Modern cruise ships are often described as “floating cities”—and like any city, they generate vast quantities of wastewater that must be managed responsibly. A typical cruise ship carrying 3,000 passengers and crew generates approximately 176,400 gallons of sewage per week. Across the entire cruise industry, this adds up to over one billion gallons of sewage annually—the equivalent of 1,515 Olympic-sized swimming pools.
Managing this immense volume of waste at sea presents unique engineering challenges. Unlike land-based municipalities with unlimited space and gravity-fed sewer systems, cruise ships must process human waste in compact, self-contained environments while navigating complex international regulations and protecting sensitive marine ecosystems.
This comprehensive guide explores everything you need to know about cruise ship sewage systems—from the fundamental distinction between blackwater and greywater, through the intricate multi-stage treatment processes, to the latest advanced technologies and the regulatory framework governing discharge at sea.
Understanding Cruise Ship Wastewater Streams
Blackwater vs. Greywater: The Critical Distinction
On a cruise ship, wastewater is categorized into two primary streams, each with different characteristics and treatment requirements:
Blackwater refers to sewage from toilets—anything flushed down a toilet, including human waste and toilet paper. Blackwater contains high concentrations of organic matter, pathogens, and nutrients that can be harmful to marine environments if discharged untreated.
Greywater encompasses everything else that goes down drains: water from sinks, showers, bathtubs, laundry facilities, and galleys (kitchens). Greywater contains organic residues, oils, fats, and surfactants from cleaning products like dish soap and laundry detergent. While generally less contaminated than blackwater, greywater can still impair marine life by disrupting oxygen exchange and inhibiting plankton photosynthesis.
Many modern cruise ships treat blackwater and greywater together in integrated systems. This co-treatment approach is becoming increasingly common as Advanced Wastewater Treatment Systems (AWTS) are designed to handle combined waste streams.
Additional Wastewater Streams
Beyond blackwater and greywater, cruise ships must also manage:
Bilge water—oily water that accumulates in the lowest parts of the ship’s hull from engine room leaks, condensation, and machinery operations. Bilge water produces visible slicks that can cause biological damage to seabirds and marine life.
Ballast water—used for vessel stabilization, which can introduce invasive species and pathogens into new environments.
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The Regulatory Framework
MARPOL Annex IV: The Global Standard
The primary international regulation governing sewage discharge from ships is MARPOL Annex IV (the International Convention for the Prevention of Pollution from Ships). This regulation applies to ships engaged in international voyages of 400 gross tons or more, and to ships certified to carry more than 15 persons. Under MARPOL Annex IV, untreated sewage discharge is prohibited within 12 nautical miles from the nearest land. The regulation establishes three scenarios in which sewage discharge is permitted:
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Beyond 12 nautical miles from land, the ship may discharge untreated sewage
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Between 3 and 12 nautical miles, the ship may discharge sewage that has been comminuted (macerated) and disinfected using an approved system
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Any distance from land, if the ship operates an approved sewage treatment plant that produces effluent not causing visible floating solids or discoloration of surrounding water
All ships must carry an International Sewage Pollution Prevention (ISPP) Certificate confirming compliance.
Special Areas: Stricter Standards for Sensitive Waters
MARPOL Annex IV designates certain marine areas as “Special Areas” where even more stringent discharge requirements apply. The Baltic Sea was designated the first Special Area under Annex IV in 2011. For passenger ships operating in the Baltic Sea Special Area:
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Discharge of untreated sewage is completely prohibited regardless of distance from shore
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Ships must be equipped with certified advanced sewage treatment systems
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Effluent must meet nutrient reduction targets: nitrogen ≤ 20 mg/L (or 70% reduction) and phosphorus ≤ 1 mg/L (or 80% reduction)
These requirements took effect for new passenger ships on June 1, 2019, and for existing passenger ships on June 1, 2021.
The Alaska waters similarly enforce some of the strictest standards globally. Cruise ships operating in Alaskan waters must use advanced treatment systems that produce effluent quality exceeding that of many shore-based municipal facilities.
Regional and National Regulations
Individual countries and regions may impose additional restrictions. For example:
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Canada prohibits discharge of greywater and sewage within 3 nautical miles from shore anywhere in Canadian waters
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Washington State prohibits blackwater and greywater discharge unless treated with AWTS and outside designated No Discharge Zones
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Alaska requires many vessels to hold wastewater onboard for discharge outside state waters
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Evolution of Onboard Sewage Treatment Systems
Marine Sanitation Devices (MSD): The First Generation
Historically, cruise ships used Marine Sanitation Devices (MSD) for sewage treatment. These systems typically employed biological treatment combined with chlorination or maceration and chlorination.
