The interlinked ecosystems of the world’s waterways enable natural wonders like the migration of salmon from freshwater to saltwater. But that same interconnectedness also makes the ocean vulnerable to harmful inputs from rivers, such as sewage. When rivers are treated as an alternative to proper waste management, massive amounts of untreated human waste end up in the ocean—with disastrous effects.

In a comprehensive global assessment of coastal pollution from human wastewater, researchers at Columbia University’s Earth Institute discovered that just 25 locations are responsible for 46% of the nitrogen and pathogen contamination in oceans. Nitrogen from human sewage reaches 58% of the world’s coral reefs and 88% of seagrass beds. Top sources of sewage nitrogen include the Yangtze, Nile and Mississippi rivers.

While the risks to swimmers and consumers of contaminated water are quite straightforward, sewage has more wide-ranging, complicated effects on ocean ecosystems. It is a multiple stressor for marine habitats, introducing a toxic cocktail of contaminants including inorganic nutrients, pathogens, endocrine-disrupting compounds and heavy metals. Modern sewage carries much more than just human waste, often containing household cleaning agents, pharmaceutical residues and chemicals from personal care products.

The potential impacts of these contaminants are broader than we may think. “It’s not only the loss of the ecosystem but the functions they provide—this suite of different pollutants can affect the reproductive output of ecosystems and species that live in them,” says Joleah Lamb, principal investigator of the Health Oceans and People Lab and assistant professor at the University of California, Irvine. “Just the baseline loss of these ecosystems can even affect weather patterns. We don’t fully appreciate the large-scale impacts that sewage pollution could have.”

The presence of pathogens is especially alarming, as the same illnesses that infect humans can also harm ocean environments. In one case, a strain of human disease found in sewage transmitted white pox to a coral reef—which are particularly vulnerable habitats, as they are mostly located on the coastlines of developing countries, where sewage treatment infrastructure is often inadequate or nonexistent. But these countries can struggle to even identify the problem. “Sampling these pollutants requires a lot of money, expertise and infrastructure, from refrigeration to clean water to centrifuges,” says Professor Lamb. “For many nations, tackling this challenge has been overwhelming.”

In high-income countries, the UN estimates that 70% of municipal and industrial wastewater is treated; that number drops to 38% in upper-middle-income countries and 28% in lower-middle-income countries. In low-income countries, only 8% of wastewater is treated. On a global scale, this means that over 80% of all wastewater is discharged with no treatment. Inevitably, a significant proportion of this water ends up in the ocean via drainage channels, rivers and lakes.

Each day 14 billion litres of untreated wastewater is created in developing countries, with little data indicating where it might end up. Its impacts on water quality can be seen in the rates of child mortality attributable to diarrhoea, which are highest in the poorest communities in countries such as Afghanistan, India and the Democratic Republic of Congo. In countries facing droughts and water scarcity, sewer systems can become inoperable. In water-scarce Zimbabwe, the mismanagement of government-operated water treatment systems put millions of people at risk of cholera in one incident in 2013, and the country again faced drought conditions in 2016, 2019 and 2024. Often, communities lack centralised water management systems and families are left to manage their own household sewage—in the Caribbean, only 25% of the population uses facilities connected to sewer networks.

Climate change will further exacerbate the sewage problem for vulnerable countries. More frequent flooding due to extreme weather will increase overflowing sanitation systems, particularly in low-lying areas like the Pacific Island nations, contaminating municipal water supplies and coastal regions with sewage. Comprehensive water quality monitoring in the Pacific Island region is rare, but nearly every Pacific nation has identified critical problems caused by human sewage, including algal blooms and eutrophication, contaminated well water, and outbreaks of cholera.

Even in high-income countries, outdated infrastructure causes problems for properly treating sewage. New York City is one of many major cities that still relies on a combined sewer overflow system from the 1800s, which combines raw sewage with stormwater from the streets. When it rains in the city, that water flows into waterways and then to 460 locations along the coast, each of which discharges millions of gallons of sewage into the ocean. As climate change increases the frequency of extreme rainfall events, flooding in the city is expected to become as much as 24% more intense, increasing the frequency of sewage overflow.

Accelerating urbanisation is compounding the problem. The ‘urban creep’ phenomenon refers to the progressive loss of green spaces within cities, resulting in more rainwater in the streets. “The more we build impermeable surfaces in cities, the more water will run off,” says David Butler, co-director of the Centre for Water Systems and professor of water engineering at the University of Exeter. “Surfaces that were once permeable, absorbing water rather than allowing it to enter the sewer system, are disappearing, replaced by paved driveways and artificial grass. On an individual basis, these seem irrelevant, but on a cumulative basis, it’s all adding up.” Every additional input of stormwater means more pressure on the combined sewers.

