I was living in Cuzco when I did that ride so I was drinking the tap water and my stomach ameobas seemed to enjoy it. But I have had visiting friends get stomach bugs. If you are there for a short time (couple weeks say). I would stick to bottled water and ditch the purifier-too much weight. You can buy bottled water pretty much anywhere, even the most basic roadside kiosk. Very important to stay hydrated at that altitude so carry more water than you think you need. Make sure the seal on your bottled water isn't broken. Kids used to find empty bottles, fill them with tap water and sell them to unsuspecting gringos then spend their ill gotten gains on Chicles and video games at the arcade!