Sewers are likely to have methane too , may affect sensor type choice . Very humid/condensing environment too , so a that could be an issue . Due to the possibility of methane you may also need an intrinsically safe/ flame proof device - not something you can make yourself.
This is really a professional type project not for a hobby forum !
I hope you are not trying to make a system to decide if it’s safe to enter a sewer . A home made device is not suitable for this task, get an approved monitor.