I'll not share my opinion of Albert Thiel. However, I won't dismiss his opinions either. Regardless, John is giving a lot of great info and you should read it. Silica is one of the first contaminants to leak past RO/DI. This is because silica is loosely bound to the resins, so when stronger binding materials come in, the silicates will lose their place and start to break free of the exhausting resin. Silica is found in 3 forms: reactive, colloidal & suspended particles. The colloidal silica causes problems when it comes to water treatment because of its stability as an un-ionized compound....the difficulty is in removal by means of ion-exchange processes. Silica breakthrough (as mentioned just before) is usually the first to occur, and the use of reverse osmosis prior to deionization aids the process by reducing the silica load on the resins. As a side note, some think silicone dioxide (SiO2) is a challenge due to a similarity of H2O. Here's the nitty gritty of it all.