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Have you ever stopped to think what’s involved in building and maintaining a kosher mikvah?  From the general structure down to the minutia, there are many things which must be considered.  Let’s suppose we get a large tank and fill it with rainwater.  Could we use that as a mikvah?  No, because a receptacle which is a ‘vessel’ is ‘mekabel tumah’.  (Mekabel tumah means that it is capable of contracting ritual impurity.)  Rainwater which is collected in a vessel becomes impure.  A vessel is defined as a receptacle which is capable of holding water, and which is detached from the ground.  However, if the tank is constructed on site, as an extension of the ground, then it is not considered a vessel.  So a mikvah cannot be manufactured and then transported to the site; it must be built in situ.  Actually, in the olden days, they would build the mikvah with a hole in the bottom.  In that way, it would not be capable of holding water and would therefore not be deemed a vessel.
 
When constructing the mikvah, care must be taken even down to the smallest detail. The tiles should be smooth and not have any grooves, which could become like a vessel which contains water and which is mekabel tumah.  Similarly a pipe with an ‘elbow-bend’ should not be used because water could collect in the ‘elbow’.  Also, the materials which are used must not be mekabel tumah.  Metal is mekabel tumah, so, if there is a stopper in the mikvah, it should be wooden, and not metal.  If there is a metal pipe carrying the rainwater down from the roof, then before the water enters the mikvah, it must go through a process called ‘hamshocho’.  The water is made to run across an area of soft earth or cement.  In this way, the water halachically becomes like spring water coming up from the ground, and can now be used.

Another problem which must be considered is that of leakages.  If there is a leak in the mikvah, the water becomes ‘flowing’, and rainwater which is flowing renders a mikvah invalid. If the mikvah has a drain, then the wooden plug must fit absolutely perfectly, otherwise any water leaking into the drain would be considered ‘flowing’.
 
Now that we have built a kosher structure, let’s have a look at how we fill it.  Before we start filling the tank with rainwater, it must be completely dry.  This is because we must begin with forty se’ah of rainwater.  There can not be any tap water in the tank before we start. To be precise, there can not be more than three ‘luggin’ of tap water.  (Three luggin is approximately one liter or five cups.)   So if, for example, the walls and floor of the receptacle are damp, there could be three luggin of tap water there.  Therefore, before it is filled or refilled, the rainwater tank must be completely dried.  So we begin with the forty se’ah of rainwater.  After we have the forty se’ah, we can  add as much tap water as we like.  The intermingled waters are then all considered kosher.
 
We can now begin to appreciate just how much care must be taken when constructing a kosher mikvah.  There are, of course, many other issues which must be considered.    There are so many intricate details that the construction must be done under rabbinical supervision.  What we have discussed here is just a drop in the mikvah!  Since Taharas Hamishpacha is such a beautiful and important mitzvah, we want to try and fulfill it in the best possible way.


Reference:  S.Z.Lesches, Understanding Mikvah: an overview of mikvah construction

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