Mikveh Construction1This is a simple summary of basic principles. Actual rules for construction are far more complicated, and building a mikveh should only be undertaken by those with expertise in this matter.
Ritual immersion needs to take place in water that has accumulated naturally (not she’uvim, or “drawn water”), and in a pool that does not have the halachic status of a kli (vessel). A mikveh is based on a minimum of 40 seah (approximately 331 liters or 87.5 US gallons2Rav Chaim Naeh, Sefer Shiurei Torah 3:29. Pinchas Even Yerushalayim, 5707.) of accumulated rainwater.3In cases of need, large blocks of ice or snow can be melted to fill the rainwater pool, and the water is not considered “drawn.” In practice, one does not immerse directly in the pool of rainwater, but in a pool of tapwater that is directly connected with the rainwater reserves. (otzrot). This allows the water used for immersion to be changed for cleanliness, even when new rainwater will not be available.
There are three ways in which the pools can be connected:
1. Hazraah (הזרעה) – If there are already 40 seah of natural water in a mikveh, one can continue to add drawn water without interfering with its status. The added water itself assumes the status of natural water. According to most opinions this can continue ad infinitum. Thus on one side of the pool there is an otzar hazraah initially consisting of rainwater. Tap water is added into this pool, which then overflows into the main pool(s) of the mikveh where immersion takes place.
2. Hashakah (השקה) – When a pool of tap water has direct contact (“kisses”) the rainwater, the drawn water attains the halachic status of mikveh water. Thus, an otzar hashakah of rainwater is located next to the immersion pool and connected to it via a small hole in the wall.4Some mikvaot also add an otzar hashakah underneath the tap-water pool, following Lubavitch practice. For further discussion of this see: Dichovsky S, Mirveh Y. “Mikveh al Gabei Mikveh.” Techumin 5751; 11:248–260.
3. Hamshachah (המשכה) – If drawn water runs along the ground for a certain distance prior to entering the mikveh, it is no longer considered drawn. Therefore, the waters of the otzar hazraah often run along the ground prior to entering the main pool.
There are advantages and disadvantages to each of these methods. Due to the grave consequences should the mikveh not be halachically valid, most mikvaot for use by women today are constructed to use all three of these methods and are thus valid according to all opinions. As men’s mikvaot are used only due to custom, the rules for their construction are more lenient. Therefore, women should not use the men’s mikveh even when the men are not present.