Learn Hardcore Afrikaans: Plastiek besoedel die omgewing; daarom skei en sorteer ons die afval. - Plastic pollutes the environment; therefore we separate and sort the waste.

Because plastiek is being used in a general sense, like English Plastic pollutes the environment. Afrikaans often leaves out the article when talking about a material or a thing in general.

Plastiek besoedel die omgewing = plastic in general pollutes the environment.
Die plastiek besoedel die omgewing would usually mean some specific plastic.

What does die mean, and why is it used with both omgewing and afval?

Die is the Afrikaans definite article, meaning the.

A useful thing to remember is that Afrikaans uses die for:

singular nouns
plural nouns
all grammatical genders

So:

die omgewing = the environment
die afval = the waste

Afrikaans does not change the the way some other languages do.

Is besoedel the normal word for pollute?