In the Second Temple era a Kohen Gadol named Ben Katin devised a machine that was used to lower the Kiyor [laver] into a well of water each night and then raise it again in the morning. This prevented the water inside from becoming disqualified and unfit for Temple use. The Sages praised Ben Katin for this great innovation that saved the Temple wasted water and the effort of refilling the Kiyor every morning. The question is, what did they do before the Muchni was invented?
Last month my wife and I attended the Talmudical Academy of Baltimore's 100th anniversary dinner. In the swag bag we took home was a sefer titled Chidushei R' Yaakov Mikamenitz, a collection of Torah thoughts written by R' Yaakov Bovrovsky who studied in the Kamenitz and Mir yeshivos. As part of a longer essay about the Rambam's view on the Muchni (מסכת זבחים סימן ח) he does attempt to answer the above question as follows:
The Gemara (Zevachim 19b, 21b) writes that a Kiyor is only valid for use if it contains at least enough water to sanctify (i.e., wash) the hands and feet of four Kohanim [a relatively small amount]. This is derived from a verse which states that the Kiyor in the Mishkan held enough water to sanctify the hands and feet of Moshe, Aharon, and his two sons (four people). Now, after the first Kohen uses the Kiyor there will only be enough water left for three people, but R' Bovrovsky maintains that the remaining water is obviously still valid because the Kiyor started out with the proper volume. Therefore, the requirement of "enough water to wash the hands of four Kohanim" is the maximum amount of water that the Kiyor would ever need to hold.
As a matter of practice, the Kiyor would never be filled past this point [even though it could hold much more water — over 250 gallons], and over the course of the day as the water ran out, more would be poured in. Thus, at the end of any given day, there would never be more than four Kohanim's worth of water in it. This relatively small amount would be left in the Kiyor overnight and — of necessity — become disqualified, and fresh water would be brought in the morning to replace it. According to this approach there was actually very little water wasted each day and this is why no one gave this system too much thought until Ben Katin came along.