What is the name of the module housing the ISS's water recycling system?

The module housing the ISS’s water recycling system is Tranquility.

Tranquility, also known as Node 3, is a U.S.-built pressurized module delivered aboard space shuttle Endeavour and installed on February 12, 2010. It contains major life-support equipment, including the Water Processing Assembly, which turns wastewater into clean water for drinking and other station needs.

The recycling system processes water from several sources, including astronaut urine, humidity condensate and hygiene-related wastewater. Recycling is essential because transporting every kilogram of water from Earth would be expensive and would limit long-duration missions. The technology also helps NASA prepare for future lunar and Mars expeditions.

A common mix-up is Harmony, or Node 2, which connects important laboratory and crew elements and contains sleeping quarters. Destiny is the U.S. laboratory, while Unity was the first U.S. connecting node. Tranquility is also associated with the Cupola, the station’s observation module, which is attached to one of its ports.