Well, to avoid the high altitude sickness during Annapurna Base Camp Trek, you need to drink sufficient water each day, if you have three liters of water capacity, would be perfect. You may use a bottle or camel bag whatever comfort for you. In most of the places at the tea houses and grocery shops, you can buy drinking water, in the Chhomrong Village, there is a safe drinking water station provided by ACAP (Annapurna Conservation Area Project). Plastic bottle is strictly prohibited beyond the Chhomrong Village to mitigate environmental pollution. For the water treatment, you may use Purification tablets or Steripen or any tools for the water treatment.