The Mishna on our daf describes how the kohen gadol, having completed the zerikat ha-dam – the sprinkling of the blood – on the parokhet of the Holy of Holies, now turns his attention to the zerikat ha-dam that he is obligated to do on the golden altar in the heikhal (see 16:18). As the Torah commands, the kohen gadol takes the blood of the par (bull) and of the se’ir (goat), mixes them together, and places blood from the mixture on each of the four karnot ha-mizbe’ah (“horns” of the altar).
According to the Tanna Kamma, the kohen gadol walks around the altar, sprinkling blood on each corner. Rabbi Eliezer disagrees, arguing that the kohen gadol stood in one spot and simply reached over the altar, sprinkling blood as necessary. To understand Rabbi Eliezer’s position, it is important to remember that the mizbe’ah ha-zahav – the golden altar – was only two cubits tall and one cubit in length and width, which allowed him to easily reach over it.
Once the kohen gadol completed the sprinkling of the blood on the corners of the mizbe’ah, the Mishna teaches that he sprinkled blood on the altar itself (see 16:19).
And he would pour the remainder on the blood on the western base of the outer altar. On a related topic, the mishna teaches that he would pour the remaining blood of an offering, after it was sprinkled, on the outer altar, on its southern base. These remainders of blood from the outer altar and those remainders of blood from the inner altar are mixed in the canal beneath the altar and flow out with the water used to rinse the area to the Kidron River. This water was sold to gardeners for use as fertilizer. The gardeners paid for this water and thereby redeemed it from its sanctity. Failure to do so would render them guilty of misuse of consecrated property.
From this topographical map, which includes, in its 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 Shiloah spring flows.