Is used water (water that has been utilized) pure and purifying?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
Water that has been used (Al-Maa' Al-Musta'mal) or water that is pure but not purifying (Tahir Ghayr Mutahhir) removes impurity from itself if it is abundant, defined as reaching two qullahs. This is based on the Prophet's saying, peace and blessings be upon him: "When water reaches two qullahs, it does not carry impurity (khabath)."
Supporting text
It is possible that used water becomes impure because it is pure but not purifying, making it similar to vinegar.