Why does one need to cover mayim acharonim water if ruach Ra'ah only applies when it's poured on the ground?
Aaron Massry, Brooklyn
The Mishnah Berurah in fact does not mention that one should cover the Mayim Acharonim.
However, the Sha'arei Teshuvah (OC 4:8) cites the Birkei Yosef who says that "the custom of zealous people" is not to learn Torah or recite a Berachah next to water which has been used for Netilas Yadayim when awakening in the morning.
The Rivevos Efraim (1:12), by Rav Efraim Greenblatt zt'l, writes that it may be that the custom to remove the Mayim Acharonim before Birkas ha'Mazon is similar to the Minhag to keep 4 Amos away from the morning water. One should be Machmir in this matter, but it is not actually forbidden according to the Halachah.
Hatzlachah Rabah,
Dovid Bloom