There's 2 rooms in the sewer, both have 3 riddler trophies inside. One of them electrocutes the floor, the other kills you with steam. I'm not sure if these are the ones you're talking about, but the electric one can be beaten by getting the line launcher and it's wire upgrade. After you hit the switch, climb on your line and wait until the electricity disappears. I'm not sure about the steam one though.
I have 81 platinum trophies! http://www.yourgamercards.net/profile/Jerry_Appleby?sort=title&order=asc PSN: Jerry_Appleby Currently playing: LBP2, MW2, SotC