Col. John Potter provides us with some great advice:
Some of my hiking buddies were unhappy with their ability to track water consumption from their single, large hydration bladders. So we investigated the eco-friendlier (reduced plastic) half-liter disposable water bottles as possible alternatives.
The idea is to use four of these instead of one 2L bladder. All four will fit in the side pockets of a Gossamer Gear overnight pack. For those of us without (enough) side pockets, two can be carried on the shoulder straps as shown in the photos. The necks of the bottles are held by water bottle clips attached to the strap buckles, and loops of shock cord around the lower straps and bottles keep the bottles in place. I used the rig shown (with plain water in one and electrolytes in the other) for four days in Grand Canyon. The bottles were secure and accessible, and consumption was very easy to keep up with.
The lightest bottles we’ve sampled (small cap, ring removed) weigh around 10g, with the Costco store brand coming in at 9.5g each. So four weigh as little as 1.3 ounces (plus 0.4 ounces for clips and shock cord, if you carry two on your pack straps). This represents a savings of 1-3 ounces over a 2L bladder with drinking tube (the difference being mostly the brand of drinking tube and the hose length).