Cruise ships dump most of their liquid waste directly into the ocean, at legally specified distances from shore. Sewage, greywater, food scraps, and oily engine water all go overboard once a ship is far enough out to sea. Hazardous materials like batteries, solvents, and expired medications are stored onboard and offloaded at port facilities. The rules vary depending on the type of waste, how far the ship is from land, and which country’s waters it’s sailing through.
Sewage Goes Overboard at Sea
Cruise ships generate enormous volumes of sewage (called blackwater) from toilets and medical facilities. This waste can contain bacteria, viruses, intestinal parasites, and harmful nutrients. Where it ends up depends on how well it’s been treated and how far the ship is from shore.
Under international rules set by the IMO, ships with an approved treatment plant can discharge treated sewage with relatively few restrictions. Sewage that has been ground up and disinfected, but not fully treated, can be released more than 3 nautical miles from land. Completely untreated, raw sewage can be discharged once a ship is more than 12 nautical miles from shore, as long as the vessel is moving at least 4 knots and releasing it at a controlled rate.
U.S. law follows a similar structure. Within 3 miles of shore, the Clean Water Act prohibits dumping untreated or inadequately treated sewage. Beyond 3 miles, raw sewage discharge is legal. Ships operating in U.S. waters must use marine sanitation devices that meet federal standards for bacteria and suspended solids before discharging closer to shore.
Greywater Has Even Fewer Restrictions
Greywater is everything that flows from sinks, showers, kitchen drains, laundry machines, and cleaning operations. A large cruise ship produces hundreds of thousands of gallons of greywater per day. Under federal law, greywater is not classified as a pollutant and is not considered sewage. There are no separate federal standards governing its discharge in most U.S. waters, meaning ships can legally release it almost anywhere.
The exception is the Great Lakes, where greywater falls under sewage regulations and must meet limits on bacteria and suspended solids. Cruise ships also face additional monitoring requirements under the EPA’s Vessel General Permit program, which sets limits on fecal coliform and residual chlorine in greywater discharges. Alaska has its own stricter rules based on U.S. Coast Guard standards. But in most open ocean waters, greywater simply goes overboard with minimal oversight.
Food Waste Is Ground Up and Dumped
Food waste follows its own set of distance rules. In designated “special areas” (ecologically sensitive zones like the Mediterranean, the Baltic Sea, and the North Sea), food scraps can only be discharged more than 12 nautical miles from land, after being run through a grinder or comminuter, and only while the ship is moving. The food waste also cannot be mixed with other types of garbage.
Outside special areas, the distance requirements are more lenient, but grinding is still generally required. In practice, cruise ships process food waste through industrial grinders before releasing it into the wake of the ship. Anything that can’t be ground up or discharged is stored and offloaded at port.
Oily Bilge Water Gets Filtered First
Every large ship accumulates oily water in its lowest compartments, called the bilge. This water picks up fuel, lubricants, and other petroleum products from the engine room. International rules under MARPOL Annex I set a strict limit: bilge water can only be discharged if it contains fewer than 15 parts per million of oil. Ships must use oily water separators and monitoring equipment to verify this before any discharge.
This 15 ppm standard has significantly reduced oil pollution from ships worldwide. However, compliance depends on the equipment being properly maintained and honestly operated, which has been a problem for some cruise lines.
Hazardous Waste Stays Onboard Until Port
Cruise ships generate a surprisingly varied stream of hazardous waste. Photo processing chemicals, dry cleaning solvents (a chlorinated compound called perchlorethylene), paint waste, aerosol cans, fluorescent light bulbs, mercury vapor bulbs, lead-acid and lithium batteries, and expired pharmaceuticals all qualify as hazardous materials.
None of this is supposed to go overboard. Industry standards require hazardous waste to be collected, stored in separate locked rooms onboard, and offloaded at licensed disposal facilities in port. In the United States, these facilities must comply with federal hazardous waste regulations. The volumes are relatively small compared to other waste streams. A fleet of 11 ships might generate around 45 pounds of medical waste and 75 pounds of battery waste per week per vessel. Hazardous waste typically accounts for less than 1% of a cruise ship’s total waste disposal.
Exhaust Scrubber Water Is a Growing Concern
Many cruise ships burn heavy fuel oil and use exhaust gas cleaning systems (scrubbers) to strip sulfur from their smokestack emissions. Open-loop scrubbers work by spraying seawater through the exhaust, which captures pollutants but produces acidic, contaminated washwater that gets pumped back into the ocean. This washwater can contain heavy metals, hydrocarbons, and other pollutants.
Pushback against this practice is accelerating. As of mid-2023, 93 regulatory measures in 45 countries restrict or ban scrubber washwater discharge. About 86% of those measures are outright bans rather than partial restrictions, and 64% specifically target open-loop scrubber discharges. Most bans apply in port areas and coastal zones, where the concentrated discharge poses the greatest risk to local water quality.
Enforcement Gaps and Violations
The rules look comprehensive on paper, but enforcement is a persistent challenge. Cruise ships spend much of their time in international waters, far from inspectors, and compliance relies heavily on self-reporting and onboard monitoring equipment.
The most prominent enforcement case involved Princess Cruise Lines, which paid $40 million after being caught illegally dumping oil-contaminated waste through a bypass pipe that circumvented required treatment equipment. While on five years of court-supervised probation, Princess and its parent company Carnival racked up six additional probation violations, including deliberately falsifying environmental training records on two ships and illegally discharging plastic into Bahamian waters. The company was ordered to pay another $20 million in criminal penalties.
Court-appointed monitors and independent auditors found “major non-conformities” across Carnival’s fleet during just the first two years of probation. The company even contacted the Coast Guard to try to redefine what counted as a major violation, after the government had already rejected the proposal. These cases suggest that the violations caught and publicized represent only a fraction of what happens at sea, where oversight is inherently limited by the vastness of the ocean and the difficulty of monitoring ships around the clock.