This piece is part of the Sidewalk Talk series "15 Innovations That Shaped the Modern City."
Modern sanitation systems are an innovation that no city or engineer ever truly set out to master. Arguably they were more or less avoided until our lives and health depended on finding new solutions. "We take things for granted," says Jon Schladweiler, curator of "The Sewer History Exhibit," a traveling exhibition of photos and artifacts that covers the evolution of sewers from antiquity to the present. "But without sewage systems, civilization as we know it would not exist."
The story of urban sanitation traces back to the Bronze Age civilizations of the Indus Valley, in what is Pakistan today. The Indus ruins reveal a remarkable feat of early waste systems: Most houses were built with horizontal and vertical drain pipes made of brick. Underground drains ran under streets, topped by stone slabs serving as early manhole covers. Toilet holes were "flushed" with a jar of well water; home drains then linked up with street drains, which fed out to brick soak pits.
But it was the Romans who first tackled sanitation at major metro scale. Etruscan engineers built what would eventually become the Cloaca Maxima ("greatest sewer") around 500 BC. Originally conceived as an open canal to drain local marshlands, the system was converted gradually into an underground sewer running through the Forum and exiting at the Tiber. (Parts of it are still functional today.) Named after the goddess of purification and powered by Rome's aqueducts, the Cloaca shuttled everything from street trash to public bath water to flows of excrement from the city's "rooms of easement."
Cloaca, alas, was not well-served: Romans still dumped refuse into the streets, open troughs teemed with discharge from outhouses, and most everything unwanted wound up in a fetid Tiber. "The main function of the sewer system was not actually to make Rome cleaner," says Marden Nichols, professor of classics at Georgetown University, "but to empty the city of the great amounts of water the aqueducts were bringing in." On the whole, she adds, Rome remained "absolutely, disgustingly dirty."
Not until the mid-1800s did the link become clear between sewage and deadly waterborne diseases like cholera. It was at that time that the River Thames in London, which had ably dealt with human waste for centuries, reached its limits. It could no longer handle the output of a booming metropolis. The most tragic consequence: three major cholera outbreaks between 1800 and 1850 that claimed more than 30,000 lives and finally forced city leaders to act.
Enter Joseph Bazalgette, one of the city's chief engineers, who in 1859 began a mammoth, decade-long project to build a network of sewers that ran parallel to the Thames, intercepting dirty surface water and underground waste and diverting it all downstream and out to sea. Three massive pumping stations fed the sewer's 82 miles of new brick-constructed tunnels and pipes. Though the steam-powered pumps themselves were impressive, the system's simplest but most critical innovation was the V (or egg) shape of the main tunnels, which maximized water flow and supported more weight above them. (Some say Bazalgette's controversial choice of portland cement, which was costlier and much trickier to lay but much stronger than conventional alternatives, was just as important.)
With an eye toward the future, Bazalgette built the system to scale. He calculated the size of the tunnels required of the current population and doubled it. Fully constructed, the new system could move 528 million gallons of water per day, sufficient to handle London's needs for another century.
While it arguably saved a city, the London model was the exception, not the rule. Installing and upgrading sewage systems is expensive and disruptive; the work requires both public tolerance and political will. Stubbornness and stinginess held back progress then, just as it often does today. As late as 1875, not a single U.S. city with a population greater than 100,000 had a uniform system for treating sewage. It took until 1926 for 20 of those same cities to install treatment plants.
Throughout the 20th century, the rapid expansion of American urban centers exposed an irony in the relationship between cities and sewers: Having helped spur urban growth by beating back disease, newer sewer systems struggled to keep pace. They also began to influence the way cities looked and functioned, demanding wider right-of-ways and remade sidewalks to accommodate sewage networks without interrupting traffic flow.
"The old 'out of sight, out of mind,' approach to sanitation gave way to an understanding that you couldn't just try to hide away your sewage in any alley or easement," says Schladweiler. "Sewer systems had to be more accessible for maintenance and repairs, which meant you had to be able to get at them easily from city streets."
As suburban and rural towns have grown, local sewers are often connected to the nearest central system in large cities. The Clean Water Act of 1972 eased pressure on sewage infrastructure by funding construction of treatment plants. But even today, sewage capacity has a profound influence on development. More people means more sewage, and installing underground pipes and pumps remains costly.
"Over the course of history, those sorts of changes have generally only happened when things got bad enough for people to do something about them," Schladweiler says. "That's still true today. You almost have a crisis for something that needs fixing to get fixed."
Next-generation sanitation systems have the potential to wield much more economic influence than their predecessors. Future sewage plants emerging in Europe won't just serve as waste management centers but as full-scale, self-powering recycling plants systems that can extract energy, clean water, and valuable commodities from human waste.
In Hamburg, Germany, a pilot waste-recovery plant that opened in 2015 incinerates all sewage sludge, treating the resulting ash with phosphoric acid. The acid then acts as a catalyst in recovering phosphorus---a critical commodity in all of agriculture---and other materials, such as calcium, gypsum, aluminum and iron. The latest dividend from all that sewage? The Hamburg plant generates 70 million kilowatt-hours of electricity per year, enough to power nearly 6,500 homes.