HOUKI, Japan >> The restorative waters that flow into the public baths in this town near the coast of western Japan originate from hot springs more than two-thirds of a mile below ground. At the surface, before the water bubbles out of the spouts, it is further heated to 107 degrees — an ideal temperature for cleansing and soaking weary muscles.

But unbeknownst to most bathers, the boiler heating the water runs on a fuel with the unlikeliest of origins: pellets recycled from soiled adult diapers.

In rapidly aging Japan, more diapers are used by older people than by babies. As the country groans under the weight of ever-rising mountains of this waste, the town of Houki has become a pioneer in trying to reduce it. By recycling the diapers, which represent about a tenth of the town’s trash, it has diverted garbage that would otherwise be dumped into incinerators.

With many other nations facing a similar demogra­phic implosion, adult diaper waste is a challenge that looms alongside labor shortages in nursing homes and insufficiently funded pension systems.

“When you think about it, it is a difficult and big problem,” said Kosuke Kawai, a senior researcher at the National Institute for Environmental Studies.

In Houki, a town of just over 10,500 people in Tottori prefecture, officials were worried about the fast-growing waste and looking at the costs to upgrade an outdated incinerator. They decided to convert one of the town’s two incinerators into the diaper recycling plant and produce fuel that would help reduce heating costs at the public bathhouse.

At the baths there is nothing advertising the provenance of the boiler fuel. Satomi Shirahase, 45, who was visiting with her husband from Kyoto, was unperturbed when she learned of the source of the heat.

The recycling effort “sounds pretty good to me,” she said in the dressing room. “I am not creeped out. It was good water.”

The diaper challenge is especially great in Japan, where more than 80% of the country’s waste goes into incinerators — more than in any other wealthy nation. While most other sources of waste are declining as Japan’s population shrinks, incontinence products are growing by the ton.

The amount of adult diapers entering the waste stream in Japan has increased by nearly 13%, to almost 1.5 million tons annually, in the past five years, according to data from the environment ministry. It is projected to grow another 23% by 2030, when those 65 and older will represent close to one-third of the population.

Because diapers contain so much cotton pulp and plastic, and swell to four times their original weight after soiling, they require much more fuel to burn than other sources of waste. That leads to costly waste management bills for local municipalities and high volumes of damaging carbon emissions.

And unlike other products, such as single-use plastics, the use of diapers cannot be restricted without compromising sanitation and health care.

A handful of other municipalities are adopting Houki’s solution, while some are experimenting with converting them into material that can be mixed with cement for construction or road paving. Unicharm, one of Japan’s largest diaper manufacturers, has built a pilot plant in Kagoshima, in southern Japan, where it is recycling diapers back into more diapers.

In Houki, individual households do not sort out diapers, but at six nursing homes, aides dispose of the diapers in odor-cutting bags that are hauled to the recycling plant every weekday.

At the Daisen Rehabilitation Hospital, where 8 out of 10 of the approximately 200 patients require disposable diapers, patients produce about 400 pounds a day.

On a recent afternoon Tatsushi Sakata, 33, one of two workers at the diaper recycling plant, collected 35 hefty bags — each containing 30 dirty diapers and all used within the previous 24 hours — and tossed them onto the bed of a Toyota pickup truck.

Sakata usually gathers close to a ton of the bags on his daily rounds. At the recycling plant, he and his co-worker, dressed in Tyvek bodysuits, rubber boots and helmets, dump the diapers into a vat the size of a small trailer. They are sterilized and fermented for 24 hours in 350-degree heat, which cuts their volume to one-third of their soiled weight. The process converts the diapers into a fluff that is processed and turned into 2-inch-long gray pellets.

The operations evoke the factory scenes from “Soylent Green,” the 1973 dystopian thriller in which nutrition wafers are made from human remains. Using filters designed to remove foul odors, the machinery emitted a faint yeasty, toasted smell as pellets rained down from a bright orange chute into a large plastic box.

“At the beginning I did think it was a little creepy because we were dealing with excrement,” said Sakata, who has worked for 10 years at the plant. “Our purpose is to turn unmanageable garbage into something manageable.”