According to the Washington Post, there is no law or regulation that says an airplane has to have a working toilet:
“We have no rules regarding restrooms,” U.S. Department of Transportation spokesman Bill Mosley said, suggesting that the Environmental Protection Agency be asked.
The EPA does set water-quality standards for aircraft, spokesman Dale Kemery said. He promised to investigate whether the agency has any further involvement. Soon he called back. “We don’t do toilets,” he said, and suggested contacting the Federal Aviation Administration (FAA).
Not us, said FAA spokeswoman Alison Duquette, who said that if there were a rule, it would be an EPA rule.
Boy, you learn something new every day, don’t you?