Bismillahi Al-Rahmani Al-Rahim

 Tahara: Water
A l-hamdu lillah, one of the brothers has many questions for the fiqh lesson. In sha Allah I will be answering these over the next few days.
Q1
 What if one inserts PURE (not purifying) water into filthy water and it
adds up to 216 liters, does this denote that the water has become pure, but
not purifying, or remains najis?

A1
 There are three basic ways for non-purifying water to become pure:
  • when the amount of water becomes approximately 216 liters (qulatan) and there is no discernable change
  • when approximately 216 liters of purifying water is added to filthy water
  • when approximately 216 liters of filthy water reverts to the way it was before becoming filthy, on its own or by removing water from it
The section in Zad Al-Mustaqni` concerning purifying water includes
[Other types of water] become purifying when it reaches approximately 216 liters--which (b: technically) is a great amount.
[A great amount of water is still purifying even if:] filth which is not urine or other liquid wastes from humans mixes with it without it changing, or (b: human) urine or waste which is difficult to remove mixes with it [...] (b: as long as it does not change).

(m: Please note the correction from "pure" to "purifying" in two places.)
And the section on filty water includes
Filthy water becomes purifying if:
(1)     a lot of purifying water is added to filthy water--not dirt and the like;
(2)     a large amount of filthy water goes back to the way it was on its own, or (m: in the case of a well or some other situation where taking out water cases new water to come) by removing water from it and a large amount of unchanged water remains afterwards.
where "a lot" is approximately 212 liters (qulatan in Arabic). The first case is more similar to what you refer to: where purifying

And Allah knows best.

Q2  'If purifying and filthy water are indiscernible then it is unlawful to
use it and one does not attempt to discern; pouring it out (m: as in the
Shafi`i mathab) or mixing it (m: in the case that it can reach 216 liters
and thus become purifying) is not a condition for making tayammum. If
purifying water is indiscernible from pure, one makes a single ablution from
them both-a hand-full from this and a hand-full from that ­and prays a
single prayer.'

Please explain this. What do you mean by 'purifying and filthy water' being
indiscernible? Do you mean you have 2 containers, but you're not sure
which one is purifying and which one is filthy?

A2
 As for the first case where purifying and filthy water are indiscernible, a possible example is putting out two buckets of purifying water and then being told that while you weren't looking someone came by and poured a cup of white wine into one of them. Here you do not add one bucket to the other, pour them out, or use the one that seems most likely to be pure; instead you just make tayammum.

As for the second case where purifying and pure water are indiscernible, a possible example is making wudu from one bucket and catching the runoff water in an identical bucket and using only half of the water, and then needing to make wudu later. Here you would make a single wudu from each bucket taking one handful from each bucket each time you wash or wipe during wudu. The reason that only one wudu is made using both buckets instead of making a complete wudu from each bucket is so that one ends up having taharah that is sure to be valid.

And Allah knows best.

wa al-salamu `alaykum
--musa