While MSDs represented an important first step in onboard sewage treatment, they had significant limitations:
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They were not designed to remove nutrients (nitrogen and phosphorus)
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They primarily focused on disinfection rather than comprehensive pollutant removal
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Effluent quality was inferior to modern standards
Advanced Wastewater Treatment Systems (AWTS): The Modern Standard
The cruise industry has increasingly adopted Advanced Wastewater Treatment Systems (AWTS), which treat sewage far more effectively than older MSD technology. According to a 2021 industry survey, 75% of cruise ships are now equipped with AWTS.
AWTS represent a significant technological leap, incorporating multiple treatment stages that can remove more than 99% of solids, bacteria, and other contaminants. These systems are designed to meet the stringent requirements of Special Areas like the Baltic Sea and Alaska.
How a Modern Cruise Ship Sewage System Works
The advanced sewage treatment process on a modern cruise ship typically involves five to seven distinct stages, transforming raw sewage into water clean enough to meet—and often exceed—drinking water quality standards.
Stage 1: Collection and Pre-Filtration
Wastewater from toilets (blackwater) and from sinks, showers, and galleys (greywater) flows through collection pipes to the treatment system. The first step is pre-filtration, where a pre-filter removes larger debris and solid particles from the wastewater stream. This protects downstream equipment from clogging and damage.
Stage 2: Equalization and Mixing
The wastewater flows into mixing tanks where flow and composition are equalized. This step ensures consistent conditions for the biological treatment that follows. On many ships, blackwater and greywater are combined at this stage for integrated treatment.
Stage 3: Biological Treatment in the Bioreactor
The heart of the treatment system is the bioreactor, where aerobic bacteria break down organic matter. In this oxygen-rich environment, microorganisms consume organic pollutants and convert them into energy, carbon dioxide, and water.
The bioreactor typically uses one of several technologies:
Membrane Bioreactor (MBR) : The most common AWTS type onboard cruise ships. MBR combines an activated sludge process with membrane filtration. The MBR process involves pre-treatment filtering, biological oxidation through activated sludge, and ultrafiltration through membranes.
Moving Bed BioReactor (MBBR) : Uses a bioreactor filled with plastic beads that support bacterial growth, combined with a Dissolved Air Flotation (DAF) unit.
Sequence Batch Reactor (SBR) : Treats wastewater in batches rather than continuously, allowing for precise control of treatment cycles. Studies have shown SBR with bio-media can achieve approximately 74% total nitrogen reduction and 75% total phosphorus reduction.
Membrane Sequence Batch Reactor (MSBR) : Combines SBR and MBR technologies. Research indicates MSBR systems can achieve 99% BOD removal, 98% COD removal, and 99% suspended solids removal—exceeding IMO regulatory requirements.
Stage 4: Flocculation and Coagulation
After biological treatment, the water enters a flocculation unit where solids are removed through a coagulation process. Chemicals such as coagulants and polymers are added to cause particles to clump together into larger, heavier masses.
Stage 5: Flotation and Solids Separation
The water then undergoes flotation in a specialized basin. In Dissolved Air Flotation (DAF) systems, tiny air bubbles are injected into the water stream, causing particles to float to the surface. The floating solids are continuously removed as sludge.
Stage 6: Polishing Filtration
Before final disinfection, the wastewater passes through a fine polishing filter that removes any remaining residual impurities. This step ensures that only clean water proceeds to disinfection.
Stage 7: UV Disinfection
The final treatment stage is ultraviolet (UV) disinfection. UV light kills any remaining pathogens, bacteria, and germs. This disinfection method is preferred over chlorination because it does not introduce chemical byproducts into the discharge water.
Stage 8: Storage and Discharge or Reuse
Clean, disinfected wastewater—called permeate—is either:
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Stored in clean water tanks before discharge
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Discharged directly into the sea
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Reused for other ship functions such as laundry or as technical water
The permeate produced by modern AWTS is so clean that it often meets drinking water quality standards. However, concerns remain about trace contaminants not removed by standard treatment processes (discussed in Part 6).
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Sludge Management and Waste Processing
The treatment process generates sludge—the solid waste separated from wastewater. Modern cruise ships employ sophisticated systems to manage this sludge.
Sludge Processing Steps
The sludge processing typically involves:
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Dewatering : Removing water from liquid sludge using decanter devices
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Homogenization : Creating a uniform mixture for consistent processing
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Thermal hydrolysis : Breaking down complex organic compounds
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Drying : Completely drying the precipitate
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Incineration : Burning the dried sludge in an incinerator
The Scanship thermal treatment system (STTP) represents a low-energy innovation for sludge processing. More advanced systems are now using microwave-assisted pyrolysis (MAP) technology, which turns waste into useful energy. This approach was first deployed on Royal Caribbean’s Icon of the Seas.
