Sewage is 99.9% water. It’s the remaining 0.1% that makes it dangerous, valuable, and fascinating. That tiny fraction contains everything a community flushes, washes, or drains away: human waste, household chemicals, medications, microplastics, heavy metals, disease-causing organisms, and even trace amounts of gold.
The Basic Makeup of Raw Sewage
By weight, almost everything in a sewer pipe is plain water. The solid fraction, just one-tenth of one percent, is a mix of organic matter (human feces, food scraps, oils), inorganic compounds (salts, grit, metals), and microorganisms. Some of those microorganisms cause disease. Despite how small that non-water fraction sounds, a city of a million people produces enormous volumes of wastewater every day, so even 0.1% adds up to tons of material that treatment plants must handle.
Bacteria, Viruses, and Parasites
Raw sewage carries a wide range of pathogens shed by the people connected to the system. Some of the most concerning are viruses. Noroviruses, the leading cause of acute stomach illness worldwide, are responsible for roughly 685 million infections per year globally and account for nearly half of all gastroenteritis outbreaks. They’re highly contagious and survive well in water. Rotaviruses are similarly tough, resisting standard wastewater treatment and persisting in the environment for extended periods.
Beyond viruses, sewage contains bacteria like E. coli, Salmonella, and Campylobacter, along with parasites such as Giardia and Cryptosporidium. Infections from these organisms range from mild diarrhea to serious liver, respiratory, and nervous system disease. This is why contact with raw sewage, whether through a basement backup or a swim near a combined sewer overflow, is a genuine health risk.
Dangerous Gases in the Pipes
Sewer systems don’t just carry liquids. As organic material decomposes underground, it produces a cocktail of gases. The two most dangerous are hydrogen sulfide and methane.
Hydrogen sulfide is the source of the classic “rotten egg” smell. At low levels it’s merely unpleasant, but at high concentrations it actually paralyzes your sense of smell, removing the very warning sign that might save your life. At extremely high levels, it causes loss of consciousness and death. Methane, which is odorless, displaces oxygen in enclosed spaces. When oxygen drops below about 12%, a person can lose consciousness and die within minutes, with no warning.
Sewer gas also contains ammonia, carbon dioxide, sulfur dioxide, and nitrous oxides. Depending on what’s been dumped into the system, you might also find vapors from chlorine bleach, gasoline, or industrial solvents. This is why sewer workers follow strict confined-space protocols with gas monitors and ventilation equipment.
Medications and Personal Care Products
Every pill you swallow doesn’t get fully absorbed by your body. The active compounds, or their breakdown products, pass through your system and into the sewer. Wastewater contains residues of antibiotics, antidepressants, hormones from birth control, painkillers, and blood pressure medications. On top of that, anything rinsed down a sink contributes: shampoos, lotions, sunscreen, and cleaning products all add a layer of chemical pollutants including salts, endocrine-disrupting compounds, and active ingredients from cosmetics.
These substances are often present at very low concentrations, but treatment plants weren’t originally designed to remove them. Their long-term effects on ecosystems downstream, particularly on fish and aquatic organisms exposed to hormone-mimicking compounds, remain a growing concern.
Heavy Metals
Sewage carries measurable quantities of heavy metals, most commonly zinc and copper, followed by lead, chromium, nickel, cadmium, and mercury. These come from several sources: industrial wastewater, corrosion of the pipes themselves, stormwater runoff from roads, and surprisingly ordinary household products like cleaning agents and pharmaceuticals. Illegal discharges from businesses that bypass proper disposal also contribute. In heavily industrialized areas, metal concentrations in sewage sludge can be high enough to limit whether that sludge can safely be spread on farmland as fertilizer.
Precious Metals in the Sludge
Here’s a detail most people don’t expect: sewage sludge contains gold. Studies from treatment plants in Australia, Germany, Austria, England, Canada, and the United States found an average of about 1 milligram of gold per kilogram of dried sludge, with some samples reaching as high as 56 mg/kg. Silver concentrations are even higher, typically ranging from 1 to 100 mg/kg. Both metals appear at levels several hundred times greater than what you’d find in average soil. Platinum and palladium have also been detected.
The gold comes from a mix of sources: jewelry manufacturing, electronics, dental work, and even trace amounts in food and cosmetics. Whether extracting it is economically worthwhile is another question, but the amounts are large enough that researchers have seriously explored mining sewage sludge as a resource.
Microplastics
Every load of laundry you run sheds tiny synthetic fibers from polyester, nylon, and acrylic clothing. Those fibers wash straight into the sewer. Add in microbeads from exfoliating scrubs and fragments from degrading plastic packaging, and raw wastewater contains a steady stream of microplastics. One study measured approximately 4 particles per liter in untreated sewage. That number sounds small until you multiply it by the millions of liters flowing through a treatment plant each day. Treatment processes catch most of them, but many end up concentrated in the sludge, which may then be applied to agricultural land.
Things That Shouldn’t Be There
A significant portion of sewer problems comes from items people flush that they shouldn’t. “Flushable” wipes are the biggest offender. While toilet paper breaks down almost immediately in water, independent testing shows that wipes change very little even after an hour of continuous flushing action. They snag on pipe joints, combine with congealed cooking grease, and form massive blockages that utilities call “fatbergs.” Condoms, cotton swabs, dental floss, and feminine hygiene products all create similar problems. These items don’t dissolve. They accumulate, clog pumps, and cost cities millions in maintenance.
What Sewage Reveals About a Community
Scientists have turned sewer monitoring into a public health tool called wastewater-based epidemiology. By collecting 24-hour composite samples of untreated sewage at a treatment plant, researchers can detect and measure the metabolic residues of drugs that an entire community has consumed. One pilot study in the western United States tracked 18 different compounds in the wastewater of two communities over three months, including methamphetamine, opioids like oxycodone and morphine, cocaine metabolites, MDMA, ketamine, and prescription medications like tramadol and fluoxetine.
The method works because your body excretes a predictable fraction of any drug you take. By measuring the concentration of those metabolites at the treatment plant and accounting for the population served, researchers can estimate total community drug consumption per day. The same approach gained widespread visibility during the COVID-19 pandemic, when testing sewage for viral fragments became an early warning system for outbreaks, often detecting surges days before clinical testing data caught up. Polio, influenza, and RSV have all been tracked the same way.