What is the common term for the structure that filters pool water?

The term "pool filtration system" specifically refers to the arrangement and components designed to remove impurities from pool water, which is essential for maintaining clean and safe swimming conditions. This system typically includes various elements such as the filter tank, the filter medium, and associated plumbing, which work together to physically or chemically eliminate debris, dirt, and contaminants from the water.

While the other options—like a water treatment system, pool circulation system, and pumping mechanism—relate to the overall management and movement of water within a pool environment, they do not explicitly denote the filtration process. A water treatment system may involve chemical treatments, a pool circulation system deals with water flow and distribution, and a pumping mechanism generally refers to the equipment used to move water, but only the filtration system is primarily dedicated to purifying the water itself. The distinction highlights the importance of the filtration system in ensuring the water remains clear and safe for users.