I’m a fan of the Sawyer Mini water filter (https://sawyer.com/products/sawyer-mini-filter), but am unsatisfied with its general reliance on collapsible bottles. I find collapsible bottles too finicky to fill up, challenging to stand upright on the ground, and cumbersome to slide into a pack’s side pocket. I’d rather use a rigid, lightweight plastic bottle.
The problem is that the design of the Sawyer Mini doesn’t allow air to get back into the bottle to replace the water being drawn out. The resulting vacuum quickly makes it hard to suck water out. You have to periodically loosen the filter to let air into the bottle to relieve the vacuum, or constantly crush and re-expand the bottle.
One simple way to fix this problem is to add a one-way “duckbill” silicone valve to a PET bottle:
Such a valve lets air in, but won’t let water out. When you drink through the filter, air bubbles indicate that air is entering the bottle to relieve the vacuum:
I haven’t yet found a cheap source of such silicone valves. Since they’re generally sold as parts for larger products, Internet sources seem to only offer them in large lots. Blue Desert will sell you a single valve as a replacement part (http://www.bluedesert-shop.com/#!product/prd1/2463039021/one-direction-valve-for-smartube%25c2%25ae-cap), although it’s ridiculously expensive: $8 for a part that probably costs a pennies. Still, a collapsible bottle can run $10–20, so the total cost here is still competitive.
Installing such a valve is trivial. First, find a PET water bottle whose threads fit the Sawyer filter. (Not all brands will work. I just bought two bottles of water, and the second one worked fine.) At the top of the bottle, drill a hole that’s just a bit wider that the valve’s narrowest part. Push the valve in the hole, and it should snap into place.
I really enjoy this water system. PET bottles are inherently light, and the valve adds negligible weight. (The Blue Desert page above states 10g, but I measure it as less than 1g.) Combined with the Sawyer Mini filter, you get a safe, low-hassle drinking system. And if the bottle ever breaks, just extract the valve and stick it in a new bottle. This could probably be done on the trail using just a knifepoint.
Give this a try, or express interest in this thread. Perhaps some DIY backpacking supplier (ZPacks, Dutchware Gear) can be convinced to order a box in bulk then sell individual valves at a more reasonable price.
