The word is probably rederers,
from the verb to render.
One meaning of this verb is [from www.merriam-webster.com] 1 a: to melt down <render suet>; also : to extract by melting <render lard>
b: to treat so as to convert into industrial fats and oils or fertilizer
So a renderer is involved in boiling down animal carcasses to extract the fat, and other by-products.