In reply to frqnt: I've carried that sort of load in a bergan before, which has metal bars running down to support your back, however, that's probably closer to an 80l pack, and from what I gather, civvi prices make them quite an expensive option.
I've done loaded runs at about 30kg dead weight in my Berghaus arete 45l bag, but that doesn't have any real back support(by which I mean, metal structural supports), and the bottom is a little flimsy, so I probably wouldn't trust putting much more kit in it, they do however, make similar style bags in slightly hardier materials that might suit your purpose. Might I enquire why you're aiming to carry 40l of water rather than 40kg of free weights, which would allow you to pad out the bag slightly, increasing comfort.