u/Downtown-Repeat-4532

This is a lower holding tank for my “clean water” after it’s filtered through my MultiFlo system. The main line out goes to garden spigot/sprinklers. Sprinkler system is currently not working and my tank is backing up. I cannot figure out what the other 2” pipe with a ball valve is for and what I need to keep it from backing up on me. the ball valve is open on the end and not connected to anything.