The traditional rules regarding the construction of a mikveh are based on those specified in classical
rabbinic literature. Numerous biblical laws indicate that one must "bathe their flesh in water" to become purified from ritual impurity. The type of bathing is specified in , which states that "a spring, or a cistern, a gathering (
mikveh) of water" is a source of purity. A
mikveh must be built into the ground or built as an essential part of a building. Portable receptacles, bathtubs, whirlpools, or jacuzzis cannot therefore function as mikvot. However, many Sephardic communities, as well as Ashkenazi Jews in America before World War 2, customarily allowed mikvehs to be filled using municipal water. Bans on such practices only became common in the US after an influx of European Ashkenazi rabbis, who saw the use of municipal water as too lenient. Some rabbis considered permitting spas to be used, but ultimately decided against it as it may encourage women to prefer warm water during immersion instead of prioritizing cold water. According to Rabbi Isaac Esrig, in 1957 most American mikvaot were filled using municipal water.
Water transport Mikveh water must have collected naturally (
bidei shamayim) rather than by human action. Thus,
mikveh water must flow naturally to the
mikveh from the source (rain or a spring). This essentially means that it must be supplied by gravity or a natural pressure gradient and cannot be pumped there by hand or carried. As a result,
tap water cannot be used as the primary water source for a mikveh, although it can be used to top the water up to a desired level provided the minimum amount (40
seah) of ritually appropriate water is in the
mikveh first; in practice, this means that for a pool of at least 80
seahs (approximately 1,150 litres) the majority of its volume can be tap water. Frozen water (snow, ice and hail) is exceptional in that it may be used to fill the
mikveh no matter how it was transported. Although not commonly accepted, at least one American Orthodox rabbi advocated a home
mikveh using tap water, for those women who did not have access to a standard
mikveh. As water flows through only pipes that open at both ends, the municipal and in-home plumbing would be construed as a non-vessel. So long as the pipes, hoses, and fittings are all freestanding and not held in the hand, they could be used to fill a
mikveh receptacle that met all other requirements. The use of tap water for such a
mikveh was controversial and was rejected by the majority of
rabbinic authorities at the time and afterwards.
Size and practical arrangements A mikveh must contain enough water to cover the entire body of an average-sized person; based on a mikveh with the dimensions of 3
cubits deep, 1 cubit wide, and 1 cubit long, the necessary volume of water was
estimated as being 40
seah of water. The exact volume referred to by a
seah is debated, and classical rabbinical literature specifies only that it is enough to fit 144 eggs; most Orthodox Jews use the stringent ruling of the
Avrohom Yeshaya Karelitz, according to which one
seah is 14.3 litres, and therefore a mikveh must contain at least some 575
litres. This volume of water can later be topped up with water from any source, but if there were less than 40 seahs of water in the mikveh to begin with, then the addition of 3 or more
pints of water that did not meet the strict requirements would render the mikveh unfit for use, regardless of whether more water from a natural source was added later; a mikveh rendered unfit for use in this way would need to be completely drained away and refilled in the prescribed way. Inasmuch as water that collects naturally according to halachic prescriptions is hard to come by in urban areas, various methods are employed to establish a valid mikveh. One is that tap water is made to flow into a kosher mikveh and through a conduit into a larger pool in which users actually bathe. A second method is to create a mikveh in a deep pool, place a floor with holes over that and then fill the upper pool with tap water. In this way, it is considered as if the person dipping is actually "in" the pool of rain water. Additionally, the
hashoko method involves using two pools: one filled with at least 40
seahs of natural water and one filled with tap water. A hole at least wide on the wall of the pool filled with tap water connects it to the pool filled with natural water. When these two collections of water touch, the tap water pool is okay to use for ritual immersion. Most contemporary mikvot are indoor constructions involving rainwater collected from a cistern and passed through a duct by gravity into an ordinary bathing pool; the mikveh can be heated to make the experience of bathing more comfortable, taking into account certain rules, often resulting in an environment not unlike a
spa. ==Background==