According to my book, it explains that the subjective pronoun 'they' and be verb 'were' are omitted between 'if' and 'processed'. 'they' refers to 'byproducts'.
What about 'they had been' ? Is this impossible?
Because it is talking about a past counterfactual event, strictly speaking the full form should be "if they had been". However, in this context "if they were" might well be written, and probably would not be noticed as an error in normal reading. Therefore, I would say that the omitted words are indeterminate.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Because it is talking about a past counterfactual event, strictly speaking the full form should be "if they had been". However, in this context "if they were" might well be written, and probably would not be noticed as an error in normal reading. Therefore, I would say that the omitted words are indeterminate.