A single large cruise ship carrying around 3,000 passengers and crew produces roughly 795 cubic meters of sewage and 3,785 cubic meters of gray water (from sinks, showers, and laundry) during a one-week voyage. That’s over a million gallons of wastewater per ship per week, and much of it ends up in the ocean after treatment. Add in oily bilge water, solid waste, food scraps, and hazardous chemicals, and the total footprint of the global cruise fleet is enormous.
How Much Wastewater One Ship Produces
The biggest waste stream by volume is gray water, the runoff from sinks, showers, dishwashers, and laundry facilities. One case study of a cruise ship’s weekly output measured about 2,358 cubic meters of combined gray water and treated sewage, along with 84 cubic meters of oily waste and 266 cubic meters of solid waste. A separate analysis estimated that a 3,000-person vessel generates nearly 3,785 cubic meters of gray water alone in a week, which works out to roughly a million gallons.
Sewage is the second-largest liquid stream, with that same 3,000-person ship producing about 795 cubic meters (around 210,000 gallons) per week. To put it in perspective, that’s comparable to the weekly sewage output of a small town, concentrated in a single floating structure that moves from port to port.
What Actually Gets Dumped Overboard
International rules set by the International Maritime Organization allow ships to discharge treated sewage directly into the ocean as long as the onboard treatment plant is approved and operational. Sewage that has been ground up and disinfected can be released as close as three nautical miles from shore. Untreated sewage can only be discharged beyond 12 nautical miles, and only while the ship is moving at a minimum speed so the waste disperses. In designated “Special Areas,” such as the Baltic Sea, discharge standards are stricter and generally require advanced treatment before any release.
Gray water has historically faced fewer restrictions than sewage, though the U.S. EPA finalized national standards in 2024 that set specific limits for cruise ship gray water discharges. Those standards cap fecal bacteria at 20 colony-forming units per 100 milliliters on a 30-day average and limit suspended solids to 30 milligrams per liter. These numbers are meant to bring gray water quality close to what municipal treatment plants produce on land. Outside U.S. waters, gray water regulations are less uniform, and in many parts of the world, ships can release it with minimal treatment.
Bilge water, the oily liquid that collects in the lowest compartments of the engine room, must pass through an oil-water separator before discharge. The international standard requires the oil content to be below 15 parts per million. Ships of 400 gross tons and above are required to have monitoring equipment that automatically stops the discharge if oil levels exceed that threshold.
Food Waste and Solid Garbage
Cruise ships generate large quantities of food waste, and the rules allow much of it to go overboard. Food scraps that have been ground to particles smaller than 25 millimeters can be discharged as close as three nautical miles from shore. Unground food waste requires a 12-nautical-mile buffer. Near fixed platforms like oil rigs, food discharge is essentially prohibited.
Non-food solid waste, including plastics, is not supposed to enter the ocean at all. Ships are required to offload it at port facilities. One case study measured 266 cubic meters of solid waste generated in a single week aboard one vessel, roughly eight tons by another estimate. That includes packaging, glass, metal, and other refuse that accumulates from feeding and entertaining thousands of people around the clock.
Hazardous and Chemical Waste
Beyond the high-volume streams, cruise ships also produce smaller quantities of genuinely hazardous material. Dry cleaning operations generate spent solvents. Onboard photo labs (less common now but still present on some ships) produce chemical fixer and silver flake. Engine maintenance creates used oils and lubricants. Incinerators that burn solid waste leave behind ash that may contain heavy metals.
These materials are supposed to be stored in locked rooms onboard and offloaded at licensed hazardous waste facilities in port. The system works when companies follow the rules, but enforcement gaps exist. In 2024, the EPA penalized Royal Caribbean $473,685 after an inspection at the Port of Galveston, Texas, revealed that the company had been offloading waste from eight vessels without proper notifications, accurate reporting, or compliant handling of incinerator ash. The violations spanned from July 2019 to July 2024, covering five years of inadequate waste tracking.
What Ends Up in the Water
Even legally discharged wastewater carries pollutants. The EPA has identified nutrients, bacteria (including Salmonella and E. coli), viruses, oil and grease, and metals in cruise ship discharge streams. Gray water in particular can contain pathogens like hepatitis and harmful single-celled organisms, which is why the tightened 2024 standards specifically target fecal contamination levels.
The nutrient load is a quieter problem. Nitrogen and phosphorus from thousands of toilet flushes, kitchen drains, and laundry cycles act as fertilizer when they hit the ocean. In concentrated doses near coastlines, coral reefs, or enclosed waterways, these nutrients can trigger algal blooms that deplete oxygen and suffocate marine life. A single ship’s discharge disperses in the open ocean, but popular cruise routes concentrate dozens of ships along the same corridors, compounding the effect in places like the Caribbean and the Mediterranean.
How the Rules Fall Short
The regulatory framework has two basic weaknesses. First, the rules change depending on where the ship is. In open ocean beyond 12 nautical miles, a cruise ship can release untreated sewage. In U.S. waters, the 2024 EPA standards impose strict bacteria and solids limits. In many other coastal nations, enforcement is minimal or delegated to the ship’s flag state, which may have little incentive to crack down.
Second, compliance relies heavily on self-monitoring. Ships record their own discharge data, and inspections happen infrequently. The Royal Caribbean case illustrates how violations can persist for years before being caught. The company wasn’t dumping waste illegally at sea; it was mishandling waste at port, a step in the process that happens on dry land where regulators theoretically have easier access. If port-side handling slips through the cracks for five years, the oversight of what happens hundreds of miles offshore is inevitably thinner.
The 15 parts-per-million oil standard for bilge water, while a significant improvement over earlier decades, still permits trace petroleum to enter the water continuously from thousands of vessels worldwide. The IMO itself acknowledges that “a greater effort to impose compliance must be carried out” for these standards to achieve their intended effect.
Scale of the Global Fleet
Individual ship numbers become staggering when multiplied across the industry. The global cruise fleet includes over 300 ocean-going ships, many carrying 5,000 to 7,000 passengers plus crew. If a mid-size ship produces over a million gallons of gray water and sewage in a week, the fleet collectively generates billions of gallons per year. The cruise industry has grown rapidly over the past two decades, with newer and larger ships entering service regularly, each one adding to the cumulative discharge volume in popular destination waters.