Sewer water is the used water that flows from homes, businesses, and sometimes streets into underground pipes leading to a treatment facility. It’s roughly 99.9% water by weight, with the remaining fraction made up of organic waste, chemicals, microorganisms, and suspended solids. Despite that tiny percentage of contaminants, untreated sewer water is one of the most dangerous substances in daily infrastructure, carrying bacteria, viruses, and parasites that cause serious illness.
What’s Actually in Sewer Water
The solid portion of sewer water, small as it is, contains a complex mix of organic and inorganic material. The organic side includes human waste, food scraps, fats, oils, soaps, synthetic detergents, proteins, and carbohydrates. These break down over time but create the odor, color, and biological activity that make raw sewage so hazardous.
The inorganic side includes sodium salts, chlorine compounds, nitrates, sulfates, bicarbonates, and phosphorus. Municipal sewage also contains trace amounts of heavy metals like lead, mercury, cadmium, arsenic, copper, chromium, and zinc. These come from household products, industrial discharge, and corroding pipes. In small concentrations they pass through largely unnoticed, but they accumulate in the environment if sewage isn’t properly treated.
A growing concern is pharmaceuticals and personal care products. Researchers have catalogued at least 190 of these compounds in water systems worldwide, spanning antibiotics, hormones, pain relievers, antiseizure medications, and blood lipid regulators. Ibuprofen, the synthetic hormone estradiol, and the antiepileptic drug carbamazepine are among the most frequently detected. Hormones and pain relievers pose the greatest ecological risk to aquatic life, even at low concentrations. These chemicals enter sewer water every time someone takes medication (the body excretes a portion unchanged), washes off topical products, or flushes unused pills.
Combined vs. Separate Sewer Systems
Not all sewer systems work the same way. In a separate sewer system, two distinct pipes run underground. One carries sanitary sewage from toilets, sinks, and drains to a treatment plant. The other carries stormwater (rainwater runoff from streets and rooftops) directly to local streams and rivers, typically without treatment. This keeps the two flows isolated from each other.
In a combined sewer system, a single pipe handles both sanitary sewage and stormwater. During dry weather, everything flows to the treatment plant as intended. During heavy rain, the volume can overwhelm the system’s capacity, forcing a mix of raw sewage and stormwater to overflow directly into rivers, lakes, or coastal waters. These overflow events are one of the main reasons untreated sewer water reaches the environment in developed countries.
Pathogens and Health Risks
Raw sewer water carries dozens of disease-causing organisms. The major categories are bacteria, viruses, and parasites, and the health effects range from mild stomach upset to fatal infections.
- Bacteria: Salmonella causes diarrhea, fever, and abdominal cramps and can spread to the bloodstream if untreated. E. coli (particularly the O157:H7 strain) can trigger bloody diarrhea and, in severe cases, kidney failure. Shigella causes bloody diarrhea and fever, with severe infections in young children sometimes leading to seizures. Leptospira, often associated with floodwaters contaminated by sewage, causes high fever, muscle aches, and jaundice, and untreated cases can progress to kidney damage, liver failure, or death.
- Viruses: Hepatitis A causes jaundice, fatigue, and nausea, with about 15% of infected people experiencing symptoms that drag on for six to nine months. Norovirus and rotavirus cause intense vomiting and diarrhea. Poliovirus, though now rare due to vaccination, historically spread through sewage-contaminated water.
- Parasites: Giardia and Cryptosporidium are the most common waterborne parasites in sewage, both causing prolonged diarrhea and stomach cramps. They form protective cysts that resist many standard disinfection methods, making them particularly stubborn to eliminate.
Contact with sewer water doesn’t require swallowing it to cause harm. Skin wounds exposed to raw sewage can become infected, and aerosolized droplets near overflows or broken pipes can carry pathogens into the respiratory system. Mosquitoes breeding in pooled sewage also transmit diseases like West Nile virus.
How Sewer Water Gets Treated
Treatment plants clean sewer water in stages, each targeting a different category of contaminant.
Primary treatment is physical separation. Sewage sits in large basins where heavy solids settle to the bottom as sludge and lighter materials like oil and grease float to the surface. Both layers are skimmed off, and the remaining liquid moves on. This step removes roughly 5% to 10% of the settleable suspended solids.
Secondary treatment is biological. Aerobic bacteria, the same kinds that decompose organic material in nature, are introduced to consume sugars, fats, proteins, and other organic matter dissolved in the water. Some plants grow these bacteria on fixed filters that the water passes through. Others mix the bacteria directly into the sewage in aerated tanks, keeping oxygen levels high to fuel decomposition. U.S. federal regulations require that secondary treatment remove at least 85% of both organic matter (measured as biochemical oxygen demand) and suspended solids, bringing each down to no more than 30 milligrams per liter in the treated water.
Tertiary treatment, sometimes called effluent polishing, goes further. Sand filtration removes remaining fine particles. Specialized bacteria can strip out excess nitrogen and phosphorus, nutrients that fuel harmful algal blooms when they reach lakes and coastal waters. Some facilities use lagoons where native plants, algae, and tiny organisms naturally filter the water over days or weeks. Disinfection with chlorine or ultraviolet light kills remaining pathogens before the treated water is released.
How Much Goes Untreated
Globally, the picture is sobering. As of 2022, 42% of household wastewater worldwide was not safely treated before being discharged. That translates to an estimated 113 billion cubic meters of inadequately treated or completely untreated sewage flowing into rivers, lakes, and oceans every year. The gap is largest in low-income countries where treatment infrastructure is limited or nonexistent, but combined sewer overflows and aging pipes mean that even wealthy nations release raw sewage during storms or system failures.
Fats, Grease, and Fatbergs
One of the biggest operational problems in sewer systems is fats, oils, and grease, collectively known as FOG. When cooking oil is poured down a kitchen drain, it doesn’t stay liquid for long. Used cooking oil contains elevated levels of fatty acids (particularly palmitic acid) created during frying, which make it stickier and more viscous than fresh oil. Once in the sewer, metal ions from food waste, detergents, and the pipes themselves trigger chemical reactions called saponification, essentially turning the grease into a soap-like solid that clings to pipe walls.
In concrete sewers, calcium ions leaching from the pipe material accelerate this buildup. Over time, layers of solidified grease, wet wipes, and other debris merge into massive blockages known as fatbergs. Some fatbergs discovered in major cities have weighed over 100 tons. These blockages restrict flow, cause sewage backups into homes and streets, and force costly emergency repairs.
Recovering Resources From Sewage
Sewer water isn’t purely waste. It contains nitrogen and phosphorus, two nutrients essential for agriculture, and treatment plants are increasingly designed to recover them. One approach converts these nutrients into struvite, a slow-release fertilizer pellet, though this works best with high-concentration waste streams like those from livestock operations rather than typical municipal sewage.
For lower-concentration flows, adsorption technologies use materials like zeolite, biochar, and volcanic tuff to capture nitrogen, while metal oxides on porous supports pull out phosphorus. Researchers have also fabricated ceramsite pellets from sewage sludge and fly ash that absorb both nutrients from treated wastewater and then serve directly as slow-release fertilizer for plants. These approaches turn a disposal problem into a resource loop, reducing both the environmental impact of sewage discharge and the need for synthetic fertilizers mined or manufactured from scratch.