Around the country, cities and towns are increasingly turning to treated wastewater to augment their supplies of drinking water. The number of drinking-water reuse projects has quadrupled over the past two decades, according to data collected by the National Alliance for Water Innovation, a research program funded by the United States Department of Energy.

“It is now necessary for us to consider options that would, in previous generations, be considered unthinkable,” said Michael Kiparsky, director of the Wheeler Water Institute at the University of California, Berkeley.