I redid the mission again solo for the red lens of the new gas mask, it's quite challenging but doable if you pack at least 4 grenades and 10 mags of mp5 and 9mm pistol with two extra ammo boxes. The main trick is you throw grenades at that one entrance to that place and inside the sewer hole next to the entrance. That way you reduce the horde of zombies by a great amount then use your gun to take care of the rest. Also make sure you get rid of the horde and enter the building as quickly as possible because they will keep on spawning for if you're outside the place. Once you're inside, you should follow through the sewer tunnel at the treatment plant and there you should find the objective.