Rate of impurity addition equals rate of impurity removal.

• A. Impurity
• B. Impurity sources
• C. Water quality equilibrium
• D. Ion Exchangers

Answer: (D)

In a steady-state operation, the system is balanced so that the amount of impurity entering per unit time matches the amount being removed per unit time, keeping the impurity level in the treated water constant. Ion exchangers achieve this by swapping ions from the water with ions bound to the resin. As long as the resin has available exchange sites and the process is run with proper flow and timely regeneration, the rate at which impurities are removed from the water matches the rate at which they enter (from the feed). If the incoming impurity load or the time between regenerations increases, the removal rate can’t keep up and the effluent impurity level would rise. This dynamic balance is a defining feature of ion-exchange systems, which is why they’re the best fit for the statement. Impurity sources describe where impurities come from, and water quality equilibrium is a broader idea about final concentrations, but they don’t specifically capture the operating balance that ion exchangers enforce.