What is the ruling on throwing leftover food and drink down the drain?

It is not permissible to throw leftover food and drink that
can be used down the drain or to put them out with the trash. 

In Fataawa al-Lajnah al-Daa’imah (22/341) it says:
Leftover food must be saved for the next time or it should be given to the
needy; if there are no needy people, then it should be given to animals,
even after it has dried out, for the one who is able to do that. End quote. 

Shaykh Ibn Baaz (may Allah have mercy on him) said:

With regard to bread, meat and other kinds of food, it is not
permissible to throw them in the dumpster; rather they should be given to
those who need them, or they should be put in a visible place where they
will not be mistreated, in the hope that someone who needs them for his
animals will take them, or they will be eaten by some animals or birds. 

It is not permissible to put leftovers in the trash or impure
places or in the road, because that is mistreatment of them, and because
putting them in the road is mistreating them and causes annoyance to those
who use the road. 

End quote from Fataawa Islamiyyah, 3/633 

Shaykh Saalih al-Fawzaan (may Allah preserve him) said: It is
not permissible to throw any food into dirty places or impure places such as
washrooms, because that is a waste, mistreatment of the blessing and a lack
of gratitude to Allah. 

The Prophet (blessings and peace of Allah be upon him) found
a date in the road and said “Were it not that I am afraid that it came from
the zakaah, I would have eaten it.” Narrated by al-Bukhaari in his Saheeh.
And he (blessings and peace of Allah be upon him) ordered the one who eats
to lick his fingers before washing them or wiping them on a handkerchief,
and he instructed that if a piece of food is dropped, it should be picked up
and any dirt should be removed from it, and it should be eaten. 

This indicates that it is not permissible to throw any kind
of food, dates or other eatables in dirty or impure places; rather the
blessing should be respected and preserved, because that is part of
gratitude for it, and because this blessing may come to one who needs it and
will eat it, even animals. So throwing it in the garbage is not permissible.
End quote. 

With regard to food that is not good or that has gone off,
and drinks that cannot be used, such as leftover tea and coffee, there is
nothing wrong with throwing them out with the trash or into drains. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: What is the ruling on students throwing leftover food and drink in
the trash? 

He replied: As for that which cannot be eaten, such as the
peels of oranges, apples and bananas and so on, there is nothing wrong with
that because this has no value in and of itself. 

But with regard to things that may be eaten, such as leftover
bread, condiments and the like, they should not be thrown in dirty places.
If they must be thrown into the trash, then they should be placed in a
separate bag, so that the cleaners will know that they are to be respected. 

End quote from Fataawa Noor ‘ala al-Darb, 6/205 

And Allah knows best.