"Garbage were collected"
I have to come back to my earlier remark. Both the Swahili text and the translation now seem perfectly fine to me. As I'm not following this course, I didn't know the verb "ku-zoa" = to gather up, whether liquids like spilled milk or rubbish. Therefore, seeing the "zilizo" part, I immediately thought of the relative infix plural of the N-Group (takataka), "zi-li-zo" meaning "they which ...". This was supported by the fact that "ku-lewa" is also a verb = to be drunk. Of course, this verb doesn't make any sense with an inanimate object. But looking at the verb "ku-zoa", which in the prepositional form is "ku-zolea" and in the passive form "ku-zolewa", it does make sense: "zi-li-zolewa" = they were gathered up, or they were collected, so after all the translation seems quite all right to me.