HONG KONG — Heinz-Peter Mang is obsessed with turning human waste into gold. As millions of Chinese move to cities, the German engineer is convinced the country is on the way to hitting the jackpot.

A growing portion of China’s toilet waste is converted into fertilizer and biogas. In Beijing, some estimates show 6,800 tons of human excrement are treated each day, enough to fill almost three Olympic-size swimming pools.

Over the past decade, China’s economic ascent has driven millions of rural workers into its cities in the largest migration in human history. In 2013, the number of urban dwellers crossed 731 million, overtaking the rural population by more than 100 million. Some fallouts: water shortages in the North and toilet waste routed into rivers in the south.

That’s forcing city planners to get creative in dealing with toilet refuse, and drawing engineers like Mang to help refine models. The push to re-purpose feces into energy resources or fertilizer is expanding across China, and Mang is advocating for the model to be copied in other parts of the world.

“The world has much to learn from China in the way it’s harnessed waste for energy,” said Mang, 57, who now works with graduate students on ecological sanitation projects at the University of Science and Technology Beijing. “With the lack of taboo around reusing fecal matter, it’s all about the science for safe reuse, and with more and more people moving into cities there’s an unprecedented opportunity.”

Globally, a variety of techniques are used to handle human waste: some cities dump it in rivers, others choose to incinerate, and still others bury it in ditches. United Utilities Group Plc, Britain’s largest publicly traded water company, handles the sewage of 1.2 million people in Manchester and operates a sludge recycling center that runs on enough human waste to power 25,000 homes.

While other parts of the world also harness fecal sludge into resources, and human beings have for centuries found ways to re-purpose their waste, China’s opportunity to do so is unmatched as the nation’s cities become more crammed.

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Mang’s life and work has centered around China, and offers a window into the country’s evolution on managing waste. He first arrived in Chengdu, China, in 1982 as part of a German government delegation, collaborating with China on a biogas project.

He was hooked from the start. The willingness of the Chinese to try different ways to use waste sustainably was amazing to Mang, then a newly graduated environmental engineer who authored a master’s thesis on sustainable uses of sewage sludge.

Through the ’80s, he worked in Africa, preaching the Chinese way of waste conservation. In the 1990s, he worked in Cuba, using Chinese models to help Cuba set up a system to funnel pig manure into growing porcine food on farms after the Russians cut off exports of pig feed.

Meanwhile, the world’s most populous nation scaled up a model used in farms. Originally used to keep humans from doing their business in pig troughs, today 40 million farm homes across China have a holding tank for human and animal waste that is partly sanitized by depriving the solids of oxygen. What’s left is then converted to liquid fertilizer for the farms.

What’s happening in Beijing is an industrialized, scaled-up version of that model, said Mang, who has lived in the capital for a decade. Across the city, which has seen its population double to 21 million in the past decade, the average annual amount of human waste processed will increase by 200 to 300 tons a day, said Zhang Jiang, general manager of Beijing Century Green Environmental Engineering & Technology Ltd., which operates night-soil treatment plants. Treating waste is set to be a growing area of business, Zhang said.

Other parts of the world are also making an effort to harness energy from fecal sludge. In recent years, the Bill & Melinda Gates Foundation invested $1.5 million in a project to explore biodiesel production from human waste in Ghana. It’s also investing in family-level biogas units or septic tanks that process human waste in Thailand and India.

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On a recent Wednesday, Mang took a group of visiting sanitation researchers and students to visit some of Beijing’s private plants. Processing at one named Sijiqing — or “four seasons green” — begins on the ground floor of a two-story, red-tiled building. The odor of human excrement in the air gives away its true purpose: some 200 trucks unload 800 tons of fecal sludge every day.

A bright yellow truck parked inside channels the human waste through a pipe into a machine, where unrecyclable solid material like tissue paper and plastic bags is separated, explained Zhang Hui, the plant’s manager.

The rest then goes for separation: the solid waste is propelled into a compost wing to ferment at 60 degrees Celsius for 10 days. The process kills harmful bacteria and ascaris eggs — parasitic roundworms that infect humans — and turns the excrement into rich fertilizers for trees and vegetables. The liquid material is routed into tanks to generate biogas, and eventually pumped to bigger water-treatment plants. Phone calls to the Beijing mayor’s office and the city commission in charge of environmental sanitation management weren’t answered.

Born into a fruit-farming family near Frankfurt, Mang’s father earned additional income as a truck driver, often transporting manure and straw. As a baby, Mang’s mother would leave him on top of manure heaps wrapped in a blanket as it was “always soft and warm.” He reckons that’s one reason he’s never been squeamish about fecal matter.

Today, he works with government officials, academics, health planners and universities. As a guest professor at the sustainable environmental sanitation department of the university in Beijing, he spends his time working with students to answer questions around waste that are getting more crucial as cities fill up.

Among other initiatives, he now provides technical assistance for biogas training to municipal governments as part of the U.S. government’s Global Methane Initiative. He also consults on economic projects related to biogas in the country.

A central part of his life’s work has been looking at how to keep the waste-recycling process hygienic, a top concern of the Chinese government from the beginning. While China is far ahead of other developing countries in terms of urban wastewater collection systems, its treatment rate is not as advanced.

“So the user doesn’t come into direct contact when expelling their excrement,” Mang said. “But where does it go after? From that sense, we will need to find safer pathways of disposal and reuse.”

The catch-up of waste treatment from low penetration rates in China is driving “a waste revolution,” with key treatment operators likely to grow 200 to 400 percent in volume in the next five years, Credit Suisse said in an Oct. 6 note on China’s environment sector. The analysts didn’t specifically mention the toilet-waste industry.

Only part of Beijing’s waste is converted into resources. Other portions are routed to treatment plants or dumped in bodies of water or directly into landfills.

New migrants to cities are one challenge to mass implementation of China’s policies, with many still seeing the toilet as a trash can and dumping everything from batteries to newspapers in the bowl.

Also, because Chinese often cook with fresh ingredients, the overall mix of waste tends to be wetter than in Europe, Mang said. “It’s very humid so it needs too much energy to incinerate, that’s why you can’t copy the European model,” he says with a hearty chuckle.

Then there is the challenge of missed opportunities. For that, Mang is advocating for better maintenance of light bulbs in public toilets.

“People have to go at all hours of the night — if you don’t give them light, how can they see where they’re going?” he says, referring to excrement that sometimes piles up in the mornings in public restrooms. “That’s a lot of wasted waste.”

— With assistance from Li Hui in Beijing.