The right tank has Centaur carbon media in it for hydrogen Sulfide. it is backwashed with the chlorine solution in the square black tank. Every 3 days it cycles about half of the 16 gallons to backwash then refills. Not sure if the same liquid comes back or if it's just water
If that's how it works, it's the craziest system I have ever seen. Centaur is a very expensive carbon for the lack of a better explanation. Works well for sulfur but doesn't need backwashing other than to remove chunks of sulfur or iron.