Is or Are? --> Every year, about 800,000 tonnes of food waste are generated

I need some help with the following sentence:

Every year, about 800,000 tonnes of food waste are generated.

Should it be "are" or "is"? (as bolded above)

My sense of grammar would have

about 800,000 tonnes of food

a complex descriptor of waste.

I can also sense

about 800,000

and

of food waste

as modifiers of tonnes.

This being so, I think the only reasonable answer would have to come by knowing the context of the sentence. That is, the general subject of the paragraph from which the sentence in question has been lifted.

Without context, for me the scales tilt toward "waste is generated".

The author structured this sentence in the passive voice, and thus omits who is generating the 800,000 tonnes of food waste. This use of the passive voice likely contributes to the confusion over identification of the parts of speech and, in turn, subject-verb agreement.

I'm a fan of sentence diagramming but, even without it, we can see that "tonnes" is the subject of the sentence (whereas it would be the object if the sentence were rewritten in the active voice, because it is the noun that is acted upon).

Stripped to its subject and verb, the sentence is "Tonnes are generated." This is because the subject, "tonnes" is plural. Everything else in the sentence is a modifier. The verb here cannot be said to modify "food waste" because that noun is preceded by the preposition "of" and comprises a prepositional phrase that functions as an adjective, modifying the subject, to signify what the tonnes are.

One also can approach the analysis interrogatively (question in italics and answer, in the form of a sentence modifier, in bold):

• Tonnes are generated. (root sentence)
• Tonnes of what are generated?
• Tonnes of food waste are generated.
• How many tonnes of food waste are generated?
• About 800,000 tonnes of food waste are generated.
• How often are about 800,000 tonnes of food waste generated?
• Every year, about 800,000 tonnes of food waste are generated.

Of course, the difficulty with subject-verb agreement here disappears if the sentence is rewritten in the active voice. With "humans" as the subject and "tonnes" becoming the object, for example, we can see immediately that the verb "are generated" becomes "generate":

• Every year, humans generate about 800,000 tonnes of food waste.

One final observation: It's worth noting that the sentence in the active voice offers more information to the reader -- and also places the additional burden on the author -- by identifying who is generating the food waste. Is this 800,000 tonnes being generated by all humans? By one nation (e.g. could we replace "humans" with a more specific noun, such as "Americans")? By one industry (replace "humans" with "restaurants")? Is it possible that the author did not know, or did not wish to call out with greater specificity, those responsible for generating the quantity of food waste at issue? The omission of such potentially important information is one of the dangers of the passive voice -- more dangerous than an entertaining question about number!