RapidPure Filters Review

Great article, thank you Roger!

One question regarding the expected filter life: In their tech data sheet, rapid pure claims 11,000 gallons of filter life (see http://www.rapidpure.net/Documents/RapidPure%20Tech%20Data%20Sheet.pdf). Your test shows a significant slow down after just a few liters. So how realistic is rapid pures claim from your point of view? What is a realistic amount of filtered water from your point of view?

And one more thing: Is there a way to backflush the filter? The Sawyer Squeeze is definitely a pain for a long thru-hike but at least you can backflush it.

Thanks for the answers in advance!

Great article as always Roger!

I've been trying to figure out if these things can be trusted for years: http://www.drinksafe-systems.co.uk/

They make great claims but do not post lab reports. I've managed to obtain a rather poor set of copies of lab reports from them but I can't really verify them myself of course. Their site claims a fair bit of testing but nothing very verifiable: http://www.drinksafe-systems.co.uk/worldwide-testing.php

I'd love to know the tech they're using but would love even more to know if their kit is as good as it claims!

This is a nice article, but it would be nice if there was a more apples to apples chart comparison with these filters and the Sawyer filter. The Sawyer filter is pretty much the standard filter for a lot of backpackers. It clogs up pretty quickly, so my guess is that you would see similar degradation numbers, if not worse than the other filters.

My setup (which I know is common) involves a Sawyer bag, the filter, a (cut down) Tornado Tube, a Platypus, and a few assorted caps. I also carry a syringe for back flushing. This setup allows me to drip filter or use the sip-and-go method if I'm in a hurry. This is a fairly light setup, and fairly convenient. For example, if I'm passing a lot of streams, I don't carry water, nor do I spend as much time as I would with a pump, let alone a drip filter. But in camp, or while taking a snack break, I let it drip.

Compared to using one of these filters, I think the Sawyer's main advantage is the housing. You can achieve the same sort of thing with plastic tubing (turning any filter into a squeeze filter or a drip filter), but there are disadvantages to that. Not only does it add weight, but it means that the clean bottle (the platypus in my case) doesn't have to sit on the ground. This makes the hanging process a lot easier.

That being said, I would love to see a weight comparison of similar systems. For the RapidPure filter, this would mean adding the set of connecting adapter tubes of minimal weight and compare that to the weight of a chopped Tornado Tube. The weight of the bag is meaningless, as you can use any bag with the right adapter. Likewise, the weight of the syringe would be the same, although the RapidPure might need another bit of tubing for this.

From what I can tell, all of the RapidPure filters are heavier, but all of them filter faster. It looks like almost all of the extra weight is due to the housing, not the filter material itself. The Tornado Tube setup (no tubing and only 14 grams) might also save a little weight.

I am using the lightest Sawyer filter at this time and have been very happy with it. I generally try to backwash clean it after every couple of sessions, depending on the water I am filtering. While the RapidPure seems to be pretty good, I'm not convinced that I'm saving weight or money by replacing my Sawyer at this time.

I share Robert's interest in the life (capacity) of the RapidPure filters. Once particles in the source water have become electrostatically associated with the filter media, those sites on the media are occupied. There are no longer positive charges at those sites reaching out to attract net-negatively charged bacteria and parasites in the stream of water passing through.

So, it seems to me that, unlike a physical filter like the Sawyer Squeeze, the RapidPure filter should start to allow pathogens to pass through at the end of its life, when all of the positively charged sites on the media are occupied. Many of the openings in the RapidPure filter media are larger than the pathogens, according to the above article. A Sawyer Squeeze that is undamaged will never allow bacteria or parasites to pass through, no matter how old it is. The RapidPure company would not volunteer this in their product literature, I assume, but is this in agreement with your understanding of the filter design, Roger?

If this is correct, users are at risk. It's not the volume of filtered water that determines filter life, but the absolute number of charged particles in the source water, which depends on water turbidity, pH, and other factors. A user has no way to know if the charged sites on the filter media are nearly used up.

Can particles entrained on the filter media be backflushed off with a concentrated salt solution?

Hi David

> http://www.drinksafe-systems.co.uk/
Yeah, NOT a very helpful web site. Very spin-oriented, which never impresses me. Usually just makes me suspicious.

They make lots of claims, but a harsh legalistic judgement would have to be that they do not meet EPA requirements. The figures they quote are not good enough as far as I can see. I may be wrong of course: I can only go on what they put on the web site.

Cheers

>> The little 2.5" Scout might be OK for a couple of nights IF you know the water will be clear. The larger ones seem to have a much better life. Certainly, the larger ones are fast!

OK, but if I read the article correctly, the little Scouts are better than a big Sawyer filter. Did I get that right? If so, you are saying that bringing a (standard) Sawyer filter is OK for a couple of nights, IF you know the water will be clear …

Did I miss something?

Hi Ross

Yes, I think you are missing the whole point. The RP filters give about ten times more flow than the Sawyer filters at the same pressure.

Cheers

I disagree with your reasoning, Roger. I've spent the last four years as a graduate student doing waterborne pathogen lab and field research and I thought carefully about my question before I posted it. It's clear that a person won't filter 11,000 gallons of water. But "bugs" aren't the only things that compete for charged sites on the filter. Tannins have net negative charges within the usual pH range, as do many species that fall within the "dissolved organic carbon" category. These organic anions will not foul the filter but will occupy charged sites. A couple of gallons of tea-colored tannin-rich water from a forest stream will contain an unfathomable number of organic anions eager to mask any positive charge on the filter media.

I'm not trying to push you for information you don't have, and I'm not suggesting that a person will ever pass thousands of gallons of water through their filter. I'm just observing that this filter design appears to have a problem: its remaining life cannot be estimated by the user because of variations in source waters, and when the filter approaches the end of its life, it starts letting pathogens through.

I'm not fond of relying on my reckoning for determining whether the filter will remove pathogens. Organic anion concentrations vary over many orders of magnitude in environmental surface waters. If published RapidPure capacity testing results come from testing against low-ion source water, the filter capacity for tea-colored forest stream water might be 20 gallons. We don't know how to guess how much water volume, or filter discoloration, indicates that it's time to replace it.

The problem remains: if you guess wrong, your filter's function appears the same, but free pathogens now flow right through into your drinking water. That's unique to this filter design.

Hi Andy and Colin

Good questions.
I will try to ask RapidPure for comment.

Cheers

Any problem with freezing?