How does osmoregulation work?
Protozoans use a contractile vacuole to get rid of excess water.
In freshwater the organisms are always up-taking water that needs to be removed.
A "hydrogen active transport protein" pumps H^+ ions into a vacuole while bicarbonate (HCO3^-) follows passively. This causes the concentration gradient of water inside the vacuole to decrease, resulting in water rushing into the vacuole.
Once the vacuole begins to reach capacity it fuses with the plasma membrane and releases its contents.