Pt 2. The two guys who did the sand and activated charcoal gravity water filter also showcased a battery powered-portable version of of the USB HOCI generator. Here is a direct link to Aqua Research and their H2Go. For $124 it self meters the solution based on the volume of water you're treating, up to 20L/5.3gal

https://store.h2gopurifier.com/collections/products/products/h2go-purifier-global
Video of how it works.
https://www.youtube.com/watch?v=C09Jr313Btw