What Is Raw Sewage? Its Makeup and Hazards

Raw sewage is untreated wastewater flowing directly from homes, businesses, and sometimes industrial facilities before it reaches a treatment plant. It is mostly water by volume, but the remaining fraction carries a concentrated mixture of human feces, urine, food waste, household chemicals, soap residues, and whatever else people flush or pour down drains. That mixture makes raw sewage one of the most reliably hazardous substances in the built environment, hosting disease-causing microbes, heavy metals, pharmaceutical residues, and gases that can injure or kill in enclosed spaces.

What Raw Sewage Actually Contains

Domestic sewage originates from toilets, sinks, showers, and laundry. When you add in contributions from restaurants, offices, and small commercial operations, researchers typically call the combined flow “municipal” or “urban” wastewater. In cities with combined sewer systems, stormwater runoff also enters the same pipes, diluting but also expanding the volume of raw sewage that needs handling.

The solid and dissolved material in raw sewage falls into a few broad categories. Organic matter is the biggest contributor by weight after water itself. This includes fecal material, food scraps, paper fibers, and biological oils. Researchers measure organic load using proxies like biochemical oxygen demand (BOD) and chemical oxygen demand (COD), both of which indicate how much oxygen microorganisms would consume while breaking the waste down. A study of domestic sewage in Basrah, Iraq, classified the wastewater as “strong” based on its high organic loading, and also found elevated dissolved solids, chloride levels, and oil-and-grease concentrations, the last of which was attributed to misuse of the sewer system for disposing of cooking fats.¹

Beyond the organic fraction, raw sewage carries inorganic material: grit, sand, metals, and dissolved salts. Heavy metals like zinc, copper, and lead enter sewage from industrial discharges, corroding pipes, and household products. Wastewater treatment plants that receive both domestic and industrial inflows have measured zinc at concentrations up to several hundred milligrams per liter in liquid samples.² These metals persist in the environment and accumulate in sediments and living organisms downstream.

The Biological Threat

The most immediate danger of raw sewage is what is alive in it. Human feces carry an enormous variety of bacteria, viruses, and parasites, many of which cause serious illness. When researchers have surveyed urban rivers receiving domestic sewage, they consistently find enteric pathogens including E. coli, Enterococcus, Campylobacter jejuni, and Arcobacter species at high concentrations, with several of these pathogens present at levels exceeding tens of thousands of gene copies per hundred milliliters of water.³ Those pathogens correlate strongly with markers of human fecal contamination, confirming that domestic sewage is the source.

Parasites round out the picture. Cryptosporidium and Giardia are two of the most important waterborne parasites globally, and both are routinely found in raw sewage. Surveys of sewage treatment works in Norway detected Cryptosporidium in about 80% of plants and Giardia in over 90%, sometimes at concentrations above 20,000 parasites per liter.⁴ A molecular surveillance study found Cryptosporidium in over half and Giardia in more than 80% of wastewater samples tested, with human-pathogenic species dominating.⁵ These organisms form protective cysts or oocysts that resist chlorine disinfection better than most bacteria, which is part of why they have caused so many large waterborne disease outbreaks over the decades.⁶

Viruses are harder to culture and count, but they are arguably the most concerning pathogen class in sewage. Norovirus, rotavirus, hepatitis A, and adenoviruses all shed in enormous quantities in the stool of infected people. Many of these microorganisms attach themselves to solid particles in the wastewater, which can shield them from disinfection even after treatment.⁷ The COVID-19 pandemic drew attention to the fact that SARS-CoV-2 RNA can be detected in raw sewage, though the virus’s genetic material decays faster at warmer temperatures, with decay rates roughly tripling between 4°C and 20°C in dormitory sewage samples.⁸

Antibiotic Resistance in Sewage

Raw sewage is not just a vehicle for known infections. It is also a breeding ground for drug-resistant bacteria. When antibiotics pass through human bodies, residual concentrations enter the sewer along with bacteria that have already evolved resistance. In that warm, nutrient-rich soup, resistance genes can transfer between bacterial species on mobile genetic elements called plasmids. Untreated sewage consistently tests positive for a wide range of antibiotic resistance genes, with genes conferring resistance to sulfonamides and macrolides among the most abundant.⁹

Hospital sewage amplifies the problem. A study of hospital raw sewage in southeastern Brazil confirmed that it acts as a major disseminator of antibiotic-resistant bacteria and their associated genes.¹⁰ In West and Central Africa, where raw sewage is sometimes used directly for urban agriculture, researchers identified 136 resistance genes commonly associated with multi-drug-resistant plasmids, alongside dozens of virulence factor genes belonging to known pathogens.¹¹ The more densely populated the area, the more diverse the resistance genes found.

