>>81619837All of that is incredibly simple, provided members of society are a certain color.
Birth rates are fake, abortions and birth control artificially lower birth rates. In a collapse type scenario access to abortions and birth control will be much lower.
Nuclear plants will require nothing, they fail safe. Once they go offline no further action is needed.
Chemical processing is extremely easy and humans have been doing it as long as chemicals have existed(in other words, forever).
Sewage might require some slight tweaks, but it is one trivial in difficulty to address. The micronation of Japan figured it out when the country was embroiled in 100 wars occurring simultaneously and the average citizen lived in abject poverty. Their solution 400 years ago was actually quite impressive and far superior to methods now used in the United States and Europe. They had dedicated poop merchants who would travel house to house, the poop was purchased and both parties were happy with the arrangement. This poop was then taken to make fertilizer which helped produce crops. Humans became part of a closed loop for food production. In cities clean water ran through channels providing water to houses and provided some protection, as houses built near the channels had cross the channel over a bridge and gate which highly restricted would be ne'erdowells. Poop merchants worked in the cities as well, because city toilet design was the same as the rural and village toilets. Even the castles and manors that chose not to use poop merchants were able to handle the poop situation. It would be directed to a decorative garden pond where it was broken down before fertilizing fields.
Dams I don't know much about other than they are extremely unimportant and that beavers will just automatically build them all on their own unprompted. The benefit of meat and fur more than offsets any inconvenience posed by the dam. If you really want it gone a stick of dynamite will clean things up.