I have a water tank at the bottom of my base that feeds my algae terrariums. Every night mechanical doors at the bottom of the tank close, pushing the water just high enough to feed the algae terrariums. Sometime in the morning the doors open again and the water falls back down. Early on a few food poisoning germs got into the tank but the polluted water from the terrariums had no germs so I sieved it back through. Even with the amount of germs in the tank the polluted water still doesn't have germs, none of the water I am feeding into the tank has germs.
43.358 million germs
87.031 million germs
Even if I was feeding highly germy clean water I doubt it'd be enough for 43 million germs in less than half a cycle. I've included the save file, not sure if I've done that quite right. I know my base is pretty unorthodox. I've also tested this in debug mode with a 2x2 4 ton water tank with a flat mechanical door at the bottom. Germs also multiplied there.
1. Build a clean water tank with a mechanical door at the bottom. 2. Introduce some germs. 3. Use mechanical doors at the bottom of the tank that open and close, pushing the water up and down. 4. Compare germ count from before and after.