Washed-up Negel Vasser
The Zohar states that water that was used to wash negel vasser (the ritual washing of hands [upon awaking]) attains kochos hatum’ah (powers of impurity) that may cause harm to a person. It must be disposed of properly; it should not be left standing where people pass, but should be poured down the drain immediately, even rinsing the sink afterwards. It should not be used for any other purpose (such as watering animals or plants), so that no benefit is derived from it.
There is a chumrah (stringency) to avoid speaking any devarim sheb’kedushah (holy matters [such as words of Torah or prayer]) in presence of standing negel vasser. The Chida (18th century authority) writes that used negel vasser is even more impure than urine—regarding which there are halachos against davening and studying Torah in its presence.
However, since there are poskim who allow devarim sheb’kedushah to be said in the presence of negel vasser, there is room for leniency in a sha’as hadechak (dire circumstances). If taking the time to dispose of negel vasser, for example, would cause a person to miss the z’man (halachic cut-off time) for reciting the morning Shema, they may recite it in the presence of the standing water. Halachah allows for davening even in the event that there is no water with which to wash negel vasser, so the trace of tum’ah that lingers in used negel vasser should certainly not inhibit a davener, me’ikar hadin (according to the letter of the law).
If a person is in a position where devarim sheb’kedushah must be said in the presence of standing negel vasser, there is the possibility of adding an unused revi’is (lit., a quarter—a halachic liquid measurement that equals approximately three ounces—see Halachah #724) of water into the used negal vasser (one revi’is for each person who used it) to “dilute” the tainted water. Other poskim require a greater volume of unused water; they claim that the pure water needs to be in the majority. There are those who maintain that these days, when we are very generous with the amount of water used for negel vasser (at least a revi’is for each hand), there is no tumah attached to the water. This gives further room for leniency in an emergency—even without adding unused water.
(For more halachos about negel vasser, see Halachah #256, Halachah #623 and Halachah #667)