When Hurricane Harvey hit Texas in 2017, at least 40 wastewater treatment plants were damaged by the floodwaters, and some leaked raw sewage for weeks. That smelly public health nightmare could become commonplace as sea levels rise along the coasts of the United States, a new study suggests. The highest increase considered—almost 2 meters—would inundate hundreds of plants nationwide, leaving more than 31 million people without any wastewater treatment services.
In most U.S. cities, wastewater from toilets, showers, and sinks travels through a network of pipes and sewers to a central treatment plant where about 60% of suspended solids are removed and microbes consume dissolved organic matter. The water is then transferred to an artificial pond or wetland where additional waste settles out or is digested, after which the water is discharged into a river or the sea. To save energy transporting effluent, most coastal treatment plants are built close to the ocean at low elevations.
Past researchers have examined how rising seas might affect these low-lying wastewater treatment plants locally, but no one has yet examined the nationwide impact. In their new study, Hummel et al. used geographic information system (GIS) mapping to see how low-lying coastal plants across the United States might be affected under several different sea level rise scenarios produced by the National Oceanic and Atmospheric Administration. They also conducted a detailed case study of 36 treatment plants in the San Francisco Bay Area, which treat an average of 600 million gallons of wastewater per day and serve more than 5.7 million people.
On a national level, the team found that a sea level rise of roughly 0.3 meters would compromise 60 wastewater treatment plants and affect 4 million people. The worst-case-scenario projection—an almost 2-meter rise—would cause 394 plants to be exposed, affecting 31 million people. Far smaller increases would cause major problems in the San Francisco Bay Area, they found. Just a 75-centimeter increase in sea level, for example, would flood wastewater plants in Silicon Valley, Palo Alto, San Mateo, and Union City, and a 125-centimeter increase would flood plants in Sunnyvale, Millbrae, Oro Loma–Castro Valley, and the San Francisco International Airport.
Rising seas will also raise the groundwater table in the aquifers that underlie much of the Bay Area, the team found. When the groundwater table rises, levees and seawalls built to keep the ocean out can actually increase floods by trapping water inland. In San Francisco and across the country, cities need to consider both types of flooding to protect their wastewater treatment systems and the network of pipes and sewers that feed them, the authors argue. In some cases, plants may simply need to be moved to higher ground.