There is a holding tank in the ship where this water is treated using natural processes (enzymes break down bacteria) or human processes (filters), and the newly-cleaned water is safe to pump back into the sea. This system is so efficient that the waste water tank only needs to be emptied and cleaned in port once per year. This incredible system supports life at sea, minimizes our human impact on this beautiful marine environment, and makes good use of the resource that the ocean has in abundance--water.
