Submarines get rid of human waste by way of a waste water tank on board the submarine.
The submarines have waste water tanks where the poop and human waste goes into.
Then once the submarine resurfaces and is near a place where the human was can be removed it is them pumped out and sent to waste water treatment plants or dumped into a septic tank.
However some submarines may also empty the human waste into the water beneath the ocean once they are well away from port.
Most bigger submarines have something called “Trash Disposal Units”, which are exactly what you think you are.
Somewhere in the submarine there is a long tube or a shaft, which has a ball valve separating it from the ocean.
Crew members have to assemble canisters from pre-punched galvanized, perforated steel sheets.
With most submarines the crew what they do is all the poop and all the excrement and all that stuff goes into the sanitary tanks.
There's three sanitary tanks onboard a submarine.
So what you have to do to discharge it is you have to pressurize a sanitary pump that blows the sanitary waste outside of the boat.
The operation of a submarine toilet is as follows.
Open lever operated drain valve.
Open flush valve and keep open long enough to thoroughly flush both the bowl and its drain line.
Close the drain valve.
Allow several inches of water to cover the drain valve before closing the flush water.