Integrated Clean Ship Solutions
Leading suppliers like Scanship now offer fully integrated clean ship solutions that process wastewater, food waste, and other biogenic waste together. All residue sludge, along with food waste and other biogenic waste from hotel operations, undergoes processing steps including dewatering, homogenization, thermal hydrolysis, drying, and pyrolysis. The end products are climate-neutral energy and carbon for capture and storage.
In emergencies, sludge can also be pumped out from the ship for shoreside disposal.
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Environmental Considerations and Challenges
The Scale of Wastewater Generation
The volume of wastewater produced by the cruise industry is staggering. Greywater discharged by cruise ships in 2023 was estimated at 14 billion liters, representing a 40% increase from 2014 to 2023, driven largely by the growing number of cruise ships.
What Remains in Treated Effluent
Even with advanced treatment, concerns persist about contaminants that survive the treatment process. A 2025 study analyzing treated wastewater from three expedition cruise ships found over 160 chemical compounds, including:
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Pharmaceuticals : antibiotics, cardiovascular medicines, NSAIDs, and antidepressants
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Personal care products
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Industrial chemicals
The presence of antibiotics in wastewater raises particular concern about spread of antibiotic resistance and potential harm to marine life. These compounds are not easily removed by wastewater treatment systems and are released into the surrounding ocean, where they can be transported even further.
Nutrient Pollution and Dead Zones
Nutrient discharges from sewage—particularly nitrogen and phosphorus—can trigger harmful algal blooms that deplete oxygen levels in coastal waters, creating “dead zones” where marine life cannot survive.
This is precisely why Special Area regulations in the Baltic Sea mandate nutrient reduction targets of 70% for nitrogen and 80% for phosphorus.
Microplastics
Cruise ships also contribute to microplastic pollution. The sources of microplastics in cruise ship wastewater include cleaning and maintenance operations, medical wastewater, personal care products, textile fibers from laundry, and plastic waste and litter. While membrane ultrafiltration (a key component of MBR systems) is associated with very high microplastic removal rates, comprehensive documentation of system effectiveness remains limited.
The Dilution Fallacy
The old assumption that the ocean can dilute all pollutants is no longer safe to rely on. As one researcher noted: “Maritime traffic is increasing at an alarming rate. Cruise ship tourism is the fastest growing industry in the tourism sector, and ships are getting bigger, with some accommodating over 6,000 passengers. Despite this, we still know very little about the types and quantities of chemicals released through wastewater discharge”.
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Cutting-Edge Technologies and Industry Leaders
Membrane Bioreactor (MBR) Systems
Most AWTS onboard cruise ships are of the Membrane Bioreactor (MBR) type. Leading suppliers include:
Hamworthy : In Alaskan waters in 2019, 14 out of 24 ships operated Hamworthy MBR systems.
Scanship (Vow ASA) : Five vessels operated Scanship MBR systems in Alaskan waters in 2019. Scanship has secured multiple contracts worth tens of millions of euros for cruise newbuilds. Their systems purify wastewater to meet the stringent Baltic Sea and Alaska requirements.
Evac : Offers MBR systems where all waste streams are treated in a single process, incorporating equalization, pre-treatment by screens, an aerated biotank, and a membrane bioreactor.
Wärtsilä : Their MBR technology produces compliant effluent samples to the highest standards.
RWO’s CleanSewage Membrane Reactor (CS-MBR)
RWO’s CS-MBR system represents one of the most advanced AWTS solutions available. Key features include:
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Removes more than 99% of solids and bacteria, including microplastics and viruses
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Uses submerged membranes in the final stage
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Conforms with IMO standards for nitrogen and phosphorus removal
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Enables vessels to meet regulations for operating in Special Areas
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Produces water pure enough to be reused for laundry or as technical water
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Features automated cleaning-in-place control for easier operation and extended membrane life
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Has a smaller footprint and uses less energy than comparable systems
Celebrity Cruises has been a pioneer in adopting CS-MBR technology, upgrading their Solstice-class vessels and placing additional orders for fleet-wide upgrades. As Celebrity’s senior ship manager noted: “The new CS-MBR goes beyond regulatory compliance to minimise environmental impacts and maximise operational efficiency”.
Emerging Technologies
Microwave-Assisted Pyrolysis (MAP) : Scanship has developed MAP technology that turns waste into useful energy. This technology was first deployed on Royal Caribbean’s Icon of the Seas.
Scanship Thermal Treatment System (STTP) : A low-energy innovation for sludge processing.
Low-energy waste treatment technologies : The industry continues to develop more energy-efficient treatment solutions.
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The Passenger Experience
What Happens When You Flush?
Many passengers wonder what happens after they flush the toilet on a cruise ship. The process is far more sophisticated than most imagine.
