Sanitary Tank #2 is located in the after end of the after battery compartment.
There is piping from the galley sink, scullery, crew's heads (toilets), and showers into this tank. To prevent floating matter from Pampanito being seen by the enemy alerting them to her presence, the sewage would be stored in the tank. When Pampanito was far from the enemy, the discharge valve was opened and compressed air used to blow the tank's contents to sea.
Because the tank has openings both inside of the boat and to sea it is a thick walled tank capable of withstanding full submergence pressure even though it would normally only be blown on the surface.
Sewage is corrosive and the interior framing of the tank has badly degraded, and the lacy material in the center cut away. These photos were taken just after cleaning, blasting to bare metal, and painting the tank.