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The EPA calls combined sewers “remnants of the country's early infrastructure.” The first sewers weren’t designed to handle the constant and huge stream of wastes from our toilets, because they were invented when we didn’t have any toilets. Sewers were originally built to solve the problems of cities that were flooded with their own refuse—garbage, animal manure, and human waste left in the open rather than in a privy or latrine—during every rainstorm. To prevent that flooding, the fouled stormwater was shunted out of town and into the nearest handy receptacle, which was often a lake, river, stream, or ocean.

When flush toilets became common in the mid-1800s, they were piped into these existing sewers, introducing much more human waste, as well as a large volume of water that had never been there before. In some ways, this was a design feature, not a bug, because the burst of stormwater flushed out pipes that might have otherwise gotten clogged. This flush of rainwater also diluted the waste before it hit a nearby river.

In time, though, dilution wasn’t enough to keep waterways safe and attractive, and sewage treatment plants were invented to clean up the waste stream before releasing it to water bodies. Newer cities, which were starting from scratch, generally handled stormwater separately from human and industrial wastes from the start, but cities whose sewer systems had always been combined continued to treat both waste streams together.

As the older cities grew larger, their combined-treatment systems struggled to keep up, and growing populations weren’t the only factor. Time itself exacerbated their woes. In Hoboken, for example, some sewer lines date back to the Civil War. Common sense says that pipes that have been buried for a century and a half tend to leak. Over time, they also get clogged with debris or even congealed cooking oil, resulting in narrowed pipes that overflow even more easily.

When narrowed pipes are already overloaded, the added influx of stormwater when it rains becomes just too much water. Now, some cities experience overflows with less than a quarter-inch of rain, with resulting risks to human health. It is common for cities with combined-sewer systems to advise citizens to stay out of the water for days after any rainfall. And now the Environmental Health Perspectives study suggests that after a very heavy rain, those overflows may be affecting their communities’ drinking water, too.

What is being done? Combined sewers have been an EPA priority for many years and, after decades of significant effort, the numbers are starting to move in the right direction, but this is not a problem that can be turned around quickly or cheaply. New York City’s combined sewers are still the single largest source of pathogens to the New York Harbor system, according to the New York Department of Environmental Protection. A single 2014 storm triggered a release into Lake Erie from Detroit, Michigan, of more than 44 million gallons of raw sewage from sanitary sewers and almost 3 billion gallons from combined sewers, and such releases from Detroit and the other cities with sewer outfalls on Lake Erie contribute to the fact that it blooms with algae every summer. Last summer, one of those algal blooms cost Toledo its drinking water for two days, and this year’s harmful algal blooms were projected to be even worse than last year’s.