I have observed in many locations that after everyone is done washing mayim achronim from the source cup into the garbage cup, the garbage cup is covered with a plate or what have you.
Is this step required and if so what is the source for it?
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According to Rabbi Dr. Ari Zivitofsky in this article, "[t]here is scant basis for covering or removing the water."
In his discussion:
The common custom is to pour the mayim achronim into a vessel. This is based on the idea that a ruach ra (evil spirit) appears on used water that is spilled onto the ground. However, if the water is poured into a utensil, vessel or even onto a pile of twigs which is on the ground, there is no danger of inviting a ruach ra.
I have found only a small number of references in the sources to removing the water, and no mention at all of the need to cover it. One halachic authority who refers to removing the water is the Kaf HaChayim Soffer (181:8). He quotes the Kaf HaChayim Palache (25:3) who says that if one does not have a special bowl for mayim achronim, or is too lazy to get the particular bowl, one can pour it into a food bowl, but then should be careful to remove it from the table before bentching. Why the need to remove it? According to the kabbalistic interpretation of the Kaf HaChayim Soffer, mayim achronim is an “offering” to the sitra achra—the “other side”—and therefore, must be removed. (This notion of the sitra achra is also mentioned by Rav Palache in the name of the Yalkut Reuvani, who states that Iyov (Job) suffered because he neglected to perform mayim achronim.) Thus, it seems that the only poskim (halachic authorities) who maintain that the water should be removed are those who are of the opinion that mayim achronim constitutes an offering to “the other side.” As already seen, however, most authorities draw on the Talmudic view that mayim achronim is for the sole purpose of cleanliness. Thus, according to the majority of halachic authorities, there is no basis for removing the water.
Many people mistakenly believe that the water must be removed due to the presence of a ruach ra. This misconception is dealt with in the Siddur Tslusa d’Avraham, where the commentary Shirusa d’Avraham (page 362) states that since there is no danger of inviting a ruach ra if the water is poured into a utensil, there is no reason to remove the water.