Penumbra: Requiem/Sewer

This is a small sewer and purification system. It is a small area, but difficult due to the jumps across water.

Bridges
Your first task is to raise a few bridges. It is not technically necessary, but provides a path faster than the associated swimming and pontoon bridges, and doesn't require any detour.

Enter the main sewage area, and head to the right - this is the generator room. Take a few crates in the generator, and throw them in the water to build a bridge across the gap - the easiest place is on the left hand side. Once you are across, flip the switch that appears on the left-hand wall to raise the first set of bridges.

Next, build a second bridge of crates. Bring two crates around the corner, and build a bridge to reach the left pathway in the main chamber. The easiest way is building a bridge first on the right-hand side of the path (accessed by a regular jump), and attempting to jump across the water once you enter the main chamber on the right.

If you fall into the water, you will have to swim back to the ladders near the generator. You can also attempt to exploit the physics system by pushing a crate downward, grabbing it and pushing it against a wall to launch yourself up. Even if the physics exploit won't land you on a raised platform, it may get you back on the crate bridge.

Once on the main chamber, flip the nearby switch to raise the second set of bridges.

Flow control
You can now turn the nearby wheel to start the flow control. The generator will stop the intake, but will cause the battery to detach from the ceiling at the end of the sewer. Take the battery and swim to the generator room, collecting the portal key at the bottom as you swim.

The next step requires quick movement for the best result.

Place the battery in the receptacle for the switch, then turn the switch off. The water will immediately being to rise. Run to the starting area and face the door. When the water is high enough, you can check the right side of the pipe, and collect the artefact hiding in the upper corner. Swim to the central chamber before you drown.

Level exit
You have one last portal key before the level exit. At the ground level, there is a switch that activates a few platforms. Flip it and quickly surface. Climb the ladder, and when the platforms are low enough, you can jump across them, and reach the portal key.

The exit is found within the sole dry exit of the central chamber.