Many Mikva’ot today have filters that clean the water. When the filter is turned on, the water of the Mikveh flows through the filter which removes impurities and then sends the water back. Are these Mikva’ot valid for use, or does the filtering mechanism disqualify a Mikveh?

In considering the validity of these Mikva’ot, we must address two issues. The first is the Halacha of "Mayim She’ubim," which means that the waters in the Mikveh must have flowed naturally into the Mikveh, without the use of a utensil. Once water has been placed in a utensil, it is not valid for use in a Mikveh. One might claim that a filter is tantamount to a utensil, and thus once the water enters the filter, it is no longer valid for the Mikveh. According to this rationale, a Mikveh with a filter may not be used, since its water has the status of "Mayim She’ubim."

However, Hacham Ovadia Yosef writes (in Taharat Habayit, vol. 3) that a filter is not considered a "utensil" in this regard. A filter is essentially just a pipe. It does not have a "Bet Kibul" – a receptacle that contains the water, which is a defining characteristic of a "Keli" (utensil) with respect to this Halacha. As such, water which enters the Mikveh through a filter is not considered "Mayim She’ubim."

There is, however, a different potential problem with filters in a Mikveh, namely, the Halacha of "Zohalin" ("flowing"), which requires that the water in a Mikveh must be stationary and not flowing. When the filter is turned on, the water moves into and through the filter. Does this movement of the water constitute "Zohalin," such that it disqualifies the Mikveh?

Rav Moshe Feinstein (Russia-New York, 1895-1986), in his work Iggerot Moshe (Yoreh De’a, 110), cites the ruling of the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) that a Mikveh is invalid while some of its water is being removed and then returned. This case is known as "Natal Se’a Ve’natan Se’a." Thus, if the Mikveh’s filter is situated outside the Mikveh, such that the water is continuously moving out of the Mikveh and then back in, one should not immerse in the Mikveh while the filter is turned on. However, as Hacham Ovadia writes, the Mikveh is valid even while the filter is operating if the filter is installed inside the Mikveh. Even though the water moves inside the Mikveh while the filter is running, the water’s movement within the Mikveh does not disqualify the Mikveh. The disqualification of "Zohalin" refers only to a situation where the water is moving to and from the Mikveh; a Mikveh is valid if its water moves within the confines of the Mikveh. After all, when two people are in the Mikveh at the same time, the water certainly moves, but the Mikveh is nevertheless valid despite the water’s movement. The case of a filter inside a Mikveh is no different. Here, too, the water moves only within the Mikveh, and the Mikveh is therefore perfectly valid.

Therefore, one may immerse in a Mikveh that has a filter, but if the filter is situated outside the Mikveh, one should not immerse while the filter is operating. Generally, the people who run Mikva’ot turn off the filters during the hours when women immerse. But men who use the Mikveh on Friday should ensure that any outside filters are turned off before they immerse. Nevertheless, if a man immersed while an outside filter was operating, his immersion was nevertheless valuable in raising his level of Kedusha in preparation for Shabbat.

Summary: One should not immerse in a Mikveh that has a filter outside the Mikveh unless the filter is turned off. If the filter is inside the Mikveh, then it may be used even while the filter is running.