DIY Platypus Pre-Filter Cap

Although I have misgivings about their durability, Platypus‘ 2L+ bottles remains the primary water reservoirs in my pack. It’s been a bit over a year now since I started using them. At the same time I switched over to Platypus, I also started treating my water with chemicals rather than filtering it. Both methods of treatment have their advantages and disadvantages, but lately I have been using chemicals almost exclusively.

A water filter, of course, filters out not only the invisible nasties that upset the stomach, but also the visible things things that don’t cause much harm but aren’t altogether pleasant: dirt, dead bugs, small rocks, and the like. When I moved to using chemicals I was just dumping the water into my drinking vessel direct from the source. Without any sort of filter, the water could sometimes be a bit gritty. Too textured for my taste.

As a first attempt to solve this I started to place a bandanna over the opening of the Platypus, and then poured the source water over that. That worked great for getting out the sediment, but then I had the problem of having a wet rag. If the sun is out, it dries, but the other 307 days of the year, the bandanna — even a synthetic Buff — became a bit of a hassle to dry. I wanted some sort of pre-filter that I could get wet without worrying about it.

The solution (like more than a few before it) came while browsing the BackpackingLight forums.

A filter washer is a rubber washer with a mesh screen in the middle. Apparently they’re used in garden hoses and washing machines to remove sediment. I was able to find them easily in the plumbing section of a local hardware store.

I took an old Platypus cap and drilled out the center of it. Then, with a little Gorilla Glue, glued the filter washer onto the cap. That’s all there is to it! The new pre-filter cap weighs 2 grams (0.07 oz) and shouldn’t cost much more than $1 to make.

The downside to the pre-filter cap is that it does noticeably decrease the flow rate of the water. To fill the Platypus, I use a scoop made out of an older Platypus bottle with the top cut off. Without the pre-filter cap, it takes all of 30 seconds to fill the Platypus bottle. With the pre-filter cap, it takes something more like 2 minutes to fill up the bottle. I have to pour the water out of the scoop much more slowly. Because of this I’ll sometimes forgo using the pre-filter cap if the water looks very clean, but the majority of the time I do use the cap. It’s become a permanent addition to my pack.