Troubleshooting Portable Mixed Bed Units
1. ORGANIC FOULING
Gradual falling off of water quality below 18 megohm can be the result of organic fouling of the anion component. Rinse down will take a long time, a brine squeeze of the anion exchange resin can help the problem.
2. SEPARATION DURING FILLING OF TANKS
Portable exchange DI tanks (PEDI) are usually regenerated in a batch process and then loaded in the vessels. If any water is left standing in the vessels, or if too much water is used in the hydraulic transfer of the resin, the resin can separate. Specifically, cation resin would form the bottom layer in the tank. Water quality would be quite bad and the pH would be low. The resin can be remixed in the vessel by lowering the water to a few inches above the bed and then introducing air up through the bottom of the bed.
3. INTERMIXING
Improper separation after backwash can result in cation resin appearing in the anion layer and anion resin in the cation layer. The cation resin that gets exposed to caustic is put into the sodium form and the anion resin that gets exposed to hydrochloric acid is put into the chloride form. This is not as bad a problem as it may appear. After the resins are mixed, a long rinse to quality may occur. After this, especially upon standing for several hours or overnight, a diffusion process neutralizes the beads, bringing them to equilibrium and the water quality will improve. It probably will not, however, attain 18 megohm.
4. DISTRIBUTOR PROBLEMS
In PEDI tanks an O-ring seals the riser tube into the top head. A leaking O-ring means raw inlet water can bypass the resin bed and appear in the effluent. Switching heads and getting different results will indicate that this is the problem. Replace O-rings frequently.
5. CONTAMINATED RESIN
One batch of badly organically fouled anion resin can destroy the capability to consistently make 18 megohm water. The difference between 18 meghom and 17 meghom is only about one tenth of one part per million, and an organically fouled anion resin can easily cause this. A caustic/brine cleaning and several subsequent clean water cycles can remedy the problem. Some PEDI companies maintain two segregated floats, one for industrial use and one for high purity (18 megohm) water to prevent organic contamination.
6. SEASONAL INFLUENCES
Mixed bed resins that rinse well in the summer may not rinse as quickly during the winter when water temperatures are lower. This is particularly true of older anion resins that have impaired kinetics. To overcome this problem and stop wasting so much rinse water, end the rinse cycle to a higher level than normal, but low enough to insure that the bulk of rinsing has been achieved, then remix the anion and cation resins and let them sit overnight.