When backwashing an ion exchange unit, how much should the bed expand from water introduced from the bottom of the ion exchange column?

• A. 20-30%
• B. 30-50%
• C. 50-75%
• D. 75-100%

Answer: (C)

When backwashing an ion exchange unit, the bed should typically expand between 50-75% as the water is introduced from the bottom of the ion exchange column. This significant expansion is necessary because it helps to dislodge and remove trapped particulates, regenerate the effectiveness of the resin, and ensure that the resin beads regain their proper spacing for optimal ion exchange efficiency.

The backwashing process involves the upward flow of water through the resin bed, which creates turbulence and helps separate the beads. This is essential for maintaining the longevity and performance of the ion exchange system. If the bed expands too little, the cleaning may be insufficient, possibly leading to channeling or reduced contact between the ion exchange media and the solution being treated. On the other hand, excessive expansion might indicate issues with the backwashing process or the condition of the resin itself. Thus, the specified 50-75% expansion ensures that the unit functions effectively while providing thorough cleaning of the resin bed.