Shlomo Zalman, you are the second person this week who has asked this question! Moshe Rubin from Brooklyn, NY, asked a very similar question on Sotah 17, and I suggest you look at what I wrote to him (which we are sending you as a separate mailing). However, since there has been a lot of discussion on this topic I may still be able to mention some new ideas, and I will try not to repeat myself too much.

1) I will mention very briefly what I wrote in the other reply. Rav Shimshon Refael Hirsch, in his commentary on Chumash, points out that Bamidbar 5:23 uses the words, "And it shall be erased to the bitter waters." The fact that the Torah did not say that it is erased inside the water but rather to the water teaches us that the erasing is not done by the water but rather into the water.

2) Notwithstanding that Diyuk in the verse, there is still a lot to be said about this topic. In the Sefer Alei Veradim on Maseches Sotah (by Rav Shlomo Levinstein and Rav Tzvi Krizer of Bnei Brak), many proofs are cited both for and against the proposal that the letters were scraped with a blade. I will now mention one proof which is cited on page 244 in the name of Rav Yosef Shalom Elyashiv zt'l, that it was done in the water and not by a knife. This is from the Mishnah (17b) which tells us that only "Dayo" should be used for the writing, not "Kumus" or "Kankantum," because we require writing which is possible to be erased. Rav Elyashiv argued that one learns from here that the erasing was done inside the water because it is possible to erase any writing with a knife.

3) However, your argument about erasing the portion written in the Torah is a very strong one, Shlomo Zalman, and is also mentioned by the Alei Veradim. He makes the fairly obvious point that it would seem that according to the opinion that they used the portion in the Torah, they were only allowed to erase the Sotah portion, not other passages from the Torah, and it is difficult to imagine how they manged to do so if they dipped the Sefer Torah in the water. The Alei Veradim writes that possibly one could say that they did not merely place it in the water but they also rubbed the letters vigrously with their hands, so perhaps this way it was feasible to erase the Sotah Parshah quickly enough before the other letters of the Torah were badly damaged.

4) I found that the Sefer Tiferes Mordechai, by Rav Mordechai Fishof of Bnei Brak, cites in the name of Rav Chaim Kanievsky shlit'a that l'Chatchilah they tried to erase the name only in the water, but if this was not possible it was valid also if the erasing was done outside the water. (This can be found in what he writes on Megilah 18b, on Tosfos DH Kankantum. In fact, the proofs cited by the Alei Veradim are chiefly based on this Sefer.) This apporach seems to solve a number of difficulties in the Sugya.

5) This seems also to be practical, because even though it might be possible to erase the Megilah actually inside the water, it might nevertheless take quite a while to do this, and this would hold up the Sotah procedure. If we say that, when necessary, one may scrape off the letters outside the water with a blade, this makes it easier to envision how the procedure was done.

Kol Tuv,

Dovid Bloom

Here are more sources about whether or not they dipped the entire scroll in the water (both taken from the Sefarim mentioned above, and comprised of my own suggestions).

1) The first source is the Gemara in Sotah, beginning of 18a, where the Gemara discusses the person who wrote one letter and then erased it, then wrote another letter and erased it, until he had written the entire Megilah in this way. This seems to fit better with the idea that the erasing was done by scraping, because it is somewhat difficult to envision how he dipped it every time into the water and then waited for it to dry out before writing the next letter.

2) Here is a suggestion of my own based on a word used by the Gemara in several places. The Gemara (Nedarim 66b and elsewhere) says that we see how important peace between husband and wife is from the fact that the Torah "said that the Holy Name of Hash-m should be erased on the bitter waters" in order to restore peace in the home.

My question is, why does the Gemara say that the name is erased "on" or "above" the waters? Why does the Gemara not say that the name is erased "inside" the waters? I suggest that this implies that it was in fact not necessary that the scroll actually be dipped into the water, but it was sufficient that the Kohen should hold the scroll "on" or "above" the water, scrape away the ink with a blade, and then make sure that the erased letters fall into the water. The word "on" used by the Gemara now may be compared with the words "to the bitter waters" used by the Torah, and we notice that neither the Torah nor the Gemara states explicitly that the scroll must be erased inside the water.

3) However, I will point out that I have found a text in Midrash Rabah, Parshas Tzav, Vayikra 9:9 which cites a Beraisa of Rebbi Yishmael which states: "Great is Peace, since ha'Kadosh Baruch Hu" said that the Great Name which was written with holiness should be erased in the water in order to make peace between husband and wife." The text here is "erased in the water." This is different from all of the other texts in Chazal (that I am aware of) that read "on the water."

4) I suggest that one may argue that there is no essential dispute between the different texts. We can use the idea I mentioned above in the name of Rav Chaim Kanievsky that it is actually better, if possible, to do the erasing inside the water, but since usually this is hard to achieve for practical reasons, the Torah says that one may do the erasing outside the water and then put the ink of the letters into the water.

5) I found another interesting addition in Rashi to Nedarim 22b (DH Chutz). Rashi cites the Gemara about erasing the Name to make peace in the home, but he adds one word -- that it is erased on the water "b'Yadayim" -- "with the hands." Possibly Rashi is saying that one scrapes off the letters with a knife, or even that one rubs the letters of the scroll with one's hands when it is in the water, in order to ensure that the letters come off properly. Rashi seems to be saying that the erasing done in the Sotah ceremony is not considered "Grama" -- indirect cause -- and therefore should be permitted (even without the special Heter of the Torah in the interest of peace) according to the Gemara (Shabbos 120b) that says that Grama is permitted with regard to erasing the Name of Hash-m. Rashi explains that the erasing of the Name in the case of Sotah is not Grama because one does the erasing in a direct way, in order to make certain that it becomes properly erased.

Kol Tuv,

Dovid Bloom