The restorative water that flows into public baths in small towns off the western coast of Japan comes from hot springs more than two-thirds of a mile underground. On the surface, before the water bubbles out of the nozzle, it is further heated to 107 degrees Fahrenheit—the ideal temperature for cleaning and soaking tired muscles.

But most swimmers don’t know that the boiler that heats the water uses the most unclean source of fuel: particles recovered from dirty adult diapers.

In rapidly aging Japan, older people with incontinence use more diapers than babies. As the country groans under the weight of the rising mountain of garbage, Houki Town has become a pioneer in efforts to reduce it. By recycling diapers, which account for about one-tenth of the town’s waste, it diverts waste that would otherwise be dumped in the incinerator and increase atmospheric emissions.

As many other countries face similar demographic collapses, adult diaper waste is a hidden challenge that coincides with labor shortages in nursing homes and insufficient funding for pension systems.

"If you think about it, this is a difficult and big problem," said Kawai Kosuke, a senior researcher at the National Institute of Environmental Studies. "Japan and other developed countries will face similar problems in the future."

In Houki Town, Tottori Prefecture, which has a population of just over 10,500, officials are concerned about the rapidly growing diaper waste and are considering the cost of upgrading outdated incinerators. They decided to transform one of the two incinerators in the town into a diaper recycling plant and produce fuel that would help reduce the cost of natural gas heating in public bathrooms.

In the bathroom, there is no advertisement about the source of boiler fuel. Satomi Shirahase, 45, was visiting with her husband from Kyoto. When she learned of the source of the heat, she was not disturbed.

Recycling "sounds great to me," she said in the locker room after hiking in the nearby mountains, which is very similar to the more famous Mount Fuji. "I was not scared. It's good water."

The challenge of diapers is particularly serious in Japan. Although Japan is almost obsessed with waste sorting, more than 80% of the country's waste goes into incinerators—more than any other wealthy country. Although the number of most other waste sources is declining as the population of Japan decreases, the number of incontinence products for the elderly is increasing by tons.

According to data from Japan's Ministry of the Environment, the number of adult diapers entering Japan's waste stream has increased by nearly 13% in the past five years, reaching nearly 1.5 million tons per year. It is expected to further increase by 23% by 2030, when the population aged 65 and over will account for nearly one-third of the total population.

Because diapers contain a lot of cotton pulp and plastic, they will expand to four times their original weight when soiled, so they require more fuel to burn than other waste sources. This has resulted in expensive waste management costs for local municipalities and large amounts of destructive carbon emissions.

Unlike other products such as disposable plastics, the use of diapers cannot be restricted without affecting hygiene and health care.

Kremena M. Ionkova, senior urban development expert at the World Bank, said: "We can easily eliminate straws and umbrellas from cocktails." "But we can't eliminate diapers."

Recognizing this growing problem, Japan's Ministry of the Environment convened a working group last year to discuss alternatives to incineration of diapers. A few other municipalities are following Houki and turning diapers into fuel pellets, and some municipalities are experimenting with converting them into materials that can be mixed with cement for construction or paving.

Unicharm is one of Japan's largest diaper manufacturers. It has established a pilot plant in Kagoshima, southern Japan, where it recycles diapers into more diapers.

One of the biggest challenges of recycling is the need for caregivers to separate dirty diapers from all other waste. Hayato Ishii, an official from the Department of Recycling and Promotion of the Ministry of Environment, said that less than 10% of municipalities require households to separate diapers from general trash.

In Houki, individual families do not sort diapers, but in 6 nursing homes, assistants put diapers in special deodorant bags for disposal, and these bags are towed to the recycling plant every working day.

In Daxian Rehabilitation Hospital, eight out of ten of the approximately 200 patients need disposable diapers, and residents generate approximately 400 pounds of such waste every day.

On a recent afternoon, 33-year-old Tatsushi Sakata was one of two workers at the diaper recycling plant. He collected 35 large bags from the cinder block storage space at the back of the facility—each bag contained 30 dirty diapers. Used in the past 24 hours and threw them on the bed of a Toyota pickup truck.

Sakata usually collects nearly a ton of bags during his daily inspections. At the recycling plant, he and his colleagues wore Tyvek catsuits, rubber boots and helmets, and poured diapers into a bucket the size of a small trailer. They are sterilized and fermented for 24 hours at a high temperature of 350 degrees, reducing their volume to one-third of their contaminated weight. This process converts the diaper into fluff, which is processed by another machine into 2-inch long gray particles.

These operations are a bit reminiscent of the factory scene in "Soylent Green", a 1973 dystopian thriller in which the nutrition film is made from human remains. Although ceramic and charcoal filters are designed to remove odors, when the particles fall from the bright orange chute into a large plastic box, the machine emits a faint smell of yeast and baking.

“In the beginning, I did think it was a bit creepy because we were dealing with manure,” said Sakata, who has worked in the factory for 10 years. "Our goal is to turn unmanageable garbage into manageable things."

Houki Mayor Tamotsu Moriyasu stated that there is no revenue from the recycling operation, although it does save fuel costs in the incineration plant and reduces transportation costs. He said that tourists who want to understand this process come from all over Japan, as well as Indonesia and Tahiti.

In the public bathroom, the operator pours the pellets into a large funnel, which is connected to the biomass boiler by a wide plastic tube. The particles are burned to generate the extreme heat required to heat the bath water. According to government calculations, although the process will produce carbon emissions, the particulate pollution is less than that of coal or petroleum gas used in boilers before.

"When I first heard about it, I thought,'um,'" said Tatsuya Sakagami, a 68-year-old retired municipal official who occasionally uses the bathroom. "But adult diapers are just items used by humans."

"In the past, people used human waste to fertilize vegetables," he added in the parking lot of the bathhouse on a recent afternoon. He said that turning dirty diapers into fuel is no different. "I think this is a good idea because it is ecologically better."

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