For those who read the earlier entry about water recycling and are now wondering why water is so important for Islamic rituals, here is something from a book of Shāfiʿī fiqh that I am working on:
Purification is cleanliness. Allah Most High said, “He sent down upon you from the sky, rain by which to purify you,” [Q8:11]. This verse is evidence for the possibility of removing ritual impurity and filth. Additional evidence comes from the Prophetic narration that “[sea water] is purifying and its dead are lawful.” (The report was transmitted by Abū Dāwūd, 84; al-Tirmidhī, 69—who said it is ḥasan ṣaḥīḥ; al-Nasāʾī, 1:50, and in Al-Sunan al-Kubrā, 58; and Ibn Mājah, 386.)
Only water raises ritual impurity, whether minor or major. Allah Most High says,
“O you who have believed, do not approach prayer while you are intoxicated until you know what you are saying or in a state of janābah [major ritual impurity], except those passing through [a place of prayer], until you have washed [your whole body]. And if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and find no water, then seek clean earth and wipe over your faces and your hands [with it],” [Q4:43].
“O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. And if you are in a state of janābah, then purify yourselves. But if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and do not find water, then seek clean earth and wipe over your faces and hands with it,” [Q5:6].
In these verses, Allah makes dry ablution [tayammum] obligatory in the absence of water, so using anything else would oppose with His command.
Only water removes filth. This is the case when the filth is slight (like the urine of a male infant who has been fed only milk), severe (like the filth of dogs, pigs, the offspring of the two of them or one of them), or intermediate (filth other than the above). Asmāʾ (may Allah be pleased with her) said that a woman came to the Messenger of Allah ﷺ and said, “One of us gets menstrual blood on her garment. What should she do with it?” He ﷺ said, “Scrape it, then rub it using water, pour water over it, and then pray [in it].” (The report was transmitted by al-Bukhārī, 277; Muslim, 1:240 #110/291; Abū Dāwūd, 364, 365; al-Tirmidhī, 138; al-Nasāʾī, 1:155, and in Al-Sunan al-Kubrā, 285; and Ibn Mājah, 629.) The reasoning is the same as it is for the verses: that using something else would oppose his ﷺ command.