Cruise ships do dump some sewage in the ocean, but not the way most people imagine

Cruise ships produce roughly 30,000 gallons of sewage per day from toilets, sinks, showers, and galleys. Most of this sewage is treated on board through marine sanitation devices before discharge, but the treated water still goes overboard in international waters. In U.S. coastal waters, stricter rules explore — ships must either hold sewage in tanks until port or use approved treatment systems that meet federal standards. The reality is more regulated than it sounds, but also more complicated than "it all gets dumped."

Key Takeaways

  • Cruise ships treat sewage on board using marine sanitation devices, but treated wastewater is still discharged into the ocean in international waters.
  • U.S. federal law requires ships to either store sewage in holding tanks or use treatment systems that reduce solids and bacteria before discharge in coastal waters.
  • Ships must be at least 3 nautical miles from shore to discharge treated sewage in U.S. waters, and 12 nautical miles for graywater from sinks and showers.
  • The Environmental Protection Agency and Coast Guard monitor cruise ship compliance, and violations can result in fines and criminal charges.
  • Older ships are more likely to have outdated treatment systems, while newer vessels often use advanced technology that produces cleaner discharge.

How sewage treatment works on a cruise ship

A cruise ship's marine sanitation device (MSD) is essentially a small wastewater treatment plant. Sewage from all toilets, urinals, and some sink drains flows into holding tanks where solids settle to the bottom. The ship then treats this sludge using one of three methods: maceration (grinding solids into smaller pieces), biological treatment (using bacteria to break down waste), or chemical treatment. The treated water is then discharged overboard, while the remaining sludge is either incinerated on board or pumped out at port.

The quality of this treatment varies widely. Older ships often use basic maceration systems that straightforward grind solids smaller but do not significantly reduce bacteria or nutrients. Newer cruise ships increasingly use advanced biological treatment systems similar to municipal wastewater plants, which remove more contaminants and produce cleaner discharge. Some newer ships also use membrane bioreactors, which filter treated water through membranes to remove even more particles and microorganisms.

Graywater — the relatively cleaner water from sinks, showers, and laundry — is handled separately and usually requires less treatment before discharge. However, graywater still contains soap, food particles, and bacteria, so it is not straightforward dumped raw.

What U.S. law requires for sewage discharge

The Act to Prevent Pollution from Ships (APPS), enforced by the Environmental Protection Agency and U.S. Coast Guard, sets specific rules for cruise ships operating in U.S. waters. Ships must be at least 3 nautical miles from shore to discharge treated sewage, and at least 12 nautical miles from shore to discharge graywater. In some sensitive areas like the Great Lakes and certain coastal zones, discharge is prohibited entirely — ships must hold all sewage in tanks until they reach port.

Ships operating in international waters beyond U.S. jurisdiction follow MARPOL Annex IV, an international maritime treaty. These rules are less stringent than U.S. law and allow discharge of treated sewage once the ship is in international waters, typically beyond 12 nautical miles from any coast.

The Coast Guard inspects cruise ships regularly and can board vessels to check sewage treatment equipment, review discharge logs, and test water samples. Violations can result in fines ranging from thousands to millions of dollars, and criminal charges have been filed against cruise lines and individual officers for falsifying records or deliberately discharging untreated sewage.

Why cruise lines sometimes break the rules

Despite regulations, cruise lines have been caught illegally dumping sewage. In 2016, Royal Caribbean pleaded guilty to deliberately discharging oily waste and sewage in U.S. waters and paid a $40 million settlement. In 2019, Carnival Corporation admitted to similar violations spanning years and paid $20 million. These cases typically involved either deliberately bypassing treatment systems or falsifying discharge logs to hide illegal dumping.

The incentive to cheat comes from the cost of proper treatment and port pump-out services. Holding sewage in tanks until port requires large tank capacity and frequent stops at facilities equipped to receive it. Treating sewage on board requires maintaining expensive equipment and trained crew. Discharging untreated or partially treated sewage saves money and time, which is why violations tend to occur on older ships with outdated systems and cost-conscious operators.

