This level has got me totally baffled. I even looked up two separate walkthroughs!
Is there a bug which keeps lowering the sewage level in the second area? There doesn't seem to be a way to keep it up after I raise it in the first area. No way to get across to the open tunnel!
I believe you have exactly 100 seconds to make it from the first area to the second area. You must run there and don't stop to fight enemies. It is quite possible to make it to the other side with time to spare.