Veer Jee, I think traditionally agan bhet and other left overs for example Langar, Degh, or fallen out Kesh, old Keskis, broken Kanghas etc etc after going through the appropriate maryada, Gursikhs usually dispose of it in running water. Same goes for phull after Antam Sanskaar, traditionally in running water. However there are some cases from last century or so where Gursikhs have buried spare langar as well.
There was a Gursikh named Bhai Mengha Jee from the time of Sri Guru Arjun Dev Sahib Jee who used to perform this seva on a daily basis with leftovers of the langar. After everyone had eaten, Bhai Jee himself would eat, and then he would take all the leftovers to a nearby forest and feed to the animals, and if there was even more left after that, he took to the river and put in there.
So I think jal parvaah is always preferred for this type of job, unless running water source is totally unavailable to jee, but I guess even then one could store in a jar or something until you go to the coast/seaside.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥
