If one finds ḥametz in his possession after midday on the 14th, he must eliminate it immediately. If he forgot to nullify the ḥametz in his possession, he is obligated by Torah law to eliminate it. Le-khatḥila the mitzva is to burn the ḥametz, but if he wants, he may eliminate it in other ways. For example, he may crumble it up and scatter it into the wind, or he may break it up and flush it down the toilet. However, if he throws it into an ownerless public domain, he has accomplished nothing. Although before the sixth hour one may still renounce ownership of his ḥametz and thus exempt himself from the obligation to eradicate it, once the sixth hour began, it became forbidden to derive benefit from the ḥametz in his possession, and it is no longer possible to get rid of the ḥametz and avoid the prohibitions it entails without destroying it completely.
Similarly, once the prohibition has taken effect, it is no longer possible to get rid of ḥametz simply by pouring a foul substance like bleach on it to make it unfit to be eaten by a dog. Only before the ḥametz becomes prohibited is it possible to do so. If the ḥametz was fit for consumption when the prohibition took effect, one is obligated to eradicate it completely. Therefore, if one finds such ḥametz 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 ḥametz. Moreover, even after the ḥametz has been burned, it is forbidden to derive benefit from its ashes.
If one finds ḥametz on the first or last day(s) of Pesaḥ or on Shabbat of Ḥol Ha-mo’ed, he may not burn it or even carry it to the bathroom to flush it down the toilet, as ḥametz 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 ḥametz before Pesaḥ, he violates two Torah prohibitions – bal yera’eh and bal yimatzei – every moment the ḥametz remains in his possession. In order for one to spare himself from these transgressions, according to many poskim he may pick up the ḥametz, 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 eliminating the ḥametz (MB 446:6).
Regarding the berakha, if one finds ḥametz during Pesaḥ, 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 ḥametz included all ḥametz in his possession that required removal. However, if one kneads dough during Pesaḥ and it becomes ḥametz, or if one accidentally bought ḥametz on Pesaḥ, he must say the berakha over its disposal since this ḥametz was not in his possession before Pesaḥ, at the time of its bedika and bi’ur, and thus the berakha he said then did not apply to it (see MB 435:5).
Once the ḥametz becomes forbidden, there is an obligation to destroy the ḥametz completely, as cited in SA 442:9 and MB ad loc. 40. Even declaring the ḥametz ownerless does not work at this point; it only works before the ḥametz becomes forbidden, as explained in MB 445:18. Even after the ḥametz is burned, one is forbidden to benefit from the ashes, as explained in SA 445:2. MB 445:5 states that if one throws ḥametz 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, I wrote that if one were to discard bread this way, he should first break up the bread into smaller crumbs.
Although in general any ḥametz found on Pesaḥ has already been nullified and would therefore only be rabbinically forbidden, the ḥametz must still be destroyed, as would any ḥametz that had not been nullified, since, as noted, the rabbis decreed that all ḥametz must be nullified and eliminated. SAH (435:4) adds that if one did not burn his ḥametz, he violates bal yera’eh and bal yimatzei on the rabbinic level and is subject to the rabbinic obligation of tashbitu.
Regarding the time for burning the ḥametz: if one finds ḥametz 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 ḥametz that he did not nullify, he has a Torah obligation to destroy it. If he does not, he violates bal yera’eh and bal yimatzei as soon as the festival begins. If he nullified his ḥametz, he does not violate bal yera’eh and bal yimatzei on the Torah level, but he still has a rabbinic obligation to burn it.