I say, and with God's success: The water that has been mixed with used water is judged by the predominance of weight. If two liters of used water are mixed with one liter of pure water, it is not permissible to perform ablution or bathing with it. Even if they are equal in weight, the ruling of the lesser prevails as a precaution, so it is not permissible to perform ablution or bathing with it. This consideration of predominance applies whether the used water is poured into the pure water, or if a person immerses himself in it, or dips his hands or feet into it; because only what drips from the limbs or touches the body is used from it, and it is, in relation to the remaining water, minimal. Refer to: Maraqi al-Falah, pp. 26-27, and the commentary of al-Tahawi on al-Maraqi, pp. 19-20, and Bada'i al-Sana'i, 1/16, and God knows best.