Yeah, that's their De-Ionizer tank they rent to window company's, it takes ALL the impurities/minerals out of the water. Depending on your water hardness, you can get roughly 600 gallons of PURE water from them... IF you feed a De-Ionizer with a water softener, it will yield 3X more pure water.
The Softener tank I have is just a De-Ionizer tank filled with softener resin. Its 20/month 40/ exchange (My water is CRUNCHY HARD at 24 grains of hardness) so I would have to exchange it once a week with a fresh tank if I did not regenerate it myself, so im saving myself $180/month.
Thinking about adding their de-ionizer in with my Softener so I can rinse carpet with ultra pure water, its only $40/month, ($60 Exchange fee) and I could offer window cleaning with it as well.
I also know how to regenerate a de-ionizer as well, for less than $10 bucks (PM me if you want to know)
I will get picks to you later today.