This is something BitterootBulls wrote a while ago on this forum and I put it into the memory bank. Pretty interesting.
I have used the Steripen Adeventurer Opti for my scouting trips this year and it is remarkably convenient. The only thing I learned (the hard way) is that the steripen won't activate the UV light if the water sensor doesn't sense water. The Water Sensor senses minerals in the water, not the water itself. If the water is TOO pure, there aren't minerals for the sensor, and the sensor won't allow the UV lamp to activate. I filled my bottle straight out of the bottom of an icefield, and the steripen blinked red at me and wouldn't illuminate in the water. I was puzzled and ended up boiling water in the jetboil for the hike out.
I got home and read the manual finding that Steripen recommends carrying a small amount of salt to pinch into the water if it is too pure, and the steripen gives you the red light.
With that figured out, I am very impressed with the ease of use of the Steripen. I also have the prefilter. Altogether the system is like 6 ounces ... and no pumping.