But the paradigm is flawed. For a start, cleaning sewage guzzles energy. Sewage treatment in Britain uses a quarter of the energy generated by the country’s largest coal-fired power station.

Then there is the nutrient problem: Human excrement is rich in nitrogen, phosphorus and potassium, which is why it has been a good fertilizer for millenniums and until surprisingly recently. (A 19th-century “sewage farm” in Pasadena, Calif., was renowned for its tasty walnuts.) But when sewage is dumped in the seas in great quantity, these nutrients can unbalance and sometimes suffocate life, contributing to dead zones (405 worldwide and counting, according to a recent study). Sewage, according to the United Nations Environment Program, is the biggest marine pollutant there is. Wastewater-treatment plants work to extract the nutrients before discharging sewage into water courses, but they can’t remove them all.

And there’s also the urine problem. Urine, like any liquid, is a headache for wastewater managers, because most sewer systems take water from street drains along with the toilet, shower and kitchen kind. Population growth is already taxing sewers. (London’s great network was built in the late 19th century with 25 percent extra capacity, but a system designed for three million people must now serve more than twice as many.) When a rainstorm suddenly sends millions of gallons of water into an already overloaded system, the extra must be stored or  if storage is lacking  discharged, untreated, into the nearest river or harbor. Each week, New York City sends about 800 Olympic-size swimming pools’ worth of sewage-polluted water into nearby waters because there’s nowhere else for it to go.

This probably won’t kill us, but it’s not ideal. Environmental scientists in California have calculated that sewage discharged near 28 Southern California beaches has contributed to up to 1.5 million excess gastrointestinal illnesses, costing as much as $51 million in health care. We can do better.

Image Credit... Katherine Streeter

Urine might be one way forward. Before engineers scoff into their breakfast, consider that since at least 135,000 urine-diversion toilets are in use in Sweden and that a Swiss aquatic institute did a six-year study of urine separation that found in its favor. In Sweden, some of the collected urine  which contains 80 percent of the nutrients in excrement  is given to farmers, with little objection. “If they can use urine and it’s cheap, they’ll use it,” said Petter Jenssen, a professor at the Agricultural University of Norway.