According to the post of Tom Freyberg, Chief Editor, WWi magazine:
the Spanish firm FCC is pioneering a drone project to speed up, facilitate and improve the inspection of Barcelona’s 1,500 km sewerage system. The Aerial Robot for Sewer Inspection (ARSI) is equipped with multiple sensors to monitor water and air quality, including a 3D camera to calculate its position and a 2D laser to detect walls and calculate flight paths in real time.
A short video (4.5 mins) at:
gives interesting details.
Cases of deaths, while descending in the manhole or sewer line, have occurred, with one in USA. Drones, in such cases, can be very useful in inspecting the sewer lines, without compromising the lives of sanitary workers.