I read somewhere (can't remember where, sorry) that depending on context, the plural of waste could either be waste or wastes. In what context would one use wastes? For some reason, I always use waste, both for singular and plural.
As tchrist notes in a comment, the plural of waste is never waste; there is no situation where these waste would be grammatical. Mass nouns (i.e. "non-count" or "uncountable" nouns) are always treated as singular.
Many non-count nouns, however, can be used as count nouns when referring to types, examples, or portions of it:
It is not especially common to pluralize waste, but in context it would be well-understood whether referring to useless material to be discarded or to an extravagant misuse of a resource.
Additionally, wastes is usually plural when referring to empty and worthless land.
- . He took with him the manuscript of The Prairie, which he would complete in a Parisian hotel – a far cry from the empty undulating wastes in which Natty first appears to the Bush family in gigantic, mythical proportions…