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…
The noun waste can be countable or uncountable. In more general, commonly used, contexts, the plural form will also be waste. However, in more specific contexts, the plural form can also be wastes.
e.g. in reference to various types of wastes or a collection of wastes.