What Happens After You Flush?

Most people probably give little thought to what happens to whatever they flush down the toilet. But the engineers and technicians who run community sewage treatment facilities think about it all the time.

Decades ago, many towns simply released untreated sewage into rivers and oceans. Because human waste harbors disease-causing organisms, this practice posed an obvious threat to public health. It was also harmful for another reason: The raw sewage killed fish and other aquatic animals. The problem was not that human diseases spread to aquatic organisms. Instead, aquatic microbes decayed the feces and other organic wastes in the sewage. As the microbes respired, they used all the available O2 dissolved in the water. Huge numbers of fishes and other aquatic organisms suffocated and died.


Federal law now mandates that communities treat wastewater before releasing it. Treatment plants harness the power of microorganisms to consume the organic matter in sewage before it enters waterways. In trickling filters, for example, sewage-eating bacteria and archaea are given “dream homes”—all the organic matter they can eat, along with plenty of moisture and O2. After the microbes have done their job, the treated water contains a very low concentration of organic matter.


The presence of these microscopic workers explains why communities prohibit dumping used motor oil or organic solvents such as paint thinner down the drain. Toxic chemicals can poison the bacteria and archaea that degrade sewage, making water treatment impossible. Thanks in part to water quality laws, U.S. river ecosystems have largely recovered from the past onslaught of untreated sewage. The most commonly used forms of sewage treatment do not, however, remove all chemicals from the waste stream. In particular, a wide array of household chemicals and pharmaceutical drugs often remain in the water discharged from a sewage treatment plant. The antibiotics in soaps and hand sanitizers are especially common, as are hormones excreted in the urine of women taking birth control pills. Ecologists are still studying the effects of these chemicals on wildlife. In the meantime, experts recommend against flushing medications—or anything other than human waste—down the toilet.

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