“Question: Is there any way of building a DIY mikva in the back garden – using a large plastic container for the bor tevila?
The following pertains specifically to building a men’s mikva; building a ladies’ mikva (because of the severity of the issurim involved) needs expertise and experience.
Size of mikva: First of all, when deciding on the receptacle that will serve as the bor tevila, attention should be given to the volume of the container; i.e. that it should be able to contain the volume of forty se’ah and the person being tovel without overflowing (see Shulchan Aruch Yoreh De’ah Siman 201 se’if 1):
The general practice regarding forty se’ah is to consider forty se’ah as 1000 liters, which is the volume that satisfies virtually all opinions. Therefore, although there are more lenient views regarding the calculation of forty se’ah, it is ideal that the mikva contains 1000 liters. As mentioned, the mikva has to be large enough to contain 1000 liters and the person being tovel without overflowing.
Nullify kli: Once we have selected a container large enough to serve as the bor tevila we have to nullify the kli status of the receptacle. This is because tevila in keilim is invalid min hatorah (Shulchan Aruch ibid se’if 6). The way that this can be implemented is to cut a hole in the kli; the diameter of the hole to be 10cm wide. The hole must be cut either in the base of the kli or in one of the sides directly adjacent to the base. The receptacle has now lost its Halachic kli status.
Attaching to the ground: In order to enable tevila inside the bored container, the hole has to be filled “in a building fashion”, and the container has to be attached to the ground (Shulchan Aruch ibid se’if 7). These two tasks are usually performed together, with cement or a similar substance used to both fill the hole in a “building fashion” and to attach the container to the ground.
From a practical point of view, one option is to place the container on the ground and pour cement inside in a way that will both fill up the hole and attach the container to the ground. Another method is to first pour a layer of cement directly on the ground and to then place the container on top, and to pour some cement inside the container in order to fill up the hole.
It should be mentioned that the hole cannot be “patched”, and not even with a material that is not mekabel tumah, but needs to be filled specifically “in a building fashion”. There is no requirement to submerge the receptacle into the ground.
It should also be mentioned that one of the most severe invalidations that can apply to a mikva is zochalin – i.e. if the water in the mikva is not still but “flowing” out of the mikva the mikva is invalid min hatorah. Therefore care must be taken to ensure that the hole at the base of the mikva is sealed in a way that water does not seep out of the container, because if water does seep out it may be invalid min hatorah.
Filling the mikva: Once we have attained a kosher site for tevila the mikva can now be filled. If there is ample rainfall it is possible to collect 40 se’ah of rainwater and the mikva is now kosher. After the required 40 se’ah of rainwater has gathered, it is possible to add any amount of regular tap water, and the mikva remains kosher (Shulchan Aruch ibid se’if 15).
Cleaning the mikva: An easy way of changing the mikva water without invalidating the mikva or having to wait for rain to fill the mikva is to add a large quantity of regular tap water to the mikva, and then to bail out some of the water ensuring that 40 se’ah will remain (Shulchan Aruch ibid se’if 24). This will allow the mikva water to be somewhat changed, while the mikva will remain kosher.
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