To disinfect your Platypus bottle, fill it with a solution of unscented household bleach and water, using a concentration of 2-5 drops per liter of water. Close and swirl for 10 seconds. Leave filled overnight. Dump solution and rinse bottle three times with hot water, then air dry.
For a more planet-friendly alternative, use a chlorine-free, hydrogen peroxide-based bleach alternative like Free and Clear™ available from Seventh Generation®.