Final answer:
An anion resin bed increases the pH of water by releasing hydroxide ions that combine with and remove hydrogen ions from the water, thus acting as a base and raising the pH.
Step-by-step explanation:
An anion resin bed controls the pH of water by releasing hydroxide ions (OH-), which combine with hydrogen ions present in the water. This action increases the pH of the water because it reduces the concentration of hydrogen ions. Bases that release hydroxide ions in water can remove protons (H+ ions) from the solution, raising the solution's pH as they form water molecules. Therefore, when an anion resin bed releases hydroxide ions into the water, it acts as a base, accepting hydrogen ions and raising the pH level.