Convenient Travel Pack to get started.
   1 x 55ml (NaClO₂) Sodium Chlorite Solution 28% 
   1 x 55ml Activator: (HCL) Hydrochloric Acid  4.5% 

Activate 1 drop of WPS with 1 drop of HCL 4.5% to purify 4L of water.
Store in a cool dark place.