Oceans cover vast areas of our planet, and there is increasing emphasis on protecting these marine environments from not just land-based pollution, but also pollution released from ships.1 Although cruise ships represent a small fraction of international shipping (see Figure 1a), they play a somewhat unique role in terms of waste generation, given that they are essentially floating cities that often operate in pristine coastal waters and sensitive marine ecosystems while offering an array of amenities in line with luxury resort hotels.2 In spite of a major decline in business during the COVID-19 pandemic, as shown in Figure 1b, the cruise industry continues to rapidly expand, and the sheer volume of waste cruise ships can produce, illustrated in Figure 1c, draws attention to their possible environmental impact.2 For example, a one-week voyage by a moderately sized cruise ship can generate the equivalent of six large swimming pools of black water, 30 swimming pools of gray water, and garbage amounting to the weight of a school bus.3,4

Figure 1. a) Global shipping lane data for all ships between 2015 and 2021.5,6 b) Trends in the annual number of ocean cruise ship passengers (data from Statista7). c) Annual estimated waste production for ocean cruise ships, calculated using data from Statista7 and the U.S. Department of Transportation.4

Wastes generated on ships are categorized as sewage, oily waste, and solid waste under the International Convention for the Prevention of Pollution from Ships, or MARPOL, adopted in 1973 by the International Maritime Organization (Figure 2). Sewage is defined as black water, whereas gray water is not included in the MARPOL standards and can be directly discharged. MARPOL Annex IV is closest to a set of universal objectives for wastewater treatment and discharge from ships, although many nations and regions also have standards in place.8

For example, relevant U.S. federal environmental statutes include the Clean Water Act, Act to Prevent Pollution from Ships, Resource Conservation and Recovery Act, Ports and Waterways Safety Act, and Certain Alaskan Cruise Ship Operations.2 Trade associations, such as the Cruise Lines International Association (CLIA), sometimes adopt voluntary environmental standards as well.2

Ship sewage can have characteristics that differ from municipal sewage generated on land. For example, salinity can be an issue when saline water is used to flush toilets; however, cruise ship sewage systems generally use fresh water to reduce corrosion risks.2 Other constituents may occur in higher concentrations in ship wastewater, given the emphasis on water conservation.

Figure 2. Classes of pollution defined by the MARPOL Convention annexes. The inset illustrates sources of black water (regulated as sewage under MARPOL Annex IV, which has been updated via subsequent resolutions, e.g., MEPC.227(64) in 2012, and gray water (not regulated by MARPOL Annex IV). Some images are from Biorender.

For example, in a 2004 survey of cruise ships operating in Alaska, vacuum-flushed toilets were found to use approximately four times less flush water compared to land-based toilets.2 These water-saving measures can lead to two to three times more concentrated sewage on ships than the land-based equivalent.9 The concentrated contaminants and small footprint available for onboard treatment pose challenges for wastewater treatment.10

Section 312 of the U.S. Environmental Protection Agency’s Clean Water Act requires that vessels with toilets be equipped with a Coast Guard-certified operable marine sanitation device (MSD) to operate in U.S. waters, defined as within three nautical miles from shore. There are three types of MSDs, of which Type II is most common for cruise ships. Type II MSDs, considered equivalent to MARPOL Annex IV effluent standards, are flow-through treatment devices that typically feature aerobic biological treatment, clarification and filtration to separate solids, and chlorine disinfection. In some designs, maceration-chlorination is used.

Type III MSDs are simply holding tanks that retain sewage until it can either be disposed of at a port reception facility or at sea beyond three nautical miles from shore. In the 2008 Cruise Ship Discharge Assessment Report, the Environmental Protection Agency (EPA) noted that no cruise ships were known to exclusively use Type III MSDs, and Type I MSDs are exclusively for smaller vessels.2

Figure 3. Comparison of cruise ship water-quality characteristics for untreated gray water and black water treated using either Type II MSD or advanced wastewater treatment (AWT) systems (data from the 2000 Alaska Cruise Ship Initiative, as summarized by the EPA (2008)).2 The original Type II MSD standards included only lower reduction targets for fecal coliform and TSS, but the revisions from MEPC.227(64),12 adopted in 2012, are shown here with the Alaskan water standards enacted in 2000, shown in the chart as Title XIV.

There is no ongoing sampling plan under MARPOL or U.S. standards, so data sets for operational treatment efficacy are largely lacking. One of the most comprehensive data sets is from the 2000 Alaska Cruise Ship Initiative, which found that the majority of fecal coliform and total suspended solids (TSS) samples did not meet Type II MSD standards (see Figure 3).2 Moreover, concentrations of fecal coliform, TSS, and five-day biochemical oxygen demand (BOD5) in gray water samples often exceeded those in black water.11 Typical Type II MSDs may mix gray water with black water but more often provide some limited treatment en route to holding tanks, where gray water may be immediately discharged or sent to longer-term storage for controlled discharge.2 Inspections of the surveyed cruise ships with the poorest effluent quality demonstrated that five of the six were operating or maintaining the MSDs improperly.2

Given the challenges associated with traditional Type II MSDs, cruise ships are increasingly implementing more-effective advanced wastewater treatment systems (AWTs). AWTs offer improved treatment (often for both black and gray water), and typically feature screening, biological treatment, solids separation using filtration or flotation, and UV disinfection to avoid excess chlorine residual.2 The filtration stage can include advanced filter configurations, e.g., membrane bioreactors (MBRs), ultrafiltration, or reverse osmosis (RO).2

