sfsuphysics
Moderator
I know quite a few watch BRS videos around here, but for those that don't
Synopsis/spoiler of video (if you don't have time/desire to watch)
Deionizing resins attach at a molecular level the stuff that gets buy the RO membrane, ammonia, silica, phosphate, nitrate, etc. However the strength of the bond depends upon what the element is, e.g. silica has a much weaker bond than phosphate.
A simplified view is that these elements bond with the first DI resin it comes across making that resin "used", however if something with a stronger affinity (attraction) comes along it will knock off the element with a weaker affinity replacing it. Normally this isn't a problem as that element with a weaker affinity just binds with the next DI resin it comes across, and this process continues as you go through resin. However when the resin is all "used up", it doesn't mean that it doesn't work anymore as elements with stronger affinity will still be absorbed but at the expense of knocking off the weaker affinity elements off the resin except now there's no more resin for those weaker affinity elements to bind to and it just starts kicking them into your RO water container, net result is you get a spike in these weaker affinity elements. Even worse is that this migration of weaker affinity elements (e.g. silica and ammonia) starts before the DI resin is used up, so if you're like me who wants to make sure every last bead changes color because "damnit that stuff ain't cheap" then you will start to "soil" your "pure" water.
Solutions to this is to double up on resin bays, that way you can run one to absolute exhaustion and the second one will pick up the slack of what gets through near the end, then swap out your second one into the first spot, and put brand new stuff in the second spot. Or you can simply deal with the elevated silica/ammonia in your RO water because you'd rather have that than nitrate/phosphate
Synopsis/spoiler of video (if you don't have time/desire to watch)
Deionizing resins attach at a molecular level the stuff that gets buy the RO membrane, ammonia, silica, phosphate, nitrate, etc. However the strength of the bond depends upon what the element is, e.g. silica has a much weaker bond than phosphate.
A simplified view is that these elements bond with the first DI resin it comes across making that resin "used", however if something with a stronger affinity (attraction) comes along it will knock off the element with a weaker affinity replacing it. Normally this isn't a problem as that element with a weaker affinity just binds with the next DI resin it comes across, and this process continues as you go through resin. However when the resin is all "used up", it doesn't mean that it doesn't work anymore as elements with stronger affinity will still be absorbed but at the expense of knocking off the weaker affinity elements off the resin except now there's no more resin for those weaker affinity elements to bind to and it just starts kicking them into your RO water container, net result is you get a spike in these weaker affinity elements. Even worse is that this migration of weaker affinity elements (e.g. silica and ammonia) starts before the DI resin is used up, so if you're like me who wants to make sure every last bead changes color because "damnit that stuff ain't cheap" then you will start to "soil" your "pure" water.
Solutions to this is to double up on resin bays, that way you can run one to absolute exhaustion and the second one will pick up the slack of what gets through near the end, then swap out your second one into the first spot, and put brand new stuff in the second spot. Or you can simply deal with the elevated silica/ammonia in your RO water because you'd rather have that than nitrate/phosphate