It's a valid inquiry, particularly when you consider the sheer scale of these colossal cruise ships that accommodate thousands of people. Take, for instance, Royal Caribbean's Icon of the Seas, boasting a staggering 7,600 passengers. With such a multitude on board, both passengers and crew members collectively generate an astounding 20,000 pounds of human waste during a week-long voyage.
So, what happens to this substantial amount of "black water"? Well, it turns out that the Icon of the Seas employs a rather sophisticated system to handle this biological byproduct. The ship features an extensive septic system where the waste undergoes thorough purification before being discharged into the sea.
From Miami New Times:
In a statement to New Times, a Royal Caribbean spokesperson explains that all of the cruise line's ships, including Icon of the Seas, process human waste — "black water" in industry parlance — via "a five-step tertiary system, which far exceeds most land-based treatment plants." The company says it runs both black water and "gray water" (from showers, baths, sinks, and laundry) through that purification apparatus, then releases the treated liquid into the open sea.
Royal Caribbean says that if a ship's treatment system is malfunctioning or bypassed, the company permits untreated wastewater to be released only if the vessel is at least 12 nautical miles from land — a policy more stringent than the three-mile distance required under federal regulations governing untreated sewage discharge.
Additionally, the company maintains that its policy is never to release wastewater (even after it has been treated and purified) from a ship when docked at port.
"Clean, treated water is discharged beyond three nautical miles, again going beyond requirements," the company tells New Times.
