What is the ruling on used (Musta'mal) water mixing with non-used water that totals two Qullahs?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
If used water combines with non-used water such that the total volume reaches two Qullahs, the entire volume becomes pure (Tāhur). This is deduced because if the used water itself were impure, the mixture would certainly be pure; thus, the used water is deemed more pure (or capable of purification). The standard for two Qullahs is five full water skins (Qirb), where each skin holds one hundred Ritals by the Iraqi measure, totaling five hundred Ritals. This definition is the apparent position within our school (Madhhab) and aligns with the position of Imam Al-Shafi'i, based on narrations regarding the measure of Qilal of Hajar.
Supporting text
If the combined volume remains under two Qullahs, and the used water is the dominant portion, purification is prevented. Similarly, if the total volume reaches two Qullahs only by the addition of used water, purification is prevented, although there is another view suggesting the prevention is removed based on the Prophet's statement: "When water reaches two Qullahs, it does not carry impurity."