However, the risk of detection has increased. The Coast Guard now uses satellite monitoring to track ships' locations and compare them against discharge logs. Environmental groups also monitor cruise ship movements and have reported violations to authorities. A single caught violation can cost a cruise line far more than years of proper compliance.

The environmental impact of treated sewage discharge

Treated sewage from cruise ships still contains nutrients like nitrogen and phosphorus, which can trigger algal blooms in coastal waters. It also contains bacteria, viruses, and pharmaceutical residues that treatment systems do not completely remove. The impact depends on the volume discharged, the treatment quality, and the receiving water's ability to dilute and break down the waste.

A single large cruise ship can carry 5,000 to 6,000 passengers and crew, producing as much sewage as a small town. When multiple ships discharge in the same area — common in ports like Miami, Galveston, and Caribbean destinations — the cumulative effect can be significant. Beaches near cruise ports have occasionally closed due to high bacteria levels, though the sewage source is not always confirmed to be from ships.

Ships operating in international waters beyond U.S. jurisdiction have fewer restrictions, so discharge standards vary by flag state (the country where the ship is registered). Ships registered in countries with weak maritime enforcement may operate with older treatment systems and less oversight than those registered in countries with stricter standards.

What happens to sewage at port

When a cruise ship docks, it can pump its holding tanks into port reception facilities — specialized pumping stations designed to receive and treat ship sewage. These facilities are required at major cruise ports, though availability and capacity vary. Some ports have modern treatment plants that can handle large volumes, while others have limited capacity and may charge high fees, which incentivizes ships to discharge at sea instead.

Sewage pumped at port goes through municipal wastewater treatment, which is more thorough than on-board treatment. However, not all ships use port facilities for all their sewage. Some ships discharge treated sewage at sea during the voyage and only pump solid sludge at port, which is cheaper and faster than waiting in line at a reception facility.

How to find out which cruise lines have better records

The Environmental Integrity Project and other advocacy groups maintain databases of cruise ship violations and environmental records. You can search by cruise line to see which companies have been cited for illegal dumping, which ships have newer treatment systems, and which lines have paid settlements. This information is public record from EPA and Coast Guard enforcement actions.

Cruise lines also publish environmental reports, though these vary in detail and transparency. Some newer ships are marketed with advanced wastewater systems as a selling point, which suggests the cruise line sees environmental performance as competitive. Asking a cruise line directly about a specific ship's treatment system and discharge practices can also yield useful information, though answers may be vague.

If environmental impact is a concern for you, choosing newer ships and cruise lines with fewer violations is one way to reduce your participation in the problem. However, the most effective change comes from regulatory enforcement and port facility investment, not individual consumer choices.

Frequently Asked Questions

Is cruise ship sewage treated before it goes in the ocean?

Yes, in most cases. Ships use marine sanitation devices to treat sewage on board, reducing solids and bacteria before discharge. However, treated sewage is not the same as clean water — it still contains nutrients and microorganisms. The quality of treatment depends on the ship's equipment and age.

Can you get sick from swimming near a cruise ship?

Possibly, though the risk depends on how close you are and how recently the ship discharged. Treated sewage contains bacteria and viruses that can cause gastrointestinal illness. Beaches near busy cruise ports have occasionally closed due to high bacteria levels, though the source is not always confirmed to be from ships.

What is the difference between sewage and graywater?

Sewage comes from toilets and urinals and contains fecal matter and urine. Graywater comes from sinks, showers, and laundry and is relatively cleaner but still contains soap, food particles, and bacteria. Ships discharge both, but graywater can be discharged closer to shore than sewage.

Do cruise ships have to pump out sewage at port?

Not always. Ships can discharge treated sewage at sea in international waters, so they may only pump solid sludge at port. However, in U.S. coastal waters and sensitive areas, ships must either hold sewage in tanks or use approved treatment systems. Some ports also require ships to use reception facilities.

How do I report illegal dumping by a cruise ship?

You can report suspected violations to the U.S. Coast Guard's National Response Center at 1-800-424-8802, or to the EPA's environmental violations hotline. Include the ship's name, location, date, and time if possible. Environmental groups like the Environmental Integrity Project also accept reports and investigate cruise ship violations.