No special excretory organs are present in amoeba.
It produces a nitrogenous waste which is called ammonia, which diffuses across the plasma membrane into the surrounding water. In case of fresh water amoeba, contractile vacuole is present which maintains the water balance through cell membrane by the process called osmoregulation.
As the water diffuses continuously through the body surface and increases the water content of the body. Contractile vacuole swells up and becomes large like a balloon, then suddenly bursts and throwing water outside.
Thus the contractile vacuole also maintains the water balance through the process called osmoregulation.