As someone who loves gear and is always looking for the next greatest ultralight backpacking gizmo, I strongly recommend you consider the SteriPEN Adventurer.
This is Hydro-Photon, Inc’s newest iteration of its proven water purification system, which was first introduced in 2000. I own the first and second generation models, and if you look at an earlier review on this site, you will see that I really love them.
As an ultralight backpacker, the only reservation I’ve ever had has been the size and weight of the earlier versions (about 7.5 inches long and 7.2 ounces with four batteries).
Compared with other water purifiers this is not exactly “heavy,” but in my quest for everything ultra, ultralight, I always wished for a smaller, lighter version. And the Adventurer is it: 6.1 inches long and just 3.5 ounces (on my postal scale) with two lithium batteries; 4.5 ounces with carrying case. The carrying case is only necessary if you want to attach the Adventurer on your belt or the outside of your pack.
The SteriPEN uses ultraviolet light (UV) to purify water. You push the button once, immerse the UV lamp, stir continuously, and you’ve got one liter of pure, safe water in 90 seconds. For a half liter you push the button twice and wait only 48 seconds. You don’t need to remember whether to push once or twice because the instructions are printed above the activation button.
What it Kills
The UV light destroys viruses, bacteria and protozoa (such as giardia and crypto), and according to company literature, exceeds U.S. EPA standards for microbiological water purifiers. The company also notes that numerous independent labs have proven its effectiveness (and it has been on the market for eight years).
Something Lighter?
The only water purification system I’ve ever seen that is lighter or smaller is iodine drops or tablets. Problem is, depending on the water temperature, you need to wait 20-30 minutes to drink after treatment and there is no guarantee that chlorine or iodine will kill cryptosporidium. Besides, who wants to wait? When you’re thirsty, you’re thirsty. Of course, you can always boil and wait. With the SteriPEN, you scoop up the water, give it a 48 or 90 second blast of UV and you’re on your way.
I’ve owned pumps, used iodine, tried in-line filters connected to water bags and none of them stack up to SteriPEN, especially the new ultralight backpacking and hiking model.
Field Testing
Since I haven’t taken it into the field yet, I can’t attest to the durability of the case and UV light cover, but it appears to be made of water-proof, high-impact plastic. I’ve dropped both of my other SteriPENs and have never had them break, so I assume they’ve made the Adventurer just as sturdy. The company notes that the U.S. military is using the Adventurer in various locations.
A Downside?
Disadvantages? Well, the company reports that like all pumps or chemical treatments, UV has a problem with treating murky or turbid water and recommend pre-filtering (with a bandana perhaps). However, the Hydro-Photon states that even with turbid water, two doses of UV light (perhaps three minutes worth) will make the water safe to drink.
Another option: Hydro-Photon has come up with a new product: a Nalgene bottle pre-filter. The Pre-Filter filters out particulates and debris. Fits on Nalgene® & wide-mouth water bottles. Water bottle pre-filter: $12.95. Weighs just 1.5 ounces.
As for battery life, my query to the company, drew this response: “In terms of the batteries, the CR123 disposable batteries will provide roughly 100 of the 0.5 L doses and the rechargeable CR123 batteries provide 60 0.5 L doses.”
This is right in line with my second generation model, which I personally tested in my kitchen over two days. (I’ve also used it on dozens of backpacks over six or seven years). It produced 60 1-liter treatments before discharging, which should be adequate for a week-long backpack or more. For longer trips — say the AT, JMT or PCT — you’ve got an optional solar charging case at $49.95. The weight is more than 7 ounces. For an ultralight backpacker concerned about every ounce, this could eliminate the need for taking extra batteries which means extra weight. You would need to balance battery weight versus solar charger wait.
One last thing: the customer service is outstanding. My first generation had a “child lock” which I didn’t want and they immediately fixed it. And responses to my questions have been really fast.
Price: $99 suggested retail with carrying case and non-rechargeable batteries
Options to consider: solar charging case – $49.95
Video Demonstration: follow this link.
In the meantime …
Be light. Be safe. Be one with the pack.
A must in every ultralight backpack is some form of first aid. I usually take duct tape (you can use to make a liquid in a 1.5 ounce and 2 3/4 inch high bottle (heavy compared with Bandaids) that you brush on: 
I discovered a “green” spork (combination fork and knife) 
My regular readers know I’m a gearhound. And I’m always looking for the latest and greatest ultralight backpacking “thing” to include in my pack.