Leaking sewer pipes create a less obvious pathway. Even in cities with functional infrastructure, aging pipes crack underground and contaminate the surrounding soil and groundwater. Research has recovered fecal coliforms, enterococci, and pseudomonads from groundwater wells near leaking sewers, and many of those isolates showed resistance to multiple antibiotics.¹² This is a route by which resistant bacteria can enter drinking water supplies without anyone noticing a sewage spill on the surface.

Chemical Contaminants You Might Not Expect

People flush or wash far more than biological waste. Pharmaceuticals are a growing concern. Drugs that pass through the body largely intact, or that people discard down the toilet, show up reliably in sewage and the waterways it feeds. Surface water sampling in New York City waterways detected the antibiotic sulfamethoxazole, the blood-pressure drug atenolol, and the painkiller ibuprofen at concentrations close to one microgram per liter each.¹³ Those concentrations sound small, but many aquatic organisms are sensitive to chronic low-level exposure, and pharmaceuticals can combine in unpredictable ways.

Per- and polyfluoroalkyl substances, commonly known as PFAS or “forever chemicals,” are another class of contaminant that treatment plants struggle to remove. A study in England’s Langstone Harbour, a marine protected area, found that PFAS concentrations spiked measurably after nearby combined sewer overflows discharged untreated wastewater. Before the discharge, only one PFAS compound was detectable; afterward, eight were, including the now-banned PFOS and PFOA.¹⁴ Combined sewer overflows in England alone discharged an estimated 3.6 million monitored hours of untreated wastewater into rivers and coasts in 2023.

Storm events intensify the release of these emerging contaminants. Pharmaceuticals, illicit drugs, and personal care products all enter the environment through untreated stormwater-sewage mixtures, and the frequency and intensity of these overflow events have been increasing in some areas due to urbanization and climate change.¹⁵

Toxic Gases and Physical Dangers

Raw sewage does not just threaten people through waterborne contact. In enclosed or underground spaces, the gases it produces can be lethal. Decomposing organic matter generates hydrogen sulfide, which smells like rotten eggs at low concentrations but deadens the sense of smell at higher ones, meaning you can stop noticing it right as it becomes dangerous. Methane, another byproduct of anaerobic decomposition, is both explosive and an asphyxiant in confined spaces. A review of occupational health hazards among sewage and sanitary workers documented exposure to hydrogen sulfide and methane alongside cardiovascular problems, musculoskeletal disorders, respiratory issues, altered lung function, and infections including hepatitis and leptospirosis.¹⁶

A systematic review and meta-analysis of health outcomes among sanitation workers found consistent evidence of increased risk for gastroenteritis and respiratory conditions. For hepatitis A, the pooled odds ratio across a dozen studies was about 2.1, meaning sanitation workers were roughly twice as likely to have been infected as comparable workers in other occupations.¹⁷ The review also flagged possible increases in musculoskeletal disorders and mental health conditions, though the evidence for those was less definitive.

How Raw Sewage Reaches Open Water

In an ideal system, all sewage flows to a treatment plant before anything touches the environment. Reality is messier. Much of Europe and older parts of North America still rely on combined sewer systems, where a single network of pipes carries both domestic sewage and stormwater runoff. During heavy rain, the volume overwhelms the system, and built-in safety valves called combined sewer overflows (CSOs) release the excess directly into rivers or coastal waters with no treatment at all.¹⁸

