Sewage is the wastewater produced by homes, businesses, and industries, carried away through a network of underground pipes called a sewer system. It includes everything that goes down your drains and toilets: water from showers, sinks, washing machines, dishwashers, and human waste. Despite its reputation, sewage is overwhelmingly just water. The solid material in it, both organic and inorganic, typically makes up less than 0.1% of its total volume.
What’s Actually in Sewage
A medium-strength sample of untreated domestic sewage contains roughly 710 milligrams of total solids per liter. To put that in perspective, a liter of sewage is about 99.93% water. The remaining fraction is a mix of dissolved and suspended material. About 500 mg/L consists of dissolved solids (salts, sugars, proteins, and other compounds too small to filter out), while around 210 mg/L is suspended solids (tiny particles that float or drift through the water).
The organic portion of those solids, meaning carbon-containing material from food scraps, human waste, soaps, and other biological matter, accounts for roughly half the load. The inorganic portion includes sand, grit, minerals, and salts that wash in from pipes, roads, and household products. Sewage also contains nitrogen and phosphorus from urine and detergents, two nutrients that cause serious environmental problems when they reach rivers, lakes, or coastal waters in large quantities.
Pathogens That Make Raw Sewage Dangerous
The real hazard in sewage isn’t the visible grime. It’s the microorganisms. Raw sewage carries bacteria, parasites, and viruses shed by every person connected to the sewer system. Viruses are considered potentially the most dangerous pathogens in wastewater because they’re difficult to remove and can remain infectious at very low concentrations.
Noroviruses and rotaviruses, both common in sewage, are highly contagious and spread through what’s called the fecal-oral route, meaning contaminated water or food reaches someone’s mouth. Rotavirus alone was responsible for over 500,000 deaths worldwide in a single year, mostly among young children in developing countries. Hepatitis A and hepatitis E viruses also survive well in sewage. Hepatitis E causes more than 20 million infections and over 40,000 deaths annually. Human adenoviruses, another frequent presence in wastewater, can trigger pneumonia, respiratory infections, and gastroenteritis.
These pathogens can cause anything from mild stomach upset to severe liver inflammation, meningitis, and respiratory disease. That’s why untreated sewage entering drinking water supplies or recreational waters is a serious public health emergency, not just an unpleasant nuisance.
Types of Sewage
Not all sewage is the same. Domestic sewage (also called sanitary sewage) comes from households: toilets, baths, kitchen sinks, and laundry. Industrial sewage comes from factories, food processing plants, and manufacturing facilities, and it often contains chemicals, heavy metals, or other pollutants not found in household waste. In many cities, a third category exists: stormwater runoff from rain that flows into the same pipes as domestic sewage. These combined sewer systems can overwhelm treatment plants during heavy rainfall, sending a mixture of rainwater and raw sewage directly into rivers or the ocean.
How Sewage Gets Treated
Sewage treatment happens in stages, each one removing a different category of contaminant.
Primary treatment is physical separation. Sewage flows into a large settling basin where heavy solids sink to the bottom as sludge and lighter materials like oils and grease float to the surface. Both layers are skimmed off, and the remaining liquid moves on. This step alone removes a significant portion of suspended solids but does little about dissolved pollutants or pathogens.
Secondary treatment is biological. Bacteria are introduced, either mixed directly into the sewage or grown on filters the water passes through, and they consume the dissolved organic material: sugars, fats, proteins, and other carbon-based compounds. Because these bacteria need oxygen to work, the sewage is typically aerated by pumping air through it or stirring it mechanically. This stage dramatically reduces the organic load and removes many pathogens as a side effect.
Tertiary treatment is the polishing step. Sand filtration catches remaining particles. Specialized bacteria can strip out nitrogen, and others can absorb and store phosphorus in their cells, pulling both nutrients out of the water before it’s discharged. Some facilities use lagoons, where aquatic plants, algae, and naturally occurring microorganisms filter the water over days or weeks. Disinfection with chlorine or ultraviolet light typically happens at the end to kill remaining pathogens.
What Happens to the Sludge
The solids removed during treatment don’t just disappear. They’re processed separately through a step called sludge digestion, where bacteria break down organic matter in enclosed tanks without oxygen. This process can produce methane gas, which some treatment plants capture and use to generate electricity, partially offsetting the facility’s energy costs.
Once treated to meet safety standards, sludge becomes what the EPA calls “biosolids.” These can be applied to agricultural land as a soil conditioner or fertilizer, supplying crops with nitrogen and phosphorus. In 2024, facilities reported approximately 2.39 million dry metric tons of sewage sludge land-applied across the United States. Uses include growing food crops, feed crops, fiber crops, and energy crops, or conditioning soil on grazing land.
The Global Treatment Gap
In wealthier countries, most sewage passes through treatment plants before reaching the environment. Globally, the picture is far worse. Over 80% of wastewater returns to ecosystems untreated or in an unusable state. In 2022, roughly 42% of household wastewater worldwide received no proper treatment before discharge, sending an estimated 113 billion cubic meters of raw or poorly treated sewage back into rivers, lakes, and oceans.
That untreated sewage feeds algal blooms that suffocate aquatic life, contaminates drinking water sources, and spreads waterborne diseases through communities that lack the infrastructure to stop it. The nitrogen and phosphorus in sewage act as fertilizer for algae, creating oxygen-depleted “dead zones” in coastal waters where fish and other marine life cannot survive. In regions without reliable sewage treatment, contaminated water remains one of the leading causes of childhood illness and death.