Which of the following is a disadvantage of zeolite softeners?

The correct choice highlights a significant process-related issue with zeolite softeners. Zeolite softeners work based on the ion exchange principle, where calcium and magnesium ions in water are replaced with sodium ions. However, in water that contains iron, particularly in the ferrous state, iron can interfere with the functioning of the ion exchange resin. When iron is present, it can precipitate out, leading to fouling. This fouling can diminish the effectiveness of the resin, requiring more frequent regeneration or replacement of the zeolite resin, ultimately affecting the system's overall efficiency and reliability.

The other options describe characteristics or outcomes that don't represent disadvantages of zeolite softeners. They are capable of functioning well with turbid water, which is beneficial for situations where clarity is an issue. While they indeed contribute to the reduction of total dissolved solids (TDS), which is advantageous in water treatment processes, this is not a disadvantage. Lastly, stating that zeolite softeners completely remove alkalinity from water is inaccurate because they primarily target hardness ions (like calcium and magnesium) without directly removing alkalinity to the same extent, making this option misleading in the context of zeolite softener performance.