Well we must make a difference between at least two things:
- Water that remains after doing wudu
This is clearly clean according to all sunni madhabs
- Water which already has been used for wudu(water that already cleaned some parts of the body):
It's considered clean but with some differences:
Maliki say: it's makroh to use if there is cleaner water!
Hanbali and Shafi'i say: it's forbidden to use and ablution with it isn't valid. طاهر وليس بمطهر
And they quoted a Hadith about ghusl al Junub as an explication:
Abu Hurairah said: "The Messenger of Allah (ﷺ) said:
'None of you should perform Ghusl in standing water while he is
Sunan an-Nisa'i or Nasa'i, ibn Majah and Sahih Muslim]
Note that the Version of ibn Majah is adding an explanation of abu Hurairah on how to perform ghusl.
But as we see this isn't exactly the same situation, there's also a sahih hadith quoting that the Messenger of Allah (peace be upon him) forbid peeing into standing water and performing ghusl al janaba with it!
Hanafi: have two statments, the documented one is that it's not clean and if a lot of it touches anything it would make it impure. (According to ibn Hazm this statement is unsustainable, ibn Hazm quoted that in a not documented statement abu Hanifa said it's clean, shouldn't be drunk and wudu or ghusl with it isn't valid)
A 3rd possibility is:
- Clean water for ghusl where some used water have been dropped in:
Again the water is considred to be clean, but Shafi'i and Hanbali say if the used water is a big quantity abolution or ghusl would be invalid, but Imam Ahmad seem to have a second statement! (See also this fatwa in Arabic)
A different view
Ibn Hazm quotes in his book al Mohalla المحلى بالآثار that used water is clean and doing ghusl or ablution with it is valid even if there's "cleaner" water he also quoted a Hadith:
Narrated Mughirah ibn Shu'bah:
Al-Rubayyi’ reported: The Prophet (ﷺ) wiped his head with water which
was left over in his hand.
Sunan abi Dawod
he added that also the Imams and Scholars al-Hassan al-Basry, Ibrahim an-Noukha'i, A'ta' ibnu abi Rabah, Sufyan at-Thawri and abu Dawod said so! (See also this fatwa in Arabic)
A practical note
Now if you have water problems you shouldn't forget a sunna which is using the less possible of water for wudu and ghusl!
As quoted by al-Bukhari:
Narrated Abu Ja'far:
Jabir bin `Abdullah said to me, "Your cousin (Hasan bin Muhammad
bin Al-Hanafiya) came to me and asked about the bath of Janaba.
'The Prophet (ﷺ) uses to take three handfuls of water, pour them on
his head and then pour more water over his body.'
Al-Hasan said to me,
'I am a hairy man.' I replied, 'The Prophet (ﷺ) had more hair than
Therefore i would say it's ok to use such a water at least if you have no better alternative and you even should, but try to avoid the most dirty parts of it.
And Allah knows best