

There are smaller hollow-fiber filters (smaller than a thimble), but the very small ones don't have much surface area, and might require frequent backflushing if the water is turbid. For clear alpine stream water, a sub-2oz complete gravity (or squeeze) setup would probably be possible with one of the very tiny ones like the Spectrum Labs "MediaKap-2".

Edited by ckrusor on 05/25/2012 12:19:15 MDT.

Ryan Nakahara
(kife42) - F

Locale: Hawaii
.2 microns on 05/26/2012 00:57:09 MDT Print View

Won't remove 100% leptospirosis that is in your area. From what I remember they are around .1 to .2 microns. Should remove most of it though.

Mary D
(hikinggranny) - MLife
Sub 3oz gravity filter setup on 05/26/2012 11:55:41 MDT Print View

Confirmed here:

"The bacteria are in general about 0.1µm in diameter and 10-20µm in length."

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Leptospira on 05/26/2012 12:07:22 MDT Print View

Yes, a 0.2 micron filter won't remove 100% of lepto. A 0.1 micron filter won't completely remove lepto, either. We use 0.1 micron eluate to inoculate lepto cultures if we need it to be very clean. Filters with 0.1 and 0.2 micron pores also won't completely remove Brachyspira, mycoplasmas, or viruses. There are some good arguments in the recent literature that intestinal colonization with Mycoplasma pneumoniae might be a component cause in some cases of Crohn's disease, inflammatory bowel disease, and colon cancers.

Viruses, spirochaetes (like Leptospira and Brachyspira), and mycoplasmas just can't be removed by any filtration method that is practical in the backcountry. When I hike in heavily used areas, lowland areas, areas that might have agricultural runoff, or areas that permit dogs or ungulates, I use iodine or aquamira in addition to filtration (the chemicals alone have questionable efficacy against Crypto). A good prefilter followed by thorough Steripen irradiation is really the only way to knock out everything, and I just don't think that is necessary in most alpine areas...yet.

Also, unlike protozoan cysts/oocysts and viruses, most prokaryotic waterborne pathogens (including spirochaetes and mycoplasmas) are not free particles in the water. They are almost always entrained on surfaces (often in biofilms). A study in India found that people could significantly reduce their risk of contracting cholera by filtering their drinking water through a piece of cotton clothing. The spaces in the fabric are much larger than a V. cholerae cell, but most of the bacteria in the water are adhered to larger particles that can become trapped in the cotton. So, filtration almost always achieves much better reductions in the burden of bacterial pathogens (not viruses or protozoans) than lab testing would suggest.

Both 0.1 micron and 0.2 micron filters will remove all protozoa and all common bacteria. The vast majority of spirochaetes and mycoplasmas will also be removed by both kinds of filters, and the small numbers that would get through (if someone encountered any) will probably be far below the minimum infectious inoculum. Viruses will pass readily through both kinds of filters. Given this, and the extraordinarily low prevalence of human pathogens smaller than 0.2 microns in alpine surface waters, there is no practical difference between the performance of 0.2 micron and 0.1 micron filters.

Ryan and Mary, This post was a bit wordy, but I don't mean it as any kind of rebuttal. You made a good and interesting observation, and my response just reflects my interest in it. I just think the pathogen burden in backcountry surface waters is a fascinating problem. It happens to be right up my alley.

Edited by ckrusor on 05/26/2012 12:21:04 MDT.

David Drake
(DavidDrake) - F

Locale: North Idaho
Re: Leptospira on 05/26/2012 12:14:36 MDT Print View

Hi Colin,
Excellent stuff. Never used a Sawyer Squeeze myself--any reason another Platy bladder can't substitute for the Sawyer bladder in your setup?

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Reservoir on 05/26/2012 12:34:46 MDT Print View

David, I think just about any water bag would work, as long as it has a good cap seal and a way to be hung upside-down.

Mary D
(hikinggranny) - MLife
Sub 3oz gravity filter setup" on 05/26/2012 12:47:34 MDT Print View

Thanks, Colin, for your excellent comments! There are a few places I go backpacking where the water sources (at least in late summer and fall, when the mosquitoes become thin enough not to obscure vision) are stagnant lakes (the outlets dry up). I was warned by my veterinarian (who also backpacks) about the possibility of lepto there. I always take along some MicroPur tabs anyway in case of filter failure, so I'll just add a tab or two.

Richard Banks
awesome filter! on 05/28/2012 21:10:48 MDT Print View

So what tubing and misc items did you need to attach this to the water bottle? I understand you got them off ebay, but what exact parts did you get?

I'd appreciate the help!


Tim Zen
(asdzxc57) - F

Locale: MI
Re: Sub 3oz gravity filter setup on 05/29/2012 12:08:53 MDT Print View

Thanks for the post. I will look for this filter once my current one expires.
I siphon, so I will stick with the current setup.

Use my locsack as a waterbag and food bag at night.
I like the siphon because I can keep the intake out of the sediment.


Colin Krusor
(ckrusor) - M

Locale: Northwest US
Parts on 05/29/2012 18:01:15 MDT Print View

Richard, the parts I used were two push-pull bottle caps, a friction-fit tubing connector, a hose clamp, five feet of 1/4" polypropylene heat-shrinkable tubing, a bit of polyester felt for the prefilter, and a couple of bits of polyethylene tubing and other odds and ends for the tubing connection at the reservoir.

A few of these items are available from many sellers on ebay:

Polypropylene tubing:

Hose clamp:

I looked for the nylon tubing connector (between the tubing and the filter) on ebay but didn't find it. That seller must have sold out of that part. I'm thinking of getting rid of it, though. I think the tubing can probably be stretched a bit to fit around the inlet on the filter.

For the connection between the filter and the platypus (clean water) bottle, I had to use a dremel and epoxy to attach the filter to the threaded part of a push-pull platy cap. If you take a look at those pieces, you will probably see how this can be done. I'll try to give a more detailed explanation if you need it.

The connection between the tubing and the dirty water reservoir is the same situation. It would take a very long forum post and many drawings to indicate exactly what I did to connect the tubing to the push-pull cap, but, if you look at the pieces yourself, you'd probably come up with a very similar method. In short: a "plenum" (a little wafer-like plastic piece) inside the push-pull cap needs to be cut/drilled out. Then a rigid or semirigid bit of something hollow (I used a fragment of a barbed nylon tube) needs to go into the end of the polypro tubing, and a soft, flexible bit of tubing goes on the outside. This creates an "anchor" for the end of the polypro tubing that can be lodged inside the push-pull cap. It needs to be a tight fit to prevent leaks.

Sorry for not providing a step-by-step protocol for making each part. I think, once you have a chance to examine/handle the parts, you'll see that it can be done in several different ways (and your method might be better than mine). I hope this helps.

Edited by ckrusor on 05/29/2012 18:03:39 MDT.

Jon Fong
(jonfong) - F

Filter price on 05/29/2012 18:07:42 MDT Print View

Colin, thanks for the parts list. I am interested in the system, however; it seems like the price of the filter is pretty high. I am seeing ranges in the $75 range. I tried to follow the link to Amazon, but all I get is the main water filter. Any other ideas as to where to look? Best regards - Jon

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Filter on 05/29/2012 19:16:02 MDT Print View

Jon, they were still available for $24.99 at the linked page a few days ago. It seems that they are no longer available through Amazon. I did a bit of looking online and I wasn't able to find any in that price range (for single filters). I think they can still be obtained for about $30 each if a bunch of BPL members want to share a package of 18. I don't see that much interest in it at the moment, though. I'll post a link if I find any places to cheaply buy a single filter.