Storing water in 55-gallon container
If you are not going to be consuming it, then you just need to keep algae from growing in it.

Make sure the container is full to the top to eliminate air from the algae. Then store it in a dark place to prevent sunshine from feeding the algae.

The other problems to consider might be contaminants in the container to start with and leeching of anything from the plastic into the water.

Good luck!