I have a 55-gallon plastic container that I'll be filling with water.
What's the best way to preserve this water? We won't be consuming the water stored in this container.
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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.
If you are looking for potable water in preparation for a storm, like Harvey was, then I highly recommend a WaterBob. It is a one-time use item, but holds a bathtub full of water in a safe, non-contaminatable container and has a hand pump to dispense water.
As for keeping water shelf stable, the bleach is a good idea depending on the container. Some plastics degrade with bleach.
There are water stabilization and purification tablets available online and at your local outdoor sporting shops that work pretty well with stackable water blocks or long-term water preservation (prepper-style) BPA free barrels that use bung holes or hand pumps. They work great.
I would suggest that you go ahead and invest in a locking roller base to put the barrel on before you fill it to make moving it easier. But that's me.