Go back to the bridge and climb across the grate using the pipe just above it. Use the wrench on the three bolts. To drain the sewer, the symbols on the LEFT side of the bolts must match up with the symbol beside it on the drainage spout. (Not the side facing you! The LEFT side! Sorry, but some people don't get this.) So basically, the symbol on the top and bottom bolts must be a single straight bar, and the one in the middle has to be a circle. It may look like the middle bolt has the right symbol on it, but it doesn't; the circle isn't centered.
You won't get instant notification if it worked or not; you have to back away from the bolts first. If there's water GUSHING out of the pipe, you got it right. Now you can go into the sewers.
I'd say more, but that would be spoiling it. You really can figure it out this way. I've all but told you exactly what to do here.