The Torah prohibits meat and milk together on three levels: Cooking, Eating and Deriving any benefit. They are only prohibited when cooked together, "Derech Bishul." If they were put together cold, that does not constitute the Torah transgression. Many authorities, including the Peri Hadash (Rav Hizkiya Da Silva, 1656-1695), hold that not only cooking in liquid, but also roasting, frying and baking constitute cooking from the Torah. However, pickling meat and milk together would only constitute an Issur D’rabanan-a Rabbinic prohibition. While both the Torah and Rabbinic violation are, of course, forbidden to be cooked or consumed, the difference between them is that it is permitted to derive benefit from a D’rabanan level mixture of meat and milk.
The Halacha requires disposing of meat and milk that were cooked together. Ideally it should be buried, but flushing down the toilet or putting securely in the garbage can is also acceptable. One must be careful not to discard it in the public domain so that dogs will not eat it. Feeding the dogs would be considered a violation of deriving benefit. Even if one burnt it, the ashes are also forbidden and may not be used as ingredients in soap.
One common case that must be avoided occurs when people have the same sink for meat and milk. They may pour cold milk in the sink and later put in a dish with meat residue. This in itself is not such a problem, because it is all cold. However, if one then rinses the sink with hot water from a Keli Rishon, such as an instant hot faucet, that would constitute a Torah violation of cooking meat and milk, even if he does not eat it!
SUMMARY
Those who use one sink for meat and milk must be careful that the sink does not contain cold meat and milk before pouring hot water into it.
