There's a lot of useful chemistry and energy in ammonia (a significant component of urine), because nitrates, or bound nitrogen, have potential energy. Gunpowder manufacture in the middle ages relied on factory-style processing of animal urine.
So the sewer bots will be powered by urine and manufacture explosives from the nitrates and will be equipped with sensors to smell each other. So the robot overlords will be under? So next we have the proof of concept hack of a sewer bot to create a distributed denial of existence (DDOE) attack.
Life is becoming a free science fiction movie. I smell a sequel with methane powered poop bots.