If one finds ĥametz in his possession after the sixth hour of the day, he must dispose of it immediately. After midday, if one forgot to nullify the ĥametz in his possession, he is obligated by Torah law to eliminate it. The most preferable form of the mitzva is to burn the ĥametz, but one may eliminate it in a number of ways, as we have learned. For example, one may crumble the ĥametz and cast it into the wind, or break up the ĥametz and flush it down the toilet.
However, if one throws his ĥametz into an ownerless public domain, he has accomplished nothing. Although one may renounce ownership of his ĥametz before it becomes prohibited and thus avoid the obligation to remove it, if one still possesses ĥametz when the prohibition takes effect, the only way he can get rid of it is by destroying it completely.
Similarly, once the prohibition has taken effect, it is no longer possible to remove ĥametz simply by pouring a foul substance like bleach on it to make it unfit to be eaten by a dog. Only before the ĥametz becomes prohibited is it possible to do so. If the ĥametz was fit for consumption when the prohibition took effect, one is obligated to dispose of it completely. Therefore, if one finds such ĥametz in his possession, he must burn it, scatter it in the wind, or crumble it into pieces and flush it down the toilet, in order to eliminate it. As stated, the best way to fulfill this mitzva is by burning the ĥametz. Moreover, even after the ĥametz has been burned, it is forbidden to derive benefit from its ashes.
If one finds ĥametz on the first or last day(s) of Pesaĥ or on Shabbat of Ĥol Ha-mo’ed, he may not burn it or even carry it to the bathroom to flush it down the toilet, as ĥametz is muktzeh and may not be handled. Rather, one must cover it with a container so that nobody accidentally eats it, and it must be burned as soon as possible after the Shabbat or holiday. If the same person forgot to nullify his ĥametz before Pesaĥ, he violates two Torah prohibitions – bal yera’eh and bal yimatzei – every moment the ĥametz remains in his possession. In order for one to spare himself from these transgressions, according to many poskim he may pick up the ĥametz, break it up somewhat, and throw it into the toilet; these poskim permit violating the laws of muktzeh, which are of rabbinic origin, in order to avoid Torah prohibitions. Nonetheless, some poskim take the stringent position that he should wait until Shabbat is over before getting rid of the ĥametz (MB 446:6).
Regarding the berakha, if one finds ĥametz during Pesaĥ, even though it is a mitzva to dispose of it, he does not recite a berakha before the bi’ur, because the blessing he said before bedikat ĥametz included all ĥametz in his possession that required removal. However, if one kneads dough during Pesaĥ and it becomes ĥametz, he must say the berakha over its disposal since this ĥametz was not in his possession before Pesaĥ, at the time of his bedika, and thus the berakha he said then did not apply to it (MB 435:5).
. See n. 3, where we learned that the main dispute between R. Yehuda and the Sages is about ĥametz that one finds after it has already become forbidden. According to the Rishonim who follow R. Yehuda, one has a mitzva to burn this ĥametz. Even though most poskim follow the Sages, as quoted in SA 445:1, burning the ĥametz is an acceptable method, especially according to R. Yoel Sirkis (Baĥ) and Gra, who maintain that the Sages agree that the preferable method to destroy the ĥametz is burning. This is what MB states in 445:6.
Once the ĥametz becomes forbidden, there is an obligation to destroy the ĥametz completely, as cited in SA 442:9 and MB ad loc. 40. Even declaring the ĥametz ownerless does not work at this point; it only works before the ĥametz becomes forbidden, as explained in MB 445:18. Even after the ĥametz is burned, one is forbidden to benefit from the ashes, as explained in SA 445:2. MB 445:5 states that if one throws ĥametz into an outhouse, it is considered destroyed. However, the outhouses in the times of MB were exceptionally filthy, and anything that was thrown there became so repulsive that it would no longer be usable in any capacity. Conversely, bread that has been discarded in a modern toilet is likely to emerge intact from the drainpipes, and would likely be fit for a dog; therefore, if one were to discard bread this way, he should first break up the bread into smaller crumbs. See also Bedikat Ĥametz U-vi’uro 7:24 n. 56 for more on this topic, and see 7:1-2 and the corresponding footnotes. In n. 7, the author writes that if one finds ĥametz on Pesaĥ and spoils it so that it is no longer fit for a dog, he still violates bal yeira’eh and bal yimatzei according to SAH, but according to Ĥazon Ish he is required to destroy the ĥametz, but does not violate bal yeira’eh or bal yimatzei. According Pri Megadim, he only has a rabbinic obligation to destroy the ĥametz, since by not completely destroying the ĥametz before Pesaĥ he missed the Torah-level mitzva (the opinions of SAH and Pri Megadim are cited in SHT 442:19).
Although in general any ĥametz found on Pesaĥ has already been nullified and would therefore only be rabbinically forbidden, the ĥametz must still be destroyed, as would any ĥametz that had not been nullified, since, as noted, the rabbis decreed that all ĥametz must be nullified and eliminated. SAH (435:4) adds that if one did not burn his ĥametz, he violates bal yeira’eh and bal yimatzei on the rabbinic level and is subject to the rabbinic obligation of tashbitu. See also Bedikat Ĥametz U-vi’uro 7 n. 40 for other sources. Additionally, see chapter 8, 24, which says that if one finds ĥametz on Pesaĥ, there are ways that he can avoid bal yeira’eh and bal yimatzei, although he cannot fulfill the active mitzva of tashbitu. For example, he can heave the ĥametz into the sea in a manner that the ĥametz would be impossible to retrieve. The mitzva, however, which he cannot fulfill, is to burn the ĥametz.
Regarding the time for burning the ĥametz: if one finds ĥametz in his possession after the beginning of the sixth hour of the day, he has a rabbinic obligation to burn it, and if he found ĥametz that he did not nullify, he has a biblical obligation to destroy it. If he does not, he violates bal yeira’eh and bal yimatzei as soon as the festival begins. If he nullified his ĥametz, he does not violate bal yeira’eh and bal yimatzei on the biblical level, but he still has a rabbinic obligation to burn it.