When you flush:
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Wastewater enters the ship’s collection system
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It’s combined with greywater from sinks and showers
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The combined stream flows to the treatment plant
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Bacteria break down organic matter in the bioreactor
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Solids are removed through filtration and flotation
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UV light disinfects the water
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Clean water is discharged or reused
The treated water is often cleaner than many municipal water supplies.
The Environmental Officer
Every cruise ship has an environmental officer responsible for environmental training and compliance with environmental laws, regulations, industry standards, and company policies. These officers oversee the wastewater treatment system and ensure proper operation.
Part 9: Challenges and Criticisms
Lack of Public Reporting
Despite the availability of advanced treatment technology, transparency remains an issue. In a Friends of the Earth scorecard, all 21 major cruise lines scored low on sewage treatment because none of them “publicly report on the performance of those advanced systems”.
Persistent Contaminants
As noted earlier, even treated wastewater can contain pharmaceuticals, personal care products, and industrial chemicals that pass through treatment systems. The long-term effects of these contaminants on marine ecosystems remain poorly understood.
Polar Regions
As polar regions become more accessible and polar tourism rises, cruise ship traffic—and wastewater discharges—increase in some of Earth’s most fragile ecosystems. Researchers have called for:
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Stronger wastewater treatment and discharge regulations in polar regions
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Targeted monitoring of persistent contaminants
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A reevaluation of current maritime wastewater treatment standards
Voluntary vs. Regulatory Standards
While some cruise lines voluntarily exceed regulatory requirements, critics argue that industry-wide standards should be strengthened. The Alaska Department of Environmental Conservation’s annual reporting on cruise ship treatment systems in Alaskan waters provides one of the few comprehensive overviews of systems in use.
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The Future of Cruise Ship Sewage Systems
Stricter Regulations
Regulatory trends point toward increasingly stringent requirements. The Baltic Sea Special Area requirements represent a model that may be extended to other sensitive waters. As one industry observer noted, the Baltic Sea and Alaska standards are the most stringent globally.
Circular Economy Approaches
The future lies in circular waste processing—transforming waste from a disposal problem into a resource. Technologies like microwave-assisted pyrolysis that convert waste into energy represent this paradigm shift.
Zero Discharge
Some ships and regions are moving toward zero discharge goals, where all wastewater is treated and either reused onboard or held for shoreside disposal. As Wärtsilä’s MBR technology producer noted, AWTS are “just the start of the cruise industry’s circular waste processing ambitions”.
Enhanced Monitoring
There are growing calls for better monitoring of treatment system performance, including:
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Real-time effluent quality monitoring
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Third-party verification
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Public reporting of treatment outcomes
Conclusion
The sewage systems on modern cruise ships represent one of the most remarkable—and least appreciated—engineering achievements of the maritime industry. From the humble toilet flush to the discharge of water clean enough to drink, the journey of wastewater through a cruise ship involves sophisticated biological, chemical, and physical treatment processes that rival the best land-based municipal facilities.
The cruise industry has made significant strides in wastewater treatment technology, with 75% of ships now equipped with Advanced Wastewater Treatment Systems that remove more than 99% of solids and bacteria. Systems like RWO’s CS-MBR and Scanship’s integrated solutions demonstrate that the technology exists to treat sewage to standards that exceed many shoreside facilities.
However, significant challenges remain. The presence of pharmaceuticals, personal care products, and industrial chemicals in treated wastewater, the lack of public reporting on system performance, and the increasing scale of cruise ship operations all demand continued attention and improvement.
The regulatory framework provided by MARPOL Annex IV—and the stricter Special Area requirements in places like the Baltic Sea and Alaska—has driven much of the technological progress. As these regulations continue to evolve and expand to new regions, the cruise industry will need to innovate further.
For passengers, understanding what happens after they flush provides a new appreciation for the complexity of life at sea—and the responsibility that comes with visiting some of the world’s most beautiful and fragile marine environments. The next time you flush on a cruise ship, you can be confident that your waste is being processed by some of the most advanced wastewater treatment technology available anywhere in the world.
References
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International Maritime Organization, MARPOL Annex IV – Regulations for the Prevention of Pollution by Sewage from Ships
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HELCOM, On-Board Sewage Treatment – Baltic Sea Special Area Requirements
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Koboević, Ž. & Krmek, I., Advanced Sewage Water Treatment Systems on Cruise Vessels
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Frontiers in Marine Science, Sources and Leakages of Microplastics in Cruise Ship Wastewater (2022)
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RECETOX/NIVA Study, Pharmaceuticals and Other Compounds in Cruise Ship Wastewater (2025)
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U.S. Environmental Protection Agency, Cruise Ship Wastewater Discharge Estimates
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Friends of the Earth, Cruise Ship Report Card
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Alaska Department of Environmental Conservation, Large Commercial Passenger Vessel Wastewater Regulations