When a CSO fires, it does more than just add pathogens to a river. The organic matter in the discharge consumes dissolved oxygen as bacteria break it down, suffocating fish and invertebrates. Modeling of CSO impacts on a large urban river showed that roughly 97% of observed drops in dissolved oxygen could be explained by three processes: the direct mixing of oxygen-poor overflow water, reduced algal photosynthesis because of overflow-induced cloudiness, and microbial digestion of the organic load, with that last factor being the most significant overall.¹⁹

What Happens When People Swim in It

The health consequences of direct exposure to sewage-contaminated water are not theoretical. A striking natural experiment took place at a triathlon in Denmark. In 2010, the open-water swimming leg coincided with peak post-flooding bacterial contamination, with average E. coli concentrations around 15,000 per 100 milliliters. Among 838 swimmers that year, 42% fell ill. The following year, when the same competition took place in clean water, only 8% of 931 swimmers reported illness, a fivefold difference in risk. Illness in 2010 was clearly linked to swallowing contaminated water, and the risk rose with the number of mouthfuls swallowed. Confirmed infections included Campylobacter, Giardia, and pathogenic E. coli.²⁰

Time matters, though. A risk assessment of swimming in surface water with aged sewage contamination found that norovirus contributed the majority of illness risk, and that after sewage had aged about three days in the environment, the median risk of gastrointestinal illness dropped below 30 per 1,000 swimmers.²¹ That is not zero risk, but it illustrates why beach advisories typically focus on the first day or two after a known overflow event. Freshly discharged sewage is dramatically more dangerous than diluted, days-old contamination.

Environmental Damage Beyond Human Health

Aquatic life does not need to swallow contaminated water to be harmed by it. Fish exposed to untreated sewage at sub-lethal concentrations show reduced body weight and length, and develop visible structural damage to their gills, including ruptured and missing gill filaments and disintegrated tissue. Cell-level analysis reveals genotoxic effects as well, with significantly elevated rates of genetic damage in gill cells of exposed fish compared to controls.²² These findings matter because gill damage compromises a fish’s ability to extract oxygen from water and regulate its internal chemistry, which cascades through the entire organism’s health.

The oxygen depletion described earlier in the context of CSO discharges is equally devastating. When organic-rich sewage enters a waterway, bacterial decomposition can strip so much dissolved oxygen from the water that fish and invertebrates die in mass die-off events, sometimes visible as floating carcasses. Nutrient loading from the nitrogen and phosphorus in sewage also drives algal blooms, some of which produce toxins of their own, creating a secondary wave of ecological harm.

Fatbergs and Physical Blockages

Not all sewage hazards play out in the environment. Some play out inside the pipes themselves. When cooking oil, food particles, wet wipes, and other non-degradable items enter the sewer, fats and grease can combine with calcium in the wastewater through a chemical process called saponification, then harden into massive blockages known as fatbergs.²³ These deposits can grow to fill entire pipe sections. Wet wipes and facial tissues substantially increase fat accumulation because their porous fiber structures trap sewer solids, with research showing that wipes nearly doubled the concentration of grease deposits compared to conditions without them.²⁴

Fatbergs contribute to sanitary sewer overflows, the same events that send raw sewage into rivers and streets. They are expensive to remove, typically requiring manual excavation, and they recur in the same trouble spots when disposal habits do not change. The practical lesson is straightforward: cooking oil belongs in the trash, not the drain, and “flushable” wipes are flushable only in the sense that they fit through the toilet, not that the sewer system can handle them.

Sewage as a Surveillance Tool

For all its dangers, raw sewage has an unusual upside for public health. Because it captures biological waste from an entire community in one stream, testing sewage can reveal what diseases are circulating before individual patients show up at hospitals. This approach, called wastewater surveillance, gained global visibility during the COVID-19 pandemic, when monitoring for SARS-CoV-2 RNA in sewage detected variants of concern earlier than clinical testing in many areas.²⁵

That same logic extends well beyond respiratory viruses. Researchers have used sewage monitoring to track foodborne outbreaks of Salmonella, supplementing traditional case-by-case reporting with a community-level signal that picks up infections people never report to a doctor. Wastewater surveillance helps public health decision-makers design and time their responses more effectively, especially in communities where clinical testing is sparse or delayed.²⁶ The irony is real: the same raw sewage that spreads disease also serves as one of the most honest mirrors a community has of its collective health.