Most older sewage systems are built on this combined model, so countries with historic infrastructure struggle the most, particularly as climate change produces new, unpredictable weather patterns. “As the climate and rainfall regimes change, we’re seeing warmer, wetter winters and hotter, dryer summers,” says Professor Butler. “Our systems weren’t built with these extremes in mind, so they haven’t kept pace.”

Addressing outdated infrastructure is perhaps even more difficult than building a new system from the ground up. “The UK had some of the first sewage systems in Europe in Victorian times, whereas now countries that used to be behind us have built modern systems while we’ve got to deal with historic ones,” says David Johnson, technical director at the Rivers Trust, an environmental charity composed of over 60 local trusts working to protect and restore the UK’s rivers. In recent years, the UK has increasingly seen beaches and bathing areas closed due to the presence of raw sewage due to outdated infrastructure and poor water management practices by some water companies.

Combined sewers are not the only legacy system causing problems. Another example is seen in Hawaii, which still relies on cesspools, holes in the ground that discharge municipal raw sewage into the groundwater with no treatment. While new cesspools are banned across the US, Hawaii’s existing 83,000 cesspools still inject 52 million gallons of untreated sewage into the ground on a daily basis. In one town, ocean bacteria levels exceeded state health standards in the areas outside 81% of homes. Systems of the past were built without a long-term lens or regard for future consequences. “We can ask whether we would have built the systems this way if we knew it would get to this point,” says Professor Butler. “But we are at this point now, and the question is, what can we do?”

We know the solutions for the various issues affecting sewer systems—and success requires mixing different approaches. “The water industry can spend a lot of money on building more underground storage for overflows and send the water to the treatment works instead of discharging it, but that will never fully eliminate the problem, because there will always be that one bigger storm that you haven’t designed for,” says Professor Butler. “The best way is to keep stormwater runoff out of the system entirely through sustainable drainage systems.” One promising concept has been China’s “sponge cities”, which use infrastructure like permeable asphalt, new canals and ponds, and restored wetlands to absorb water. A similar idea has been deployed in Montreal, Canada—another city facing the consequences of legacy combined sewers—where sponge parks and sidewalks are being built to naturally absorb excess rainfall.

Another way to improve existing infrastructure is to integrate technological controls like digital twins, which can anticipate how sewer systems will respond to extreme weather conditions. “A key area for the water companies is maintaining their systems, a lot of which are legacy, in order to have faster responses to failures,” notes Professor Butler. Many water companies already have electronic software representations of their systems, but the predictive abilities of digital twins could be hugely beneficial. “We’re talking about a pre-emptive approach—monitoring sewer water levels and investigating quickly if they deviate from expected patterns, before things get out of hand,” says Professor Butler. One wastewater treatment plant in Germany has used machine learning to model incoming loads of carbon, nitrogen and phosphorus, optimising the overall treatment process.

On top of infrastructure modernisation, there are a variety of end-of-pipeline strategies for cleaning water, according to Mr Johnson at the Rivers Trust. Two of these options are UV disinfection and wetlands treatment. A UV disinfection plant exposes wastewater to UV light, which renders microorganisms unable to reproduce, neutralising bacteria, viruses and parasites. This treatment enables disinfection without harmful chemicals, and results in clean water that can be used for irrigation, industrial processes and water reuse programmes. UV technology is currently expensive and energy-intensive, however, so it is best deployed in specific areas like bathing sites. Wetlands that naturally filter water are another option—they require large areas of land but have a far lower carbon footprint. “One of the things I always try to think about is using as little energy as possible for water treatment,” says Mr Johnson. “Not only do wetlands provide treatment, but they potentially provide recreational benefits and sequester carbon. They take less energy overall.”

This type of nature-based solution is a hugely promising approach. A 2017 study in Indonesia found that seagrass ecosystems in areas with no wastewater treatment reduced human pathogens in the water column by 50%, with the potential for a twofold reduction in disease levels. Seagrass meadows are declining at a rate of 7% per year; preserving them not only protects the grasses and the species that rely on them, but also provides natural disinfection of the surrounding waters. “Seagrasses produce chemicals that prevent fouling so that they can stay upright in the water—they inhibit any land-based pathogens that haven’t evolved alongside them,” explains Professor Lamb. “They can also pulse oxygen, and oxygen pulsation is one of the mechanisms used in wastewater treatment facilities. Mangroves and wetlands also have these properties.”

Addressing sewage pollution requires solutions as diverse as the communities they serve. Nature-based approaches, such as sponge parks and seagrass restoration, offer cost-effective and environmentally sustainable options suited to lower-income regions. These can also be complemented by cutting-edge technology like digital twins to optimise system management. Although the practice of combining stormwater and sewage is part of the problem, an approach thoughtfully combining multiple strategies will be the solution.