A sidestream off this line feed a DI bottle service for DI water. Membranes are very good at rejection high molecular weight compounds and multivalent ions. The amount of leakage is determined by the reject ration. The reject ratio changes over the life of the membrane and leakage increases. When they do, this will plug the very small pores of the membrane.
Organic compounds can also plug the pores.
Once plugged, the flow decreases and the membrane must be cleaned. Hardness can be eliminated by softening or continuously dosing a chemical chelating agent. A. Initially the incoming water if filtered to remove particulates and colloidal substances. After this there are two ways to reduce the chance of plugging. A chemical can be added to the feed stream that keeps the hardness from precipitating out.