What is the method for purifying water that is less than two qullahs (approximately 120-200 liters) and has become impure?

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Water less than two qullahs is purified by augmentation using two full qullahs of pure water. This is achieved either by pouring the pure water into it or by a continuous flow into it, until the impurity (i.e., change in characteristic) is removed. If the water was not changed by the impurity, purification occurs merely by the augmentation to the quantity of two qullahs, because two qullahs do not carry filth and are not rendered impure except by change. Consequently, if impure water falls into water constituting two qullahs without causing change, the large volume is not deemed impure, and similarly, when water is being added to purify it, the added water aids in purification.