Hollow Fibre Water Filtering

Ahh, the joy of being in the mountains. The pure scenery, the pure air, the pure water. Oh, actually, scratch that last bit, because all-too-often the enticing, cool water in that apparently clear and beautiful stream is not exactly pure. It’s often full of all kinds of unseen microbes, protozoa, bacteria and more, and if you’ve ever suffered the ill-effects of gastro out bush, you’ll know it’s an experience you won’t want to repeat (read the winning entry in our Fiasco Stories Competition as a case in point).

There are many ways of treating water so that it’s safe—boiling it, using tablets, UV radiation, filtration and more; they all work, but they have their cons, too, among them time, cost, weight and effectiveness in turbid water. One of the most convenient and weight-effective methods is filtration using hollow fibre technology. Like ceramic filters, hollow fibre filters rely on size exclusion to make water safe to drink. Essentially, pathogens are too big to pass through the filter, but water still can. Unlike ceramic filters, though, hollow fibre filters tend to be lighter, often much less so. And unlike, say, UV filters or chemical tablets, their effectiveness isn’t compromised when the water is heavily contaminated or full of sediment.

But what exactly is a hollow fibre? Essentially, it’s a less-than-a-millimetre-in-diameter, straw-like tube, with the outer walls of the ‘straw’ being a membrane made porous by the presence of thousands of tiny holes. These holes are literally microscopic, although exactly how small they are depends on the filter model and the quality of hollow fibres being used. But for reference, MSR’s Guardian Purifier, for example, which is at the upper end in terms of filtration effectiveness (it incorporates technology adapted from kidney dialysis) has hollow fibres with pores less than 0.02 of micron. If you’ve forgotten how small a micron is, this equates to 1/50,000th of a millimetre. That’s small enough to not only filter out protozoa and bacteria, but viruses as well.

You don’t always need that level of protection, however. While a purifier that gets rid of viruses may be necessary in populated areas or more heavily visited spots out bush, for less-trafficked or more remote areas—where viruses and chemical contaminants are rare—a filter will usually suffice. In these circumstances, hollow fibre filters with larger-pore diameters—say like 0.1 or 0.2 micron—allow for extremely lightweight, compact filters. Platypus’s QuickDraw Filter, for example, weighs just 69g, despite being able to have a throughflow rate of up to three litres a minute. And it’s so small that, while you can attach it to a Platypus bladder, you can attach it to any drink bottle with a 28mm opening—ie most standard soft-drink or bottled-water containers—and drink straight from it. MSR is another company with an equally light and small hollow fibre filter, the 71g Thru-Link Inline Filter.

It takes more than one hollow fibre, however, to provide protection; a filter has hundreds of hollow fibres bundled together, through which water flows. One key advantage with hollow fibres, besides their light weight, is that water can flow through the filter in both directions. Let the water flow in reverse, and you can ‘flush’ any trapped contaminants and/or sediments back out of the filter. This not only keeps the filter clean, it keeps the flow rate high.

Speaking of flow rate, that’s one other advantage of hollow fibres, and it’s not merely that they have a high throughflow; the flow rate—when it drops and cannot be restored by back flushing—gives a useful end-of-life indicator for the filter. Many other filter types don’t offer such an easy gauge.

There is a drawback to hollow fibre filtering, though. If the filter is dropped, or even if it freezes, the hollow fibres can break, thus compromising the filter’s integrity. Worse yet, it’s not immediately obvious this has happened. Some hollow fibre filters and purifiers, however, like the Platypus QuickDraw and the MSR Guardian mentioned earlier (in fact, this goes for all hollow fibre filters from both companies) allow you to perform integrity tests in the field that indicate if damage has occurred. And the Guardian—which was initially designed for military use—is unique in that it can withstand sizeable impacts; apparently, a six-foot drop onto concrete won’t damage it.

So there’s a rough guide to hollow fibre technology. For me personally, the weight advantages of hollow fibre filtering is key. There have been instances in years prior, before hollow fibre filters were widely available, where I’ve simply avoided taking a filter because of the weight penalty. I’ve also, it should be noted, suffered from giardia and dysentery on extended trips. And on those trips, the bottom fell out of my world. And vice versa. Try and avoid the same happening to you.