By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
Tevilas Ezra—so called since it was instituted by Ezra Hasofer—which applies when a man immerses in a mikveh before praying because he requires purification, should ideally be in a mikvah filled by rainwater.
Where rainwater is unavailable, forty se’ah of mayim she’uvim (tap water) qualify, but it should be bekarka (in the ground); velo bekli (but not in a receptacle above ground); a kiddie pool, for example, is unacceptable, whereas a receptacle built into the ground, such as a pool, usually qualifies as bekarka.
The water should also not be zochalin (running)—it should be stationary. If there is a filter, it should be turned off prior to immersion; in case the filter cannot be turned off, it is still kosher.
However, if someone is immersing in a mikvah for tosefes taharah (additional measure of purity), as some people do with varying frequencies—every day, on certain days of the week, on Friday, on Shabbos, or before Yom Tov—there are some poskim who maintain that it must be in rain water.
Nevertheless, in this case too, if rainwater is unavailable, it is permissible to use mayim she’uvim—but it should preferably go behamshacha al gabei karka (run a bit on the ground) for a distance of three tefachim (handbreadths).
There is another option called tish’ah kabin, which involves pouring nine kav (approx. 12-16 liters) of water on a person in an uninterrupted flow, which can be used if a person does not have access to a rainwater mikvah, a natural body of water, or even a pool.
This method can be achieved by taking a shower for several minutes. To ascertain how many minutes are necessary, one can put a pail where it can catch the water to measure how long it takes for the water to reach the required shiur.
There are additional details about this method that are beyond the scope of today’s halachah. If a kosher mikvah is available later on, one should immerse again.