Sewage entering the ocean is a multifaceted environmental challenge, impacting marine ecosystems and human well-being. While treated wastewater should ideally integrate safely into natural water bodies, various factors lead to the discharge of inadequately treated or raw sewage into coastal waters worldwide. This issue stems from complex urban infrastructure, population growth, and insufficient wastewater management.
Pathways for Sewage Entry
Sewage can find its way into the ocean through several distinct pathways, often linked to infrastructure limitations or malfunctions. One common route is through Combined Sewer Overflows (CSOs), prevalent in older urban areas where stormwater and sanitary sewage share the same pipes. During heavy rainfall or snowmelt, these systems can be overwhelmed, leading to the direct discharge of untreated or partially treated sewage mixed with stormwater into nearby waterways, eventually reaching the ocean. The U.S. Environmental Protection Agency (EPA) identified approximately 770 communities in the U.S. that have combined sewer systems.
In contrast, Sanitary Sewer Overflows (SSOs) occur in systems where sewage and stormwater are typically separated. SSOs result from blockages, such as accumulated fats, oils, and grease, or tree root intrusion, as well as pipe breaks, equipment malfunctions, or power failures. These overflows can release raw sewage from manholes, cleanouts, or even back up into homes, subsequently flowing into stormwater systems and then into surface waters. Approximately half of SSOs in the United States are caused by blockages.
Beyond system overflows, direct illegal discharges or improper dumping from land-based sources can also contribute. This includes illicit connections from homes or businesses that bypass treatment systems, or the improper disposal of waste. Malfunctioning wastewater treatment plants present another significant pathway; if treatment processes fail or are overwhelmed, inadequately treated or raw sewage may be discharged directly into rivers or coastal waters. For instance, a major malfunction in Warsaw, Poland, in 2019 led to over 4.8 million cubic meters of untreated wastewater being discharged into the Vistula River, ultimately affecting the Baltic Sea.
Finally, in many developing regions, a lack of adequate wastewater collection and treatment infrastructure means that a substantial portion, estimated globally at 80%, of sewage is discharged into the environment untreated.
Consequences of Ocean Sewage Pollution
The discharge of sewage into the ocean leads to a range of environmental and human health consequences. A primary impact is eutrophication, where excess nutrients like nitrogen and phosphorus from sewage stimulate the rapid growth of algae. This overgrowth, known as an algal bloom, can block sunlight from reaching underwater plants, and when the algae decompose, they consume large amounts of dissolved oxygen, creating hypoxic or anoxic “dead zones” where most marine life cannot survive.
Sewage also carries a variety of pathogens, including bacteria like E. coli, viruses such as Norovirus and Hepatitis A, and parasites. Exposure to these contaminants through recreational activities like swimming or surfing, or by consuming contaminated seafood, particularly shellfish that filter water, can lead to gastrointestinal illnesses, skin infections, and other health issues.
The impact on marine life extends beyond oxygen depletion and disease. Corals are particularly vulnerable, suffering from bleaching, disease, and reduced reproductive success due to sewage pollution. Wastewater can introduce disease-causing microorganisms to corals, with some outbreaks directly linked to human gut pathogens. Fish populations can experience stress, lower reproduction rates, and declines due to degraded water quality and habitat destruction. Mangroves and seagrass meadows, vital coastal habitats, can also be degraded by the influx of contaminants.
Furthermore, sewage can introduce emerging contaminants like pharmaceuticals and microplastics into marine environments. Wastewater treatment plants can remove about 90% of microplastic particles, but the remaining amount is still significant. These substances, originating from household products, medications, and synthetic textiles, can persist in the environment and accumulate in marine organisms, raising concerns about their long-term ecological effects and potential entry into the human food chain.
Efforts to Prevent Ocean Sewage Discharge
Significant efforts are underway globally to mitigate sewage discharge into marine environments, focusing on improving infrastructure and regulatory oversight. Advanced wastewater treatment plants play a central role, employing primary, secondary, and tertiary treatment processes to remove pollutants before discharge. Primary treatment removes solids, secondary treatment uses biological processes to break down organic matter, and tertiary treatment further removes nutrients like nitrogen and phosphorus, and can include disinfection.
Upgrading and expanding existing sewer systems is another important measure. This includes repairing aging pipes to prevent leaks and blockages, and separating combined sewer systems to manage stormwater and sewage independently. Many communities are investing in large-scale infrastructure projects to increase system capacity and reduce overflows during heavy rains. For instance, the EPA has noted a $23 billion backlog in needed sewer improvements in the Great Lakes states alone.
Regulatory frameworks and their enforcement are also key in preventing pollution. National laws like the Clean Water Act in the U.S. establish discharge limits for pollutants and require permits for wastewater discharges into navigable waters. International conventions, such as MARPOL Annex IV, regulate sewage discharge from ships, prohibiting it within certain distances from land unless treated by approved systems. These regulations aim to control the quality of discharged effluent and hold polluters accountable.
Public awareness campaigns and citizen action also contribute to prevention. Educating individuals on proper waste disposal, such as avoiding flushing non-biodegradable items or pouring fats, oils, and greases down drains, can reduce blockages and overflows. Reporting illegal dumping or unusual discharges empowers communities to act as stewards of their local waterways. Finally, green infrastructure solutions, such as permeable pavements, rain gardens, and constructed wetlands, help manage stormwater runoff naturally, reducing the volume of water entering sewer systems and decreasing the likelihood of overflows. These nature-based approaches complement traditional engineering solutions, offering a comprehensive strategy for protecting coastal waters.