In this part of the game you will need to cooperate with Elle.
Go forward and jump down. You will notice a gate with number 3 (it's marked as Gate 13 on your map).
There's a wheel on the left side. Use it to open the gate. Elle should go through and open it for you from the other side.
Walk forward, then left and climb onto the higher level.
Walk in the water with the axe equipped. There are two Lurkers here so be careful. Kill them and leave the water.
One way goes nowhere but you can find 2 x Pistol Ammunition there.
Go down, then left at the fork, climb up and pry the door with the crowbar. Rotate the wheel to get rid of the water. Go to the main tunnel and go forward.
You should reach a save point.
In the room nearby you will find Shotgun Ammunition and Sewers Map. Go to the corridor marked as Bilige Tank.
Go east, then north. Turn to the east and go south. In the corridor on the left there is a passage leading to a Health Drink.
Go south until you reach a gate with number 8.
Rotate the wheel, Elle will go on to do.... nothing. Go back north and to the save point.
During your travel to the save point Alex will need to fight with Needlers. From the save point go north and then turn west.
Go south until you reach the gate with number 4. Elle will open it from the other side.
In the corridor find a lane on the right. You will find some Shotgun Ammunition. In the Spillway kill two Lurkers and climb the ladder.
In the corridor heading west another Needler will attack. Take the ammo from the corner (Pistol Ammunition and Shotgun Ammunition).
Road to the west is blocked, pull the lever nearby and go north. Jump down and save the game.
Go north, then east to the place marked as Main Drain Chamber.
Main Drain Chamber
Rotate the wheel near the gate - Elle will disappear on the other side. Kill three Needlers and the Sion. You can use the axe or simply shoot them with your shotgun.
Go east through the destroyed gate. In the corridor on the left you can find the Photo #7. Climb up at the end of the corridor.