The Mishna on today’s daf teaches the following in the name of Rabbi Shimon:
With regard to misuse of the blood of offerings that is to be sprinkled on the altar, the is lenient with regard to the status of the blood at the outset and stringent at its conclusion. With regard to misuse of the wine of the libations that accompany the offerings, the is stringent with regard to the status of the wine at their outset and lenient at their conclusion. With regard to blood before it is sprinkled on the altar one is not liable for misusing it, but once it has flowed away to the Kidron Valley one is liable, while libations are subject to the law of misuse at their outset, but are exempted from it after they flowed down into the shittin.
Regarding sacrificial blood, the Gemara in Massekhet Yoma (daf 58b) teaches that once the kohen gadol completed the sprinkling of the blood on the corners of the mizbe’aḥ, the Mishna teaches that he sprinkled blood on the altar itself (see 16:19) before pouring the remainder of the blood down a drain that was built into the foundation of the altar itself. This blood mixed in the plumbing pipes of the Temple with the remainder of other blood that had been poured into a similar drain in the outer altar. From there they emptied into the Kidron Valley, where their remains were sold as fertilizer.
On topographical maps which include, as their center, the Second Temple-era platform on which the mikdash stood, it is clear that the Kidron Valley, running to the east of the Temple Mount, is the natural run-off point for sewage from the Temple. The walls of the Temple Mount actually stand at the very edge of the banks of the dry river, in which the Shiloaḥ spring flows.
The definition of shittin appears in Massekhet Sukka (daf 49), which discusses the two bowls on the altar – sefalim – that drained into the foundation of the Temple. Rabba bar bar Ḥanna quotes Rabbi Yoḥanan as interpreting a passage in (7:2) as teaching that these drains – shittin – existed from the time of creation.
The Rishonim and Aharonim point out that it is difficult to reconcile Rabbi Yoḥanan’s teaching that the shittin are part of God’s creation with a statement made by him later on in the Gemara that describes King David as having them dug. Many answers are given to this question – e.g. that they were closed up at some point and that King David reopened them, or that Rabbi Yoḥanan is presenting the opinions of two different Tanna’im. The Maharsha explains simply that the term shittin refers to different things. In our discussion they are the pipes through which the wine and water that are spilled on the altar drain down into the Kidron Valley; in the later statement the shittin (or shattot) are the foundation of the altar itself.