Looks like a mineral sanitizer (I think), like the Nature2 system.
That is incorrect. I would start by reading "The ABC's of Pool Water Chemistry" up in Pool School, then reading about the various ways chlorine can be delivered to your pool. There is no "low chlorine" pool......nor is there any other chemical sanitizing your water except chlorine.So that's how the pool uses so little chlorine (the previous owner told me it was a "low chlorine" pool. Whatever mineral is being supplied through here is helping sanitize the water.