On big days out in the summer heat you can expect to use 1l of water an hour. That means that even if you use a 3l camelbak, theoretically you could be dry in three hours and pretty dehydrated in four, not to mention the potential for cramps from the loss of body salts all that sweating can cause.
Historically I’ve scheduled stops at civilisation to replenish, but I do have an ‘aquastraw’ and plastic bag to allow me to scoop and drink water in streams and whatnot. Its not as convenient as a pump actuated water purifier or tablets but its lighter. The disadvantage is that it only purifies what you drink at the time you’re drinking, so its no good for completely re-filling a camelbak for example.