MBR technologies have been effectively employed in ships,13 and most AWTs remove pathogens, oxygen-demanding wastes, suspended solids, grease, and particulate metals quite well, with moderate nutrient removal.2 The treatment efficacy of emerging contaminants, e.g., microplastics, pharmaceuticals, and personal care products, is less well established.1,14 Some approaches considered or implemented for improved AWT performance include ammonia removal by biological nitrification, ion exchange for nitrate or metal removal, phosphorus removal by chemical precipitation, and metals removal by RO.2

Figure 4. Percent of Cruise Lines International Association (CLIA) reporting fleet capacity, based on number of lower berths, with AWT systems on board capable of meeting or exceeding MARPOL Annex IV discharge levels. The subfraction signified with diagonal lines is the fraction of the fleet capable of meeting or exceeding the MARPOL Annex IV discharge levels for the Baltic Sea Particularly Sensitive Sea Area. Data reported by CLIA (2023).17

Implementation of AWTs is highly relevant in particularly sensitive marine environments, such as in Alaska and the Baltic Sea.2,15 In 2000, the U.S. Congress enacted Title XIV into law, which includes more robust discharge standards for black and gray water from many cruise ships operating in Alaskan waters, including targets for fecal coliforms, TSS, BOD5, residual chlorine, and pH (see Figure 3).2

Additionally, in 2005, the Baltic Sea was designated as a Particularly Sensitive Sea Area with special protection. This designation introduced more stringent discharge regulations, including one milligram per liter (mg/L) of phosphorus and 20 mg/L nitrogen.15 As of 2020, one study reported that there were 52 commercially available AWTs that could meet these sensitive area requirements.16 Figure 4 shows increasing adoption of AWTs capable of meeting MARPOL and Baltic Sea standards.

The trend toward AWT implementation on cruise ships will continue in the coming years, making the design, testing, and implementation of AWTs relevant to ship constraints crucial to the burgeoning cruise industry.


  1. Westhof, Lena, Stephan Köster, Margrit Reich. “Occurrence of Micropollutants in the Wastewater Streams of Cruise Ships,” Emerging Contaminants 2, no. 4 (December 2016): 178-184. https://doi.org/10.1016/j.emcon.2016.10.001
  2. U.S. Environmental Protection Agency. Cruise Ship Discharge Assessment Report, 2008.
  3. Klein, Ross. “Greening the Cruise Industry,” Journal of Ocean Technology 5, no. 1 (2010): 7-15.
  4. “Table 1 Summary of Cruise Ship Waste Streams,” Bureau of Transportation Statistics, U.S. Department of Transportation, updated May 20, 2017. https://www.bts.dot.gov/bts/bts/archive/publications/maritime_trade_and_transportation/2002/environmental_issues_table_01
  5. Symington, Adam. “Mapping Shipping Lanes: Maritime Traffic Around the World,” Visual Capitalist, accessed January 20, 2024. https://www.visualcapitalist.com/cp/mapping-shipping-lanes-maritime-traffic-around-the-world/
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  9. Huhta, Hanna-Kaisa, Jorma Rytkönen, Jukka Sassi. Estimated Nutrient Load from Wastewaters Originating from Ships in the Baltic Sea Area, VTT Technical Research Centre of Finland, Otakaari, 2007.
  10. Chen, Qing, Wanqing Wu, Yafei Guo, Jingtai Li, Fang Wei. “Environmental Impact, Treatment Technology and Monitoring System of Ship Domestic Sewage: A Review,” Science of the Total Environment 811 (March 2022): 151410. https://doi.org/10.1016/j.scitotenv.2021.151410
  11. Eley, W.D., C.H. Morehouse. “Evaluation of New Technology for Shipboard Wastewater Treatment,” Ocean. Conf. Rec. 2003, 2, 748-753. https://doi.org/10.1109/oceans.2003.178405
  12. 2012 Guidelines on Implementation of Effluent Standards and Performance Tests for Sewage Treatment Plants, Marine Environmental Protection Committee. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MEPCDocuments/MEPC.227(64).pdf.
  13. Zhu, Linan, Hailing He, Chunli Wang. “COD Removal Efficiency and Mechanism of HMBR in High Volumetric Loading for Ship Domestic Sewage Treatment,” Water Science & Technology 74, no. 7 (October 2016), 1509-1517. https://doi.org/10.2166/wst.2016.271
  14. Peng, Guyu, Baile Xu, Daoji Li. “Gray Water from Ships: A Significant Sea-Based Source of Microplastics?” Environmental Science & Technology 56, no. 1 (January 2022), 4-7. https://doi.org/10.1021/acs.est.1c05446
  15. Vaneeckhaute, Céline, Ali Fazli. “Management of Ship-Generated Food Waste and Sewage on the Baltic Sea: A Review,” Waste Management 102 (February 2020): 12-20. https://doi.org/10.1016/j.wasman.2019.10.030
  16. “On-board sewage treatment,” Baltic Marine Environment Protection Commission–HELCOM, accessed January 21, 2024. http://www.helcom.fi/action-areas/shipping/sewage-from-passenger-ships/on-board-sewage-treatment/
  17. “CLIA Global Oceangoing Cruise Lines – August 2023,” Environmental Technologies and Practices, Cruise Lines International Association (CLIA), accessed January 20, 2024. https://cruising.org/en/Sustainability-Data

About the author

Dr. Brooke K. Mayer is an associate professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds bachelors, masters and doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.


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